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What is a Valve Train?

By Jessica Reed
Updated May 17, 2024
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A valve train consists of all the parts in an internal combustion engine which open and close the engine's valves. Without the valves, the engine cannot receive air and fuel necessary to power the car. The valves also allow the used air and fuel mixture to leave through the exhaust system after combustion. A valve train typically consists of the valves themselves and the camshaft used to turn them. Other parts vary depending on the specific design, but each performs a specific function in moving the valves.

When the engine needs to create power, it must do so by pressing an air and fuel mixture into a small space and igniting it with a spark. The cylinders of the engine perform this job inside an enclosed chamber. To allow the air and fuel into this chamber, a valve opens. Once the mixture is inside, the valve closes to create a tight seal. After the mixture ignites and releases energy to turn the car's wheels, a different valve opens to allow the exhaust to leave.

Three different common designs are used in making the valve train function, with the overhead camshaft being used most often on new vehicles. In this type of valve train, small lobes on the camshaft push against the valves as the camshaft rotates and this causes the valves to temporarily open. The camshaft is connected by gears or belts to the crankshaft, which allows it to turn in time with the engine. This ensures the valves open and close on time and prevents the engine from misfiring. Other versions of the overhead camshaft use extra parts instead of lobes to open and close the valves.

The second type of camshaft design is the cam-in-block. It operates in much the same way as the overhead camshaft and opens the valves either by pushing on them or activating extra parts to do the job. The difference is the cam-in-block design places the camshaft inside the engine instead of over it. Commonly, parts known as rocker arms and pushrods activate the valves. The third design uses a camless engine, which runs without the use of a camshaft and uses different methods, such as sensors, to activate the valves.

If the valve train isn't working properly, it can cause the engine to misfire. The explosion in the chambers may occur too early or too late. In either case, the engine doesn't produce as much power and parts may become damaged over time from the force of the explosion. When a driver notices the car engine producing lower power, misfiring, or making strange noises, he should have the engine checked out by a mechanic to avoid causing further costly damage.

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