Piston Engines – Compression Ratio

Why are we interested in the compression ratio of a piston engine? Density.

The power produced by a piston engine is directly proportional to the air/fuel mixture that is burnt on the power stroke. The mixture is achieved BY WEIGHT. Therefore, if the density of the air is high it follows that more fuel can be mixed with it and therefore more power is produced.

Are there any problems associated with high compression ratio?

There are – if you manage to increase the compression to the point where the construction of the engine cannot cope you will have destroyed your own engine. This is not considered an intelligent thing to do!

The more the compression ratio the more stain on the working parts, and although you may not have reached the destruction point there will be a maintenance penalty with worn out bearings requiring replacement sooner than would normally have been the case.

How can we calculate Compression Ratio?

There is a formula, but first some terminology.

Where the diagram shows BDC, all that area from the top of the cylinder to the top of the piston is call the Total Volume.

The area shown at TDC is the Clearance Volume.

The Swept Volume is the difference between the two as shown.

The formula for calculating Compression Ratio is:

Compression Ratio = Total Volume

Clearance Volume

As you can see, given 2 of the 3 volumes you can calculate the volume that you need to complete the formula.

Example 1:

Swept Volume of a cylinder is 1500 Cubic Centimetres (CCs)

Clearance Volume is 500 CCs

What is the Compression Ratio?

Formula   =   Total               =   1500 + 500             =   2000    =   4:1

Clearance                 500                         500

Example 2:

Total Volume 3000 CCs

Swept Volume 2500 CCs

Total – Swept = Clearance

300 – 2500 = 500

3000 = 6:1

500

Your Instructor will (no doubt) make you practise this until it “clicks”.

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