Torque Question

[jamie g]

The picture below shows the torque curves of 3 different bikes. The Ducati has the highest torque figure but is closely followed by the slightly smaller Honda and Suzuki.
The ducati has a 2 cylinder engine and the other 2 bikes have 4 cylinders.
So here is my question:
The ducati has twice the capacity for every power stroke to make its torque, so why doesn't it produce twice as much maximum torque?

[Matt UK]

jamie g said:

The ducati has twice the capacity for every power stroke to make its torque, so why doesn't it produce twice as much maximum torque?

Because it has half the cylinders?

Or am I missing something obvious here?

[intrepid44]

Becuase it's torque at an RPM. If it was twice the torque at the same rpm as the other engines, then it would produce twice the power. And quite simply you would end up with the most efficient engine being a single cylinder, which obviously isn't the case.

[busta]

More torque per bang but with bigger gaps between bangs.

[Boba Fret]

and it'd be pretty horrible to ride.

[icepop]

A laymans explanation might be that, torque is a measure of the turning force of an engine, and as the Ducati has 2 Huuuuuge pistons, and hence more mass per piston, and the other bikes have 4 smaller chuncks of metal, less mass, albeit occupying a similar displacement, then the Ducati will have more propencity to produce "turning force". Probably produces more flywheel effect too.

Could be a load of bo**locks though, I'm sure an engineer will be along shortly to clarify.

[GroundEffect]

Torque is related to BMEP. Efficiency dictates the torque output, not the configuration.

Also, think of it as the averaged torque over 4 revolutions of the crank.

Edited by GroundEffect on Wednesday 4th May 23:23

[icepop]

I think from the vague posts, it's obvious we're not mechanics here, what the heck is BMPF or whatever.

[tvrolet]

Given they're all 4-strokes, for each 2 revolutions of the crank you're going to get 1000cc's worth of combustion. On the Ducati it's via 2 x 500cc 'bangs', and on the others it's 4 x 250cc 'bangs', but still 1000cc per 2 revolutions.

[jamie g]

But torque has no time or revolution reference in its value. Torque is an instantaneous value measured at one point in time.
Imagine pushing down on your bicycle pedal, the force applied is transferred into torque. In this case the piston is replacing your leg, and surely one big piston can push harder than one half the size?

[GroundEffect]

icepop said:

I think from the vague posts, it's obvious we're not mechanics here, what the heck is BMPF or whatever.

Well, I am an engineer but I'm struggling to think how to word it...

BMEP is Brake Mean Effective Pressure. It's basically the efficiency of an internal combustion engine and is directly related to torque output. What I was using it for was to highlight that it's the efficiency that dictates maximum torque output rather than how the engine is configured. A one litre engine with the same BMEP will produce the same peak torque no matter what configuration you put it in.

[icepop]

I always looked at it from the view that, torque was a measure of the amount of force required to stop that engine from turning. And as 2 cylinder had more mass in the pistons than the 4 cylinder, of the same capacity, it required more effort to stop the engine, and hence returned a higher torque figure.

[intrepid44]

But the torque that is being measured is coming from the flywheel. Which sort of combines the torque into one, and seeing as they are all roughly the same size engine, is why the torque is roughly the same.

[jamie g]

GroundEffect said:

icepop said:

I think from the vague posts, it's obvious we're not mechanics here, what the heck is BMPF or whatever.

Well, I am an engineer but I'm struggling to think how to word it...

BMEP is Brake Mean Effective Pressure. It's basically the efficiency of an internal combustion engine and is directly related to torque output. What I was using it for was to highlight that it's the efficiency that dictates maximum torque output rather than how the engine is configured. A one litre engine with the same BMEP will produce the same peak torque no matter what configuration you put it in.

So, are you saying the bigger volume is only filled with a charge of, say, 65% volume and the smaller piston is 100% filled?

[intrepid44]

icepop said:

I think from the vague posts, it's obvious we're not mechanics here, what the heck is BMPF or whatever.

I'm actually a second year Mechanical Engineering student.

[GroundEffect]

jamie g said:

So, are you saying the bigger volume is only filled with a charge of, say, 65% volume and the smaller piston is 100% filled?

No. Not at all. I don't really know where you got that to be honest...

I'm really having a hard time trying to think how to describe the images I have in my head! It's honestly a question I've never been asked before.

The torque output is time-averaged. i think that's the best way I can word it.

Just as an anecdote from my own experience: I have ridden and raced motocross bikes from a very young age and you can drastically sense the difference between a two stroke and a four stroke bike by their torque delivery. The four strokes have better traction as they have larger gaps between power strokes, giving the rear tyre a little bit of relief and the ability to gain purchase. This can win or lose a race depending on the surface.

Edited by GroundEffect on Wednesday 4th May 23:38

[jamie g]

GroundEffect said:

jamie g said:

So, are you saying the bigger volume is only filled with a charge of, say, 65% volume and the smaller piston is 100% filled?

No. Not at all. I don't really know where you got that to be honest...

I'm really having a hard time trying to think how to describe the images I have in my head! It's honestly a question I've never been asked before.

The torque output is time-averaged. i think that's the best way I can word it.

But as I said before, there is no time reference in the units of measure.

[anonymous-user]

Simply because it is only pressing on the crank for half as many times per cycle.

in very layman terms:

say the force on the piston is 2, and on a 2 cyl eng it happens twice per cycle (2revs) the "total" is 2 x 2 = 4


for the 4cyl, force per cyl is only 1 (half) but it happens 4 x per cycle, so 1 x 4 also = 4

hence same torque output

[deveng]

jamie g said:

But torque has no time or revolution reference in its value. Torque is an instantaneous value measured at one point in time.
Imagine pushing down on your bicycle pedal, the force applied is transferred into torque. In this case the piston is replacing your leg, and surely one big piston can push harder than one half the size?

That's not right. Torque may not be in reference to a given time period but the nature of an ICE means that you get an average torque coming out of the engine which is smoothed by the flywheel. So even though the engine turning is powered by discrete combustions, it effectively has constant torque output.

[intrepid44]

jamie g said:

But as I said before, there is no time reference in the units of measure.

Fine, the torque being exerted on the crank by an individual piston in the Ducati is twice that of the Honda/Yamaha. But the combined torque due to there being twice the number of pistons in the Yamaha/Honda is not. Remeber the torque being measure is coming from the flywheel, which sort of averages it out.