Quote:
Originally Posted by moTthediesel
So is the EGT reading a reliable measure of how much fuel is being burned? Or - does the reading go down in 3rd just because the engine is breathing more and cooling the combustion chamber, even though more fuel is being burned?
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In order to get a more efficient extraction of useful work out of an internal combustion engine, you would want a larger temperature differential between the intake and exhaust temperatures. Indeed, the equation that predicts the theoretical maximum efficiency of any given heat engine (such as your diesel) is:
n(th) = 1 - T(hot)/T(cold)
Where the Ts are both given in Kelvins (or both in Rankines).
There is a practical limit, of course, and that is the limitation of the materials making up the heat engine. You don't want your heat engine to turn its pistons into so much melted slag, nor do you want it to suddenly shatter its pistons, or ruin its valves. Therefore, there's a practical upper limit to what you should have your EGTs at.
So, to answer your question from this perspective, you are not really gauging the amount of fuel you're burning. Rather, by increasing your EGTs by shifting into higher gears, you are making your engine more efficient at higher gearing than at lower gearing. At lower gearing under load along with lowering EGTs, your engine can't get as much work extracted because of the lower temperature differential, and therefore has to burn more fuel to get the same amount of performance at the higher gear.