Torque Power etc

[TonyLees]

About time this forum had something interesting posted?

Torque, power and performance

The facts are:

The torque developed by the engine is multiplied by the gearbox and axle ratios to produce the drive wheel torque, which when then divided by the tyre rolling radius gives the tractive effort.
To drive the car at a constant speed requires a certain tractive effort, to overcome wind and rolling resistances, the major part of which varies as the square of the speed. If the applied tractive effort exceeds the required effort the car accelerates at a rate equal to the available effort minus the required effort divided by the mass of the vehicle. At maximum speed, or at any lower constant speed at part throttle, applied effort equals required effort and the acceleration is zero.
Engine torque times engine speed equals engine power, which, apart from transmission losses, is the power driving the car, which equals the tractive effort times the vehicle speed. It therefore follows that max acceleration at any vehicle speed is achieved when the engine is developing its max power.
Changing up when over the peak of the torque curve to pick up at the same level on the rising torque curve in the higher gear isn?t the route to best performance. Although the engine torque remains high, in the higher gear it has been multiplied by a lower number to produce the prop shaft torque; a higher prop shaft torque and therefore tractive effort is maintained by hanging on to the lower gear up to at least max power speed.
In fact, to stay as close as possible to the max power and acceleration condition as the car accelerates means actually going over the peak of the power curve until the power has fallen to the same level as on the rising power curve in the higher gear, mechanical sensitivity or a deep enough pocket permitting!

What this simple analysis does not take into account is the engine torque required to accelerate the rotary inertia of the engine and flywheel. This can be expressed as an "additional equivalent mass", the value of which is different for each gear. It is probably only significant in reducing the torque reaching the prop shaft in the lower gears, when, for example, the engine may have to be accelerated from 700rpm to 4000rpm whilst the car goes from walking speed to 24mph.

Apparently...

[John Emberson]

Tony,

Are you having a detox January (like me) which has brought about this amazing clarity of technical thought or have you a new job in the Mclaren engine dept.............

[TonyLees]

Hi John,
January detox is bad for you - February shocks the system too much
Don't forget the internet mantra - steal, steal , steal.
Cheers, Tony

[Bob Bull]

Tony,
Quite a nice presentation, however, I think you are over simplyfieing it a bit.

Bob

[Andy Green]

Tony

All good stuff,seems you got a new book for Christmas on engines while I got one concerning aero dynamics

But if you drive "over" the peak power curve this actually means reving higher,which will ruin the engine and on a V8 mean dropping a valve

The effect of changing gear means briefly no drive and higher revs, then the resistance slows the car,meaning for the first few micro seconds the clutch/engine when re ingaged have to take up the slack in the drive train(shunt)
Therefore the engine in the new gear should be engaged in the mid power range to deliver smooth acceleration

If you have over revved prior to the gearchange the revs will not have dropped enough,then with the force of the slack etc you will actually get poor pick up

So better to change early,especially in a V8,then you take advantage of the torque curve not just the power curve

The power curve is really missleading in a lower reving engine like the V8

But I look forward to following you at Donington with your new technique,could you please park to the right when the engine goes bang!

[TonyLees]

mechanical sensitivity is my mantra

[Philip Tisdall]

Help, all these big words… and I failed physics “O” level ( did get a few marks for spelling my name correctly though)
I think the simple formula and soulition is “ there is no substitute for Cubes”

[Chris Acklam]

Interesting Tony. Please expand.

[TonyLees]

Interesting Tony. Please expand.

mantra = "ommm" ?

[Jim Mountain]

Hands up all thows that have been having trouble sleeping

[JERRY KNIGHT]

About time this forum had something interesting posted?

Torque, power and performance

The facts are:

The torque developed by the engine is multiplied by the gearbox and axle ratios to produce the drive wheel torque, which when then divided by the tyre rolling radius gives the tractive effort.

no
wrong . its divided not multiplied ,theoretically only 4th gear is equal
and dont forget friction losses in the drivetrain.
I bet you are in to computers !

[TonyLees]

Blame the author
Perhaps worth adding that brake mean effective pressure (BMEP) times engine swept volume equals engine torque?

[JERRY KNIGHT]

Blame the author
Perhaps worth adding that brake mean effective pressure (BMEP) times engine swept volume equals engine torque?

now thats interesting Tony , perhaps you could explain that for me.

[TonyLees]

Blame the author
Perhaps worth adding that brake mean effective pressure (BMEP) times engine swept volume equals engine torque?

now thats interesting Tony , perhaps you could explain that for me.

I could... if I knew what it meant

[Bob Bull]

Brake! Mean effective pressure on appropriate pedal or car not stop, Kimo Sabbi! comprendi!

Another equation worth noting is 'Any two sides of a triangle are together greater than the third.'

Bob Bull Professor of Hard Sums