Q Is any harm done if a petrol or diesel
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engine is allowed to idle for a few minutes to warm up before starting off? Is it preferable to drive off immediately the engine is started?
AIn both cases it is better to drive off immediately the engine is started, as long as it is firing evenly. But the engine should not be operated under heavy load until it has been running for some time.
It is desirable to warm-up the engine as quickly as possible: if it is run at a reduced temperature the acidic products of combustion are not evaporated and rapidly corrode the pistons and piston rings. The rate of piston and ring wear of an engine that is running well below the optimum temperature may be as much as five or six times the normal rate. And blow-by of the acids can lead to rapid deterioration of the lubricating oil.
Operating an engine under heavy load before it is warmed up may damage the working parts because of sluggish circulation of the oil. A low-viscosity oil will circulate more freely when cold than a high-viscosity oil: the use of a highly detergent oil improves lubrication at low temperatures. It is of allied interest that excessive choking of a petrol engine can cause a very high rate of wear and of lubricant contamination.