Q What is the power loss of an engine with increasing
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altitude? Can the loss be corrected by increasing the compression ratio of the engine?
AThe reduction in output of a petrol or
diesel engine with altitude is approximately 21 per cent for every 1,000 ft. above sea level. In the case of a petrol engine, the compression ratio could be increased as a means of restoring at least part of the power lost, but in a typical case, raising the compression ratio of a diesel would have little or no compensatory effect. Obviously it would be inappropriate to raise the ratio of a petrol engine unless it operated at the higher altitude at all times.
It is pertinent that turbocharging automatically compensates for loss of power with altitude up to the limiting r.p.m. of the rotor assembly. In practice, compensation is provided up to, say, 8/9,000 ft.. above which the speed of the turbocharger exceeds the rated r.p.m. and failure is likely to occur.