How do the compression ratios of conventional petrol engines generally compare with diesel engines?
Conventional petrol engines have lower compression ratios than diesel engines.
Compression ratio is the ratio of the cylinder volume at bottom dead centre to that at top dead centre, and it differs sharply between the two ignition systems.
A petrol engine is spark-ignition; it draws in an air-fuel mixture, so it must limit how far it compresses that mixture or it will pre-ignite and knock.
For this reason petrol engines use moderate compression ratios, typically about 8:1 to 12:1.
A diesel engine is compression-ignition; it compresses air alone to a very high degree so the air becomes hot enough to ignite the fuel that is then injected.
Diesels therefore run much higher compression ratios, commonly 16:1 to 22:1, with no fuel present during compression to knock.
Comparing the two, the conventional petrol engine clearly has the lower compression ratio, so the options claiming petrol is higher, higher only at idle, or equal all contradict this basic difference.
Hence, the answer is Conventional petrol engines have lower compression ratios than diesel engines.
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