Worth noting. There are actually two sparks, per fire. One spark is at
TDC when the gasses are compressed. The second spark is also at TDC at
the end of the exhaust stroke. The second spark is wasted, but does no
damage.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"Steve B" wrote in message
...

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.

Steve