GBM wrote:
"RW Salnick" wrote

Assuming we are talking about a 2GM, displacement of say 600 cc, idling
at 1000 RPM, it is pumping 0.6/2*1000 = 300 liters/min of air out the
exhaust pipe. This is approximately 10 CFM - not bad for idle and a
small engine...


bob,
If the 2GM is a 4-cycle engine, is the above correct if it only draws in
air every other revolution?

Shouldn't air flow to engine room be calculated to keep temperature down ?
As the combustion air temperature goes up, engine efficiency goes down - I
read somewhere that power loss could be 11% if temperature rises from 80F to
150F.

Perhaps having the exhaust fan run all the time the engine runs would be a
good idea?

GBM

Correct - that is why I divided by 2 in the equation. I doubt that the
engine room temperature would reach 150 F under normal conditions
(although the engine external surfaces certainly could). If it did get
this hot, improved ventilation is certainly in order!!

The amount of air we are discussing here is a little more than what
would fill a 2' x 2' x 2' cube. On the small boat we are discussing
here, I am guessing that the engine compartment is pretty small and
crowded, and that the 10 CFM flow, at engine idle, would completely
replace the air in the engine compartment every minute or two, or less.

bob

The amount of air we are discussing here is a little more than what
would fill a 2' x 2' x 2' cube. On the small boat we are discussing
here, I am guessing that the engine compartment is pretty small and
crowded, and that the 10 CFM flow, at engine idle, would completely
replace the air in the engine compartment every minute or two, or less.


bob,
I have the same size engine (older model) - The engine compartment is
3'highx2.5'wide and is open aft to the cockpit lockers etc, so enclosed
space is quite large. Nevertheless, after a long run (without blower
running), the engine "room" IS quite hot - I must measure it, but it must be
20-40F above the cabin temperature. Most of this is likely due to radiation
from the block which is at 150-160F, the exhaust manifold and the
alternator.

Because the refrig unit is nearby, I plan on rewiring so my exhaust fan will
run whenever the engine is running (so I don't have to remember to turn it
on). I have already insulated the wall between the engine and the refrig
unit. Hopefully these two things will keep the engine from adding further to
the already high refrig heat load.

GBM