Why can't gasoline engines be compression ignited?

[Razorwyre]

It is difficult to explain without a lengthy post, but gasoline is too volotile and burns too quickly to make an efficient diesel. If you have the balls, you can put out a match in a can of diesel, try doing that with gasoline...boooom.

This is also the reason that you cannot have a high RPM diesel engine, the diesel does not burn fast enough to keep up with the speed of the engine. Easy to make big torque on a diesel, not so easy to make big HP.

 

[Evadman]

Howard: I think you are trying to apply gasoline engine dynamics to a diesel engine running on gasoline. I believe this is why you are not "catching on" so to speak. They way the fuel burns in a gasoline and diesel engine are fundimentaly different. You must understand these differences before any of this stuff will make sense.

 

[silverpig]

Gasoline = burns very quickly. It wouldn't use the longer lever arm as well as a slow burning fuel.

Diesel burns for a good portion of the time the piston head is moving downwards, resulting in a relatively constant push on the piston. Gasoline burns up very quickly, giving a quick hard slam on the piston head, and then having the force drop way down immediately after (there's still the added pressure of the hot gas though). You can either push the piston down, or smash it with a hammer.

 

[geno]

Originally posted by: Evadman

Originally posted by: geno

And in the case of modern diesels, 17:1 is a static ratio, you have to add in a 15PSI turbo.

can you elaborate? I'm not sure what you're tryin to say there 🙂

a 15 PSI turbo is actualy doubling the amount of air the engine can take if NA. (14.7 psi at sea level + 15psi)
So the perceieved CR is closer to 34:1 because there is 2x the NA amount of air in the Cyl. 30+ PSI is not out of the question. an off the shelf kit from Banks will up the PSI to 34.

When this is done, the engine can actualy last LONGER because the extra air has a cooling effect, so the EGT's are lower. Unlike a gas engine, it is impossible to run a diesel too lean.

This is why some people have been experementing running nitrous on diesels.

*Perceived* CR, that's more like it - I was about to point out that the compression ratio doesn't change a bit no matter how much boost you're running

 

[Wolfsraider]

add also;

when a spark plug fires ...it starts a controlled fire at the percise time the piston is starting the downstroke.

when gasoline is compressed ...it can ignite and burn , but controlling that burn in conjunction with the downstroke of the piston would be very hard to do.

no to mention the damage to the pistons/rings/block etc... if it fired as the piston was on the upstroke

mike

 

[Howard]

Originally posted by: Wolfsraider
add also;
no to mention the damage to the pistons/rings/block etc... if it fired as the piston was on the upstroke

mike

I wouldn't be asking why something UNPRODUCTIVE would not be done.

 

[FritzTheCat]

A gas diesel is possible, the only real problem is in pollution. Since the ignition of the fuel/air (gas or deisel) mixture is triggered by the extremly high temp in the cylinder (caused by the high compression [compress air and it gets hot]). Gasoline when burned that hot puts out considerably more pollution. The engine in your car uses the EGR valve (exhaust gas recirculation) to inject some spent exhaust gasses into the fresh air/fuel mixture to help cool down the combustion temps, reducing the pollutants level.

 

[Eli]

Uncontrolled means just that, uncontrolled.

When the spark ignites the fuel/air mixture, the flame front moves outward from the electrodes in a smooth, even movement. It is not violent.

As said before, gasoline is much too volatile. It would be too difficult to control the points of ignition in a compression engine.

 

[FritzTheCat]

It would be too difficult to control the points of ignition in a compression engine.

There are no "points of ignition" in a diesel engine as there is no air/fuel mixture in the cylinder during the compression stroke, just air. The compression stroke compresses the air making it very hot. At or about top dead center, fuel (gas or diesel) is injected into the cylinder (using a very powerful mechanical injecter). The fuel (gas or diesel) is ignited by the super hot air and burns in a single wave front, starting at the injecter and moving out. That is why diesels do not use spark plugs, carburaters, tuned port intakes, ect. like a standard gas engine.

Here's a picture that shows what im talking about.

As I said in my earlier post, a gas powered "diesel engine" is possible, it just pumps out to much pollution.

 

[Pacfanweb]

A few points here:
A normally aspirated diesel is usually around 22-1 compression.

The 17-1 engines have to account for the turbo pressure, as mentioned, just like you don't see gas engines with 12-1 compression and a supercharger.

Easy way to see why gas won't work in this fashion:

Gas goes KABOOM! when ignited.
Diesel goes KAAAABOOOOOOOOOOOOOOOOOOOOOM! when ignited.

Much slower burn.

Ever hear a gas engine keep running after you shut it off? Usually in hot weather. Kind of knocks and shudders but keeps turning over. It's dieseling. Either the gas is igniting from a combo of compression and heat, or there's a carbon hot spot in the combustion chamber that's igniting the fuel.
That's why higher compression engines use higher octane; it's harder to ignite, and burns longer than lower octane.