Tuesday, January 19, 2010

How Variable Cam Timing Works (VCT)

Car manufacturers have developed a variable cam timing system that adjusts  the cam to crankshaft timing depending on change in the engine speed and load.  The engine computer plays a vital role in the engine's performance as it adjusts  the camshaft timing depending on 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 control  by the engine computer through an oil control valve which allows engine oil to  flow to the cam timing actuator (or phaser), as the oil is forced into the actuator the  camshaft timing advances, when the pressure is released a return spring supplies  force to return the actuator to standard position.

Variable Timing Camshaft

There are many types of camshaft arrangements; some common types are single overhead  cam or SOHC, double overhead cam or DOHC and pushrod, rocker arm style of systems.  In a single overhead cam type, the engine is equipped with only one cam per cylinder head. Therefore, there is one cam in a four cylinder engine, also with straight  six cylinder engine. In v6 or v8 engine configurations there will be one camshaft  for each cylinder head, two camshafts and double the camshafts for DOHC engines.  The valve spring maintains pressure against the valve forcing the valve to contact  the valve seat, 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.

Variable Cam Timing Phaser Operation 
The specialty of a double overhead cam engine is that it has two cams per head.  This means that the inline engines are equipped with two cams, four cams in V style  engines. The use of double overhead cams is more common in engines which have more  than two valves per cylinder. This is because a single camshaft doesn't have the  ability to include sufficient cam lobes that can accommodate all the valves present  in a three or four valve per cylinder engine. Double overhead cams give the advantage  of adding more exhaust and intake valves. More valves allow the exhaust and intake  gases to flow freely because of the increase in the number of openings, this in  turn improves engine power and economy.

Four Valves Per Cylinder Configuration

A pushrod style of engine has a different valve opening 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 through a push rod.

Typical In Block Camshaft Configuration

This system is not as efficient because of the increase in weight of the system.  In turn, increases the valve springs load, slowing the engine down. Overhead camshaft  engines are more efficient and can produce more power.
A Camshaft is commonly used to operate poppet valves in a piston engine. A cylindrical  rod is situated in the cylinder block or cylinder head which has oblong lobes or  cams which push a tappet or lifter to raise and lower the intake and exhaust valves.  This force is applied on the valve directly or through an intermediate mechanism  such as a rocker arm, lifter (cam follower) and push rods are used to press against  the valve for movement. Each valve utilizes a spring which will return them to their  original position (closed) after the force is removed.

Cams are designed according to the RPM and horsepower range desired. When the  intake valve is pushed open, the piston travels down pulling an air/fuel charge  into the cylinder. This intake charge is a mixture of air and fuel and is ready  for combustion. The faster the engine is running the faster the air and fuel mixture  moves into the engine which also means that the intake and exhaust valves open and  close quicker. This parameter is known as the valve duration and is controlled by  the cam lobe width profile.

Basic Maintenance
A camshaft is driven by the crankshaft of the engine by a timing chain or timing  belt. The timing belt or chain needs to be replaced per manufacturer's specifications  because they wear out and fail without warning and stall the engine. Because a timing  chain configuration is more durable when compared to the timing belt style a timing  belt will need to be replaced more often, comparatively. Timing belts are more common  in overhead cam engines and are more easily serviced.
Cam followers or lifters have the capability to automatically adjust themselves,  removing any access clearance in the system. Maintain manufacturer recommended engine oil service.


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