Mechanics

Camshaft Variable Valve Timing

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This article explains how variable camshaft timing works and the roll a camshaft plays in the internal combustion engine.

Automobile manufacturers have developed a variable cam timing system that adjusts the camshaft to crankshaft timing. The engine computer plays a vital role in the engine's performance as it adjusts the camshaft timing depending on changes in the vehicle's engine speed and load. In any range of engine speeds only one camshaft position (in relationship to the crankshaft) is optimum for power and economy. Pressurized engine oil is controlled by the engine computer through an oil control valve which allows engine oil to flow to the camshaft timing actuator (or phaser), as the oil is forced into the actuator the camshaft timing advances, when the pressure is changed to the opposite port the actuator is returned to standard position.

Variable Cam Timing Actuator-Phaser
There are several types of camshaft arrangements; some common types are single overhead cam or SOHC, double overhead cam or DOHC or pushrod, rocker arm style of systems. In a single overhead camshaft type, the engine is equipped with only one shaft per cylinder head. The valve spring maintains pressure against the valve forcing it into the valve seat while sealing the intake or exhaust port. If the valve spring is weak or is broken the valve will lose pressure and will cause the engine to run poorly. Visit - Abnormal engine noises 

Variable Timing Camshaft
Variable Timing Camshaft

The advantage of a double overhead cam engine is that it can accommodate more than two valves per cylinder. More than two valve per cylinder allows the exhaust and intake gases to flow more freely because of the increased number of openings, inturn improving engine power and economy. A pushrod style of valvetrain configuration the camshaft is housed in the engine block, instead of the cylinder head. The camshaft utilizes lobe followers or lifters that connect to the rocker arms using a push rod. This system is not as efficient because of the increased weight of the system which increases valve springs load, slowing the engine down. Overhead camshaft engines are more efficient and can produce more power.

Camshaft

A Camshaft is commonly used to operate poppet valves in an internal combustion engine. A camshaft is situated in the cylinder block or cylinder head which has oblong lobes or cams which causes a tappet or lifter to open and close the intake and exhaust valves. This force is applied on the valve directly or through an intermediate mechanism such as a rocker arm, lifter, follower or tappet. Push rods are used to connect the camshaft to the rocker arm via the lifter. Each valve utilizes a spring which returns the valve to their original position (closed.)

Camshafts are designed according to the RPM and horsepower range desired. When the intake valve is opened, the piston travels downward pulling in an air-fuel charge into the cylinder. This intake charge is a mixture of air and fuel which is ready for combustion. The faster the engine is running the faster the air and fuel mixture moves into the cylinders. This parameter is known as the valve opening duration and is controlled by the cam lobe width profile.

Basic Maintenance

  • A camshaft is driven by the crankshaft via timing chain or belt. The timing belt or chain needs to be replaced per manufacturer's recommended intervals because they can wear out and fail without warning which will stall the engine.
  • Camshaft followers or lifters have the capability to automatically adjust themselves utilizing motor oil pressure. To maintain proper operation service the engine oil regularly. Visit - Engine oil change 

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-21)