FROM: arcaeh@aol.com (Andrew) SUBJECT: Re: diesel question DATE: 15 Sep 2000 01:04:03 GMT ORGANIZATION: AOL http://www.aol.com NEWSGROUPS: rec.autos.4x4 > NO DIESEL ENGINE MANUFACTURER recommends idling its > engines for extended periods of time. Directly from the Cummins FAQ: << Q: Can I Leave the engine idling for long periods? A. Do not idle the engine for excessively long periods. Long periods of idling (more than 10 minutes) can damage an engine because combustion chamber temperatures drop so low the fuel will not burn completely. This will cause carbon to clog the injector spray holes and piston rings, and can cause the valves to stick. If the engine coolant temperature becomes too low (60 degrees C [140 degrees F]), raw fuel will wash the lubricating oil off the cylinder walls and dilute the crankcase oil; therefore, all moving parts of the engine will not receive the correct amount of lubrication. (For more information, refer to Cummins Operation and Maintenas. nce Manual section 1-5, Bulletin 3810205-12.) >>> From Josh Berman to the Cummins mail list: << The only time when extended idling becomes a problem is when it is cool-to-cold outside. At that point, the engine may not be able to generate enough heat to stay warm inside. When that happens, fuel can condense on the inner walls of the cylinders, washing away the oil film and causing accelerated wear. Carbon from the incompletely burned fuel can also clog injector tips and stick the piston ring >>> Andrew FROM: "Mike Locke" SUBJECT: Re: diesel question DATE: Thu, 14 Sep 2000 22:02:38 -0700 ORGANIZATION: http://extra.newsguy.com NEWSGROUPS: rec.autos.4x4 Douglas A. Shrader wrote in message news:ss2e6rpuct6105@corp.supernews.com... > > > -- > Douglas A. Shrader & Jake (RIP) > "Mike Locke" wrote in message > news:8ppnb701lgm@enews1.newsguy.com... > > > > Douglas A. Shrader wrote in message > > news:ss0c9o1gct618@corp.supernews.com... > > > > > > > > > -- > > > Douglas A. Shrader & Jake (RIP) > > > "Mike Locke" wrote in message > > > news:8pn0kd014vq@enews3.newsguy.com... > > > > > > > > Actually, this fuel-air-diesel Idle myth is just that and it needs to be > > > > stopped right now. > > > > > > > > Each of your statements alone are correct except the final conclusion. > > > > That's because you left out the all important fact: at Idle a gas engine has > > > > nearly no air at all going through it, > > > > > > A gas engine always has 14.7 parts air for every one part fuel. Engine speed > > > has no bearing on this. Diesels vary from 100 parts air at idle to around > > > 20-25 parts at full throttle. Less fuel per air equals less fuel total. > > > Simple math really. You can find all this in any competent diesel manual, > > > you really should try reading a few. > > > > Don't be so presumptious about what I have or have not read. > > Comparing idling gas and diesel engines of equal displacement, the diesel > > engine will have *MORE AIR* going through it. *A LOT* more air. > > 4 times more air in fact, they still burn less fuel, hence the proven > increase in fuel economy. Just using your numbers, 14.7:1 with 1/4 the air vs 100:1 with the full air The gas engine to diesel engine ratio of idle fuel consumption is 59:100. OK the diesel gets better idle fuel economy, just like it gets better on road fuel economy no surprise there. This ratio is pretty close to the difference in compression ratios. This is not enough to make the really huge difference you claim in idle times. It is also not even close to the 1:7 that just discussing the fuel-air mixture ratios implies. > > The gas > > engine has a butterfly throttle (or similar) that restricts the air intake > > so that the correct amount of fuel energy required to turn the engine over > > at idle can be reasonably efficiently burned. > > > > Your statement is equivalent to something like > > > > A gold coin is 100% gold. > > A gold mine is 0.1% to 0.01% gold (that would be a really good mine :-) > > > > Therefore (you conclude) if I own a gold coin I have more gold than if I own > > a gold mine. > > > > You wanna sell me that mine for a coin? > > There is more energy in a gallon of diesel than a gallon of gas, thus again > less fuel used, gold has nothing to do with it. You obviously have never heard of an analogy. Remember 8th grade English? > > Because that diesel truck of yours has an enormous fuel tank > > Oh it does not. It has a twenty gallon tank, same as our gas chevy with a > 350. The gas uses far more fuel to idle than the diesel, proven fact. But you said your truck "drinks fuel" on the highway, implying that it uses a lot of fuel. OK, maybe your Chevy diesel does. Have you actually measured the idle rate of fuel consumption for these two engines? > > NO DIESEL ENGINE MANUFACTURER recommends idling its > > engines for extended periods of time. > > They don't recommend it for gas engines either, in fact they warn against > idle gas engines for more than I beleive ten minutes due to exhuast > overheating. Diesels are the ooposite. Because they burn so little fuel at > an idle they cool off to the point where the fuel maybe able to slip past > the rings and dilute the oil. I've seen no evidence of this happening on > mine but the amount of fuel would be so small I doubt it could be noticed. On gas engines it is the catalytic converter that overheats, due to excessive unburnt fuel making it to the converter. If a diesel engine had a catalytic converter it might have this problem too. Adding an exhaust brake to a diesel engine (a throttle butterfly in the exhaust pipe) will stop the diesel engine cool off problem and reduce the idle engine air. I don't know if it reduces idle engine fuel usage. > > If you try driving your truck as fast as it will go at idle you will most > > likely get horrible fuel economy. At idle, the only thing your engine is > > doing is turning itself, and that is getting you 0 MPG. > > Not if it is in gear and moving. It can idle in gear. Sheesh, read it again > and pay attention this time. I stand by my statement. At idle your engine is only making enough power to turn itself, it cannot move your vehicle. The fuel injection system increases the fuel rate when the engine is loaded so that the RPM starts to fall below idle, thus explaining how you can let the clutch out at idle RPM. The engine is not really idling in this condition (in gear at idle RPM), it is just running at the idle RPM. The engine is grossly inefficient at this RPM, though and you will get terrible fuel economy. > > Max fuel economy in my truck is at 1400 RPM in 6th gear at ~45MPH. I get > > around 30MPG. > > > That's great. Those economy figures also apply when you are idling as well. > An idleing diesel uses less fuel than an equiavilent gas engine. Maybe, but not 1/7 the fuel use as implied by the numbers that you quoted. -- S'later, Mike Locke FROM: James Clark SUBJECT: Re: diesel question DATE: Fri, 15 Sep 2000 18:56:29 -0700 ORGANIZATION: Global Network Services - Remote Access Mail & News Services NEWSGROUPS: rec.autos.4x4 Mike Locke wrote: > > The gas engine to diesel engine ratio of idle fuel consumption is 59:100. > OK the diesel gets better idle fuel economy, just like it gets better on > road fuel economy no surprise there. This ratio is pretty close to the > difference in compression ratios. > > This is not enough to make the really huge difference you claim in idle > times. It is also not even close to the 1:7 that just discussing the > fuel-air mixture ratios implies. > Think of a diesel as a big air spring. Most of the energy is used to compress the spring is returned on the power stroke, even with no fuel present. Adding fuel just increases the "bounce" of the spring. You just have to add enough fuel to overcome the friction of the bearings and rings as well as the pumping losses from filling and purging the cylinders. Not being throttled, these pumping losses are negligible. A gas engine has to pull air through the throttle, incurring pumping losses. Then there must be enough gas in the mix to sustain combustion. > Adding an exhaust brake to a diesel engine (a throttle butterfly in the > exhaust pipe) will stop the diesel engine cool off problem and reduce the > idle engine air. I don't know if it reduces idle engine fuel usage. Only by increasing the load on the engine. So, yes, it does increase fuel consumption. I understand that the T444E EMC uses an exhaust brake to reduce warm up time of the engine.