Which relates to my statement:
"If all cases were special cases, then there would be little point in formulating general principles at all, would there ?"
( A response to your above claim that "all cases are special cases" )
and to which I assume you were responding, how exactly ?
michael adams
...
In article , Thomas Prufer writes
Let's bring clutches into the argument just for a laugh.
It seems that it is agreed that Max Speed is only attained at Max Power. At any speed less than Max Speed, Max Power is unavailable. WOT (Wide Open Throttle) is available at any speed, but does not equal Max Power, but, at, say, 3000 RPM, WOT would supply Overall Max Torque. Remember, WOT will give maximum effort at EVERY engine speed. So, the above quotation should be re-written to read "... maximising the amount of power, available at that engine speed by opening the thottle fully, transmitted to the wheels."
No true. Max speed is usually defined as the maximum speed which can be obtained on a level road on a windless day - and occurs when the max available driving forces match the resistive forces from aerodynamic drag, etc.
But you may need max power to climb a steep hill at a lower road speed. In this case, the driving force is also being resisted by the component of the vehicle mass acting down the slope.
And you can use maximum power - albeit transiently - whilst accelerating through the gears. In this case, the driving force is rested by the inertia of the vehicle - F = M x A etc.
Wide open throttle at any engine speed will give the same torque at the crankshaft as you could measure on a dynamometer at the same steady speed. But it may not all get out to the outside world in a dynamic situation because some of the torque will be absorbed accelerating the engine itself.
Well, if Dave wishes to continue arguing otherwise, I don't see any point in further correspondence on the matter.
The long and the short of it is that accelerating a mass is a matter of raising its kinetic energy and the quickest way to achieve that is the use of a range of gear ratios such that the engine operates as close as possible to its peak BHP rpms during the change up sequence required to accelerate the vehicle as quickly as possible.
Peak torque revs with any practical piston ICE are unlikely to produce more than 60 or 70 or so percent of its maximum power output so that's most definitely the wrong choice of gear change point as far as maximizing a car's acceleration performance is concerned.
It's a pity I couldn't have distilled it to just those two paragraphs sooner rather than later. :-(
What you say is absolutely right, but I'm afraid that you're wasting your breath as far as Mr P is concerned. He believes that torque is GOD
- period!
I wonder how he would view the electrical analogy - where torque equates to current and speed to voltage. Would he consider that 10 amps at 12 volts is just as useful as 10 amps at 240 volts?
"Torque, moment, or moment of force (see the terminology below) is the tendency of a force to rotate an object about an axis,[1]
Given that torque is the power transmitted via the drive axle which causes the wheels to rotate, its difficult to see what's wrong with his claim, quite honestly.
Except maybe in cars fitted with sails.
michael adams
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Energy of moving object = mass x velocity squared.
If you want it to go faster you have to input energy.
If the energy source has higher output at high revs you have more to put in.
In a car this quite often means changing gear and possible slipping the clutch and is therefore a dynamic system and not a static one like having an engine running at one speed at maximum torque.
See its easy to argue.
Yes.
Only if you choose to deliberately conflate engine torque, which is presumably measured at the flywheel, with driven axle torque which actually turns the wheels.
I'm not concerned about the engine.
For the wheels to be turning or accelerating fastest - in the absence of the car being powered by external factors such as the wind propelling the body, they need to be subject to the maximum available torque.
Quite how it got there, is an entirely different question.
michael adams
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>
That's the problem - Mr Plowman *is*.
Indeed. And you obtain that by running the engine at max power and using a suitable gear ratio. But Mr Plowman doesn't accept that. He says that you have to run the *engine* at max *torque*. So he would end up using a lower engine speed - producing less *power* - with higher gearing which provides *less* torque multiplication - resulting in less torque at the wheels. Mad!
Sorry, but maybe I'm missing something here.
Basically the engine is pistons on con rods going up and down which is converted to rotary motion by cams on the crankshaft which is attached to a flywheeel.
And as I understand it anyway, its from the flywheel that engine torque, is measured.
And so my question is this.
If the engine is running at maximum power, and thus the crankshaft is turning as fast as possible, then how is it possible for less than maximum torque to be delivered to the flywheel ?
michael adams
...
Wow! it seems I missed a few dozen posts due to excessive indentation in the thread list. I mention this to explain my absence from this part of the discussion.
What you mention in the paragraph above is all true but the argument by Dave over max torque versus max power rpm change up points for maximum acceleration didn't need these extra real world complications which *do* further aid the fact that best acceleration is achieved by changing to the next higher gear at or just above the peak power revs.
If you don't mind abusing the clutch, that does provide additional acceleration impetus. It was a trick I often used with the works van when accelerating up to speed after hopping onto a motorway section. :-)
Yet another consideration. Most noticed when driving a naturally aspirated diesel powered 3 1/2 tonner when starting off in bottom gear. You soon learned to live with the almost total absence of perceivable acceleration you'd normally experience with a lightweight petrol engined van.
Again, I excluded this extra consideration from my arguing the case for max power revs. Indeed, the perfect automatic continuous ratio gearbox holding the engine to its max power revs rather neatly eliminates this specific issue. :-)
There are many real world effects that complicate the issue being discussed here. One of them being Dave's assertion that the DAF's CVT system was optimised to hold the engine to it's max torque rpm under full throttle acceleration or hill climbing conditions. If true (and it might well be), this optimises for best fuel efficiency / acceleration ratio because, almost without exception, peak torque revs in a piston ICE coincides with maximum efficiency. This won't maximise acceleration performance but it does reduce strain on the transmission components somewhat which would be a major consideration in such a choice.
In short, to boil it down to its essence, accelerating a body from rest to a different speed imparts kinetic *energy* to that body. The key thing in this being the requirement to impart *energy* into said body. The greater the acceleration you wish to impart, the greater the rate of energy delivery that needs to be imparted. Energy delivery rates are expressed as 'power' Kilowatts or horsepower. The higher the power level, the greater the acceleration hence the choice to keep your prime mover (the engine) operating at or close to its maximum power output revs by use of a variable ratio transmission system.
Torque != angular momentum.
You could have it spinning at 6k RPM but the slightest load (in terms of a braking torque opposing the motion) slows it down.
You could have the same engine running at 3k RPM and it takes a much much larger braking torque to slow the engine.
Thanks anyway, I've answered my own question.
" Typically, the torque peak will occur at a substantially lower RPM than the power peak."
At least, somebody else has.
michael adams
...
That's because it's the most effective way to measure the performance of a jet engine. However, the use of 'thrust measurement' isn't exclusively reserved for jet engines. You can measure or calculate thrust imparted by the driven wheels of a road vehicle readily enough and is a valid way to measure performance. It can easily be measured on a rolling road dynamometer for example, which when multiplied by the rollers' circumferential speed, calculates the effective output power being employed to accelerate the vehicle under test after the losses in the transmission system have taken their toll.
For the purpose of this particular argument, the real world consideration of rolling resistance and aerodynamic drag on a car's performance is an unnecessary complication even though these factors are essential in determining the required engine power to drive the car to a specified maximum speed.
True enough when all that's being discussed is the optimum change up point as you accelerate the car to some predetermined speed in the shortest time possible, assuming the target speed is below the drag limited speed.
When this assumption is met, the best acceleration performance, regardless of fuel economy, will always occur when the engine is allowed to operate at or close to its maximum power output revs since accelerating a mass involves raising its kinetic *energy* at as high a rate as possible.
Since energy delivery rate is simply a matter of *power level* usually expressed in KW or BHP, it follows that the best acceleration, regardless of other complications, can only be achieved by operating the prime mover (car engine) at its maximum power revs courtesy of a variable ratio transmission system.
In practice, no piston ICE reaches its limiting maximum power output at the same maximum torque rpm speed unless deliberately governed to prevent it going faster than the maximum torque rpm speed (which is a distinct possibility with the diesel engines used in 38 tonners). Even in this case, all that achieves is the coincidence that the optimum change up point matches both max torque and max bhp rpms.
As did I elsewhere in this thread. However, it's an exercise that's rather akin to (and almost as distasteful as) picking at a scab. Somehow or other, we just can't resist 'having one more go'. :-(
Ah! I see your problem now. If, as you've seemingly posed the question, we're discussing, "When does the maximum instantaneous acceleration over the operating rpm range of the prime mover occur in any *one* gear ratio at a time?", then the answer to *that* question *will indeed* be at maximum torque rpms.
However, since you used this little factoid to claim that the fastest acceleration of a car can therefore only be achieved by making each successive up-change at peak torque revs rather than at peak power revs, you've immediately posed an entirely different more complex question which now involves best gear selection choice versus engine revs to maximise the acceleration of the whole vehicle.
I'm sure that, if you care to pose this completely different question to the engine building and tuning forum, you'll get a different response, one that matches what most in this NG have been trying to tell you, seemingly to no avail over the past 8 days ever since you posted these statements:
"And you'd get even more force at peak torque in that gear..."
You get the best acceleration with the maximum torque *at the wheels*. And in any given gear, this will be when the engine develops maximum torque."
Whilst, on careful examination, both statements *are* strictly true, the implied maximum acceleration requiring the engine to be held to its max torque rpm as you progress through the necessary gear changes is patently
*untrue* since accelerating a mass involves raising its kinetic energy which means its acceleration depends on the how swiftly you can increase its kinetic energy which in turn is a function of energy rate or power.I've been re-reading your subsequent posts and you seem to be purposely avoiding this particular aspect of your implied question over best use of gear changes versus max torque or BHP rev points to accelerate a car to speed. Indeed, I spotted a statement where you seem to be trying to limit the discussion to the engine alone sans the complications introduced by the pesky transmission system.
Whether you were merely trying to introduce a confusingly phrased assertion to stir things up or were confused yourself still isn't entirely clear but I do have my suspicions that you were trying to 'Stir up a Lively Debate' between mostly just yourself and the rest of the group. If my suspicion is correct, may I be the first to congratulate you "On a Job Well Done". :-)
Yes but Dave was. If you carefully examine his statements in the post he made 8 days back which I've quoted here:
"And you'd get even more force at peak torque in that gear...
You get the best acceleration with the maximum torque *at the wheels*. And in any given gear, this will be when the engine develops maximum torque."
You'll observe that, strictly speaking, they're actually true! :-)
He can't help it if others want to misinterpret the second statement as implying best acceleration of a car being achieved by choosing to maintain the engine speed at or close to peak torque revs rather than peak power revs by choosing gear ratios to maintain this state. :-)
I suspect Dave was merely trying to 'pep up' the debate, relying on our human propensity to read into such bare statements their own interpretation of the 'question' thus posed. Even I failed to observe the strictness applied to those statements (Damn! Where *is* a "Sheldon Cooper" when you need one?)
More accurately, that last statement would be better paraphrased as,
"they need to be subject to the maximum torque available at that road speed."
Quite! For most of us here, including myself, this is a matter dealt with by optimal use of the gearbox to operate the engine as close to its maximum power output revs rather than the, as implied by Dave's statements, maximum engine torque revs.
Wrong. If you want max speed that will correspond to max power with exactly the right gear ratio.
If the gear ratio is not optimal, it wont, but that's not what was said
What was said was that you couldn't use max power at any speed lower than max speed. That is wrong - and I explained why, but you conveniently snipped the relevant bit of my post!
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