Name the factors an engine designer considers in the selection of the compression ratio for a compression ignition engine. Give some examples of compression ratio values.

The ratio between the total surface area of the cylinder space and the volume of the space is such that as the cylinder dimensions increase the ratio between the values decrease.
In a small engine this means more heat is lost to the cylinder space surface during compression than in a larger engine. For this reason smaller engines require a higher compression ratio than larger engines.
An engine started in low ambient temperatures without preheating requires a higher compression ratio than an engine started in higher ambient temperatures.
The factors considered by the designer are therefore the cylinder dimensions and the ambient starting temperature of the engine’s operating environment. Common values of diesel engine compression ratios are as follows.
Slow-speed two-stroke cycle engine used for ship propulsion 12:1. Medium-speed turbocharged four-stroke cycle engine used for propulsion purposes 12:1.
Emergency electrical generator set 14:1 to 16:1.
Small, high-speed, naturally aspirated four-stroke cycle automotive engine fitted with glow plugs up to 23:1.
Note Large engines are usually preheated by raising the temperature of the cooling water. This aids starting and reduces cylinder liner wear. The lubricating oil is also preheated to reduce its viscosity and to assist starting by reducing the friction in bearings.
For cylinders with identical proportions, the total area of the cylinder surfaces varies as the square of the linear dimensions, and the volumes vary as the cube of the linear dimensions .

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