How to measure engine blow-by ?

What is engine blow-by?

A combustion engine is burning fuel in a cylinder in which a pistion is moved up and down. The piston is sealed towards the cylinder by piston rings. An certain amount of exhaust gas is escaping from the combustion chamber to the crankcase housing. This is the so-called blow-by. At you can find a nice drawing of it.

Why and how testing an engine for blow-by

During a used machine inspection it makes sense to check the engine for the amount of blow-by. With checking or a measurement of the amount of escaping gas an experienced inspector can estimate the engine wear. In most cases it is sufficient checking with a hand on top of the oil refiller tube for the amount of blow. At the same time the inspector would check the smell of the gas for burned oil or smell of coolant in the engine lubrication system.

On larger engines it might be reasonable to measure the blow-by and compare it with factory specifications of engine manufacturer. Some of our inspectors are equipped with the gauge, a so-called anemometer.

Engine blow-by measurement

However, the inspector should know about engine configuration. Not all gas is escaping through the refiller tube if the measures are not correct. Some engines have a certain amount of blow-by even when new. Mevas inspectors who checked Perkins engines and stood too close can tell a story about it. Talk to an expert before blow-by is evaluated.

What can the exhaust fume tell us about engine condition?

Technicians with some experience would smell if an engine is burning oil. The color and the amount of smoke is telling a story as well. Engine smoke can appear white, grey, grey-blue or black. If the appearance of the smoke is not normal an inspector would check the engines systems carefully for problems such as injectors not working well or coolant draining into the engine greasing.

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