Kleetus said:
dang it... you're gonna make me go get a thermodynamics book... I can't see how a fuel burning in an excess of oxygen can be colder than a lot of fuel burning in the same amount.
Because in a properly tuned diesel engine, there is ALWAYS an excess of oxygen. You're confused because you are thinking about a throttle-plated gasser burning at stocihometric (ie, the fuel/air ratio is the exact amount of oxygen needed to burn that exact amount of fuel). If you go OVER stoicihometric in a gasser, you have unburned fuel taking heat out the exhaust. If you go UNDER stociometric in a gasser, the excess oxygen burns ALL of the fuel and things start heating up. The ratio in a gasser has to stay around 14 parts air to one part fuel for it to work properly, so the throttle plate "meters" the amount of air intirduced into the cylinder by making the cylinder draw from a vacuum of varying levels of intensity depending on how wide the throttle is opened.
Diesels do not have a throttle plate like gassers do, so there is no way to "meter" the amount of air pulled into the cylinder with each intake stroke like there is in a gasser. Therefore, on a naturally aspirated diesel engine, all other things being equal, the amount of air drawn into the intake is the EXACT SAME AMOUNT with every intake stroke, whether the throttle is WIDE OPEN, or closed all the way... The speed an power output of the engine is regulated by regulating the AMOUNT OF FUEL INJECTED into the cylinder. So, if you are in a low-demand situation, say, coasting on a flat road, your air to fuel ratio could be 200 parts air to one part fuel, because the pump/injectors is only injecting a TINY amount of fuel on each TDC to maintain the coast. In fact, the air to fuel ratio can become INFINITE when coasting down a hill with the throttle closed, since most modern diesels inject NO fuel under these conditions. Therefore, in a *properly tuned* diesel, there is ALWAYS more air in the cylinder than is needed to burn the fuel injected. That means, the only way to make more heat is to inject more fuel, right? So, the leaner the ratio in a diesel, the COOLER it runs, because there is less fuel being burned to make heat. The closer it gets to stociometric and a "rich" condition, the more heat it makes, since there is more fuel to make more heat. Keep in mind that up until stociometric, ALL of the fuel is being burned ALL the time, so the only way to make more heat is to ADD MORE FUEL.
Dunno... I know it's a different heat cycle and all, but still, eventually you're gonna run out of consumable O2.
Since the amount of air taken in by a diesel in EVERY intake stroke, even at IDLE, is the SAME, and since this amount is the same as you get at WIDE OPEN throttle in a similar displacement gasser, a properly tuned diesel engine will not reach this point until full throttle.
A modified diesel can and will reach this point pretty quickly with more agrressive fuel curves, and when it does, the result is unburned fuel in the form of BLACK SMOKE. At this point, adding more fuel does not generally create more power, of more heat, since the fuel is being wasted, not burned.
This is why adding a turbo to a diesel works so well. It allows you to cram more fuel in and still have enough air to burn it all, and the most lovely thing about it is that the more fuel you cram in, the more air IT crams in in return. That is why it is so easy to make power in a turbocharged diesel. All you have to do is add fuel (to a point, of course) and it naturally takes care of the rest by it's very nature.
Anyway, this doesn't solve our man's problem here... But I do agree something doesn't add up...
It adds up perfectly if you can get rid of the throttle plate that you have in your minds eye. Without that throttle plate, it is a completely different ballgame...
