"Sweet Spot"

[Greg Hermann]

>-> Right though, inaccurate thermodynamics. The gas loses temperature as
>-> it expands, as well as losing pressure, so the drop in pressure is
>-> considerably more precipitous than what you describe, Bob!
>
> The lower combustion limit of several of the end-gas reactions is
>around 700F.

Depending on inlet air temp, and effective compression ration, the peak
temp at TDC, again assuming no water jacket losses, is in the 6000 degrees
R (absolute) neighborhood

Didn't give this one before, but:

(V 1 / V 2 ) exp. ( 0.4 ) = ( T 2 / T 1 ) ---with the temperatures being in
absolute, same as the pressures were.

So-- for a 2 : 1 expansion, and if T 1 = 6000R, T 2 = 4547 R = 4087 F

and for a 3 : 1 expansion, and if T 1 = 6000R, T 2 = 3866 R = 3406 F.

Even if you assume that 500 degrees off of these temperatures is lost to
the water jackets, the lowest temperature here would be 2906 F, which is,
first, plenty hot enough for the end reactions to continue, and second,
still approximately 2200 degrees above the critical temperature of
water--and therefore plenty hot enough for an ideal gas approximation to be
fairly decent for the steam in the mixture.

Another feature of these numbers is that the work given up by the
combustion products in expanding will be equal to the change in their heat
content:

(6000 - 3866) x specific heat of the gasses (about .26 for what you
typically have) x (mass) x (778 foot pounds/BTU)


 Also consider the combustion products contain liberal
>amounts of water as steam, which is a radically non-ideal gas.

See above--at these temps, it is pretty close.
>
> The temperature/pressure drop may not be as severe as you indicate.

Really, it is quite a bit worse--because of the heat loss to the water jackets.

>And, of course, various fangles could be conceived to keep the
>temperature at any desired level.

As noted above, the temps are plenty high!

Regards, Greg