Timing gear of an internal combustion engine: design, function, principle of operation

Modern car engines can be equipped with different types of timing mechanisms. But if any of them are damaged and can not be repaired, as well as if together with it damaged the main units of the car, the car should be scrapped: https://scrapmycarnear.me.

The timing mechanism is classified into four categories:

By the location of the camshaft – upper or lower location;

By the number of camshafts – one (SOHC – Single OverHead Camshaft) or two (DOHC – Double OverHead Camshaft);

According to the number of valves – 2, 3, 4, 5;

by camshaft drive – chain, pinion, and toothed-belt drive

The top position of the shaft in the cylinder head is the most common and efficient. The valves are opened and closed from the camshaft by means of the drive levers (tappets). This arrangement of the camshaft helps to simplify the overall design of the engine, reducing its mass, reducing the inertial forces.

Timing Mechanism Design

The timing mechanism consists of a camshaft, pushrods, valves, rocker arms, rods, and a drive.

The diagram is based on ©Volkswagen


The camshaft ensures the timing of the closing or opening of the timing valves in accordance with the engine cylinder sequence and the timing of the gases in the mechanism. The camshaft is made from high strength steel (with additional hardening) or cast iron. The shaft is equipped with bearing journals and cams. The shape of the cams influences the operating phases of the gas distribution, valve frequency, and valve timing.

A chain drive sprocket is mounted on the end of the camshaft. The shaft is mounted in a bearing housing that is attached to the cylinder head. To prevent axial movement of the camshaft, a thrust flange is used which is connected to the end of the bearing housing.


Tappets are components of the timing system whose primary purpose is to transmit forces from the cams of the camshaft to the rods. They are made from high tensile steel or cast iron.

There are three types of tappets – mushroom, roller and cylindrical. The movement of the tappets can be on guides in the cylinder block or in small housings attached to the cylinder block.

Valves are designed to provide fuel to the engine cylinders and exhaust.

The valve design consists of a stem and a flat head. The valve head has a flat beveled edge at a 45 degree angle. The diameter of the inlet valve head is much larger than that of the outlet valve, since the volume of gases discharged from the combustion chamber exceeds the volume of the FAM.

The timing valves are installed in the cylinder block head, and their connection point is also tapered and called a seat.

The intake valves are made of chrome plated steel and the exhaust valves are made of heat resistant steel. Heat-resistant cast iron is used for the valve seats.

The valve spindle is cylindrical in shape, with a special groove at the top for fixing the valve spring.

The movement of the valve spindles is carried out exclusively by guiding sleeves made of cast iron or steel. The guides themselves are connected to the cylinder head.

Each valve is equipped with an inner and an outer spring. The springs are mounted by means of washers, discs and bumpers.

The valves are opened by the actuator, which transmits the force from the camshaft to the valve.

Modern automotive engines, most commonly used in production vehicles, have two inlet valves and two outlet valves mounted on each cylinder.


Rods are used to transmit action from the pushrods to the rocker arms. These parts can be in the form of hollow cylindrical rods with steel tips.

The rods are made of a wear-resistant aluminum alloy and are connected to the rocker arm on one side and the tappet on the other.

Rocker arm

The rocker arm transmits force from the boom to the intake/exhaust valves. The rocker arm is a double-arm lever that sits on an axle. One arm (near the valve) is longer than the other (near the boom).

The rocker arms are made of solid steel and are mounted on special sleeves on an axle attached to the cylinder head. Between the rocker arm and the axle there is a sleeve designed to reduce the friction between them.

Camshaft Drive

The camshaft is driven from the crankshaft by an actuator that drives the camshaft in a rotary motion. The speed at which the camshaft rotates is half the speed of the crankshaft.

Thus, in two crankshaft rotational motions, the camshaft will make only one rotation, providing one inlet and exhaust valve opening per duty cycle.

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