Seamless Gearboxes (aka blowing in the shifts)

[Tommy Cookers]

Here's a good test to prove the torque multiplication effect... take your family hatch (I.e. low powered car) and gently get the car rolling, and fully engage the clutch. Now quickly open up the throttle... you will almost certainly not break traction (spin the wheels)
Now get the car rolling gently again, rev the engine up high, feed in the clutch and open up the throttle to maintain high revs... wammo: spinning wheels, even in your family hatch. The spinning wheels occur because you are attempting to transmit more motive force than the tyres can handle, showing that you've increased the force by use of the clutch.

Basically you've temporarily created a high ratio difference between engine speed and road speed, and thereby increased force multiplication. However, whereas a normal gear is about 98% efficient the use of the clutch in this way is a lot less efficient because of the heat generated....

this 'torque multiplication' effect is not really torque multiplication via changed velocity ratio
it is increased torque produced by the engine being allowed to run at higher rpm ie closer to maximum torque
addition of clutch slip (at a given throttle opening) reduces the load on the engine, so increases the rpm and engine torque
much of this increased torque drives the car, some is lost in clutch slippage ie converted to heat
the clutch slippage is not in reality held exactly ie the torque transmitted by the clutch is allowed to rise in this case
this is learned behaviour reinforced by apparently endless repetition

if the clutch really has a fixed limit of torque transmission ie due to a fault or by design the situation is different
this applies by design in helicopters, where there is a slipper clutch driving the rotor
the load being potentially greater in accelerating the rotor on the ground than in flight
the clutch is designed to slip on the ground to limit the load experienced by the gearbox

[machin]

Another method of proving the torque multiplication effect is to drive a car up a steep hill while towing a heavy trailer... if it is steep enough the car might not be able to drive up the hill because it can't produce enough motive force, even in 1st gear. However if you introduce some clutch slip you can increase the motive force and get up the hill. This test is better than my first example because any effect of rotating inertia doesn't come into play as its a constant speed (no acceleration) test.

Of course driving your car for extended durations this way is not good for the clutch and therefore I don't recommend you do this too often (if at all if you can avoid it!).

[rjsa]

Last try:

A leverage does force multiplication.

A damper does energy dissipation by way of heat loss.

A gearbox is a leveraging mechanism.

The clutch is a dampening mechanism.

A clutch does no force multiplication.

That's physics 101.

[rjsa]

http://en.wikipedia.org/wiki/Torque_con ... iplication

The maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications (although multi-element designs as used in the Buick Dynaflow and Chevrolet Turbo glide could produce more). Specialized converters designed for industrial, rail, or heavy marine power transmission systems are capable of as much as 5.0:1 multiplication. Generally speaking, there is a trade-off between maximum torque multiplication and efficiency—high stall ratio converters tend to be relatively inefficient below the coupling speed, whereas low stall ratio converters tend to provide less possible torque multiplication.

http://en.wikipedia.org/wiki/Clutch#Torque_limiter

Vehicular (general) [edit]
There are different designs of vehicle clutch but most are based on one or more friction discs pressed tightly together or against a flywheel using springs. The friction material varies in composition depending on many considerations such as whether the clutch is "dry" or "wet". Friction discs once contained asbestos but this has been largely eliminated. Clutches found in heavy duty applications such as trucks and competition cars use ceramic clutches that have a greatly increased friction coefficient. However, these have a "grabby" action generally considered unsuitable for passenger cars. The spring pressure is released when the clutch pedal is depressed thus either pushing or pulling the diaphragm of the pressure plate, depending on type. However, raising the engine speed too high while engaging the clutch causes excessive clutch plate wear. Engaging the clutch abruptly when the engine is turning at high speed causes a harsh, jerky start. This kind of start is necessary and desirable in drag racing and other competitions, where speed is more important than comfort.

Im's sorry I can't find a quote regarding clutch torque multiplication. But then again, I'd guess that's because there is no such a thing.

[Clew]

[Richard]

I agree that "torque multiplication" isn't the right term, only the gear box can do that.

The caravan up a hill example is where slipping the clutch does allow the engine to operate at higher revs than the wheel revs will allow, and those higher engine revs allow the engine to generate higher torque so it doesn't get bogged down.

Its about avoiding the engine getting bogged down in low revs & torque. If you avoid that then you are able to accelerate out of the problem. However, the overall traction is still limited by the grip of the tyres.

[autogyro]

Hmmm, so this must mean that the hatched area in the graph would be much improved by using a 'real' torque multiplyer.
i.e at least one much lower gear ratio.
If that were the case, to massively increase initial acceleration all that would be needed as well as the lower ratio/s would be wider or dual tyres on the drive axle to give better traction.

Now why didnt I think of that.

Mind you I would still like to know why a car accelerates faster from stationary with a slipping clutch or spinning tyres.

Hmm, what is the ideal wheel rpm / road speed for the best traction?

[autogyro]

Here is another way to look at the issue.

We have a nice engine dyno that uses a disc brake/clutch to hold engine revs.
We also have one that uses a fluid flywheel and another that uses a torque converter.

Can you tell me where and how we measure the engine torque on each machine?

I think you will find that a friction clutch is indeed a damper.
However this damper does allow for variations in the torque transfered through it.

That being the case it does work as a ratio changer to augment the gearbox.

[rjsa]

I think you will find that a friction clutch is indeed a damper.
However this damper does allow for variations in the torque transfered through it.

That being the case it does work as a ratio changer to augment the gearbox.

Yes it does, but the torque ratio is always <=1, sisnce you are only dissipating energy. It just reduces, does not augment anything.

[Richard]

The energy ratio will be <1, but the input energy is much higher. Hence it is possible to get more torque to the wheels.

[rjsa]

Ok, bad example, you're right. Let's try again:
EDIT:(fantasy numbers, I didn't do the reduction math - AND slipping at peak torque us quite unreal but... for the sake of the example please let it pass)




You are uphill hauling heavy cargo with you 1.6L hatchback. Let's say 30Kph in 1st gear and you need at least 130NM to keep going

Situation A - Clutch slipping 4/5th:
At the crankshaft you have 4000RPM, 150NM.
At the gearbox you have 2000RPM, 150NM.
Works

Situation A1 - Clutch engaged:
At the crankshaft you have 2000RPM, 120NM.
At the gearbox you have 2000RPM, 120NM.
Car will halt.

Situation B - Torque converter
At the crankshaft you have 4000RPM, 150NM.
At the gearbox you have 2000RPM, 220NM.
Works very well.

So while slipping the clutch will allow you to go uphill by keeping the engine at a better point in the torque curve, the clutch has no torque multiplying effect and is dissipating half the energy provided by the engine.

Now the torque converter will have a torque multiplying effect and will affect the effective reduction rate, even if still dissipating part of the energy.

[autogyro]

The energy ratio will be <1, but the input energy is much higher. Hence it is possible to get more torque to the wheels.

How can the energy ratio be <1, when you are converting a part of that energy into heat through the process?
On the other hand the input (engine) rpm is higher than the output (to the gearbox)rpm.
That is a ratio of rpm. (if not a gear ratio)

Slipping the clutch increase the engine rpm.
If that increase also increases the torque at the clutch, the process is torque multiplication.
If the process increases the torque at the gearbox input it continues to be a torque multiplication process.

Fluid flywheels and torque converters are also torque multiplyers, (they are not described as gear ratios).
They heat up the fluid they use as their torque transfer medium.
A fluid flywheel is one to one, a torque converter has a variable ratio dependent on the stator, turbine and impellor blade angles and the efficiency dictated by its size tolerances and the fluid used.
A slipping clutch heats up the friction material through which the torque is transfered.
All three are inefficient ways of transfering torque to a drive axle, just as slipping tyres during a start is an inefficient method of using available torque.
An electric drive is far more efficient and has torque available at zero rpm.

I mentioned engine dynos that use all three of these slipping components for the same result.
Measuring torque.
The explanation should be obvious, it is the math that is much more complex than you have as yet defined.

[machin]

If that were the case, to massively increase initial acceleration all that would be needed as well as the lower ratio/s would be wider or dual tyres on the drive axle to give better traction.

Now why didnt I think of that.

You didn't need to; I've been telling you that all along

Your initial acceleration is limited by your traction... if you increase the tractive capacity then the limit is now higher and the car will now benefit from a "shorter" (larger ratio) first gear. Of course there are limits: an F1 car has mandated tyres of a certain size/coefficient of friction, hence an F1 car wouldn't benefit from a "shorter" (larger ratio) 1st gear than the one they already use. Even in a (largely) unrestricted category like hillclimb and sprinting you have to contend with the law of deminishing returns regarding tyre foot print vs traction, not to mention the extra mass and inertia of larger wheels and tyres.

There is one way to massively increase the car's tractive capacity; 4wd... although the downsides to that are transmission weight, efficiency loss and packaging issues.

I'm glad you finally get it, how many threads have we ruined in the process?!

[autogyro]

You didn't need to; I've been telling you that all along

Your initial acceleration is limited by your traction... if you increase the tractive capacity then the limit is now higher and the car will now benefit from a "shorter" (larger ratio) first gear. Of course there are limits: an F1 car has mandated tyres of a certain size/coefficient of friction, hence an F1 car wouldn't benefit from a "shorter" (larger ratio) 1st gear than the one they already use. Even in a (largely) unrestricted category like hillclimb and sprinting you have to contend with the law of deminishing returns regarding tyre foot print vs traction, not to mention the extra mass and inertia of larger wheels and tyres.

There is one way to massively increase the car's tractive capacity; 4wd... although the downsides to that are transmission weight, efficiency loss and packaging issues.

I'm glad you finally get it, how many threads have we ruined in the process?!

Hahaha
It has been an interesting journey Machin.
I wish you would sort out your high gears from your lows though.
I know what you mean but it can get confusing.

[Tommy Cookers]

A fluid flywheel is one to one, a torque converter has a variable ratio dependent on the stator, turbine and impellor blade angles and the efficiency dictated by its size tolerances and the fluid used.
A slipping clutch heats up the friction material through which the torque is transfered.
All three are inefficient ways of transfering torque to a drive axle, just as slipping tyres during a start is an inefficient method of using available torque.
An electric drive is far more efficient and has torque available at zero rpm.

the 'torque converter' (strictly a 'converter coupling') with its related gearbox can work very well in this sort of motor sport
the high stall torque competition type will keep the engine in its powerband throughout the event (eg the Chaparral 2F)
this is well known in the USA and it works in other countries too
the 'fluid flywheel' (strictly a 'coupling') is 94% efficient when working as intended
current car 'torque converters' in cruise (mechanically locked) are 98% efficient (better than typical gearboxes)

such couplings were intended for traditional mechanical configurations, end-on transmissions allowing direct drive top gear
they are disadvantaged when forced into equivalency with transverse transmissions (needing step-down etc gearing added)
unfortunate, as they work brilliantly with simple modern control
as would the first generation true automatics, in top gear these had only 25% of the torque transmitted hydraulically
so were then about 98% efficient

a good use for electric drive is in hybrid integration with mechanical, even fluid drives, eg facilitating lossless 'geared neutral'
there is nothing efficient or economic about using EMs to give high torque, low in the speed range
this approach was beaten 100 years ago by the ICE (ask Camille Jenatzy aka 'la jamais contente' and the UK replica at Brighton)
worthwhile EVs have gearboxes, don't they ? (to avoid high torque/low speed operation of the EM)