Does clutch control affect torque output in cars?

[LostConjugate]

Does anyone understand this? How do you get more torque by slipping the clutch? I slip the clutch to reduce torque to the wheels until I get going while maintaining power by keeping the rpms high.

As the clutch slips, engine speed is lost but torque is not lost except through the effect of the new engine speed on the engine torque itself. In most car engines the torque output is higher as the engine speed increases up to 4500RPM or more [1]. So slipping the clutch actually gives more torque to the wheels even though the fraction of power wasted in the clutch increases much faster.

 

[Drakkith]

As the clutch slips, you can increase the engine speed to any RPM while the tire RPM stays the same. The increased RPM of the engine can put MORE torque on the clutch while it is slipping than when it is locked. For example, let's say your engine puts out (totally made up numbers) 100 ft lbs of torque at 2,000 RPM and 200 ft lbs at 4,000 rpm. If you are going down the road at 2,000 RPM and you want to accelerate, the maximum torque available at 2,000 RPM is 100 ft lbs. However, if you let the clutch out a bit so that it starts slipping and rev up to 4,000 RPM it is possible you will have more than 100 ft lbs of torque available from the engine. MOST of that extra torque is lost due to the slippage, but not all.

USUALLY you don't use the clutch in this manner. A clutch really isn't designed for that specific purpose, it is instead used because without a clutch you wouldn't be able to accelerate from a stop as the engine cannot operate at 0 RPM. Once you are up to speed it is 100% possible to shift gears without using the clutch at all.

 

[LostConjugate]

Ah that is a better explanation. I wish my engine had 100ft lbs of torque at 2,000 rpm! Sign me up!

My gears don't shift unless I disengage the clutch.

USUALLY you don't use the clutch in this manner.

People often say I am not very usual.

 

[Drakkith]

Ah that is a better explanation. I wish my engine had 100ft lbs of torque at 2,000 rpm! Sign me up!

My gears don't shift unless I disengage the clutch.



People often say I am not very usual.

You should be able to shift without disengaging the clutch, but you have to match the transmission and engine speeds. For example, start in 1st and accelerate. At about 3-4k RPM there should be a point that you can simply pull the shifter down out of 1st and into neutral. Immediately after that let the engine rev down to 2k or under I believe and there should be a spot in that RPM range that you can push the shifter into 2nd without using the clutch. Note that you cannot still be putting pressure on the transmission and engine, you have to *match* the speeds, not simply hit that RPM with the engine still trying to accelerate. I don't know the actual RPM ranges, but the general idea is the same.

 

[nitsuj]

The comment "So slipping the clutch actually gives more torque to the wheels even though the fraction of power wasted in the clutch increases much faster."

The clutch doesn't give torque. The engine can increase torque output by using more input, gasoline.

1.)Torque is measured at the wheels, cause that's what matters.
2.) Engine speed has little correlation to torque output.

The torque measurement requires a force input, 4k rpm is not a force, and rotation speed is not a measure of torque.

 

[Drakkith]

Nitsuj, I think they mean that the MAX torque at a given RPM is set, and you would need to increase engine speed to put out more torque. The torque from the engine is output to the wheels via the clutch and rest of the drivetrain. Slipping the clutch allows you to get to that higher RPM to get that higher max torque.

 

[nitsuj]

Nitsuj, I think they mean that the MAX torque at a given RPM is set, and you would need to increase engine speed to put out more torque. The torque from the engine is output to the wheels via the clutch and rest of the drivetrain. Slipping the clutch allows you to get to that higher RPM to get that higher max torque.

Yea, so if the clutch is releasing the resistance to the engine and as a result the engine speeds up, doesn't mean more torque is at the wheels. Giving the engine more gas & engaging the clutch puts more torque to the wheels.

at wide open throttle trying to go up hill in 5th gear with the engine speed slowing will not be cured by slightly disengaging the clutch. What would cure it is greatly improving torque to the wheels by shifting into first gear and easily climb up the hill with the new found torque via gear ratios, not specifically increasing engine speed by disengaging the clutch.

To ensure you are transferring maximum available engine torque to the wheels keep the clutch engaged.

 

[Drakkith]

Yea, so if the clutch is releasing the resistance to the engine and as a result the engine speeds up, doesn't mean more torque is at the wheels. Giving the engine more gas & engaging the clutch puts more torque to the wheels.

at wide open throttle trying to go up hill in 5th gear with the engine speed slowing will not be cured by slightly disengaging the clutch. What would cure it is greatly improving torque to the wheels by shifting into first gear and easily climb up the hill with the new found torque via gear ratios, not specifically increasing engine speed by disengaging the clutch.

To ensure you are transferring maximum available engine torque to the wheels keep the clutch engaged.

You are mentioning two different effects here. At any given RPM you WILL have more torque applied to the wheels if you have the clutch engaged fully. But that is not what the discussion is about. Nor it is about what the BEST way to get more torque to the wheels. It is about whether this method CAN give more torque to the wheels. I believe it can as long as the clutch is engaged enough and the engine RPM is high enough.

 

[rcgldr]

A mechanical clutch doesn't change the amount of torque, the output torque is the same as the input torque. The input torque can be higher if a fast spinning engine is being slowed down by the clutch due to angular momentum, used to help with launches on pro strock drag cars.

Most clutches on street cars have a relatively low kinetic (sliding) coefficient of friction, perhaps to reduce the shock on the drive train during a launch. Many automotive magazine testers find it's faster and more consistent to launch by dropping the clutch at some engine rpm and spinning the tires instead of slipping the clutch. In some cases the kinetic coefficient of friction decreases as the speed differential of the plates increases and also as heat increases.

The top classes of drag cars use a single forward gear with a mechanically programmed clutch that slips allowing the engine to run a reasonbly fast rpm at high torque output with the goal of keeping the tires from spinning.

 

[nitsuj]

You are mentioning two different effects here. At any given RPM you WILL have more torque applied to the wheels if you have the clutch engaged fully. But that is not what the discussion is about. Nor it is about what the BEST way to get more torque to the wheels. It is about whether this method CAN give more torque to the wheels. I believe it can as long as the clutch is engaged enough and the engine RPM is high enough.

Consider the clutch limiting torque to the wheels, it relieves that force by slipping.

 

[Drakkith]

Consider the clutch limiting torque to the wheels, it relieves that force by slipping.

IF the torque from the engine stays the same, yes. But what happens if you let the clutch start to slip a little and then increase the RPM and torque from the engine?

 

[tasp77]

Man, I am NOT letting any of you drive my truck, I hate the smell of burning clutch and replacing them is a pain.


Interesting discussion though. On one of those "build something out of the stuff in our junk yard shows" one group used a rear end differential with a brake on one wheel as a clutch to control the speed of the other wheel. Yeah, it got very hot, but it did what they wanted.

And they were not too worried about the longevity of their cobbled device.

 

[nitsuj]

Consider the clutch limiting torque to the wheels, it relieves that force by slipping.

IF the torque from the engine stays the same, yes. But what happens if you let the clutch start to slip a little and then increase the RPM and torque from the engine?

The engine speed increased, the torque ouput is the same if you haven't applied the accelerator more, the engine sped up because there is no longer as much resistance on the engine.

If an engine is reporting it max torque is at 4k rpm, it doesn't mean if the engine isn't under a load and you rev it to 4k then it's producing the max engine torque.

 

[LostConjugate]

IF the torque from the engine stays the same, yes. But what happens if you let the clutch start to slip a little and then increase the RPM and torque from the engine?

This makes sense to me. My car will accelerate faster if I slip the clutch and rev first, rather than flooring it and waiting for it to rev up. I can chirp the tires going into 2nd.

@Drak
I can't shift without disengaging the clutch at least a little. Must be a Japanese thing.

 

[russ_watters]

at wide open throttle trying to go up hill in 5th gear with the engine speed slowing will not be cured by slightly disengaging the clutch.

Try it sometime. Or try applying your logic to the extreme case: Can you apply more torque to the ground with a slipped or engaged clutch when stationary?

Also, for people who drive stick, making smooth but fast shifts requires easing off the gas to lower the rpm. Failure to do that will make the car lurch forward due to the extra torque.

 

[Drakkith]

The engine speed increased, the torque ouput is the same if you haven't applied the accelerator more, the engine sped up because there is no longer as much resistance on the engine.

If an engine is reporting it max torque is at 4k rpm, it doesn't mean if the engine isn't under a load and you rev it to 4k then it's producing the max engine torque.

Let the clutch start to slip, hit the gas, then ease off the clutch a little bit. Not enough to cause it to fully engage, but just enough so that when you hit the gas pedal fully down you can get to 4k RPM and keep it there. You will be applying more torque through the clutch to the wheels even though the clutch is slipping.

WARNING: I don't recommend this, as I like my clutches NOT on fire.

 

[russ_watters]

WARNING: I don't recommend this, as I like my clutches NOT on fire.

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

It has a mild slope, then a sharp left curve, then a steep slope. I'll be going 40+ in 3rd gear at the beginning, then coast through the curve to 30mph. Now I'm in the wrong gear. Occasionally, I'll keep my gas pedal foot on the floor and push in the clutch a little to let the engine rev up, rather than shifting into 2nd.

 

[Drakkith]

Actually I think I do something similar myself russ. I'll be in 2nd at about 20 or so, go around a curve and slow down to 10ish by pushing the clutch in and coasting. Instead of shifting I'll just let the clutch out and stay in 2nd.

 

[LostConjugate]

Actually I think I do something similar myself russ. I'll be in 2nd at about 20 or so, go around a curve and slow down to 10ish by pushing the clutch in and coasting. Instead of shifting I'll just let the clutch out and stay in 2nd.

That is just to prevent the engine from stalling though.

 

[russ_watters]

That is just to prevent the engine from stalling though.

Same diff, isn't it? You're running the engine at a higher RPM to keep it at a better power point, so you can better deliver torque to the wheels.

 

[Drakkith]

That is just to prevent the engine from stalling though.

Nah, I could run at 10 mph in 2nd gear, but the power and torque to the wheels is horrible. Revving the engine up with the clutch partially disengaged is much better than doing that.

 

[Drakkith]

I did apply my logic, and I think a dropped clutch will deliver more torque to the wheels then a slipping clutch, especially from stationary. Since the dropped clutch will deliver all of the engines power (including torque) to the wheels.

I'm not disputing that the torque starts at the engine. Engine momentum, while part of power, is not torque. Again engine momentum is not torque.

All else equal, increasing torque to the wheels will cause acceleration. Do you disagree?

You aren't even on the same page as us. If you keep the engine at the same RPM, same torque, ETC, then of course the clutch being fully engaged will deliver more torque to the wheels. But we are not doing that in this case.

Take my example in an above post. In 2nd gear at 10 mph, I WILL get more torque and power and whatever to the wheels by letting the clutch slip and revving up compared to just giving the engine more gas at 1,000 rpm. I do it nearly every day.

 

[nitsuj]

You aren't even on the same page as us. If you keep the engine at the same RPM, same torque, ETC, then of course the clutch being fully engaged will deliver more torque to the wheels. But we are not doing that in this case.

Take my example in an above post. In 2nd gear at 10 mph, I WILL get more torque and power and whatever to the wheels by letting the clutch slip and revving up compared to just giving the engine more gas at 1,000 rpm. I do it nearly every day.

opps, sorry for the disagreement.

I get it now. Changing the ratio between engine speed and wheel speed by slipping the clutch is equivalent in increasing torque to the wheels as changing gears.

 

[rcgldr]

Changing the ratio between engine speed and wheel speed by slipping the clutch is equivalent in increasing torque to the wheels as changing gears.

A clutch can't multiply torque even though there's an effective gearing when the clutch is slipping. The torque output is the same as the torque input. The only advantage of letting the engine spin faster is if the engine is able to produce more torque at a higher rpm.

 

[256bits]

And then there is a slip-clutch used in rotating machinery that is torque limiting so you do not break shafts and mess up machinery.

 

[Drakkith]

And then there is a slip-clutch used in rotating machinery that is torque limiting so you do not break shafts and mess up machinery.

Yes, when it starts to slip the torque is reduced as long as the engine or motor is applying a steady torque to the clutch.

 

[rcgldr]

Yes, when it starts to slip the torque is reduced as long as the engine or motor is applying a steady torque to the clutch.

A clutch doesn't change the torque, in this case, by allowing the clutch to slip, the excess torque from the engine ends up as angular acceleration of the engine, flywheel, and driven side of clutch, and eventually the increase in rpms of the engine will result in reduced torque (either from partial throttle setting, reaching the rev limiter, or running at an rpm where torque output is reduced).

 

[Drakkith]

A clutch doesn't change the torque, in this case, by allowing the clutch to slip, the excess torque from the engine ends up as angular acceleration of the engine, flywheel, and driven side of clutch, and eventually the increase in rpms of the engine will result in reduced torque (either from partial throttle setting, reaching the rev limiter, or running at an rpm where torque output is reduced).

Of course, I meant that if the clutch starts to slip the torque applied to everything behind the clutch is reduced unless for some reason the engine starts to apply even more torque through higher RPM or something.

 

[256bits]

Of course, I meant that if the clutch starts to slip the torque applied to everything behind the clutch is reduced unless for some reason the engine starts to apply even more torque through higher RPM or something.

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.

 

[sophiecentaur]

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. 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 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.
There is a limit to how far you can take this because there is a limit to the engine output power and power is only deliverable to the gearbox as torque times revs. Once you have reached maximum engine output power (like when you are labouring uphill in third), you cannot afford to be wasting any energy in the clutch by going into fourth and slipping because the available torque times revs will be less than when you were in third.

So it's Power and not Torque that is the limiting factor in this business.

 

[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.