Rob:

I agree with all of your points but one.

**. As rpm and/or combustion duration approach zero the ideal theta PP approaches TDC. As theta PP drifts away from TDC the engine becomes less efficient.**

Unless I'm missing something about your point, I don't think that's quite correct. The optimum thetaPP for maximum mechanical advantage is always about 15-16dATDC. By definition and measurement, approximately half of the charge is burned at the peak pressure. If I've misunderstood, please try again!

Thanks. And, Merry Christmas and Happy Holidays to you all.

Walter


On Dec 24, 2005, at 1:30 AM, REHBINC@aol.com wrote:

Skip/Walter,
 
 

**Can you explain why spark advance is a good idea in auto engines and
a bad one in airplanes?**

It's not necessarily.   It all depends on the power and fuel being used.

You can actually gain power in an aircraft engine at takeoff power by
RETARDING the timing from the historically chosen timing.  That has
several advantages.

OTOH, you can gain power in cruise in most situations (NA) by advancing
the timing during that power demand.

If the thetaPP is 10 like it is at takeoff, retarding the timing will
make more HP.  If it is 25, like it can be in cruise, advancing the
timing will increase output.   IOW, if the thetaPP is 16 and you
advance OR retard the timing, the power output goes down.

The objective is to have a thetaPP of 16dATDC in all conditions.  That
results in the most mechanical advantage for the fuel charge being
burned.  It improves the BSFC.

Water-cooled, iron-head engines have a wider detonation margin and
withstand detonation a lot better than our aircraft engines, so we need
to pay closer attention to these issues.I supect Skip really wants to know why your car operates with a more or less continuously variable spark advance verses a static advance in an airplane. The principle reason is that rpm and cylinder filling are constantly changing in your car, while these values are almost constant in a typical aircraft engine.

 
Engine rpm is important because as the engine turns faster, the crank swings more degrees during the combustion process. This has two effects on the combustion process. First, for a given ignition point the peak pressure occurs occurs later in the power stroke (theta PP). Second, the shorter time at peak chamber pressure reduces the time for the fuel mixture to pyrolize (decompose into its combustible components). With less pyroysis, the detonation margin is increased. This is one reason why your old car may nock a bit at 45 mph steady state and not at 65 mph.
 
Cylinder filling is also important for two reasons and they work opposite one another. As you open the throttles, manifold pressure increases and the engine is fed denser air fuel mixture. This results in a higher cranking pressure. As the pressure increases, the flame speed decreases, requiring more spark advance to develop best power. (counter intuitive)
 
The other thing that happens with increasing cranking pressure, and the temperature that comes with it, is that we increase pyrolization and thus get closer to the detonation limit.
 
In an airplane with a constant prop, the engine barely changes speed the entire time the wheels are off the ground. With a fixed prop, the engine speed still changes much less than in a car. Thus the engine speed effects can be ignored without much loss.
 
In a typical airplane the most critical engine operation is at takeoff and the critical limitation is detonation. If the timing is adequately retarded to prevent detonation (or at least keep it under control) at full power take off, then it should also be safe when cylinder filling is reduced to 75% power. The reduced cranking pressure also slightly reduces the spark advance for best power/detonation limit because the flame speed is now faster (not a huge increase). Whether the increase in flame speed makes up for the increased detonation margin depends on the engine characteristics, particularly how it heats up.
 
Keep in mind also that the theta PP for best power is a function is of the combustion duration and of engine speed. As rpm and/or combustion duration approach zero the ideal theta PP approaches TDC. As theta PP drifts away from TDC the engine becomes less efficient. This is why open combustion chambers are so inefficient, the flame has to travel a greater distance and increases the combustion duration.
 
Ya'll have a merry Christmas,
 
Rob