Engien Temp?

[SteveAnsley]

1st: I'm officially a diesel convert, I've seen the light.

I've had my '98 12v QC for about 6 weeks now and really enjoy driving it. I was a little concerned about the 5 spd during D/FW rush hour traffic, but I actually look forward to the stop-n-go as I enjoy 'rowing' the gears.



Questions: (forgive, I'm still a newb)

I've been under the impression that a diesel will disipate engine heat better than a gasser (ie: less of an issue of overheating while idleing).

Is this true? And if so, Why?

You would think the opposite would be true w/ the higher compression ratio.



I plan on getting a pyro & boost guage and I'm aware of keeping an eye on the EGT's. But, can I idle safely during Texas August heat? The temp gauge reads a constant 180-185 so far.

What's the ideal range for these motors?

 

[rbattelle]

Whereas most spark ignition engines run to a constant fuel/air ratio, compression ignition engines run with a variable fuel/air ratio. That means at idle the fuel/air ratio is very small (lots of air, very little fuel). Such little fuel means you're not generating much combustion heat.



In fact, there is a phenomenon sometimes referred to as "stacking" wherein the low fuel/air ratio causes the cylinders to cool so much that you begin getting incomplete combustion and the unburned fuel washes down the beneficial lubricating layer in the cylinders and runs down into the oil sump. That's bad. Some people install high-idlers to prevent this condition.



Bottom line: there should be absolutely no problem running that engine at idle no matter the ambient temperature.



Someone will certainly correct me if I've said anything incorrect...



-Ryan

 

[PC12Driver]

Higher compression ratios generally make internal combustion engines more efficient. As the air volume compresses in the cylinder it is heated, and the higher the ratio the more heat. But as the piston travels back down and the air charge returns to its original volume, and the expansion causes the charge to cool (not counting fuel or friction it would be the same temp). So the compression ratio alone won't do it.



A gasser at idle is sucking against the throttle valve (butterfly) which is barely open. The engine requires more fuel to suck against this restriction, which generates more heat. A diesel has no throttle valve - its wide open all the time (intake wise). The throttle valve on the gasser is also it's source of "compression braking" (which has nothing to do with compression at all - "vacuum braking" is more correct. Ever notice that your high-compression diesel doesn't slow much when you let off the throttle?). When you close the exhaust brake on a diesel you get the same effect as closing the throttle valve on a gasser (but on the exhaust side instead of the intake), and you'll typically see a 200 degree-ish rise in exhaust temps at idle as the engine needs to do more work to pump air.



A gasser needs to maintain a 15:1 (about) air:fuel ratio to run properly no matter what the operating mode is (idle, cruise, full load). A diesel can vary from 80:1 at idle to 18:1 at full load (the full load ratio will vairy quite a bit; I'd swear there's some guy's on here running 1:1 with the smoke they put out). So, at idle the diesel is barely sipping fuel.



When you get your egt gauge installed you'll see idle temps around 250 degrees or so (when it's cold up here I can hardly get 200). That's barely enough to warm the water in the radiator. Gassers will have way more than that. In winter my water temps will actually drop to 160-170 if I idle for more than a few minutes (thermostat is just fine - there's just not enough heat being produced). And that's also why you don't want to idle these things for very long, since there's not enough heat being produced to burn all the fuel. The unburnt fuel will wash the lubrication off the cylinder walls and will dilute the engine oil. And the 24 valve engines can also get a damaging amount of carbon build up on the exhaust valves, too.



While pulling a trailer in 100 degree heat for a few hundred miles my water temps got to 205-210. After a couple minutes of idleing they dropped right back down to 190 (a/c on the whole time). Essentially, if you can get these things to overheat at idle or light throttle, something is broke.

 

[PC12Driver]

Heh, nothing like saying the same thing twice. Just took me 3x as long to say it!



Ryan, I think I'm that "someone"...

I think "wet stacking" occurs in diesels with vertical exhaust pipes. The exhaust cools during idle as it travels through the pipe, the cooling causes condensation, and there's not enough heat to evaporate the water or velocity to blow it out of the pipe. It just kinda pools at a low spot in the exhaust, thus, wet stacks.



"Fuel washdown" takes the lubricant off the cylinder walls, which leads to "fuel dilution" in the engine oil.



But then again, I could be completely wrong and someone will correct me!



-Wally

 

[rbattelle]

"Fuel washdown" takes the lubricant off the cylinder walls, which leads to "fuel dilution" in the engine oil.

I stand corrected! I had heard other people call the cylinder washdown effect "stacking" and I assumed they were right. Never made any sense to me why it would be called stacking... your explanation makes more sense.



-Ryan