The combustion properties of commonly available fuels (various grades of gasoline and different weights of diesel fuel oil) are remarkably similar. There are however some slight differences that can in some cases make a dramatic difference in how an engine runs.
What are the different combustion properties of fuels?
Different Strokes for Different Folks
Physical Properties of Fuel
Superior Vs. Inferior Fuels
Of course the most talked about property of any fuel is it's energy content, that is how much heat it can release when burned. Number two diesel oil as is commonly used to power boats and over the road vehicles is seven pounds per gallon, and has an energy content of somewhere around 120,000 BTU/gallon. In the past the energy content of diesel fuel was usually listed as 140,000 BTU/gallon, but this was heavier oil. The energy content of fuels tends to be approximately proportional to their density, but there is also a pronounced tendency for less dense fuels to have higher energy contents per pound. Propane for example weighs only four and a quarter pounds per gallon yet has an energy content of over 90,000 BTU/gallon. Making good use of that released heat though requires matching the engine parameters to the other combustion properties of the fuel. There are essentially two combustion properties of fuel that are significant for the way that engines run. The first of these is the easiest to define and also is easier to measure. That is the temperature and pressure required for the fuel to light off on compression ignition. For gasoline this property is described as the octane rating. For diesel fuel this is rather vaguely described by the cetane rating, although the cetane rating is often said to be a measure of the ignition temperature of the fuel at atmospheric pressure. The other significant combustion property of fuel is much harder to define and also much harder to measure. This property would best be called the maximum temperature potential of the fuel, and has to do with how much work a certain amount of heat can do. In an internal combustion reciprocating piston engine this property of the fuel has to do with how fast the pistons should travel.
The main engine parameter having to do with the combustion properties of the fuel is the piston speed (usually described as the "mean piston speed"). Because the temperature potential of all combustion fuels is quite high it is normally the case that higher piston speeds yield higher efficiency, so long as the weight of the pistons and rods can be kept low enough that reciprocating losses remain just a small fraction of the total power output. For diesel engines this means a six inch stroke engine running at about 1200 to 2500RPM, or a three inch stroke engine running at about 2400 to 5000RPM. What is a bit confusing though is that higher piston speeds actually appear to work quite well, provided that the pistons and rods are light enough. For gasoline engines the piston speed issue gets very muddled and confused by the fact that gasoline engines of any stroke do not run well at less than about 4000 or 6000RPM. In fact 6000RPM is probably the "bottom basement low end" to quote Dirt Bike Magazine. Aircraft engines have long been run down as low as about 4300RPM, but this is probably a bit too slow for a gasoline engine. The advantages of running down at 4300RPM though are that a larger displacement per cylinder can be used and more efficient large diameter propellers can be used without reduction gears, and these advantages have tended to outweigh the disadvantage of running the engine somewhat too slowly. The reason that gasoline engines cannot run more slowly is that all the fuel burns all at once no later than about 10 or 15 degrees past top dead center. This also is rather confusing and widely misunderstood since the spark plug fires somewhere around 15 to 35 degrees before top dead center (depending on the displacement per cylinder, the compression ratio, engine speed, engine load the shape of the combustion chamber, and of course the combustion properties of the fuel). What is happening is that a small portion of the fuel burns in a flame front travel mode starting as soon as the spark plug fires. This flame front travel combustion raises the temperature and pressure in the combustion chamber up to the point where compression ignition can take place. If the timing of the spark is perfectly matched to the operating conditions of the engine then this late compression ignition can take place a bit after top dead center. There is however no way to get the late compression ignition to take place substantially later than about 15 degrees past top dead center, as by that time the piston has begun to move down far enough that the temperature and pressure cannot build quickly enough to cause later compression ignition. Just how late the compression ignition can take place in a gasoline engine has to do mostly with how well the spark timing can be matched to the operating conditions of the engine, but also has to do with both the compression ratio of the engine and the combustion properties of the fuel. The compression ratio of the engine and the octane rating of the fuel have mostly to do with what spark timing values will be required for various engine speeds and loads, but the compression ratio and octane rating do also play a role in how well the engine can run. Higher compression ratios and higher octane rating fuels do not require as much fuel to be burned in flame front travel mode, and this makes the engine run smoother and more efficiently. Higher compression ratios and higher octane rating fuels also can allow the time of compression ignition to be somewhat later than is possible with lower compression ratios and lower octane rating fuels. There is considerable confusion about what fuels have what octane ratings. The reality is that the highest octane rating premium fuel makes lower compression ratio engines (less than about 11:1) run better. Lower octane rating regular gasoline can never attain quite as good efficiency in low compression ratio engines because more of the fuel has to be burned earlier. Ethanol runs even worse in low compression ratio engines, but is said to be able to be run in higher compression ratio gasoline engines without undergoing destructive early compression ignition. Because of starting and spark plug fouling problems oil is normally not run in gasoline engines, but if oil is run in a gasoline engine it requires even more spark advance than ethanol. One might wonder how it is that a 14:1 gasoline engine can run without experiancing destructive early compression ignition when a diesel engine can start from cold with a 14.5:1 or even 14:1 compression ratio. This would seem to indicate that diesel oil actually lights off under the heat of compression more easily than does gasoline. What has to be kept in mind though is that the high pressure spray of fuel creates a localized higher pressure area which allows the fuel to light with a rather modest cranking pressure. Another point that should be kept in mind is that the actual rate of flame front travel may be slightly different for different fuels, and it is ultimately a combination of flame front travel speed and the pressure and temperature required for compression ignition that dicates how much spark advance a gasoline engine needs to run. If the spark plug fires too early then late compression ignition takes place sooner, and the engine will run loud and harsh and will be somewhat less efficient. If on the other hand the spark plug fires too late then late compression ignition will not take place at all, and the engine will hardly run. If the spark timing is just a very small bit too late then a gasoline engine will hesitate and miss as late compression ignition takes place only some of the time. This hesitation and missing is typical for gasoline engines with fixed advance curves (or no advance mechanisms at all) if they are run at higher engine speed as soon as they are started cold. For extremly high engine speeds (greater than perhaps about 11,000RPM) a gasoline engine will make more power and attain a higher efficiency if the time of compression ignition is allowed to become earlier than 15 degrees past top dead center.
The temperature of combustion potential of the fuel is also significant for gasoline engines. The highest speed that a gasoline engine could run efficiently at would tend to be limited by the temperature of combustion potential of the fuel. For this high speed limit to be reached though requires an extremely high performance engine with very light pistons and rods. Most engines run into problems of excesive reciprocating losses and an inability to flow long before the piston speed becomes too high. On the other end there would also be limits to how slowly a gasoline engine could run because of the temperature of combustion potential of the fuel, but for this limit to be reached the stroke of the engine would have to be rather short. A two inch stroke gasoline engine does run noticeably worse at down at 4000RPM than does a three and a half inch stroke engine, but the difference is surprisingly slight. The fact that the piston speed on the two inch stroke engine is a bit too slow at 4000RPM is eclipsed by the fact that both the two inch stroke engine and the three and a half inch stroke engine will run a whole lot better up at 6000RPM.
Older low compression ratio long stroke gasoline engines ran in a totally different way, and would be called full flame front travel gasoline engines. Even a 9:1 gasoline engine can run at very low power output at low engine speeds in full flame front travel mode, but the efficiency attained at this very low power output is abysmally low. Lower compression ratios allow more power to be made in full flame front travel mode, but the combination of the still very low power output and the very low compression ratio makes for a spectacularly inefficient engine. Because gasoline engines could make so much more power and attain so much higher efficiencies by running in late compression ignition mode the old standard of 4:1 compression and a five inch stroke slowly changed to 6:1 compression and a four inch stoke by the 1930's. These six to one engines still were best run in full flame front travel mode at 1500 to 2000RPM, but they could also make a substantial amount of power at 3000-4000RPM in late compression ignition mode. Operational efficiencies were however still abysmally low. By the 1950's an 8:1 compression ratio and a three to four inch stroke became standard. These 8:1 engines could still run in full flame front travel mode, but they made a whole lot more power and usually attained significantly higher maximum efficiencies in late compression ignition mode up at 3000 to 6000RPM. Even three inches of stroke is however far too much to get a gasoline engine to run well over a wide range of speeds and loads. Since 6000RPM is the "bottom basement" for good efficiency it requires a much shorter two inch stroke to allow for efficient reduced load operation.
Even a four inch stroke gasoline engine can be made to make big power all the way up to 6000 or even 7000RPM, but efficiency tends to be quite poor at any engine speed. A three inch stroke gasoline engine can attain much higher maximum efficiency at 6000RPM, but reduced load efficiency still remains very low. The shorter two inch stroke allows both a somewhat higher maximum efficiency at 6000RPM and a spectacularly higher reduced load efficiency. Diesel engines run at much lower piston speeds simply because they can. Lower piston speeds with taller and heavier pistons allows for much longer engine life in demanding applications. As far as differences in the temperature of combustion potential for diesel oil versus gasoline goes little solid information seems to be available. What is clear though is that the temperature of combustion potential for heavy oils is still very high.
Physical properties of fuels don't have so much to do with how an engine runs or what efficiency it can attain, but rather have to do with the setup of the injection system. On a diesel engine the speed of sound through the fuel is significant in that lag in the high pressure lines is approximately proportional to the speed of sound through the fuel. At higher pressures the speed of sound through fuel is slightly faster, but even up to the 10,000psi maximum pressure that injection systems sometimes operate at the increase in the speed of sound through diesel oil is rather modest. At atmospheric pressure the speed of sound through diesel oil is about 1200 meters per second, and at 10,000psi this is increased to perhaps 1400 meters per second. Not all that significant of a change, especially considering that most injection systems operate at considerably less than 5,000psi. Different fuels not only have different speeds of sound, but also respond somewhat differently to changes in temperature and pressure. The differences tend to be rather small though. The compressibility of the fuel also has something to do with the setup of a diesel engine, and the degree to which the compressibility of the fuel is significant has to do with the ratio of the volume of the high pressure circuit to the volume of fuel injected at maximum power output. The most significant thing here is that the compressibility of the fuel goes up at higher temperatures. As long as the volume of the high pressure circuit is not hugely larger than the maximum volume of fuel injected then the compressibility of the fuel is only a very minor setup concern. If however the volume of the high pressure circuit is ten times the maximum volume of fuel injected then the heating of the fuel from the freezing point of water to the boiling point of water would cause about a three degree additional lag in the time of the start of injection. See K.S. Varde's test of oil's compressibility on
pages 713-715 of the May 1984 issue of Fuel
published by Butterworth & Co. Ltd. This temperature dependant lag is not necessarily a bad thing, as engines tend to need somewhat earlier start timing to run well when they are cold. This should not however be confused with the fact that the speed of sound is lower in hot oil than in cold oil, which only exacerbates the problem of wave travel lag causing later timing at elevated engine speeds. The compressibility of the fuel causes a certain fixed number of degrees of crankshaft rotation of injection lag, where the wavefront travel delay causes a certain fixed time lag which corresponds to twice as many degrees of crankshaft rotation at double engine speed.
Another property of fuel that actually is somewhat significant for the way an engine runs, or at least how easily it starts, is the volatility of the fuel. Gasoline engines generally need highly volatile fuels to be able to easily start. Diesel engines do not require a high volatility to start, but the volatility of the fuel does somewhat affect the setup required to get the engine to run at it's best. This would be particularly true on an engine with a linear distributor injection pump which has a fixed injection flow rate but can match the injection start timing to the requirements of the engine for more efficient low speed operation. Again this is mostly just a setup issue, and problems would only be encountered if an engine was run on a different type of fuel that it was setup for.
The last physical property of fuel has nothing to do with how an engine runs, but has been known to be excruciatingly significant from time to time. This is the cloud point of the fuel, the temperature at which the fuel begins to solidify. If the temperature drops below the cloud point of the fuel engines cannot be started from cold. If the fuel has only just begun to cloud, then removing the fuel filter element might be all that is required to start an engine. If the fuel in the supply line has fully solidified there is nothing short of a huge bed of coals that is going to get the engine going before warmer weather arrives. Flax seed oil solidifies quite easily at about 20 degrees Fahrenheit, but adding just 20% number two diesel fuel keeps it liquid down to less than 10 degrees Fahrenheit. Normally number two diesel fuel is considered to be good down to about zero degrees Fahrenheit, and winterizing additives can keep it liquid down to much colder temperatures. Gasoline will not solidify at any normal temperatures, but it's ability to vaporize does drop off at very low temperatures making carbureted engines somewhat difficult to start. Thickening of lubricating oil and diminished current delivery from lead acid starting batteries leading to slower cranking speeds tends to be an even more severe problem for making engines difficult to start at very low temperatures.
Even though different combustion fuels have different energy contents per pound the energy that a particular amount of oxygen can release when reacted with combustion fuel remains amazingly constant. This is why a normally aspirated engine of a certain displacement operating at a certain engine speed could potentially produce essentially the same maximum power output on any combustion fuel. This of course does not apply to the nitro methane used in top fuel drag racing because nitro methane is a high explosive which releases energy independent of the presence of oxygen. For combustion fuels there are however still some reasons why certain fuels would tend to produce more power. The most obvious example here would be less dense fuels such as ethanol that take up more space as they pass through the intake valves leaving less room for intake air to enter the engine. This problem would however not apply to direct injection engines, and even port injection engines that shoot a timed jet of fuel through the open intake valves make this difference between fuels mostly insignificant. There is also a slight difference in both maximum efficiency as well as maximum power production based on how high of a compression ratio can be run. This is however not the same thing as a mismatch between the compression ratio of an engine and the fuel that it runs, which makes a much more dramatic difference in efficiency and maximum power potential. And taking this one step farther neither of these should be confused with the quite common and potentially much more severe problem of gasoline engines running with the wrong amount of spark advance for the fuel and compression ratio that is being used. Fuels that have to use lower compression ratios to avoid destructive early (full) compression ignition cannot attain quite as high of a maximum efficiency or quite as high of a maximum power output simply because higher compression ratios are more efficient in heat engines regardless of anything else. This is however only a very slight difference since for the most part there are no combustion fuels that will experience full compression ignition at extremely low compression ratios. Just what the lowest compression ratio for full compression ignition would be is not entirely clear, but it has been widely believed for many decades that there really are no common fuels that will pop off on full compression ignition at less than about an 11:1 or 12:1 compression ratio. And of course in recent years 13:1 and even 13.5:1 gasoline engines have become quite common. If there is no fuel that will experience full compression ignition at less than 12:1 then any engine that has a substantially lower compression ratio than 12:1 will always run at a reduced efficiency simply because extra spark advance will be required to get late compression ignition to occur. For this reason there is always a dramatic difference in efficiency and power output when compression ratios are increased from say 7:1 to 8:1 or even from 9:1 to 11:1. If a fuel is however available that will require only a small amount of spark advance with an 11:1 engine then there will be only very modest improvements in efficiency when increasing the compression ratio from 11:1 to 13:1.
Differences in flame front travel speed would also make a substantial difference in how well a gasoline engine could run. It is not that slower flame front travel speed fuels could not be made to run in gasoline engines at just as high peak efficiencies as faster flame front travel speed fuels, but rather is a case of faster flame front travel speeds allowing for better efficiency over a wider range of engine speeds and loads. The reason for this is that even with a sophisticated ignition system that can provide the perfect spark timing for any speed, load, altitude and temperature conditions the spark lead has to be increased for lighter loads as well as at the high end of the range of operable engine speeds. If the compression ratio is set so that a minimum of spark lead is required at the lowest operable engine speed with a full load on the engine then any increase in engine speed or decrease in load is going to require more spark lead causing the engine to run less efficiently. A faster flame front travel fuel simply makes these problems less sever by increasing the range of engine speeds and loads that can be attained with some maximum amount of spark lead. It should be noted that a smaller bore diameter also makes these problems of excessive spark lead less severe on any fuel. Poor combustion chamber shape can also cause an engine to appear to be running with a slower flame front travel speed. The combustion chamber shape is however a rather contentious subject because an ideal combustion chamber shape for full flame front travel combustion at low speed and low power output is not necessarily the ideal combustion chamber shape for minimizing spark lead time when running in late compression ignition mode. One of the big problems with full flame front travel combustion is that at higher engine speeds the flame front tends to not have enough time to make it to the “far corners” of the combustion chamber which causes a small amount of raw fuel to be blown out the exhaust leading to both reduced efficiency as well as radically increased exhaust emissions. A combustion chamber that is shaped for best possible full flame front travel combustion has the spark plug located directly in the middle with a generally round combustion chamber. When running in late compression ignition mode though only a small amount of the fuel needs to be burned in flame front travel mode, so only the shape of the combustion chamber in the immediate vicinity of the spark plug is significant for determining how much fuel can be burned in a short period of time. Regardless of combustion chamber shape the size of the engine is manifestly significant for how it will respond to different compression ratios and different fuels. For late compression ignition operation it is the displacement per cylinder that is significant, but in full flame front travel mode it is only the bore diameter that is significant. The reason for this is that in late compression ignition mode it is the length of time required to burn some small percentage of the fuel that dictates the required spark timing value. The volume occupied by this small percentage of the fuel is directly proportional to the displacement per cylinder (when the compression ratio, mixture ratio and fuel properties remain constant). For full flame front travel operation the flame front has to burn all of the fuel, meaning that the flame front must make it's way all the way across the combustion chamber before the piston moves down so far that pressure and temperature have dropped off to the point where combustion can no longer take place. In full flame front travel mode the duration of combustion is directly proportional to the bore diameter. What this means is that the speed at which a full flame front travel engine runs best is directly proportional to the bore diameter and also is directly proportional to the flame front travel speed of the fuel.
Higher flame front travel speed fuels are generally considered to be far superior both because they allow shorter spark lead times and because they allow for a smoother transition from full flame front travel combustion to late compression ignition combustion. With this in mind it should be pointed out that there would be a general tendency for faster flame front travel fuels to also have lower temperature and pressure requirements for compression ignition. What this would mean would be that a premium fuel with a faster flame front travel speed would need to be run in a low compression ratio engine (less than about 11:1) where a less desirable fuel with a slower flame front travel speed would also tend to require higher temperature and pressure to light off on compression ignition so it could be run in a higher compression ratio engine. Running a richer air/fuel mixture can allow an engine to run in late compression ignition mode with less spark advance, and this richer mixture tends to seem like a faster flame front travel speed fuel. The problem with this of course is that if the air/fuel mixtures is made too rich some of the fuel does not burn causing both increased fuel consumption and dirty exhaust. Any gasoline engine can be made to make more power with a richer mixture up to the point where the engine "loads up" fouling the spark plugs and causing stumbling. If the compression ratio of the engine is radically too low for the fuel being used then a very rich mixture will yield much higher power output simply because an ideal leaner mixture would require extremely excessive amounts of spark advance. If on the other hand the compression ratio is close to correct for the fuel being used then richening the mixture up to the point where efficiency drops off will yield only very slight increases in power output. In full flame front travel mode the mixture ratio only slightly affects the flame front travel speed, so there is usually no good reason for a gasoline engine to run rich in full flame front travel mode.
For a diesel engine the only property of the fuel which is really significant is the temperature of combustion potential of the fuel. It is not that a lower temperature of combustion potential is desirable in any way, because a fuel with a higher temperature of combustion potential can always attain a higher efficiency and a higher maximum power output. What is significant about the temperature of combustion potential of the fuel is that it is an indicator of how high of a mean piston speed is required to make the best possible use of the fuel. If however diesel fuel contains small amounts of compounds that require higher pressures and temperatures to burn than the bulk of the fuel does then smoky and inefficient operation can be a problem. Normally a diesel engine runs best and attains the highest efficiency when the cylinder pressure remains just high enough for combustion to take place. When a portion of the fuel requires higher temperatures and pressures to burn though there is a mismatch which cannot be fixed with a different engine setup. If the injection system parameters are changed so that the temperature and pressure remain high enough to burn the small portion of combustion resistant fuel then the engine will be unnecessarily loud, harsh and inefficient for burning the bulk of the fuel which would not otherwise require such high temperatures and pressures. If on the other hand the injection system is setup to run smoothly, quietly and efficiently on the bulk of the fuel that burns at lower temperature and pressure then the portion of the fuel that would require higher temperatures and pressures to burn is simply blown out the exhaust mostly unburned. For inline injected engines the temperature and pressure usually peak early at quite high values and then drop off dramatically towards the end of the injection period. At higher engine speeds and loads where the end of injection is becoming dangerously too late for combustion to occur any problems at all with the fuel being a mixture of different temperature and pressure requirement fuels shows up as black smoke production. A better injection system means that slight problems with mixed fuels causes little or no problems for the operation of the engine. From another perspective though severely mismatched mixed fuels will not show up as such a huge problem on inline injected engines because the peak pressure and temperature during the early part of the injection event is normally high enough to deal with stubborn fuels that require higher temperatures and pressures to burn. It is only towards the end of the injection event at higher speeds and loads that the severe mismatch of mixed fuels will show up as a big cloud of black smoke and reduced maximum power output. An engine with a competent injection system that runs really very well on homogenous fuel might experience smoking problems even at lower speeds and loads when a severely mismatched mixed fuel is used.
When it is a small amount of a fuel that requires a lower temperature and pressure for combusion mixed in with a much larger quantity of fuel that requires a higher temperature and pressure for combustion only relatively slight problems with early peaking of temperature and pressure arise. A lower initial injection flow rate that ramps up to a higher injection flow rate to deliver the bulk of the fuel can go a long way to mitigating any slight problems caused by a small amount of easier burning fuel mixed in with the bulk of the fuel. It should also be said that lower temperature and pressure requirements for combustion would generally be considered desirable because it would allow an engine to run more smoothly and quietly while also not putting as severe loads on internal engine components. Diesel fuel that requires higher temperatures and pressures for combustion can still however for the most part attain equally high maximum efficiency, it just might need to be run in a higher compression ratio engine. The most severe problem here though would be a mismatched mixture of fuels requiring radically different temperature and pressure levels for combustion. And finaly another important point is that with the large molecules that make up heavy oil it is sometimes the case that partial combustion can take place at a lower temperature and pressure while complete combustion of the entire molecule may require higher temperatues and pressures in the combustion chamber.
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