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There is an important chore by the fuel in preventing combustion from becoming too fast during the early first stage after the spark has started the burn or after the diesel fuel is injected. Gasoline is designed to not burn too quickly while diesel fuel is designed to ignite as soon as possible without lag or with a very short lag time. When gasoline burns too easily, there is knock near TDC. When diesel fuel burns too slowly, there is knock near TDC. Knock is the general category for abnormal combustion, such as detonation (with gasoline) and fuel knock with diesel fuel. When fuel burns uncontrollably, pressure piles up on top of the piston near TDC where the excess pressure cannot do much to turn the crank and make torque. Plus this condition causes damage to engine parts and is inefficient as well. It seems the engine gets the longest life from its parts when it runs most efficiently and yields the best mileage. Runaway combustion happens from a bad combination of fuel components for the application. BTW there is a weird spark plug being promoted in Detroit that claims to ignite all the fuel and air in the chamber in a single (instantaneous) burst at TDC thereby creating enormous efficiency and economy. Yeah, right.

With gasoline we seek a gradual flame propagation from spark ignition in first stage and rapid propagation in second stage. With a diesel we seek rapid flame propagation in both first and second stage. The diesel fuel is supposed to ignite immediately as it gets squirted into the hot cylinder. That is what we mean by compression ignition. The combustion is regulated by the rate of injection from the pump and injectors while the piston is moving down. With gasoline the rate of combustion is regulated by a kind of "peeling the banana" effect whereby outer layers of more stable ions are stripped from the rather large original fuel particles by the violent collision process. These initial particles combust somewhat gently even at the high temperature and pressure near TDC. The burn is controlled by peeling the banana and uncovering faster burning fragments under the top skin. Then more peeling reveals very fast burning Freds that urgently seek any Opel to mate with in the scarce condition part way down the cylinder where fewer and fewer free Freds and free Opels can be found. Any aid that could assist combustion in that part of second stage will be very welcome indeed because that is where many engines and fuels go wrong. That is where they allow combustion to peter out, even well before the exhaust valve opens. In gasoline engines, we need to get maximum pressure to occur at about 15-20 degrees past TDC. In diesels, we will see a "constant" pressure from about 15 to 75 degrees after TDC. Gasoline combustion is termed "constant" volume while diesel combustion is termed "constant" pressure. The quotes mean it is ideally constant, not actually.

You can appreciate that diesel fuel is not as complex as gasoline which consists of hundreds of ingredients that contribute to a very carefully designed layered burn scenario that we commonly find available in three different grades of octane rating. Too much octane rating and the burn may be slow while too little octane rating and the burn may proceed too quickly from lack of tolerance for heat and pressure. Altitudes affect the rate of burning as atmospheric pressure drops and we need a lesser octane rating in the mountains. Superchargers raise the demand for higher octane because of the higher intake pressures from the blower. To protect engines from knock, electron absorbers are put into gasoline--such as metals or fuel additives with a lot of methyl ions such as xylene, toluene, acetone, neopentane or neohexane--my choices. These additives and methyl ions add a great deal of vaporization to the fuel and that in turn can create cooling due to the latent heat of evaporation. IOW evaporation and vaporization cool the mixture as larger Fred fragments are allowed to form into many small fragments and improve efficiency. It takes energy to force the breakup. That energy comes from the linear and spin kinetic motions of the particles in the furnace. So cooling occurs and the mix is less likely to detonate and will in fact allow a smoother operating, more efficient engine.

Octane and speed of combustion are only superficially related. Octane rating is best seen as the tolerance for heat and pressure of a fuel. Octane rating is NOT a simple reference to the speed at which fuel propagates a burn because that speed is never a constant but always very different and random.

When good fuel layering exists and combustion progresses nicely, most if not all of the fuel in the cylinder gets burned and the specific fuel consumption rate is the lowest possible--perhaps below .400. Average good gasoline is .500. When fuel layering is poor and combustion falters part way through the burn, the specific fuel consumption rate is very high--perhaps over .700 as may be the case with ethanol blends. When atomic hydrogen or CH particles are peeled or released too soon, an explosion could be triggered and the engine might ping like the old cliche--an ashcan full of marbles. Hydrogen gas burns 8-10 times faster than gasoline which makes for many headaches of engine design. Each hydrogen molecule or H2 has absolutely no layering and resembles a naked stick of dynamite with a microscopic fuse. Hydrogen is an ideal fuel for fuel cells and rockets rather than internal combustion engines.

There was a temptation to employ large and heavy fuel molecules as mileage fodder. Heavy molecules yield more BTU when they burn and can give a longer range. However they might be sensitive to cracking in the chamber and might produce undesirable products (such as heavy alkanes that refuse to disassociate) during the peeling, may cause detonation and/or create sludge and varnish problems in the engine. They are okay for asphalt. The fuel student must know his stocks intimately and have tested the components carefully in the lab before selling the stuff. During the fuel crisis of 1973, many bad gasolines were thrown out into the street.