Engine Camshafts & Components
A camshaft is a vital component in an engine, and it controls the positioning of the poppet valves in the engine. It is vital that these valves are opened and closed at the right time to produce maximum efficiency and power out of the engine. The camshaft is basically a long metal rod that has lobes positioned on it. As the camshaft starts to spin, these irregular shaped lobes will spin which is what opens and closes the valves. Each valve on the engine will have its own lobe on the camshaft which allows valves to be opened and closed at different times.
The positioning of the camshaft in an engine is also crucial when higher amounts of power have to be produced. Either the camshaft lobes or a system of pushrods and rockers will control the valves depending on the position of the camshaft. Direct operation between the lobes and valves is preferred because it is simpler and eliminates many common failures in this part of the engine. Having this direct operation between the lobes and valves requires that the camshaft be positioned at the top of the cylinders which is a common position in many modern vehicles produced these days.
Timing between the camshaft and crankshaft is extremely critical in an efficient engine. To keep the timing between the two units at a proper amount, a timing belt or chain is used to connect them. This ensures that the valves which control the gas and air into the engine won’t open or close at the wrong time during the piston stroke. In a two stroke engine that uses a camshaft, each valve is opened once for each rotation of the crankshaft. In a four stroke engine, the valves open half as much which means that two full rotations of the crankshaft occur for each rotation of the camshaft. Timing of the camshaft can be used to change the torque ratio at different RPM’s which is needed when balancing a car for max efficiency or when making an engine that needs maximum mower at certain RPM’s.
The camshaft lift is used to categorize camshafts by how far they can lift the valve off its seat. This is extremely important because a valve that is lifted higher will allow for higher amounts of fuel and air to flow which is generally more beneficial. The lift of a valve is limited by things such as the increased proximity of the valve head to the piston crown, and lobe clearance in the cylinder head construction. Camshafts that have too high a resultant valve lift at high RPM’s can because a condition called valve bounce. This is when the valve spring tension is insufficient to keep the valve following the cam at its apex. This condition is typical in motors that have been over revved and on motors that are ran on a higher speed than what they are designed for.
Duration of the crankshaft is determined by the number of crankshaft degrees of engine rotation during which the valve is off the seat. Higher duration generally lead to higher performance because of the increased amount of fuel and air allowed to flow. The lift of the camshaft and duration are often confused and mixed up because the both deal with the lifting of a valve off its seat. The performance of the engine is increased with higher durations. For example, a camshaft rated at 0.006 will produce more power than one rated at 0.002. An effect of increased duration is increased overlap. This is the number of crankshaft degrees during which both the intake and exhaust valves are off their seats. This overlap is easily seen and heard when the engine is at a low operation speed.
Camshafts are often made out of many different metals depending on the application of the vehicle. One of the most common materials used to make a camshaft is chilled iron castings because of its easy strength and ability to be mass produced for an assembly line. This type of camshaft gets its name because the lobes are gently chilled which makes them harder and more resistant to wear. Other materials are usually added into a chilled iron camshaft to add more strength and durability. In high performance engines that are produced in low volume, a billet steel camshaft may be used. This type of material is far superior to the other materials but it is time consuming to make and fairly expensive.
Most modern passenger vehicles made these days rely on an engine with a single camshaft per cylinder bank, which is known as a single overhead camshaft. The dual overhead cam engine design which is more common in larger vehicles and high performance vehicles will feature two camshafts per cylinder bank, one of which is for the intake valves and the other is for the exhaust valves. Having an overhead camshaft design adds more valvetrain components which are more complex and add increased manufacturing costs. The advantages of an overhead camshaft such as the ability to freely place valves and higher RPM capabilities outweigh the disadvantages which is why it is so widely used.
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