BusBuilding

Bryan asks in a recent post on one of the bus boards, “How long should a diesel engine be warmed up before driving off?”

Among the many considered answers is Ed’s post with a great explanation about diesel idling in general, from someone who oughta know: Detroit Diesel’s Mike Meloche, Senior Reliabilt Sales Manager:

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 five 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., obtained by compressing air in the compression cycle, 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 are 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!!

Can I get a chorus? If the engine is going to be idling, low idle, longer than five minutes, shut it off.

Oh by the way, on the how long before I drive off question? I think Andy came up with the best commonsense answer:

“Let it run for 30 sec to a minute to insure that the entire engine has oil pressure then pull out easy. I like to take it pretty easy until the temp gauge starts moving then drive in your normal manner.”

Clarke adds the following as always well-written everything-you-wanted-to-know about idling:

Don’t idle engines.

By Clarke Echols on February 12, 2006:

The worst thing you can do to any engine is let it sit and idle.

For gasoline engines (cars, pickups, light trucks), start it. If the weather’s cold, let it idle briefly until it is firing on all cylinders, then ease out into the street and let it warm up while working. Idling a cold engine in cold (Rocky Mountain or Northern States areas) weather until it warms up is murder on the insides. It casues high acid build-up in the oil, and generally destructive. A fuel-injected gas engine will start immediately, even in cold weather (down to zero or ten below — minus 40 might be a bit tougher). As soon as it’s running, get going. Don’t idle. If mama wants a warm car, you may have to compromise, but it’s idling that contributes to pollution because there’s not enough exhaust to get the catalytic converter running properly.

Even a warm/hot engine is damaged by idling. One minute at idle is as bad on most engines as two miles of driving at 60 mph on the interstate, whether gasoline- or diesel-powered.

Diesels pull a full gulp of air on every intake stroke. They have no throttle butterfly like gas engines to restrict air flow for idling. They just change the amount of fuel being fed into the injectors to control speed/torque. If the engine isn’t working hard, it gets a smidgen of fuel, not enough to maintain block temperature, and the intake air is like a big blast of refrigerating (cool) air, especially if the turbo isn’t spinning at full compression. Thus the engine cools to below its design operating temperature and you get the high wear factors again.

Idling wastes fuel. It takes about 0.3-0.4 pounds of fuel per hour to produce on HP of output from a diesel. There are 7 pounds of diesel in a gallon. It takes at least 10-15 HP or more to idle a diesel, depending on size, including alternator, compressor, etc. At 0.3 lb/hr, 10 HP = 1/2 gallon of fuel per hour; 20 HP = 1 gallon per hour. At $3/gallon, that’s a lot of bread to be blowing out the stack for no good reason.

If you’ve been pulling hard down the highway and pull in for a stop, let the turbo cool down, but in most situations, by the time you get from the exit ramp through the traffic signals and into a parking spot, you’ve already burned of 1-3 minutes which is adequate for turbo cooling, so just shut down the engine. When you restart, it will be fully warm and ready to roll immediately, assuming you didn’t park for an extended time of hours, even in cold weather. Especially in cold weather, you really knock the temp down on an idling engine.

So start it, get the air tanks up to an adequate pressure (some air bags don’t air up until a minimum-pressure valve opens at 90 pounds or some similar level), then ease it out. As long as you’re not blowing air off of the brakes faster than the compressor can air the tanks up, you should be fine, and the tanks will fill faster as you are driving down the road at the start of the trip.

Once it’s warming up, don’t be afraid to lean on the throttle if you’re so inclined. But you’ll save fuel if you keep your foot out of it.

If you’re hot-footing it and you’re blowing black smoke, lay off. As soon as it starts smoking, that says it’s getting more fuel than it can burn, and you’re just wasting diesel. You won’t go any faster, because the engine’s already at its peak.

For best engine life, don’t idle, and treat it with respect while not warmed up, and again after it’s fully warmed up as well.

CE

[Bus Conversions Magazine Forum]

This entry was posted on Saturday, February 11th, 2006 at 11:48 pm and is filed under Bus Conversion. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.