A question of horse power

[cchoate]

A simple question and probably a not-so-simple answer:

Why is diesel engine much stronger than a gas engine with equal horse power?

 

[RoyJackson]

Boy, this is a tough question...

First, let us assume normally aspirated engines only (no turboes, no superchargers).

For a given displacement, diesels put out less horsepower then a gasoline engine. One reason for this is diesels operate at lower RPM then gasoline engines due to the higher stresses inherent in a compression engine versus an ignition engine (also, this is the reason diesel are built so much more robust then an equivalent gasoline engine). We could get into reciprocating masses, but I'm not competent to get into that (and the math required).
Diesels are also used more in commercial applications in which fuel consumption is more critical (i.e. costs).

However, torque, a more usable output of internal combustion engines, peaks at a lower RPM then an equivalent gas engine. Diesels also tend to be more of a constant RPM engine (higher efficiency), rather then varying up and down the RPM range as the way gas engines are used.

Bottom line, diesels are more robust due to being built to withstand higher compressions.

By the way, this is somewhat generalized. I'm sure someone can find instances in which diesels have a higher power output then a gasoline engine of the same displacement. But those would be the exception, rather then the rule.

 

[Hawkflyer]

It really isn't. Horsepower is horsepower weather it is generated by gas, diesel, or steam. The differance is torque and the relationship between horsepower and torque for a particular engine. If you look at a graph of torque vs horsepower for engines using different fuels you can see how these power measurements differ for each type of engine. A diesel has a broader power band than a gas engine (it generates it's power starting at lower rpm than gas).Torque is really what we are interested in when we look at engines for applications requiring low and moderate RPM power. On the graph I mentioned above, once the horsepower and torque lines cross, very little additional usefull work can be gotten out of the engine by running it faster. However this crossing point usually occurs at some point below where the engine is actually generating it maximum horsepower. The torque curve on a diesel crosses the horsepower curve at a point very close to the engines maximum horsepower curve. On a gas engine this crossing occurs well before the maximum horsepower is reached.On two stroke engines this situation is very noticable, and as a result they produce usefull horsepower in a very narrow RPM range.Remember also that diesel engines have a much higher compression ratio. This has a direct relationship to how the power is generated. Usually longer stroke engines can generate more torque. This is why the 300 CID Ford in line 6 was so popular in pickup trucks for so long. The same 300 CID in a v8 did not generate the same torque and hense did not provide low down, lugging pulling power. Why same displacement shorter stroke length.

 

[dgood1]

I am not an expert, but am willing to add my .02 in this thread.

As previously mentioned, diesels have more mass. The pistons, connecting rods and crankshafts are much heavier and therefore have more mass. If you think about the effect of a bus hitting another bus at 30 mph and the amount that the bus would be moved as compared to the how much it would be moved by a compact car hitting the same bus at 50 mph. You will see that a large moving mass has a great deal of kinnetic energy.

The other thing to consider is that diesel has many more BTU's per gal. than gasoline. More BTU's = more energy. The 2 fuels also burn at much different rates. The gasoline explodes and thereby burns itself out much faster than diesel. The diesel burns through more of the power stroke.

Don

 

[centex]

Isn't a diesel engine a two-cycle engine? It fires every cycle vs every other cycle for four-cycle engines. A two cycle engine has more power than the same size four cycle engine thank goodness or weedwackers and chain saws would require Paul Bunyon to lift them.

 

[s1120]

<font color=red>Isn't a diesel engine a two-cycle engine</font color=red>

Some are, but most are not. A lot of the big prime movers in trains are two stroke, now they make a LOT of power. I don't think they will fit in a tractor though./w3tcompact/icons/smile.gif

 

[MJB]

The vast majority of diesel engines are four stroke. The big exception for mass produced diesels are the old Detroit diesels, aka "Screamin' Jimmys". These were all supercharged two strokes. The Detroits produced alot of power for their size and were very reliable, but exhaust emmissions were not one of their strong points. You could always tell if one was around based on the high pitched whine from the supercharger.

 

[centex]

Good thing they didn't have me designing engines or I would have gotten them all screwed up!

 

[Anonymous Poster]

Explodes is not a good word, I think you mean combustion. There is 3 phases of combustion 1. Neucleus 2.Hatching out "flame front" 3. Propigation . Its the heating and expansion of air that drives the piston down. John

 

[dgood1]

John,

"Its the heating and expansion of air that drives the piston down. "

Something like the expansion of gases when dynomite or gunpowder combusts?

Don

 

[Hawkflyer]

Same idea only slower./w3tcompact/icons/smile.gif

 

[Anonymous Poster]

Hawkflyer is correct its much slower and also controlled, I don't know how you determine when it becomes a explosion off hand but I would guess that it has to do with how violent the expansion/reaction is? Engine theory text books do not use the word explode in the description of the combustion process. I still remember the GM training I had many many years ago on engine theory, the first day they set you straight on this topic. John

 

[dgood1]

Ok! I'll agree that explosion was probably the wrong term to use and I have no idea of what the threshold is for the difference between explosion and combustion. I'll have to remember that gasoline and diesel only combust.

Don

 

[BigDave]

A few years ago, I asked a Kubota salesman about the usefulness of 16hp in the B7400, he told me that diesel horsepower was different than gas horsepower. I still try to hope that he was trying to refer to the increased torque that diesel engines produce for the same amount of horspower, but I doubt it. That statement still gives me a chuckle every now and then.

 

[RoyJackson]

"I asked a Kubota salesman about the usefulness of 16hp in the B7400, he told me that diesel horsepower was different than gas horsepower."


Actually, he was right in a way...

There's gross HP, net HP, JIS HP (Japanese standard), Din HP (German standard used throughout Europe) all of which will give you different results for the same engine.

And, of course, we also now use Kw to measure engine output.

 

[BigDave]

I seriously doubt it.

The 16 gross HP as Kubota states in thier catalog is the larger of net & gross HP used in the U.S. by major manufacturers. I recently downloaded a European Shibaura catalog that states power in KW, SAE HP & DIN HP. Maybe European salesmen just pick thier favorite unit of measure, or maybe some of them make-up thier own units of measure like the previously mentioned Kubota salesman.

I'm not even remotely trying to pick on Kubota, but "dummer than dirt" comes to mind when I think about this particular salesman. He could have been working anywhere, well hopefully not NASA.