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Knock Resistance

There is acceleration knock and high speed knock,
the latter is more difficult to note ...


The knock resistance test engine should check the antiknock properties of petrol fuel under precisely determined conditions.

How it works

The knock resistance-test engine does not differ from other single cylinder engine petrol engines except that its compression ratio can be varied infinitely during the engine run. This is possible, because the bearing of the crankshaft is adjustable against the cylinder and the cylinder head.

The engine is operated with a mixture of isooctane (octane rating 100) and n-heptane (octane rating 0). The engine's compression is raised so long, until the engine starts knocking. If the engine knocks earlier, the comparative fuel must contain more N heptane, if it knocks later, more Isooctane. If both fuels trigger a knocking engine, finally, with the same compression, the percentage of Isooctane corresponds to the octane rating of the fuel that is tested.This test can be carried out with different, in each case exactly determined conditions. If the engine octane rating (MOZ) is tested, the conditions are harsher and closer to reality as compared to the research octane number (ROZ). The test engine produces higher revs and the mixture is warmed up more. The engine knocks earlier, the octane ratings are lower. Even closer to reality is the street octane rating (SOZ).

Here the comparative values:

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