Generally speaking bore diameters of two inches are much better than bore diameters of four inches for gasoline engines. Smaller bore sizes allow the cheapest types of slower flame front travel gasoline to run better over wider ranges of engine speeds and loads, and even on the fastest flame front travel speed premium gasoline types two inch bore engines are generally more friendly and easy to deal with than four inch bore engines. There are however some potential problems with smaller bore engines, particularly as the bore size is reduced bellow about two inches. The important point is that these problems can rather easily be overcome. It is however worth considering what kind of difficulties may show up in very small bore diameter gasoline engines.
The Advantages of Small Bore Diameters
Getting Top Heavy
Tuning Small Bore Engines
A Model for the Future
There are two separate ways to look at the advantages of smaller bore sizes in gasoline engines. One is that smaller bore engines run better with better torque production over a wider range of engine speeds and higher efficiency over a wider range of engine speeds and loads. The other perspective is that smaller bore engines displace less, allowing for smaller power output high cylinder count engines for improved transmission efficiency in land vehicles.
The main problem with very small bore diameter engines is that they have a tendency to only run well up at higher engine speeds. This is partially because smaller bore diameter engines also tend to have shorter stroke lengths, and shorter stoke lengths favor higher engine speeds to keep the mean piston speed up to where peak efficiency can be obtained. The small bore diameter itself does also contribute to a top heavy power band though.
The main reason that small bore engines get top heavy and will only make power at the top of the engine speed range is that the camshaft is only delivering high cylinder filling over a rather narrow range of engine speeds. The faster an engine spins the more difficulty it has flowing well enough to attain high cylinder filling, and this tends to result in very long duration camshafts being used on small engines. A more competent valve train can mitigate these problems by allowing for high cylinder filling over a wider range of engine speeds. If it is a two valve per cylinder engine then canted valves allow for much better flow than a parallel valve arrangement. Of course four valves per cylinder flows much better than two valves per cylinder, but very small bore diameter engines with four valves per cylinder end up with some extremely small valve sizes. Ratio rockers and roller followers also allow for more aggressive camshaft profiles that open and close the valves more rapidly. Flat tappet or cam and bucket valve trains are a top cause of narrow top heavy power bands in small bore engines. Ratio rocker valve trains work much better over a wider range of engine speeds. Variable valve timing systems as have become nearly universal on automotive engines can also deliver high cylinder filling over a much wider range of engine speeds. The main disadvantage of variable valve timing systems being just the added complexity.
More spark advance makes an engine run as if it is running on slower flame front travel speed gasoline. This only applies when the engine is running in late compression ignition mode. This seems backwards of course, but it is true. Larger amounts of spark advance mean that an engine will not pop off on late compression ignition as easily at higher engine speeds but will become very crisp and even excessively harsh at lower engine speeds.
A big question that keeps coming up over and over again in relation to gasoline engines, and motive power in general, is as to what level of sophistication in electronics and computerized controls is desirable. The easy answer for many electronics luddites is "none". No sophistication and no computerized controls. The easy answer for those already invested in the electronics industry is "as much as is required". As much sophistication as is required to get the job done, or as much sophistication as can be reasonably managed. Whichever comes first. Reality is however somewhat different. Sophisticated electronics and computerized controls are already required for all sorts of aspects of industry, so the luddite perspective is not exactly realistic. On the flip side though ever increasing levels of complexity and sophistication that fewer and fewer people understand or can deal with is also not realistically an option. As far as gasoline engines are concerned a happy medium is a solid state electronics control module with only a bare minimum of customizable "software". Something like the CDI control boxes that have been, and continue to be, popular on many types of small gasoline engines. This level of electronics control means that breaker points and mechanical advance mechanisms are not required, a big plus for ease of ownership and reliability. This level of electronics control also allows certain higher level features that just are not possible with mechanical control. Chief among them being load dependant spark timing. It could be as simple as a limit switch on the throttle to back off on the spark timing down to an alternate advance curve. Onc advance curve for normal light to medium load operation and another advance curve for near full load operation. Very simple and very effective. Of course calling any CDI control box with an advance curve simple is perhaps a bit of an exaggeration. Just getting spark timing to change with engine speed requires a fair amount signal processing. It is however a level of sophistication in electronics that has proven over about four decades to be cheap, reliable and easy to deal with. Adding a second advance curve simply adds one small additional aspect of complexity, it is still the same basic device with all of the same reliability and ease of use.
One severe limitation for the dual curve ignition using a simple limit switch is of course that the amount of throttle opening that results in slightly reduced cylinder filling is dramatically different for different engine speeds. This means that the adjustment of the limit switch ends up also setting the range of engine speeds where the load dependant advance mechanism works well. Mitigating this problem is possible in a number of ways, but it is an inherent limitation. The obvious partial solution is just to accept that the single step load dependant spark timing mechanism is going to be for different levels of engine load at different engine speeds and adjust the advance curves accordingly. This would mean that at the lowest engine speeds for late compression ignition one advance curve would be for a full load and the second advance curve would be for just a slightly reduced load where up at the top of the engine speed range one advance curve would be for full load operation and the second advance curve would be for very light load operation in full flame front travel mode. The dual advance curves are still a dramatic and very significant advantage across all engine speeds, it is just that this simple load dependant timing mechanism does considerably different things at different engine speeds. At the lower end of the range of engine speeds for late compression ignition the load dependant advance mechanism helps the engine stay in late compression ignition mode and run efficiently under slightly reduced loads. Up at the top of the engine speed range the load dependant advance mechanism just dials in more spark advance under very light loads so that the engine does not have such a tendency to stumble, load up or foul the spark plugs if the throttle is slowly closed at a sustained high engine speed. Despite this limitation the dual advance curve ignition system is still much better than a vacuum advance mechanism simply because both advance curves can be shaped to actually deliver approximately the correct spark timing for both sides of the limit switch at each engine speed. Right in the middle where peak cylinder filling occurs the second advance curve of the simple dual advance curve ignition control module may allow the engine to stay in late compression ignition mode down to substantially reduced load levels. A big advantage both for light load efficiency and clean operation.
The biggest disadvantage of such a simple ignition control module is that it is not flexible enough to work over a wide range of sizes and types of gasoline engine. The advance curves need to be fairly reasonably matched to the engine that they are to be used on. For overall simplicity though roughly the correct spark timing is actually just as good as exactly the right spark timing. What is most important is that the shape of the advance curves be roughly what is required for a certain size and type of gasoline engine. Adjusting the static timing setting and the position of the limit switch allows some fine tuning to get this simple dual advance curve ignition module to work on slighlty different engines and engine setups. To do it's best though the simple dual advance curve ignition module needs to be used on the actual model of engine that the advance curves are set for. Swithching the ignition module to a different model of engine that has similar but not identical bore, stroke, compression ratio, valve timig and carburetor/throttle body size would be possible but might result in some slight tuning difficulties.
A model for a practical, usable and efficient gasoline engine based on a dual curve ignition module is fairly easy to envision. The first design criteria is application, I'll just stick to land vehicles since ocean going vessels have such dramatically different requirements and possibilities. The main thing for land vehicles is engine size, both physical dimensions and power output. Smaller physical dimensions generally are better for land vehicles, but a practical gasoline engine tends to be rather small so going for the smallest attainable physical dimensions is not really necessary.
The most common required size is for moving a two or four seat enclosed automobile at speeds that dwarf the size of a town or small city within a half hour. That means cruising at somewhere around 35 to 50mph, which is fast by most realistic measures of transportation speeds. By this I don't mean just getting from one side of a city to another. Rather it is also getting into and out of the immediate vicinity of a city. A city requires real connections with large areas of rural and semi-rural land, so being able to get 15 or 30 miles out into the country side in a reasonably short period of time is a requirement. For practical transportation most trips are over distances of just a few miles, but a practical vehicle also travels dozens or hundreds of miles from time to time. Localization of means of production, agriculture, employment, education, health care, entertainment ext. are generally good for efficiency of transportation and are also good for most aspects of human life. Being able to travel longer distances is however also a basic requirement of most types of modern existence.
Is a 130mph top speed and peak fuel mileage at 70 to 80mph necessary or desirable in any way? Not really. A practical gasoline engine does not need to be anywhere near that big.
More like 25 or 30hp peak output and best fuel mileage in the 25 to 40mph range of speeds with the capability to reasonably effectively cruise at around 50mph. The 25hp peak output in a roomy four door four seater is a blazing fast 70mph or so, but that is really only required for compatibility with existing vehicles. A practical engine would be working very hard to go 70mph. It is a pretty small gasoline engine, especially when a high cylinder count of four or six is used to get transmission efficiency up. Lots of cylinders, but not much displacement. It is a total displacement well under a half a liter. Realistically it is more like 300cc with a radically under square bore and stroke configuration. Say a six cylinder engine with 35mm bores and a 52mm stroke length. And that peak 30hp output comes way up at 8,000RPM. Does it have to be revved to 6,000 and 8,000RPM through each gear though? No, not if the ignition system actually works.
That 52mm stroke length allows good usable and fairly efficient torque down to around 3,500 or 4,000RPM with really substantially efficient operation everywhere from about 4,500 to 7,000RPM. The trick is the transition from the latest possible time of late compression ignition to the earlier and easier to hit 5 degree ATDC time of late compression ignition. This transition has to happen at the correct engine speed for the stroke length, engine load level and temperature of combustion potential of the gasoline that is actually being used.
If the gasoline is exactly the same all the time then tuning is a matter of matching the advance curves to the altitude and driving style where the car is actually used. Something needs to be user adjustable, or gas station attendant adjustable, if huge levels of complexity and sophistication in electronics controls are to be avoided. Mixture ratios don't really need to be adjustable if the gasoline does not change much. A fairly lean mixture is great for efficiency, low emissions and the ability to change altitude with a minimum of fanfare. Keeping the mixture lean requires a bit more displacement, but that is not a problem if the weights of the pistons and rods are kept down so that reciprocating losses remain low all the way out to 8,000RPM. It is not that hard to get a 52mm stroke length engine light enough to do well to 8,000RPM, it is the same mean piston speed as high output turbocharged six inch stroke length marine diesel engines. Gasoline does not have as high of a temperature of combustion potential as diesel oil, but gasoline does go bang and burn all at once when it pops off on late compression ignition. Diesel might be a hotter burning fuel, but it burns gradually as it is injected into the combustion chamber of a diesel engine over a period of 20 or 30 or even degrees of crankshaft rotation at low engine speeds. And even after the diesel oil is injected it has to disperse and mix with the available oxygen before it can burn. Diesel engines are slow and gasoline engines are fast, the fact that gasoline necessarily is a lighter and colder burning fuel does not change this fact.
It is spinning a gasoline engine up to 4,000 and 6,000 and even 8,000RPM that allows it to run at thermodynamic efficiency levels similar to a diesel engine. When gasoline engines spin only 2,000 and 3,000RPM they have no chance of attaining reasonably good thermodynamic efficiency. Is that 300cc and 52mm stroke length six cylinder configuration the smallest that is possible for a gasoline engine? Nope, it'll go smaller. Shortening the stroke length bellow 52mm may push the minimum engine speed up a bit, but not all that dramatically. The 3,500 or 4,000RPM minimum engine speed is a fairly fixed limit. Shorter stroke length engines get less efficient at 3,500RPM, but they can still run down nearly as low as long as they are actually able to attain the latest possible time of late compression ignition.
Going substantially less than about a 50mm stroke length can complicate tuning though since the earlier and easier to hit 5 degree ATDC time of late compression ignition then does need to stay up at correspondingly higher engine speeds. Shorter stroke lengths stretch out the range of engine speeds where the latest possible time of late compression ignition works well, but if the earlier and easier to hit 5 degree ATDC time of late compression ignition is required to really make best use of the gasoline then a shorter stroke length engine does need to twist out higher.
If small is the goal some compromises can be made. One such compromise is to accept an in-between range of engine speeds where the latest possible time of late compression ignition is too late and the earlier and easier to hit 5 degree ATDC time of late compression ignition is too early. This is not necessarily a deal breaker for a small gasoline engine, but it may be considered undesirable from a "Fahrvegnuegung" perspective. In a very small practical gasoline engine it might just mean a flat spot where torque dips slightly and power output goes flat before the mean piston speed gets up enough to where the earlier and easier to hit 5 degree ATDC time of late compression ignition can be used without a bearing pounding harshness. This all may sound suspiciously like a description of a non-functional long stroke length gasoline engine what with the flat spot and bearing pounding harshness. It is however a very different sort of phenomenon. A short stroke length engine gets harsh and inefficient when the time of late compression ignition is too early or when the spark timing is dramatically too early at low engine speeds. A long stroke length engine just gets harsh because sufficiently high engine speeds would require mean piston speeds that are too high for medium load efficiency. Long stroke length gasoline engines get flat spots because they will run over only narrow ranges of engine speeds. Typically the flat spot on a long stroke length gasoline engine is in full flame front travel mode before late compression ignition can take place, or the flat spot may actually be while the engine is running in late compression ignition mode at such a low engine speed that it is not doing any good to actually make torque. Long stroke length gasoline engines do sometimes run in late compression ignition mode down to 2,700 and even 2,300RPM, but torque production down there remains stubbornly low. In a gasoline engine of any stroke length torque production tends to skyrocket somewhere around 3,000 or 3,500RPM. Slightly higher for shorter stroke length engines and slightly lower for longer stroke length engines.
Longer stroke length engines run over much narrower ranges of engine speeds at the latest possible time of late compression ignition. This has traditionally been interpreted as having some advantages. One is that the transition from the latest possible time of late compression ignition to the earlier and easier to hit 5 degree ATDC time of late compression ignition is less choppy. I would not go so far as to say smoother though, because long stroke length gasoline engines tend to be extremely harsh as they hit earlier times of late compression ignition at much too low of engine speeds to support the elevated mean piston speeds required to get the engine speed up above that 3,000RPM threshold for all gasoline engines. And it is not just getting above 3,000RPM as a maximum engine speed, long stroke length gasoline engines have to spin out much farther than 3,000RPM so that upon shifting the engine speed does not drop much bellow 3,000RPM. Land vehicles do require that the engine run over a range of engine speeds, and unless an eight speed automatic transmission that shifts constantly is used that means a rather wide range of engine speeds.
The important point here is that it is a fallacy to think that longer stroke gasoline engines with stroke lengths of more than about two and a half inches are any kind of an advantage. They are not. There are certain aspects of operation that can seem more desirable with longer stroke lengths, but any advantage derived from a three and a half or four inch stroke length gasoline engine always comes with much larger tradeoffs that result in overall worse performance and less efficient operation with a narrower range of operable engine speeds.
Perhaps it could be said that the flat spot in the middle between the latest possible time of late compression ignition and the earlier and easier to hit 5 degree ATDC time of late compression ignition on very short stroke length gasoline engines is not so much a sign of trouble, but rather is an indication that shorter stroke length gasoline engines are capable of efficiently running over much wider ranges of engine speeds than longer stroke length gasoline engines.
Going down to a 40mm stroke length may be seen as somewhat undesirable in terms of pushing top end power up to higher engine speeds, but it certainly is possible to go down to rather short stroke lengths to make gasoline engines smaller.
Going with a more radically under square configuration can also reduce displacement per cylinder. The 35mm bore by 52mm stroke length engine is a radically under square configuration, but gasoline engines could go even more radically under square with some tradeoffs. Smaller bore sizes and smaller valves don't flow as well, so a smaller engine that needs to rev higher does eventually run into difficulties as displacements per cylinder get smaller.
If very small sizes are desired then it is even possible to give up on the earlier and easier to hit 5 degree ATDC time of late compression ignition all together and focus on the increasingly broad torque at the latest possible time of late compression ignition. A 4,500 to 8,000RPM range of engine speeds at the latest possible time of late compression ignition in a 35mm stroke length engine certainly gets the job done. Especially if the required job is to deliver small amounts of power with reasonable efficiency.
Sort of the end of the line for minimizing gasoline engine size would be to give up on late compression ignition all together and focus on full flame front travel mode operation. It is the next step beyond giving up on the earlier and easier to hit 5 degree ATDC time of late compression ignition that may be required to get the most out of any gasoline engine. The main problem with full flame front travel mode combustion is that it is dirtier than late compression ignition. Late compression ignition burns all the fuel in the combustion chamber, and provided that the engine speeds are high enough late compression ignition does not produce higher peak cylinder pressures that cause nasty high temperature pollutants.
Smaller bore sizes certainly do make full flame front travel mode combustion cleaner and more efficient, but there are limitations on how well full flame front travel mode combustion can work. A big concern is that the flame front actually make it all the way out to the far corners of the combustion chamber to burn all the fuel. Smaller bore sizes help greatly with this. Basically what it comes down to is that full flame front travel mode needs a small enough bore diameter that the gasoline can burn over a short duration of crankshaft rotation. Full flame front travel mode gasoline engines certainly favor radically under square configurations with very small bore sizes.
Another very significant concern with full flame front travel mode combustion is that the first of the fuel burns at a much lower temperature and pressure than the last of the fuel. This is just unavoidable. Pressure has to build to push the piston down, so the last of the fuel is going to be burning under much higher temperature and pressure conditions than the first of the fuel. Again smaller bore sizes help to mitigate this problem, but it is a problem that is sort of inherent to full flame front travel combustion.
Ultimately it can be said that late compression ignition is inherently much better than full flame front travel combustion. Full flame front travel combustion is something that tends to come with any gasoline engine, but a practical gasoline engine is able to stay in late compression ignition mode over a range of normal operating conditions. One of the keys of avoiding large amounts of full flame front travel operation is getting a gasoline engine to run well over a wide range of loads in late compression ignition mode. Small displacements per cylinder are very good for this. It may be somewhat annoying to have a giant flat spot between the latest possible time of late compression ignition and the earlier and easier to hit 5 degree ATDC time of late compression ignition, but that enormous range of operable engine speeds in late compression ignition mode is actually very good for staying in late compression ignition mode. A land vehicle uses a whole lot more power to go faster, so any vehicle that needs to operate over a range of vehicle speeds requires an engine that is capable of operating over a wide range of engine loads.
A very small gasoline engine that can purr along at reduced torque output at 4,000RPM at the latest possible time of late compression ignition while also being able to scream big power output up at 9,000RPM at the earlier and easier to hit 5 degree ATDC time of late compression ignition is sort of inherently well matched to the requirements of being able to cruise along down to 25mph and also being able to do reasonably well up to 50mph and faster. Incidentally being able to efficiently cruise along down to about 25mph in late compression ignition mode with low emissions is something that can be seen as immensely desirable for maintaining good air quality and preserving public health in towns and cities where combustion powered vehicles are widely used.