Why don't diesels make heat at idle?

[SmokinU]

Hey fellows,



I was just wondering why a diesel engine doesn’t make any heat at idle speed? A gas engine has no problem doing it. It is pretty dang cold here where I live, and when setting at a red light, every car around me has condensation coming out of the exhaust, but my truck has zero condensation coming out of the tailpipe. This is a dumb question, but Id really like to know why that is.



Brandon

 

[JoeBioDiesel]

Hi Brandon,



There are a couple of reasons.

1) Not much fuel is being burned. Gassers have a fuel misture of 14:1 at idle. Diesels approach 100:1 at idle. Little fuel means littel heat.

2) Diesels burn their fuel more efficiently. Less heat is lost to the cooling system. More of the heat created is used to move the piston.



Joe

 

[Vaughn MacKenzie]

From what I understand a typical gasser has around 800-1000F EGT at idle, diesels around 250F.



Another reason going along with what Joe pointed out is diesels always run without a throttle restriction (full air flow) so the air has a cooling effect compared to the low airflow on a gasser at idle. And to go along with that diesels don't have a throttle plate to work against so it takes even less fuel to spin them over at idle.



I have always wondered why Dodge doesn't build the HVAC system on the Cummins trucks with an electric heater coil that would kick in as soon as the grid heaters go off so that you could get heat into the cab relatively quickly and let the engine have more heat so it can warm up quicker?



Vaughn

 

[Forrest Nearing]

high resistance heater coils like that draw a LOT of juice... the grid heaters draw a LOT... the amount of current required to actualy provide heated air would wipe out the charging system in short order.



Forrest

 

[ata]

Probably because all the cars and trucks around you have catalytic converters on them that turn their unburnt fuel into water and sulferic acid. Ever notice how water is always dripping out of their tail pipes. At least that would be my explaination for it.

 

[SmokinU]

Thanks for the replies. All of what you said makes sense. I didn't know that the mixture was so lean at idle. I can see where the converter would hold water too. Mine bit the dust a week after I got the truck, along with the muffler.



Brandon

 

[Blakers]

Originally posted by ata

you have catalytic converters on them that turn their unburnt fuel into water and sulferic acid. Ever notice how water is always dripping out of their tail pipes.

And we never heard of acid rain before the catalytic converter came out. But the EPA doesn't want to talk about that one.

 

[Tomscreek]

Makes quite a difference to bump up the idle to 1000 or a little more if you let it run a little when it's cold out. I have a very high tech piece of equipment (a stick) that I place between the brake pedal and the gas pedal and the engine does start to produce some heat if the rpm's are 1000 or higher.

 

[CATCRACKER]

long stroke high compression more time to cool cyinder walls and also cooled charge air coming in from the turbo. my truck will see 210egt when idling for a while.

 

[EricBu12]

I use the Jacobs brake to put a load on the engine and that makes it warm up faster than high idle

 

[JPLB22]

I usually follow the owners manual guide for idle before shutdown. It says for unloaded freeway to idle for 2 min. before shutdown. I am usually around 400* when I pull in to the driveway. At the end of two minutes I'm at 200*.



My question is..... What is the lowest safe EGT to opperate at? I have an auto transmission, and there is a pretty long and steep canyon close to my home. When coming down this canyon the average speed of folks is about 70 mph. I can coast with no throttle pressure and be doing 70 mph easily. Now when I do this my EGT's really start to drop, close to 150 *, at which point I use a little throttle pressure to bring them back up. But I am ready to start running over folks when I throttle up. So is it O. K. to coast all the way down ?? It doesn't feel O. K. to see EGT's at 150 ????



Jeff

 

[RustyJC]

Low EGT's when running downhill (especially towing) are of no concern. The engine is injecting minimal or no fuel (many gassers shut off fuel on deceleration until 2000 RPM or below), and the unthrottled diesel engine is pumping ambient air through it like heck won't have it. It's normal and doesn't hurt a thing.



Rusty

 

[JPLB22]

So, is it because the VP is injecting so little fuel when coasting down hill, that cylinder wash-down is not to worry about ?



90% of the time I go through said canyon, I am not towing.



Not trying to hi-jack thread, just trying to learn as much as possible .



Jeff

 

[RustyJC]

Yep, washdown is not a problem under these conditions.



Rusty

 

[rbattelle]

One thing no one mentioned was the large size of our cooling reservoir. These trucks have an awful lot of coolant they need to heat up. My wife's Civic heats up within 5 minutes, but it's coolant capacity is probably less than my oil sump capacity!

 

[fkovalski]

Originally posted by RustyJC

Yep, washdown is not a problem under these conditions.

Rusty- is this true? I thought that this is why we are told not to idle our CTD's. I let mine warm up in the AM about 10- 15 mins as we do hit -10 several times in the winter (and recently), but I offset it by religiously changing dino oil every 3K miles. It really needs more time here than to just start and get up to proper oil pressure then drive slowly till normal operating temps.



If what you say is true, then why should we not idle out trucks?

 

[nickleinonen]

have always wondered why Dodge doesn't build the HVAC system on the Cummins trucks with an electric heater coil that would kick in as soon as the grid heaters go off so that you could get heat into the cab relatively quickly and let the engine have more heat so it can warm up quicker?

how about a factory option for an engine pre heater [like an espar or webasto] that new dodge sprinter has that as an option [like $700 canadian pennies] they can be left running when the engine is idling to keep the heat up

 

[bmoeller]

Originally posted by fkovalski

Rusty- is this true? I thought that this is why we are told not to idle our CTD's. If what you say is true, then why should we not idle out trucks?

The conditions that were mentioned above are different than running at idle. At idle, there is fuel being injected to keep the engine running. Going down a steep grade, there is only enough fuel going through the system to lube the pumps, not much more, if any more than that. If you ever hear a Powerstroke truck coming to a stop with the drivers foot off the go pedal, the electronics shut the fuel deliver off 'til the engine speed drops low enough that the engine needs some fuel to keep it from shutting off. Even semi truck engines are almost dead quiet going gown a grade with the foot off the go pedal, whether mechanical or electronically controlled.

 

[Hohn]

Originally posted by fkovalski

Rusty- is this true? I thought that this is why we are told not to idle our CTD's. I let mine warm up in the AM about 10- 15 mins as we do hit -10 several times in the winter (and recently), but I offset it by religiously changing dino oil every 3K miles. It really needs more time here than to just start and get up to proper oil pressure then drive slowly till normal operating temps.



If what you say is true, then why should we not idle out trucks?

I think that the conditions are different. When Idling, the VP44 injects just enough fuel to keep the engine running at idle.



When coasting, the VP44 injects almost NO fuel- thus the really low EGTs.



When idling, you have very little airflow though the engine. But when coasting, you have quite a bit more air.



The combination of the two seems to keep washdown from being an issue.



IMHO, I would only be concerned with washdown if running large injectors that are honed (read:sloppy and smoky). With those, I wouldn't let it idle much more than the time it takes to get through the drive-through at Mac and Don's.



Justin

 

[Hohn]

beat me to it