"Larry" wrote



Altitude makes much more difference than temperature on a diesel without a
turbo or supercharger. Yes, temperature changes the density altitude, but
in comparison to the 22:1 compression just before the fuel sprays into it,
it's quite insignificant. Of course, altitude isn't much of a problem for
boats at sea.


The compression ratio causes the temperature to rise, but it would do this
even if there was no oxygen present.

Air density (and therefore oxygen content) is affected by temperature and
pressure. If my high school physics is correct (and it may not be), the
affect of going to an altitude of 4000ft, is about the same as changing the
temperature from 80F to 150F at sea level. Both will reduce the oxygen in
the cylinder by about 11%. Most engines have no fuel management system to
handle this so they over inject fuel which causes environmental problems
(trucks/stationary diesels).

If we want more power out of a diesel, we can use a turbo compressor to
raise density, but it also causes air to heat up. Therefore, intercoolers
are sometimes added to reduce air temperature and get density back up. If we
overdo boost, exhaust temperatures get too high and we risk ruining the
turbo and the engine (My car has 3 safety systems just to prevent
over-boost!)

If my 14BHP non-turbo boat engine loses 11% power because the engine intake
air is hot, it WOULD be significant - Now I only have 12.5BHP and if my
alternator takes another 2BHP I am down to 10.5. But this is at max RPM! At
cruising speed, I may lucky to get 8BHP!

In reality, I won't see 150F at the engine intake and I won't be motoring at
altitude unless I go and cruise Lake Titicata.

It is something to be aware of but can be controlled by good ventilation.
Having a clean filter and unrestricted inlet air flow is also important
because the pressure loss also causes lower inlet air density. Maybe someone
sells a performance kit for Yanmars

GBM