Each piston makes four strokes:
- Intake
Air is drawn into the cylinder through the open inlet valve. Fuel is sprayed into the engine by the fuel injection nozzle.
- Compression
The mixture of air and fuel is compressed in the cylinder in order to heat the mixture to the temperature of combustion.
- Power
The mixture of air and fuel ignites at the top of the compression stroke. The expansion of gases from the combustion forces the piston downward. This force creates the power of the engine.
- Exhaust
The piston moves upward in order to force the gases of combustion from the cylinder through the open exhaust valve.
The sequence of the strokes by all of the pistons in all of the engine cylinders provides constant air flow from the air inlet system during the engine operation.
The exhaust stroke and the timing of the valve mechanism pushes combustion gases out of the open exhaust valve through the exhaust manifold. The exhaust gases flow from the exhaust manifold to the outside through a muffler.
The valve mechanism cover contains an open breather assembly. This device vents the gases, which are caused by blowby, from the valve mechanism cover to the outside air. The breather reduces emissions without reducing the power of the engine.
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| Illustration 1 | g00468536 |
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Cross section of the inlet valve and the exhaust valve in the cylinder head | |
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| Illustration 2 | g00458090 |
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Cylinder head and valves (typical example) (1) Collet (2) Valve spring retainer (3) Valve spring (4) Valve guide seal (5) Recess for valve spring (6) Valve guide (7) Exhaust valve (8) Inlet valve (9) Valve face angle (10) Head of exhaust valve (11) Head of inlet valve (12) Cap (13) Valve guide seal (14) Cylinder head (15) Cylinder head gasket (16) Pushrod (17) Valve seat insert (18) Valve lifter | |
The valves and the valve mechanism control the flow of air into the cylinders during engine operation. The valves and the valve mechanism control the flow of exhaust out of the cylinders during engine operation. The cylinder head assembly has two valves for each cylinder. Each valve has one valve spring (3). The ports for inlet valve (8) and exhaust valve (7) are on the left side of the cylinder. Steel valve seat inserts (17) are installed in cylinder head (14) for both inlet valve (8) and exhaust valve (7). The seat face angle is equal to valve face angle (9). The valve seat inserts can be replaced.
The valve moves along a steel valve guide (6). The valve guides can be replaced.
The inlet valve and the exhaust valve are opened and closed by the rotation and movement of the following components:
- Crankshaft
- Idler gear
- Camshaft
- Valve lifters
- Pushrods
- Rocker arms
- Valve springs
The camshaft gear is driven by the idler gear. The camshaft gear, the idler gear and the crankshaft gear are timed together. When the camshaft turns, the valve lifters are moved up and down. The pushrods move the rocker arms. The rocker arms make the inlet valves and the exhaust valves open and close. This is in sequence with the firing order of the engine. The valve springs push the valves back to the closed position.