Does clutch control affect torque output in cars?

[Drakkith]

Torque is not a function of the difference in radial velocity between the slipping clutch and mating pressure plates. The torque will not change to the driven shaft if the clutch is slipping whether or not the engine speeds up, as the friction within the clutch would be independant of the difference in velocity. Power on the other hand does depend on the radial velocity. With a slipping clutch, the excess power that is not transmitted to the driven shaft from the engine will end up as heat within the clutch and mating parts.

I don't follow you.

I think that there is not just a simple answer to this but I think you really have to consider Power as well as torque.

I think it is very simple. If you are not at max torque, then allowing the clutch to slip and the engine to rev up to get to max torque CAN produce more torque to the wheels through the drivetrain.

Clearly, a IC engine can't go slower than about 1.5k with any appreciable power output. You can get around this, at takeoff, by revving the engine and slipping the clutch (wasting power but producing more torque into the gearbox).

This is exactly the phenomenon this thread is about.

This will work in first gear and even, if you're being sloppy, in second. If you are in third, by mistake, then it's very hard and you can only accelerate (pull away) along the flat. As you go up through the gears, you need more torque at the input to the gearbox for a given amount of acceleration (torque at the wheels) - at a given speed - so the engine needs to be revving faster and the clutch needs to be dissipating progressively more power.

Gears are irrelevant to the discussion. This is solely about torque input to the clutch. The amount of torque applied will change depending on how much the clutch is slipping and how much torque the engine is providing.

 

[sophiecentaur]

I don't see how gears are irrelevant at all. The torque delivered to the wheels is the bottom line and that is affected by the gear. There may be a fairly wide range over which you can benefit by slipping the clutch but there is an upper limit to the ratio you can do it with, for a given torque required at the road wheels. The question was about driving a car and my response was putting things in context, which involves more than just the torque delivered to the gearbox. Starting or driving on any slope involves power and, if you can't deliver the power, the car won't do what you want.

 

[Drakkith]

I don't see how gears are irrelevant at all. The torque delivered to the wheels is the bottom line and that is affected by the gear. There may be a fairly wide range over which you can benefit by slipping the clutch but there is an upper limit to the ratio you can do it with, for a given torque required at the road wheels. The question was about driving a car and my response was putting things in context, which involves more than just the torque delivered to the gearbox. Starting or driving on any slope involves power and, if you can't deliver the power, the car won't do what you want.

Because we aren't worried about torque delivered to the wheels, we are worried about torque delivered to/from the clutch. The wheels are just the final step in the chain AFTER the clutch. The context of the question is about the way torque is delivered to the clutch from the engine, not how the entire drive-train works.

 

[D H]

Not that it's a good idea, but there is this certain hill on my way home from work...

If I coast in neutral for a few seconds with the clutch fully disengaged and then shift into first my car goes "wub wub wub wub ..." The car's nickname is of course Curly.

Slipping back on topic, there are lots of occasions where you do want to feather the clutch. You can't smoothly start from a dead stop without feathering the clutch, particularly uphill. I got whiplash multiple times while teaching my kids how to drive a manual.

Feathering can also be useful while the car is moving, particularly if you don't mind paying for a new clutch now and then. (And drivers of race cars don't care, so long as the clutch doesn't give out during the race.)

 

[nitsuj]

Because we aren't worried about torque delivered to the wheels, we are worried about torque delivered to/from the clutch. The wheels are just the final step in the chain AFTER the clutch. The context of the question is about the way torque is delivered to the clutch from the engine, not how the entire drive-train works.

Well, the wiki quote says it increases torque to the wheels, which now I would agree with. But simply because of engine speed to wheel speed ratio, and only under specific conditions.

However I disagree with you and the wiki quote that higher RPM (upto max torque RPM)means more torque regardless of energy input.

The engine on the other hand does not increase in torque simply because it revs more closely towards the RPM that produces max engine torque under WOT. The engine speeds up when the clutch clips because of reduced resistance on the engine, not because the engines current power output increased. An engine reving without any load isn't producing rated torque.

That wiki quote is poorly worded.

I'd guess if you go WOT in a car with a CVT transmission, the computer is programed or the engine naturaly revs at its max torque RPM as the car accerlerates. Because it provides the highest toque output. My tractor is throtle controlled by a rev limiter. I put it at WOT and now my power plant will (thanks to the govener that maintains engine speed) always produce max RPM to the drivetrain. This is simular to what the slipping the clutch thing is, only a very inefficient and costly way to achieve an inferior (as opposed to just changing gears) increase in wheel torque.

 

[sophiecentaur]

Because we aren't worried about torque delivered to the wheels, we are worried about torque delivered to/from the clutch. The wheels are just the final step in the chain AFTER the clutch. The context of the question is about the way torque is delivered to the clutch from the engine, not how the entire drive-train works.

The gears are very relevant to operating a car. The ratio actually affects how much power you can deliver to the wheels, with or without slipping. Torque is only part of the story because revs will be different for each gear you use.
This thread is clearly about driving cars. There is a good reason why you take off in a low gear. It's because there is an optimum engine speed for starting off and a higher gear would involve too much loss and a super low gear would mean you'd have to change up by 10mph; not useful.
You seem to want to ignore the relevance of Power to the situation. But are there any of my 'facts' that you disagree with? I don't see why you don't want to widen the discussion to explain the practicalities of use of clutch and gears.

 

[Drakkith]

Well, the wiki quote says it increases torque to the wheels, which now I would agree with. But simply because of engine speed to wheel speed ratio, and only under specific conditions.

Exactly! Normally you wouldn't worry about the clutch after you start moving and instead rely on different gears.

However I disagree with you and the wiki quote that higher RPM (upto max torque RPM)means more torque regardless of energy input.

Note that they are only referring to torque, not to power or energy. The engine itself is locked at a set Torque/Power output that depends on RPM and how much fuel is burned each time a cylinder fires. The transmission multiplies this torque by varying the RPM of the axle and wheels. Obviously in 1st gear you generate MUCH more torque at the wheels than you do in 4th gear, yet the engine itself always outputs the same at a given RPM and fuel use.

The engine on the other hand does not increase in torque simply because it revs more closely towards the RPM that produces max engine torque under WOT. The engine speeds up when the clutch clips because of reduced resistance on the engine, not because the engines current power output increased. An engine reving without any load isn't producing rated torque.

You are correct, but that is not what the OP's quote is referring to. If you keep the gas pedal in the same place but push in the clutch, then yes the engine revs because of less resistance. It is when you push the gas pedal in further and ease off of the clutch to keep the RPM near max torque that you see the effect.

That wiki quote is poorly worded.

I agree.

I'd guess if you go WOT in a car with a CVT transmission, the computer is programed or the engine naturaly revs at its max torque RPM as the car accerlerates. Because it provides the highest toque output. My tractor is throtle controlled by a rev limiter. I put it at WOT and now my power plant will (thanks to the govener that maintains engine speed) always produce max RPM to the drivetrain. This is simular to what the slipping the clutch thing is, only a very inefficient and costly way to achieve an inferior (as opposed to just changing gears) increase in wheel torque.

Yep. Usually you can simply shift to a lower gear to get the increased torque you desire. Using the clutch is normally just a waste.

This thread is clearly about driving cars. There is a good reason why you take off in a low gear. It's because there is an optimum engine speed for starting off and a higher gear would involve too much loss and a super low gear would mean you'd have to change up by 10mph; not useful.

I disagree. I see it as solely a engine-clutch issue. The gears are irrelevant in this case because of that.

You seem to want to ignore the relevance of Power to the situation. But are there any of my 'facts' that you disagree with? I don't see why you don't want to widen the discussion to explain the practicalities of use of clutch and gears.

Because I believe it confuses people. It is very easy to misunderstand and think that we are talking about torque applied to the wheels, which would have to take into account the rest of the drive-train. Nitsuj's deleted posts are a case in point. The issue that the OP was not understanding was specifically about the way the engine provides torque to the clutch and how slipping affected that. I'm not trying to be rude, I am simply trying to avoid a 10 page long thread that got off topic on page 2-3 and does nothing but confuse twenty people.

Edit: Look, I've made my point and explained it the best way I can. If you want to branch off to something else, feel free. I'm not going to argue anymore.

 

[LostConjugate]

a super low gear would mean you'd have to change up by 10mph; not useful.

Its useful for me, this is how I got my ratios configured. I got 6 speeds though and 8k red-line. I just need a short shifter. If I am shifting at 2k I shift into 2nd at 10m though :P

 

[rcgldr]

Well, the wiki quote says it increases torque to the wheels, but simply because of engine speed to wheel speed ratio, and only under specific conditions.
...

Torque is not a function of the difference in radial velocity between the slipping clutch and mating pressure plates. The torque will not change to the driven shaft if the clutch is slipping whether or not the engine speeds up, as the friction within the clutch would be independant of the difference in velocity. Power on the other hand does depend on the radial velocity. With a slipping clutch, the excess power that is not transmitted to the driven shaft from the engine will end up as heat within the clutch and mating parts.

I don't follow you.

A slipping mechanical clutch (as opposed to a fluid clutch) is no different than any other kinetic (sliding) friction situation. For linear motion between two sliding surfaces, you have equal and opposing forces (Newton third law pair). For angular motion between two sliding surfaces, you have equal and opposing torques (again a Newton third law pair). Kinetic friction consumes power, converting some of it into heat. So torque isn't changed by a mechanical clutch, but some power is lost when a clutch is slipping: torque output == torque input, but revs output < revs input.

A bit off topic, but a fluid clutch can a torque converter, wiki article:

http://en.wikipedia.org/wiki/Torque_converter

However I disagree with you and the wiki quote that higher RPM (upto max torque RPM) means more torque regardless of energy input. ... WOT

We're assuming WOT in this case.

I'd guess if you go WOT in a car with a CVT transmission, the computer is programed or the engine naturaly revs at its max torque RPM.

The engine should rev at the higher rpm of peak power. You'll end up with more torque output out of the CVT if it's programmed to operate the engine at peak power. (You can confirm this by doing the math for a range of various output rev rates).

And drivers of race cars don't care, so long as the clutch doesn't give out during the race.

A bit off topic here.

The mechanical clutch used in typical race car has a very high coefficient of friction, so the clutch is very grabby, and the flywheels in race car engines are light and provide little angular momentum, which is why the race car drivers drop the clutch and spin the tires when exiting the pits as opposed to slipping the clutch, which is difficult to do. Generally without computerized assists, race car drivers don't slip the clutch. In the case of DCT (dual clutch transmissions), the dual clutches will be slipping during shifts. In the case of Formula 1 like "seamless" shifts, the single clutch is slipping during shifts. Traditional racing transmissions use straight cut gears and can be optionally shifted without using the clutch. Some drivers downshift conventionally, using "heel and toe" method for brake and throttle with the right foot, and using the left foot for the clutch, while others left foot brake and downshift without using the clutch, just blipping the throttle.

 

[Drakkith]

A slipping mechanical clutch (as opposed to a fluid clutch) is no different than any other kinetic (sliding) friction situation. For linear motion between two sliding surfaces, you have equal and opposing forces (Newton third law pair). For angular motion between two sliding surfaces, you have equal and opposing torques (again a Newton third law pair). Kinetic friction consumes power, converting some of it into heat. So torque isn't changed by a mechanical clutch, but some power is lost when a clutch is slipping: torque output == torque input, but revs output < revs input.

Ah, ok. I see what he was saying now. Yeah, the torque doesn't change simply due to the engine velocity, however the torque delivered from the engine itself can change if you give it more/less fuel.

 

[mender]

On race cars there isn't very much clutch material available for slipping, which is one of the main reasons for not doing so. Typically, each plate of a triple disc clutch has a wear limit of about 0.015" for a total of 0.045" wear before needing replacement.

The other reason is that the plates are metal-faced rather than the street disc with a composite material, which makes the race discs very sensitive to heat build-up and warpage. With very little throw available to disengage the clutch, any disc warpage would make complete clutch disengagement impossible. Much better to slip the tires than the clutch, as was stated.

Also, while the kinetic Cf of the disc is lower than the static Cf, don't forget that the clutch is usually sized to handle considerably more toque than the engine is capable of producing, and because of that the engine/clutch combo is capable of transferring more torque when slipping than the engine is when the clutch if fully engaged. If that weren't so, there would be no advantage in dumping the clutch, right?

As a result more power from the engine can flow through the clutch when slipping despite some being wasted as heat. That's why launching a car at higher rpm will result in longer black marks even if the rpm isn't dragged down during the launch; more power through the clutch (which in this case is more torque and more rpm) means more power to the wheels.

The key point is that the clutch torque capability is more than the engine's torque output, and by slipping the clutch that can be taken advantage of - for short durations.

 

[rcgldr]

On race cars there isn't very much clutch material available for slipping, which is one of the main reasons for not doing so.

This depends on the race car. In race classes that allow computerized shifting, such as DCT (dual clutch transmissions, one clutch for odd gears, the other clutch for even gears), the clutches slip on every shift (two gears engaged at the same time). Formula 1 cars aren't allowed to use DCT's, but they implement "seamless" shifts where the ECU cuts fuel, and slips the clutch enough during the short shift (50ms to 30ms) time so that the output torque is fairly smooth, taking into account angular decleration during a shift.

no advantage in dumping the clutch, right?

Dumping the clutch for race cars is partially due to the limited wear factor of a clutch, but also due to the reduced angular momentum (light flywheel) which helps shift speed, but makes easy to stall the engine if trying to launch by slipping the clutch. As mentioned before some street car clutches sustained kinetic friction is relatively low, so it's better to dump the clutch as a somewhat low rpm (1500 rpm to 2000 rpm depending on the car), to create enough jerk to spin the tires.

More power from the engine can flow through the clutch when slipping despite some being wasted as heat.

Also as mentioned before, a good example of this is the top classes of drag racing cars, a single forward gear and a clutch mechanically programmed to slip for about 1/2 of a drag run to prevent tire slippage or shaking.

 

[mender]

This depends on the race car.

The classes that I'm involved in are GT-1 and NASCAR (oval and road course); they use dog-style engagement ("clutchless") boxes and small diameter triple disc clutches, still much more common than the cutting edge systems that you mentioned.

Dumping the clutch for race cars is partially due to the limited wear factor of a clutch, but also due to the reduced angular momentum (light flywheel) which helps shift speed, but makes easy to stall the engine if trying to launch by slipping the clutch.

For our classes, we really only have one scenario when the car needs to be launched: leaving the pits after a tire change. Getting up to pit speed quickly is important but not at the expense of the clutch, so spinning the tires is the obvious choice for us.

Given that, flywheel weight really only helps if the driver doesn't coordinate properly with the throttle, and at our level that had better not be an issue! Besides unleashing a bit more of the engine's power during acceleration, low flywheel weight also reduces shock on the drive train during shifting.

I frequently drive our NASCAR road course car to pre-grid for the driver and have no trouble easing the car through the cold pit area (no lighting up the tires allowed!) with a tall 1st gear and a very short throw pedal despite the low flywheel weight and lots of cam timing. By changing the gear ratio to a more normal range, lengthening the clutch pedal throw and adding mufflers, I'd have no trouble running the car on the street except for the clutch wear and heat factors already mentioned.

As mentioned before some street car clutches sustained kinetic friction is relatively low, so it's better to dump the clutch as a somewhat low rpm (1500 rpm to 2000 rpm depending on the car), to create enough jerk to spin the tires.

Could you describe the scenario for this a little more?

 

[LostConjugate]

Dumping the clutch for race cars is partially due to the limited wear factor of a clutch, but also due to the reduced angular momentum (light flywheel) which helps shift speed, but makes easy to stall the engine if trying to launch by slipping the clutch.

Wait, doesn't a light weight flywheel make it easier to stall by NOT slipping the clutch. Since the flywheel does not hold as much energy to get the wheels moving, if you let the clutch out too fast from a hard stop the engine would just stall.

 

[rcgldr]

This depends on the race car. In race classes that allow computerized shifting, such as DCT ... the clutches slip on every shift ...

The classes that I'm involved in are GT-1 and NASCAR ...

ALMS GT1 class and perhaps some other USA GT classes (Grand Am?) allow "no lift" (throttle) sequential shifters. I don't know if the high end ones involve computer controlled slipping of the clutch like Formula 1 or DCT's, or if they just cut fuel like the XTRAC.

Dumping the clutch ... at some rpm ... launch

Could you describe the scenario for this a little more?

I had a 1997 Trans Am with a clutch that had very little clutch grip if you tried a high rpm launch by slipping the clucth. Other cars I've owned didn't have this problem. I'm not sure how many cars today have this issue, but for the ones that do, the owners and automotive magazine testers will resort to dumping the clutch at some rpm (found by trial and error), to spin the tires for a better launch when doing 0 to 60 mph or other forms of drag racing.

Wait, doesn't a light weight flywheel make it easier to stall by NOT slipping the clutch. Since the flywheel does not hold as much energy to get the wheels moving, if you let the clutch out too fast from a hard stop the engine would just stall.

This mehod relies on the sudden jerk to break the tires loose so that the tires are spinning instead of the clutch slipping.

 

[LostConjugate]

This mehod relies on the sudden jerk to break the tires loose so that the tires are spinning instead of the clutch slipping.

It seems like that always causes such a loss in rpms that you end up not being able to accelerate afterwards causing the familiar acceleration followed by jolting deceleration that a new driver does when learning the clutch and letting it out too fast.

I have been thinking of going from 17lbs to 11lbs for daily driving but never tried such a light flywheel. Some people swear by 8lbs even for daily but I don't think I would like that on the ice.

 

[rcgldr]

Dropping the clutch ... this method relies on the sudden jerk to break the tires loose so that the tires are spinning instead of the clutch slipping.

It seems like that always causes such a loss in rpms that you end up not being able to accelerate afterwards ... light flywheel.

You need to find the right rpm to do this. In the case of that 1997 Trans Am (5.7 liter engine, 305 hp), where the slipping clutch was the issue and not a light flywheel, the ideal "drop the clutch" rpm was about 1700 rpm for stock tires, higher for sticky drag racing tires. The driver would go full throttle just after dropping the clutch, almost at the same time. Race cars with light flywheels, sticky clutch, and sticky tires, are launched out of the pits by dropping the clutch at very high rpm.

Flywheel - if it's too light, you'll have to increase the idle rpm to keep the engine running, and launching will require more precision to avoid stalling the engine.

 

[mender]

ALMS GT1 class and perhaps some other USA GT classes (Grand Am?) allow "no lift" (throttle) sequential shifters.

Yes; just got back from Montreal and some of the Grand Am classes had the fuel cut.

 

[Ranger Mike]

mender..where did you finish?

 

[mender]

34th, ten laps down.

 

[JeffKoch]

A couple things to keep in mind. First, gasoline engines produce no torque at no RPM, so what you would want to do to launch or climb a hill in a conventional car might be different from what you would want to do in an electric car like a Tesla, which produces gobs of torque at no RPM. Second, not all gasoline vehicles are created equal. Most cars can't sit at the lights and spin the tires, so clutch slippage is necessary to go anywhere - this becomes a huge issue with motorcycles, which I used to race. A sportbike (as opposed to a custom-built drag racing bike) definitely will not sit there and spin the rear tire, instead it will either stall or loop over backwards if you dump the clutch, so feathering the clutch is critical to good starts and people pay a lot of attention to the slip and engagement feel of the clutch. You burn out the clutch after a fairly short while, but that's part of the cost of doing business.