On Thu, 6 Nov 2003 23:14:57 +1100, "David Flew"
wrote:

Can anyone provide sizing info for wet exhausts on diesel engine. My recent
purchase originally had 80 HP petrol engine, then 30 HP Lister, now 80 HP
converted Nissan diesel. The Nissan is OK up to about 2200 RPM, but over
that the exhaust has both a little smoke and what looks like steam to me.
And the exhaust if absolutely rocketing out the pipe ! I've seen
somewhere that diesels have twice the exhaust volume of similar sized petrol
engines, which suggests the exhaust may be half the required area ( I bet
it's the original diameter ...) but I'd love either rule of thumb or
rigorous methods for sizing exhausts.

Thanks

David



Here's something.
Note the full throttle power rating.
Say 80HP
That represents about 30% of the power available from the fuel
in a gas engine. It represents perhaps 40% (say) of the fuel in a
diesel - it is a more thermally efficient device.

But use that 30% value.
Say that the remaining heat exits via the exhaust (it doesnt)
that's 70/30 of 80HP = 187 HP = 187 X 750 watts = 140.3 kW

How much water flow is needed to carry off 141 kW at moderate
temp rise?
If you allow a temp rise of 50 to 180 degF (that's 10degC to 82degC)
141 kW is mass rate (kg/sec) times temp rise (72degC) times calorie
to watt conversion (1/4.2)
So mass rate is (141 X 4.2 )/72 = 8.23 kg/sec
or 2.25 gall/sec = 136 gall/min

Size a pipe to carry this water flow at moderate speed while occupying
20% of the pipe cross section, and you have a rather conservative
rule of thumb to size the exhaust pipe (and the cooling pump). The
flow rate is dramatically reduced if you allow some boiling to occur.
Does this help?

Brian Whatcott Altus OK