The weird thing is you've just agreed with my statement in your previous post.
is a longer way of saying what I've just said, and you replied saying "Yes".
Why agree with one yet disagree with the other?
Sigh. If you measure the time taken for say 5 mph increments in speed in any one gear, the shortest time will be at peak torque. Not peak BHP.
You decided to choose a road speed where the engine was at peak BHP in that gear. The only way to get the same road speed at peak torque would be by using a higher gear, since peak torque is always at lower RPM than peak BHP.
That doesn't explain why you disagree with what I said yet not with what Roger said. None of what you've said just there disagrees with what I said - best rate of change of speed is achieved with the gear chosen so the engine at its maximum power, not maximum torque.
Or is this entire argument about you saying that the fastest instant acceleration at all the road speeds between 0 and maximum speed will be at the road speed where the engine is at peak torque in 1st gear? I won't disagree with that, but don't think it's terribly useful, because generally people are concerned with the fastest way of getting between two road speeds (eg 20-60). And for that, the fastest way is to choose the gear such that the engine is at or closest to maximum power, not maximum torque. Ie change up late, not early.
Well you are simply WRONG.
Consider: at a given speed you want to maximise thrust. Thrust is linearly related to torque, so what you want is to maximise the product of torque and the speed, which happens to be the dimension of POWER.
In order to get the best acceleration you set the engine at peak POWER and use a particular gear ratio to match its RPM to the desired road speed.
Knowing when to stop thinking in terms of force and or torque, and start thinking in terms of power and energy, is actually a key thing you learn as an engineer.
Acceleration is adding kinetic energy, to get best acceleration you need to deploy the best deployment of energy with time, that's power.
The mere mechanical detail of whether that energy is being deployed at a wholly inappropriate crankshaft speed is why you employ mechanical engineers to design gearboxes.
Yes.
Well yes, if they have a need for peak acceleration.
Ok...
ok...
Why don't you want to maximize the torque --hence the thrust-- anymore, like you did above?
Thomas Prufer
Because torque does not stay constant with SPEED.
So to get the best torque at any given speed, you need the most power out of the engine.
If that's too hard, think of energy and power and the relationship of rate of change of kinetic energy to them
I.e to move from say 50 mph to 70 mph requires an *energy* input of 1/2 mass x (70^2-50^2) (excluding drag)
The faster that energy goes in, the faster you get that job done, and the rate of energy is POWER.
Consider a water wheel which gives as much torque as a formula one engine, but rotates once every 10 seconds. Which do you think would accelerate a block of whatever the fastest?
You want torque? How about a worm drive. Given a couple of worm drives I can by hand generate more torque than a car engine.
Does that mean I can out accelerate the car on a bike?
At anything faster than 1/10 mph?
You are not alone in the confusion. I spent years patiently explaining that 'static thrust' on a model aeroplane propellor is only useful if you want it to emulate a helicopter, because static thrust drops away at speed.
Power delivery at the speed you want is what you want.
Indeed. If you want to accelerate the car as fast as possible, you need to maximise the power available at the wheels. This invariably means taking the engine up to at least its maximum power speed before changing up to the next gear. I say "at least" because quite often - unless you have lots of closely spaced gears - you still get more acceleration in the gear you're in by keeping going beyond max power speed until you run our of revs than by changing to the next gear which drops you a long way back down the power curve.
To see this properly, you need - using the engine torque curve and the gearing - to plot thrust vs road speed in each of the gears.
In each gear the thrust drops off when you're above the max torque point
- but it's invariably higher than it would be if you were to change up to the next gear.
[I mis-spent a lot of my youth doing this sort of thing in the motor industry many decades ago!]
Then you haven't understood what I've written. Many many times.
Can I not get through to anyone that the gear used is irrelevant - assuming it allows the vehicle to reach maximum BHP in that gear?
The discussion was about where on the *engine's* output maximum acceleration occurs. Not about achieving the best 0-60 mph or anything else.
No, You're wrong. For one who claims to have a degree of engineering knowledge, it's unforgivable. There is no opinion involved. Just facts.
But BHP does? You're excelling yourself with this one...
And idea set of gearbox ratios would allow you to change up round about peak BHP and the next higher gear would plonk you at near enough maximum torque...
I'd guess you're wasting your breath. ;-)
It would help if you weren't assuming everybody in the discussion is an idiot. Pretty much none of the people are, not even you.
To most of the people in this discussion, the discussion is about how to achieve maximum acceleration during normal (or rather 'spirited') driving, at whatever start and end speed the situation requires - and the answer to that is to choose the right gear to get as near to maximum power as you can, not maximum engine torque.
To you it seems to be only about at which point when you're racing from
0 to whatever are you accelerating fastest - which I agree is max torque in first gear.Is that an adequate summary of the two positions?
I did a little test with my Defender which has max torque at about 2000 rpm and max power at about 3500 rpm. Going up the gears and accelerating as fast as I could I never got anywhere near max power in any gear before I could feel the acceleration dropping off.
But did changing to a higher gear result in faster acceleration?
Peak power revs is normally a point where your acceleration is dropping right off. For the best 0-60, standing quarter, or whatever you would normally change up before you reach peak power (as long as you have another gear available). You change up somewhere beyond peak torque so that in the next gear you are going back through the torque peak again. Exactly where you change is also partly determined by how much time you lose in the gearchange; you might decide to start in second if you have a low first ratio, or to reduce wheelspin. If you are drag racing in a Landy you probably would not use low ratios!
Power, revs, and torque are inextricably linked but I think that most mechanical engineers who go back to the basics think of it being the torque which provides the mechanical force at the wheels (and hence the acceleration).
I don't think that is true. Peak power will give you the fastest change in speed - if you have a choice of gear X at peak power and a higher gear Y at peak torque for that road speed, you will accelerate fastest by being in gear X.
I don't find any of the relationships confusing.
Power = engine torque * RPM
Acceleration = wheel torque * constant
wheel torque = engine torque / gear ratio
(where higher gear = higher gear ratio)
That's all there is. The available torque at various RPM is of course unknown, depending on the engine itself.
No. Vir Campestris started it again after say the same as many on here seem to think. That an engine accelerates at its best at peak BHP. But was wrong before and is wrong now.
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