Torque Question???

[tomTR6Fan]

Theoretical question: first let me say I understand torque (Torque = force x radius) and I understand how horsepower is calculated (HP = T x RPM/5252).  Obviously, we all know typical race engines are designed to operate at high RPM to increase their horsepower and their thrust down the race track.  Of course this means the torque curve and peak is moved higher in the RPM range than in a typical road car.  Here is the question – given the same amount of torque, why does moving the torque peak and usable range to a higher RPM result in more thrust to the car?  Considering the fact that there is more friction, heat and likely more vibration at higher RPM, why does an engine accelerate a car quicker when spinning faster?

[GT6Steve]

Errmm, cuz all that G force and vibration goes straight to our gonads making us push the pedal harder? :

[Gerard]

It doesn't if you have a diesel.

 

So there.

[GT6Steve]

Well maybe,

 

But who would drive a diesel unless paid for it?...

[Gerard]

Me.

 

I don't live in the US of A where they give petrol away.

 

And I distinctly remember an Audi winning Lemans burning peanut oil...

[Nick Jones]

There's no substitute for cubic inches....... that generates torque - but big engines often don't rev well.  The trick is getting big torque out of a small engine at lowish rpm - which usually requires forced induction.  Turbo diesels can often manage HUGE torque but often over a rather short rpm range.  A good modern TD (3.0 BMW, 2.7 or 3.0 Audi for example) is a fine engine but requires a different driving style - you have to change up just as  petrol engine experience suggests things should be getting interesting!

 

Nick

[GT6Steve]

Well, I reckon diesels are a bit more developed on your side.

 

  In the US a diesel is in a monster pickup truck with a halfwit behind the wheel thinking he's lord of all he surveys.  Or a proper lorry.  Diesel autos are relatively rare.

 

That said, back in the 80's I asked a Merc diesel driver how he could endure that horrible row from his engine bay.  His response, with a smile " I can't hear it on the inside."   ;D ;D

[Gerard]

Trust you to be sensible Nick

 

Indeed, My good old Berlingo rarely need to go over 2300 rpm, though I've been told to push it over 3000 for the turbo...

 

Maybe a proper sensible answer would be "it's all a compromise". The torque will fall off at a given point due to the limitations of the engine setup.

 

PS, Steve, my engine is noisy on the outside, and I don't care. 5.2L/100km

[sorbs]

There are lots of factors but generally power tends to rise with engine speed in a fairly linear fashion so higher rpm = more power.  Such engines tend to be fairly weedy in terms of torque at low rpm.  Engines with high torque at low rpm tend to have limited power at high rpm so although you get an intial burst of acceleration, they simply run out of steam  and you have to grab the next gear at the point where a revvy engine is just getting into its stride.  If you have an engine with lots of power at high rpm you can have longer gearing to take advantage of the power produced at high engine speeds.

 

My old Passat TDI had 210 ft/lb of torque at 1800rpm and 130bhp at 4000rpm.  I always enjoyed annoying the drivers of Impreza Turbos which had similar torque characteristics.  I could match them for acceleration until I hit 4000rpm / 130bhp in the Passat but then, the fact that the Imprezas would rev to 6000rpm+ and produce 220bhp or so meant that they just disappeared at this point while I fumbled around grabbing the next gear.

 

I always remember having a drive of my mate's Renault 19 16 valve*.  I'd been used to cars in which there was no real point in exceeding 5500rpm as the power just died away after this.  The Renault didn't really get into it's stride until it was doing 5000rpm at which point it would spin it's wheels in 2nd and 3rd if the roads were a bit damp.  The fact that it had a wide rev range and produced a lot of top end power meant that it could employ fairly long gearing giving a large speed range in each gear.

 

Hopefully this makes sense, I know what I mean.  Maybe someone will be able to put it into words a little better......

 

 

 

 

*  Those were the days; when a hot hatch (or in this case 4 door saloon) only needed 140bhp to be quite seriously rapid.

[flatter4]

 

The faster the engine spins the more opportunities to put fuel in it, so you can burn more = HP

 

....however friction goes up with rpm too.

[GT]

The ONLY thing that matters is torque.

Peak horsepower, is only where torque is falling off faster than revs rise.

 

As Horsepower is only a function of the RATE of doing work then it follows the longer peak torque (or close to it) is maintained, the flatter the torque curve.

Modern turbo diesel engines in this respect are a model of virtue.

Because of the monster compression ratio, they provide huge torque at low rpm which is usually maintained because of the turbo boost until really quite high revs by diesel standards.

Most of these engine size for size are producing MUCH better torque and Bhp/L these days than petrol engines of a mere 10 years ago......AND using less than half the fuel.

I've been in a modern large Beemer Turbo diesel and the performance is literally AWESOME.

[logicaluk]

if i remember correctly VW removed all its diesel cars from the US market because they were having big problems with crap quality diesel fuel with way to much sulpher in it.

Dan

[sorbs]

The BMW straight six diesels are amazing engines, I loved driving the 530d, it was a very quick car but even so, they have a relatively limited powerband and although they'll run to 5000rpm or so, the main action is over by 4500rpm.  It's just difficult to burn fuel quickly enough using the compression ignition process to give high engine speeds.  In the case of the BMW engines, they produce so much torque it's possible for them to have gearing which allows pretty good maximum speeds in each gear, thus disguising the limited rev range.  Even so, the effective powerband is from around 2000 - 4000rpm during which period the torque curve will be pretty flat.

 

Top Gear compared the BMW 535d with the 545i petrol a while ago.  On paper, both cars had similar performance, 0-60, top speed etc and the diesel version had a lot more torque, the petrol version more power but on the track, the petrol car was quicker by a considerable margin.  I imagine that the variable valve timing etc. of the petrol BMW would allow it to produce a decent and fairly consistent, albeit lower, amount of torque from 2500 - 6000rpm ish?

[AJ.Lintern]

It's torque at the wheels which counts. So if you compare two identical cars, one a low-revving TDi with 200lb ft of torque and 150bhp, the other a high-revving Honda Vtech with 150lb ft and 200 bhp. The more powerful car can make use of gearing to have decent torque at the wheels, so will be able to perform better, albeit with a different more hectic driving style - lots of gear changing to keep the engine revving! Generally though, having a torquey engine is preferable as it gives more flexibility - something particularly desirable in a road car, where you don't necessarily want to be hitting the red line with every gearchange to find any performance!

[GT]

If it's all about torque and acceleration, the argument is a little different.

 

Acceleration of the moving components with a "fixed" torque value varies enormously from one engine to another.

Eg. With a heavy crankshaft and flywheel those parts will accelerate slower than the weight of the car, so gearing them down is pointless.

This is why diesels usually use very high gearing and the engine/crank doesn't accelerate fast.

 

A racing type Vtec engine will accelerate fast because most of the components are lightweight, and the amount of inertia (read energy) required to accelerate that lot at 4000rpm is lower than the heavy diesel (read large engine 3.5L+).

 

These are different things, but the proof is, who won le Mans the last few years?

DIESELS.  

(with lightweight STEEL pistons)

[AJ.Lintern]

I guess a diesel engine is more suited to an endurance race really - more likely to survive longer with the lower revs? What sort of power output did they have compared to the petrol LMP cars? If it's anything like similar then they would have a clear advantage.

[sorbs]

I guess a diesel engine is more suited to an endurance race really - more likely to survive longer with the lower revs? What sort of power output did they have compared to the petrol LMP cars? If it's anything like similar then they would have a clear advantage.

 

Aren't they a bit more economical than the petrol cars too?  Fewer fuel stops?

[tomTR6Fan]

Interesting discussion on diesel engines and all, but I am not sure if anyone really answered my theoretical question - maybe I missed it: given the same amount of torque in an engine, why does moving the torque curve to a higher RPM result in more thrust to the car?

[charlieb]

Power = Torque x speed so if you have the same torque at twice the speed you have twice the power.

[tomTR6Fan]

charlieb, but that doesn't, of course, mean you have twice the acceleration.  Put it this way, in a given engine, what effect does moving torque up the rpm range have on acceleration?  Does it actually increase acceleration or does it make it possible to accelerate longer or does it make it possible to use a shorter gear longer - what is the acceleration advantage?  Example: assume for a moment you are driving a car in a any gear at 2000 rpm and then floor it.  You will feel a varying amount of acceleration over the power band until you reach the maximum rpm range for that engine, say 6500 rpm.  Assume the max torque is at 4000 rpm and max hp is at 5700 rpm.  Now lets move the same amount of torque to 5500 rpm.  We can assume that by mathmatical association with torque, horsepower goes up.  But what about acceleration?  Does peak acceleration go up or does it just stay high longer or both?

 

(If you could gragh acceleration on a curve like torque and horsepower, how would its peak and duration change when you move the torque up the RPM range?)

[spitNL]

Here is the question given the same amount of torque, why does moving the torque peak and usable range to a higher RPM result in more thrust to the car?  Considering the fact that there is more friction, heat and likely more vibration at higher RPM, why does an engine accelerate a car quicker when spinning faster?

 

A car doesnt accelerate faster at a higher rpm.

If you stay in the same gear, maximum acceleration occurs at the engine rpm with maximum torque.

I made an excel sheet which shows this.

Plotted in the figure below are:

-accel3: acceleration in 3rd gear.

 

-Engine torque

-Engine power

 

What we see is that maximum acceleration occurs at the engine rpm with the greatest torque, not maximum power.

 

 

However maximum possible acceleration at a given speed occurs at engine revs with maximum power. So in this case the maximum possible acceleration for this vehicle at 130 km/h is 2.1m/s^2 .

And although the maximum acceleration in this figure is 2.6 m/s^2. But this is not the maximum acceleration possible at this speed. For that you need to change down to a gear where power is bigger.

For that lets look at the figure below.

 

 

In this figure we see the acceleration curves in each of the five speeds.

Letst look accel2-acceleration in second gear.

We see that maximum acceleration in this gear occurs at just over 50 km/h.

But this is not the maximum acceleration possible with this car.

If we change down to first gear we see that we can attain a greater acceleration at 50km/w dispite of not having maximum torque in that gear.

So maximum acceleration for that vehicle at a certain speed occurs at engine speed with maximum horsepower.

 

What we also see is that the greatest possible acceleration is reached at the lowest vehicle speed and lowest gear.

Why? Because that is when the resistance forces are smallest. (air and rolling resistance)

 

In the figure below I have plotted the acceleration curves in each of the five gears against the engine speed.

 

Again we see that the maximum possible acceleration in a gear is at maximum torque and not maximum power.

 

Now let's look at the top speed of the car.

We see that the resistance force and power increases dramatically as vehicle speed increases.

In fact for double the speed you need four times the power.

We see that maximum speed is reached at maximum engine power, not maximum engine torque.

The vehicle speed where the engine power and resistance power cross each other is the topspeed of this car. If we were to change the final gear ratio we shift the power curve left of right. Now if we would change the gearing to get more engine torque at maximum speed, the maximum speed would drop because there would not be enough power to overcome the wind and rolling resistance powers.

In the figure below I have changed the diff ratio from 3.9:1 to 2.8:1

We look to see where the engine power and resistance lines cross, we see that they cross at 150km/h. So we have dropped the top speed from 220km/h to 150km/h by changing the diff ratio!

 

 

 

So concluding we can say that:

- Maximum acceleration without changing gear occurs at the engine speed with maximum torque.

In formula:

F=force

m=vehicle mass

a= acceleration

F=m*a

a=F/m

 

- Maximum acceleration at a certain vehicle speed is possible if we select a gear with the engine rpm with greatest power.

In formula:

P=power

T=engine torque

Rad/s= engine revolutions in rad/s (1 rev/min = 1* (2pi/60) rad/s)

P=F*v

F=m*a

P=m*a*v

a=P/(m*v)

P=T*rad/s

a=(T*rad/s)/(m*v)

 

Frederick

[Jango]

When i'm in vegas fuel is just so cheap, so out there i would have gas car or van any day but my Berlingo 1.6 does 50 miles to a gallon if i keep it below 2000rpm

[Gerard]

but my Berlingo 1.6 does 50 miles to a gallon if i keep it below 2000rpm

 

Yes, they really could do with a sixth gear!

 

Bloody hell Fred! That must have taken some time...

 

[spitNL]

Yeah Gerard, took me a few hours late at night. (computer)

 

If we translate this to diesel and petrol cars we can say that if they both slam their pedals from low rpm, with the same overal gearing, the diesel will accelerate faster.

The petrol car will catch up as they get higher in the rev range.

Diesels generally have more torque, but not as much horsepower.

Because it has more torque it will provide more force at the wheels so a greater acceleration at low rpm.

How can the petrol car make up for the bigger torque of the diesel?

By changing down a gear, going higher in the rev range.

Inspite of having a lower torque at the higher engine rpm, the torque at the wheels will increase because of the lower gear he selected.

So increasing the force at the wheels and therefore acceleration.

 

Frederick

[Gerard]

e can say that if they both slam their pedals from low rpm, with the same overal gearing, the diesel will accelerate faster.

 

In theory.

 

In real life, the Diesel will billow out black smoke and probably stall. The petrol car will spin a wheel and career off sideways into a ditch due to the driver trying to keep both of his elbows in the sun.

 

A triumph would just lose a wheel/break a trunnion/have an electrical problem/ spit the diff all over the road. ;D