Why all the torque from the diesel engine?

[snowracer69]

Hey all,



I know this has probably been asked 100 times, but I couldn't find it in the search, and a friend has me baffled with this question...



Why does the diesel engine, with comparable HP levels to a gas engine, produce so much more torque?



I know I should know this, but it's just slipping my mind right now. Moderators, please leave this in 2nd gen forum so it can be seen, until I get an answer... please? Thanks.



Josh

 

[rspinks]

HP is a number defined by torque x RPM. Diesels generate more torque but at lower RPM so the HP number can be identical between say a Magnum 360 and Cummins HO at 245hp. In the real world though, they are obviously very different engines in terms of pulling, longevity, mileage, etc. Like they say HP sells vehicles but torque wins races.

 

[illflem]

Try this http://www.howstuffworks.com/question381.htm



Sorry, moved it already- now it will be in two forums so you will get twice as many replies.

 

[Hohn]

I love that website!! I have spent hours reading lots of stuff on there. . !



Hohn

(the geek)

 

[illflem]

Noticed that that How stuff works topic had a link to TDR at the end.

 

[gocirino]

Most of the torque from the diesel engine comes from the fuel burning much slower and helping push the piston downward while on the downstroke. On a gas engine the fuel ignites much quicker at or before top dead center and does'nt contribute much to torque production and relies more on piston stroke and RPM.

 

[CRSmith]

The compression ratio is also about 2-3 times that of a gas motor. I'm sure someone will jump in with the cummins compression ratio, but modern gas motors are in the neighborhood of 8:1. The way I understand it is more compression = more downward force on the piston = more torque on the crankshaft.

 

[Hohn]

Welcome, Copter

Looks like you are new to the TDR! Welcome aboard!



SO if we want to get more in depth with why a diesel make more torque for a given HP, here we go:



First is the RPM factor. HP is time derivative of torque, so the more often you can make a given amount of torque (rpm) the more HP you have. Diesel make more torque less often (lower rpm). Gassers make less torque more often (higher rpm). They differ in which they rely upon more to make their HP (since time and torque are the only 2 ingredients in HP)



The slower burning of diesel has a lot to do with it too. It's helpful to think in terms of "average" pressure on the piston over its stroke. The gasser burns fast and early, and applies most of its work to the crankshaft when their is little leverage to help (i. e. closer to TDC). The diesel spreads the force out over the travel of the piston, creating less peak load (proportionally) on the piston, and applying work to the crankshaft closer to the optimum of 90° angle (note: this is NOT the halfway point of the piston travel, due to the engine geometry-- piston is always "below" the halfway point at 90° crankshaft position).



The difference is like the contrast between when a gasser detonates versus when it runs normally. Detonation "slaps" the piston down instead of pushing it down. A diesel "pushes" is down even smoother, so that tends to be a more efficient transfer of energy from the combutions pressure to the mechanical energy turning the crankshaft.





Hohn

 

[Hohn]

close, but not quite

Originally posted by reduaram

The compression ratio is also about 2-3 times that of a gas motor. I'm sure someone will jump in with the cummins compression ratio, but modern gas motors are in the neighborhood of 8:1. The way I understand it is more compression = more downward force on the piston = more torque on the crankshaft.

Most gassers are a little higher than 8:1, since technology now allows 9-10:1 on crappy gas. New 'vettes are 11:1 on pump 93.



More compression doesn't really mean more torque. Why? Because all the torque you GAIN from the higher pressure on the piston's way down, you LOSE on the piston's way up when it has to fight all the compression to get to the top.



Higher compression DOES increases efficiency, and THAT'S where your extra power comes from. Efficiency is basically that difference between the hottest temp and the coldest. The bigger the difference, the more efficiency. Ideally, you would have VERY hot combustion and VERY cool exhaust.



Higher compression keeps EGT about the same, but has hotter peak combustion temp, thus the greater difference in temps, thus the greater efficiency, thus the greater engine output.



Kinda complicated.



Hohn

 

[Joe G.]

Another thing that contributes to the high torque is that diesel fuel has more heat energy (it's heavier). Thus more power from the same amount of fuel. This results in better fuel milage at the same power output. Pretty nice. Hot rods that get 20 MPG.

 

[RustyJC]

It doesn't hurt that most diesels are turbocharged and most gassers aren't. Compare the 5. 9L Cummins to the 5. 9L Dodge V8 gas engine. 20 PSIG boost can help torque a bunch.



Rusty

 

[Dennis Garrett]

Gasoline explodes quickly in the cylinder, but by the time the piston is about half way down it's energy is about gone. So, they usually have a shorter stroke. In the diesel, the fuel burns slower, and is still pushing on the piston through out it's travel, so they can use the longer piston travel more effectively. The longer the stroke, the bigger the diameter of the crank throws, & more twisting effect on driven parts. Think of ridding your bicycle up a hill with little short cranks, compared with ridding the same bike with long cranks. The longer the cranks, the more twisting effect or torque on the sprocket.

 

[rbattelle]

Actually, diesel fuel has less energy content than gasoline. Check out the lower heating values, it's true!



Beware Hohn's second post is talking about thermal efficiency, which has no effect on power or torque... only on fuel economy. The ideal efficiency (or Carnot efficiency) of any cycle operating between two thermal reservoirs is 1 minus the ratio of cold-to-hot temperature. It is the absolute maximum achievable thermal efficiency of a perfectly reversible cycle. A common metric for determining how thermally efficient your cycle is, is to compare it's real thermal efficiency with that of it's Carnot efficiency.



As Hohn points out, if we can cool the cold reservoir or heat the hot reservoir the Carnot efficiency will increase, thereby allowing a well-designed engine to operate more efficiently and use less fuel for the same power output.



God I love these technical threads.

 

[Big_Daddy_T]

I thought because diesel has more btu's that meant it stored more energy per gallon. I remember reading it had about 5 to 10 percent more btu.

 

[illflem]

diesel has more energy

On average, a gallon of diesel fuel contains approximately 147,000 Btus, while a gallon of gasoline contains 125,000 Btus ...

 

[rbattelle]

diesel has less energy than gasoline

You guys are comparing energy content on a volumetric basis, which has no real meaning. If we examine the lower heating value of medium (#2) diesel fuel and that of gasoline we find:



Gasoline: 19,020 Btu/lbm

#2 Diesel: 18,000 Btu/lbm



Now, on a volume basis diesel is more dense than gasoline, so comparing energy content by volume has little meaning (1 gallon of diesel contains more mass than 1 gallon of gasoline). The only correct way to do it is to compare on a mass basis: gasoline has more energy than diesel.



About one of my earlier comments: that thermal efficiency has no effect on power output. I'm no longer sure that is 100% true. There is another way to look at thermal efficiency: in it's most basic form it's expressed as a ratio of work output to heat input. Then, to increase thermal efficiency we must either decrease heat input (increase energy content of the fuel) holding work output constant, or increase work output holding heat input constant. What we're really talking about here is the engine's real ability to extract energy from the fuel: for two identical engines running on identical masses of fuel, the one that develops more power is said to be more thermally efficient. So how do you increase power output for the same mass of fuel? One way is through compression ratio.



Based on the cold-air standard analysis, we find that for the same compression ratio, thermal efficiency of the air-standard Diesel cycle is less than that of the air-standard Otto (gasoline) cycle. The cold-air standard analysis tells us that in order to get higher thermal efficiency, we must increase compression ratio and/or cutoff ratio.

 

[illflem]

If you want to look at that way gasoline and diesel are equal as far as energy goes http://www.prime-mover.org/Engines/GArticles/bsfc.html



''Diesel fuel weighs more than does gasoline per volume but its energy density is approximately the same. That makes it possible to compare the relative efficiencies of gasoline vs. diesel engines simply by comparing their BSFC numbers. ''

 

[rbattelle]

That article doesn't say anything about diesel having more energy content than gasoline. The diesel will tend to have a lower BSFC because it's more efficient (primarily due to higher compression ratio) than it's gasoline counterparts. What that article is saying is that although there is less available energy in diesel, the higher efficiency of the engine allows proportionally more energy to be extracted as compared with the amount extracted by a spark-ignition engine running on gasoline. BSFC comparisons should never be used to compare fuel energy content, except under very specific conditions (identical engine, compression ratio, ignition timing, load, RPM, etc. ).



It comes down to chemistry, really. Chemistry tells us that if you burn one pound of #2 diesel completely, you can expect to get 18000 Btu from it. If you burn one pound of gasoline completely, you can expect to get 19020 Btu from it.

 

[fbaurley]

Rbatelle,is right on the money with the heating values of the fuels. alcohol has even less heating value, than diesel. The performance of an engine is directly related to the compression ratio. If you you could design a gasoline engine that could run at 16:1 with out knocking itself apart it. would be more efficient than a diesel at 16:1.

 

[fbaurley]

Another thing with the efficiency of a diesel, on an equal cubic inch comparison to gas basis is that in a diesell you are only compressing the air, therefore it is possible to get more air through your engine. in a gas you are compresssing both air and fuel. This is why a gas engine knocks or predetonates when the compression is to high, becaise it is "dieseling" This is why the modern gas engine technology is aiming towards direct injection gasoline engines, where the fuel is injected in like a diesel, right before the spark fires. It is not drawn through the intake valves such as on a carburated, or multiport, or throttle body fuel injection. This lets them bump up the compression more which = more efficiency. If it were possible to design a diesel that ran on gasoline, it would be a very efficient engine, but because gasoline burns so fast it doesn't work. Therefore diesel is used, because it burns at a slower more controlled rate.