Ask the Mechanic

Steve Thompson, Contributor

READER: I've been around engines and liked to race cars in high school and even built a few engines. I've often heard the terms blowby and blowdown that are related to an engine. I don't know if these are terms to help us know when an engine is going bad or if these engine terms mean something that is supposed to happen in an engine. Can you explain these words in a way that they are easy to understand?

STEVE: Both of these terms are things that are going on in an internal combustion engine while it's running. But, blowby and blowdown are completely different things happening as the engine runs.

Blowby is compression that escapes by the rings when the explosion occurs during the power stroke, and the combustion is forcing the piston down to move the crankshaft. Some blowby is normal; it's burned gas that is mainly free of oxygen. But, when blowby gets excessive, it's time for an engine overhaul. In older engines, blowby was vented to the atmosphere; but in newer engines, it's recirculated back through the engine through the EGR (exhaust gas recirculation) valve. If you have ever had to replace this valve, you paid enough to know its purpose. Research has found that the cooler the burn in the engine, the less pollution it produces. This burned gas now goes in the engine again through the EGR valve, it's not vented into the atmosphere. The blowby routed back through the engine by the EGR only takes up space for compression, does not burn again and create heat, and is not vented to the atmosphere.

Blowdown is a completely different animal. It's burning fuel that is "blown" out the exhaust valve at the end of the power stroke when the burning is still taking place. When there is still pressure in the cylinder from the burn, the exhaust valve opens at a precise time, allowing the pressure from the explosion to be "blown" out the exhaust valve. This premature opening of the exhaust valve is necessary to help all the burned fuel exit the cylinder just before the piston comes up on the exhaust stroke. Just as the piston nears the end of its exhaust stroke, the intake valve opens, allowing a fresh, cool fuel mixture (or air on a diesel) to enter the cylinder. Actually, both the exhaust valve and the intake valve are open briefly at the same time. If an engine is running without an exhaust manifold on it, you will see the blowdown exit in the way of a flame. Yes, blowdown is wasted power, but it is necessary to "scavenge" the engine properly.

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-- Email Steve Thompson at mechanic@progressivefarmer.com, and be sure to include your contact information and phone number.

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