[thoma]

Originally Posted by Sutripta

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.

In this era when even science is rewriting itself, the only thing that is a constant is change. Nevertheless, I had always believed in this quote from Bernard Shaw - "If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas." So, one or more, right or wrong, aren't discussions always positive and helpful.

Personally, I am a hardcore fan of low end torque than max speed. And it was this post from noopster that set me thinking why he could not get a straightforward answer from this thread. That was when I went through all the pages and failed to reach a definitive conclusion.

Comparing equal displacement engines, it is true that if we double the stroke hypothetically, the area on which the gas pressure (force produced by the burning fuel per unit area, assumed to be constant) acts gets reduced by half. And hence, the total force acting on the piston gets reduced by half, though the crank radius doubles. This makes me too to think that torque (force X crank radius) remains the same.

But there is much "writing" (might be we can't call them authentic) on the net generically in favor of increased torque when the stroke is increased. Or I wonder whether these authors have considered the factor of a level playing ground in terms of keeping the displacement constant. Or is there something else that we could be missing?

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

But, are any of the below in any way contradictory to this?

"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."

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

Is not max torque proportional to the torque from an average combustion stroke? Looking at the figures below, I am confused whether, for the same or a bit lesser amount of fuel (assuming same displacement), much torque (or combustion force?) per stroke is possible from the diesels?

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

Vento Diesel 1.6
Power (PS@rpm) 105PS @4400rpm
Torque (Nm@rpm) 250Nm @1500rpm

Vento Petrol 1.6
Power (PS@rpm) 105PS @5250rpm
Torque (Nm@rpm) 153Nm @3800rpm

Beat Diesel 1 L
Power (PS@rpm) 59PS @4000rpm
Torque (Nm@rpm) 150Nm @1750rpm

Beat Petrol 1.2 L
Max Power (PS) 80.5@6200 80.5@6200 80.5@6200
Torque (Nm) 108@4400

Or there could there be still some other design factors that make these engines differ?

[rangakishen]

Originally Posted by Sutripta

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.

Consider a bicycle. We apply torque to the pedals and the bicycle moves.

Now just imagine you have a huge load or are climbing a steep hill. You would have to apply a lot more force onto the pedals to move up the hill. Now if only you had pedals which were a bit bigger. Your efforts would pay off better. I.E, with the same force but increased radius, torque would increase and it would be easier. This is torque. And in our analogy this is lengthening the stroke.

Now try to remember your old days when you would be so happy to see a steep downhill section to just breeze through. Now just try and recollect what would happen when you tried to peddle when you were already going downhill. YES!! You were not able to. Because you are traveling much faster than you can peddle. Now what would be ideal is we had a small pedal. With small pedal with decreased radius you would be able to accelerate even in that situation. This in our analogy would be decreasing stroke length.

Now here's my point, the effect of stroke is not just torque produced but more importantly, when that torque is generated.

In the above analogy, when pedal radius is increased the max torque is produced at that low RPMs. I.E load lugging capability increases. Real world example would be Royal Enfield bikes. Longer strokes allow them to produce a lot of torque low down in the rev range.

At high speeds or RPMs though (above example's downhill riding), Torque produced by the same setup (longer stroke or large pedals) is no longer enough.

You'll question that with large radius, torque should be more. No. Just think. Radius is more. BUT!! You are just not able to move fast enough to have any impact on the pedal. It is moving faster than your leg!! So basically your force downwards has decreased to nil!! So obviously no TORQUE!! Now what is required is a decrease in radius such that your legs can move faster than the pedals at that radius. Now you can apply force and hence apply torque.
So you decrease your pedal radius or you stroke length. AND VOILA!! You're able to pedal and accelerate. I.E now your engine produces that very torque much higher up the RPMs. Hence it can go faster. Formula One cars have incredibly short strokes and produce their peak torques very high up the rev range.

Hope I was able to help!!

Of course this plays a part on the power produced. Just haven't gone into details. Will do when i have my concepts clear.

[thoma]

Originally Posted by rangakishen

You would have to apply a lot more force onto the pedals to move up the hill. Now if only you had pedals which were a bit bigger. Your efforts would pay off better. I.E, with the same force but increased radius, torque would increase and it would be easier.

I guess we were discussing equal displacement engines and hence Sutripta's reply.

Originally Posted by Sutripta

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.

Originally Posted by thoma

Comparing equal displacement engines, it is true that if we double the stroke hypothetically, the area on which the gas pressure (force produced by the burning fuel per unit area, assumed to be constant) acts gets reduced by half. And hence, the total force acting on the piston gets reduced by half, though the crank radius doubles. This makes me too to think that torque (force X crank radius) remains the same.

[Diesel_convert]

Diesel's generate more torque because the combustion pressures are much higher, typically 220 bar ( or 2200 meters of water column)

Petrols on the other hand have peak pressures of only 60 bar or so (600 meters of water column)

This is first hand data.


So all other things being equal the diesel will develop a lot more torque.

[Aroy]

Diesels will generate more torque simply because the force on the piston is higher (due to higher pressures during power stroke)

Long stroke engines will also generate more torque as the diameter of the crank is more than in a short stroke engine, hence the moment arm (the radius of the crank) is longer resulting in greater torque.

Combine both these and you have - long stroke diesels will have more torque at a give RPM compared to a shot stroke petrol. That the long stroke diesel cannot operate at higher RPM is another matter.

Race tuning increases the combustion pressure in a petrol engine, which depending on the anti knock properties (octane rating) of the fuel, can have the compression ratios of 1:11 for 100 octane and go as high as 1:15 for 124 octane rated fuel. Higher compression ratios translate to higher torque

http://www.daytona-sensors.com/tech_tuning.html

http://www.bp.com/retail/liveassets/...nd_PowerV2.pdf

[Sutripta]

Originally Posted by thoma

But, are any of the below in any way contradictory to this?

Is not max torque proportional to the torque from an average combustion stroke? Looking at the figures below, I am confused whether, for the same or a bit lesser amount of fuel (assuming same displacement), much torque (or combustion force?) per stroke is possible from the diesels?

Diesel and petrol have similar calorific values and stoichiometric ratios.

For this discussion torque is a function of how much air is used for combustion. As I have mentioned before, the increased compression of a diesel is not enough to compensate for the fact that a diesel is unable to use all its air.

Regards
Sutripta

Originally Posted by Diesel_convert

Diesel's generate more torque because the combustion pressures are much higher, typically 220 bar ( or 2200 meters of water column)

Petrols on the other hand have peak pressures of only 60 bar or so (600 meters of water column)

This is first hand data.


So all other things being equal the diesel will develop a lot more torque.

BMEP/ IMEP figures please.
Even better, indicator diagrams.

@Aroy:
Examples of diesels with high max torque/ litre. (NA engines for proper comparison)

Regards
Sutripta

[thoma]

Originally Posted by Sutripta

torque is a function of how much air is used for combustion.

Could you please elaborate how torque is a function of the air used? I'm blank.

a diesel is unable to use all its air.

How? I guess the below could be an explanation for that, but I didn't get one bit of it.

Originally Posted by vina

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.

--

Originally Posted by Aroy

Long stroke engines will also generate more torque as the diameter of the crank is more than in a short stroke engine, hence the moment arm (the radius of the crank) is longer resulting in greater torque.

But if we keep the displacement constant while increasing the stroke, won't the piston area reduce? And hence the total force acting on the area too? As quoted in my post (#153) above? Assuming force per unit area same in both the cases.

[thoma]

Originally Posted by Sutripta

Examples of diesels with high max torque/ litre. (NA engines for proper comparison)

Sorry, the Vento and Beat examples in my previous post are definitely not NA. I am not sure about the Indica though. The Indica example is taken from greenhorn's post in the initial pages of this thread.

But, I was naturally thinking that if a turbo increases torque, won't the higher compression ratio of a diesel over a petrol too increase torque? Oh no! I'm wrong there. Higher compression ratio does not mean more air (rather oxygen) molecules per volume. It just means more pressure and thus temperature of the same number of molecules. And what does a turbo do? Increase the density of oxygen to the intake. So more air indeed means more torque and now I got why the statement.

Originally Posted by Sutripta

torque is a function of how much air is used for combustion.

But how exactly? What could be happening behind the scenes? Is it that more fuel can be burned using the more air available and thus increased torque?

[Aroy]

Originally Posted by thoma

But how exactly? What could be happening behind the scenes? Is it that more fuel can be burned using the more air available and thus increased torque?

If the Fuel to Air ratio is maintained, more air due to turbo will require more fuel, so essentially you will be burning more fuel, which if fully consumed by the combustion will result in more torque.

[Sutripta]

Originally Posted by thoma

But how exactly? What could be happening behind the scenes? Is it that more fuel can be burned using the more air available and thus increased torque?

Yes. Nothing behind the scenes!

Regards
Sutripta

[Diesel_convert]

Originally Posted by Sutripta

Diesel and petrol have similar calorific values and stoichiometric ratios.

For this discussion torque is a function of how much air is used for combustion. As I have mentioned before, the increased compression of a diesel is not enough to compensate for the fact that a diesel is unable to use all its air.

Regards
Sutripta


BMEP/ IMEP figures please.
Even better, indicator diagrams.

The diesel engine is designed to not use all the air, its a lean burn engine.
Unfortunately I can't share the first hand info (NDA's and all that jazz).
And you wont be able to find too many modern engine diagrams on the internet as these are proprietary. Manufacturers guard these as trade secrets.

But I will try and explain,

For a petrol engine the compression ratios are typically 10:1

Implies peak pressure without ignition of slightly more than 10 atm or 10 bar.

it is more than 10 bar because of the gas law (for air) =

P * (V^1.4)= const * temp

For a diesel engine of CR 22:1

Pressure with out ignition is slightly more than 22 bar.

When you add ignition to the picture:

When the fuel ignites the volume of fuel in liquid form is about a 1000 times less than the gases that are formed after combustion. The temperature is also raised significantly.

Keeping all other things constant and varying only temperature, we get a linear relationship. Like you are suggesting with same fuel per stroke.

As soon as you take into account the extra volume of gas generated, it is non-linear.

So more initial compression higher the difference the volume change (combustion) makes to the system. Hence higher the peak pressures.


Results in much higher peak pressures for a diesel than for a petrol.

This is also why diesels can't rev very high, the torque plus peak pressures ensure that the combined centrifugal and linear forces acting on the crankshaft are very high.

I hope I have added something to the discussion.

Cheers,

[nim_peter]

Another question . How does CNG buses produce Torque if it is not running on Diesel? Does it mean it is the compression which is producing the torque or it is still the fuel and I'm missing the complete picture?

[Sutripta]

Originally Posted by Diesel_convert

The diesel engine is designed to not use all the air, its a lean burn engine.
Unfortunately I can't share the first hand info (NDA's and all that jazz).
And you wont be able to find too many modern engine diagrams on the internet as these are proprietary. Manufacturers guard these as trade secrets.

Indicator diagrams proprietary, possibly. But BMEPs? Seriously?

But I will try and explain,

For a petrol engine the compression ratios are typically 10:1

Implies peak pressure without ignition of slightly more than 10 atm or 10 bar.

?

P * (V^1.4)= const * temp

?

For a diesel engine of CR 22:1

Pressure with out ignition is slightly more than 22 bar.

??
CR=22 for a DI?


Considering this thread is more than a year and a half old, I don't think repeating things is going to resolve the issue. So lets try some different tacks.

A) We could compare values from some (apples to apples) engines. ie a NA diesel and a (non screamer) petrol. The American pushrod V8 rumblers come to mind. We would have to normalise/ standardize the results. BMEP, or say Kg-m/ ltr or something like that.

We could start with the Indian example which comes readily to mind: the Indica engine. What could account for the torque values of the petrol and diesel?
http://www.team-bhp.com/forum/techni...ml#post2357828

B) Is there a difference in torque output between a diesel at light load and at full load, rpm remaining constant? Why? In both cases the peak pressure before ignition (fuel delivery) remains the same.

Regards
Sutripta

[spkingsley]

Originally Posted by Sutripta

Indicator diagrams proprietary, possibly. But BMEPs? Seriously?

?

?

??
CR=22 for a DI?

Considering this thread is more than a year and a half old, I don't think repeating things is going to resolve the issue. So lets try some different tacks.

A) We could compare values from some (apples to apples) engines. ie a NA diesel and a (non screamer) petrol. The American pushrod V8 rumblers come to mind. We would have to normalise/ standardize the results. BMEP, or say Kg-m/ ltr or something like that.

We could start with the Indian example which comes readily to mind: the Indica engine. What could account for the torque values of the petrol and diesel?
http://www.team-bhp.com/forum/techni...ml#post2357828

B) Is there a difference in torque output between a diesel at light load and at full load, rpm remaining constant? Why? In both cases the peak pressure before ignition (fuel delivery) remains the same.

Regards
Sutripta

2

Torque = Explosion pressure above piston x crank throw

Petrol engines - peak explosion pressure is 30-40 bar
Diesel engine - peak explosion pressure is above 80 bar; reaches above 120 bar if it is a turbocharged engine.

So if the crank If the crank throw is same, diesel will generate much more torque..
RPM Has no role in torque - it is related to power

[Sutripta]

Originally Posted by spkingsley

Petrol engines - peak explosion pressure is 30-40 bar
Diesel engine - peak explosion pressure is above 80 bar; reaches above 120 bar if it is a turbocharged engine.

So per litre, a NA diesel should generally have twice the torque (or more, considering diesels normally are long stroke engines) of a petrol?

Regards
Sutripta