The main piece of traditional folk knowledge about the properties of fuel states that diesel oil has a higher energy content than gasoline, and also that regular gasoline has a higher energy content than premium gasoline. Both of these pieces of information are true and correct, but there are other important trends in the properties of combustion fuels as well. For more on what the various properties of fuels mean for engine design and engine performance see Combustion Properties of Fuel.
Maximum Temperature of Combustion Potential
Pressure and Temperature for Compression Ignition
Flame Front Travel Speed
It is easy to see that light volatile compounds would tend to be less dense in liquid form than oils. Heavy oils can weigh as much as 10 pounds per gallon, and the rather thin and light number two diesel fuel weighs just over seven pounds per gallon. Gasoline is said to normally weigh about six pounds per gallon, although this certainly does vary slightly with the particular grade of gasoline. Liquid isobutane is 4.7 pounds per gallon and liquid n-Butane is 4.9 pounds per gallon. Propane is considerably lighter at about four and a quarter pounds per gallon (often listed as 4.11 pounds per gallon at standard temperature of 25 degrees C). Liquid methane is even lighter at three and half pounds per gallon. If these density numbers seem strange to you, you are not alone. The reason that there is some confusion surrounding densities of liquefied gases is that densities are sometimes listed for the 25 degree C liquid after pressure has been removed, which is a rather strange way to rate the density of a liquefied gas. For the lighter gases such as methane this rating problem can represent a substantial error in the listed density of the liquefied gas.
For the purposes of comparing the energy density trends in combustion fuels these lower instantaneous density numbers after the release of pressure are however actually quite interesting. It is these lower density numbers that give a more realistic impression of the relative weights of the fuels. Methane might compress down to a significantly higher density at the large pressure required for liquid storage at ambient temperature, but that does not change the fact that methane is in fact a very light fuel compared even to propane.
Even though the lighter fuels do have slightly higher energy contents per pound the heavier fuels still provide considerably more energy per gallon than the lighter fuels. A gallon of diesel fuel releases a whole lot more heat when burned than a gallon of liquid propane.
How hot a combustion fuel can burn is also extremely significant for engine design and engine performance. Hotter burning fuels can attain higher thermodynamic efficiencies in combustion engines and hotter fuels also can support higher piston speeds for bigger power output. The maximum temperature of combustion potential of combustion fuels certainly does vary somewhat, and there would be some general trends. The heavier fuels would tend to be able to burn hotter than the lighter fuels, but this is only a very general trend. The actual maximum temperature of combustion potential of a particular compound depends mostly on what chemical bonds are broken and formed during the combustion reactions.
For the hydrocarbon series of combustion fuels (methane (CH3), ethane(C2H6), propane (C3H8), isobutane (C4H10), n-Butane (C4H10), isomers of pentane (C5H12), isomers of hexane (C6H14), isomers of heptane (C7H16) and some longer hydrocarbon chains) combustion mostly just involves breaking of carbon-carbon bonds and hydrogen-carbon bonds and the formation of carbon-oxygen bonds and hydrogen-oxygen bonds. Said another way the hydrocarbon fuels break down to form water and carbon dioxide with oxygen from the air. The release of energy is mostly from the formation of the strong carbon-oxygen bonds, but the formation of hydrogen-oxygen bonds also provides a very significant release of energy. Since all of these combustion fuels form the same products the maximum temperature of combustion potential of the various fuels remains rather similar.
Even within this hydrocarbon series of combustion fuels there are however some slight differences in how hot the fuels will burn. Butane for example can be seen to burn considerably hotter than propane when trying to solder copper plumbing fittings together with an undersized torch. The difference here is that butane has more carbon and less hydrogen, therefore butane forms more carbon dioxide and less water compared to propane.
The longer the hydrocarbon chains the less of a difference there is in relative carbon and hydrogen content from one fuel to another. Butane has 6.7% more carbon than propane, but an isomer of pentane has only 4.2% more carbon than butane. There is a noticeable difference in temperature of combustion between propane and butane, but differences in temperature of combustion between the longer hydrocarbon chains would generally tend to be smaller and less noticeable.
Undoubtably the most significant property of combustion fuels (at least for port injected and carbureted engines) is the temperature and pressure requirements of the fuel for compression ignition. For a gasoline engine to run well a good match between the compression ratio of the engine and the temperature and pressure requirements of the fuel is required. If the fuel lights off on compression ignition at too low of a temperature and pressure point then full compression ignition will occur and the engine will quickly be damaged or destroyed. If on the other hand the fuel requires a substantially higher temperature and pressure point for compression ignition than would be ideal for a particular compression ratio then excessively large amounts of spark lead are required to attain late compression ignition and the engine will be loud, harsh and inefficient.
The temperature and pressure requirements of a fuel for compression ignition have to do with what bonds are broken during the combustion process. Again since all hydro carbon series fuels involve the breaking of the same hydrogen-carbon and carbon-carbon bonds the temperature and pressure requirements of all of the hydro carbon fuels remains rather similar.
There are however some differences in temperature and pressure requirements for compression ignition even within the hydro carbon series of combustion fuels. For the most part these difference involve the different temperature and pressure requirements for breaking hydrogen-carbon bonds and the temperature and pressure requirements for breaking carbon-carbon bonds. Since all of the hydro carbon chain fuels have both types of bonds there are some general trends that can be predicted.
Since the hydrogen-carbon bonds are weaker they would break at a lower temperature and pressure point, this is the pressure and temperature point at which late compression ignition can take place. As soon as the hydrogen-carbon bonds begin breaking the whole molecule quickly comes "unraveled" and the carbon-carbon bonds break as well. Because the longer hydrocarbon chains have more carbon and less hydrogen they tend to be just a bit more resistant to compression ignition and may appear to require slightly higher pressures to burn well. This is tricky though, as the initial compression ignition can actually take place at the same lower temperature and pressure point as for shorter hydro carbon chains with less carbon and more hydrogen.
The structural arrangement of bonds also plays a role in how easily they will break down. The end of the hydro carbon chain will break down more easily than the middle sections. The first hydrogen atom to break off of each end of a hydro carbon chain will come off at a slightly lower temperature and pressure point than the rest of the hydrogen atoms. Shorter hydro carbon chains have more ends and will break down more readily. Longer hydro carbon chains with fewer ends will be a bit more resistant to compression ignition and will appear to require slightly higher pressures to burn well. Again though this is tricky because the initial compression ignition can still begin at the same lower temperature and pressure point as for shorter hydrocarbon chains.
The best example of how the structure of hydrocarbon fuels affects combustion properties is the octane ring. With no "ends" the octane ring is markedly more resistant to compression ignition and requires a significantly higher temperature and pressure point to burn well.
If the temperature and pressure requirements of a fuel is undoubtably the most important fuel property for gasoline engines then the flame front travel speed is undoubtedly the second most important fuel property for gasoline engines. The flame front travel speed of a fuel is very significant in that it determines how well a gasoline engine will be capable of running over a wide range of engine speed and engine load conditions. Slower flame front travel speed fuels make it difficult to get a gasoline engine to run over a wide range of engine speeds and a wide range of engine loads, and unavoidably cause loud, harsh and inefficient operation under reduced loads at elevated engine speeds regardless of how well the engine management system works.
Just what the flame front travel speed of a fuel happens to be depends again on the bonds involved and the structure of the molecules. For the hydrocarbon chain series there would again tend to be some general trends in flame front travel speed. More hydrogen and less carbon is going to tend to result in faster flame front travel speeds, and more ends is also going to tend to result in faster flame front travel speeds. The longer hydrocarbon chains would then tend to have slower flame front travel speeds and the shorter hydrocarbon chains would tend to have faster flame front travel speeds.
Pure hydrocarbon chains with only hydrogen and carbon are not the only compounds that will burn in combustion engines. Quite a few other atoms and molecular groups may be found stuck onto hydrocarbon chains in place of one or more of the hydrogen atoms. For the most part these other elements and molecules stuck onto hydrocarbon chains would be considered contaminants and undesirable in motor vehicle fuels. That is however an overly simplistic and unrealistic view.
From a pollution and public health perspective it might be considered desirable to have fully refined hydrocarbon fuels with no other elements other than hydrogen and carbon. This certainly would simplify the chemistry involved in analyzing fuels, engine performance and emissions.
The reality though is that petroleum derived fuels do tend to have some elements other than hydrogen and carbon in them, and adding additional mineral materials might even be considered desirable.
The main thing that other atoms or molecular groups stuck onto hydro carbon chains do is lower the boiling point. This is significant because it allows shorter hydrocarbon chains to be used as motor vehicle fuels without the need for a high pressure fuel system.
Some types of gasoline will evaporate rather readily, and a slightly pressurized fuel system can help with reducing emissions due to evaporation. These slightly pressurized fuel systems that operate at up to a maximum of about 5psi are however considerably different than the 150psi fuel system that is required to use straight propane as a motor vehicle fuel.
However it is accomplished using shorter hydrocarbon chains tends to result in gasoline with a higher flame front travel speed, which is highly desirable for gasoline engines that have to run over a wide range of engine speeds and a wide range of engine loads.
The problem with additional elements present in combustion fuels is that combustion byproducts other than just carbon dioxide and water end up produced. If the additional elements are only nitrogen and oxygen then little in the way of additional exhaust emissions pollutants are likely to occur because air is after all about 70% nitrogen. Add some sulfur though and significantly worse pollutants are likely to be generated in the high pressure and high temperature combustion chamber.
It is not clear just what specific compounds have been used as specialty fuels over the decades. All that is clear is that specialty fuels of some kind have in fact been popular. The two most obvious specialty fuels are low pressure and slow flame front travel speed regular gasoline that will run in 9:1 or 10:1 low compression ratio engines and high pressure race gas that has a very fast flame front travel speed but can also be used in high compression ratio engines up to 13:1 or 14:1 and possibly even up to 14.5:1.
The low pressure and slow flame front travel speed regular gasoline has probably been produced by mixing a rather small quantity of a lower pressure "octane improving" additive in with long hydrocarbon chain regular gasoline that would otherwise only work in higher compression ratio gasoline engines. This octane improving additive is in fact premium gasoline. A modest 10 or 20% of this premium gasoline mixed in with regular gasoline lowers the pressure and temperature requirement of the fuel so that it will work in lower compression ratio engines but is not enough to significantly raise the flame front travel speed of the fuel.
If on the other hand a modest 20 or 30% of regular gasoline is mixed in with premium gasoline then the temperature and pressure requirements of the fuel hardly change at all and the flame front travel speed is seen to be just a bit slower.
The fast flame front travel speed race gas for high compression ratio engines may be mostly octane. This is however just a guess. The fact that fast flame front travel speed fuels with several discrete levels of temperature and pressure requirements have been available over the years indicates that there actually are a number of different compounds that can be used for race gas.
Premium gasoline with a fast flame front travel speed that works well in 10:1 and 10.5:1 engines is probably just normal gasoline made up of the shortest hydrocarbon chains that are practical in non-pressurized or lightly pressurized fuel systems without the use of large quantities of nasty mineral based additives. It has long been believed that this same standard premium gasoline can also be used in 11:1 or even 11.5:1 engines with very small amounts of spark lead, but just what the actual maximum compression ratio for standard premium gasoline in fact is remains somewhat of a mystery.
What is clear though is that this standard premium gasoline has not often been available in recent years. What has been coming out of the pumps instead is race gas for much higher compression ratio engines. The problem is that race gas is probably much less efficient to produce in the refining process. No good information seems to be available on this matter either, but race gas was in the past much more expensive than premium gasoline. When premium gasoline for 10:1 and 10.5:1 engines was selling for close to $2 a gallon throughout the 1990's and into the first decade of the 21st century race gas with a fast flame front travel speed for 12.5:1 and 13.5:1 engines sold for more like $5 to $8 a gallon. Part of this difference of course was based on volume. Many times more premium gasoline was sold than was race gas. Still though it certainly appeared that race gas was much more expensive to produce than premium gasoline.
Another thing about traditional gasoline that has been a bit difficult to reconcile is the fact that throughout the middle and later part of the 20th century and into the 21st century regular gasoline was observed to at least sort of work in extremely low 8:1 compression ratio engines even if it did require large amounts of spark advance.
This very low pressure regular gasoline was probably attained by using rather small quantities (perhaps only 10%) of even lower pressure specialty additives. These even lower pressure specialty additives were probably produced using substantial quantities of mineral derived materials to attain properties well outside of what is possible just with the hydrocarbon series. Run 100% of these specialty compounds as a supper low pressure fast flame front travel speed premium fuel and you can even get a six to one flathead into late compression ignition mode.
The thing is though that these supper low pressure additives don't actually accomplish anything beneficial. All they do is keep the compression ratios of engines irrationally low, and on top of that they are probably significantly worse environmental and public health hazards than just standard premium gasoline for 10:1 and 10.5:1 engines.
What would the maximum compression ratio be for these lower pressure additives? That is not entirely clear. A guess might be about 9:1 or 9.5:1. The 9:1 and 9.5:1 compression ratios were very popular for many types of gasoline engines throughout the late 1980's and 1990's. These 9:1 and 9.5:1 engines typically used quite a lot of spark advance and were intended to run on the slightly higher pressure standard premium gasoline mixed with a large portion of regular gasoline. At the same time that most automotive engines were produced with 9:1 and 9.5:1 compression ratios there were always some automotive engines that were at 10:1 and 10.5:1, and all of the turbocharged automotive engines from the 1980's and 1990's ended up running with effective compression ratios in the same neighborhood of 10.5:1 or slightly higher.
It seems like it was this period from the 1980's through the first decade of the 21st century when this standard gasoline was in widespread use. It was really for 10:1 or 10.5:1 engines and could be run up to about 11:1 maximum but most of the automotive engines remained down at 9:1 and 9.5:1 and ran rather poorly. Running poorly at 9:1 on fuel for 10.5:1 engines is relative though. Try to run that same 9:1 engine on race gas for 14:1 engines and truly spectacularly poor performance results.
The last big question: What maximum compression ratio could just regular slower flame front travel speed gasoline with no pressure lowering additives run in? Certainly more than 12:1. I would guess actually more like 14:1 or even slightly higher. The reality is that there would be a number of different compounds that could make up regular gasoline, and some would have higher pressure and temperature capabilities. Bigger hydrocarbons would be able to handle more pressure and temperature, and would also tend to provide slightly higher energy densities.
Just how "high" on the hydrocarbon series gasoline can go ultimately depends on how gasoline like the fuel needs to be. That is how well a gasoline engine will start on that fuel. And this is not just starting from cold first thing in the morning either. Big oily hydrocarbons can actually cause a gasoline engine to be hard to re-start hot if it stalls. The reason for this is that spark plugs will foul with oily compounds when the engine stalls. Of course an electronically controlled port injection system could potentially allow engines to run on somewhat oilier gasoline by stopping the injection of fuel as soon as engine speed dropped bellow some minimum low idle speed. If a mechanically controlled gasoline engine is run on this same oily gasoline then the spark plugs might need to be cleaned each time the engine stalls in order to get the engine to restart.
An extreme example of this is running a gasoline engine on diesel oil. As long as the engine is running in late compression ignition mode it runs cleanly on the diesel oil with no smoke and the spark plugs do not foul. If the engine is allowed to run in full flame front travel mode on diesel oil though a cloud of blue smoke is produced and the spark plugs quickly foul. If the engine is not gotten back into late compression ignition mode quickly the spark plugs will foul to the point that the engine stalls, and it might not be able to be restarted without cleaning the plugs and switching back to gasoline. This is true at any normal compression ratio from 9:1 up, but the higher compression ratios certainly do help to keep the engine running in late compression ignition mode more easily and would therefore better be able to run for long periods of time on diesel oil. Running diesel oil in a gasoline engine at any compression ratio also tends to require considerably more spark advance than for any type of gasoline run in the same engine. As with any fuel there would be a maximum compression ratio that diesel fuel could be run in a gasoline engine at. This does not actually appear to be all that high. Perhaps just 14:1 or so.
With no specific information about the relative proportions of specific compounds found in crude oil and with no information about which specific compounds make up the various grades of gasoline it is extremely difficult to come to any theoretical approximations of refining efficiency.
Of course diesel fuels yield the most bang for your buck in terms of refining efficiency. Some types of light crude oil can be run directly in large diesel engines, and even other types of crude oil require only minimal reigning to be run as "bunker oil" in large diesel engines used to drive ships. Lighter diesel fuels such as number six and number two diesel fuel require more reigning, but still attain quite high overall refining efficiencies. A gallon of number six diesel fuel probably requires only slightly more than a gallon of crude oil to produce.
Any gasoline is of course considerably less efficient in the refining process. The traditional knowledge was that in refining crude oil into diesel oil, lube oil and road tar a certain amount of gasoline and a certain amount of LPG and methane would inevitably be produced. In recent decades there has been considerable talk of more sophisticated refining processes that can get a whole lot more gasoline out of the same amount of crude oil. The fact remains though that gasoline is a less efficient fuel, no matter what refining processes are used a barrel of crude oil is going to yield a considerably smaller volume of finished gasoline than it could diesel oil.
It is a given that gasoline is somewhat less efficient, but just how inefficient gasoline is also depends greatly on what type of gasoline is used. I would suspect that regular gasoline with no pressure lowering additives would be the most efficient gasoline to produce. This would be the slower flame front travel speed gasoline that is still volatile enough to reliably start gasoline engines that do not have weak cranking spark magneto ignition systems.
Premium gasoline would inevitably be somewhat less efficient in the refining process, although there would always be a tendency for a certain amount of the lighter compounds to come off in any refining process. The most efficient premium gasoline would be the standard premium gasoline that attains a fast flame front travel speed without the use of large quantities of mineral derived materials for the production of specialty low pressure additives.
In a certain sense these two most efficient types of gasoline could be said to be homogeneous fuels. From this perspective a non-homogeneous fuel would be a low pressure regular gasoline that uses pressure lowering additives. Regular slower flame front travel speed gasoline mixed with 10, 20 or 30% premium gasoline to run in lower compression ratio engines would be a non-homogeneous fuel. This form of non-homogeneous regular gasoline is essentially just as efficient in the refining process as any regular gasoline, but it tends to be less efficient in the gasoline engines themselves. Regular gasoline mixed with specialty supper low pressure additives for use in 6:1 and 8:1 engines would be an even worse non-homogeneous fuel. The specialty supper low pressure additives are expensive because they require mining and processing large quantities of additional materials, and those large quantities of mineral derived materials tend to cause much more severe environmental and public health problems when they are burned in combustion engines. On top of this the lower compression ratios contribute to even lower thermodynamic efficiency in the gasoline engines themselves.
The worst type of gasoline would be the supper low pressure premium fuel made up entirely of the supper low pressure additives produced with the use of large quantities of mineral derived materials. Standard premium gasoline for slightly higher compression ratios is somewhat less efficient to produce than just regular gasoline, but at least it does not have the large additional environmental and public health hazards of the high mineral content supper low pressure additives.
The other worst type of gasoline would be race gas. Gasoline made up of the few rare specialty compounds that can attain both a high flame front travel speed and a very high temperature and pressure capability. There will always be a demand for race gas, because it wins races. A 14:1 engine running fast flame front travel speed fuel is certainly going to have an advantage over a 10.5:1 engine running similarly fast flame front travel speed fuel. Dispensing race gas at the pumps as premium motor vehicle fuel is however incredibly stupid.
One might ask why race gas is a problem. I don't know exactly. It might be said that a certain small amount of these specialty race gas compounds would tend to be present in refined petroleum products. Isolating these specialty compounds and selling them as race gas seems to make sense. Why waste a good thing? The problem though is that if only a very small amount of race gas is produced then it becomes extremely valuable. Everyone involved in open class motorsports racing where any and all combustion fuels are allowed has to buy this race gas in order to be competitive. Since testing and motor development normally also needs to be done on the same fuel that will be run in competition it is easy to see that there would be a substantial demand for race gas.
The problem comes when this large demand for race gas causes petroleum refiners to modify their processes to produce more of it. Of course it is possible to produce larger quantities of race gas, anything can be synthesized. If the refining efficiency of race gas is compared to the refining efficiency of standard premium gasoline though I suspect that a really dramatic difference would be found. Probably somewhat similar to the old $2 a gallon premium gasoline and $5 a gallon race gas proportionality. What looks so incredibly stupid is selling race gas as motor vehicle fuel so that it takes two and a half times as much crude oil to go a mile on the freeway using similarly performing gasoline engines.
If I am correct in my guesses and assumptions about the various grades of gasoline then it could be said that there are certain good and bad compression ratios for gasoline engines. The worst compression ratios would be anything less than 9:1, the very low 6:1 and 8:1 compression ratios are horrible because they require the use of large quantities of environmentally hazardous mineral derived materials for the production of supper low pressure additives. These low compression ratios are the worst because they tend to result in nasty pollutants coming out of the tailpipes of cars. From a performance perspective the best compression ratios would be ones that could run on standard premium gasoline, probably in the 9.5:1 to 11:1 range. Somewhat higher compression ratios could also be highly desirable for running on cheaper and more efficient (at least refining efficiency) regular gasoline.
In order for gasoline engines to run on the higher pressure and lower flame front travel speed regular gasoline though a meaningful rating system would be required so that the wrong fuel would not end up in the wrong engines. Put standard premium gasoline in a high compression ratio engine for regular gasoline and it will be kaput in a very short period of time. Think "grenade on the onramp" sort of problems. Going the other way if you put regular high pressure gasoline in a 10.5:1 engine for standard premium gasoline and it just won't work well at all. If the engine management system is capable of compensating for the higher pressure and lower flame front travel speed then the engine might be able to sort of run over a narrow range of engine speeds for a short period of time, but performance and efficiency will be abysmal. If the engine management system is not capable of compensating for the higher pressure and lower flame front travel speed fuel then the engine might not be able to run at all. On mechanically or electronically controlled engines where the static timing setting can easily be manually adjusted then a lower compression ratio engine for standard premium gasoline would always be able to made to at least idle and run at low power output at low engine speeds on higher pressure and slower flame front travel speed regular gasoline. A lower compression ratio engine can nearly always be "tuned up" to run on a higher pressure fuel, but the mismatch between the compression ratio and the pressure and temperature requirements of the fuel causes dramatic reductions in performance and efficiency. A higher compression ratio engine just flat out cannot run on lower pressure fuel, as soon as the throttle is opened full compression ignition damages or destroys the engine.