Brother Raccoons:
Over the years I've seen and heard lots of different misconceptions about
the way engines work. Ideas such as:
- The flow through an engine is practically continuous, especially at high
RPM.
- Building an exhaust system is all about reducing backpressure.
- Flow testing a head will always yield good results.
- Higher octane fuel will produce more power.
- Orienting the spark plug gap a particular direction will increase power.
- Etc.
all seem, at first blush, reasonable.
To be sure, any true gearhead understands the workings of the four-stroke
engine, at least on the macro scale. But it's when things turn to the micro
scale that the story becomes far more complex. And in those cases, common
sense and good mechanical skills aren't sufficient.
In the case of our current discussion on spark plug orientation, the thought
that the orientation will have an effect makes sense when one thinks, "Well,
the mixture comes in through the intake valve, and it's going to leave
through the exhaust valve. So, pointing the plug gap a particular direction
could improve on things." But what really happens inside of the cylinder
leads to another conclusion.
The time it takes, from the time the intake valve closes to the time the
piston reaches the top of the compression stroke, is extremely short to us.
At 5,000 rpm it's only about 3 milliseconds. But to the intake mixture,
it's a long time. And over that time, a lot happens. Consider the charge
within the cylinder just after the intake valve closes: The charge at the
cylinder head is constrained. But the charge at the piston crown is moving
along with the piston. So during compression, the piston is forcing some
charge, at pretty high velocity, into charge that is essentially stationary.
As the piston rises, the charge will possess the swirl that was imparted on
it by its flow into the cylinder. But at a certain point, the entire charge
changes complexion. It undergoes "transition to turbulence", and the entire
charge suddenly breaks out into a mass of small, energetic eddies. The
higher the piston speed, the more energetic the eddies are. The eddies
provide for very efficient mixing of the mixture. And they also rapidly
dissipate any of the mass flow--the swirl--that the charge had after the
intake stroke. When the piston approaches TDC and the spark is finally made
across the plug electrodes, those same eddies are what propagate the flame
through the mixture. At the spark plug's electric arc, a small bit of the
charge ignites. The eddies that are near the plug carry the flame around in
a small circular pattern at high speed, igniting fresh charge in the nearby
eddies, which continue the process all the way through the intake charge.
This process creates very high flame speeds, on the order of hundreds of
feet per second, and because the eddies increase in energy with RPM, so does
the flame speed. This is how an engine can run at a wide RPM range even
without any form of spark advance. So the idea that, by TDC, the charge
will still possess some influence imparted on it by the intake process, and
that there is some degree of non-homogeneity in the compressed charge isn't
accurate. There is a whole world in there that we can't appreciate at our
timescale. And it makes us come up with incorrect conclusions.
Some time ago I wrote a short white paper for a prospective customer. It
shows some of the results that I've generated with my engine simulation and
reveals more of this hidden life of engines. It will help you to gain a
better understanding of what goes on at this micro scale. If you're
interested, go to http://radicalnovelties.spaces.live.com. Look under the
"Engine Simulation" heading. Those of you who own a GTV6 may recognize the
dimensions of the test engine in the paper.
Rich Wagner
Montrose, CO, USA
'82 GTV6
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