Ported engines Two-Stroke engine showing ports in the cylinder walls. Variable valve timing can give both maximum power at high rpm and smooth idling at low rpm by making small changes to the relative angular position of the camshafts and thereby varying the valve overlap. Road car engines are different because they are required to idle at less than 1000rpm, and excessive valve overlap would make smooth idling impossible because of the mixing of fresh and exhaust gases. Variable valve timing Įngines that always run at a relatively high speed, such as race car engines, will have considerable overlap in their valve timings for maximum volumetric efficiency. Primary cooling is accomplished by dissipating heat to the valve seats. Valve overlap is a secondary means to cool exhaust valves with intake air during valve overlap. The amount in crankshaft by which the valves close after top dead centre or bottom dead centre is reached is known as valve lag. The amount in crankshaft degrees by which the valves open before top dead centre or bottom dead centre is reached is known as valve lead. In the diagram, the valve overlap periods are indicated by the overlap of the red and blue arcs.Įither valve opens before the piston head reaches top dead centre or bottom dead centre. Engine designers aim to close the exhaust valve just as the fresh charge from the intake valve reaches it, to prevent either loss of fresh charge or unscavenged exhaust gas. The intake valve is opened before the exhaust gases have completely left the cylinder, and their considerable velocity assists in drawing in the fresh charge. With traditional fixed valve timing, an engine will have a period of 'valve overlap' at the end of the exhaust stroke, when both the intake and exhaust valves are open. Valve overlap Valve timing diagram for a 4-stroke engine However, this variation is normally unwanted. It is also affected by the adjustment of the valve mechanism, and particularly by the tappet clearance. It can be varied by modifying the camshaft, or it can be varied during engine operation by variable valve timing. In four-stroke cycle engines and some two-stroke cycle engines, the valve timing is controlled by the camshaft. In an internal combustion engine those are usually poppet valves and in a steam engine they are usually slide valves or piston valves. In a piston engine, the valve timing is the precise timing of the opening and closing of the valves. Please help improve this article by introducing citations to additional sources. Relevant discussion may be found on the talk page. Keep in mind that this timing diagram is only used to explain the principles of operation and the general timing of these events in relationship to the position of the crankshaft.This article relies largely or entirely on a single source. Although each point on the timing diagram is just an example, all four-stroke events generally occur somewhat near the points shown. The exhaust stroke continues to about 10 degrees BTDC at which time the cycle starts all over again.

The power stroke goes to about 12 degrees BBDC. The compression stroke then continues to the ignition point, about 15 degrees BTDC. To begin, the intake valves open about 10 degrees BTDC. During the four cycles of operation, the crankshaft revolves two complete revolutions or 720 degrees of rotation. Two circles are shown to represent two complete revolutions of the crankshaft. The mark at the bottom of the graph would illustrate the position of the piston at BDC. Any event that happens after top dead center is called ATDC or after top dead center. Any event that happens before TDC is referred to as BTDC or before top dead center. For example, at the top of the diagram, the piston would be located exactly at TDC (Top Dead Center). One way to look at this diagram is to think of these events in terms of the position of the crankshaft and 360 degrees rotation. The diagram is set on a vertical and horizontal axis. Describing the Principle: A timing diagram is a method used to identify the time at which all of the four stroke events occur on a typical four-stroke engine.

Valve and ignition timing diagram