Not surprisingly the most striking thing about the new Volkswagens is the increased power output of both the gasoline and diesel engines. Along with this higher output, fuel mileage in the German highway and city fuel consumption tests is also considerably improved. The biggest gains in fuel mileage (and really quite impressive performance) are from the little 1.4 liter turbo charged four cylinder gasoline engine. This 1.4TFSI has a direct injection system, and is able to belt out 148hp at just 5000RPM with maximum torque available down to 1500RPM. Performance of the flagship two liter turbo diesel has also been improved, with the highest output version doing 236hp at 4000RPM. The array of power output options on this 2.0 TDI is bewildering, with 134hp, 148hp 161hp, 174hp and 188hp versions all available in some models. The highest output 236hp version appears to be available only on the four wheel drive Volkswagen Passat sedan and station wagons. The 2015 model year is particularly interesting for readers familiar with the early works of the great science fiction writer Isaac Asimov. The Asimov story that references 2015 model year automobiles is titled "Sally", and was first published in the May/June 1953 issue of "Fantastic".
The Passat and the A4
The Golf and the A3
Full Time Four Wheel Drive
Fuel Consumption Testing
Improved Volkswagen Gasoline Engines
Speculation on Further Improvements
For 2015 the 1.4 TFSI turbocharged gasoline engine also makes itís best showing in the Volkswagen Passat, but this time in the two wheel drive version with a manual transmission. What is so impressive is that such a large and heavy vehicle can do 55mpg (4.3l/100km) on gasoline in the highway test. And even in the city the mileage stays up at 40mpg despite the car weighing in at 3000 pounds. The Volkswagen Passat and the Audi A4 are not the same cars for 2015 as they have been in the past, but they are in fact extremely similar cars with approximately the same exterior dimensions and weights. With the same drive trains performance is also nearly identical. With the two liter turbo diesel fuel mileage is a very impressive 65mpg (3.6l/100km) for the Passat, and the slightly heavier but also sleeker A4 is able to do 67mpg in the highway test. It is interesting that the heavier (3200 pound) A4 gets the 134hp output 2.0TDI while the lighter (3100 pound) Passat gets the slightly higher output 148hp 2.0TDI. The zero to 100kph times of the Passat are therefore a bit quicker at 8.9 seconds to the 9.3 seconds of the A4. The slightly lower and slightly narrower A4 is also able to attain the same 134mph top speed as the Passat despite the lower power output rating. Interestingly though the interior dimensions for the area of the front seats is actually larger for the A4. In this case heavier and more expensive yields a roomier and faster car with the same motor. The extra weight of the A4 hurts city fuel mileage only very slightly with 49mpg to the 50mpg of the Passat with the 2.0TDI.
The other models of greatest significance seem to be the very similar Volkswagen Golf and Audi A3. These are in essence the smallest cars because the smaller models do not use less fuel. The smaller and much less expensive Volkswagen Polo actually uses more fuel with itís three cylinder diesel. Likewise the Audi A1 does not get even slightly better mileage than the bigger A3. The very lightweight Volkswagen Up! does deliver an impressive 64mpg with it's 74hp port injected one liter three cylinder engine, but this is not very good compared to the 55mpg that the much larger Passat gets with the 1.4TSI gasoline engine. Even the 46mpg city mileage rating is not very good compared to the 40mpg that the much heavier Passat does on gasoline. The Up! is however an interesting vehicle with it's low 1800 pound weight yet still very substantial interior dimensions with seating for four generously sized adults. Going along with seating for four hefty humans the payload rating is nearly a thousand pounds and the top speed is 107mph. For a different perspective on small Volkswagens see The Volkswagen XL1. The Golf and A3 are not the same cars, but the dimensions and weights are very similar. And again the Audi is a bit heavier and a bit sleeker. This time though the mileage and performance numbers are exactly the same with the same drive trains. With the 148hp 1.4 TFSI the 57mpg highway and 41mpg city mileage is really not much better than for the Passat and A4. With the 109hp 1.6 liter turbo diesel though these smaller cars get a whole lot better mileage at 78mpg on the highway and 62mpg in the city. This 78mpg is really a very large improvement in performance, as previously this kind of mileage was only attained with a smaller 90hp 1.4 liter four cylinder diesel in the even smaller Volkswagen Polo. At 13'11" long, 5'11" wide, 4'9" tall and with a GVWR of 4200 pounds the Volkswagen Golf is not a small car. These days just 109hp does not sound like much for a car, but the 2015 Golf 1.6TDI will go 124mph on that 109hp and the zerro to 100km time of 10.5 seconds is only a bit slow compared to the nine second 0-60mph time of a 140hp 1.8 liter Honda Civic.
For 2015 it appears that only a very limited number of the Volkswagen and Audi models are being offered for sale in America. Even though almost all of the model names are accounted for, the U.S. offerings are available with extremely limited engine and transmission options. In fact in the U.S. the only transmission available for most models is the torque converter equipped automatic, where in Germany most models are available with either a manual transmission or a torque converterless automatic. In Germany the torque converter equipped automatic transmissions are for the most part only used on the extremely high power output V8 engines. With four or five hundred horse power on tap in a still rather light weight 4100 pound Audi A8 the seamless and smooth power delivery is no doubt a significant advantage. Not surprisingly the A8 is also only available as a full time four wheel drive, where most of the smaller models are available either as two wheel drive or full time four wheel drive models. There are however a few models offered in Germany with the same engines and either a torque converterless automatic transmission or a torque converter equiped automatic transmission. On these models there is certainly a difference in fuel consumption, with the torque converter sucking down about 5 to 10% more fuel in both city and highway tests. This is a rather modest difference really, which indicates that the new torque converter equiped eight speed automatic transmissions are making good use of the torque converter lockup clutch even in lower gears. Also of interest is that the zero to 100kph times suffer somewhat with the torque converter. Again though the difference is small, with the torque converter equiped models taking about seven percent longer to get to 100kph with the excact same engine and power rating.
One model conspicuously absent from the U.S. model lineup is the somewhat ironically named Volkswagen California and it's more utilitarian siblings the Caravelle and Transporter. The Vee Dub bus has been a fixture of the mark for quite a long time, and it's latest iteration being absent from California seems very strange indeed. The new front wheel drive light truck from Volkswagen is in weights and dimensions similar to the previous models from the past quarter century, but performance is considerably boosted with the 200hp 2.0TFSI four cylinder gasoline engine delivering as good as 30mpg (7.8l/100km) on the highway with a five speed manual transmission. There are only the two four cylinder two liter engines available in the vans, gasoline and diesel, and the 2.0TDI delivers as good as 40mpg with a five speed manual. Also absent from the U.S. model line are all of the other Volkswagen trucks ranging from an american style pickup truck to the big rear wheel drive flatbeds and cargo vans with payload ratings of 1500 pounds up to 6000 pounds. The most interesting thing about the trucks is that they all use the same two liter turbodiesel which is ubiquitous throughout the Volkswagen and Audi model lineups. The output rating is just 161hp for the big trucks, but the power output curve is somewhat different with 3600RPM being the maximum engine speed and 295 foot pounds of torque peaking at 1800 to 2000RPM. Highway fuel mileage is 38mpg with the smaller cargo van body providing a 5'10" wide by 8'6" long cargo area and 5'5" headroom, and even the highest 11,000 pound gross vehicle weight rating yields the same 38mpg on the highway. With the optional IFS four wheel drive system fuel mileage inexplicably drops all the way off to 30mpg (7.9l/100km).
The pickup truck with a 177hp rated 2.0TDI and the standard 6 speed manual transmission tests out at 36mpg (6.5l/100km) on the highway and a full time four wheel drive system drops the mileage only slightly to 6.6l/100km. A selectable four wheel drive system is also available, and although fuel consumption results inexplicably apear to remain mostly unchanged there are some differences in 0-100kph times. The two wheel drive pickup will reach a hundred kilometers per hour in ten seconds flat and the full time four wheel drive system takes 10.6 seconds. The selectable four wheel drive system comes in right in the middle with a 10.3 second 0-100kph time. Not too surprisingly the American style pickup is also available with an American style eight speed torque converter equipped automatic transmission, but only with the full time four wheel drive system. The auto trans drops the highway mileage only very slightly to 35mpg (6.8l/100km), but 0-100kph times also rise to 10.9 seconds. At 76.5 inches wide with room for a 4x8 sheet of plywood between the wheel wells and with a payload rating of up to 2700 pounds the Volkswagen pickup is as big as a full size pickup truck, but it's empty weight of 3500 to 4200 pounds is less than most small pickups weigh. What is a bit hard to understand is that the Volkswagen pickup gets no better than 36mpg on the highway and 29mpg (8.0l/100km) in the city where the heavier and slightly wider full time four wheel drive Audi A8 somehow manages to get 46mpg on the highway and 32mpg in the city with the 255hp three liter turbodiesel. The difference on the highway is obviously at least in part due to the lower and sleeker shape of the sedan versus the pickup. The fact that the A8 gets better mileage in the city with an engine displacing half again as much is probably due mostly to larger wheel bearings and heavier duty drive line components used on the pickup.
For 2015 the gap in fuel mileage for the full time four wheel drive cars and the two wheel drive cars has also been narrowed, but the full time four wheel drive cars still use quite a lot more fuel for a variety of reasons. Of course the more complex transmission with two power outputs along with a second differential and two extra drive shafts are going to suck down a bit more power. The big difference in fuel mileage has however traditionally been due to lower overall gearing for the four wheel drive models combined with a tendency for the four wheel drive models to be available only with the higher output engines and automatic transmissions.
The torque converterless Volkswagen automatic transmissions have been doing an excellent job for nearly a decade of narrowing the gap in performance and fuel mileage between the automatic and the manual. In the past though even the torque converterless automatics tended to use a bit more fuel than the simpler manual transmissions. For 2015 there are some models where the torque converterless automatic does essentially just as good as the manual transmission. The best performing models with the really high fuel mileage numbers though all benefit from the simpler and more efficient manual transmissions. For the full time four wheel drive models there is little or no difference between the manual and torque converterless automatic transmissions in performance and efficiency.
Still though the full time four wheel drive models somehow manage to suck down considerably more fuel. One major oversight appears to be that the Passat is not available with both four wheel drive and the 1.4 liter gasoline engine. To get four wheel drive on the Passat the only option is the big 236hp high output 2.0TDI with an automatic transmission and fuel mileage drops off all the way down to 51mpg. For the Audi A4 there are more full time four wheel drive options, but none of them do better than the 53mpg that is possible with the 188hp 2.0TDI and a manual transmission. Going to the torque converterless automatic on this 188hp full time four wheel drive model yields the same 40mpg in the city and an essentially identical 52mpg on the highway. The difference between 67mpg for the two wheel drive A4 and 53mpg for the full time four wheel drive A4 is huge, and is probably still in large part due to lower overall gearing for the four wheel drive model.
In the smaller A3 the difference in fuel mileage between the two and four wheel drive models is even more dramatic as the four wheel drive A3 is not available with the 1.6 liter diesel that is available in the two wheel drive model. What this means is that the four wheel drive 2.0TDI A3 does only 55mpg to the 78mpg of the two wheel drive model with the 1.6TDI. The four wheel drive A3 being available only with the big two liter diesel seems like a rather large oversight considering that the same 2.0TDI is the engine used in the Volkswagen trucks with a GVWR of up to 11,000 pounds.
Where the difference in mileage between the two and four wheel drive models is smaller is with the gasoline engines. The A4 with the 221hp 2.0TFSI four cylinder turbocharged gasoline engine can do a quite impressive 49mpg (4.8l/100km) on the highway and 32mpg (7.4l/100km) in the city, and with the full time four wheel drive system this drops off to only 44mpg (5.3/100km) and 28mpg (8.5l/100km). It is not that 44mpg is any kind of good mileage, but for a big heavy full time four wheel drive four door sedan that can attain a top speed of 155mph and can reach 62mph (100kph) in just 6.4 seconds it is much better than was possible in the past. The same 2.0TFSI powered full time four wheel drive A4 is also available with a seven speed torque converterless automatic transmission which drops the highway mileage to 42mpg (5.6l/100km), but in the city test the mileage remains at 28mpg and the zerro to 100kph time also remains unchanged. Interestingly the two wheel drive 2.0TFSI powered A4 with the same 221hp output rating takes very nearly a half a second longer to reach 100kph; 221hp is quite a bit more than a front wheel drive car can handle even on the stickiest dry pavement available at the test track.
The bigger 3800 pound A6 also does pretty good with the full time four wheel drive system. With a higher output 248hp version of the 2.0TFSI and a torque converterless automatic transmission the two wheel drive A6 does 46mpg(5.1l/100km) on the highway and 32mpg (7.4l/100km) in the city. Again there is that trend of the smaller cars not doing much better on fuel. The bigger A6 with the 2.0TFSI will also reach the 155mph electronic limiter and at 6.7 seconds actually goes zero to 100km quicker than the two wheel drive A4 with the 2.0TFSI. The full time four wheel drive A6 is not available with the 2.0TFSI, but with the big 328hp 3.0TFSI V6 and a torque converterless automatic the mileage stays impressively high at 39mpg (6.0l/100km) on the highway and 24mpg (9.8l/100km) in the city while the zero to 100km time drops all the way down to a blistering 5.1 seconds. The engine that the A6 is available in both two and four wheel drive versions with is the 3.0TDI V6 diesel rated at 214hp, 268hp or 315hp. The two wheel drive 214hp 3.0TDI A6 with a torque converterless automatic gets 55mpg (4.3/100km) on the highway and 43mpg (5.5l/100km) in the city and will reach a top speed of 152mph just shy of the 155mph limiter. The four wheel drive version also with a torque converterless automatic somehow manages to reach the same 152mph top speed with the same 214hp 3.0TDI, but gets only 51mpg (4.6l/100km) on the highway and 40mpg (5.9l/100km) in the city. With the higher output 268hp rated version of the 3.0TDI and again with a torque converterless automatic transmission the full time four wheel drive A6 gets the same 55mpg on the highway and 40mpg in the city but will do zero to 100km in just 5.5 seconds. To best the acceleration performance of the 3.0TFSI the 3.0TDI is also available in a 315hp output rating in the A6 with a torque converter equipped automatic transmission. This racing version of the diesel A6 gets only 44mpg (5.3l/100km) on the highway and 32mpg (7.2l/100km) in the city.
The German fuel consumption test (NEFZ) for highway mileage is a short six minute test cycle that starts from a stop but has no stops in the middle. The maximum speed is 75mph, but this high speed is held for only a short period of time with the average speed being about 45mph. This test has been used since 1996, previously fuel consumption was listed at a constant 56mph highway cruise or a constant 75mph autobahn cruise. The city simulation test has remained about the same since 1978.
From the introduction of U.S. EPA fuel consumption testing in 1978 a simulation on a dynometer has been used. Although wind tunnel testing to go allong with this simulated testing yielded fairly realistic results slight anomalies did occur. The highway fuel consumption test involved several near stops with a maximum speed of 60mph and an average speed of 48mph. The car was ďdrivenĒ on the dynometer by a paid professional driver who had considerable leeway in terms of deciding how each individual car should be driven. In 1985 a correction was added to the published results so that the number listed on the window sticker represented a 22% higher fuel consumption than what the testing for that model had actually yielded. From 1985 to 1996 both the corrected and uncorrected fuel consumption numbers were listed together, and then starting in 1997 only the lower corrected MPG figure was published. The same simulated highway fuel consumption test was used through 2007, but then in 2008 a totally new test cycle was introduced. This new test cycle was widely publicized as taking into account use of the air conditioner as well as the realities of much higher speed freeway cruising. What has not however been widely available is information on just what the new test cycle is, even the average speed and maximum speed for the new test cycle has not been available. It appears that the new highway fuel consumption estimates include a correction similar to the old 22% increased fuel consumption tacked on from 1985 to 2007, but information about this has not been available either.
The current U.S. EPA tests certainly do come up with radically different numbers for fuel consumption than the German NEFZ tests. The Audi A6 with full time four wheel drive for example is available both in the U.S. and in Germany with the 3.0TDI V6 turbodiesel and the eight speed torque converter equipped automatic transmission. The U.S. EPA estimated fuel mileage is 24 mpg city and 38mpg highway where the German NEFZ test results are 32mpg (7.3l/100km) in the city and 44mpg (5.3l/100km) on the highway. And to add insult to injury the 3.0TDI in this A6 with the torque converter is rated at 315hp (235kw) output in the German model and just 240hp in the U.S. model. The zero to 100km (0-62mph) time of the German model is five seconds flat where the 0-60mph time of the U.S. model is 5.5 seconds. For the two wheel drive Volkswagen Passat which is available with a manual transmission on both sides of the Atlantic the difference in fuel consumption is even more dramatic. The Passat with the 147hp 2.0TDI turbodiesel and a six speed manual transmission that is rated by the German NEFZ to get 65mpg (3.6l/100km) in the highway test and 50mpg (4.7l/100km) in the city test gets a U.S. EPA mileage estimate of just 44mpg on the highway and 33mpg in the city. That is just an absolutely huge difference.
The only small (if you can call 200+ horsepower small) gasoline engine available for direct comparison is the 2.0TFSI in the Audi Q5 or the Volkswagen EOS. The full time four wheel drive Q5 with the 2.0TFSI four cylinder gasoline engine is available with the torque converter equipped eight speed both in the U.S. and Germany. The German NEFZ test yields 25mpg (9.6l/100km) in the city and 34mpg (6.9l/100km) on the highway where the U.S. EPA estimated fuel mileage is just 20mpg in the city and 28mpg on the highway. In the Volkswagen EOS the 2.0TFSI is available in both Germany and the U.S. with the six speed torque converterless dual clutch automatic. In the German NEFZ test the EOS does 40mpg (5.9l/100km) on the highway and 23mpg (10.2l/100km) in the city while the U.S. EPA estimated highway mileage is all the way down at 30mpg and the city mileage is about the same at 22mpg. The other gasoline powered Q5 available on both sides of the Atlantic is the hybrid with the same 2.0TFSI gasoline engine. In the German NEFZ test the Q5 hybrid gets 36mpg (6.6l/100km) in the city and actually does not go as far on a gallon in the highway test with 33mpg (7.1l/100km) results. The U.S. EPA estimated mileage for the Q5 hybrid is an unbelievably abysmal 24mpg in the city and 30mpg on the highway. The 2.0TFSI really is a very big four cylinder gasoline engine with a 3.65 inch stroke and a 248hp output rating in the Audi A6. Interestingly the A6 is able to do 46mpg on the highway and 32mpg in the city with this higher output 2.0 TFSI even though at 3700 pounds it weighs just as much as the Q5. The Q5 is an inch wider and considerably taller than the A6, but this extra size would not be expected to cut into city fuel mileage where the speeds remain extremely slow compared to the top speed of 138mph. Even with the half again larger displacement 328hp six cylinder 3.0 TFSI gasoline engine and full time four wheel drive the A6 still does 39mpg (6.0l/100km) on the highway and 24mpg (9.8l/100km) in the city with an automatic transmission. Somehow the Audi Q5 just seems to be a particularly fuel hungry model.
For larger engines there are many models to compare as it is the high output engines that the eight speed torque converter equipped automatic transmission is most often found behind in the German model lineup. The Audi A8 is available with both the 306hp 3.0TFSI V6 gasoline engine and the 429hp 4.0TFSI V8 gasoline engine in both Germany and the U.S. and only comes with the eight speed torque converter equipped automatic transmission and full time four wheel drive. In the German NEFZ test the 3.0TFSI does 22mpg (10.5l/100km) in the city and 37mpg (6.7l/100km) on the highway where the U.S. EPA estimates are 19 city 29 highway. For the big V8 the German NEFZ test goes down to 19mpg (12.6l/100km) in the city and 33mpg (7.1l/100km) on the highway where the highway mileage is actually estimated higher than for the V6 by the U.S. EPA at 30mpg. The smaller discrepancy between German and U.S. testing for the big V8 powered model is due to the fact that bigger engines generally don't get much better mileage at low speed. The extremely high overall gearing for the V8 powered Audi which yields 78mph at 2000RPM undoubtably has something to do with this smaller discrepancy as well.
The gasoline engines in the Volkswagen and Audi cars are definitely doing better on fuel than in past years. The 55mpg gasoline powered Passat is in fact running just about as efficiently as the 65mpg diesel powered Passat during the German highway test averaging about 45mph. The difference between 65mpg and 55mpg is hardly more than the difference in energy content between the diesel fuel and the gasoline. What is interesting here is that it takes putting the little 1.4 liter gasoline engine in the big Passat and running it on the highway test to get this impressive efficiency of nearly as good as the two liter diesel. In the city test the mileage of the Passat drops off to 39mpg for the 1.4 liter gasoline engine and 50mpg for the two liter diesel. Under a lighter load the diesel engine remains more efficient.
This trend of lighter load operation working better on the diesel engines continues with bigger engines in smaller cars. In the big 3700 pound Audi A6 the 214hp output three liter V6 turbodiesel yields 55mpg on the highway. The little 188hp 1.8 liter turbocharged direct injection gasoline engine however can only do 47mpg with the same torque converterless automatic transmission. In this comparison the gasoline engine is not holding up as well against the diesel engine despite the displacement of the gasoline engine being hardly more than half as much as the displacement of the big three liter diesel. This bigger gasoline engine really falls flat on itís face in the city fuel consumption test getting only 32mpg to the 43mpg of the big three liter diesel.
For the full size Audi A8 which is only available with full time four wheel drive and a torque converter type automatic transmission there seems to be no fair comparison between gasoline power and diesel power. The most reasonable direct comparison seems to be between the high output 255hp 3.0TDI and the 306hp 3.0TFSI gasoline engine. What is not fair about this comparison is that the gasoline engine is of just as large a displacement as the diesel engine. A gasoline engine of the same displacement would normally be considered a larger engine since it revs higher and makes more power. In this case though both engines are high output turbocharged engines, and the fact that the diesel uses more boost at a lower engine speed makes them roughly equivalent engines. With the three liter diesel the A8 gets 46mpg on the highway and with the three liter gasoline engine it gets 37mpg on the highway. That is a rather large efficiency advantage for the diesel engine, and in the city the difference is of course much more dramatic. On the low speed city test the three liter diesel does 32mpg to the 22mpg of the three liter gasoline engine. For the A6 the 3.0TDI gets a 315hp rating, but this higher output rating is not available in the bigger A8. Presumably the idea being that the big A8 is intended for very high speed cruising, and loading the 3.0TDI more heavily up near the top of itís speed range is not as good as going with the big 4.2 liter 380hp V8 turbo diesel.
It might also be wondered why the 2.0TDI has up to a 236hp output rating where the 4.2TDI remains below 400hp. The answer obviously is that no car has any use for that many hundreds of horsepower, and if it is absolute high power output from a small and light engine that is desired the 512hp twin turbo V8 gasoline engine certainly does deliver the goods. Incidentally the 32mpg that the A8 gets in the highway test with the 512hp V8 is a substantial improvement over other current muscle cars and supper cars.
So how is it that the gasoline engines have improved so much. The prime suspect would be the direct injection system. A gasoline engine with a direct injection system should be able to run much more efficiently at low engine speeds under a light load than a regular gasoline engine can. The reason for this is that injecting the fuel at the time of combustion at low engine speeds allows for a prolonged combustion event that allows the pressure in the cylinder to remain high later when better conversion efficiency at the crankshaft is possible. This is why diesel engines can run all the way down to 150RPM where gasoline engines donít do well at all at less than about 3000RPM and gasoline engines continue to do better and better up to at least 6000RPM.
With the gasoline being injected at the time of combustion at low engine speed a gasoline engine would essentially be running as a diesel engine. Up to this point gasoline direct injection engines have not done very well with running in time of combustion injection mode, and the efficiency gains over port injection engines have been only rather modest. Now with a 55mpg gasoline powered 3000 pound Volkswagen Passat though it appears that the gasoline direct injection engines are finally doing better.
The confusing point here though is that the better performing Volkswagen TSI engines still appear to suffer immensely when they are required to run under radically reduced loads for city driving or when big engines are put in small cars. This hints that perhaps the performance gains of the current generation of Volkswagen TSI engines is due instead to incremental improvements in engines and transmissions as well as overall improved vehicle design. From this perspective it would then be said that the diesel engines are actually doing worse than they did in the past since a 1998 Passat with the 1.9TDI was also able to deliver 65mpg in highway fuel consumption tests. That same 1998 Passat could not do better than about 34mpg with the turbocharged gasoline engines of the time.
Even when time of combustion injection is not used at all gasoline direct injection systems have several advantages over port injection. A direct injected normally aspirated engine can make a lot more power than a normally aspirated port injected engine simply because the fuel does not get in the way of the intake air being sucked through the intake valves. This is however not nearly as significant on a turbocharged engine. Direct injection systems can however get a gasoline engine to actually run better under light loads even if time of combustion injection is not used. By injecting much of the fuel late in the compression stroke a rich air/fuel mixture can be established in the area of the spark plug so that the flame front travel combustion will burn more fuel in a shorter period of time even when the engine is running very lean under a light load. Burning more fuel in a shorter period of time during flame front travel combustion allows less spark advance to be used which both allows the time of late compression ignition to be somewhat later and of course also reduces the amount of fuel that has to be burned in flame front travel mode. This localized rich air/fuel mixture that can be created in the immediate vicinity of the spark plug accomplishes the same thing as a faster flame front travel speed fuel being used, and it certainly can get a gasoline engine to run better over a wider range of engine speeds and loads. For more on flame front travel speed and late compression ignition see Combustion Properties of Fuel.
Even with these advantages of direct injection the engine is still running as a gasoline engine and all of the fuel burns all at once no later than about 15 degrees past top dead center unless time of combustion injection is used. Any fuel that can be injected after late compression ignition takes place would help a gasoline engine to run better at lower engine speeds of less than about 6000RPM, but this is in fact time of combustion injection and time of combustion injection works better when all (or nearly all) of the fuel is injected during the combustion event.
Ultimately 55 or 65mpg is not all that impressive for a full size sedan traveling at 45mph. Pushing the Passat at 45mph might require only 7 to 10hp, which could be as little as a third or a half a gallon per hour of number two diesel fuel. That would be 90 to 135mpg! The 1.4 to 2.0 liter direct injection gasoline and diesel engines should be able to do much better at supporting these light loads. What it comes down to is that neither the direct injection gasoline engines or the direct injection diesel engines are delivering the correct injection flow rate and timing for optimum light load efficiency. Better injection systems could allow better fuel mileage.
It might also be pointed out that even when running in time of combustion injection mode the gasoline engines are hampered by the low compression ratios they have to run. A 17:1 diesel engine is going to have an easier time running under a very light load than a 9:1 turbocharged gasoline engine. Recently the compression ratios of gasoline engines have been dramatically increased, but more boost and higher power output from a slow turning gasoline engine means that a lower compression ratio must be used.
So ultimately the limitation of the Volkswagen TSI engines is that they donít spin fast enough. The 1.4TSI that makes 148hp at 5000RPM is impressive, but it requires a rather low compression ratio. Going with a shorter stroke and a larger bore could easily yield the same 148hp from 1.4 liters without forced induction. A normally aspirated 1.4 liter four cylinder capable of making 148hp would have to spin up to 7000RPM. The problem with this is that the higher spinning normally aspirated engine would tend to work much better with a shorter stroke, and a shorter stroke is going to severely cut into light load performance down at 1800RPM with a direct injection system.
The solution of course is even more radically undersquare forced induction engines with longer strokes and smaller bores. To get a gasoline engine to work as well as it possibly could with a competent direct injection system the forced induction would not be used to make more power at lower engine speeds, but would rather be used to allow a radically undersquare engine to flow well and make big power up to 8,000 or even 10,000RPM. If the forced induction is used not to build big boost at low engine speed but rather to get the engine to flow at high engine speed then the compression ratio can be kept just as high as would be used on a normally aspirated gasoline engine running the same fuel.
That same maximum compression ratio for a gasoline engine could potentially also allow higher torque production down at low engine speeds if the direct injection system was up to the task of supplying a large amount of fuel during time of combustion injection. There are however a couple of potential problems with this. One obviously is that a forced induction system does not tend to be able to supply a large amount of boost at a low engine speed and then still efficiently supply a small amount of boost at high engine speed. The other main problem with a radically undersquare high speed gasoline engine is thermal loading. Generally the maximum amount of power that an engine can make is proportional to the area across the tops of the pistons. A water cooled engine of course can run at much higher power output without thermal problems than can an air cooled engine, and likewise aluminum cylinder heads allow much higher maximum power output than do iron heads. Still though a long stroke engine with a two inch bore cannot make all that much more power than a short stroke engine with a two inch bore without something getting too hot.
The maximum power output for reasonably long engine life with a certain size bore is in fact rather high. Three inch bore and radically oversquare engines can run quite well at 40hp output per cylinder. A longer stroke does increase the heat transfer away from the piston somewhat, and the three inch bore engine could safely make somewhat more power. A two inch bore though would tend only to be able to handle about half as much power as a three inch bore engine. This is however less of a problem than might be expected since higher cylinder count engines allow for much higher transmission efficiency. A six cylinder engine with a two inch bore and a three and a quarter inch stroke would displace one liter, and could be expected to safely run at 120hp output with little in the way of thermal loading problems. In all reasonableness 120hp is sufficient for a passenger car. That 120hp allows about a 120mph maximum speed in a roomy four door sedan and quite brisk acceleration even with a full thousand pound load. Such an engine would rev to at least 8000RPM, and some overrev up to 9,000 or 10,000RPM could be provided for as well. The meat of the power would come between 6,000RPM and 8,000RPM, but fairly good usable medium power output could also be used down to perhaps 3500 or 4,000RPM. With a competent direct injection system substantial efficient torque could also be generated from 2,000RPM to 3,500RPM. The in between area from 3000 to 4,000RPM would best be provided for by the time of combustion injection, but this would require a high capacity injection system capable of stepping up to a rather high injection flow rate to deliver a large amount of fuel in a short period of time.
Since the three and a quarter inch stroke would allow for such good torque generation from 2,000 to 3,500RPM if a competent injection system were used it does beg the question of how large boost could be provided at this much lower engine speed. The obvious answer of course is a dual stage forced induction system. This dual stage forced induction system would actually be two compressors, one to provide the perhaps one to two atmospheres of boost at 2,000 to 4,000RPM and another compressor to provide the perhaps a third of an atmosphere of boost required to attain a 100% volumetric efficiency up at 8,000RPM. If both of these compressors were driven by exhaust gas turbines then switching valves would be required both in the intake tract and in the exhaust system. Another option of course would be clutch driven compressors. Clutch driven compressors lose the advantage of cogeneration, but a combination of one turbocharger and one clutch driven compressor could regain some of the advantage of co-generation. The question then would be which compressor should be on a clutch and which should be exhaust gas turbine driven. The obvious answer is the one that runs more of the time should be the turbocharger. For automotive use it would seem that the engine would run most of the time in the lower speed range with time of combustion injection, but there is another reason why it might work considerably better for the high engine speed low pressure compressor to be exhaust gas turbine driven instead. If a turbocharger was sized to provide just a small amount of boost up at 6,000 to 9,000RPM then it would not be expected to get in the way much down at 2,000 to 4,000RPM. Looked at another way though the large boost and high torque generation would be producing large pulses of exhaust gas, and the turbocharger for higher speed operation would lead to some small amount of undesirable exhaust restriction. A variable compressor geometry type turbo charger could be used to reduce this drag by opening up the compressor to allow the exhaust turbine to spin free up to a medium speed where there would be less exhaust restriction.
From the perspective of the automotive engine being required to run along most of the time under light to medium loads where no boost would be required probably the best overall solution would be just two clutch driven compressors. The one liter engine could easily make 20hp normally aspirated at 2000RPM, and up at 3000RPM the maximum normally aspirated output would likely be more than 35hp. A roomy medium sized four door sedan could easily travel in excess of 70mph on 35hp.
Essentially the engine would be a diesel engine that would provide all sustained cruising power between 2,000 and 3,000RPM as a normally aspirated engine. Any time that a bit more power was required for quick acceleration or hill climbing the high pressure compressor would be clutched in to provide somewhere around one and a half times as much output. That would be 30hp at 2,000RPM, 40hp at 2,500RPM and over 50hp at 3,000RPM. From 3,000RPM to 4,000RPM the maximum output would probably remain pretty flat at not much more than 50hp and then from 4,000RPM the output would build to 90hp at 6,000RPM and 120hp at 8,000RPM.
With slight risk of elevated piston and cylinder wall wear a 130hp maximum might be available from 9,000 to 10,000RPM. This slight extra power and over rev would make the engine work well for the hardest acceleration in racing type driving where it would be desirable to shift down into the meat of the power at no less than 7,000RPM. The engine would probably rarely or never be run as a normally aspirated gasoline engine. About the only reason that the engine might be run as a normally aspirated engine would be for spinning over under a light load up at 3,500 to 5,500RPM. These engine speeds might be encountered if the engine was held in one low gear for negotiating variable terrain where shifting would be undesirable for some reason but only small amounts of power were required. Of course small amounts of power could also easily be supported by the injection system in time of combustion injection mode all the way up to 6,000RPM.
There would be two different ways that the engine could run as a diesel engine in time of combustion injection mode. One would be that a small amount of fuel would be injected early on the last part of the compression stroke, and then the spark plug would fire this fuel off sometime around or slightly before top dead center. The atomized fuel would burn in flame front travel mode until the temperature and pressure in the cylinder was high enough for injected fuel to ignite. This type of time of combustion injection would work with how ever low of a compression ratio might be used, and could also be used if for some reason a fuel with a higher temperature and pressure requirement for compression ignition had to be used.
The other way that the engine could run in time of combustion injection mode would be fully as a diesel engine, and the spark plugs would not even need to fire. This would be possible once the engine was warmed up and under boost. The extra pressure in the cylinders from the boost combined with the localized high pressure area caused by the fuel injected at high pressure would allow compression ignition even with a modest 11:1 compression ratio. Depending on the fuel that was to be used though the compression ratio of the engine might be as high as 14:1.
Interestingly this type of engine could potentially run on any fuel even if it had a high 14:1 compression ratio. If the fuel had too low of a temperature and pressure of compression ignition then the engine could not run as a gasoline engine, and power output would be limited to what the engine could make in time of injection combustion mode under boost up to approximately 4,000 or 4,500RPM. This could potentially be quite a bit of power, although both compressor efficiency and engine efficiency would drop off at higher speeds and very high power output. There would also be good reason to limit maximum power output down at 4,500RPM in time of combustion injection mode so as not to unduly stress the lightweight pistons, rods and bearings with high loads at low engine speed. Even if stress on lightweight pistons, rods and bearings was of no concern the maximum reasonable output in time of combustion mode would only be about 90hp at 4,500RPM.
When running on a fuel that would light off with destructive early compression ignition if the fuel was injected early the engine would be a full diesel engine, and the spark plugs would not be required at all. There might still however be some use for the spark plugs for cold starting on a fuel that was just a bit too easily ignited on compression ignition to allow the engine to run as a gasoline engine.
All of the difficulties with forced induction and compression ratio matching go away if just a pure diesel engine is used. Such an engine can run on any fuel, even the extremely clean burning gasoline favored by most city dwellers. In fact because getting a diesel engine to run well over a wide range of speeds and loads requires a very flexible injection system one single engine could run on any type of fuel. There is still some reason to match the compression ratio of the engine to the fuel being used, but the penalties for a mismatch between the compression ratio and the fuel are extremely mild compared to the large penalties in performance and efficiency for running a gasoline engine with a mismatch between the compression ratio and the fuel. What it comes down to is that a diesel engine can run on any fuel with about a 16:1 or 18:1 compression ratio and attain extremely high efficiency.
For a small diesel engine the ideal stroke is about four inches. Again it is a radically undersquare configuration with a much smaller bore that works best for getting the engine small without losing efficiency. Even a normally aspirated diesel engine can flow well enough for 3,000 to 4,000RPM operation with a radically undersquare configuration, but forced induction is highly desirable for stretching the maximum engine speed up to 4,500RPM while also allowing even more radically under square configurations with smaller bores. Even with forced induction an extremely radically undersquare configuration can lead to loss of efficiency at high engine speed simply because it takes more power to push the intake air through the small valves, so there is a limit to how small the bore can be. The faster the engine is required to spin the less under square it can be, but four valves per cylinder helps considerably with flow at higher engine speeds.
A two inch bore and four inch stroke six cylinder engine would displace 1.2 liters (75 cu-in). At 1800RPM this 1.2 liter engine would be able to do something close to 25hp normally aspirated and up at 3,000RPM it would be more like 40hp. That is actually a lot of power just normally aspirated, and with substantial boost power output could easily be over 100hp at up in the 4,000 to 4,500RPM neighborhood. Ultimately the limitations on power output would be of the same nature as with the radically under square forced induction gasoline engine. With the two inch bores much more than about 120hp from the six cylinder engine would be pushing the limits of heat dissipation to keep the pistons cool enough for long engine life. This 120hp maximum could of course be attained at even lower engine speeds with more boost, but this would in fact be contrary to good light load efficiency. Big power at lower engine speed would mean that the pistons, rods and bearings would have to be burlier. Heavier pistons and rods and larger capacity bearings means lower efficiency under light loads. Stretching the maximum engine speed out to 4,500RPM means that peak torque does not have to be as high, so lighter pistons and rods can be used.
The four inch stroke engine would run best at about 2,500RPM, although anywhere from 2,000 to 3,000RPM efficiency could be extremely good. Light load operation would best be kept below 2,500RPM and all the way down to 1,500RPM the engine would run quite well under a light load. A 1.2 liter engine at 1500RPM is just the ticket for good efficiency during low speed cruising where really only very small amounts of power are required to push even a quite large four door sedan.
The small gasoline and diesel engines in the 2015 Volkswagens and Audis are going in the right direction, and it appears that the injection systems are now doing a better job of matching the injection flow rate to the requirements of the engines over a wider range of engine speeds and loads. The gasoline engines appear to be doing quite well compared to what has come before, but that big turbocharged power at 5,000RPM is the wrong thing to get a gasoline engine to work as well as it possibly can. Unless that big power at 5,000RPM can be provided by time of combustion injection it is necessary to shift the power to a higher engine speed with a volumetric efficiency down closer to 100% so that the compression ratio can be the same as for a normally aspirated gasoline engine. Running with big boost as a gasoline engine at 5,000RPM unavoidably necessitates a radically reduced compression ratio, and this hurts both performance and efficiency quite a lot.
The VW 1.4TFSI gasoline engine making huge 148hp output from 5,000 to 6,000RPM is impressive, but possibly of even more interest is the fact that it can do 184 foot pounds of torque from 1500 to 3000RPM. That is 105hp output at 3000RPM, twice as much power as any normally aspirated 1.4 liter gasoline engine would be able to make at that engine speed. Unless the compression ratio is less than half of the maximum compression ratio for the fuel being used this has to be done with time of combustion injection. At least up to 3000RPM the 1.4TFSI is running as a diesel engine. Under a heavy load with full boost the spark plugs are probably not even used, but under a lighter load where less boost is produced the spark plugs might be used to get the fuel to light off. Up at 5000RPM the 148hp is more than one and a half times as much as any normally aspirated gasoline engine can produce. To be doing this as a gasoline engine the compression ratio would have to be just two thirds of the maximum compression ratio for the fuel being used. Since production gasoline engines are being sold with up to 14:1 compression ratios these days this would mean that the 1.4TFSI would need to have a 9:1 compression ratio to run as a gasoline engine at 5000RPM. Up at 148hp output at 6000RPM the 1.4TFSI could run as a gasoline engine with up to an 11:1 compression ratio if the maximum compression ratio for a normally aspirated gasoline engine is 14:1. Since the 1.4TFSI actually has a 10:1 compression ratio this would mean that it could only run as a gasoline engine up at about 5,500RPM or higher. Essentially the VW 1.4TFSI is a gasoline engine in name alone since it runs most of the time as an extremely low compression ratio diesel engine. With such a low compression ratio the spark plugs are indispensible for starting and light load operation but otherwise the 1.4TFSI apears to truely be a diesel engine. If any doubt remains about the Volkswagen direct injection systems allowing the gasoline engines to run in time of combustion injection mode the 221hp 2.0TFSI gasoline engine should be considered. Maximum torque is 258 foot pounds, and is available from 1500 to 4500RPM. By comparison a totally worked out four inch stroke big block Chevy bored out to 468 cubic inches (7.7l) running on pump gas has a torque peak of 558 foot pounds at 4500RPM (Test D on page 169 of "Big Block Chevy Performance" by Dave Emanuel published in 1995 by The Berkley Publishing Group, New York). In this comparison the VW 2.0TFSI is running at 177% of the specific output of the normally aspirated race engine. This would seem to require a compression ratio of 56% of the compression ratio for the normally aspirated engine. If 14:1 is the maximum compression ratio for a normally aspirated gasoline engine then the 2.0TFSI would need less than an 8:1 compression ratio to do this kind of output while running as a gasoline engine. The 2.0TFSI actually has a 9.6:1 compression ratio. The higher output version of the 2.0TFSI produces just a slightly higher 273 foot pounds of torque which is available from 1600 to 4700RPM, but it's 248hp comes at a somewhat higher 4900RPM versus the 4500RPM power peak for the 221hp version. Both versions rev out to 6,000RPM where they still make their full rated output.
For the full time diesel engines there is obviously much room for improvement in the injection systems to get both better light load efficiency and a wider range of engine speeds where near maximum power is available. Going to a slightly longer stroke might also help the diesel engines, although the three and three quarters inch stroke of most of the Volkswagen TDI engines is already pretty close to correct for a small diesel engine. The bore sizes certainly could be reduced considerably though to allow high cylinder count with low displacement. Two liters is just way too much displacement for automotive use, and any vehicle really could benefit from six cylinders instead of just four.
A competent injection system that can deliver a largely increased injection flow rate for high engine speed maximum power output would also allow higher power output than even the highest output VW TDI engines currently attain. With the big bore sizes currently used it would be no problem to belt out even more power up to higher engine speeds, but this is the wrong direction to go for diesel engines. If really huge power output is required from a small lightweight engine then a traditional gasoline engine with about a two or two and a half inch stroke running at around 6,000 to 10,000RPM is undoubtably the way to go.
The direction of development that seems to be hinted at by Volkswagen's now longstanding insistence on sticking to high output four cylinder diesel engines in rather heavy trucks is a switch to two stroke engines. The four stroke two liter turbodiesel already can belt out huge power at 2,500 to 4,000RPM, and better injection system performance would allow even more power. Four cylinders for a four stroke engine is however too low of a cylinder count for really good transmission efficiency. Going to a two stroke two liter four cylinder diesel engine power output could be increased with lighter pistons and rods and smaller bearings while also allowing the transmissions to be smaller, lighter, more efficient and longer lasting. The major obstacle to going two stroke would be that some means of starting the engine would be required since turbochargers don't make boost at cranking speed. An electric motor driven starting compressor would seem to make the most sense, although other strategies might be used instead. Two stroke diesels have usually used intake ports in the cylinder walls, but a forced induction two stroke might instead use all in head valves. The advantage of all in head valves would be that the pistons could be shorter and lighter for better light load performance. All in head valves on a fully variable valve timing system would also allow the engine to start and run under light loads as a four stroke. This would have the same advantages of less air pumped under light loads that cylinder deactivation systems seek to attain, but without the necessity of the deactivated cylinders bumping over as air springs and wasting power. For more on camless engines see Camshafts and Valve Timing Systems.
That is not to say that cylinder deactivation itself cannot be useful. A really good cylinder deactivation system would open the exhaust valves a bit towards the end of each exhaust stroke so that a vacuum would develop in the deactivated cylinders. The pistons bumping over against a vacuum would waste much less power than compressing a bunch of air each revolution of the engine. The advantage of burning the fuel in fewer cylinders under very light loads is another aspect to cylinder deactivation. Generally the fuel that burns early in any engine does not do much useful work in turning the crankshaft, but rather simply heats and pressurizes the combustion chamber so fuel can be burned later when it can do a more effective job of turning the crankshaft. Since it is a fixed amount of fuel that needs to be burned early just to keep the combustion chamber hot and pressurized, deactivating some of the cylinders under a light load allows more of the fuel to be burned later when it is more effective at turning the crankshaft. The only type of engine that does not benefit from the fuel being burned in fewer cylinders is a low compression ratio gasoline engine that runs in full flame front travel mode under all lighter loads. It might be thought that turning a four cylinder engine into a two cylinder engine would cause poor transmission efficiency, but this is not the case. A transmission is sized for high output on all cylinders, and under a light load it takes about the same amount of power to spin it over regardless of how many cylinders are firing.