Instant Torque (Electric Cars vs. Gas/Diesel Cars)

[Ryuk1990]

So I hear a lot about how electric cars are great because electric motors get instant max torque at 0 RPM.

However, can't the same be done with an internal combustion engine if you have a clutch?

 

[Kozy]

No. Run an ICE to slow and it will stall. Even below about 2000rpm in most engines, there is very little torque.

 

[SteamKing]

That pesky transmission is there for a reason, and it's not to give you an extra hand rest.

 

[Ryuk1990]

I'm not familiar with stick shift cars at all (or car mechanics in general) so bare with me as I have no idea what I'm talking about. But can't you rev up the engine to its max torque almost instantly using a clutch without moving the car and THEN hit the pedal to get that same feeling of instant torque that you would get from an electric car?

 

[OmCheeto]

I'm not familiar with stick shift cars at all (or car mechanics in general) so bare with me as I have no idea what I'm talking about. But can't you rev up the engine to its max torque almost instantly using a clutch without moving the car and THEN hit the pedal to get that same feeling of instant torque that you would get from an electric car?

Yes, but this involves the clutch slipping. If you do that a lot, the clutch will wear out prematurely. Have you ever changed brake pads? Changing the clutch disk is about 100 times worse, as it involves removing the engine, transmission, or both.

Typical costs:
Having a clutch replaced can cost $400 -$3,000 or more, depending on the make, model and type of vehicle;

 

[SteamKing]

More likely, you'll stall the engine.

Hit what pedal?

With a manual transmission, the clutch is disengaged to allow the engine to run without load. Consequently, it takes very little throttle to rev the engine to its max. torque RPM. If you put the transmission into its highest (1:1) ratio and engage the clutch, the engine still must overcome the inertia of the car before movement can take place. More often than not, doing this will stall the engine rather than getting the car to move. Also, since the clutch is a friction device, engaging it repeatedly at high RPM will rapidly burn out the friction plates, requiring a costly clutch replacement.

Doing the same thing with an automatic transmission may not stall the car, but soon you will need a new transmission, which is even more costly to replace than a clutch.

 

[xxChrisxx]

I think the main point to take is that there is a difference between having torque being produced at a specific high RPM and slipping something to achieve pull away, and having produced at zero RPM.

 

[Highspeed]

Ryuk1990, Yes, what you're talking about is possible. You can just dump the clutch at peak torque. You will get the results above though...fast wear and broken parts. You could also use a slipper clutch that will only lock-up above a certain rpm. It's not instant, but will allow the engine to rev up before being loaded at peak torque (or whatever rpm you set it for). You could also use a high-stall torque converter. It doesn't hammer the drivetrain like a clutch, but creates a whole lot of very hot transmission fluid, so you'll need a good cooler. Driveablitlity would be poor with a slipper clutch or converter that stalled that high, so daily use wouldn't be possible. Is there a point beyond a theoretical question? It's going to be a loooooooong time before a Prius can pull a travel trailer, or before an electric truck can pull a semi-trailer for 500 miles a day. ICE vehicles overcome the lack of instant torque through gearing, so there is really no need to turn an ICE into an electric motor.

 

[Ryuk1990]

Is there a point beyond a theoretical question? It's going to be a loooooooong time before a Prius can pull a travel trailer, or before an electric truck can pull a semi-trailer for 500 miles a day. ICE vehicles overcome the lack of instant torque through gearing, so there is really no need to turn an ICE into an electric motor.

It was more about having that fast smooth acceleration that the Tesla Model S offers with an ICE car.

 

[SteamKing]

Most cars with automatic transmissions accelerate quite smoothly from stop to highway speeds. The shift points of the transmission provide a momentary change in the sound of the engine as the revs drop, but other than that, everything is quite smooth. Also, new transmissions have up to eight speeds, rather than the three or four or older transmissions, making the process even smoother, as the engine revs do not need to drop as much between shifts.

It might be you are thinking of the so-called Continuously Variable Transmission (CVT), which doesn't operate using conventional reduction gears.

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

These transmissions allow the engine to maintain a constant speed as the vehicle accelerates. Although such transmissions have been used in a variety of automotive applications, several factors have kept them from replacing conventional transmission designs (see the article above for more details).

 

[AlephZero]

It was more about having that fast smooth acceleration that the Tesla Model S offers with an ICE car.

Tesla S: 0-60 in 5.4 sec, top speed 125 mph, "handles like a sedan" (to quote the Tesla website)
Porsche 911S turbo: 0 - 62 mph in 3.1 sec, 0-124 mph in 10.3 sec, top speed 200 mph, handles like a car.

 

[AlephZero]

But can't you rev up the engine to its max torque almost instantly using a clutch without moving the car and THEN hit the pedal to get that same feeling of instant torque that you would get from an electric car?

Yes, but this involves the clutch slipping. If you do that a lot, the clutch will wear out prematurely.

That depends on the design of the clutch. Back in the 1970s I used to drive a british-designed sports car with a 6 speed gearbox. More accurately, it had a 4 speed conventional box and pedal-operated clutch, and a separate epicyclic gearbox and clutch that was operated electrically from a switch in the top of the gearstick, to switch between gears 3 and 4, or 5 and 6. (The manual box and clutch selected either 1,2,3,5 or 1,2,4,6).

The epicyclic gear change was designed to operate at any engine RPM and throttle position, and changed gear in less than a second. Floor the throttle, flip the switch, and get a kick in the back ... (or on a bad hair day, over-rev the engine when changing down without first engaging brain).

 

[xxChrisxx]

That depends on the design of the clutch. Back in the 1970s I used to drive a british-designed sports car with a 6 speed gearbox. More accurately, it had a 4 speed conventional box and pedal-operated clutch, and a separate epicyclic gearbox and clutch that was operated electrically from a switch in the top of the gearstick, to switch between gears 3 and 4, or 5 and 6. (The manual box and clutch selected either 1,2,3,5 or 1,2,4,6).

The epicyclic gear change was designed to operate at any engine RPM and throttle position, and changed gear in less than a second. Floor the throttle, flip the switch, and get a kick in the back ... (or on a bad hair day, over-rev the engine when changing down without first engaging brain).

It doesn't make any difference on design, a slipping friction clutch transmissting lots of torque will wear prematurely.

And a British 70's sports car in the US... an MGB?
What you are describing is an overdrive in 3rd and 4th.

The solenoid should automatically disengage the overdrive in 2nd and 1st. If/when it fails, and you can engage the overdrive in the lower gears, it slips and wears the cone clutch out. Also the overdrive is not designed to give you 'a kick in the back'. It's designed for cruising.

The reason for the lurch when you engage the overdrive whilst still having the throttle open is that wheels suddenly wanted to be traveling much faster than the road speed (as overdrive is effectively a longer gear). Two things can happen in this case, the wheels slip or the overdrive clutch slips to synchronise the road and engine speed. As it's rather dangerous for the wheels to start slipping at high speed, the clutch was designed to slip first.

This abuse of the overdrive clutch is what caused MG to stop the overdrive from engaging in 3rd on later cars

 

[sgb27]

More likely, you'll stall the engine.

Hit what pedal?

With a manual transmission, the clutch is disengaged to allow the engine to run without load. Consequently, it takes very little throttle to rev the engine to its max. torque RPM. If you put the transmission into its highest (1:1) ratio and engage the clutch, the engine still must overcome the inertia of the car before movement can take place. More often than not, doing this will stall the engine rather than getting the car to move.

The technique asked about by the OP involves flooring the throttle and then controlling the clutch pedal to ensure RPMs stay around max torque (gradually engaging it as car speed builds) - if you're stalling the engine you're engaging the clutch too quickly.

 

[SteamKing]

The technique asked about by the OP involves flooring the throttle and then controlling the clutch pedal to ensure RPMs stay around max torque (gradually engaging it as car speed builds) - if you're stalling the engine you're engaging the clutch too quickly.

Perhaps so, but it is a maneuver which I think could only be accomplished with not a little skill and a fairly robust clutch. All in all, even with the best technique, I still see a much shorter clutch life, perhaps shorter engine life to boot.

 

[cjl]

Tesla S: 0-60 in 5.4 sec, top speed 125 mph, "handles like a sedan" (to quote the Tesla website)
Porsche 911S turbo: 0 - 62 mph in 3.1 sec, 0-124 mph in 10.3 sec, top speed 200 mph, handles like a car.

That's not really a fair comparison though, since the Tesla is a much different car in a completely different category of size and performance (and is half the price of a 911 Turbo S). A BMW M5 or Audi RS6 would be a much better comparison (and should still win in all performance metrics).

 

[Averagesupernova]

No. Run an ICE to slow and it will stall. Even below about 2000rpm in most engines, there is very little torque.

Interesting. I just drove a Volvo semi-tractor with a Detroit diesel in it and it governs out when it approaches 2000 RPM. ALL the torque is below 2000. Yes, the thread is about diesels too.

 

[xxChrisxx]

That's not really a fair comparison though, since the Tesla is a much different car in a completely different category of size and performance (and is half the price of a 911 Turbo S). A BMW M5 or Audi RS6 would be a much better comparison (and should still win in all performance metrics).

It's ironic that you picked two cars with pretty much the same power as the 911 turbo s. New Golf R. 300bhp and 0-60 in 4.8 with the DCT.

Hot hatch progress... its mind bending.

 

[NigelTufnel]

Generally with ICE the larger the displacement the lower the rpm at which it makes max torque. If you want more torque without having to rev the engine just get a bigger engine.

Here's a 14 cylinder diesel that makes over five million foot pounds of torque at only 102 rpm
http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96-C

 

[sgb27]

Perhaps so, but it is a maneuver which I think could only be accomplished with not a little skill and a fairly robust clutch. All in all, even with the best technique, I still see a much shorter clutch life, perhaps shorter engine life to boot.

Isn't this what everyone does when they try to pull away "quickly" in a manual transmission (eg pulling out onto a busy road)? You rev the engine up a bit (to around max torque), then release the clutch in a controlled manner whilst pressing the accelerator harder to keep the rpms more or less constant. In most cars the clutch will only slip for a fraction of a second until the car is moving fast enough to match the 2000-3000rpm. Yes it does wear the clutch more than a slower pull-away, but it's not going to destroy anything. Try the same technique in 2nd or 3rd gear, then you'll smell the clutch burning away :-)

 

[SteamKing]

Isn't this what everyone does when they try to pull away "quickly" in a manual transmission (eg pulling out onto a busy road)? You rev the engine up a bit (to around max torque), then release the clutch in a controlled manner whilst pressing the accelerator harder to keep the rpms more or less constant. In most cars the clutch will only slip for a fraction of a second until the car is moving fast enough to match the 2000-3000rpm. Yes it does wear the clutch more than a slower pull-away, but it's not going to destroy anything. Try the same technique in 2nd or 3rd gear, then you'll smell the clutch burning away :-)

Maybe you can do this all day in a little four-banger car, but you take a big torquey V8 and drop the clutch at 3000 rpm, you will wear out your tires. You don't always have to smell the clutch plate burning to bring the day for a new clutch closer to today.

 

[SteamKing]

Generally with ICE the larger the displacement the lower the rpm at which it makes max torque. If you want more torque without having to rev the engine just get a bigger engine.

Here's a 14 cylinder diesel that makes over five million foot pounds of torque at only 102 rpm
http://en.wikipedia.org/wiki/W%C3%A4rtsil%C3%A4-Sulzer_RTA96-C

The Sulzer engine is specially designed to operate at low RPM because it is directly connected to the propeller of a ship. A propeller of the size used to drive the ship operates most efficiently at this low speed. For smaller craft, higher speed diesels are used with reduction gears so that the engine can operate at its most efficient speed while the propeller operates at its most efficient but slower speed.

 

[sgb27]

Maybe you can do this all day in a little four-banger car, but you take a big torquey V8 and drop the clutch at 3000 rpm, you will wear out your tires.

The question was whether you can make an ICE generate peak torque at 0mph (as electric motors can), what happens at the tyres is exactly the same for electric and ICE. If you had a powerful enough electric motor and commanded it to generate peak torque from 0mph all day you would also wear out your tyres.

 

[mheslep]

so there is really no need to turn an ICE into an electric motor..

Besides the 0 rpm torque question there is the advantage of efficiency. The electric motor is far more efficient tank to wheels than the ICE, and always will be.

 

[rcgldr]

So I hear a lot about how electric cars are great because electric motors get instant max torque at 0 RPM.

Peak torque for an electric motor occurs at 0 rpm and decreases linearly until it become zero torque at max rpm. Internal combustion engines produce peak torque at some rpm, but with an ideal CVT (continously variable transmission), peak torque delivered to the rear wheels occurs at the rpm corresponding to peak power.

However, can't the same be done with an internal combustion engine if you have a clutch?

Yes, or with a CVT.

Note that the limiting factor for peak torque at the driven tires is limited to the amount of traction available. Dropping the clutch at high rpms or attempting to apply peak torque from an electic motor onto the driven wheels could result in the wheels spinning, assuming sufficient torque at low speeds is enough to spin the driven wheels.

Diesel electric locomotives use diesel engines to drive generators which in turn deliver electricity to electric motors directly attached to the driven wheels of the locomotive. It's a heavy equivalent to a CVT, but heavy is a good thing for locomotives (to get enough traction to pull the remainder of a train).

 

[SteamKing]

Besides the 0 rpm torque question there is the advantage of efficiency. The electric motor is far more efficient tank to wheels than the ICE, and always will be.

I don't know what you mean by the statement that "The electric motor is far more efficient tank to wheels than the ICE, ..."

The electric motor is not a prime mover; it depends on an external source of energy, be it a battery, a fuel cell, Fred Flintstone running on a treadmill, whatever.

If you want to measure the efficiency of the electric motor compared to an ICE, you must compare like with like, i.e., for the electric motor, you've got to throw in the efficiency of the generating plant as well.

 

[OmCheeto]

I ...
The electric motor is not a prime mover; it depends on an external source of energy

And that would be...

If you want to measure the efficiency of the electric motor compared to an ICE, you must compare like with like, i.e., for the electric motor, you've got to throw in the efficiency of the generating plant as well.

Feynman's dad!

------------------------
It's ok to ban me. Really. I've things to do.

 

[mheslep]

I don't know what you mean by the statement that "The electric motor is far more efficient tank to wheels than the ICE, ..."

The electric motor is not a prime mover; it depends on an external source of energy, be it a battery, a fuel cell, Fred Flintstone running on a treadmill, whatever.

If you want to measure the efficiency of the electric motor compared to an ICE, you must compare like with like, i.e., for the electric motor, you've got to throw in the efficiency of the generating plant as well.

"tank to wheels" is a common standard of efficiency found all throughout the literature. Its importance lies is due to the finite amount of energy is carried on board the platform. The efficiency of conversion of the on board energy source to motion is the critical measure of efficiency in determining both maximum range per energy carried and cost per unit distance traveled. Note this is the case regardless of the efficiency of the process used to make the carried energy.

So as I say, the typical electric motor as found in the currently available EVs has an efficiency of over 90%. The batteries are likewise about 90% efficient on discharge. The typical combustion engine and drive train currently on the road is around 15% efficient, moving to around 20% for the fleet of all new vehicles. The gap between the two, tank to wheels EV and ICE efficiency, will always be large due to thermodynamics.

Finally, we don't commonly see demands that ICE mpg calculations include the large energy overhead in refining oil to gasoline or the energy consumed in oil extraction.

 

[SteamKing]

That's what a prime mover is. You add fuel to the engine and get the work out. An electric motor has no tank, and you can't consider the charge in the battery to be the equivalent.

 

[mheslep]

An electric motor has no tank, you can't consider the charge in the battery to be the equivalent.

Why not? It is commonly done.