Team-BHP - Do Diesel engines generate more Torque than Petrol engines?

Thanks Arka. Interesting topic.

To add to Arka's questions:
5) How do some engines manage to generate more torque at much lower rpms than others?
6) Flat torque curves are better but how much % should be available at idle and slightly above idle
7) How much is good enough for a good offroader e.g. for a SWB, a MWB and a LWB with say 1200 kg, 1600 kg and 2000 kg kerb weights respectively? Infinite torque is obviously not an answer as I understand that torque can only be put to good use if there's enough weight keeping the vehicle on the ground.

Ballpark numbers should be a good start too.

Looking forward to this!

Cheers,
Adi

I'm no auto guru but i'll try to explain in layman terms

Quote:

Originally Posted by ex670c (Post 2355342)

1) What is torque(Engine)?

Torque is the pulling power of any vehicle. It is the grunt which is responsible for acceleration. The more torque in the engine, the faster you accelerate.

2) Do Diesel Engines generate more torque that petrol engine?
Yes

3) If so then why?

Diesel engines have high compression values inside the cylinders when compared to a petrol engine. You may refer to this link Why do diesel engines deliver more torque?

I'll give you an easier explanation. Let us compare a diesel engine to an elephant and a petrol one to a cheetah. We all know who is the fastest. But, have you ever seen a cheetah running fast with a load :D. Elephants (diesel engines) can accelerate pretty fast even when there is load put on them.
I'm sorry for the lame and somewhat funny explanation.

Hi, based on my understanding of engines, am taking a shot at answering your questions.

Quote:

Originally Posted by ex670c

Hi Guys,

I was meaning to start this discussion for a sometime.

1) What is torque(Engine)?

Torque is the turning moment that the engine produces. Torque is responsible for pulling power/acceleration. While BHP determines the max. speed, Torque determines the time taken to reach that (there are other factors as well determining that, for instance Gear-Ratios, but I am assuming them constant for the moment, and hence not a factor affecting acceleration). Also, since Torque is the turning moment that the engine is producing, and not the max. power, the torque vs rpm curves are much flatter compared to bhp Vs rpm curves.

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2) Do Diesel Engines generate more torque that petrol engine?

Yes, for a given displacement, diesel engines would produce more torque than petrol engines, but that's because of High compression ratio. As a rule of thumb, higher the compression ratio, higher is the torque, lower the max. rpm and hence, lower the max. power. Roughly speaking, bore being same, torque is directly proportional to stroke. As a further example, F1 engines have very small stroke, (reqd. for higher rpms,) low torque (relatively), and hence reliance on gearboxes to provide acceleration.

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3) If so then why?

Answered above

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

Again, answered above.

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Originally Posted by AVR (Post 2355422)

Thanks Arka. Interesting topic.

To add to Arka's questions:
5) How do some engines manage to generate more torque at much lower rpms than others?

Partial answer above. Also, would want to add, that generally direct injection engines have higher torque than the counterpart indirect injection (eg Tata Spacio had a higher torque than Tata Sumo on launch, and Spacio had a direct injection engine, as opposed to Sumo's indirect injection block)

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6) Flat torque curves are better but how much % should be available at idle and slightly above idle

To the best of my knowledge, its not related to idle, rather the measure is what's the % of max. torque that is available across 90% of rpm range. I believe (am not sure), ball-park is 70%

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7) How much is good enough for a good offroader e.g. for a SWB, a MWB and a LWB with say 1200 kg, 1600 kg and 2000 kg kerb weights respectively? Infinite torque is obviously not an answer as I understand that torque can only be put to good use if there's enough weight keeping the vehicle on the ground.

This is a tricky bit, as its not only weight, but tyres (to provide traction), as well as gear-strength (to handle the turning forces) that would determine this. Am not sure, if there are ball-park numbers for this

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Ballpark numbers should be a good start too.

Looking forward to this!

This is what I found from Why do diesel engines deliver more torque?

A gas engine compresses a fuel-air mixture and then ignites it with a spark. A diesel engine compresses just air, to such a high pressure and temperature that when fuel is then injected, it ignites automatically with no need for a spark. This is the fundamental difference between the two engines.
...so, a diesel engine has to do more compression than a gas engine does, to get the fuel to ignite. Conversely, a gas engine cannot do as much compression as the diesel engine does, because the fuel-air mixture it is compressing would ignite too early, at the wrong moment. Remember the diesel engine just compresses air, so it doesn't have that problem.
...so, the piston in the diesel engine has to travel further, in order to compress the air more. So the piston stroke is longer in a diesel engine.
...and, a longer piston stroke means a larger diameter crankshaft. Assuming the force coming from the gas piston and the diesel piston is equal, then the diesel piston has a longer lever arm and is turning its crankshaft with greater torque (but, necessarily, at fewer revolutions per minute). The gas piston may be delivering the same power, but it is delivering it by turning a crankshaft at more revolutions per minute, with less torque.

In fact, the diesel engine burns up the fuel a bit more efficiently, and diesel fuel has a bit higher energy content than gasoline, so a diesel piston should actually deliver more power from burning the same volume of fuel. But this is less important than the above argument in explaining the torque difference.

If and when you do want less torque and more speed, or more torque and less speed, than your engine is naturally inclined to give you, you can get it by gearing the engine up or down. That's why cars have gears, and why big trucks have lots of gears. But of course, in any of various senses of the word "free", transmissions don't come free.

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Originally Posted by SPIKE ARRESTOR (Post 2355476)

Till that time, let the tawa heat up.

Jeepers have a lot of patience.
Do you want the honour of the first dance or last dance!

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Originally Posted by anupamjain2k (Post 2355477)

What is the relationship between CR and max rpm?
If bore is the same, displacement is directly proportional to stroke.
Could never quite follow the logic of short stroke = No LET. Guess my doubts will be cleared now!

Did the Spacio and the Sumo have the same displacement?

Regards
Sutripta

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Originally Posted by Sutripta (Post 2355512)

Jeepers have a lot of patience.
Do you want the honour of the first dance or last dance!

What is the relationship between CR and max rpm?
If bore is the same, displacement is directly proportional to stroke.
Could never quite follow the logic of short stroke = No LET. Guess my doubts will be cleared now!

Did the Spacio and the Sumo have the same displacement?

Regards
Sutripta

Sutripta,

Higher the CR, longer is the distance needed to be traveled by piston. Higher the rpm required, lesser is the time to do it (1 to-and-fro motion, per rev.), hence higher the speed (linear speed) at which the piston is traveling. Beyond a point, linear piston speeds can't be increased with corrosponding favorable gains on power produced (Losses in the setup start increasing at a higher rate etc.). Hence, there is a rough limit on liner piston speed, and consequently the rpm. This is what connects CR and RPM

You're right on Displacement being directly prop. to stroke.

Short Stroke essentially means that the distance traveled by the piston in a revolution is less (relatively) and because of this low distance, lesser is the turning moment on crank and hence lower torque. So, short stroke engine on low rpms would have very little torque, and hence such engines would typically have to employ high gear-ratios to compensate.

I stand corrected on Spacio/Sumo bit........They didn't have the same displacement.....

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Originally Posted by cooljai (Post 2355496)

+1 great topic indeed. Looking forward for the discussions.
Would be great if someone can also explain
1) Torque Curves

If you find for a given engine, for a handful of rpm values, you find the max possible torque generated at each one of those rpm values, and then plot them (x-axis = rpm, y-axis = torque) you get the torque curve for that engine.

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Originally Posted by cooljai (Post 2355496)

2) Redline

All engines have a max rpm limit beyond which the engine can/should not be run. that is the engine's redline

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Originally Posted by cooljai (Post 2355496)

3) Power and Torque- Whats the difference

Power = troque*rpm (in appropriate units)

First of all, I'm thinking how is this thread different from http://www.team-bhp.com/forum/techni...e-vs-bhp.html?

further one:

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Originally Posted by ahm (Post 2355510)

This is what I found from Why do diesel engines deliver more torque?

...
...
...so, the piston in the diesel engine has to travel further, in order to compress the air more. So the piston stroke is longer in a diesel engine.
...and, a longer piston stroke means a larger diameter crankshaft. Assuming the force coming from the gas piston and the diesel piston is equal, then the diesel piston has a longer lever arm and is turning its crankshaft with greater torque (but, necessarily, at fewer revolutions per minute). The gas piston may be delivering the same power, but it is delivering it by turning a crankshaft at more revolutions per minute, with less torque.

No. 3 makes not much sense, bore/stroke ratio has to be considered.

No. 4 includes "for the same force on the piston" - here's the thing why would the force be the same? For a given engine displacement you can build two petrol engines one with longer stroke than the other - both will generate similar torque at a given rpm. The one with longer stroke simply will have a proportionally smaller force on the piston.

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Originally Posted by ahm (Post 2355510)

...

But this is less important than the above argument in explaining the torque difference.

the above argument is wrong

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Originally Posted by ahm (Post 2355510)

If and when you do want less torque and more speed, or more torque and less speed, than your engine is naturally inclined to give you, you can get it by gearing the engine up or down. That's why cars have gears, and why big trucks have lots of gears. But of course, in any of various senses of the word "free", transmissions don't come free.

While part 1 is true to a good extent, another reason big trucks come with lots of gears because unlike car engines, truck engines generate power in very narrow rpm bands.

Let me add to the confusions and contusions. Horsepower is an older measurement of work related to dragging a weight, e.g. a horse drawn wagon. Torque is a newer measurement of work related to turning something round like a crankshaft, driveshaft or wheel. Both being measuremets of work, they are related. You can not have tremendous horsepower and no torque or vice versa.

The timing of the introduction of the fuel/air mixture into the cylinder is another factor and variable in determining torque. The quality of diesel fuel is more-or-less a constant. So is the time it takes for fuel to be pumped to the injectors and into the cylinder. A key variable in the ignition cycle is the speed of the piston moving in the cylinder. As the speed of the engine increases the piston is moving faster. This means the fuel/air mixture input has to start earlier to get into the cylinder in time to ignite. An analogy is throwing a ball to a running man. The faster he runs, the more you have to lead him, throw in front of him, for man and ball to happily intersect.

Modern CRDi engines are an attempt to solve the diesel ignition timing by having compressed fuel on hand in a common rail or pipe and control its input into the engine via snazzy ECUs and injectors.

Spike Arrestor knows all about this and I am content to let him wax poetic on the subject and learn from him, as it makes my head hurt and my teeth itch.

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Originally Posted by DirtyDan (Post 2355688)

sir, with all due respect, neither of them is a measure of work. Horsepower is power (i.e. work done per in a given time) - and with suitable conversion factors it is synonymous with Watt or even Ergs/second.

torque is similar to force - you can have huge torque with no power whatsoever (in fact you can have huge torque with no movement whatsoever)

Torque and power are of course related but they are different physical quantities.

Why diesel produces more torque hmm..
Lets say Honda makes a beautiful engine 1.5L petrol producing 118bhp at 6600 rpm and 146nm at 4800rpm. I on the other hand who knows very little about making good engines produce a 1.5 liter engine, Due to my inexperience the engine has bad breathing characteristics because I made it small bore and large stroke.So my engine produces something like 70 bhp @ 3000 rpm and 135nm @ 2000 rpm (my torque figures will be close as it depends on the VE at low rpm) . So to beat Honda I turbo charge my 1.5L to 14psi which is like double the pressure of the NA engine.
So what happens now is my torque will jump to something like 250nm @ 2000 rpm and power to say 110bhp @ 4000rpm(wild guess).
So why diesel makes more torque ?
Because it has bad breathing characteristics and is compared with NA petrols' of same power when it is turbocharged effectively to double its capacity or more.:Frustrati

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Originally Posted by anupamjain2k (Post 2355570)

I don't know where the limit on rpm comes from, I guess the limiting factors vary from engine to engine, but your argument is confusing to say the least. For a given displacement and bore size, stroke length is fixed, irrespective of rpm, and CR can still be increased by, for example, turbocharging (in fact CR is equal to ratio of volume at TDC and volume at BDC, merely by reducing the volume at TDC you can increase the CR, without changing displacement and, by the arguments you have given above, the rpm).

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Originally Posted by anupamjain2k (Post 2355570)

You're right on Displacement being directly prop. to stroke.

Short Stroke essentially means that the distance traveled by the piston in a revolution is less (relatively) and because of this low distance, lesser is the turning moment on crank and hence lower torque. So, short stroke engine on low rpms would have very little torque, and hence such engines would typically have to employ high gear-ratios to compensate.

I stand corrected on Spacio/Sumo bit........They didn't have the same displacement.....

For a given displacement, CR and fuel burnt, short stroke (vs. long stroke) also means that the force on the piston is higher - almost exactly cancelling the effect of a shorter stroke - torque will be the same.

I'm not sure about your gear ratio stuff (though in theory it is correct) - there are several applications where a low rpm engine is connected via gears to generate higher rpm outputs (diesels mainly) but do you know of any significant commercial application where the other way round also happens (e.g. a 8000rpm engine used with a reduction gear arrangement).

I know they do it with turbojets in military helicopters, but don't know any other application.