Engines At Idle

By on June 12, 2008

I witnessed something this afternoon that made me a little angry, and I need some help in knowing what to do about it.

I stopped to fill up my tank on my way back to work after lunch ($3.85 this afternoon, up 5 cents from yesterday, and an average of about 26mpg on this tank; not bad for a 13 year old Suzuki Sidekick). I drove into the station behind a city-owned street maintenance truck, and while I was filling up (and checking the oil & washing my windows) the truck sat there in the parking lot, idling. A 2 1/2 ton diesel-engined truck, idling for at least five minutes.

Living in the upper midwest, I know that there are times when it’s not a good idea to start & stop a diesel engine; usually in the dead of winter when the cold temps make restarting a diesel engine difficult (years ago I worked with a guy who drove an old Mercedes diesel, and he let the thing run 24 hours a day from mid-November through March every year so that a. he wouldn’t have trouble starting it in the cold, and b. so he’d have a nice warm vehicle on those cold cold mornings.)

But the temperature today is in the low 70’s and the sun is shining. So I’m left wondering; is there a compelling reason for this guy to have left his engine running during a five minute snack stop? Or is this an example of fraud/waste/abuse in our fine city’s workforce that warrants a letter/email of complaint to the street department?

I would’ve said something to the guy at the time — after all, he is working for me — but… Can anyone help me out here?



  1. no compelling reason to leave the car running. He was probably just being lazy, although you really don’t know unless you talk to him.

  2. Truckers aren’t lazy; they’re sensible. It costs way more to overhaul one that to keep it well-lubricated and running.

    Truckers leave diesel engines idling all the time because it reduces wear and tear. Most engine wear is incurred at start-up: cold engine, oil at the bottom of the pan, etc. That’s also when the worst emissions come out of diesel engines. Ever see them spew black soot when cranking-over the first time?

    Smaller trucks may have dozens of deliveries a day. And since the diesel has an expected service life of 2-3 times longer that a plain gasoline engine, it makes sense to keep it running, as the cost of maintenance over that time frame is less than the half gallon of gas per hour that it takes to idle the diesel engine.

  3. Thanks, Benxamin (how do you pronounce that?) That makes sense, and if that’s the case I’ll leave the guy alone.

    But that brings up another question; if a cold engine with the oil at the bottom is the cause of most engine wear, would it make sense to dump the engine-driven oil pump for an electric pump that can get oil moving through the engine before it cranks? And would it make sense to have a block heater on it to keep the block at the optimal startup temperature? I guess it depends on the payback of the added complexity under the hood, which depends on how often the truck would be left idling and how much fuel could be saved by shutting it off instead.

  4. Like Benxamin said, the trucker was actually saving you money by doing the best practice on a diesel which is to keep it running. ‘Cold’ isn’t gauged by the ambient temperature, but rather below operating temperature, so even in 70 degree weather you’d have the requirement he explained.

  5. It actually does more damage to engines leaving them idle for more than a few minuets, it glazes the cylinders, the main cause of Smokey motors, most truckers are not mechanics.

  6. I live near a big port and know that diesel pollution is a big health/environmental problem but wanted to understand more about why truckers let the vehicles idle before getting to irate. After reading this and other helpful articles online i found an interesting website about a system developed by a trucker to solve these problems. http://www.idlefree.net/ Pretty inspiring… I wonder how feasible it is for most truckers out there to afford making the change.

  7. Hey Joe, How is what you put into Dave’s lungs not his business? And how was Dave’s quest for some answers so threatening?

  8. I’m amazed at all the ignorant opinions out there. As an engineer I can tell you the 3 actual reasons to leave a diesel engine running.

    1. TEMPERATURE – Diesel engines do not use spark plugs but rather rely on heat and compression in order to attain combustion inside the chamber. (see reason 2 for more on temperature)

    2. FUEL EFFICIENCY – A diesel engine is a very sturdy engine that requires an immense ammount of energy to get started. When a gasoline engine starts up, it’s equivalent to using a simple BIC lighter where you stream some fuel and introduce a spark in order to ignite the butane. I don’t know about anyone else but I find it much more difficult to compress butane until it reaches critical mass and explodes on its own. (see reason 3 for more on fuel efficiency)

    3. RESONANCE!!!!!!! – Every time a diesel engine starts it must pass through its resonant frequency. To put this simply, when a child is on a swing set, the parent will push them when they reach the the top of their swinging motion and are just about to start on the down swing. If the parent were to push them while they’re still swinging backwards, the child would simply slow down and swing less. Every piece of moving equipment has a resonant frequency and some static structures as well (bridges, buldings, etc.). When a diesel engine is started it must first pass through this resonant frequency, which causes a lot of wear and tear on the mechanical pieces, before it becomes easy to maintain its running motion. Again, it takes a lot for the parent to get the kid swinging, but once they are swinging, it takes minimal effort to maintain the swinging motion. Therefore, LESS EFFORT = LESS FUEL CONSUMPTION!!!

    So from now on when you see a diesel engine running with no one inside, instead of bitching about it, you should fight for them to get the laws repealed that make leaving a vehicle running while unattended illegal. After all, these drivers are simply trying to reduce they’re maintenance costs as well as fuel usage. I’m sure they don’t give a crap about the environment, and rightfully so as hydro-carbons are PROVEN in every study to do nothing to the environment, but in the end they are actually using LESS fuel in the few minutes they’re inside a store than they would have to use to start the engine up and pass through the engine’s resonant frequency. And by the way, if you don’t believe me that it takes a lot more energy to start a diesel engine than a gasoline, go look under the hood at the TWO batteries that it requires. And don’t try to argue that the batteries are doing all the work, because the engine requires a specific ammount of energy to start whether it comes from the batteries or the fuel. After energy is drained from the batteries it must be recharged from somewhere. And I’m pretty sure you can all figure out what source of energy is used to recharge both those batteries.

  9. Older diesel did have all sorts of issues with cold start and it was perhaps a sensible precaution to let them idle if the stop was no more than a few minutes. Modern engines, however, use numerous techniques to mitigate these issues, and it doesn’t make sense to let a diesel engine idle for anything beyond a couple of minutes.

    BMW has introduced diesel engines that stop the moment brakes are applied, and then kick back to life upon acceleration. They do know their diesels – their engines are some of the finest around.

  10. How is it that this question is still being asked? How can an engineer repeat the old nonsense years after diesel experts tried to set the record straight?

    Check the following:

    EPA, April 2002: http://www.epa.gov/region1/eco/diesel/pdfs/DieselFactsheetTruck_Idling.pdf

    A senior engineer at Detroit Diesel: Feb. 2006 http://busbuilding.com/bus-conversion/diesel-engine-idling-from-an-authority-detroit-diesel/

    Basically all this stuff about letting diesel engines idle for long periods is just nonsense and has been for many years (I’m not sure about back in the 1950’s). But we can be sure that people who like to leave engines idling won’t stop just because the myths have been busted.

    Where I live (in western Canada) it seems to be a way for pickup drivers to show that they are serious about having a big diesel pickup, even if they aren’t so serious about understanding the mechanics of their diesel engines.

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