This is a copy of email I found in my archives
hope it helps some NEWBIEs
I'd say Andy G. is pretty close to "Right on" with the
DD recomendations. A few years back I wrote DD
regarding idling of our engines and here is the reply
I received.
Ed,
Your question ranks right up there with the most commonly asked questions we receive. It is also, at least in my opinion, the most commonly misunderstood aspect of the operating parameters of a diesel engine.
The short answer is we have always recommended that if the engine is going to be idling, [low idle] longer than 5 minutes that it be shut off. The reasoning is a little more complicated but without getting into engineering 101 basically the following is the layman's reasoning.
Diesel engines were designed to operate at peak efficiency running wide open. As there are no spark plugs; cylinder temp. is obtained by compressing air in the compression stroke, this heat is necessary to ignite the fuel. At low idle the cylinder temp. drops leading to incomplete combustion. This incomplete combustion leads to carbon build up on the valves, in the oil
(crankcase), on the fire deck of the head and the dome of the piston along with fire rings on the piston.
Engine "slobbering", what appears to be oil, out of the air box drains and the exhaust are other side effects. As worn oil control rings will also give you
both of the last two items this is often cited as the
cause of an engine "slobbering". The irony is that the carbon build up in the oil will lead to worn oil
control rings.
Now the reasons folks give for letting the engine idle. "I need to keep up cab heat / AC in the winter / summer". If you going to be away for less than 10 minutes the engine will keep more heat being shut off than idling for that amount of time. The AC unit will not maintain temp at low idle. "Truckers all leave their rigs running while at a truck stop". They aren't
paying for the fuel or the repairs. "When you've been running all day you need to let the engine "cool" down for at least ten minutes before shutting it down". Simply not true!!
I hope this has been of some help to you. If I can be
of any further assistance please contact me.
Mike Meloche
Senior reliabilt Sales Manager
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A couple of comments. Truckers leave their engines running at truck stops-this is only for either heating or especially keeping the cab cool in summer. When I was driving I would put the engine on fast idle and leave it run while I had lunch to come back to a cool cab (trucks take a much longer time to cool down than a car). And even owner/operators like I was do keep their engines idling during the night burning their own fuel (I had a generator-didn't believe in running a 4-500hp engine just to run a 5hp air conditioning compressor).
With the older engine designs we have (read 2 strokers) it is important to not let them idle long-since at 650rpm most put out only 10-15psi of oil pressure. If you want to idle your engine more than 5 minutes, it should be put to a fast idle-around 9-1000rpm-then you'll slow down the chances of wet stacking. If on the other hand you have a 4 stroke electronic engine like a Series 60, Cat C12, Cummins ISM/ISX, then the likeliness of wet stacking is almost non existing.
As to cooling the engine at the end of the day-if you have a 2 stroke engine, it flows so much air that by the time you get to the bottom of the off ramp from the freeway, it is cool enough to shut down. If you have a mechanical 4 stroke engine-then yes you have to be very careful not to shut it down to soon-hopefully you have a pyrometer and not shut the engine down until under 450 degrees or less. If you have an electronic 4 stroke-they never get hot enough on the exhaust side to worry about it. Good Luck, TomC
My concern in shutting down a hot engine would be cooking the oil in the turbocharger bearings, if the turbo is shutdown hot. Some install a post lube pump to maintain oil flow until the exhaust turbine cools.