0 . The term "highest useful compression
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ratio" is sometimes used in papers of the learned societies dealing with the design of petrol engines. Is it not also applicable to diesel engines?
A The highest useful compression ratio of a petrol engine is determined by the onset of detonation, which can prevent the use of a higher ratio. In practice, the HUCR depends on the form of the combustion chamber and the octane rating of the fuel. Detonation is caused by auto-ignition of the -end gas" in the combustion chamber when it is compressed by the advancing flame front.
In a diesel, the fuel is burnt as it leaves the injector nozzle by virtue of the high temperature of the air in the cylinder (resulting from high compression) and there is no fuel mixture in the cylinder that can detonate by auto-ignition befoie it burns (at a lower rate of combustion) in the normal way. Diesel knock results from combustion lag during the initial phase of injection. When fuel is injected with the engine running under light toad, the air is locally cooled by the droplets of solid fuel during this phase and combustion is delayed until an appreciable quantity of fuel has been injected. Burning of the fuel is uncontrolled in that all the injected fuel starts to burn at the same time, instead of burning progressively as it leaves the injector. And although the amount of fuel involved is small compared with the total amount injected, the rate of pressure increase locally is very high and a knock is heard.
Raising the compression ratio of a diesel is not, therefore, restricted by detonation and in theory there is no limit to the compression ratio that can be employed. In practice, a very high compression ratio is undesirable because the small gain in thermal efficiency that it provides is more than offset by the high (peak) cylinder pressure and the increased frictional losses. A high peak pressure that is developed over a very small part of the combustion cycle cannot be converted into useful work but can overload the working parts. A number of manufacturers are experimenting with lowcompression highly turbocharged engines that have a high power output but do not develop high peak pressures.