If lowest possible engine speeds are considered desirable then the latest possible time of late compression ignition around 15 or 20 degrees ATDC would be the goal. If on the other hand peak efficiency and high performance is the goal then the earlier and easier to hit time of late compression ignition around 5 degrees ATDC might be considered the ideal goal. Hot Rod Magazine once (at least once that I know of) wrote: "Ideally the gasoline would all burn right at top dead center". They went on to qualify this by saying that unfortunately it takes considerable time for the flame front to reach across the combustion chamber. They were beating around the bush on late compression ignition to be sure, but they did mention an important point. A time of late compression ignition near top dead center yields the best performance. It is all about getting the pressure in the cylinder to build at the correct time and stay high for the correct number of degrees of crankshaft rotation. When the pressure versus crankshaft position profile is correct peak thermodynamic efficiency can be obtained. Just what time of late compression ignition yields this ideal pressure versus crankshaft position profile depends on the speed of the engine, the stroke length of the engine and the temperature of combustion potential of the fuel being used.
The Latest Possible Time of Late Compression Ignition
The Easy to Hit Time of Late Compression Ignition
Even Earlier Times of Compression Ignition
Cold Burning Specialty Fuels
The most important piece of information about the latest possible time of late compression ignition around 15 or 20 degrees ATDC is that it can only be obtained with spark timing later than about 25 degrees BTDC. At 27 degrees BTDC on slow flame front travel speed gasoline the latest possible time of late compression ignition around 15 or 20 degrees ATDC might sometimes be obtained when the tune of the engine is absolutely perfectly matched to the operating conditions. The reality though is that it takes getting down to 23 degrees BTDC or later to reliably hit the latest possible time of late compression ignition.
The latest possible time of late compression ignition around 15 or 20 degrees ATDC is not always easy to obtain because it requires a rather good match between the compression ratio of the engine and the fuel being used. When the latest possible time of late compression ignition around 15 or 20 degrees ATDC can be obtained it works well over a certain range of engine speeds and mean piston speeds. Just what engine speeds the latest possible time of late compression ignition around 15 or 20 degrees ATDC works well at depends somewhat on the stroke length of the engine and the temperature of combustion potential of the fuel. Longer stroke engines can run down to somewhat slower engine speeds, and colder burning fuels can also run down to somewhat slower engine speeds. Likewise a shorter stroke length needs to stay at a somewhat higher engine speed to run in late compression ignition mode. The interesting reality though is that there is actually only a slight stroke length and temperature of combustion potential dependence to the minimum engine speed that will work for the late compression ignition. This minimum engine speed for late compression ignition tends to be around 3,000 to 3,500RPM. Very long five and six inch stroke length engines can sort of run in late compression ignition mode down to around 2,500RPM, but they are extremely inefficient. Three and a half inch stroke automotive engines on the other hand need to stay above about 3,200RPM to work at all well in late compression ignition mode. Shorter stroke length engines tend to need to stay up at even higher engine speeds to work well in late compression ignition mode. It is not at all uncommon for two inch stroke engines to need to stay above about 4,000RPM to work even at the latest possible time of late compression ignition.
How high an engine can spin while attaining high efficiency at the latest possible time of late compression ignition around 15 or 20 degrees ATDC depends on the stroke length and the temperature of combustion potential of the fuel. Likewise the minimum engine speed for late compression ignition also depends somewhat on the temperature of combustion potential of the fuel. As with stroke length only slightly changing the minimum engine speed for late compression ignition, the temperature of combustion potential has much less effect on the minimum engine speed for late compression ignition than it does on the maximum engine speed for the latest possible time of late compression ignition.
On the hottest burning race gas engines will not work in late compression ignition mode down to quite as low engine speeds as outlined above. There is also colder burning gasoline. Other people may know more about this colder burning gasoline than I do, but I have seen it. The dramatically colder burning gasoline will sort of work all the way down to about 2,700RPM in a 2.5 inch stroke engine. At those very low engine speeds efficiency in late compression ignition mode is still rather poor, but the colder burning gasoline does reduce harshness and engine damage simply because the peak cylinder pressure is somewhat lower.
Where the colder burning gasoline really seems dramatically different is at the top of the engine speed range. Normally a three and a half inch stroke automotive engine will run up to about 4,500RPM at the latest possible time of late compression ignition on normal gasoline. Likewise a two inch stroke engine will run up to around 8,000RPM at the latest possible time of late compression ignition around 15 or 20 degrees ATDC. For higher mean piston speeds an earlier time of late compression ignition is required.
What is so confusing about the difference between the latest possible time of late compression ignition at 15 or 20 degrees ATDC and the earlier time of late compression ignition around 5 degrees ATDC is that it is sometimes difficult to tell them apart. If an engine is running spark timing in the 27 to 33 degree BTDC range then it is the earlier time of late compression ignition around 5 degrees ATDC that is going to appear to be the latest possible time of late compression ignition. With that much spark advance the later time of late compression ignition at 15 or 20 degrees ATDC is just not attainable.
This earlier time of late compression ignition works up to about 7,000RPM or so in a 3.5 inch stroke automotive engine and up to about 12,000RPM in a two inch stroke engine. That is a rather dramatic difference from the latest possible time of late compression ignition around 15 or 20 degrees ATDC. Of course the exact time of this earlier time of late compression ignition may sometimes also be a bit earlier around TDC or 5 degrees BTDC once an engine really gets going and is well heated up. Just as the 5 degree ATDC time of late compression ignition is easy to hit compared to the 15 or 20 degree ATDC latest possible time of late compression ignition the time of late compression ignition also rather easily climbs to TDC or even 5 degrees BTDC with just a bit more heat build up. It is only times of late compression ignition substantially earlier than TDC that take a lot more heat build up to obtain. It is easy to see that there is a significant difference between the piston still traveling up at 15 degrees BTDC versus the piston hanging essentially stationary at top dead center from 5 degrees BTDC to 5 degrees ATDC.
Two years ago in Combustion Fuel Properties I wrote that the latest possible time of late compression ignition around 15 degrees ATDC was good up to perhaps about 11,000RPM. This was a bit misleading because I did not specify any engine stroke for this estimate. And as it turns out even for the short two inch stroke engine I had in mind this estimate is not entirely correct. The problem was that all the engines that I was running had such low compression ratios that spark timing was up around 33 to 35 degrees BTDC and they were never hitting the latest possible time of late compression ignition. I knew that the excessively early spark timing was causing harsh operation bellow 6,000RPM, but I did not really put much thought into the fact that in those engines running 33 to 35 degrees BTDC spark timing the latest attainable time of late compression ignition was around 5 degrees ATDC not 15 degrees ATDC. This approximately 5 degree ATDC time of late compression ignition was good up to around 9,000RPM in the heavy, poorly running 2.5 inch stroke engines (2.44" stroke 1980 Kawasaki KZ440 and stock 2.48" stroke Husqvarna 350) and although advancing the spark timing did push the peak power point up higher by reducing power output at 7,500 to 8,500RPM it did not deliver more power above 9,000RPM.
Especially the rod bolt equipped KZ440 just did not want to run above 9,000RPM. Advancing the spark timing did deliver 9,500RPM but it was just a bunch more noise and harshness without even a slight bit more power. The KZ440 made maximum power at 7,500 to 8,500RPM when the spark timing was backed off to the point that it would just barely enter late compression ignition once fully warmed up. Earlier spark timing got the engine to make power when first fired up cold, but once warmed up it was just much harsher operation and the fuel mileage dropped from 65-70mpg to 50-55mpg. Backing off about 4 or 5 degrees of crankshaft rotation again on the spark timing got the mileage back up to 65-70mpg with dramatically more power everywhere from 4,000RPM up to about 8,000RPM.
The lighter, higher performance 350 Husqvarna motors would rev to 10,500RPM, but again advancing the spark timing did not deliver more power. The stock 1991 WXE 350 made maximum power when the spark timing was set just early enough that the engine would run without huge amounts of hesitation. Normally backing off 2 degrees on the spark timing caused the engine to totally loose power at all engine speeds from 4,500RPM up. And since that stock engine that was always requiring spark timing in the 32 to 35 degree range to run would not make torque bellow 6,000RPM it really was a total loss of power.
As it turns out it is the easier to obtain 5 degree ATDC time of late compression ignition that is good up to approximately 11,000RPM in about a two inch stroke engine. The latest possible time of late compression ignition around 15 or 20 degrees BTDC does not support such high engine speeds, at least with anything longer than an unheard of short stroke length. The latest possible time of late compression ignition around 15 or 20 degrees ATDC in a two and a half inch stroke engine really only works up to about 7,500 or 8,000RPM, and the earlier and easier to hit 5 degree ATDC time of late compression ignition will sort of work down to around 6,500RPM. It is only when the earlier and easier to hit 5 degree ATDC time of late compression ignition is used bellow about 6,000 or 6,500RPM in a 2.5 inch stroke engine that extreme harshness and dramatically reduced efficiency becomes a problem.
And it does also have a lot to do with how much fuel is burned before late compression ignition takes place. Slow flame front travel speed fuel absolutely can tolerate more spark advance around 4,500 to 6,500RPM. Even if slow flame front travel speed fuel is running with a spark timing of 30 degrees BTDC which cannot hit the latest possible time of late compression ignition around 15 or 20 degrees ATDC that slow flame front travel speed fuel still tends to be a bit less harsh because less fuel burns early in flame front travel combustion. Fast flame front travel speed gasoline running even at 27 degrees BTDC tends to be extremely harsh and inefficient at 5,000 to 6,500RPM because such a large portion of the fuel is burning in flame front travel mode before late compression ignition takes place. Not only is a large portion of the fuel burning in flame front travel combustion way earlier than does any good, but then when late compression ignition does take place it is happening at 5 degrees ATDC which is somewhat too early for 5,000 to 6,000RPM operation in a 2.5 inch stroke engine.
The numbers above are all for the normal gasoline that has been coming out of the pumps most of the time over at least the past 20 years or so. Then there is also the dramatically colder burning gasoline that I have seen a few times. The dramatically colder burning gasoline is considerably different as it requires earlier times of late compression ignition down to lower engine speeds. In a 2.5 inch stroke engine that is running spark timing of 33 degrees BTDC the dramatically colder burning gasoline will make better torque down in the 4,000 to 5,000RPM range of engine speeds than normal slow flame front travel speed gasoline is capable of at that excessively early spark timing value. In the same engine running the same excessively early 33 degree BTDC spark timing the dramatically colder burning gasoline will always make more torque at all lower engine speeds than normal gasoline provided of course that the spark timing remains well matched to the compression ratio on both types of fuels. It many cases it would be a matter of an engine running harshly and inefficiently with 30 degree BTDC spark timing at 3,500 to 5,500RPM on normal gasoline and then running much more smoothly with more torque at 35 degrees BTDC in the same 3,500 to 5,500RPM range on dramatically colder burning gasoline. What it comes down to is that spark timing earlier than about 25 degrees BTDC just does not work bellow 6,000RPM, and an engine running earlier than about 25 degrees BTDC spark timing at less than 6,000RPM is going to favor colder burning specialty fuels.
Perhaps an even clearer picture of the timing of late compression ignition is provided by how the three inch stroke Husqvarna 610 motor runs. The defining feature of the 610 motor is the short seven inch total intake stack length that provides a bit of intake stack boost above about 7,500 or 7,600RPM. This means that the 610 motor is essentially always able to give a good strong pull from 7,600 to 8,500RPM when it is running well on any fuel. I would say that under all conditions the earlier and easier to hit 5 degree ATDC time of late compression ignition comes on no later than 7,600RPM on the Husqvarna 610 motor, and upon close consideration I would have to say that this earlier and easier to hit 5 degree BTDC time of late compression ignition is in fact being used down to even lower engine speeds around 6,500 to 7,000RPM much of the time.
The best place to start when considering what engine speed works for the earlier and easier to hit 5 degree ATDC time of late compression ignition on the three inch stroke engine is with the stock engine with the 406g Mahle piston running a points ignition system. With that very heavy 406g piston the engine speed where the wide open throttle crankshaft wiggle advance occurred was down around 5,500RPM. When this 3 or 4 degrees of crankshaft rotation of crankshaft wiggle advance hit it nearly always resulted in loud and harsh operation from 5,500 up to about 6,500RPM. Then above about 6,500RPM the engine usually once again smoothed out for an even more powerful pull up to around 8,000RPM. At least the combustion smoothed out above 6,500RPM, that heavy 406g stock piston did a lot of crashing around up at higher engine speeds and smooth is hardly how the poorly balanced engine would be described.
When I cut that stock Mahle piston down to 368g with the same 10.2:1 stock compression ratio the crankshaft wiggle advance obviously was coming up at a considerably higher engine speed around 6,000 or perhaps 6,300RPM. This increase in the engine speed where the crankshaft wiggle advance hits nearly totally eliminated the excess harshness with static timing values less than about 24 degrees BTDC. With larger amounts of spark advance the engine was still very harsh, but it was then harsh and inefficient all the way from 3,500RPM up to 6,000RPM. It seems clear that the earlier and easier to hit time of late compression ignition at 5 degrees ATDC is too early for any engine speeds bellow about 6,000 or 6,500RPM on normal pump gas in the three inch stroke engine. This has been true for almost 20 years on the exact same motor.
When I cut the aftermarket Woessner 610 piston down to 332g things really started to go wacky. On the stock CDI ignition running a fixed spark timing of about 25 degrees BTDC the engine pulled really hard from 3,500RPM all the way up. The interesting thing though was that there was no excess harshness anywhere, it just pulled smooth and strong all the way up. With the stock ignition and about 29 degree BTDC spark timing there was however a bit of a flat spot in the power from 7,000 to 7,500RPM. Then when the intake stack boost hit at around 7,500 or 7,600RPM the engine really took off for a big pull up to 8,500RPM. Backing off to 25 degrees BTDC on the spark timing with the stock CDI ignition the flat spot at 7,000 to 7,500RPM still existed, but it was less noticeable and also less of a problem as the engine then ran even better over an even wider range of engine speeds from 3,000 to 9,000RPM.
When I put the points ignition system on the hot rod 610 motor with the 332g cut down Woessner piston it ran even better. The flat spot at 7,000RPM was gone and the engine pulled even harder from 7,000 to 8,500 and even 9,000RPM. It seemed like the wide open throttle crankshaft wiggle advance was coming right at 7,000RPM. The really big difference though was the crankshaft wiggle advance coming down to perhaps as low as 4,500RPM with small throttle openings. This extra spark advance down to much lower engine speeds under reduced loads made the engine more responsive with better instant torque available over a wide range of engine speeds from 3,000 to about 6,500RPM even after the engine had cooled off from coasting down a hill.
Then I started getting slow flame front travel speed gasoline even right from the gas stations though and it all went to hell. On the slow flame front travel speed gasoline the engine did not want to run above 5,500 or 6,000RPM, and it would start hesitating horribly and cutting out around 6,500 to 6,800RPM and would often just continue to hesitate it's way up to about 7,500 or 7,800RPM once the engine was well warmed up on a big pull.
Advancing the spark timing got the slow flame front travel speed fuel to pull pretty hard from 6,000 to 8,000RPM with only little bits of hesitation and cutting out here and there mostly right around 7,000RPM until the engine was fully warmed up. But then the engine tended to just be totally unusable harsh down at less than 4,000RPM with the advanced spark timing. Not just a bit of harshness from too low of an engine speed for late compression ignition, but huge amounts of harshness from a very early time of late compression ignition as soon as the throttle was cracked open a small bit. It was the difference between pretty good torque from 3,000 to 5,000RPM, then no torque at all bellow 3,700RPM on the same tank of gasoline once I bumped the static timing setting up four degrees. This was true even when the bump in the static timing setting was from 20 degrees BTDC to 24 degrees BTDC. If I had to go beyond about 26 degrees BTDC on the slow flame front travel speed fuel then the engine got rather harsh everywhere bellow about 6,000RPM.
On the slow flame front travel speed fuel the big problem was cutting out. The engine would just sharply stop making power anywhere from 6,500RPM up, and then a half second later it would just as sharply cut back in and make power again. The sharp, but erratic and somewhat random, cutting out and cutting back in sometimes sounded a bit like a Morse code message on an old telegraph buzzer. Kind of like Y,Z,F each time I hit 6,700RPM and then sometimes W,M,X right at 7,600RPM with the dashes representing making power and the dots representing cutting out. It seemed like the engine was falling off of late compression ignition and running in full flame front travel mode, and this may actually have been happening sometimes. What also might have been going on though was that the slow flame front travel speed gasoline also had a slightly lower temperature of combustion potential and the engine was requiring the earlier and easier to hit 5 degree ATDC time of late compression ignition way down at 6,000RPM.
The temperature of combustion potential of the fuel was not all that much lower, but it was perhaps a bit lower. The engine really was running amazingly well all the way down to 3,000RPM on the slow flame front travel speed fuel with spark timing in the 16 to 20 degree BTDC range. I thought this was just due to the small amount of spark advance, but there may actually have been a slight difference in the temperature of combustion potential also. Normally even with rather small amounts of spark advance there is some considerable harshness that increases as the engine speed is decreased from 3,500 to 3,000RPM on the three inch stroke engine. The three inch stroke engine will still make some torque down to 3,000RPM on normal gasoline, but it gets quite harsh all the way down there at the bottom bellow about 3,300 or 3,400RPM. For most purposes about 3,200 or 3,300RPM would be considered the bottom basement minimum engine speed for any three inch stroke engine running small amounts of spark advance.
If the temperature of combustion potential of that slow flame front travel speed fuel was a bit low then it would have been requiring the earlier and easier to hit 5 degree BTDC time of late compression ignition down somewhat lower than the normal 6,500 or even 7,500RPM "shift point". This shift from the latest possible time of late compression ignition at 15 or 20 degrees ATDC to the earlier and easier to hit time of late compression ignition at 5 degrees ATDC is an all or nothing shift. There does not tend to be any in-between time of late compression ignition between about 5 degrees ATDC and about 15 or 20 degrees ATDC. When the earlier and easier to hit 5 degree ATDC time of late compression ignition is being required down to 6,000RPM then running at the latest possible 15 or 20 degree ATDC time of late compression ignition up at 7,0000 and 7,500RPM is going to make a whole lot less power.
The reason that I never really noticed that shift from the latest possible time of late compression ignition around 15 or 20 degrees ATDC to the earlier and easier to hit 5 degree ATDC time of late compression ignition is just that the shift was taking place at just the right time around 6,500 to 7,000RPM on the three inch stroke engine where there was little or no change in power output between the two times of late compression ignition. If the shift occurs at too low of an engine speed then the engine gets very harsh and power output drops somewhat. If the shift occurs at too high of an engine speed then power output suddenly dramatically increases when the earlier 5 degree ATDC time of late compression is attained. When the shift happens at just the right engine speed though it is seamless and hardly noticeable. The engine neither gets much harsher nor makes dramatically more power, it just continues to rev and power continues to build in a linear manor.
Even higher mean piston speeds can be supported with even earlier times of compression ignition before top dead center, but these earlier times of compression ignition work very poorly on longer stroke engines. A short 1.75 inch stroke engine might spin up to 19,000RPM with an even earlier time of compression ignition, but a 3.5 inch stroke engine stubbornly does not want to make power much above 8,000RPM no matter how early the time of compression ignition is made. Unsurprisingly earlier times of compression ignition only work well at higher engine speeds.
Erlier times of late compression ignition certainly can support somewhat elevated mean piston speeds on long stroke engines, but it is not a good sort of power. Even on a long stroke engine earlier times of late compression ignition down to low engine speeds result in high spikes in cylinder pressure which are very hard on parts. A 5 or 10 degree BTDC time of late compression ignition at 14,000 or 15,000RPM on a two inch stroke engine might safely make huge amounts of power, but that same 5 or 10 degree BTDC time of late compression ignition at 8,000RPM on a four inch stroke drag racing engine just smashes things. It is about the pressure versus crankshaft position relationship. At very high engine speeds early times of late compression ignition still result in an even sort of a pressure curve where pressure stays rather evenly high late enough for the crankshaft to move around to where good conversion efficiency is possible. When an early time of compression ignition is used at lower engine speeds, even to support high mean piston speeds on a long stroke engine, the pressure spikes up early and then drops off as the crankshaft comes around to where good conversion efficiency is possible.
Full compression ignition engines operate at times of compression ignition no later than around 15 degrees BTDC, and this is so early that it only works up at very high engine speeds. Usually up in the 15,000 to 25,000RPM range. Colder burning specialty fuels can make full compression ignition appear to work down to lower engine speeds, and full compression ignition engines have been advertised as being able to idle all the way down to 6,500RPM. Other tricks can also get full compression ignition to work down to even lower engine speeds. A three and a half inch stroke pre-combustion chamber diesel engine can be made to idle down to about 4,000RPM even when the fuel is being injected on the intake stroke. The pre-combustion chamber isolates the rapidly burning fuel from the main combustion chamber and introduces a dramatic delay in the pressure rise in the cylinder. Idling is however different than making power, light loads always favor reduced engine speeds and reduced mean piston speeds for any type of combustion.
It is not at all clear just what the natural temperature of combustion potential of gasoline would be. What comes out of the pumps supports engine speeds as outlined above. Race gas supports even higher mean piston speeds, and dramatically colder burning gasoline also sometimes exists. Very rarely dramatically colder burning gasoline is available at gas stations. Is this dramatically colder burning gasoline what gasoline is supposed to be? That is hard to say since what has normally come out of the pumps is so much hotter burning.
The dramatically colder burning gasoline can support somewhat lower engine speeds in late compression ignition mode, and this may in some instances be considered desirable. With the proliferation of anemic automotive engines fitted with obscenely heavy pistons and rods incapable of running above about 4,000RPM a colder burning gasoline might be considered a desirable premium product. Engines that won't run over 4,000RPM benefit from any little bit of reduction in minimum engine speed bellow 3,200RPM that can be obtained. The 3,200 to 4,000RPM range of engine speeds is just not wide enough to do anything useful so being able to run down to 2,800RPM pays big dividends in performance and utility.
The problem is that dramatically colder burning gasoline works best at dramatically reduced mean piston speeds, but the minimum engine speed for late compression ignition changes only slightly. It is analogous to how longer stroke lengths dramatically reduce maximum engine speeds but only allow very slightly lower minimum engine speeds. Doubling the stroke length may reduce the minimum engine speed from 3,700 to 3,000RPM, but that is only a 20% reduction in minimum engine speed for a 50% reduction in maximum engine speed. Longer stroke lengths over about two and a half or three inches unavoidably result in narrower ranges of engine speeds. Likewise colder burning gasoline unavoidably results in a narrower range of engine speeds unless the stroke length of the engine is correspondingly reduced.
The dramatically colder burning gasoline that will work down to 2,800RPM in a three and a half inch stroke automotive engine would need a two inch stroke length to yield a wide range of engine speeds and overall reasonably good performance. In the three and a half inch stroke engine the dramatically colder burning gasoline that reduces the minimum engine speed from 3,200RPM to 2,800RPM also reduces the maximum engine speed from 7,000RPM to about 5,000RPM. That is fine if the anemic automotive engine won't run above 4,000RPM on any type of fuel, but for any sort of good performance and efficiency the 3.5 inch stroke engine just does not work on the dramatically colder burning gasoline.
Just how low of a temperature of combustion potential does the dramatically colder burning gasoline have? In a 2.5 inch stroke length engine it is the difference between a 10,000 or 10,500RPM maximum engine speed on normal pump gas and a 7,500 or 8,000RPM maximum engine speed on the dramatically colder burning gasoline. About a 25% difference. And that normal pump gas is by no means the hottest burning fuel around. The same 2.5 inch stroke engine can twist 12,000RPM on race gas! These are not maximum no load engine speeds. These are the engine speeds where the power begins to drop off so dramatically that it is not worth trying to rev higher. The upper end of the usable over rev one might say.
There is a bit more to this example than first meets the eye. It is dirt bike engines that I am comparing here, and they do not tend to have hugely long duration camshafts. Dirt bikes always favor big torque down in the 6,000 to 8,000RPM range even if they will make power to much higher engine speeds. It is a lot easier for the engine to flow well and make big torque down at 7,000 and 8,000RPM than up at 12,000RPM or even 10,500RPM. What this means is that the dramatically colder burning gasoline that will only spin 8,000RPM actually has more than a 25% lower temperature of combustion potential than the normal pump gas that will make power at 10,500RPM. The peak cylinder filling is higher at 8,000RPM than 10,500RPM, and this favors 8,000RPM operation on any fuel.
The other thing that can be confusing about comparisons like this is variations in the time of late compression ignition. To make a fair comparison each test has to also be run with more spark advance just to make sure that an even earlier time of late compression ignition won't deliver higher engine speeds and more power output. In the case of the dramatically colder burning gasoline it actually comes to a screeching halt at just 7,000RPM in the 2.5 inch stroke engine, and it is only with a bunch more spark advance to push the time of late compression ignition over well before TDC that 8,000RPM can be obtained.
How does one tell that the time of late compression ignition is going far over to the early side of TDC? That's easy, the engine begins to whine like a turbine and tends to surge rhythmically if not enough spark advance is applied. It is a crappy way to tune an engine, but a 2.5 inch stroke engine that is designed through and through for 8,000RPM and up engine speeds is going to make more and more power up to 8,000RPM on any fuel, no matter how cold that fuel burns. The colder burning fuel just makes a whole lot less power than normal gasoline and the whining and surging results in very poor power delivery with poor throttle response and a slow revving feel. Even when more spark advance is piled on to eliminate the surging the power deliver is still very poor, with a delayed and rather unpredictable slow building of power after the throttle is opened at high engine speeds. This rhythmic surging and unpredictable delayed throttle response is the result of the time of late compression ignition going over before top dead center at less than extremely high engine speeds. The shift from the latest possible time of late compression ignition at 15 or 20 degrees ATDC to the easier to hit 5 degree ATDC time of late compression ignition is an abrupt shift with nothing in between. The shift from 5 degrees ATDC to 5 degrees BTDC or even 10 degrees BTDC is not abrupt, times of late compression ignition everywhere in between are possible and this is where the rhythmic surging comes from. At extremely high engine speeds in excess of about 12,000RPM that will only work with before top dead center times of compression ignition the rhythmic surging would not occur. It is the before top dead center time of compression ignition at more moderate 5,000 to 9,000RPM engine speeds that causes cylinder pressure to spike up early reducing efficiency and putting severe stress on engine components.
Even though very short stroke engines can run fairly well and make big power at extremely high engine speeds in excess of 12,000RPM those BTDC times of compression ignition would always result in reduced peak thermodynamic efficiency compared to later times of compression ignition. Clearly peak efficiency would be obtained with ATDC times of late compression ignition, but which has more performance potential the earlier and easier to hit 5 degree ATDC time of late compression ignition or the 15 or 20 degree ATDC latest possible time of late compression ignition? That is not at all clear, and it may actually end up one way or the other depending on the actual temperature of combustion potential of the fuel used. As backwards as it sounds hotter burning fuel would tend to favor the earlier 5 degree ATDC time of late compression ignition and colder burning fuel would favor the 15 or 20 degree ATDC latest possible time of late compression ignition. This sounds really extremely backwards when only one stroke length is considered. On any particular three inch stroke engine the earlier and easier to hit 5 degree ATDC time of late compression ignition is absolutely required to get good performance on the dramatically colder burning gasoline, where the same engine on more normal hotter burning gasoline runs pretty well from 3,500 to about 6,500RPM just at the 15 to 20 degree ATDC latest possible time of late compression ignition. Again this seems totally backwards, the hotter burning fuel works well down to 3,500RPM or even 3,200RPM at the 15 or 20 degree latest possible time of late compression ignition, where the dramatically colder burning fuel will only run well down to about 4,000 or 4,500RPM at the earlier and easier to hit 5 degree ATDC time of late compression ignition. The only way to get any kind of a range of engine speeds out of the dramatically colder burning fuel in the three inch stroke engine is to use both the 15 or 20 degree ATDC latest possible time of late compression ignition AND the easier to hit 5 degree ATDC time of late compression ignition. By using both times of late compression ignition the colder burning gasoline can be made to work from 3,000RPM up to about 5,000 or 6,000RPM in the three inch stroke engine. That is however still a disappointingly narrow range of engine speeds compared to the 3,200 to 8,000 or 9,000RPM range of engine speeds attainable when both times of late compression ignition are used with the more normal hotter burning fuel in the same three inch stroke engine.
It is probably sounding repetitive by now, but the dramatically colder burning gasoline absolutely needs a stroke length shorter than three inches to deliver a reasonably wide range of engine speeds.
It is possible to get a long stroke engine to operate at higher mean piston speeds with times of late compression ignition before top dead center, but it is not good for efficiency or performance. Peak cylinder pressures spike up very high and more heat is transferred to the cooling jacket resulting in lower efficiency and lower torque production.
If the three inch stroke engine has too long of a stroke for the colder burning gasoline but is about right for high performance racing on the hotter burning gasoline then what stroke length might deliver peak thermodynamic efficiency at the 15 or 20 degree latest possible time of late compression ignition on the dramatically colder burning gasoline? Perhaps a inch and a half or two inch stroke engine running at about 4,000RPM. The dramatically cold burning gasoline obviously needs quite low mean piston speeds to do as well as it can.
What sort of stroke length and engine speeds might deliver peak thermodynamic efficiency on the more normal hotter burning gasoline that has usually come out of the pumps? At the 15 or 20 degree latest possible time of late compression ignition it would probably be about a 2.5 or perhaps three inch stroke engine running at about 4,000 or 4,500RPM. The engine speed at which the 15 or 20 degree ATDC time of late compression ignition delivers peak thermodynamic efficiency would tend to be close to the same for any temperature of combustion potential fuel, the colder burning fuel just needs a shorter stroke length. The hotter burning fuel does favor a slightly higher engine speed for the same time of late compression ignition, but mostly it is just a higher mean piston speed that the hotter burning fuel requires.
The next question would be what sort of stroke length and engine speed would deliver peak thermodynamic efficiency at the earlier and easier to hit 5 degree ATDC time of late compression ignition on the more normal hotter burning gasoline? That would probably be about an inch and a half or two inch stroke engine running at 7,000 or 8,000RPM. The earlier 5 degree ATDC time of late compression ignition certainly favors much higher engine speeds.
So that is an extremely dramatic difference in the engine speed where peak thermodynamic efficiency occurs on the same two inch stroke engine when the temperature of combustion potential of the fuel changes by just 30%. On the same two inch stroke engine a 30% reduction in the temperature of combustion potential of the fuel results in more than a 50% reduction in the engine speed where peak thermodynamic efficiency occurs if both times of late compression ignition are considered.
And finally, what sort of stroke length and engine speed would the dramatically colder burning gasoline running at the easy to hit 5 degree ATDC time of late compression ignition do best at? The earlier and easier to hit 5 degree ATDC time of late compression ignition probably does favor slightly higher mean piston speeds on the same fuel, but the dramatically colder burning gasoline is still going to require rather low mean piston speeds to do it's best. Probably something ridiculous like an inch and a quarter stroke length at 7,000RPM.
These numbers are sort of just guesses, but it would tend to be something like this. An important point to keep in mind when considering these numbers is that peak thermodynamic efficiency tends to come towards the lower end of a range of full load engine speeds that work well. To support reduced loads an engine might run down to engine speeds somewhat lower than these, and to make power over a range of engine speeds an engine would rev considerably higher than these points of peak thermodynamic efficiency.
If a compromise stroke length had to be picked for the dramatically colder burning gasoline it would probably be the longer stroke length to favor operation at the latest possible time of late compression ignition. An inch and a half to two inch stroke engine that would do best down at the bottom of the engine speed range around 3,500 to 5,000RPM but would also be able to make power somewhat less efficiently up to perhaps 8,000RPM.
If a compromise stroke length had to be picked for the more normal hotter burning gasoline it might instead favor the higher engine speeds at the earlier and easier to hit 5 degree ATDC time of late compression ignition. Again it would be the inch and a half to two inch stroke length, but peak thermodynamic efficiency might tend to come up at 7,000 or 8,000RPM with considerable big power production potential up to around 11,000 or 12,000RPM. If the 15 or 20 degree ATDC latest possible time of late compression ignition were used then this short stroke engine running on the more normal hotter burning gasoline would also be able to run down to about 4,000RPM, but it would strongly favor reduced loads way down at the bottom of the engine speed range.
The surprising conclusion here is that when peak thermodynamic efficiency is considered the ideal stroke length might end up being about the same for gasoline engines regardless of what the temperature of combustion potential of the fuel happened to be. The hotter burning fuel just delivers a wider range of engine speeds and very significantly also a wider range of engine loads where reasonably high efficiency is possible. It is however about a two inch stoke length or a bit shorter that looks appropriate, not a three inch or three and a half inch stroke even for running the hotter burning gasoline.
That same dramatically cold burning gasoline that would favor a short inch and a half or two inch stroke even to run at the 15 or 20 degree ATDC latest possible time of late compression ignition would begin whining and surging down around 4,000 or 4,500RPM in a four inch stroke drag racing engine and would be all done making power by about 5,000 or 5,500RPM. On race gas a four inch stroke drag racing engine pulls to 8,000RPM, and on pump gas a four inch stroke drag racing engine makes power to around 6,000 or 6,500RPM with some over rev to perhaps as much as 7,000RPM.
In the three inch stroke Husqvarna 610 motor this dramatically colder burning gasoline tends to need the earlier and easier to hit 5 degree ATDC time of late compression ignition way down at about 4,500RPM and the rhythmic surging and whining starts at about 5,500RPM. Because the big cam four valve per cylinder engine flows very well up in the 5,000 to 7,000RPM range of engine speeds the surging and whining just keeps making more and more power up to about 6,500 or 7,000RPM. When I pile on more spark advance to get the surging to go away on the dramatically colder burning gasoline the three inch stroke engine will then rev all the way out to about 7,800 or even 8,000RPM, but there is just not much power up there and torque from 5,000 to 6,500RPM drops off dramatically. It is interesting that the hot rod 610 motor with the bigger 36mm/32mm valves and big aggressive 250 degree at 1mm valve lift camshaft can always be made to rev to 8,000RPM on any type of fuel. How well it runs and how much torque and how much power it makes though depend greatly on just what type of gasoline is in the tank.
Some clues about what the dramatically colder burning gasoline might be come from when and where it shows up. When a few times in January and February of 2016 I have gotten the dramatically colder burning gasoline directly from a gas station it is always rather fast flame front travel speed gasoline that runs well in full flame front travel mode at moderate engine speeds with small amounts of spark advance. When on the other hand the dramatically colder burning gasoline has showed up in my gas cans overnight it is always also rather slow flame front travel speed gasoline, such slow flame front travel speed gasoline in fact that it has a hard time running at all in the big four inch bore engine with less than about 25 degrees BTDC on the static timing setting. In general the dramatically colder burning gasoline always seems to be very low pressure fuel that has no difficulty running in low compression ratio engines. One batch of this dramatically cold burning gasoline that showed up overnight was such low pressure fuel that it was running and making torque nearly all the way down to 2,600RPM in the 9.7:1 386 stroker 1991 WXE 350 motor. On that gasoline that would make torque in the 2.68 inch stroke motor down to 2,600RPM it was seeming like the earlier 5 degree ATDC time of late compression ignition was being required all the way down to about 4,000 or 4,500RPM and the rhythmic surging was starting at 5,000RPM.
That was the coldest burning gasoline that I have ever seen, and it was also about the lowest pressure gasoline I have ever seen. Even though it was rather slow flame front travel speed fuel the 9.7:1 engine was very easily lighting off on late compression ignition all the way up to 5,000 feet of elevation with just 20 degrees BTDC on the static timing setting. And lighting off easily down to 2,600RPM on the 386 stroker motor is very low pressure gasoline. With the 0.020" base gasket removed the 245 degree at 1mm valve lift camshaft is installed a few degrees retarded so that it is more like a 250 degree at 1mm valve lift camshaft in terms of very low engine speed performance. Way down at 2,600RPM that 9.7:1 compression ratio might be equivalent to about an 8.25:1 automotive engine with a stock 190 or 200 degree at 0.05" camshaft. A 20 degree BTDC static timing setting is a rather small amount of spark advance on the Husqvarna motors, but that 20 degrees BTDC way down at 2,600RPM is in fact more similar to what some automotive engines have run. Automotive engines traditionally topped out at about 33 or 35 degrees BTDC up at 3,000 or 3,500RPM. Down at 2,600RPM a mechanical advance that tops out at 33 degrees BTDC at 3,000RPM might be at just 27 degrees BTDC. With the spark plug offset to the side of a four inch bore that 27 degrees BTDC might be about equivalent to 20 degrees BTDC on the 3.3 inch bore Husqvarna in terms of how easily the engine enters late compression ignition mode.
Even on this extremely unusually cold burning gasoline the high performance 2.68 inch stroke engine was able to make some substantial power up to about 6,500RPM, and with more spark advance piled on it was able to rev a bit higher although there really wasn't more power up there and the rhythmic surging and whining was horrendous. On another day I was running faster flame front travel speed dramatically colder burning gasoline that showed up overnight and it was able to surge and whine it's way up to 8,000RPM in the 2.68 inch stroke 386 stroker motor with the whining and rhythmic surging starting a bit higher at about 6,000RPM. As a point of comparison the 2.68 inch stroke 386 stroker motor normally makes maximum power up at 8,000 to about 9,000 or 9,500RPM on more normal gasoline as has usually available from the gas stations, and it does this with no whining or rhythmic surging at any spark timing value.
What would the dramatically cold burning gasoline work well for? It would pull great in a 1.75 inch stroke engine running between 3,500 and 9,000RPM. More normal gasoline benefits from stroke lengths down around two inches because medium load efficiency can be so much better over a reasonably wide range of engine speeds as compared to three or three and a half inch stroke engines. The extremely dramatically colder burning gasoline absolutely has to have stroke lengths shorter than two inches or it just won't work well over any sort of reasonable range of engine speeds even under a full load.
Reiteration of another important point is appropriate in relation to the unusually cold burning gasoline. Even though the dramatically cold burning gasoline will run just fine if the stroke length is made short enough it should be kept in mind that maximum thermodynamic efficiency can never be as high as compared to gasoline with a higher temperature of combustion potential. A higher temperature of combustion potential fuel can do more work on the same amount of heat, that means not only higher mean piston speeds and higher maximum output but also higher maximum thermodynamic efficiency.
The trick is figuring out what gasoline product delivers the best performance and efficiency when petroleum use is factored in. Does the dramatically colder burning gasoline actually require sufficiently less petroleum to produce to make up for the performance and efficiency deficiency? Another way of looking at it would be miles per barrel of crude oil. This is, to be blunt, a crude way of looking at the situation but it does have some merit. The dramatically colder burning gasoline works just fine, but maximum attainable thermodynamic efficiency is lower so it will always take more of the stuff to do the same job that hotter burning fuel can do. If for the same 80mph cruising speed fuel mileage is 30% lower for the dramatically colder burning gasoline then petroleum use per gallon of gasoline has to be able to be somewhat more than 30% lower for it to be worthwhile. If maximum attainable thermodynamic efficiency drops off by 30% and petroleum use per gallon only drops by the same 30% then it is absolutely not worth it. The colder burning gasoline requires bigger tanks, higher flow rates and does not have as much performance potential.
The only way that colder burning gasoline could be considered worth the drawbacks on a 30% reduction in thermodynamic efficiency and 30% reduction in petroleum use per gallon basis would be if much smaller engines for much lower travel speeds were being considered. For any particular fuel there is an approximate minimum displacement per cylinder bellow which maximum attainable thermodynamic efficiency drops off. That displacement per cylinder is in fact pretty low though, a whole lot lower than the engine sizes that are in common use. Even on hot burning gasoline that won't work at less than about 4,000RPM in a two inch stroke engine there is no problem with using a 2 inch bore on that two inch stroke for a 100cc per cylinder displacement so long as a competent valve train is used. Even with just two valves per cylinder that 2x2 engine can spin fine up to 9,000RPM so long as the valves are canted away from each other and are driven by a rollerized overhead camshaft or some other competent valve train such as two giant hollow camshafts acting on lifters nearly as large in diameter as the bore of the engine.
Colder burning gasoline would allow some additional slight reductions in engine speed on that two inch stroke engine, but the question remains: Is less than 100cc per cylinder even worth considering at this point? With automotive engines currently at around 500 to 600cc per cylinder the ability to easily and reliably go down to 100cc per cylinder is a 5x or 6x improvement without having to do anything crazy with less efficient gasoline.
A 600cc V6 that can run efficiently under a range of loads from 4,000 to 7,000RPM and can twist up to 9,000RPM or a bit higher under a full load to make a maximum of about 80hp sounds like a dramatic improvement over a 3.5 inch stroke 2.4l four cylinder that will only run under a full load from 3,500 to 5,500RPM or idle inefficiently in full flame front travel mode from 2,000 to 3,000RPM. The 3.5 inch stroke 2.4l four cylinder monster might be able to belt out 160hp at 5,500 to 6,500RPM with a variable valve timing system, but that is a drag racing engine not a practical form of motive power. For normal cruising at 35 to 50mph the 600cc V6 could easily use a third as much fuel as the drag monster, and 80hp is enough to drive a full size four door sedan up to 100mph if the need were to arise.
It is easy for someone to immediately argue that 100mph is faster than the car needs to go, and they would be correct. What is not correct though is to argue that the 600cc V6 is not a dramatic improvement over the 2.4l four cylinder just because 100mph is excessively fast. Any way it is looked at 110mpg on the same gasoline that currently delivers 35mpg is a dramatic improvement. Of course at an 80mph cruise the improvement is not going to be so dramatic. Sill though even all the way up to 80mph the 2.4l displacement is extremely excessive for good efficiency and the smaller engine is going to be able to do somewhat better. It is only about 40hp that is required to cruise at 80mph in a current type four door sedan, and that could easily be provided by the 600cc V6 at about 5,000RPM. Just lopping along really for a two inch stroke engine.
There is certainly something given up though cruising at 80mph on 600cc, and that is the immediate brisk acceleration that people have begun to become accustomed to. The 600cc V6 needs to shift down and spin up to 7,000RPM to accelerate hard from 80mph. That is much different than a 2.4l engine cruising at 3,500RPM at 80mph which is all ready to go to deliver big torque with just a push of the peddle.
If cruising at 80mph is really what a car is designed for then an in-between engine displacement around 1200cc would probably be more desirable. The 1200cc engine could deliver the required big power for brisk acceleration without having to spin up so high. It is not that there would really be much in the way of an efficiency advantage to the 1200cc engine at 80mph over the 600cc engine, it is just that the 1200cc engine would feel more capable of easily doing the job at more familiar sorts of engine speeds around 3,500 to 4,500RPM. What would the stroke length of the 1200cc engine be? Probably about 2.5 inches with 2.5 inch bores for a V6 configuration, but it could be anything from two to three inches with anywhere from four to six cylinders. At the 1200cc size none of the parameters are all that critical, it is an oversized engine.
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