It's all Greek to me! On the island of Saint Johnís in the USVI we were very amused by overhearing a, rather staged sounding but nonetheless poignant, conversation between a father and a ten year old daughter. The topic was the symbols carved in the rock near a series of deep water pockets along the seasonal creek by which we were hiking. The gist of the conversation was a foible where "petroglyphs" were somehow called "pyroglyphs" and I walked on along the trail chuckling about synthesized rocks and flaming letters written in burning oil on the ground. If the term petroleum is to be criticized for incorrectly associating oil with rocks it should be kept in mind that ancient Greek writing had no punctuation or even spaces between the words which invariably led to an abundant amount of ambiguity and "poetic" interpretations of works of writing. My favorite pop culture reference to this strange linguistic term that has such huge importance for modern civilization are the dinosaur logos on Sinclair gas stations throughout much of the eastern part of the country. As a young child seeing these big friendly looking green depictions of dinosaurs on gas stations for the first time I was reminded of a teacher in school pointing out that petroleum is almost entirely composed of fossilized plant material with only extremely small amounts of fossilized animal remains.
What is Petroleum
From Coal to Oil
Hotter and Faster
The Speed Debate
The Compression Ratio Debate
Types of Gasoline
Additives and Mixtures
The Diesel Engine Alternative
It is in fact commonly known that petroleum is the result of hundreds of millions of years of plant life on the planet. What is important in understanding where the petroleum came from is the fact that life on earth got a very slow start. Even though the origin of life on earth probably dates back a significant portion of the nearly four billion year solid history of this third rock from the sun not much happened until just recently. The age of the dinosaurs is very recent history compared to the many long hundreds of thousands of years of almost entirely animal free plant domination of the planet. In all likelihood it was during this very long period of plant life that the vast majority of the carbon in petroleum deposits was fixed to hydrogen through biological photosynthesis using the energy of the sun to drive the reactions. It is the hydrogen to carbon and carbon to carbon bonds of fixed carbon that represent the stored chemical potential energy that the modern world has been running on for a few hundred years now. So petroleum is solar energy, it is just a battery that takes an excruciatingly long time to recharge.
The way that this played out was a story of carbon dioxide, water sunlight and the slow development of sophistication in life on earth. The earliest life like activity was the self replicating proteins that really did nothing but build complexity in the natural environment. When a sufficient level of complexity existed the self replicating proteins began to "collaborate" and the life cycle was born. We are still made up largely of self replicating proteins that have been enslaved and put to work by cells that are the units of life. There are many levels to this theme of larger systems enslaving smaller and less complex systems, but the self replicating proteins are at the bottom of it all. As life on earth got more complex it also became a more and more voracious generator of complexity, and the building blocks of this complexity are structures made up largely of fixed carbon. As life on earth diversified those strains that were able to build complexity faster thrived, and the increases in the rate of fixing of carbon to hydrogen increased faster and faster as well.
Eventually this long period of increasing rates of carbon fixing used up the carbon dioxide in the atmosphere and life on earth would have stagnated were it not for the advent of a significant additional level of sophistication. This additional level of sophistication was animal life which feeds directly on plant life. It is mostly the metabolic process of animal life that differentiates it from plant life, although the extremely larger amounts of sophistication attained by animal life is what is unavoidably striking. Animal life essentially "burns" plant life, releasing energy by breaking the carbon-hydrogen bonds and re-forming carbon dioxide. Swallow a pound of dry pasta with a half gallon of water and take a four hour walk to see firsthand how important burning carbohydrates is for animal life on earth.
Animal life reversed the trend of depletion of atmospheric carbon dioxide, and we are still in the honeymoon period of animal feasting on abundant plant life. As has been the case throughout the history of life on earth additional large increases in sophistication dramatically and more rapidly than ever in the past change the face of the earth. The first really big change came when man learned how to burn wood for heat, light and to cook food. At first mans consumption of wood for fires was small and mostly insignificant, but as cooked food and the security of a warm hearth allowed for even more rapid developments of sophistication this consumption of wood began to change the face of the earth. With entire forests being wiped out to fuel the fires of industry civilization was at the brink of collapse. The solution was the exploitation of coal deposits.
It is often said that natural petroleum seeps were known back to ancient times, and exploited mostly as novelties due to their extremely limited availability. The first use of fossilized plant material as a fuel was the mining of coal that began slowly both in china and in the ancient Roman world. Then from about 1800 the use of coal as the main combustion fuel really took off. With huge consumption of coal for space heating, steel making and to fuel large fleets of inefficient early steam ships the easily accessible reserves were quickly depleted. By the late 1800's civilization was once again in a state of crisis surrounding energy supplies.
Salvation came this time in the form of development of liquid petroleum reserves. The main advantage of liquid petroleum was that minors did not have to travel into the bowls of the earth to extract it. Liquid petroleum being pumped to the surface, or in some cases simply being forced up by released geologic pressure was abundant and easy to get at.
It was this ease of access that made liquid petroleum such a powerful energy source. Petroleum was also a lot more convenient to use because it could readily be refined into a number of combustion fuel products that could be used in ways that coal could not. Not only could petroleum power atmospheric pressure burners for space heating and steam engines, but it also could be pumped into pressurized combustion chambers to attain higher temperatures of combustion and more efficient release of energy.
Pressurized burners for steam engines were able to boost thermodynamic efficiency to over 40%, which was an unbelievable improvement over the abysmally inefficient atmospheric pressure coal fired burners for earlier lower pressure steam engines. The inevitable further sophistication was the internal combustion engine which directly uses the high pressure of pressurized combustion to drive mechanical output. It is not that higher pressure combustion can extract more heat, but rather that the same amount of heat released at a higher temperature has the potential to do more mechanical work.
The temperature of combustion potential of coal and petroleum derived fuels are similar, and modern coal fired power plants also attain greater than 40% thermodynamic efficiency with the use of pressurized burners for coal. It took the ease of pumping and pressurizing oil though for mankind to arrive at this realization that there was more energy stored in hydrocarbons than what could be gotten out of them just by throwing a log on the fire.
The first widespread use of petroleum in the mid to late 1800's was as a replacement for whale oil lighting. Simple heat distillation of crude oil readily produced large quantities of kerosene that was a clean and bright lighting fuel. As the story goes the first internal combustion engines were embraced as a good way to make use of the otherwise excessively dangerous light volatiles that came off during the production of kerosene. This is a bit of a chicken and egg story though, as the quantity of gasoline produced depends greatly on the method of refinement of crude oil.
Modern oil refining produces a wide array of products for combustion and as raw materials for industrial processes. The types of products produced and the relative quantities of many of these products can be greatly varied with the introduction of non-petroleum derived compounds under various temperature, pressure and catalyst conditions. It is often said that anything that can be synthesized using petroleum can also be synthesized purely with mineral and plant derived feed stocks. Equally significant though is the fact that a huge variety of materials and products can easily and inexpensively be produced using petroleum.
Since a wide variety of different combustible compounds can be extracted from petroleum or synthesized using petroleum it is necessary to consider the ways in which fuels are burned in order to understand how petroleum is used. In a diesel engine or a pressurized burner for a steam engine all that is ultimately significant about the combustion fuel is how cleanly it burns and how much heat of what temperature can be extracted from it. More heat obviously means more useful power output, but higher temperatures also mean that the same amount of heat can do more work. From a perspective of petroleum refining diesel engines and steam engines are simple, just squeeze the oil has hard as it takes to get the most work out of it. The way that this has played out is that diesel engines are run as hot and fast as is convenient to extract the energy stored in the carbon-hydrogen bonds, and this has often meant that temperatures and pressures are somewhat lower than would be ideal for maximum efficiency.
When petroleum is used to power gasoline engines though there is a whole lot more difficulty in deciding just what fuel to use and why. The root origin of this difficulty is that hotter not only means a faster piston speed in a gasoline engine, but hotter also means more revolutions per minute. In a diesel engine a longer stroke can be used to attain a higher piston speed and extract more work from fuel with a slower spinning engine. In a gasoline engine though both a higher piston speed and a faster spinning engine are required to extract more work from fuel with a high temperature of combustion potential.
The fact that gasoline engines have to spin up to very high speeds of somewhere around 4,000 to 6,000RPM to make best use of the stored chemical potential energy of the carbon-hydrogen bonds has lead to a situation where there is significant dissent about how petroleum should be used. Fast spinning engines have just not seemed aesthetically pleasing, and fast spinning bearings have also been difficult to make work efficiently and last well. With a widespread desire for slower turning engines that sound more tame and are easier to get to last well there is an inevitable drive in society towards supplying lower temperature of combustion potential fuel for gasoline engines. Lower temperature of combustion potential gasoline that works acceptably well down to 3,000 or even 2,000RPM might be aesthetically pleasing, but it is abysmally inefficient from a refining perspective. Combustion fuels tend to have a certain rather narrow range of temperature of combustion potentials, and this seems to correspond to about 6,000 to 8,000RPM with a two to three inch stroke in a gasoline engine.
Specialty fuel that runs smooth in a four inch stroke gasoline engine at 2,000RPM might sound nice, but it takes more petroleum to make a gallon of it and other more severe environmental and human health problems may exist as well. Going the other way specialty fuels can also be produced that will allow a high performance three and a half inch stroke gasoline engine to romp stomp up to 10,000RPM and it is not at all clear just how abundant or efficient they are from a refining perspective. What is clear is that the "standard" gasolines that tend to be available regardless of how much desire there is for slower gasoline just keep delivering more work up to at least 8,000RPM in a two inch stroke engine.
It seems that it is the ideal piston speed that is more easily attained at 6,000 to 8,000RPM for a two inch stroke engine or 3,000 to 4,000RPM for a four inch stroke engine and the ideal maximum engine speed is in fact extremely high for essentially all possible combustion fuels. The big clue pointing to this is the fact that on essentially all possible combustion fuels gasoline engines continue to work better and better up to higher and higher engine speeds provided that the stroke is made shorter. There would be some maximum engine speed for gasoline engines to make big power, but it appears to be in excess of 18,000RPM. The ideal stroke for an 18,000RPM gasoline engine however appears to be only about an inch and a half.
In diesel engines the pressure and temperature required for a fuel to burn is significant, but most fuels can be made to run really pretty well over a certain range of compression ratios from about 16:1 up to about 20:1. The reason that diesel engines are less sensitive to compression ratio changes and fuel changes is that the pressure during cold cranking already has to be high enough for the injected fuel to begin burning. This requires very high compression ratios, but there is also no firm limit to how high the compression ratio of a diesel engine can be made since there is nothing to burn before the fuel is injected at the time of combustion.
A gasoline engine on the other hand must have a compression ratio precisely matched to the fuel being used if it is to run efficiently. Two high of a compression ratio simply will not work because the fuel will light off on compression ignition at about 20 degrees BTDC destroying the engine. The compression ratio of a gasoline engine absolutely must be bellow some maximum value for a particular fuel. If however the compression ratio of a gasoline engine is radically lower than this maximum value for a particular fuel then a large amount of fuel must be burned early in flame front travel mode before late compression ignition can take place. Over a certain small range of compression ratios reductions in the compression ratio just require moderate amounts of spark lead that results in slightly reduced efficiency. If however the compression ratio is so low that the majority of the fuel is having to be burned in flame front travel mode before late compression ignition can take place then the engine is going to be very loud, harsh and inefficient with a pronounced inability to run well down at lower engine speeds around 4,000RPM.
It is not entirely clear just why gasoline engines from the first half of the 20th century nearly universally used extremely low 6:1 compression ratios, but the aesthetics of engine speed probably play a central role. In the face of the reality that gasoline engines need to spin up to 4,000 to 8,000RPM to run well there was probably a backlash against late compression ignition in general, with 6:1 engines running nearly entirely in full flame front travel mode stealing the show.
There were however a lot of problems with trying to keep gasoline engines in full flame front travel mode. Most significantly there is the simple fact that the lowest compression ratio gasoline engines that start easily on normal fuel will also tend to be able to attain late compression ignition under some circumstances. Six to one engines require way too much spark lead to reliably get into late compression ignition, but on the lowest pressure fuel they will pop off when well heated up and being run hard.
Faced with the inevitability of late compression ignition the march towards higher compression ratios began. In the early years it was probably a case of 6:1 engines accidentally lighting off on late compression ignition on the lowest pressure fuels, and these lowest pressure fuels were then run in 7:1 engines with much better results because less spark advance was required. Since 7:1 engines also worked better as full flame front travel engines there was then a demand from the "late compression ignition avoiders" for higher pressure fuel that would not so easily light off on late compression ignition to be supplied. Inevitably though the 7:1 engines still often got into late compression ignition and then 8:1 engines were developed to even more easily attain late compression ignition. This trend of ever increasing compression ratios continued up until the late 1950's and early 1960's when 9.5:1 and even 11:1 high performance "muscle car" engines were available. It was probably somewhere around the 9:1 level that full compression ignition first began to crop up as a problem on the lowest pressure fuels but the debate did not end there.
Just because a specialty fuel was available that would destroy 9:1 engines did not mean that 8.5:1 was the desirable maximum compression ratio. Then and now the question was what the maximum compression ratio for "standard" gasoline would be.
The ideal compression ratio is not as simple as just running slightly less than the maximum compression ratio for a certain standard gasoline because there would actually tend to be at least two radically different standard grades of gasoline. The absolute lowest pressure fuel that will reliably run in late compression ignition mode in 7:1 engines is probably not good stuff. This lowest pressure fuel would probably tend to be inefficient in the refining process, requiring more crude oil to produce a gallon of gasoline. The most efficient fuel from a refining perspective would be an oily mess that would require about a 14:1 gasoline engine to run at its best. Because this high pressure oily mess would tend to have problems like slow flame front travel speeds, low volatility and associated hard starting and unusually dirty operation in flame front travel mode it would never be the only gasoline available for carbureted or port injected engines. Lower pressure fuel with higher flame front travel speeds, higher volatility and overall cleaner burning would be much more desirable from many different perspectives. The question then is just how much lower of a pressure does this "premium" fuel operate at and more importantly what is the absolute maximum compression ratio for this "premium" gasoline without full compression ignition occurring in the highest performance normally aspirated engines.
For any particular fuel laboratory tests of the pressure and temperature curve for compression ignition would be fairly straight forward to generate. It would just be a matter of using a pressure measurement device that was both sensitive enough to accurately measure the approximately 200 to 250psi required for compression ignition but that was also able to withstand the much higher pressure that would be encountered after ignition. A large capacity blow off valve set at perhaps 400psi would help with reducing the range of pressures that the pressure measurement probe would have to sustain. A high frequency data acquisition system that could pinpoint the exact pressure at which the air/fuel mixture ignited would be the most user friendly means of reading the pressure, but simply making a number of runs at incrementally increasing pressures until the air/fuel mixture ignited would be a simpler and more reliable if somewhat more labor intensive approach.
Running these tests at room temperature would be good enough to get some idea about the relative properties of various fuels, but a series of tests at elevated temperatures would give detailed information more directly applicable to engine performance. The elevated temperature could be attained just with an isolated heated side chamber, which would be much easier to attain than heating the entire test apparatus up to 250 or 300 degrees Fahrenheit.
It has long been said that the actual tests of gasoline for the official octane rating numbers are carried out using variable compression ratio test engines. The problem here is that a variable compression ratio test engine can be used to perform any number of tests on the fuel, and because there is so much interest in the flame front travel speed of the fuel any single test carried out on a variable compression ratio test engine will tend to be designed to make some measure of the flame front travel speed of the fuel. Testing only for the temperature and pressure capability of the fuel would involve running the test engine until full compression ignition occurred. The way that this would be done would be to run the test engine wide open with a rich mixture and wait to see if full compression ignition occurred once the engine was well headed up. This would be done by starting at a low compression ratio, then the engine would be stopped and adjusted to a slightly higher compression ratio and run again. This procedure would be repeated until full compression ignition was attained. The compression ratio at which full compression ignition occurred would then be just beyond the absolute highest possible compression ratio for that fuel. Determining when full compression ignition occurs is not difficult because the engine makes such a horrible loud clanking that it is absolutely impossible to mistake it for anything else as long as the engine speed is up above about 2,500RPM. The confusion comes from running the test engine down at only 1,000 or 1,500RPM where late compression ignition itself sounds so horribly loud and violent that it is often mistaken for full compression ignition.
The testing itself is however not the real problem. Either the motor test or the laboratory test can easily be used to measure the temperature and pressure required for compression ignition of a particular fuel and any low compression ratio motor test can be used to make some sort of a determination of flame front travel speed. The problem lies in determining what sample of gasoline to run the test on, because this brings up the question of just what the "standard" gasoline is or should be. Ultimately this determination of what the standard premium gasoline should be can only be made by an analysis of the crude oil composition and a BMP (best management practices) study of both the desirability of certain properties in the gasoline product and the refining efficiency and environmental and human health impact of the use of candidate compounds in the gasoline product.
The basic idea with a best management practices study would be to determine what compounds that are potentially useful as combustion fuels are the easiest, safest and most efficient to produce from the crude oil. Once a list of these easy to obtain compounds is available each one can be analyzed to determine its desirability as a motor vehicle fuel. The final gasoline product would in all likelihood be a mixture of a variety of desirable compounds, although a certain grade of gasoline might be only one compound or a mixture of very similar compounds. The goal would be four things: Performance and efficiency from the fuel, efficiency and safety of refining, lowest practical environmental and human health impact and consistency of the gasoline product from one batch to the next.
The consistency is the tricky part, as the results of the best management practices study might be slightly different depending on the actual composition of the crude oil being used. A comprehensive best management practices study would have to take into consideration crude oil derived from various locations. This may seem like whole lot to accomplish with little prospect of good results, but it has to be kept in mind that any sort of a systematic approach with some resources behind it is bound to come up with a system that is far superior to the non-functional single number rating system and resultant expensive "strange brew" race fuels currently in use.
The alternative to a methodic approach to determining what to call standard gasoline would be a market based approach. If a meaningful rating system was instituted then personal buying choices would eventually result in a cost versus benefit driven process of the refineries producing the products that are cheap and easy while also being desirable from a performance perspective. The danger here is of course that consumers might mistakenly pay whatever they had to in order to attain the absolute best performance fuel they could get, and this would lead to "strange brew" fast flame front travel speed gasoline for 14:1 engines. In order to get good information out of consumers good information would have to be provided to the consumers. For people to really make good decisions about what fuel to buy information about absolute maximum compression ratios would of course be mandatory, flame front travel speed ratings would be important for marketing purposes and energy density ratings would also be very useful.
The concept of additives being used to modify the combustion properties of gasoline is a particularly contentious subject. It has long been known that additives can be used to boost the octane rating of gasoline, but this practice does not necessarily accomplish anything useful.
The basic idea is that for fuels that will mix but do not chemically react with each other adding a rather small quantity of a lower pressure fuel will lower the temperature and pressure capability of the fuel mixture all the way down to the temperature and pressure capability of the lower pressure additive. In other words the temperature and pressure capability of a fuel can be lowered by using an additive, but the temperature and pressure capability of a fuel cannot really be increased by using a non-reacting additive. Because only a rather small amount of some lower pressure additive would be required to reduce the temperature and pressure required for late compression ignition the flame front travel speed of the fuel does not necessarily change significantly. Because the lower pressure additives do tend also to have much higher flame front travel speeds though it is inevitable that the flame front travel speed of the fuel will be increased at least by a small amount. Adding more of the lower pressure additive does not necessarily lead to significant additional reductions in the temperature and pressure capability of the fuel, but these larger quantities of the lower pressure and higher flame front travel speed additives do of course further increase the flame front travel speed of the fuel mixture.
Lowering the pressure and temperature capability of a gasoline with the use of a non-reacting additive will get a lower compression ratio engine to run more smoothly and more efficiently, but ultimately does not accomplish anything beneficial. Raising the compression ratio of the engine so that the base fuel can be run without the use of a low pressure additive will yield a slightly higher volumetric efficiency and an even more significant boost in thermodynamic efficiency.
In most moderate speed gasoline engines a mixture of fuels with different temperature and pressure capabilities does not cause any large operational problems, but when the engine speed is increased up towards the high end of the speed range where latest possible late compression ignition is ideal the mixture of fuels does cause a significant problem. Down at lower engine speeds where late compression ignition can first be used between 3,000 and 6,000RPM the temperature and pressure spike up so high immediately after late compression ignition occurs that pretty much any combustion fuel present will fully burn. Up at the higher engine speeds where latest possible late compression ignition is still ideal around 8,000 to 11,000RPM there can however be a problem with part of the mixture of fuels not burning at all in a lower compression ratio engine. What this means is that when a mixture of fuels with different temperature and pressure capabilities are run at high engine speed the time of late compression ignition may have to be made earlier to attain full combustion. This is then a type of problem related to a mismatch between the compression ratio of the engine and the temperature and pressure capabilities of the fuel being run. On a mixture of fuels with different temperature and pressure capabilities the time of late compression ignition may have to be made earlier down at say 9,000RPM, where an engine running on a homogeneous fuel may be able to rev all the way out to above 11,000RPM on the latest possible time of late compression ignition.
A small difference in temperature and pressure capabilities of mixed fuels would cause only small reductions in efficiency, and these small reductions in efficiency would occur only up at the highest engine speeds. A severe mismatch between mixed fuels though where late compression ignition is first taking place at a much lower temperature and pressure than is required to get the bulk of the fuel to light off will cause severe operational problems, and these problems will crop up at significantly lower engine speeds. If the time of late compression ignition has to be made earlier all the way down at 7,000RPM then the engine is going to be louder, harsher and less efficient than an engine running at the latest possible time of late compression ignition at the same 7,000RPM.
In general it could be said that the use of non-reactive additives to lower the temperature and pressure capabilities of gasoline is not a good idea. There could however be some instances where a variety of similar fuels would need to be run in gasoline engines of some standard compression ratio, and in this case it could be beneficial to slightly lower the temperature and pressure requirements of some of the fuels to run as well as they can at low engine speeds in the slightly lower standard compression ratio engines. For high engine speed operation it would in most instances work better to simply use a bit more spark advance and maintain the homogeneity of the fuel. The irony here is that it is down at the lower engine speeds where late compression ignition is loud, harsh and inefficient that a mismatch between the temperature and pressure requirements of mixed fuels is no kind of a problem. In order to get any gasoline engine to run as efficiently as it can homogeneous fuel is required.
In the absence of a meaningful rating system gasoline engines might be abandoned altogether, although this is not as desirable as it might sound. The biggest problem with a full switch to diesel engines is the elimination of the simple and attainable lightweight high performance mechanically controlled engine. If every engine uses a computerized injection system then 18:1 compression ratios and direct injection is undoubtably the way to go and variations in fuel properties are only a minor concern. What is lost with a complete elimination of carbureted and port injected engines is a form of motive technology within the control of the people. Any form of internal combustion engine requires factory level support, but the number of people who are willing to fight with the corporations to keep the technology viable and appropriate is much larger when they can use mechanically controlled engines. So ultimately it is a freedom issue. Availability of gasoline equates to a free and dynamic population and total reliance on computer controlled engines equates to a population beholden to large corporations or overbearing "Orwellian" governments.
There is however a "middle ground" on the issue of gasoline and freedom or diesel and slavery, and that is the electronically controlled engine management systems of the late 1980's and early 1990's. It is not the exact engine management systems from this era, because they were only for rather poorly running low compression ratio port injected engines. Rather it is the concept of those pre OBD II engine management systems. They were simple enough to be comprehended by a wide range of mechanics and hobbyists, reliable enough to function for many decades with little difficulty and sophisticated enough to be able to deliver darn good control of even the most difficult to manage types of internal combustion engines. It would not be the same port injected engines that would be used, rather it would be direct injection diesel engines that would be used. All that direct injection engines need to be able to attain high efficiency, good performance and acceptable cleanliness is sophisticated electronic management. The level of sophistication is however not all that high, certainly not higher than what the pre OBD II engine management systems were capable of doing.
The other big problem with diesel engines is that cleanliness issue. Nothing is going to run as clean as a gasoline engine operating in late compression ignition mode up at a reasonable engine speed of 4,000 or 6,000RPM. The engine speed is critical for port injected or carbureted engines because insufficient engine speed causes high peak cylinder pressure which in turn causes the formation of oxides of nitrogen and other nasty high pressure pollutants. The pathetic long stroke low compression ratio port injected automotive engines of the late 20th century are not cleaner than good running diesel engines, but that is no reason to dismiss the extreme clean running capabilities of gasoline engines running in late compression ignition mode.
Gasoline direct injection engines hold some promise for the "best of both worlds" in that they can run cleanly in late compression ignition mode up at over 4,000RPM but can also run efficiently and fairly cleanly down at lower engine speeds as diesel engines. The problem with gasoline direct injection for cleanliness is of course that they are only supper clean up at high engine speed, and down at lower engine speed where they attain good light load efficiency they are only as good as a diesel engine running on gasoline. The low compression ratio required to attain late compression ignition also severely interferes with efficient operation as a diesel engine. Just running a well managed diesel engine on gasoline instead of heavier and dirtier oil is a big step towards cleanliness, and it is widely believed that gasoline fired diesel engines could potentially be good enough for the strictest standards of cleanliness.
The big question though is whether the reduced refining efficiency of gasoline production is worth the higher level of cleanliness. If the only goal of producing a lighter and more volatile fuel is cleaner operation in diesel engines then the flame front travel speed of that lighter and more volatile fuel is of less importance. In other words it might not ultimately matter how well a fuel can run in a gasoline engine as long as it can run as cleanly as possible in a diesel engine.
Really the biggest cleanliness problem with good running diesel engines is that the last of the fuel injected at the end of the injection event does not have time to fully mix with the small amount of remaining oxygen in the combustion chamber and incomplete combustion leads to soot formation. A good running and efficient diesel engine forms only small amounts of soot, but soot formation is an inevitable problem with time of combustion injection. The important thing to keep in mind is that a good running diesel engine is cleaner and forms less soot than a gasoline engine running in full flame front travel mode on the same fuel. Try to run diesel oil in full flame front travel mode in a gasoline engine and it will smoke like a pile of smoldering tires.