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Old 11-26-2007, 03:43 PM   #28
Marin
Scraping Paint
 
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Join Date: Oct 2007
Posts: 13,745
Boating and fuel cost

To me, proper temperature in a marine diesel is a combination of at least two factors. In the case of our boat, it's the coolant temperature and the exhaust gas temperature. On our boat, the coolant temperature of each engine will not come up to the lower end of the proper temperature range until we're running at least 1200 rpm under load.

We don't get into the desired exaust gas temperature range until we're at a cruise power setting of about 1600 rpm. Obviously the harder the engine works the higher the EGT will be. But it's possible to find out what the correct EGT temperature range should be for the engine and then operate it inside that range.

In the plane I fly, I'm used to using EGT, cylinder head temperature, and oil temperature gauges to judge the temperature of the engine. It would be nice to have a CHT sensor on the diesels in our boat but the Ford Lehman 120 isn't set up for them. So the EGT is the next-best thing.

Most engine manufacturers select a thermostat that will keep the engine in it's proper operating range (once the temperature gets high enough to open the thermostat). So if the proper temperature in the combustion chamber for complete and efficient fuel burn is "x," and when "x" is achieved the coolant temperature is 180 degrees, then the coolant temp gauge becomes a pretty accurate way of knowing when the engine's combustion chambers are at "x". The only way I can see that you'd get a different coolant reading when the combustion temperatures were at "x" degrees is if the thermostat, temp sensor, or gauge was malfunctioning.

But it's much better, I think, to have at least two ways of reading engine temperatures, which is why it's handy to have both coolant temperature and exhaust gas temperature readouts.

An oil temperature readout would also be very handy, and would probably be much easier to add onto and engine like a Lehman than a CHT. Oil temperature is like coolant temperature but without the potential variable of a thermostat. But I believe a coolant temperature readout will react faster than a lube oil temperature readout to a cooling problem, which is probably why the manufacturers of liquid cooled engines use them.
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