Timing mechanism (another name – gas distribution system, short name – timing) is designed to ensure the timely flow of air or fuel-air mixture (depending on the type of engine) into the engine cylinders and release of exhaust gases from the cylinders. If it is not only this mechanism that has failed, but also other major parts of your car, then go to scrapyard toronto.
Widespread four-stroke piston internal combustion engines use valve timing mechanisms, so the device of the timing mechanism is considered exactly on its example.
The timing mechanism has the following general structure:
Valves directly supply air (fuel-air mixture) into the cylinders and exhaust gases. The valve consists of a pallet and a stem. On modern engines, the valves are located in the cylinder head, and the place where the valve comes into contact with it is called the seat.
A distinction is made between intake and exhaust valves. For better filling of the cylinders, the diameter of the inlet valve disc is larger than that of the outlet valve.
The valve is held closed by a spring and opens when the stem is pressed. The spring is secured to the stem by the spring plate and the bumpers. The valve springs have a certain stiffness to ensure that the valve closes during operation. To prevent resonant vibrations, two springs of lower stiffness can be mounted on the valves with opposite coils.
Valves are made of alloys. The valve head is reinforced. The intake valve stem is usually full bore and the exhaust valve stem is hollow, with a sodium fill for better cooling.
Most modern engines have two intake valves and two exhaust valves per cylinder. In addition to this timing scheme, three-valve (two intake, one exhaust), five-valve (three intake, two exhaust) schemes are used. The use of more valves is limited by the size of the combustion chamber and the complexity of the actuator.
The opening of the valve is accomplished by an actuator, which provides force transmission from the camshaft to the valve. Two basic valve actuation schemes are currently in use:
Roller levers are preferred as the actuator because they have lower friction losses and less mass. The roller lever (other names are rocker, rocker, from the English “rocker”) rests one side on the valve stem and the other on the hydro-compensator (in some designs on the ball bearing). To reduce friction losses, the mating point between the rocker arm and the camshaft cam is designed as a roller.
With the help of hydro-compensators in the valve train, a zero thermal clearance in all positions is realized, less noise and softer operation is ensured. The expansion joint consists of a cylinder, piston with spring, check valve and oil channels. The hydraulic compensator located directly on the valve tappet is called a hydraulic pusher.
The camshaft ensures the operation of the timing mechanism in accordance with the cylinder order and timing of the engine. It consists of a camshaft with arranged cams. The shape of the cams determines the valve timing, namely the valve opening-closing moments and the valve timing. A significant increase in the efficiency of the timing system, and thus an improvement in engine performance gives the use of different cam timing systems.
On modern engines the camshaft is located in the cylinder head, and there are two such schemes:
Single shaft – SOHC (Single OverHead Camshaft);
DOHC (Duble OverHead Camshaft).
Due to the use of four valves per cylinder, a dual-shaft timing scheme is preferred (one camshaft drives the intake valves, the other camshaft drives the exhaust valves).
The camshaft is driven from the crankshaft by an actuator that rotates it twice as slowly as the crankshaft (in one engine cycle a particular valve opens only once). The following types of gears are used to drive the camshaft:
timing belt; chain; and toothed.
Belt and chain gears drive a camshaft located in the cylinder head. A toothed gear typically rotates the camshaft in the cylinder block. The colloquial name for a timing gear is “guitar” (based on the shape of two connected gears).
Belt and chain gears have both advantages and disadvantages, so they are used equally in the timing system. The chain drive is more reliable, but the chain is heavier than the belt, so it requires additional devices for tensioning and damping of vibrations.
The belt drive does not require lubrication, so it is installed openly on the pulleys. At the same time, the belt compared to the chain has a limited life. Toothed belts are widely used as a belt drive for the camshaft.