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-   -   Do Diesel engines generate more Torque than Petrol engines? (https://www.team-bhp.com/forum/technical-stuff/101417-do-diesel-engines-generate-more-torque-than-petrol-engines-10.html)

back to the topic, here's a question:

Everybody "knows" that diesels have more torque. In fact I got flamed months ago on that (to a good extent my fault - I wrote some technical BS). At the time some people wrote that even NA diesels generate more torque than petrols.

Now turbocharged diesels generating more torque I can digest. But NA diesels? I mean they get the same amount of O2 and in theory they can at best have similar efficiency as petrols while generating max. rated torque (at less than max. torque, they may again get away by having a leaner fuel-air ratio than petrol would require, and hence reducing throttling loss) so unless diesel had more calories per unit of O2, there is no way they could generate more torque at the same rpm.

But people claimed their's did.

What could be the reason?

Some more information

AutoZine Technical School - Engine
Detroit Diesel - DD15 Diesel Engine Performance Here you can see that the torque curve starts practically at zero and then tapers off.
Focus - Automotive Engineer
Why aren't you driving a diesel?
Torque & Power

Hi,
Ten page long thread. You were a participant. Revision time? :)

Regards
Sutripta

Thanks dada

http://www.team-bhp.com/forum/techni...ml#post2355899

Looks like I forgot my own discussion - I guess persistence of diesel lovers does make it an accepted (but not real) truth in one's head that diesel's have more torque.



EDIT:

at one point you had mentioned that petrol/diesel have same stoichiometric ratio. However AFAIK diesel has more carbon atoms and less hydrogen atoms per gram of fuel compared to petrol, and hydrogen needs less O2 per atom than does carbon. Then how can petrol and diesel have the same O2 requirement?

Agreed100%. The fuel-air ratio in a diesel engine has be be lower, as diesel is a higher/heavier hydrocarbon.

agree:
Here is quote from my previous link AutoZine Technical School - Engine
"Even without direct injection, diesel inherently delivers superior fuel economy because of leaner mixture of fuel and air. Unlike petrol, it can combust under very lean mixture."

I think I mentioned almost the same! No matter.

Could those with access to a reference library, or those who are good at trawling the internet (and able to separate wheat from chaff) establish the correctness (or otherwise) of that statement.

If incorrect, discussion ends there I guess.
If correct, I'll leave further discussion as to why to more learned men. Beyond my level.

Relevance to immediate question, which is the stoichiometric values for petrol and diesel?

Regards
Sutripta

here is a table of values. delcohacking.net • View topic - Stoichiometric values for alot of different fuels, not sure if it is reliable or not.

Hi,
It is 14.7 and 14.5 are the figures bandied about, so in that it is correct. But a few more 'authoritative' references would seal the deal.

Regards
Sutripta

Well, you did mention almost the same, but nevertheless my question was how come (I wasn't nitpicking)?

As for more authoritative sources here you go: Stoichiometry - Wikipedia, the free encyclopedia

petrol- 14.6, diesel -14.5 and the article references it to Heywood's book, interestingly diesel needs less air (rather than more) than petrol to burn for the same weight of the fuel.

Now my question was again - bloody hell, how come?




So I went in and did some search. Bottomline both are primarily alkanes of the C(n)H(2n+2) variety with petrol having n=8 and diesel having n=16. So one molecule of the major ingredient of diesel is just shy of two molecules of the major ingredient of petrol by two hydrogen atoms. The one molecule of cetane in diesel will then need 16 molecule of O2 for the 16Cs and another 17 atoms of O2 for the 34H atoms. The two molecules on isooctane in petrol will need 16 molecules of O2 + 17 atoms of O2- Overall difference is 1 atom of O2 in 50 - about 2% for roughly the same mass of two fuels.

Once you have too many carbon atoms per molecule, the overall balance is roughly 2 hydrogen atoms per carbon atom in hydrocarbons, no matter how complex a mixture/molecule it is. And this means roughly 3 atoms of O2 are needed per atom of carbon - roughly same amount of O2 per unit weight of the fuel.



Also air-fuel ratio is given as a weight ratio - this means that a hydrocarbon fuel that has a higher percentage of Carbon will end up needing less O2 per gram of fuel - this is because even though per atom of carbon you need more O2 (one C needs 2 O atoms vs. one H needing 0.5 O atom on an average) vs. per atom of hydrogen, 12 grams of C will need about 32 grams of O2, while 12 grams of H2 will need about 96grams of O2 ! - simple because C weighs 12 times as much as H.



Long story short - all liquid hydrocarbon fuels will end up needing similar air-fuel ratio when measured as weight ratio (because their C:H ratio will be very similar), but between two fuels of different C:H ratio, the one that has higher fraction of C will end up needing less O2 per gram of fuel.

^^^
Great.
Note the similarity (calorific value, stoichiometric ratio) between petrol, diesel, and its major adulterants!

Regards
Sutripta

Gone through the 10 pages, but could the experts please enlighten the lesser mortals in simpler terms, on any conclusions?

- Keeping the cubic capacity constant, increasing the stroke does not increase the torque? Because, though the radius increases, the force exerted per unit area gets reduced when the bore is decreased to make the stroke increase?

- Keeping the capacity constant, a petrol and a diesel engine delivers almost similar torque or not?

- If not, is it that the higher compression ratio in a diesel engine leads to better efficiency? By better efficiency does it mean that more force is exerted on the piston per combustion stroke? And thus torque?

- Diesels produce less peak power because they don't rev as high as a petrol engine due to heavier components and longer stroke?

My answers in bold. Hope it helps you.

Torque = Average explosive force on top of piston (aka mean effective pressure) multiplied by the crank throw.

Crank throw is half the stroke. So increasing stroke increases torque. Engines of 20 years back were long stroke engines with plenty of low end torque.

Long stroke engines slowly died down due to need for higher engine speeds. With long stroke the avearge piston speeds increase leading to lubrication problems.

Solution reduce the stroke but increase the torque by increasing the mep (Turbo) or manage through gearing..

10 pages because threads were merged. Which also accounts for its disjointed flow. But your questions you ask were discussed (maybe not answered) in the first part.

Please treat the following as my opinion. I know I am in a minority of one!
There will be no essential change in torque. Increase in stroke > decrease in bore. (less piston surface area). For same gas pressure, moment remains exactly the same.
Incidentally, force per unit area = gas pressure remains the same. It is just that the area decreases.


Assuming NA, petrols generate more torque. Because diesels cannot use all their air. The increased CR is not enough to compensate.


A diesel is far more efficient. Efficiency has to do with input (fuel consumed) - output (power produced). Not with maximum power and torque produced.

True. RPM also limited by the nature of its combustion process.

Regards
Sutripta