Are extended idle times really OK?

[DennisG01]

Something I've been wondering about for a while - Is it really OK to leave the truck idling for 30 minutes to an hour? How about longer? And what about fuel usage - I originally thought it wasn't supposed to use much at all at idle, but based on my recent mileage in colder weather, I'm not so sure.

[AndyL]

My 99 uses about 6cc per stroke at idle.

[DmaxMaverick]

Originally posted by AndyL:
My 99 uses about 6cc per stroke at idle. I think you mean 6 mm

[charliepeterson]

Extended idling can load up the cylinders and cause a host of other issues besides wasting fuel. In my area five minutes is the law for the big trucks. If you're warming up in the morning just remember that the only thing warming up is the engine. The trans., axles, rear end are all still cold. A nice easy ride for the first few miles is good.

[AndyL]

Originally posted by DmaxMaverick:
</font><blockquote>quote:</font><hr />Originally posted by AndyL:
[b] My 99 uses about 6cc per stroke at idle. I think you mean 6 mm

[computer monkey]

I installed the fast idle switch in my 98. It helps keep the dogs cool inside in the summer when they are along for the ride when I have to run inside the store for something.

[markrinker]

Originally posted by charliepeterson:
Extended idling can load up the cylinders and cause a host of other issues besides wasting fuel. In my area five minutes is the law for the big trucks. If you're warming up in the morning just remember that the only thing warming up is the engine. The trans., axles, rear end are all still cold. A nice easy ride for the first few miles is good. Idling does warm up automatic transmissions and is one of the main reasons I am a proponant of 'extended idling before rolling' on mornings &lt; 15F. I do agree on your 'nice easy ride' recommendation for a few miles before adding any load to the drivetrain.

As for loading up - I've never seen any evidence of that with my 6.5s. After the initial smoking at cold startup - unburned fuel. The exhaust is clear and engine rattle consistant. If they were loading up, the idle would change and first exhaust smoke upon takeoff would be black.

Cummins, Cat, and other diesels may well be different when it comes to extended idle. I can only speak for my experience with the GM 6.5s in severe winter operations snowplowing.

Axles bearings and diffs are cold regardless, and deserve those first few no-load miles to warm up from friction alone. I am amazed that they survive the cold up here in the northland, but they do.

[DennisG01]

AndyL-

Can you explain "6mm per stroke" in layman's terms? Also, how does that relate to a gas engine - I had heard that diesels use much less fuel at idle than a comparable gas engine. Thanks!

[AndyL]

Originally posted by Dennis Galligani:
AndyL-

Can you explain "6mm per stroke" in layman's terms? Also, how does that relate to a gas engine - I had heard that diesels use much less fuel at idle than a comparable gas engine. Thanks! Sure!

Per stroke means that each combustion event consumes about 6 cubic millimeters of fuel.

Lets assume a 6.5 litre diesel idling at 600 RPMs.

6mm^3 * 8 Cylinders * 300 half of 600 RPM because it is a 4 stroke engine = 14400mm^3 per minute, or 0.0038041 gallons per minute or 0.228 gallons per hour. :D

[DennisG01]

Thanks! That makes more sense. .2 gallons per hour ain't a whole lot! Have you ever measured your Burb w/ the 350?

[AndyL]

Originally posted by Dennis Galligani:
Thanks! That makes more sense. .2 gallons per hour ain't a whole lot! Have you ever measured your Burb w/ the 350? That is a good question! I have plugged into it before but I don't recall if it displayed fuel rate. I will check in the near future and post my findings.

[BuffaloGuy]

That sounds about right. I have slept many a night in my truck with it idling for warmth. I guessed that it took about 2 or 3 gallons per night. A whole lot less than a hotel!

[moondoggie]

Good Day!

[AndyL]

Creating a vacuum does consume efficiency, but diesels have to compresses a full chamber 22 times over ATM. Here is some math:

Vacuum force is limited to the available atmospheric pressure of about 14.7 PSI. A 6.5 bore is 4.055", or 3.14*r^2=12.9 cu in * 14.7PSI= 189 pounds of pressure per cylinder. Assuming it pulled a perfect vacuum.

On the 22:1 6.5: 14.7PSI * 22 = 323 pounds of pressure per cylinder! This combined with the power required to turn the IP makes the diesels losses considerably higher.

On the plus side, diesels are capable of running at extremely lean ratios of about 110:1, whereas gasers are only capable of about 15:1. For every point of compression an IC engine gains about 3.5% in efficiency.

[moondoggie]

Good Day!

Wow, thanks! :D Most of the mpg benefit of diesels must be the extra BTU's available in diesel over gasoline then. I had always thought some was due to the vacuum pump thing.

Perhaps you can explain something else then. If it takes so much more energy to turn over the diesel engine than the gasoline (which is reflected in idling fuel consumption), why do diesels have almost no engine braking, while gas engines have lots?

Boy, at times like this, it sure is a bummer not having internet access at home. I'm leaving work pretty soon, so won't get to see your reply until Monday morning! :(

TIA & Blessings!

Brian Johnson, #5044

[gymcarm]

I have one of those PS Fords. It idles up to ~1000 RPM when the temp is below some magical level. The whistle is apparently a backpressure valve. They restrict the turbo exhaust trying to get the engine to warm up faster. The idle speed drops to a normal level when any of the pedals are touched.

Overall, I like the truck. This cold weather package makes it sluggish and very noisy until it warms up.

[markrinker]

why do diesels have almost no engine braking, while gas engines have lots? [/QB]I always assumed it had to do with very different cam profiles - i.e. overlap.

[DmaxMaverick]

Originally posted by Mark Rinker:
</font><blockquote>quote:</font><hr /> why do diesels have almost no engine braking, while gas engines have lots? I always assumed it had to do with very different cam profiles - i.e. overlap. [/QB]</font>[/QUOTE]Gassers have a throttle. Diesels have no appreciable restriction in the intake path to cause a state of vacuum, which labors the intake stroke.

[Bobbie Martin]

I think this begs the question, do Diesels really consume less fuel at idle? This might be a good question for Dr. Lee or Ronniejoe. Heres my thinking - flawed as it may be.

Gasoline and like fuels must be held to a very narrow fuel to air ratio. While there is a bit of leeway, it cannot vary much more than a couple of points from 15 parts air to 1 part fuel. This is why gas engines have throttle plates. There must be some way to limit the amount of air at low speed operation. The throttle plate (if everything is working properly) will allow the correct amount of air for the amount of fuel needed. So even though only a small amount of air may be going into the engine, the air/fuel mixture is still around 15-17:1.

On the other hand, diesel engines work much different. Diesel fuel can burn at very wide fuel/air mixtures. There is no throttle plate. The only way for a diesel engine to vary the power output is to change the air/fuel mixture. It uses the same amount of air per stroke regardless if idling or max RPM. By injecting a small amount of fuel, the engine will idle. The air/fuel ratio may only be 60:1 or less. Inject a little more fuel and you get more power, inject a lot more fuel and you get a lot more power. At full throttle, you are probably more than 20:1. So diesels, by their nature run much lower air/fuel ratios at idle.

And there is where most people fall into the trap thinking that Diesels use much less fuel at idle than their spark ignition counterparts. However, they don't take into account the increased airflow in the Diesel. And this is where I don't know the correct answer. The Diesel is using much more air at idle, but is the lower fuel ratio enough to overcome the increase in airflow? For sake of argument, let say the Diesel has an air fuel ratio of 60:1 at idle, while the gas engine has 15:1. That's about