Severe non-homogeneity of combustion fuel can cause operational problems in any heat engine. Ironically it is rather slow turning gasoline engines operating below about 5,000 that have the least problems with non-homogeneous fuel. Down at the lowest engine speeds for late compression ignition around 2,500 to 4,000RPM as soon as any of the gasoline pops off on late compression ignition the temperature and pressure in the combustion chamber spikes up very high and all of the combustion fuel pops off on late compression ignition right away. At higher engine speeds gasoline engines may experience severe problems when running on non-homogeneous gasoline. Particularly up above about 7,000 or 8,000RPM gasoline mixtures made up of fuels with radically different combustion properties can cause harsh and inefficient operation while also making tuning very difficult. Just how low of an engine speed non-homogeneity problems first show up depends on three things: How different the various components of the gasoline mixture are, what the relative quantities of different types of gasoline in the mixture are and what the maximum temperature of combustion potential of various components are. Depending on the properties of the non-homogeneous fuel and the performance of the engine two quite different types of non-homogeneity problem may occur. At high engine speed part of the fuel may pop off on late compression ignition soon than the rest of the fuel, an even more severe type of non-homogeneity problem may occur where part of the fuel does not pop off on late compression ignition at all.
Types of Non-Homogeneity Problems
Non-Homogeneity as an Advantage
There are two main reasons why non-homogeneous gasoline might be used. The most obvious is simply that petroleum is made up of a wide variety of different hydrocarbon molecules, and refining this mixture of hydrocarbons into a gasoline fuel would tend to produce a number of different distinct compounds. Some single most desirable compound could be synthesized exclusively, but this would be wasteful and would require much larger amounts of petroleum to produce the same amount of finished gasoline. The most efficient refining processes would tend to result in large quantities of a few compounds and smaller quantities of a large number of other compounds. Making use of all of these different compounds requires either a large number of grades of gasoline or requires that mixtures of compounds be sold as gasoline.
The other reason that non-homogeneous gasoline might be used is when a specialty pressure lowering compound is required so that unusually low compression ratios can be used. A 7:1 gasoline engine requires a fuel that will light off on late compression ignition extremely easily at rather low pressures and temperatures. Since compounds that will light off on late compression ignition at such low temperatures and pressures are unusual and expensive these gasolines for extremely low compression ratio engines have of course used small quantities of pressure lowering additives.
When pressure lowering additives are used a particularly severe non-homogeneity occurs where just a small quantity of a fuel with dramatically different combustion properties is blended in with the bulk of the combustion fuel. Just how much pressure lowering additive is used depends mostly on what type of pressure lowering additive it is that is used. When premium gasoline is blended in with regular gasoline to allow the regular gasoline to run in the same 10:1 and 11:1 engines it does not really mater a whole lot just how much of the premium gasoline is used. Since the premium gasoline is only moderately more expensive than the regular gasoline using 10 or even 20% premium gasoline as a pressure lowering additive increases the cost of the blended regular gasoline product by only a small amount. If on the other hand exotic mineral based additives are used to produce very low pressure gasoline for 7:1 engines then the smallest amount of the additive that will produce the desired pressure lowering would tend to be used. The exotic mineral based pressure lowering additives are expensive and environmentally hazardous, so they have tended to be used in smaller quantities.
It can sort of be seen intuitively that a mixture of different combustion fuels will cause some sort of combustion problems. For gasoline engines it is when the mean piston speed is increased up close to where an earlier time of late compression ignition would be required that non-homogeneity problems crop up. One type of non-homogeneity problem would be that the pressure lowering additive would pop off on late compression ignition but the bulk of the fuel would not light off on late compression ignition. This would occur when the mean piston speed was so high that the pressure and temperature did not continue to rapidly increase after late compression ignition first occurred. Another type of non-homogeneity problem involves a delay between when the small amount of pressure lowering additive pops off on late compression igniton and when the bulk of the fuel pops off on late compression ignition. The higher the engine speed the more significant this small delay is.
The likelihood of a non-homogeneous fuel causing problems, and also the severity of problems caused, depends mostly on how dissimilar the constituents of the blended fuel are but also depends on the relative quantities in the blended fuel and also on the performance of the engine.
In most engines a homogeneous fuel will result in highest efficiency, highest power output and best overall performance. There are however some specific circumstances where a non-homogeneous fuel might be used to deliver slightly better engine performance. Particularly down at the lower engine speeds where late compression ignition can be made to work a non-homogeneous fuel might deliver smoother and more efficient operation in late compression ignition mode. The way that this would work would be for a specialty additive with both a lower temperature and pressure requirement for late compression ignition and a lower maximum temperature of combustion potential to deliver a delayed time of late compression ignition so that the bulk of the fuel could be made to light off on late compression ignition somewhat later than would otherwise be possible. For this to work the engine design and setup would have to be carefully matched to the specific combustion properties of some particular non-homogeneous fuel. Basically what would happen would be that a very small amount of a lower temperature and pressure requirement pressure lowering additive would be used. This very small quantity of pressure lowering additive would light off on late compression ignition at the normal latest possible time of late compression ignition around 15 or 20 degrees ATDC. Because only a very small amount of this lower pressure requirement pressure lowering additive would initially be burning the rise in the temperature and pressure in the combustion chamber would not be large and dramatic. Because this small amount of pressure lowering additive pops off at considerably lower temperatures and pressures than the bulk of the fuel the temperature and pressure in the combustion chamber has to build considerably higher before the bulk of the fuel will pop off. Essentially it takes twice as long for the temperature and pressure in the combustion chamber to build as would be the case with a homogeneous fuel. The pressure lowering additive has to ignite, burn and impart it's stored chemical potential energy to the hot gases in the combustion chamber before the bulk of the fuel will pop off. It is easy to see how this would be difficult and very tricky to get to reliably work from one batch of fuel to another. It really is splitting hairs in terms of the timing of the various combustion processes. Still though this phenomenon is real, and has something to do with why gasoline engines have sometimes been made to run acceptably well down to 3,000RPM and even slightly slower.
For the vast majority of gasoline engines, especially if they regularly operate up at above 6,000RPM, non-homogeneous fuel does not improve performance. Because it is so likely though that standard cheap gasoline which is efficient in the refining process is at least slightly non-homogeneous it is of interest to explore exactly what various types of fuel blends might mean for engine performance and efficiency.
The main point about non-homogeneous gasoline is that the smaller the amount of some dramatically different fuel blended in the more likely it is that problems will crop up. This is true for both of the basic types of engine performance problems caused by non-homogeneous gasoline. A smaller amount of a pressure lowering additive is more likely to pop off on late compression ignition without causing the bulk of the fuel to immediately pop off also. The consequence of this is that smaller amounts of a pressure lowering additive would tend to cause non-homogeneity problems down to lower mean piston speeds and lower engine speeds.
It is however very interesting that the two different types of non-homogeneity problems respond very differently to larger or smaller quantities of a pressure lowering additive being used. Even though the likelihood of non-homogeneity problems occurring always tends to be greater when smaller quantities of pressure lowering additives are used the severity of problems caused either increases or decreases with smaller quantities of pressure lowering additives depending on which of the two types of non-homogeneity problems are taking place.
When the problem is that only the pressure lowering additive is popping off on late compression ignition and the bulk of the fuel is only burning in full flame front travel mode then using larger quantities of the pressure lowering additive both decreases the likelihood of the non-homogeneity problem occurring and also reduces the severity of that non-homogeneity problem. If the quantity of pressure lowering additive is increased to 50% then nearly half of the fuel is burning in late compression ignition mode so the engine makes substantially more power than if only 10 or 20 % of the fuel is burning in late compression ignition mode.
When on the other hand the problem is that the pressure lowering additive is lighting off slightly earlier than the bulk of the fuel then increasing the amount of pressure lowering additive used does make the non-homogeneity problem less likely to occur, but that larger quantity of pressure lowering additive can also make the non-homogeneity problem more severe. If just 10% of the fuel pops off a bit early at say 6,000RPM then the engine will be a bit louder and harsher. If 50% of the fuel pops of a early at just 6,000RPM then the engine will be extremely loud and harsh.
Smaller quantities of a pressure lowering additive always make non-homogeneity problems more likely to occur. It should be noted though that if the quantity of a pressure lowering additive is reduced to the point that it does not actually pop off on late compression ignition then there is essentially no pressure lowering additive. If just 1% premium gasoline blended in with regular gasoline does not actually get the gasoline to pop off at a lower pressure and temperature then that 1% premium gasoline is insignificant. These insignificantly small amounts of pressure lowering additives don't do anything, they don't change the overall combustion properties of the fuel and they don't cause any engine performance problems either.
Small quantities of higher pressure compounds mixed into gasoline can cause dramatic problems. Think 1 or 2% two stroke oil mixed into gasoline. Even at 100:1 two stroke oil stinks and smokes when burned in any gasoline engine. That 1 or 2% additive is so resistant to burning in full flame front travel mode that it mostly just gets blown out the exhaust lightly toasted. In late compression ignition mode the two stroke oil may or may not burn depending on it's relative temperature and pressure requirements as compared to the gasoline. In some cases a 25:1 mixture of light oil might actually act as a pressure lowering additive if extremely high pressure gasoline is being used. Normally this is not the case though. Normally the oil requires somewhat higher temperatures and pressures to pop off on late compression ignition than does the gasoline. And normally even 4% two stroke oil is not enough to actually function as a pressure lowering additive even if extremely high pressure gasoline is being used.
When using extremely low pressure gasoline for 7:1 engines and running the engine up at above about 8,000RPM the two stroke oil may not burn even when the engine is running in late compression ignition mode. In this case the 1 or 2% oil mixed in is blown out the exhaust mostly unburned as if the engine were running just in full flame front travel mode. The longer the stroke of the engine the more likely it is that two stroke oil will not burn in a low compression ratio engine. This is the main reason why long three inch stroke 250 and 500cc two stroke dirt bikes tended to blow much more smoke than two inch stroke 125cc dirt bikes. Back in the era of two stroke dirt bikes low 8:1 and 9:1 compression ratios were still very common. The typical 8.75:1 compression ratio of 1990's Japanese two stroke dirt bikes was roughly equivalent to 7:1 or 7.5:1 compression ratios in 1950's through 1980's automotive four stroke gasoline engines. Of course if the two inch stroke 125cc two stroke dirt bike is twisted out to 12,000RPM then it is just as likely to smoke under full power as a 250cc two stroke running at 8,000RPM. The longer stroke 250cc two strokes did however tend to operate at somewhat higher mean piston speeds, at least when they were tuned to really belt out the power for racing.
And why is it that all the factory race team 250cc two strokes ran clean without smoking while the bikes straight off the showroom floor smoked like coal fired battleships? That is easy, all the race teams were running higher pressure gasoline and had the compression ratios bumped up to 12:1 or even 13:1. In a 13:1 engine, even a two stroke, oil doesn’t stand a chance of making it through unburned.
The case reed induction two stroke engines are a good example of what happens when a small quantity of dramatically different fuel is blended in. The same things can also happen with gasoline intended for use in four stroke engines if small quantities of higher pressure compounds are present. Think 10% race gas blended in with standard premium gasoline. If the 10:1 engine for standard premium gasoline operates up at very high mean piston speeds then that 10% race gas for 14:1 engines just gets blown through unburned like two stroke oil. If the quantity of race gas for 14:1 engines is increased to 30% then it actually stands a better chance of getting burned. With 30% of the fuel requiring a higher pressure to light off there is actually a noticeable increase in power output associated with more spark advance to dial the time of late compression around earlier to get all of the fuel to burn. With just 10% race gas blended in advancing the spark timing results in a reduction in power output even though more heat is being released.
This situation where a small quantity of the gasoline does not burn at high mean piston speeds is only sort of a non-homogeneity problem. These small quantities of higher pressure fuels being blown out the exhaust unburned would better be classified as a contamination problem. The gasoline is contaminated with something that tends to not burn, and quickly the environment becomes contaminated with the unburned fuel blown out the exhaust. Non-homogeneity problems are different in that non-homogeneous fuel tends to be rather common, and the negative consequences of running non-homogeneous fuel can rather easily be avoided. When an engine is running on gasoline with 10 or 20% pressure lowering additive and only that pressure lowering additive is popping off on late compression ignition while the bulk of the fuel burns only in full flame front travel mode it is quite obvious that the engine just needs a bit more spark advance or a bit higher compression ratio to get all the fuel to burn. The difference in power output, efficiency and exhaust emissions are extremely dramatic when that last 80% of the fuel is caused to burn in late compression ignition mode. Just about anyone would be able to identify an running at 6,000RPM and burning only 10 or 20% of it's fuel in late compression ignition mode as not running well. Bumping the spark timing up so that 80% of the fuel pops off on late compression ignition is going to cause such a huge increase in power output that anyone would realize that something was severely wrong.
Pressure lowering additives in 10, 20, or even 50% quantities cause non-homogeneity problems that are quite obvious and relatively easy to solve. It should however be noted that not everyone has just automatically come to the correct conclusions in regard to what was wrong with these engines when the bulk of the fuel was not burning in late compression ignition mode. Just as when an engine is running just in full flame front travel mode at 6,000RPM and making a small amount of power the extremely large and dramatic power increase associated with a bit more spark advance or a bit higher compression ratio to get the gasoline to pop off on late compression ignition can be a bit shocking. One common misconception is that it is something about the type of fuel that makes such a difference. Many observers would claim that it is not the same gasoline in the engine when it is making four times as much power at 6,000RPM. When faced with overwhelming evidence that it is in fact the same gasoline because the gas tank was undisturbed while the spark timing was bumped up a few degrees many people then tend to think that it is something else in the gasoline that is causing the dramatic increase in power generation. A common idea is that a high explosive such as nitro methane was present in the gasoline, and only when the temperature and pressure in the combustion chamber was increased by increasing the spark advance did that high explosive ignite and cause a dramatic increase in power output.
In the case of a non-homogeneous gasoline the analogy is similar. It really is part of the gasoline popping off on late compression ignition while a large part of the gasoline does not pop off on late compression ignition. The difference is that the non-homogeneous gasoline is just the cheapest combustion fuel that will work in a 9:1 or 10:1 gasoline engine, where nitro methane on the other hand is an expensive and difficult to obtain high explosive that would increase the total cost of the fuel dramatically. The difference can also be observed by changing jetting. If twice as much fuel is dumped in then twice as much energy can be released from the nitro methane. A normally aspirated engine running nitro methane can make huge amounts of power essentially right up to the limits of the strength of the pistons, rods and crankshaft. Combustion fuel on the other hand makes only very slightly more power with richer mixtures. A gasoline engine jetted just right for a particular combustion fuel might be able to be made to deliver 5% more power by dumping in 10% more fuel. Dumping in 15% more fuel though does not result in any noticeable power increase and dumping 20% more fuel in may actually cause a reduction in power output.