[vina]

Originally Posted by SPIKE ARRESTOR

^^ Yes, I think so, do you think these values are not correct?

Spike

I don't know. But I wanted to know what puts the upper limit (have asked before) - I don't believe the usual answers on the lines of mass of the piston, stresses etc. Materials have changed a lot in time, peak piston velocities seem to have not.

fuel burn rate, flame velocity etc. can be valid reasons - but they are no more than mentioned in the passing, and that too rarely.

[SPIKE ARRESTOR]

True, materials have changed but some constraints remain -

• Friction / Wear
• Noise
• Fill ratio

BTW, forgot to mention, you can calculate piston speed using v = 2*l*n, where l is stroke and n is engine speed.

Spike

[Jeroen]

Actually the biggest problem / constraint with pistion speed is often the piston rings and how they behave under (de)-acceleration.

Things might have moved on since I was taught this, but here goes:

Pistion rings force themselves against the cilinderwall due to mechanical pressure. Outside the ciliner the rings are bigger, it is a bit like a spring. However, that mechanical force is by no means sufficient in sealing the pistion against the cilinder liner during the compression or ignition stroke. Piston and piston rings are designed in such a way that the pressure actually pushes the rings tighter into the piston and against the cilinder lining wall.

When piston acceleration exceeds the sum total of gas
pressures holding the ring in place, the ring will lift upward (as the piston nears the top of its stroke, and is being braked to a halt). Instantly, as the ring lifts, the gas pressure
previously applied above and behind is also applied underneath the ring, at which point
its inertia takes over completely and the ring slams up hard against the top of its groove.

This last action releases all pressure from behind the ring, leaving it entirely to its own
feeble devices in holding back the fire above. This all happens in the space of a miliseconds.

This phenomena is also know as piston ring flutter. Google it and you'll find a wealth of information.

Jeroen

[vina]

Originally Posted by Jeroen

Actually the biggest problem / constraint with pistion speed is often the piston rings and how they behave under (de)-acceleration.
...

Jeroen

Thanks a lot

found this on search:

http://www.lortim.demon.co.uk/vsih/pistons.htm

Since the motion of the piston is simple harmonic, peak acceleration will also limit peak velocity (for a given stroke length).

Also explains the trend in going for shorter strokes.



By the way, people can now compare torque numbers between Ford Ecosport turbo diesel and turbo-petrol and see which one gives more torque for a given displacement

[thoma]

Originally Posted by vina

By the way, people can now compare torque numbers between Ford Ecosport turbo diesel and turbo-petrol and see which one gives more torque for a given displacement

EcoSport's turbo petrol is a 1L and the turbo diesel is a 1.5L. Is it fair to derive and compare a torque per liter-of-displacement between the two?

Should not the apt comparison for the thread be between an NA petrol and an NA diesel?

[vina]

Originally Posted by thoma

EcoSport's turbo petrol is a 1L and the turbo diesel is a 1.5L. Is it fair to derive and compare a torque per liter-of-displacement between the two?

Should not the apt comparison for the thread be between an NA petrol and an NA diesel?

well, I wrote on a given displacement basis - which means the diesel should have more than 1.5x the torque to be called "torquey".

The ratio is more like 1.2times - so much for the extra torque of diesels.



NA petrol vs. NA diesel would throw up similar results, it is just that I don't know of any NA diesel being sold these days.

[Sutripta]

Originally Posted by vina

Since the motion of the piston is simple harmonic,

What! No 2nd order terms! (OK, I'm being pedantic)

Originally Posted by vina

so much for the extra torque of diesels.

NA petrol vs. NA diesel would throw up similar results, it is just that I don't know of any NA diesel being sold these days.

Go through the whole thread.
Esp. http://www.team-bhp.com/forum/techni...ml#post2968866 onwards.

Regards
Sutripta

[vina]

Originally Posted by Sutripta

...

Go through the whole thread.
Esp. http://www.team-bhp.com/forum/techni...ml#post2968866 onwards.

Regards
Sutripta

Dada

I remember the post. but the naysayers can always say "new vs. new"

[thecoolsundar]

Originally Posted by ex670c

Hi Guys,

I was meaning to start this discussion for a sometime.

1) What is torque(Engine)?

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

3) If so then why?

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

Sutripta, Spike, Pjbiju, Star Aqua, DB Sir please pitch in.

Regards,

Arka


Related thread : Power characteristics of long and short stroke engines: Cast in stone?

At the risk of being long winded; here goes;

What is torque?

Torque is defined as the moment of a force, or the force applied times the distance between the point of application of the force and the center of rotation.

In engine terms, you can think of it as the net force applied by the piston on the crankshaft, times the crankshaft throw.

The pressure exerted by the gases on the piston head, are responsible for the force exerted, now only the pressure exerted by the combustion gases during the power stroke is doing any useful work whereas the pressure exerted during the other strokes (compression, intake) are eating up energy.

The average pressure exerted on the piston, is a figure that is important to engine designers, it is called BMEP or brake mean effective pressure.

now force is pressure times area, so multiply the the BMEP by the area of the piston head and you have the the average force applied on the piston.

Multiply this average force by the crankshaft throw (which is half the stroke) and take into account the fact that the angle of the connecting rod is changing constantly and you have the torque per cylinder, multiply that by the number of cylinders and you have the torque of the engine.

Do Diesel engines produce more torque than petrol engines, if so why? What if the displacement of both are same

Power can be defined as Torque times angular velocity, in other words, the more the torque and the faster the crankshaft is rotating, the more the power of the engine.

So as a corollary, for two engines delivering the same power, the one rotating slower is delivering more torque.

All engines, have 'Torque Curve', Depending on the design, they deliver their max torque in a certain RPM range.

So the short answer to your question is NOT NECESSARILY. Even if the bore and stroke are the same, other factors like the compression ratio, charging, injection/ignition timing, valve timing etc can change the performance of either engine.

Usually Diesel engines are designed to deliver their peak torques at lower RPMs. This is because, the components of diesel engines are usually heavier to withstand the higher pressures, these heavier components do not like being moved around fast.

[vina]

Originally Posted by thecoolsundar

...
So as a corollary, for two engines delivering the same power, the one rotating slower is delivering more torque.
...

how would you take this for torque at the wheels (as against at engine shaft?)

[amit_purohit20]

Originally Posted by thecoolsundar

At the risk of being long winded; here goes;

What is torque?

The average pressure exerted on the piston, is a figure that is important to engine designers, it is called BMEP or brake mean effective pressure.

now force is pressure times area, so multiply the the BMEP by the area of the piston head and you have the the average force applied on the piston.

Very good explanation indeed. But one more thing to be remembered is that Pressure= Force/Area but without resistance there is no pressure as in hydraulics.

Same applies for engine, without resistance the engine does not create torque. Therefore BMEP will also be low if there is no resistance to the crankshaft movement.

As you start creating resistance to rotation of the Crankshaft there is increase in BMEP and it peaks at one point. This is the point where you will get Max. Torque at given rpm.

So to conclude engine generates torque depending upon the resistance from the wheels.

Off-course for all calculations and testing the resistance is considered maximum at the point where engine should not stall afterwards. You go on increasing the resistance in the dyno to get the max. torque figure.

Another analogy. A pump doesnot create pressure, its the resistance to the flow of fluid which creates pressure. A pump only displaces the fluid.

Similarly a Engine only displaces the piston (torque required to overcome friction+ torque required to run the engine auxillaries) but as you load the engine it starts developing more and more torque till a maximum point is reached.
This concept is often misunderstood by many people.

[a4anurag]

Originally Posted by thecoolsundar

Usually Diesel engines are designed to deliver their peak torques at lower RPMs. This is because, the components of diesel engines are usually heavier to withstand the higher pressures, these heavier components do not like being moved around fast.

Good explanation!

What will you say of high revving diesel engines (high performance)?

What will be difference in components when compared to a normal engine versus a high revving (performance) engine?

Anurag

[amit_purohit20]

Originally Posted by a4anurag

Good explanation!

What will you say of high revving diesel engines (high performance)?

What will be difference in components when compared to a normal engine versus a high revving (performance) engine?

Anurag

A high revving engine be it either petrol/diesel has to be necessarily made up of lighter components. Higher revving in diesel engines has only be possible thanks to CRDI and Multiple Injections per cycle - Fiat MJD engine.
Higher revving means less time for complete combustion so we need better control on injection timings and the no. of injections per cycle is needed.
It has also been aided by innovations in Material technology, light weight forged pistons and lighter high strength connecting rods and crankshafts.

Diesel has higher calorific value than petrol. Diesel is not easily combustible so less prone to detonation allowing higher compression ratios and use of turbochargers. All this leads to an increase in net BMEP which is nothing but the force on pistons. Add to it diesel engines are comparitively low revving engines which means if longer crank throws are used (Long stroke) we get higher torque. I have hardly seen any Diesel engines which are oversquare to a very large extent ie. Short stroke.

[thecoolsundar]

Originally Posted by vina

how would you take this for torque at the wheels (as against at engine shaft?)

That depends on two things;

a) Transmission efficiency
b) Gear ratio

Let's take the example of a typical diesel car like the fiat Punto with a 1.7 L turbo D engine.

It delivers a max torque of 137 Nm at 2500 RPM.

Now since power P = 2*pi()*RPM/60, the power being produced by the engine at this point is 35.87kW.

Lets assume that the transmission efficiency is about 90%, then the power at the wheels is about 32kW.

Now, assume that the car is in first gear, with a gear ratio of about 4:1 and has a final reduction ratio of about 6:1

Then the RPM at the Wheel would be about 2500/(6*4)= 104 RPM.

So working backwards, the Torque would be about 2938 Nm.

[thecoolsundar]

OOPs! that's P = Torque * 2*pi()*RPM/60

sorry