"Steve B" ) writes:
"Oren" wrote
.



The spark ignition happens at the top of the compression stroke TDC
(valves closed). When fuel/air is compressed and fired piston drops
to BDC ready for the exhaust stroke (valves open). No?


No. Spark happens, piston travels to bottom, on the upstroke, the exhaust
valve opens, piston comes up, pushes out spent gas, at TDC, intake valve
opens, piston drops pulling in air, comes up on compression stroke, and a
few degrees BTDC, fires again. One spark per two full revolutions of crank.


Simple small four stroke engines (lawnmowers etc.) spark every crank
revolution. The spark is usually triggered by the crankshaft on small
four stroke engines. Larger four stroke engines (automobiles) spark
every second crank revolution per cylinder. The spark is usually triggered
by the camshaft on large four stroke engines. The camshaft rotates
at half the speed of the crankshaft.



Steve