General Diesel Question

[Noface]

I've got a friendly debate going on in another group about whether or not diesel engines run hotter than gasoline ones. (It all started from an oil thread doh!)

It's not really a debate, but none of us are particularly familiar with diesels (although I love my Duramax). Mostly relying on heresay.

So, do diesels burn hotter than gasoline engines? And if yes/no why?

Thanks a bunch!

Jody S.

[mackin]

Originally posted by Noface:
I've got a friendly debate going on in another group about whether or not diesel engines run hotter than gasoline ones. (It all started from an oil thread doh!)

It's not really a debate, but none of us are particularly familiar with diesels (although I love my Duramax). Mostly relying on heresay.

So, do diesels burn hotter than gasoline engines? And if yes/no why?

Thanks a bunch!

Jody S. Hotter combustion ..... Engine temp (coolant) is same as gasser .....

Mac

[Idle_Chatter]

I dunno about the hotter combustion, Mack. The diesel fuel has a higher ignition temperature, but compression ignition makes the process more efficient, which significantly lowers exhaust temps over a gasser. So it's really a matter of "hotter where?" Higher ignition temperature in the cylinder, but bigger and more massive heads and blocks for pressures that run cooler due to more heat mass. Cooler exhaust gases and headers/pipes. I think the oil burners run cooler than gassers overall! ;)

[TBC]

Exactly what is meant by hotter is not defined. My Duramax runs the same coolant temp as the 454 and 350 gassers I have previously owned. That is determined by the temp gauge on the dash. Based on the coolant temp indications they are the same.

Tom

[hoot]

A diesel is more efficient and gives off less heat. Heat is wasted energy not necessarily converted into work.

Start your engine and watch your egt gauge. As the engine warms up, egt's go down (idle). Why do you think diesels need help in the winter to warm up?

Gas engine egt's can get much hotter than diesel.

EGT is combustion temp.

[DMAX_Phil]

Hmm, running hotter could mean a couple of different things. My diesel certainly doesn't run any hotter than my '98 K1500 w/ 350, using the water temp. as the reference. However, the diesel combustion temperatures should be higher than a gas combustion cycle for two reasons: 1) the higher energy density of diesel vs. gas; and 2) the higher compression in the combustion process. Even though diesel is a more efficient combustion (then gas) I'd still think the higher compression (air gets much hotter the more you compress it). Interesting question...

I have not heard/read of concerns (similar to us diesel owners) over high EGT temps. on modified gassers when towing, or any other time... I know I never worried about this with my modified gasser.


Here is some information from "howstuffworks."

"Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.


The main differences between the gasoline engine and the diesel engine are:

* A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously.
* A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency."


Phil B.

[ 06-11-2003, 10:32 PM: Message edited by: DMAX_Phil ]

[Noface]

This is good stuff. I knew I could count on you guys.

Most of this makes sense.

So what I gather so far, although diesel has a higher flash point or burn rate, the efficency of the engine is greater, and the energy it takes to make power is less than that of a gasser, which results with a cooler exhaust temp.

The thing that started our conversation's on oil was the difference between automotive and diesel oils. Specifically, Harley Davidson air cooled engines. HD recommends if you can't buy their oil, use a diesel oil instead.

Someone mentioned diesels run hotter, so diesel oil would be better suited for an air cooled Harley engine. I stated diesels don't run hotter than gassers. Which from my experience, and from what others have told me, they don't. In fact, my Duramax takes forever to get warm in the winter time, and I've never seen more than 200-205 on the temp gage in the summer.

My "theory" on why HD would recommend diesel oil for their engines is because of diesel oils ability to deal with contaminates or ignition bi-products, and shear strength properties of the oils.

Is that anywhere in the ballpark?

Thanks,
Jody S.

[mackin]

Well of course I was talking about MY truck ....


I thought with a lower flash point of diesel compiled with a higher compression ratio and higher BTU rating would surely create more heat somewhere ....

My tailpipe sure gets warm ...... :eek:


Mac tongue.gif

[chuntag95]

Jody, I think you are on the right track with the diesel oil properties for contaminates. The addative package is different for a diesel and I expect that is what they want. Send a note to HD and ask them. I'm sure someone would tell you why.

[jbplock]

Jody,

This would be a good question for the "Ask Dr. Lee" forum.