To: tooze@vinny.cecer.army.mil (Marcus Tooze)
Subject: advance curves
>>>But WHY does a lot of advance work at higher rpms (it must be something
to do with when the *bang* happens in the cylinder).
Exactly. Let's hope that you have a mild bent for math and a spread sheet on
your computer.
I had a sense of the answer for your question from a long time ago, but I
wanted to see if I could quantify it. It turns out to be quite simple.
Everyone on this list should be familiar wihth the four stroke cycle. The
piston goes down, drawing the air and fuel mixture through the carburetor,
valves close, the piston goes up compressing the mixture, the spark ignites
it and the pisto .. wait! stop! freeze frame! rewind! Instant replay in
slo-mo!
The fuel/air mixture is drawn in. The valves close and the piston goes up.
Somewhere on the way up the spark plug fires. A flame front moves from the
spark plug, across the combustion chamber towards the dead end space. This
moving flame front requires a discrete amount of time. No matter how fast
the engine is spinning, the compression at the top will be about the same and
the fire will cross the combustion chamber just so fast.
Your goal is to have the expanding, burning mixture start to push the piston
down when it is very close to the top of its stroke. It is obviously bad if
the gasses want to push the upcoming piston back down. At the same time it
just about as bad if the p
ush doesn't come till the piston is half way back down.
My next step was to consult the Haynes manual for the old anachronism, the
Land Rover. (Vanity Plate: LOTECH)
And I quote (put this stuff into a spread sheet):
Column 1:Crankshaft angle <tab> Column 2:Engine RPM <tab> Column 3: C2/60 =
revolutions per sec <tab> Column 4: 1/C3 = seconds per revolution <tab>
Column 5: Average of crankshaft angles from C1 (eg 38 to 42 = 40) <tab>
Column 6: (C5/360) = crankshaft angle as a decimal fraction of 360 degrees
<tab> Column 7: 1000 x C3 x C6 = thousandths of a second before TDC.
DATA
Column 1 <tab> Column 2
38 to 42 <tab> 4500
30 to 34 <tab> 3500
22 to 26 <tab> 2500
12 to 16 <tab> 1200
4 to 12 <tab> 900
0 to 4 <tab> 600
no advance below <tab> 450
Plot Column 1 v. Column 6
Lo! and behold! The numbers form a nice curve at an average of about 0.0016
seconds. It appears that the flame front takes a bit more than one and a
half thousandths of a second to cross the cylinder. In order to get the fire
get burning well by the time the piston peaks, you've got to fire it early.
Well, at least you do in a Land Rover. Maybe with high tech . . .
If you set the fire much too soon, the fuel mixture is burning strongly as
the piston is still on the way up. The radiated heat from this fire heats
the unburnt gasses ahead of it and pretty soon they explode, rather than burn
smoothly. And that is called PING! If the compression ratio is high, that
compression heats the gasses further, helping the end gasses to explode.
Some fuels, like iso octane, resist this tendancy to explode better than
others. A comparison of a given fuel to iso octane provides its octane
rating.
The history of gasoline engines is the history of trying to run higher
compression ratios, at higher RPM, at greater spark advances without PING!
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Malcolm R Forbes Merrimack NH
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