Hi folks, this topic of the pressure on the piston has always been an
interesting one, since that's what makes the car or bike go fast! Let
me throw my two cents in.
My understanding has always been that useful work on the piston is
impossible until about 15 degrees after top center on the power stroke,
simply because the conrod is almost straight up and down with the
crankshaft until then and there's no leverage. Since the spark may
occur at say 38 degrees before top center, there may be a very high
pressure before and after top center, on the piston, but it does no good
for torque production at the crank, it's just a necessary preliminary to
what happens next. Spark timing is advanced to get the most favorable
pressure on the piston when it can do the most good in the rpm range
that is most beneficial.
>From 15 to 90 or 100 degrees after top center you have all the torque
production created by the combustion("expansion") atop the piston,
pushing down hard on the crankshaft. Following this the conrod starts
to lose its leverage on the crank, and, very soon the exhaust valve must
start to open; as soon as it does the pressure starts to blow out the
exhaust port rather than continuing to push the piston down. And it
must or it will try to hold the piston back on the exhaust stroke.
Further, my understanding of why blown or nitrous engines make more
torque is that a larger amount of a given fuel and oxygen is ingested,
and assuming detonation does not arise, there is a much larger net push
on the piston during the same push-period (15 to 90/100 degrees after
top center on the firing stroke), hence more torque to the crank. The
peak pressure on the piston may not be greater with the blown or nitrous
engine than the unblown, but a higher pressure may be sustained longer,
giving a greater net torque. High-compression unblown engines may give
a very high peak pressure on the piston during the push-period, but the
pressure drops quicker, compared with the blown, nitrous, or
nitromethane engine. We are told that for a given unblown engine,
methanol only gives 10% more power than gasoline, not such a big deal
compared with the other approaches to power increase.
Now if detonation does arise through the tuner's mismanagement(!),
instead of a controlled pressure on the piston crown as described above
which does no harm to the engine, you have a violent explosion in the
combustion chamber, probably near the point where the exhaust valve is
getting ready to open(or the exhaust port opens on a two stroke). This
violent explosion creates extremely high pressures in the cylinder, and
often does break pistons, rods, cranks, bearing caps, or even blocks.
It can work the same way in either two stroke or four stroke engines.
What kind of pressures are we talking about? Authorities with which I'm
familiar show that with unblown engines running without detonation, the
cylinder pressure at full throttle may be three to four times what you
see on a compression test, and it might(but shouldn't according to the
above theory)go up from there when you get into the blown and nitrous,
and God knows what it does with nitromethane! I recall a "National
Dragster" article where "somebody" claimed that under detonation
conditions with top fuel dragsters, the cylinder pressure was 90,000
psi. Maybe he meant 9,000. We do know that "something" often makes
them run over their cranks!
Books have been filled with this subject, but these are the outlines as
I know them. Cheers Bill
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