The biggest single problem with gasoline engines that have been used over the past half a century has been excessive levels of spark advance. A spark timing of 40 degrees BTDC at 3,000RPM is extremely excessive any way it is looked at, and even 30 degrees BTDC under a full load is so early as to cause severe problems with poor engine performance, low efficiency, high exhaust emissions and premature engine failure. These excessive levels of spark advance go hand in hand with excessively low 6:1, 7:1, 8:1, and 9:1 compression ratios that have also been widely used. It can be argued either way: That excessively low compression ratios caused the excessive advance, or alternately it can be argued that compression ratios were kept low because of large amounts of spark advance. In either case both the excessively low compression ratios and the excessively large amounts of spark advance have caused severe problems with high emissions, dirtier nastier more polluting emissions, low efficiency, poor performance, excessive fuel consumption and premature engine failure.
The Myth
The Reality
Slow Flame Front Travel Speed Fuel
Load Dependent Spark Advance
The Difference Between Fast and Slow Flame Front Travel
Conclusions
The myth is that large amounts of spark advance are required for clean and efficient operation of gasoline engines. This comes from the fact that full flame front travel combustion tends to be so incredibly dirty and inefficient. Getting all the fuel to burn in full flame front travel mode on a four inch bore engine at 4,000RPM does in fact require rather large amounts of spark advance. If the spark plug is located off to the side of the combustion chamber then full flame front travel combustion is going to be even dirtier and less efficient than in an engine where the spark plug is located directly in the center of the combustion chamber. Where the real problems arise is in confusing this particular problem of dirty full flame front travel combustion with engine operation in general. For a four inch bore engine to run cleanly in full flame front travel mode under a light load at 4,000RPM it is certainly going to take some substantial spark advance, but that does not mean that gasoline engines should always run with excessive amounts of spark advance.
The obvious solution is a load dependent timing mechanism so that light loads with the throttle plates firmly closed can be supported more cleanly and more efficiently. By advancing the spark timing under a light load there is both more time for full flame front travel combustion, and there is also the possibility of supporting late compression ignition under much lighter loads.
Even in the absence of a load dependent timing mechanism though there is a compromise between clean light load operation and smooth and efficient full load operation. That compromise is 20 degrees BTDC at 4,000RPM in a four inch bore engine on a reasonably fast flame front travel speed fuel. The key to getting this compromise spark timing to work is a good enough match between the compression ratio of the engine and the temperature and pressure capabilities of the fuel so that late compression ignition can easily and reliably be attained over a range of engine speeds. This means that as soon as the throttle is substantially opened at 3,000 to 4,000RPM the engine will light off on late compression ignition.
The result is that although the lightest of loads in full flame front travel mode at 4,000RPM might be a bit dirty and inefficient the engine is able to get into late compression ignition mode easily enough that all substantial power generation is done in late compression ignition mode. Very light loads then are best supported down at around 2,500 to 3,500RPM. For this to work in real applications two things have to exist. First the engine has to be sized small enough that the substantial amount of torque generated as soon as it enters late compression ignition mode at 3,000RPM is not excessive for the application. This means that a two or three hundred cubic inch engine in an automobile just is not a good idea in most cases. The engine also has to be able to run over a wide enough range of engine speeds that there is room for both full flame front travel mode operation down at 2,000 to 4,000RPM as well as late compression ignition operation from 3,000 to 6,000RPM. If the engine won't run efficiently up to at the very least 6,000RPM then there just is not enough room in the engine speed range to get between gears. If the jumps between gears are made much smaller then too much shifting is involved and the vehicle becomes a chore to drive and in most cases ends up being slower and less efficient as well. That is not to say that the jumps between gears should be two to one, certainly not. Two to one jumps are excessively large. Ideal gear spacing is however not all that much narrower than two to one jumps, something like 1.5:1 between the lower gears and sometimes a bit narrower between the higher gears. The two to one range of engine speeds with the 1.5:1 jumps between the gears is required so that shift points don't have to be precisely the same each time. Some wiggle room either to adjust to different conditions or simply to allow for a bit of operator error is required to make a vehicle reasonably usable.
Just what fixed advance curve is required to attain this sort of performance depends on the bore size of the engine and the flame front travel speed of the fuel. If the spark plug is located off to the side of the combustion chamber then the engine will inevitably be dirtier and less efficient at least while it is in full flame front travel combustion mode. The ideal shape of the advance curve for 4,000RPM and higher engine speed operation depends also on the valvetrain parameters and to a lesser extent the intake runner design and exhaust system design. Fixed spark timing from 4,000RPM up can work if the engine has a rather long duration camshaft and is also capable of flowing well at higher engine speeds provided of course that a fast enough flame front travel speed fuel is used for the bore size of the engine.
If all of these things are in place and tuned to work with each other then the compromise 20 degree BTDC at 4,000RPM spark timing can work quite well. For slightly slower flame front travel speed fuels somewhat more spark advance may be required. A four inch bore engine can still be made to sort of work with a compromise fixed advance curve on slower flame front travel speed fuel with up to 30 degrees BTDC at 4,000RPM. There are however some very significant differences when using a slower flame front travel speed fuel. First and foremost it has to be realized that slower flame front travel speed fuel on a fixed advance curve is going to result in quite dramatic reductions in the range of engine speeds and engine loads that will appear to work well. In order to get the engine to still rev all the way out and make power up at the higher engine speeds on a slower flame front travel speed fuel the advance curve absolutely must continue to advance from 4,000RPM up to maximum engine speed. There is no way around this other than such an excessively long camshaft duration that both torque production and efficiency will suffer dramatically. Regardless of how an engine with a fixed advance curve is made to run over a wide range of engine speeds on a slower flame front travel speed fuel that advance curve has to continue to be well matched to the properties of the fuel being used. The slower the flame front travel speed of the fuel the more slight shifts in the properties of the fuel will affect engine performance. Slow flame front travel speed gasoline for engines with fixed advance curves must be very nearly exactly the same all the time, batch after batch, if the engines are to work reasonably well. Faster flame front travel speed fuels also benefit from consistency, but there is more wiggle room and it is easier to compensate for slight differences simply by changing the static timing setting. On slower flame front travel speed fuels changing the static timing setting on the same fixed advance curve only changes the way that the engine runs at lower engine speeds without providing much influence on higher engine speed operation. What this means is that a mismatch between the properties of the fuel being used and the shape of the fixed advance curve means that it will be impossible to get the engine to run well at higher engine speeds.
When a system for providing load dependent spark advance exists engine performance and efficiency can be substantially improved. particularly for slower flame front travel speed fuels a load dependent advance mechanism can make the difference between the engine working fairly well and appearing to not work at all. With slower flame front travel speed fuels it is the light loads at higher engine speeds that just cannot be supported at all. If a fixed advance curve engine for slow flame front travel speed fuel is run under a light load up near maximum engine speed it will not be able to light off on late compression ignition and there will also not be enough time for even a substantial portion of the fuel to burn. The result is large amounts of raw fuel blown out the exhaust and there is the potential for problems with backfiring and spark plug fouling.
A good load dependent advance mechanism will provide substantially more spark advance under light loads up near maximum engine speed which does two separate things to improve engine performance. More spark advance under light loads means that the engine will be able to stay in late compression ignition mode down to much lighter load levels. If the engine management system is really competent then even more spark advance can be applied when the load has been dropped off so far that late compression ignition can no longer be supported. This large additional amount of spark advance (up to perhaps about 50 degrees BTDC on a four inch bore engine at 4,000RPM) can allow much better full flame front travel mode operation under the lightest of loads at high engine speed on a slower flame front travel speed fuel. The tricky thing though is that this additional large amount of spark advance would require a sophisticated engine management system that is capable of identifying the precise load level where late compression ignition can no longer be supported. This means either extremely consistent fuel and temperature and pressure inputs, or a closed loop system capable of measuring exhaust composition or engine vibration characteristics to identify late compression ignition.
With faster flame front travel speed fuel a load dependent timing mechanism is easier to get to work well. It simply dials in a bit more advance when the throttle is closed, and this small additional amount of spark advance for late compression ignition under reduced loads is also the spark timing value that the engine runs at in full flame front travel combustion mode at even lighter loads. This is the type of load dependent advance mechanism that was found on many dirt bikes throughout the first decade of the 21st century before the advent of EFI dirt bikes. The early open loop EFI dirt bikes have been essentially the same, they are just more sophisticated and offer some additional levels of control and adjustability.
The real reason that a closed loop computerized engine management system is required for good full flame front travel mode operation on slow flame front travel speed fuel is that the slower flame front travel speed fuel dictates a better match between the compression ratio of the engine and the temperature and pressure capabilities of the fuel. If the compression ratio is more than a small amount too low for the fuel being used then it simply will not be possible to get the fuel to light off on late compression ignition over a wide range of operating conditions. This better match between the compression ratio and the temperature and pressure capabilities of the fuel means that rather small amounts of spark advance will be used for full load operation at sea level once the engine is fully warmed up. This might mean spark timing as late as 5 degrees BTDC or even 5 degrees ATDC at some engine speed under a full load, which is radically different than the 40 or 50 degrees BTDC that may be required to avoid a large portion of the same slow flame front travel speed fuel being blown out the exhaust pipe under a light load in full flame front travel mode at elevated engine speeds.
For the reasonably fast flame front travel speed premium gasolines the ideal spark timing for full flame front travel mode combustion are not all that early really. The tricky thing here though is that the engine will normally sound and feel just a bit harsher than it should when the spark timing is just right for full flame front travel combustion. Backing off to the 20 degrees BTDC at 4,000RPM on a four inch bore engine results in smoother and quieter full flame front travel operation. Going up to 25 degrees BTDC at the same 4,000RPM may result in louder and harsher full flame front travel mode operation with only a slight bit more power, but the amount of unburned fuel blown out the exhaust is substantially reduced. What is happening is that at 25 degrees BTDC (or even 20 degrees BTDC) at 4,000RPM the small amount of fuel that is burned as soon as the spark plug fires is doing no useful work. In fact that fuel which is burned early is actually working to slow the engine as the piston approaches top dead center. The reason though that going up to 25 degrees BTDC at 4,000RPM yields slightly more power from the same amount of fuel in full flame front travel mode and substantially reduced exhaust emissions is that even on the fast flame front travel speed fuel it is taking too long for the flame front to get out to the "far corners" of the combustion chamber. Advancing the spark timing from 20 degrees to 25 degrees BTDC at 4,000RPM on fast flame front travel speed fuel helps assure that the flame front actually makes it out to all areas of the combustion chamber before the piston has receded so far that combustion can no longer take place. In other words even fast flame front travel speed fuel is pretty slow when it is burned in full flame front travel mode.
The difference between faster flame front travel speed fuel and slower flame front travel speed fuel may seem extreme when spark timing values of 25 degrees BTDC and 50 degrees BTDC are being thrown around. The reality though is that the period of time for full flame front combustion is in fact much longer than just the 20 or 50 degrees of crankshaft rotation. The flame front can continue to burn quite a while after the piston has begun to recede from top dead center. Even 40 degrees ATDC the piston has receded only 12% of the stroke of the engine. Obviously full flame front travel combustion can continue past 40 degrees ATDC. All the way around to 60 degrees ATDC the piston has moved down only one quarter of the stroke distance. Considering that more and more fuel is being burned to keep the combustion chamber hot and pressurized it seems likely that full flame front travel combustion can in fact occur at least all the way around to 60 degrees ATDC.
From 50 degrees BTDC to 60 degrees ATDC is 110 degrees of crankshaft rotation for slower flame front travel speed fuel to burn at 4,000RPM in a four inch bore engine. Where 25 degrees BTDC to 60 degrees ATDC is 85 degrees of crankshaft rotation for faster flame front travel speed fuel to burn at 4,000RPM in a four inch bore engine. In this example the faster flame front travel speed fuel in fact has just a one third faster linear flame front travel speed than the slower flame front travel speed fuel; not nearly as dramatic as might be expected when the 25 degree BTDC and 50 degree BTDC numbers are looked at without considering how long the period of flame front travel combustion actually is.
This one third faster linear flame front travel speed is actually extremely significant for how well an engine will run in late compression ignition mode over a wide range of engine speeds. Because the flame front expands out spherically from the spark plug this one third faster liner flame front travel speed is actually going to be able to burn more than two times as much fuel in the same amount of time. That is a big difference, and this is why faster flame front travel speed fuels run so much better over a wide range of engine speeds in a gasoline engine.
Of course the other thing that the slower flame front travel speed fuel can do is be run in smaller bore engines. If the 25% slower linear flame front travel speed fuel is run in a three inch bore engine it will perform similarly to the faster flame front travel speed fuel being run in the four inch bore engine. That seems supper easy, just use smaller engines. And this is in fact a very good idea to use smaller engines with higher cylinder counts, but it is not just as simple as reducing the bore diameter. If the stroke length remains the same with the reduction in bore diameter then the engine becomes less oversquare, or even undersquare, and is not as well able to flow acceptably well at higher engine speeds. If the stroke is reduced to maintain the same bore to stroke ratio then of course the stoke ends up shorter. If it is three and four inch bore engines that are being discussed then they probably need shorter strokes anyway. The standard three and a half inch stroke for automotive engines is really too long for gasoline engines. Going all the way down to a two and a half inch stroke that is proportionally the same for a three inch bore as the three and a half inch stroke is for the four inch bore is however a dramatic reduction in stroke length. Two and a half inch stroke gasoline engines certainly can be made to work well, but it is not going to be anything like the three and a half inch stroke engine. The two and a half inch stroke engine would have to spin a third faster than the three and a half inch stroke engine to attain the same mean piston speed. If the engine has to spin a third faster then the advantage of the smaller bore goes out the window in terms of the flame front travel speed of the fuel, and the engine is back to running as poorly as the four inch bore engine. Of course a reduction in bore size from four inches to three inches is a good idea because the three and a half inch stroke was too long for gasoline engines anyway. That long three and a half inch stroke was itself limiting the range of engine speeds over which the engine could run, so a smaller bore and a smaller stroke is in fact going to work better for a gasoline engine.
Large amounts of spark advance under light loads can be great for keeping exhaust emissions low. Excessive spark advance under heavy loads though just makes for loud, harsh, inefficient engines that actually end up being worse pollution hazards. The solution is a load dependant timing mechanism. In the absence of a load dependent timing mechanism just getting the spark timing correct goes a long way towards getting a gasoline engine to run well. The best compromise spark timing is approximately 20 degrees BTDC at 4,000RPM for a four inch bore engine running fast flame front travel speed fuel. This can be increased to 25 degrees BTDC if the engine runs predominantly under dramatically reduced loads, but louder, harsher and less efficient operation will result. Going the other way reducing the spark timing to 15 degrees BTDC at 4,000RPM on a four inch bore engine accomplishes less positive results. There is just not as much difference in the way that the engine runs in late compression ignition mode between 15 degree BTDC spark timing and 20 degree BTDC spark timing, while the amount of fuel that can be burned in full flame front travel mode does drop off some small but significant amount with the even later spark timing. The point here is that it is 20 degrees BTDC that is the critical timing value for late compression ignition. Small increases in spark advance beyond 20 degrees BTDC result in dramatic increases in noise, harshness and fuel consumption, were reductions in spark advance less than 20 degrees BTDC makes for an engine that is only moderately quitter, smoother and more efficient. As far as full flame front travel combustion goes the 20 degree BTDC spark timing value is not so meaningful. Five degrees in either direction makes some small difference, but it is similar in both directions.
There is in fact some critical spark timing value for the flame front to reach the far corners of the combustion chamber, but this is much more engine speed dependent. Slowing the engine 10% while in full flame front travel mode makes as much difference as increasing the spark advance from 20 degrees BTDC to 28 degrees BTDC. That 10% reduction in engine speed while in full flame front travel mode is only a moderate reduction in vehicle speed, but increasing the spark timing from 20 degrees BTDC to 28 degrees BTDC on fast flame front travel speed fuel is the difference between an engine running darn well with near peak power output and near peak efficiency and an engine that is so loud and harsh that it sounds like it is going to blow up. This is assuming that the compression ratio is lowered or the pressure capabilities of the fuel are raised to allow the latest possible time of late compression ignition with the earlier 28 degree BTDC timing setting. If an engine was running well on fast flame front travel speed fuel in late compression ignition mode at a spark timing value of 20 degrees BTDC and the spark timing is bumped up to 28 degrees BTDC without changing anything else then the thing probably really will blow up (or at least cause severe damage to the reciprocating assembly if it is run like that for long).
It would have to be said that the range of acceptable spark timing values for a four inch bore engine with a fixed advance curve at 4,000RPM is 15 to 20 degrees BTDC. This spark timing could go either direction five degrees, but neither of these five degree shifts are desirable. The bump up from 20 degrees to 25 degrees BTDC causes large and dramatic increases in noise and harshness where going down from 15 to 10 degrees BTDC on the spark timing makes essentially no difference other than the engine is harder to tune and a bit less able to support light loads at elevated engine speeds. The reduction from 15 to 10 degrees BTDC on the spark timing however only represents a seven percent decrease in the engine speed at which light loads can cleanly be supported in full flame front travel mode. The real reason that it is undesirable to go down to less than 15 degrees BTDC on the spark timing at 4,000RPM in a four inch bore engine with a fixed advance curve is that it becomes harder to tune because small changes in spark timing yield large changes in engine performance. Smaller bore engines running the same fuel can of course go down to even smaller spark advance values, but again the sensitive twitchy nature of tuning at these small spark advance values makes 15 to 20 degrees BTDC the desirable range of spark timing values at 4,000RPM for a fixed advance curve on fast flame front travel speed fuel.