Valve timing and ignition

The picture of four neat 180° strokes with valves that snap open and shut exactly at TDC and BDC is a simplification. Real engines open and close the valves a few tens of degrees early or late, because air and burnt gas have inertia: once they are moving they keep moving, and a valve that waited for the piston would waste that flow.

The list below uses typical, nominal values for an ordinary petrol engine. Real engines vary a lot, and sporty ones use more extreme timing.

• Intake valve opens (IVO): about 10° before TDC
• Intake valve closes (IVC): about 40° after BDC
• Exhaust valve opens (EVO): about 40° before BDC
• Exhaust valve closes (EVC): about 10° after TDC

Two things follow. First, each valve stays open for about 230° rather than 180°. Second, around TDC at the end of exhaust and the start of intake both valves are open at once, for about 20° in this example. That is called valve overlap. At high speed the fast-moving exhaust gas can help pull fresh air in behind it. At idle, overlap lets some exhaust back into the intake, which is why a very "lumpy" idle goes with a lot of overlap.

The intake valve closing after BDC looks odd, since the piston is already rising. But at moderate and high speed the incoming air is moving so fast that it keeps filling the cylinder for a while after the piston turns round. The exhaust valve opens before BDC because the gas is still at high pressure at the end of the power stroke; letting it blow down early means the rising piston has less pressure to push against.

Modern engines often use variable valve timing, which shifts these angles with speed and load. A fixed timing is always a compromise between low-speed smoothness and high-speed power.

Now the spark. A common guess is that it fires exactly at TDC. It does not: it fires before TDC, by an angle called the ignition advance, usually somewhere between about 10° and 40° BTDC depending on speed and load.

The reason is that burning is not instant. After the spark, a flame has to spread through the mixture, which takes a couple of milliseconds. The best result comes when the pressure peaks a little after TDC, typically in the region of 10° to 15° ATDC, so the push on the piston starts just as it begins to descend. If the spark were at TDC, the peak would come when the piston is already well down, and the work would be wasted.

Because the delay is a time, not an angle, faster engines need more advance. At 6 000 rpm the crankshaft turns 36 000° per second, which is 36° per millisecond; a 1 ms delay is 36° of crank. At 1 500 rpm the same millisecond is only 9°.