MEASUREMENT OF DIESEL BLOW-BY GASES 2

[harrisj]

We have been asked by a client to add some instrumentation to an engine test rig to measure 'crankcase blow-by' in order to assess cylinder wear, ring condition etc.

The client has only asked for a measurement of pressure, but in a vented crankcase this is likely to be so near atmospheric as to be immeasurable. I believe that what is needed is a measure of volumetric flow, ie the rate at which gas leaks past the pistons.

I have no idea what is a reasonable figure for a good engine, or what you might expect from a seriously worn one. The manufacturers seem to think zero is a good figure - I'm sure it is, but what is acceptable?

The engine is a turbocharged intercooled 20 litre V-12 diesel producing 900kW @ 2300 rpm.

Any guidance? Or experience of this type of testing?

 

[ivymike]

The crankcase ventilation pressure will probably be measurable in the "inches of water" range (1-5). Blow-by gas flow (at STP) will probably be in the neighborhood of 0.7 to 1.5 ft^3/hr per horsepower. Zero is not a practically attainable value. Not all of the blow-by comes from the cylinder packs - some will also come from the turbo drain tube (1% to 40%, depending on number of turbos, their configuration, and their condition) - clarify whether you're expected to separate this! If you measure volume flow, it is critically important that you either (1) measure the blow-by flow at the end of a long tube such that the temp is close to room temp or (2) measure the temperature of the gas, so that mass flow can be back-calculated, and so that you can compare values directly.

 

[harrisj]

ivymike:

Many thanks for these useful numbers. I now have an approximate range to select an airflow meter. Point taken on temperature compensation - we can easily do this.

I still don't think that crankcase pressure tells us much unless the breather(s) are calibrated. A restrictive breather will give a substantial pressure but a large diameter hose will maintain the crankcase pretty much at atmospheric pressure.

I am not familiar with a turbo drain tube - does this normally vent significant quantities of compressed air into the crankcase? Why? Is this air which gets into the turbo lube system when turbo bearings are worn?

 

[sbmar]

Follow Ivymike's advice.....Crankcase pressure is a very significant test for measuring engine condition. As for the turbo oil drain, yes, this must be isolated if you want to see if it is cylinders or bad seals in the turbo. Your testing must be done at various loads all the way to WOT at rated HP if you want to get a real picture..

Many manufacturers, including Cummins, use crankcase pressure as the gage for TBO's.. A manometer is tycally the tool of choice..........

Tony



Tony Athens

http://www.sbmar.com

 

[dgallup]

When I did that 25 years ago we used a natural gas meter (like on the side of your house) to measure blowby on 600 to 1000 CID turbo diesels. Simple & cheap.