A leak-down tester is a measuring instrument

Measuring instrument

In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...

 used to determine the condition of internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

s by introducing compressed air

Compressed air

Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....

 into the cylinder and measuring the rate at which it leaks out.

Compression testing is a crude form of leak-down testing which also includes effects due to compression ratio

Compression ratio

The 'compression ratio' of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity...

, starter/battery condition and other factors. Leak-down testing confines the results to cylinder leakage alone.

Testing is done on an engine which is not running, and normally with the tested cylinder at top dead center, although testing can be done at other points in the compression and power stroke.