Diesel over gasoline engines 1

[guimba]

Hello, there are probably a zillion entries to my search for this question so I'll stick my neck out and ask it. I've always wondered what the big deal is with diesel engines and why they are so rugged and preferred for all manner of rough and tough industrial applications over gas engines. Are they not designed in proportion to the additional stresses from the higher combustion ratio? Are there other factors that I'm not aware of that would make it a better choice? Thanks!

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[MikeHalloran]

Diesels are preferred in yachts because the fuel is somewhat less dangerous than gasoline. They are despised in yachts because the soot and slobber gets all over the nice white surfaces.





Mike Halloran
Pembroke Pines, FL, USA

 

[BrianPetersen]

They are indeed designed more ruggedly due to the higher compression ratio. The nature of the diesel combustion process lends itself to low-RPM high-torque applications, like pulling something heavy. Diesels like running steadily at a constant speed (gasoline engines have poor efficiency at high speed and low torque output), but they can immediately take up the load when it is demanded (turbines and the like are not happy about that, gasoline engines if allowed to run at low revs when unloaded first have to gear down to increase revs, etc). But the main reason that diesels are preferred in those applications, is that the fuel consumption is lower, particularly when running lightly loaded, which happens in a lot of industrial/commercial applications. The lighter weight of a gasoline engine of equal horsepower is of no advantage if you are building a bulldozer or earth mover or locomotive, which needs to be heavy anyhow in order to do its job.

 

[JSteve2]

I have a few additions or modification to what has already been suggested.
One, it is more efficient in the global oil cycle to have a significant fraction of the fuel oil go to each diesel and gasoline due to the natural fractions of hydrocarbons in crude oil. Thus, cost competition between diesel and gasoline drives marginal applications one way or the other over time.
Two, the nature of the diesel engine makes it generally more expensive, especially when parts are added for responsiveness (e.g. turbocharger and robust transmission). Combined with the improved fuel economy, you have a high capital cost and low operating cost situation that naturally drives commercial applications to diesel.
Three - to add to what Brian said, diesels have a narrow range of operating speeds and gasoline engines have a wide range of operating speeds (I differ with Brian, because both engine types "like running steadily at a constant speed"). Again, professional drivers or expensive transmissions have no issue with a narrow engine speed range, but most consumers don't want to deal with more than a few gear shifts (and often pay extra for zero).
Four - again enhancing what Brian said - gasoline engines run stoichiometric, which is a severe limitation on available torque. On the high end, you need engine speed to generate power. On the low end, you need to fuel significantly (except when motoring or in a hybrid). With a diesel, you can select your torque on virtually every engine cycle, with (practically) the only limitation being available air.

 

[SomptingGuy]

Additional points...

While it's not quite so true these days, diesels never used to suffer from electrical gremlins.

Driving on the governor is fun - getting it into top gear using only the left foot.

And they smell nice.

- Steve

 

[moon161]

Thermal efficiency is a reason they're used, turbodiesel cars are much more common in europe, cleaned up at lemans last year as well. At the extreme, I think the most efficient engines are the monster diesels that power ships. Being able to run on bunker fuel helps too.

 

[GregLocock]

They cleaned up at lemans because the rules were changed to improve their chances. You could argue the same about ECE.

Cheers

Greg Locock


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[Warpspeed]

And the calorific rating of the fuel is higher as well as it being potentially cheaper to produce from crude oil.
The main advantage with diesel is with the fuel, not with the engine.
It is the same reason kerosene is burned in gas turbines instead of gasoline. More heat per gallon.

 

[JSteve2]

Warpspeed,
There is more energy per unit volume in diesel, but less energy per unit mass. That sort of wipes out the airplane theory. And the difference is modest (maybe 10% net specific energy per volume). The bulk of the fuel efficiency difference really is the combustion mechanism.

See my earlier post about being "potentially cheaper to produce from crude oil." This is true only if you are talking about the naturally occurring fractions of the oil. However, if you try to make more diesel than the naturally occurring fraction of that type of hydrocarbon, you are having to add energy to get diesel relative to gasoline. It's cheaper and more environmentally friendly to break diesel down into gasoline than to synthesize gasoline into diesel.

 

[moon161]

"While it's not quite so true these days, diesels never used to suffer from electrical gremlins.

Driving on the governor is fun - getting it into top gear using only the left foot.

And they smell nice. "


It is fun to have a car that will run with a dead or missing battery.

 

[Warpspeed]

It is fun to have a car that will run with a dead or missing battery.

Not if you have to start your diesel with a crank handle.

 

[patprimmer]

I have a spark ignition car that runs but has no battery. Once upon a time that was all that was available.

Regards
Pat
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[patprimmer]

I am also pretty sure Sprint Cars mostly have SBC engines, methanol fuel and no battery.

Regards
Pat
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[SomptingGuy]

But the electrical systems of a spark-ignited engine always used to be the weak link, compared with the mechanical pumps in previous generation diesels.


- Steve