[Born2Slow]

Originally Posted by sgiitk

While diesels do have a higher torque, due to higher gearing the gain on the road may not be as high as it may appear at first sight.

+1
And if it is short geared, it will loose out on high speed torque. But diesels can afford short gearing because of their higher efficiency. Any idea why a diesel mostly is short geared compared to similar petrols(Taking into account their max power rpms). Is it to compensate for the turbo lag ?

[motorpsycho]

Diesels need to have a heavier crank to compensate for the higher Compression ratio. Which results in better torque but lower acceleration.

Also the energy generated by the fuels.
Quoting ACEA

The calorific value of diesel fuel is roughly 45.5 MJ/kg (megajoules per kilogram), slightly lower than petrol which is 45.8 MJ/kg. However, diesel fuel is denser than petrol and contains about 15% more energy by volume (roughly 36.9 MJ/litre compared to 33.7 MJ/litre). Accounting for the difference in energy density, the overall efficiency of the diesel engine is still some 20% greater than the petrol engine, despite the diesel engine also being heavier.

Originally Posted by Born2Slow

+1
And if it is short geared, it will loose out on high speed torque. But diesels can afford short gearing because of their higher efficiency. Any idea why a diesel mostly is short geared compared to similar petrols(Taking into account their max power rpms). Is it to compensate for the turbo lag ?

Since diesel develop better torque( Flat torque band?) it would make it suited for tall gearing as the torque is available at all RPMS.
Diesels have been low speed engines and don't rev as high as petrol engines ( I could be wrong here) this may translate lower top speed.

Cheers

[SPIKE ARRESTOR]

Please exclude gearing out of the equation, why not discuss only "Petrol Engine Vs Diesel Engine" ?

Spike

[AVR]

Is it also always the case that Diesels produce max torque at much lower rpms than Petrol? IIRC, that's what I have seen on engine specifications of many cars. Why?

[greenhorn]

Originally Posted by ex670c

4) What if the displacement of both the engines are similar/same?

To add what I know

Indica Diesel 1.4
1405 cc 4 speed, 53.5 bhp, 8.7 kg-m

Indica Petrol 1.4
1405 cc 60 bhp 4 speed carb, 10.7 kg-m torque
1405cc 75bhp 16 bit mpfi (hitachi) 11.2kgm
1405cc 69bhp 32 bit mpfi (Bosch - Indica) 12.6kgm

a bit more to add ( the DOHC from the indigo XL and the indigo engine which was in a different tune)

but these are essentially the same engine block. I didnt add the turbo/dicor figures since i was not sure you could compare a blown and NA engine

[Sutripta]

Originally Posted by ex670c

Please explain IMEP & BMEP.

BMEP:- Calculated from the brake dynamometer.
IMEP:- Calculated from the indicator card diagrams.

BMEP + Losses (friction, pumping. Converted to pressure) = IMEP.


So what's the consensus conclusion? Do diesels generate more, less, or equal torque compared to SI?

Will this hold if instead of comparing for equal displacement, we use equal power? Or equal energy (calorific value of fuel used) input?

Regards
Sutripta

[MileCruncher]

I'll try to explain "Why Diesel Engines generate more torque than Petrol Engine" using basic Physics. This is going to be a long post so please be forewarned

Lets understand the characteristics of the concerned fuels first!

Contrary to popular belief, both Petrol and Diesel have similar calorific values. Actually Diesel has a lower calorific value at 45.5MJ/Kg compared to 45.8MJ/Kg of Petrol. But diesel has higher density of 0.832 Kg/L compared to 0.740Kg/L of petrol. Furthermore boiling point of Petrol is from 40 - 205 Deg Celsius whereas for Diesel its 250-350 Deg Celsius
Ref: (ACEA - European Automobile Manufacturers' Association )

So now we know why diesel fuel has more energy packed in its 1 litre compared to petrol.

As most of us know according to Law of Conservation of Energy, energy can neither be created or destroyed. It just changes form. Like from Kinetic to Potential to Rotational etc etc.

As Diesel has a very high combustion point, lot of work is done to get the diesel to burn. This work gets converted to along with the higher calorific value to provide higher amount of energy which is nothing but the higher torque at the flywheels.

Hope its clear. Please feel free to ask any further questions. I have kept the loses out of the equation to keep the explanation simple

[vina]

Everyone

I think the question in the thread is "do diesel engines produce more peak torque than petrol engines".

Let me ask a different question : why do diesels (especially turbodiesels) start losing their torque at lower rpm numbers than do petrol engines?


This question is related to another thread ( http://www.team-bhp.com/forum/techni...on-engine.html ) but since we are doing it here already, we don't want to turn that one into a diesel vs. petrol thread.

[Born2Slow]

Originally Posted by MileCruncher

I'll try to explain "Why Diesel Engines generate more torque than Petrol Engine" using basic Physics. This is going to be a long post so please be forewarned

Lets understand the characteristics of the concerned fuels first!

Contrary to popular belief, both Petrol and Diesel have similar calorific values. Actually Diesel has a lower calorific value at 45.5MJ/Kg compared to 45.8MJ/Kg of Petrol. But diesel has higher density of 0.832 Kg/L compared to 0.740Kg/L of petrol. Furthermore boiling point of Petrol is from 40 - 205 Deg Celsius whereas for Diesel its 250-350 Deg Celsius
Ref: (ACEA - European Automobile Manufacturers' Association )

So now we know why diesel fuel has more energy packed in its 1 litre compared to petrol.

As most of us know according to Law of Conservation of Energy, energy can neither be created or destroyed. It just changes form. Like from Kinetic to Potential to Rotational etc etc.

As Diesel has a very high combustion point, lot of work is done to get the diesel to burn. This work gets converted to along with the higher calorific value to provide higher amount of energy which is nothing but the higher torque at the flywheels.

Hope its clear. Please feel free to ask any further questions. I have kept the loses out of the equation to keep the explanation simple

Seriously friend you really need to get your basics right. I can vouch for it that most of what you have written does not obey the laws of physics.

Originally Posted by vina

Everyone

I think the question in the thread is "do diesel engines produce more peak torque than petrol engines".

Let me ask a different question : why do diesels (especially turbodiesels) start losing their torque at lower rpm numbers than do petrol engines?


This question is related to another thread ( http://www.team-bhp.com/forum/techni...on-engine.html ) but since we are doing it here already, we don't want to turn that one into a diesel vs. petrol thread.

I thought it was widely known, due to turbo lag I guess.

[MileCruncher]

Originally Posted by Born2Slow

Seriously friend you really need to get your basics right. I can vouch for it that most of what you have written does not obey the laws of physics.

Please let me what doesn't obey the law of Physics? I would be more than willing to learn.

[oxyzen]

Too many Questions:
I will answer one by one.

Originally Posted by AVR

How do some engines manage to generate more torque at much lower rpms than others?

I dont have much experience with diesels so starting with Petrol.
In case you wnat to increase low RPM torque the most fundamental part is the intake manifold. Changing the runner length runner diameter and plenum volume it is possible to adjust the max torque at some RPM. How it is achieved is a phenomenon of resonance. The air coming in the combustion chamber has pulsating effect. You need to match this frequency with that of the manifold at the RPM where you want more torque.
The next thing is engine caclibration. That includes air fuel ratio and ignition timing. Ignition timings can be advanced a bit more at low RPM to get more torque but at the expence of slight knocking. (happens only at full throttle though.)
VVT can drastically change torque at any RPM. It basically can optimise the intake valve opening at every RPM. Initialy engines like Kappa 1.2 and 1.2K series engines have their camshaft timing in such a wat that max torque is obtained at higher RPM thus producing more BHP and that directly is a Marketing tool. Drivability be damned. Now with VVT they can optimise the timing in all RPMs.
And of course Turbos will definately increase low end torque. It also varies according to boost etc.

[SPIKE ARRESTOR]

Originally Posted by oxyzen

In case you wnat to increase low RPM torque the most fundamental part is the intake manifold. Changing the runner length runner diameter and plenum volume it is possible to adjust the max torque at some RPM. How it is achieved is a phenomenon of resonance.

Could you explain more elaborately how this is done? How the intake manifold pressure is fine tuned?

Initialy engines like Kappa 1.2 and 1.2K series engines have their camshaft timing in such a wat that max torque is obtained at higher RPM thus producing more BHP and that directly is a Marketing tool. Drivability be damned

Gurgaon, huh, I know where you are coming from, can understand your situation. Same here!

Spike

[vina]

Originally Posted by Born2Slow

I thought it was widely known, due to turbo lag I guess.

Well, I thought turbo lag happens at lower rpms. My question is why does diesel lose torque at lower rpm number (I guess I confused you) i.e. why do diesels redline at lower rpms.

e.g. most passenger diesels redline before 5000rpm, while most passenger petrols redline after 5500 on Indian cars.


Also even NA diesels apparently have their redline rpm lower than petrols.

[oxyzen]

Originally Posted by SPIKE ARRESTOR

Could you explain more elaborately how this is done? How the intake manifold pressure is fine tuned?

I have tried to explain as much as possible from a lehman;s point. Anything more is beyond the scope of this forum. No point confusing people here.
In case you want to know more, kindly visit
Intake Manifold Tech: Runner Size Calculations - Team Integra Forums - Team Integra

Awesome explanations.

And by the way intake manifold pressure is not fine tuned. It is the torque that is optimised by changing the dimensions. May or may not have direct effects on manifold pressure.

[bzr77k]

Doesn't "more torque" in the question refer to more "overall torque" and not just the peak torque?

So, looking at the overall "area under the curve" of a torque curve: do diesel engine torque curves have more "area under the curve" than the petrol engines? (Given all other points equal, displacement, stroke, NA etc...)

Mostly because of the higher "energy content" of diesel as compared to petrol. Right?

On the below point, I still don't understand the explanation!! Am i stil missing something???
Higher compression ratio of diesel engine leading to a larger and heavier Flywheel.
- The Flywheel acts as a "capacitor" (in electrical terminology) which "filters" the torque. This spreads the torque more evenly but does not result in increase in "overall torque"