Why all the torque from the diesel engine?

[rbattelle]

I couldn't have said it better myself!

 

[Diesel Freak]

Re: diesel has less energy than gasoline

Originally posted by rbattelle

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.

Great Post!



Just be careful about increasing Cut off Ratio... it can send EGT through the roof!

 

[rbattelle]

Thanks, Diesel Freak!

 

[Joseph Donnelly]

A few points should be added to this fray:



Fuel is metered by volume not mass.



The Cummins has a 4. 72" stroke; the 360 gasser has 3. 58".



The diesel injection process results in more fueling at lower rpm.



Diesel burns slower than gasoline so pushes on the piston longer.



Diesels are more efficient than gas engines, less waste heat.



Horsepower is defined as the rate of doing work. Gears multiply torque but not horsepower. A gasser with 500 hp will do the same work as a diesel of 500 hp, if the gearing keeps the engine at peak hp while working.



HP x 5252 = torque x rpm

 

[Diesel Freak]

Originally posted by Joseph Donnelly

Fuel is metered by volume not mass.

this is true, but it is a GFCE (Gross F#$@%!& Conseptual Error) to compare the energy content of fuel on a volumeteric basis... . it is nonspecific!

 

[Hohn]

Originally posted by 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.

Thermal efficiency DOES have an effect on output. But, only IF you keep the fueling level the same. More efficiency, more output. You get better mileage simply because you are using LESS of the potential output than you were before. It's a chicken or the egg kind of thing. Your second post on this is accurate, AFAIK. Think of it like a muscle: a muscle that is capable of lifting 300 pounds is far more efficient at lifting 50 pounds than one that is maxing out at 50. Especially if they are using the same level of "fuel" to lift their respective maxes (300 pounds and 50 pounds).

I am not sure if you are correcting me (take a number since i am wrong a lot!), but I can't see anything i said that was untrue. Joe Donnelly explained the heating values for us (VOLUME, not MASS), so for our purposes, diesel does have a higher heating value- though I was not the person to state so.

As you said, it is the diesel ENGINE that is more efficient, not the diesel FUEL, per se. The Fuel itself DOES have some advantages in that is has larger molecules, on average, in its mixture of hydrocarbons, i. e. longer HC chains. More bonds= more potential energy. Along with these larger molecules, density (or, for rbatelle, specific gravity) increases. In fact, it increases faster than the heating value does, thus you have lower heating values when measured BY MASS. Not true by volume.

As for Joe's comment about gears, it's like I said: HP is composed of the tq and time elements. Gears trade one for the other. That's why you can multiply torque, but not HP.


My Thermo education is no doubt NOT as extensive as some other people's is, so let me ask the question:

What is cut-off ratio? I never heard of that. All this thermo stuff so far I am just extrapolating from my turbine engine theory class. i don't recall ever hearing of a cutoff ratio! I do remember Carnot and all that other jazz.

Leave it to the TDR to teach me something new every day!

Thanks, fellas!

HOHN

 

[Hohn]

By the way, the Carnot formula neglected to mention that the temperatures of the thermal reservoirs must be in absolute scale (Rankin or Kelvin)



HOHN

 

[Diesel Freak]

Cut Off Ratio

hehe... going to make me dig a little arent ya



Cut off Ratio (CR) :a ratio of cylinder volumes.



It can be stated two ways



Cylinder volume at the end of combustion/Cylinder volume at the initial introduction of fuel



the ratio of the cylinder volumes after and before the combustion process in diesel cycle.

 

[rbattelle]

Hohn,



I didn't want to come across that I was "correcting" you, and I don't think anything you said in any post was wrong. . my apologies if I offended you. That's why I made my second post where I corrected my earlier incorrect statement that thermal efficiency has no effect on output (it certainly does!).



Anyway, it's all come together nicely. We see how the diesel has higher efficiency because of it's higher compression ratio and it's higher cutoff ratio (due to larger stroke). This allows it to extract more energy from a fuel that otherwise has less energy content than gasoline.



It would be interesting to see someone build a gasser with a compression ratio of 16 and a cutoff ratio closer to that of a diesel, just to compare it's characteristics to that of a similar diesel.

 

[rbattelle]

And one other thing,



The equation:



HP x 5252 = torque x rpm



is a "rule of thumb" that GM came up with some time ago (or so I thought). That constant 5252 is a rough estimate of a very complex relationship between HP and torque. I thought it was applicable only to spark-ignition engines?

 

[Hohn]

So this is why advanciing timing reduces EGT? I mean you advance timing, piston is further below TDC, thus you have more volume at the point of combustion ignition. That means the denominator gets bigger, ratio is smaller.



I knew the phenomenon, but not the term.



Geeks of te world UNITE!



hehehe



HOHN

 

[Hohn]

let me pre-empt myself by saying that the lower cutoff ratio does NOT decrease EGT. In fact, it would tend to increase EGT were it not for the extra TIME that the carge has to burn.



That'w what was meant when you said that EGT can go through to roof, right? higher cutoff ratio LOWERS EGTs, to a point, and then they go sky high!



Sorry for thinking out loud here-- next tiime I'll keep the monologue to myself.



HOHN

 

[Diesel Freak]

Originally posted by rbattelle

And one other thing,



The equation:



HP x 5252 = torque x rpm



is a "rule of thumb" that GM came up with some time ago (or so I thought). That constant 5252 is a rough estimate of a very complex relationship between HP and torque. I thought it was applicable only to spark-ignition engines?

It is applicable to all engines. I have seen the derivation..... but it is a lengthy one

 

[Diesel Freak]

Originally posted by Hohn

let me pre-empt myself by saying that the lower cutoff ratio does NOT decrease EGT. In fact, it would tend to increase EGT were it not for the extra TIME that the carge has to burn.



That'w what was meant when you said that EGT can go through to roof, right? higher cutoff ratio LOWERS EGTs, to a point, and then they go sky high!



Sorry for thinking out loud here-- next tiime I'll keep the monologue to myself.



HOHN

Higher CR can raise EGT..... worst case would be... when the fuel is still burning at BDC... ... bad juju



Can you say "Overfueled"

 

[Hohn]

I know that higher compression ALWAYS reduces EGT in a gasser, up until the point of detonation (then watch out)



I always thought that higher compression lowered EGT in a diesel as well, because of the higher expansion ratio, and the hotter temp at TDC meant the fuel burned faster. I could be mistaken yet again. This assumes, of course, that fueling is constant (and reasonable).



Yeah, if fuel is still burning @ BDC (or after) then you are SERIOUSLY overfueled.



BTW-- That's what happens on Top Fuel Nitro dragsters. Their exhaust is a flaming spray, that's why they shoot fire 30ft out the pipes. That's also why they are so loud. Talk about over fueled! They are running 1:1 ratios of nitro:air. The use the extra liquid nitro for cooling the engine as it passes through. THAT'S CRAZY!! (but that's also a 6000HP blown Hemi)



Hohn

 

[fbaurley]

From my old thermo book 1 HP =550lbf *ft/s=42. 41 Btu/min. So if you can lift 550 lbs one foot in one second you have developed one horsepower, or can make 42. 41 btus in one minute, but considering a diesel is only about 30 % efficient your talking about 130 btu/min to develop 1 hp so at 250 hp= 32500 btu/min

 

[Diesel Freak]

Originally posted by Hohn



I always thought that higher compression lowered EGT in a diesel as well, because of the higher expansion ratio, and the hotter temp at TDC meant the fuel burned faster. I could be mistaken yet again. This assumes, of course, that fueling is constant (and reasonable).

Hohn, I was refering to Cut Off Ratio... "CR" or should it be "Cr"... . but that would be Chromium...



Crap... both my Thermo texts and Chem textbook are at home...

 

[Big_Daddy_T]

The 5252 is just a conversion of the original 33000 lbs per minute to a circle that way it applies to an engine.

"That 5,252 number, by the way, comes from dividing 33,000 by (2 * pi). Imagine taking 33,000 foot-pounds and walking it around in a circle rather than a straight line. For example, if you took a 10 foot pole and attached it to a vertical axle, the circumference of its circle is:

circumference = 10 * 2 * pi = 62. 8 feet

If one horse is pushing on the pole with 100 pounds of force (1,000 foot-pounds of torque), it can move the pole at 5. 25 RPM. Torque and horsepower are directly related to each other. "

 

[Diesel Freak]

Originally posted by Big_Daddy_T

hate to split hairs but hp doesnt do work. Its just a way of stating the speed the work gets done.

woo hoo we have a winner



Work= Force x Distance... ... ... . FTxLBF



work also has the same units as Energy or Torque



Power= force / time... ... ... ... ... . (FTxLBF)sec



Force= Mass x Acceleration..... (FTxLBF)sec^2



hmmm I see a pattern here

 

[Joseph Donnelly]

If torque relates to the work, horsepower refers to rate of doing it. Multiplying torque with gears allows you to do more work, but slower. If you geared the gasser to be at the rpm where hp peaks, it could pull the same load at the same speed as a diesel at the rpm where the diesel makes its peak hp, an equal hp. The diesel will probably be going 2200 rpm instead of 7000 rpm. Furthermore, the diesel may have a better hp curve at other rpms so when you shift gears, it won't suffer as much as the gasser might. Thus the argument may move to flexibility meaning flatness of the hp curve over rpm.



Now lets quit arguing and dyno at May Madness. Oo.