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Old 22nd May 2011, 11:27   #46
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by bzr77k View Post
...
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"
Forget the capacitor part - while it is absolutely true, most electronics guys don't have a clue about it.

In a four stroke engine, only the ignition/combustion stroke generates energy, all the other strokes (exhaust, intake, compression) expend energy.

Compression stroke in particular not only expends enrgy (more than the other two - via heat transfer) - it requires more energy than the other two strokes because compression of air (or air/fuel for petrols) also stores energy into the air.


Flywheel (and a capacitor in electrical system) stores the energy from the ignition/combustion stroke and provides it for the other three strokes. While it does that, it loses speed (it regains the speed during combustion). If it is heavier it loses less speed, if it is lighter it loses more speed. Losing speed is not good.

Since diesel engines need more energy in the compression stroke - a heavier flywheel is needed to reduce variation of rpm.


By the way, for four cylinders every cycle is a power stroke for one cylinder - flywheel is needed to smoothen out the power variation within a cycle only. With 8 cylinders it is even better and so on.

This is also the reason why 3 cylinders run rough.
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Old 22nd May 2011, 14:27   #47
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by bzr77k View Post
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?
No diesels don't have more energy content than petrol infact it has less energy content when you consider that 14.7 grams of air burns with 1gram petrol or diesel for complete combustion.
No diesels dont produce more torque or power than petrol (even if you take the area under the curve) if both are NA and same cubic capacity, they produce less.
But what they do is that they have wonderful efficiency and there is no theoretical limit on the boost they can run.
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Old 22nd May 2011, 15:21   #48
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Default Re: Torque

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Originally Posted by AVR View Post
Flat torque curves are better but how much % should be available at idle and slightly above idle
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.
Well lets say a naturally aspirated petrol generally produce around 75% of their peak torque between 1000-1500 RPM. Max torque at atound 4000 and again 70-75 % at the redline. However the advantage is in the range between 2000 and 5000 you can get approximately 90% of peak torque.

Turbo diesels (engines) when dynoed (not the whole car on on innertia dynamometers) may show decent amount of torque at the bottom end (1000 to 1500). But surprisingly do not have the same effect on road. Even if the manufacturer claims max torque is at 1500 RPM you will feel the punch only after 2000. Take Vento for example.
The OEM specified value is a steady state condition. In other words if you are running your car at 1500 RPM at full throttle for a long time then you will be producing the OEM specified torque. Unfortunately this is highly impractical in real world. In fact the 1.6L TDi engine from VW can produce 250Nm at 1500 RPM. However it will need some time (lets say 2 sec for the turbo to spool up completely) in that 2 sec you are doing 2000RPM rms. So in addition to how much torque, the lag is also important.

Too much torque will cause wheelspins. However that can be avoided by keeping the gear right. Using wider tyres. etc. So bottomline: more the torque better it is.
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Old 22nd May 2011, 18:26   #49
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Default Re: Torque

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In other words if you are running your car at 1500 RPM at full throttle for a long time then you will be producing the OEM specified torque.
Errrr, how can one drive on full throttle and be on 1500rpm in the same time?
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Old 22nd May 2011, 18:31   #50
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Default Re: Torque

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Errrr, how can one drive on full throttle and be on 1500rpm in the same time?
On an incline lugging the engine in a high gear
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Old 22nd May 2011, 18:43   #51
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Default Re: Torque

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On an incline lugging the engine in a high gear

But that is not on full throttle i believe, full throttle means at max rpm right?
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Old 22nd May 2011, 18:48   #52
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Default Re: Torque

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But that is not on full throttle i believe, full throttle means at max rpm right?

AFAIK full throttle means foot completely pressing the pedal - whether engine can reach its full rpm or not.
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Old 22nd May 2011, 20:46   #53
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Default Re: Torque

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Originally Posted by Born2Slow View Post
No diesels dont produce more torque or power than petrol (even if you take the area under the curve) if both are NA and same cubic capacity, they produce less.
Great! How much less (ballpark figures). Why?

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Originally Posted by DRIV3R View Post
But that is not on full throttle i believe, full throttle means at max rpm right?
No. The torque/ power curves you see quoted are all WOT.

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Old 22nd May 2011, 21:26   #54
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Default Re: Torque

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Originally Posted by vina View Post
AFAIK full throttle means foot completely pressing the pedal - whether engine can reach its full rpm or not.
Quote:
Originally Posted by vina View Post
On an incline lugging the engine in a high gear
Spot on

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Originally Posted by DRIV3R View Post
Errrr, how can one drive on full throttle and be on 1500rpm in the same time?
Full throttle is pedal to metal. So practically only in inclines and higher gear may put you in such situations. Practically normal driving will never encounter this.
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Old 22nd May 2011, 23:33   #55
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Default Re: Torque

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Originally Posted by Sutripta View Post
Great! How much less (ballpark figures). Why?


Regards
Sutripta
All depends. All I can say they produce less power and torque. (cubic capacity ,bore/stroke,no.of cylinders all equal or similar). Until proven otherwise .
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Old 23rd May 2011, 00:33   #56
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by vina View Post
Forget the capacitor part - while it is absolutely true, most electronics guys don't have a clue about it.

In a four stroke engine, only the ignition/combustion stroke generates energy, all the other strokes (exhaust, intake, compression) expend energy.

Compression stroke in particular not only expends enrgy (more than the other two - via heat transfer) - it requires more energy than the other two strokes because compression of air (or air/fuel for petrols) also stores energy into the air.


Flywheel (and a capacitor in electrical system) stores the energy from the ignition/combustion stroke and provides it for the other three strokes. While it does that, it loses speed (it regains the speed during combustion). If it is heavier it loses less speed, if it is lighter it loses more speed. Losing speed is not good.

Since diesel engines need more energy in the compression stroke - a heavier flywheel is needed to reduce variation of rpm.


By the way, for four cylinders every cycle is a power stroke for one cylinder - flywheel is needed to smoothen out the power variation within a cycle only. With 8 cylinders it is even better and so on.

This is also the reason why 3 cylinders run rough.
Great, I understand that
A capacitor does exactly the same thing while filtering ripples.

I dont think the flywheel will 'lose more speed' in a diesel engine to generate more compression. In fact, thats why a heavier flywheel helps in 'storing more energy' from the power stroke (since it is heavier) which can be lost in the compression stroke.

Going by this line of thought, the fact that the diesel engines require more compression means they consume more of the energy they generate. Means there is less energy available for use to drive the wheels. Right?

Quote:
Originally Posted by Born2Slow View Post
No diesels don't have more energy content than petrol infact it has less energy content when you consider that 14.7 grams of air burns with 1gram petrol or diesel for complete combustion.
No diesels dont produce more torque or power than petrol (even if you take the area under the curve) if both are NA and same cubic capacity, they produce less.
But what they do is that they have wonderful efficiency and there is no theoretical limit on the boost they can run.
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Originally Posted by Born2Slow View Post
All depends. All I can say they produce less power and torque. (cubic capacity ,bore/stroke,no.of cylinders all equal or similar). Until proven otherwise .
I am still not convinced that diesels generate less power/torque! Expert opinions on this?
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Old 23rd May 2011, 20:32   #57
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Default Re: Torque

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Originally Posted by Born2Slow View Post
All depends. All I can say they produce less power and torque. (cubic capacity ,bore/stroke,no.of cylinders all equal or similar). Until proven otherwise .
An empirical study will show that you are right. (So little chance of being proven wrong). A theoretical reason would conclusively close the thread.

Interestingly, diesel and petrol have essentially the same calorific value, and stoichometric ratio. (Well to nitpick, diesels calorific value is slightly less. And for the ratio, we are not talking of biofuels.)

Quote:
Originally Posted by SPIKE ARRESTOR View Post
Could you explain more elaborately how this is done? How the intake manifold pressure is fine tuned?
Don't tell me you never thought of designing (your own) chambers for your RX.

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Old 23rd May 2011, 21:32   #58
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Default Re: Torque

Quote:
Originally Posted by Sutripta View Post
An empirical study will show that you are right. (So little chance of being proven wrong). A theoretical reason would conclusively close the thread.

Interestingly, diesel and petrol have essentially the same calorific value, and stoichometric ratio. (Well to nitpick, diesels calorific value is slightly less. And for the ratio, we are not talking of biofuels.)
...

Regards
Sutripta
To take it to a purely theoretical level, we should compare IHP (indicated horse power) rather than BHP to see which engine produces more torque. But I doubt if we can get these figures. I agree that at the end of the day, it is the torque available at the wheels that matters. But this discussion should start from what happens inside the cylinder and then progress towards Friction Horse Power and then BHP.

As far as the energy values are concerned, we need to remember that we buy diesel/petrol by volume. And the density of diesel is higher than petrol. So in effect we get more energy/kg for diesel.
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Old 24th May 2011, 03:45   #59
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

One of the main reasons being diesels run quite lean even at full power. From whatever information I could get on the internet it is never richer than 1:20 as the diesels starts to smoke. So that is 30% less fuel compared to a petrol.
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Old 24th May 2011, 13:51   #60
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Default Re: Do Diesel engines generate more Torque than Petrol engines?

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Originally Posted by Born2Slow View Post
One of the main reasons being diesels run quite lean even at full power. From whatever information I could get on the internet it is never richer than 1:20 as the diesels starts to smoke. So that is 30% less fuel compared to a petrol.

Wouldn't a lean mix cause higher NOx emissions?

From what I have read CO, SOx and soot particles get oxidised and NOx get reduced in the catalytic converter provided the average lambda is 1 (i.e. fuel air mix is, on average over a few seconds, stoichiometric).

I guess if there is more O2 than required to burn the fuel, O2 + NOx will be more than is needed to oxidise the CO etc. and a good amount of NOx will then remain in the exhaust.

What am I missing?
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