T&T: Ambient Air Temperature and Diesel Engines

[Mike Maurice]

You may recall that the book says that you should choose your oil 
viscosity by the AAT(Ambiant Air Temperature). There are two parts to 
this problem. The AAT that the engine will start up from, sitting in a 
cold engine room for instance. And, the AAT that will be ingested while 
it is running.

Most diesels that we think about run in trucks or in boats.
In the truck case the AAT that it runs in is pretty similar to that 
which it starts up in.

In the case of a boat, the engine starts up at whatever temperature 
exists in the engine room, which may or may not be the same as the 
outside air, especially if the engine room is heated.

Once the boat's engine is running, the engine room generally heats up 
such that the temperature is always above 80 F. At which point the air 
that the engine is ingesting is most likely that of the air temperature 
in the engine room, not that of the air outside the hull. This might not 
be the case if there are direct ducts from outside to right near the 
engine air intakes.

In addition the boat engine will be sitting in what amounts to a heated 
room. In other words, the AAT situation is entirely different between 
that of a truck and a boat.

Except for the short period before the boat's engine room heats up, that 
engine is pretty much running in an AAT of 80 F or above. Which argues 
for using a 40 weight oil, in all but the most severe cold conditions.

Of course, if the engine room is not heated and it is below about 35 F., 
the oil may not flow as desired and could cause problems at startup and 
until the engine and room are heated up.

I think you can see where this is leading. We may be using lighter 
weight oils than would be most desirable, but if you use heavier ones, 
you may have to baby the engine on startup, if the AAT is cold.


Mike
_____________________________________
Capt. Mike Maurice
Beaverton Oregon(Near Portland)