# SI engine compression

## Main Question or Discussion Point

in SI engines fuel/air mixture is compressed. The disadvantage is that one can't achieve high compression ratios. I was wondering if it was possible to compress fuel/air mixture to the maximum possible ratio without fear of ignition and compress a pure air mixture to very high ratios 25:1 and then mix the two together. The fuel/air mixture would ignite and burn at nearly constant volume. This way could we get high compression ratios with SI engines?

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Clausius2
Gold Member
sid_galt said:
in SI engines fuel/air mixture is compressed. The disadvantage is that one can't achieve high compression ratios. I was wondering if it was possible to compress fuel/air mixture to the maximum possible ratio without fear of ignition and compress a pure air mixture to very high ratios 25:1 and then mix the two together. The fuel/air mixture would ignite and burn at nearly constant volume. This way could we get high compression ratios with SI engines?
You are describing a Direct Gasoline Injection Engine, aren't you?

No, not a Direct gasoline injection engine. I'm saying that if a rich fuel/air mixture is made and compressed by a ratio of say 9.5 and pure air compressed by a ratio of say 25:1 and then both are mixed together and the fuel/air mixture is ignited with a spark, won't we have a high effective compression ratio engine running on the otto cycle? Won't this increase efficiency because of high compression and the fact that further energy can be extracted from exhaust gas?

Clausius2
Gold Member
sid_galt said:
No, not a Direct gasoline injection engine. I'm saying that if a rich fuel/air mixture is made and compressed by a ratio of say 9.5 and pure air compressed by a ratio of say 25:1 and then both are mixed together and the fuel/air mixture is ignited with a spark, won't we have a high effective compression ratio engine running on the otto cycle? Won't this increase efficiency because of high compression and the fact that further energy can be extracted from exhaust gas?
I don't understand your question. How are you going to mix two gases of different pressures so suddenly?. Also aren't you diluting the mixture? in addition to: won't be the pressure above autoignition limits anyway?

I can see what you mean sid, but i cant visualise how it you would make it work.

But I think that the temperature of the air at that pressure would be too high to allow the fuel to mix with it before ignition, and you would end up with detonation.

Clausius2 said:
I don't understand your question. How are you going to mix two gases of different pressures so suddenly?. Also aren't you diluting the mixture? in addition to: won't be the pressure above autoignition limits anyway?
I don't think there is any need to mix the two gases completely suddenly. It's in effect a piston engine which is getting it's heat from a burning fuel/air mixture at a lower pressure. There will be efficiency losses since the compressed gas will be diluted, but the efficiency gains will also be good since diesel like compression will be possible. The fuel/air mixture will have to be very rich to minimize the dilution losses.
It won't be direct fuel injection because the burning will more closely imitate the Otto Cycle while in direct fuel injection you have more of a Diesel Cycle.

Andy said:
But I think that the temperature of the air at that pressure would be too high to allow the fuel to mix with it before ignition, and you would end up with detonation.
You mean the fuel/air mixture right?

Also why will detonation occur? If I am correct, then in normal piston engines detonation occurs because a small amount of fuel/air mixture gets collected above the piston after the spark and then detonates due to thermal radiation ,etc. In this version, due to the high temperatures of the compressed gas, fuel/air mixture will not have much chance to collect and detonate.

Another option is to burn the fuel/air mixture separately before TDC of the pure air compression piston. When the piston reaches TDC, the fuel/air mixture can be allowed to mix with the air by opening a valve or something. Of course the surface area of the valve will have to be big.

Clausius2
Gold Member
sid_galt said:
Also why will detonation occur? If I am correct, then in normal piston engines detonation occurs because a small amount of fuel/air mixture gets collected above the piston after the spark and then detonates due to thermal radiation ,etc. In this version, due to the high temperatures of the compressed gas, fuel/air mixture will not have much chance to collect and detonate.
Your statement is totally wrong. Detonation occurs because of the sum of two main causes:

i) high pressure and temperature in the combustion chamber
ii) large time of residence of the mixture inside the chamber. I mean, if the mixture remains in chamber enough time to initiate the chemical reaction of explosion, it will detonate. The more temperature and the less rpms, the more chance to detonate. Usual SI engines detonates when rpms are low.

Thank you for the correction.

One more question, can a chamber (not an engine) be made which can withstand repeated detonations?

Clausius2