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Patrols have higher self-ignition temperature than diesel, but still, petrol engines have lower compression ratios than diesel engines. As the self-ignition temperature of petrol is shouldn't we be able to compress it more. I am not saying compress petrol to the point of auto-ignition which can cause knocking, but just have a higher compression ratio than diesel.

Is it because when the combustion of petrol-air mixture starts the moving flame front starts to compress the remaining air-fuel mixture which could reach a point of self-ignition when using higher CR?

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  • $\begingroup$ Your first line answers this question and it has been covered before. $\endgroup$ – Solar Mike Aug 5 '18 at 9:58
  • $\begingroup$ @SolarMike Having higher self-ignition temperature should result in a higher compression ratio. But that is not the case with petrol why is that? $\endgroup$ – GRANZER Aug 5 '18 at 10:23
  • $\begingroup$ see : quora.com/… $\endgroup$ – Solar Mike Aug 5 '18 at 10:43
  • $\begingroup$ @SolarMike I do understand what self-ignition temperature is and that petrol has higher self-ignition temperature than diesel. I don't understand why petrol engine uses lower compression ratio than diesel engine in spite of petrol having higher ignition temperature. Doesn't higher self-ignition temp means that petrol can undergo higher compression without undergoing self-combustion and so not cause knocking? $\endgroup$ – GRANZER Aug 5 '18 at 11:10
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    $\begingroup$ @GRANZER Well, I don't understand what SolarMike's comments and the link to another forum are trying to tell you either. They don't explain WHY a higher self-ignition temperature means you have to reduce the compression ratio. Maybe that's obvious to a thermodynamics guru, or a chemist, but it's not obvious to me! $\endgroup$ – alephzero Aug 5 '18 at 15:54
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It's not because of the fuel, but because of the process. The diesel process differs fundamentally from the otto(petrol/gasoline) cycle.

In the otto cycle, where fuel is present in the cylinder while compressing, the compression is limited by the auto ignition temperature of the fuel, whatever fuel is used. The compression warms up the mixture, ideally just before auto-ignition. The spark plug adds the needed flame source to start the combustion.

In the diesel cycle, fuel is added only when compression has already taken place, the temperature in the combustion chamber is way higher than the auto-ignition point of the fuel, which is why the fuel combusts as soon as it's injected into the cylinder. This removes the limit set by the auto-ignition temperature when choosing a compression ratio. Thus, the ratio can be higher, up to where materials begin to form a problem.

Compression is the very reason diesels are more efficient; the compression ratio can be higher, and they also always run at 'wide open throttle' giving high compression.

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  • $\begingroup$ Yup I was looking at it the wrong way. As only air is compressed in diesel it can be compressed to a much higher degree and when diesel is injected it undergoes stratified ignition and so is exactly SCCI. And this is the reason even in a gasoline engine with direct-injection-system we can achieve higher compression ratios as the fule is itself not present with the compressed air for enough time to undergo spontaneous combustion. HCCI engines are still a mystery though. Thank you for a fresh perspective. $\endgroup$ – GRANZER Aug 6 '18 at 5:48
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    $\begingroup$ @GRANZER Stratified injection is a more advance method of petrol injection, where there is more than one injection. VAG is most well known to use this. Not compression, but leaner mixtures are the key to efficiency here. HCCi is when a diesel process is so sophisticated, and the compression so well tuned/maintained, that compression will cause the mixture to be just hot enough at TDC to ignite the entire mixture all at once. There is no flame front here, the charge will combust much faster, since everything is homogenous. $\endgroup$ – Bart Aug 7 '18 at 18:24
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Gas engines are spark ignited. Diesel engines have no spark, but achieve ignition through compression alone. It's really that simple.

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    $\begingroup$ Perhaps you should modify your answer from "...through compression," to, "...through heat of compression..." $\endgroup$ – BillDOe Aug 5 '18 at 20:17
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    $\begingroup$ So you are saying the reason petrol engines have lower compression is simply because spark ignition means they don't need high compression to function? That makes a lot of sense - a lower compression engine is easier to design mechanically and cheaper to manufacture, for example, so long as the lower compression doesn't degrade the performance and/or fuel economy too much. $\endgroup$ – alephzero Aug 5 '18 at 20:29
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    $\begingroup$ @BillDOe saying through compression is accurate enough $\endgroup$ – joojaa Aug 5 '18 at 20:33
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    $\begingroup$ @alephzero I was asking more from an efficiency perspective than from a functional perspective. As efficiency increases with CR. Yup, I have my answer though. $\endgroup$ – GRANZER Aug 6 '18 at 5:42
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    $\begingroup$ @joojaa, for you and me, agreed; we both know that compression creates the heat to ignite the fuel mixture, but for someone who doesn't know that, they might be left puzzled why compression alone is sufficient. $\endgroup$ – BillDOe Aug 6 '18 at 19:23

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