A diesel engine is designed to work with spontaneous ignition, gasoline engine is designed not to. I was wondering what is it that makes it impossible (or difficult?) to have fuel in gasoline engine ignited spontaneously due to (almost) adiabatic compression rather than with a spark plug?. Or to rephrase this question, could we have diesel engine working on gasoline? Is this something in the properties of gasoline that excludes taking advantage of spontaneous ignition even though gasoline is much more volatile/flammable than diesel fuel?
Yes. The gasoline has a different (lower but more random) flash point. They used to run gasoline engines up around 12:1 compression, but still ignite with spark plugs. This cycle was not good for emissions.
They are working on high pressure injection, but I don't know if you could get the correct injector lubrication with gasoline.
In theory you could run a diesel engine on petrol. There would be a few practical differences such as the poor lubricating properties and lower viscosity of the petrol but it could be done. You are missing the point about diesel vs. petrol engines. There are more accurately described as "compression ignition" and "spark ignition" engines respectively. The critical point is the timing of the burning of the fuel. In a diesel engine, combustion begins when the fuel is injected into the cylinder. In a petrol engine, the fuel is drawn in with the air and is ignited by the spark plug. If you try to draw in a mixture of fuel and air and then ignite it purely by compression temperature, the timing of ignition is extremely variable and depends upon many other factors not least of which is throttle setting. Basically, it won't work. Incidentally, the fuel in an engine burns, it doesn't explode. There should be a controlled burn with a defined flame front travelling across the combustion chamber. There has been work done on this subject with plastic engines and high speed cameras. Don't ask me where the results are though. It was many years ago that I saw them.
Thanks for your comments, especially for more accurate engine classification by "compression ignition" and "spark ignition".
Are you sure that fuel in compression ignition engine does not explode? It would appear that spontaneous ignition takes place instantly in the entire volume of compressed fuel and air mixture. On contrary, in spark ignition engine flame propagates with a finite velocity from the spark plug. This is why when you step on gas when idling the spark engine it will rev up only to a certain speed (although very high) while compression ignition engine (without governor) will rev up until destruction.
The detonation (spontaneous ignition) you refer to is called "knocking". It still usually starts from one or more "hot spots", and does not propagate much differently than a spark driven ignition.
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classification
As you say, a definite flame propagation takes place within a spark ignition engine. In the compression ignition engine, the mechanism is somewhat different. Certainly the combustion will take place over a much larger total area. As each droplet of fuel evaporates, the vapour will burn. This results in a number of centres of combustion. I suppose it depends upon the definition of explosion. When this occurs in a petrol engine, the fuel mixture self ignites generally due to the increase in pressure and temperature either before or during the spark initiated combustion. Here there is frequently no centre of combustion and no associated flame front, combustion happens spontaneously throughout the mixture. In a compression ignition engine, the fuel droplets evaporate as they travel around the combustion chamber leaving a trail of burning vapour in their wake. Since combustion chambers are designed to be turbulent producing near TDC, the result is near chaos and could be construed as being explosive. With regard to your comments concerning the limiting speed of the two types of engine, I would add a couple of comments. A compression ignition engine is designed to run slower than a spark ignition one. (Lets call them petrol and diesel, less typing). The main reason for this is that it takes time to inject the fuel into the cylinder. This has to happen during the burn time and should take place over as few crankshaft degrees as possible in order to produce the best power at the highest combustion chamber pressure. The longer the injection time, the less efficient the engine as the last increase in pressure will be acting on the piston for less time. As a result, these engines are designed to rotate slowly with relatively heavy moving parts and can be easily destroyed by over speeding. Petrol engines add their fuel to the air before it enters the cylinder so can take more time for the process. As a result, they are theoretically limited by flame propagation speed. That is far faster than the limiting strength factors though. Petrol engines can be destroyed by over speeding as I have found out to my cost. That's another story though. Are you sure you are not thinking of modern car engines with rev limiters here?
There is an even more fundamental difference in thermodynamics cycles between the two. The SI engine operates on the OTTO cycle, while the CI engine operates on the DIESEL cycle. The major difference between the two is the heat addition (combustion process) portion of the cycle. The OTTO cycles uses a constant volume process, while the DIESEL uses a constant pressure process. So the all the combustion occurs in the SI engine near top dead center of the piston motion, while in the CI engine it occurs on the downstroke . The constant pressure acting on the piston during the major portion of the down stroke it was leads to the high torque output normally associated with diesels.
I thought that detonation was when the flame front (pressure wave) trapped and compressed a small volume of mixture in a corner of the combustion chamber--causing that small volume to explode. But it's been a long time for me too, so I could be mistaken...
You could use gasoline in a diesel engine only it would the engine would detonate. The two fuels have completely different burn characteristics. They would both ignite, only gasoline would not be a controlled burn and would detonate. This is essentially what knocking in a gasoline engine is. That is where the compression ratio is too high and you end up with multiple ignition points causing an uncontrolled burn. Anti-knock agents would not have that much effect without being is such large proportions that the efficiency would be greatly reduced. High compression ratio gasoline engines are around 12:1 and these cause significant problems. Diesels around 20:1 as the norm.
Multiple hot spots will generate lots of wavefronts. Whereever these wavefronts impinge, I think you get the effect you note. Wonder if it is akin to bubble collapse in liquid...
There are other ways of increasing the Anti-Knock Index of gasoline. 100 and 110 "octane" unleaded is readily available. Given modern engine controls, you can also run stratospheric (like 12:1) compression on standard pump gas these days.
How fast is the gas injected in the fuel injected gasoline engine?
If it is added all at once, then it is the Otto thermo cycle, a more efficient cycle than the Diesel cycle for a given compression ratio.
If it is injected over time like in a Diesel engine than the fuel injected gas engine is effectively operating on a Diesel cycle.
The only real difference is the fuel, the only reason to use ignition in the fuel injected gas engine is that gasoline doesn't pressure ignite as easy as # 2 fuel oil.
Ed Ruf . . .
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