Turbo cool down

Rod

• Lubrication Engineers

What I was getting to with the idle speeds and cooling. Hmmm. To eloborate more. Think of the engine as a big air pump, right? So for every rev, it moves so many CFM, and that would be it's air exchange. Turning at 1250 is roughly twice 650, so it should have roughly twice the air exchange. I couldn't tell you the exact parameters of how much heat the engine and manifold etc. is giving off, but the basic idea is that the more air that goes through the engine, the more heat it's going to carry off. Now, off to pump theory. An engine like the Ford 268 turbo that I used as an example will have and idle fuel delivery of 2-3 cc/200 shots. At high idle I think it was something like 4-5cc/200 shots. Wheras the factory delivery at maximum on that engine was ~21cc/200 shots, and I'm thinking I've got it somewhere around 25cc/200 shots. So basically there is a very very small amount more energy going into the engine to turn it at 1250 rpm than at low idle, and really not much more heat being generated. So with the higher air movement, it should cool itself much quicker. This I have observed on the pyro on that tractor. If I let her idle at 650, it might take 10 minutes and it's still around 300 F. Idled up a bit, and she goes to ~100F in a couple minutes, depending on how hot it was. As I said, I've seen that thing hit 1150F in the pipe after the turbo. The info from Hewitt that came with the pyro suggested a 200-250 F drop across the turbo.... so that's 1350+ in the manifold. I try not to go there to often, or for too long. Aluminum has that smell about it..... To take your question further, it doesn't heat back up noticeably at low idle on that engine, because it's not making much heat. Just the higher rate of heat removal at a fast idle that causes the quicker cooling.
To answer the other question. The turbo we lost on that tractor was an AiResearch. It was replaced with a Garret branded turbo.... but basically the same turbo. Came through Ford, so it should be. Again, it was just a matter of whoever was in the seat would drive into the yard, after dragging who knows what, full bore, and the cooling and spooldown time it had was the amount of time it took to set the brakes. Then out comes the shutoff. I'd like to see how long that spun. Probably quite a while. One turbo I do know that spins and spins is on the 3204. Often hear that for 20 seconds or more. Talking 3306's reminds me of the milk trucks that used to pick up here. Dairy was big on Cat's. Run nothing else in recent years. I guess in days gone by, they used to rag the drivers about ideling the old Detroits... Shut them off. shut them off. So, they shut them off. Up the driveway the come, lugging and blowing smoke. The key went off 10 feet from the door, and roll to a stop. WWWWHHHHHIIIIIRRRRR Rrrrrr rrrrr . I can still hear the damn thing. They ate a few turbo's over the years too, and never learned. Oh well.
Anyway, I'll give you one thing. You got a good discussion going. Take er easy.

Rod

Rod

I have an example to use, my place is atop a hill and I've got a small car with a turno on it, with a vacuum gauge, no pyrometer, if I really hit the fuel on the way up in a higher gear I can really get the needle to advance on the vacuum gauge, assuming if it had a pyrometer, it would show the dramatic increase in temperature while climbing the hill and the pedal to the floor, in a higher gear with plenty of r.p.m.s left before shifting, (even in this little mercury capri that turbo can be noticed, snappy little car actually) As soon as I get to the top and pull into the driveway, just a few seconds before, it was hot and working hard to do its job, I never just shut it off, on this one MFR. says to let it idle for at least 30 sec. I give it at least a minute, but the reasoning is for oil to keep circulating until it cools and stops spinning, for reasons mentioned above, if you shut it down you could seize a bearing, due to the heat and lack of lubrication, assuming that to be caused by the oil coking up as mentioned.

So in theory it's best not to shut an engine down after a hard acceleration or right after it's been under a heavy load, if I understand what I've learned from others on this and what I've read on different manufacturers warning stickers. This little car has 124,000 miles, orginal turbo still, knock on wood.

I remember how powerful some of the tractors I used to drive were with no trailer, many times I'd get locked out of the yard when coming in late and just drive the tractor home, really feel that turbo with no load to pull, they are amazing for what they do, sets you back right in the seat if you get on it.

Same was true for extreme cold starts, on the Mack's I remember we had a fuel shut off in the cab, used to crank the engine for a bit to get oil up to the turbo before giving it fuel in low winter temperatures, that was to allow the turbo to get some oil into the bearings, assuming the oil would flow, really not sure if this was helping it any, but a co-worker back then who used to run a few trucks coast to coast said he did it for this reason. Made some sense.

I have heard of turbos failing in some large Cat dozers while in a heavy push after the operators stalled them out and could not get them immediately started again, that makes sense, I have ran plenty of D8K's and when you are working hard they get real hot, used to heat my lunch on the exhaust manifold, it did not take long for a hot lunch to be ready. It's a good thing to learn and know about if you are running any engine with a turbo, lot easier to let it cool than replace.

If you let the engine idle untill the exhaust gas which is in the 500-600 degree range has cooled the exhaust turbine and housing to that temp,the turbo lube oil will not burn and coke up the bearing.

Myself, I like to let all engines get to an idle and maybe idle for 30 seconds or so before I shut it off.