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

[xxChrisxx]

A lot of things are commonly done, but that does not mean it is necessarily the correct way of doing things.

Look, a prime mover takes the potential energy stored in a quantity of fuel and turns that potential energy into useful work. We look at the work obtained versus the amount of energy contained in the fuel. The ratio of work obtained to energy input is the definition of thermal efficiency of a prime mover.

On the other hand, the electricity which charges a battery in an electric car doesn't come straight out of the ground. It has to be generated somewhere. In the US, with certain exceptions, electricity is generated by burning coal or natural gas. The potential energy contained in that coal or natural gas is the basis for ultimately generating the electricity which goes into the battery of an electric car. That must be the basis for realistically assessing the relative efficiency of two vehicles which use different means of propulsion.

There is a pretty good reason people use tank to wheels. Any why it's the more useful metric for cars.

1. Noone who is diving round in their car gives a toss where the energy comes from. Mr. D. River wants to know how far he can travel without having to fill up with juice.

2. Well to wheel is an utter nightmare to work out, both for petrol and for electric. Electric because you have different methods of generation. Does your argument for efficiency fall down when you consider renewably generated electricity? This is not dismissive, it's a very real question for the 'greeness' of EV. It's fairly obvious that an EV powered by a windmill and sunshine is far better on CO2/km than one powered by black coal.

Also, what about CHP plants? How do we take our value for efficiency then, do we dismiss the heat as only the electricity is useful to us? If so, when distilling fuels, do we dismiss the energy that isn't in the form we want?

One can become infinitely pedantic about well to wheel calculations. Especially for comparison of two different fuel types. It's why they are generally avoided, or treated separately.

You people really know how to kill a good car thread with boring stuff.

Good god. That's 3.3 gallons per minute.

Thank god it reaches 60 mph in only 2.5 seconds. Slightly less than a second slower than Mr. Wayland's '72 Datsun.
(Ha ha!)

Only about 150 more than the $2,500,000.00 Bugatti Veyron Super Sport.
*Trying to bring us back on topic.

Glad someone is. Even if you completely miss the point of the Veyron. :P

The reason that the Veyron is so impressive is not that it's fast. It's because it's a luxury barge that goes like a speedboat.

If you just wanted to build a fast car, you wouldn't do draw the Veyron. It was conceived by the marketing men first, given it's vital stats, then given to the engineers to say 'make it work boys'.

 

[Ryuk1990]

"superior" is a subjective term.

f=ma

which yields: a=f/m

therefore, with two vehicles with similar motive forces, the one with the smaller mass will have a higher acceleration value.

Tesla S:

Curb weight: 4,647.3 lbs
torque 430 Nm​

Porsche 911 S Turbo:

Curb weight: 3680 lb.
torque 710 Nm​

Given that the Porsche has both higher torque and weighs less, it's a no brainer that it's faster.

I don't believe the Tesla S was designed as an adrenalin pumper.

Tesla S mpge: 95
Porsche mpg: 19

You can try and power a car with a 99.99% efficient electric sewing machine motor.
That's much superior to the 25% efficiency of a 530 hp gasoline engine.

Yes?

Yeah, but batteries are the culprit for why electric cars are heavy and thus aren't quite as fast as the high performance gas cars of today, right?

I always thought that electric motors can give more power than a typical IC engine for the same mass.

Oh and the Model S has 600 N*m of torque. Or were you talking about torque at the wheel?

 

[rcgldr]

Therefore, with two vehicles with similar motive forces, the one with the smaller mass will have a higher acceleration value.

Tesla S:

Curb weight: 4,647.3 lbs
torque 430 Nm​

Porsche 911 S Turbo:

Curb weight: 3680 lb.
torque 710 Nm​

Given that the Porsche has both higher torque and weighs less, it's a no brainer that it's faster.

One issue here is that engine torque is multiplied by the overall gearing, and assuming a car is reasonably geared, the car with a higher power to weight ratio (as opposed to a higher torque to weight ratio) will accelerate faster.

For example, two ICE that produce the same torque, but the second engine produces it's torque at twice the rpm of the first engine, and makes twice the power as the first engine throughout the range of operating rpm. Assuming the gear ratio for the second engine is double that of the first engine, then the driven wheel torque of the second car is double that of the first car.

 

[mheslep]

the electricity which charges a battery in an electric car doesn't come straight out of the ground.

Nor does gasoline or diesel.

...In the US, with certain exceptions, electricity is generated by burning coal or natural gas. ...

Coal 32%, natural gas 32%, other non-fossil sources (nuclear, hydro, wind, other) ~33%. And, in areas where the automobiles densities are highest in the US (California, Atlantic seaboard), the ratio of non-fossil sourced electricity is higher still.

 

[zanick]

One issue here is that engine torque is multiplied by the overall gearing, and assuming a car is reasonably geared, the car with a higher power to weight ratio (as opposed to a higher torque to weight ratio) will accelerate faster.

For example, two ICE that produce the same torque, but the second engine produces it's torque at twice the rpm of the first engine, and makes twice the power as the first engine throughout the range of operating rpm. Assuming the gear ratio for the second engine is double that of the first engine, then the driven wheel torque of the second car is double that of the first car.

One of the problems of comparing performance of the electric Tesla and gas cars , is that the tesla only uses one gear ratio and a very broad HP curve. (forget about the flat torque and all the marketing hupla) its HP curve dies out at 130mph and past 80mph is pretty anemic . However, if it had just one more gear, (and here were versions in the past that did have 2 gears), it would be a high speed monster too, being able to take its 420hp engine , to near 180mph
I like this look at an actual tesla dyno run... It clearly shows its advantages and disadvantages over a comparable powered V8 gas engine.
what it shows it that the penciled HP curves vs the tesla dyno run, can actually make the same rear wheel torque initially (due to gearing) as the tesla... and it has power to use at much higher vehicle speeds.

 

[rcgldr]

One of the problems of comparing performance of the electric Tesla and gas cars , is that the tesla only uses one gear ratio and a very broad HP curve.

Not sure why this old thread was brought up again, but assuming that the Tesla motor is similar to typical DC motors, then the torque is greatest at zero rpm and linearly decreases to zero at maximum rpm. Peak horsepower occurs at 1/2 of peak rpm. To prevent wheel spin, the torque is limited to some maximum value, so you get a flat torque versus rpm curve until the maximum motor torque versus rpm decreases below that of the limited torque.

 

[zanick]

Not sure why this old thread was brought up again, but assuming that the Tesla motor is similar to typical DC motors, then the torque is greatest at zero rpm and linearly decreases to zero at maximum rpm. Peak horsepower occurs at 1/2 of peak rpm. To prevent wheel spin, the torque is limited to some maximum value, so you get a flat torque versus rpm curve until the maximum motor torque versus rpm decreases below that of the limited torque.

actually, most AC servo motors, as what is powering the Tesla has a flat torque curve, until the inductive losses start effecting current rise in the motor windings where the torque will fall off linearly.
as far as comparisons to a gas engine.. starting torque in most V8 cars can be as high or higher.. most of the time, its tire grip limiting for both car's power plants.

 

[cjl]

One of the problems of comparing performance of the electric Tesla and gas cars , is that the tesla only uses one gear ratio and a very broad HP curve. (forget about the flat torque and all the marketing hupla) its HP curve dies out at 130mph and past 80mph is pretty anemic . However, if it had just one more gear, (and here were versions in the past that did have 2 gears), it would be a high speed monster too, being able to take its 420hp engine , to near 180mph
I like this look at an actual tesla dyno run... It clearly shows its advantages and disadvantages over a comparable powered V8 gas engine.
what it shows it that the penciled HP curves vs the tesla dyno run, can actually make the same rear wheel torque initially (due to gearing) as the tesla... and it has power to use at much higher vehicle speeds.

The penciled in curve for an engine/transmission combo is incredibly closely geared, and doesn't really look right to me. 4th gear runs out at 85mph, and 5th at 110, which is pretty unreasonable for any high performance vehicle.

 

[zanick]

The penciled in curve for an engine/transmission combo is incredibly closely geared, and doesn't really look right to me. 4th gear runs out at 85mph, and 5th at 110, which is pretty unreasonable for any high performance vehicle.

Actually, its a 6 speed based on another dyno run. its safe to use a post shift RPM level to be about 75% of redline. with 1st gear redlining at 40mph or so, the rest is just simple math... yes, this would be a little close in gear ratio, but widening it up, to say 70% (pure street car ratios), wouldn't change things all that much.
The point seems to be clear, the gas engine has quite a bit of advantage passed 80mph and the same starting forces. the advantage the electric car has, is that it can be a closer performer, with similar max HP output, without a gear box. (only a single reduction)

 

[cjl]

What car is that based on that has 300+hp, but runs out of gears with a 6 speed at 140mph? Even in a racecar, there are several tracks where that 6th would be a bit too short. I'd expect the gearing for a sleekish 300hp car to be more along the lines of (gear: speed at redline):

1: 40
2: 70
3: 95
4:115
5: 145
6: 170

 

[zanick]

What car is that based on that has 300+hp, but runs out of gears with a 6 speed at 140mph? Even in a racecar, there are several tracks where that 6th would be a bit too short. I'd expect the gearing for a sleekish 300hp car to be more along the lines of (gear: speed at redline):

1: 40
2: 70
3: 95
4:115
5: 145
6: 170

you are too focused on the micro details of the graph. sure, its not a realistic spread as I worked backward with a close ratio gear box , more like a 7 speed.
the point is, your gear ratios are not too viable either.
most 6 speeds look like this:
8:1 1st
7:1 2nd
6.:1 3rd
5.:1 4th
4:1 5th
3:1 6th
this is a 80-75% drop (apprx) per gear.

regardless, you can see the average HP that is kept pretty consistent out to redline. that's the point. the tesla has a flat HP curve, which is ideal for wide spacing in gears , or in its case, NO GEARS. :)

 

[cjl]

the point is, your gear ratios are not too viable either.

You should let Porsche know - those are very close to the ratios in my 6 speed box in my Cayman S...

 

[zanick]

You should let Porsche know - those are very close to the ratios in my 6 speed box in my Cayman S...

I thought those were gear ratios, not speeds in gears as they are. yes, they are normal.
regardless of the actual rations, its the space you need to look at and if you do, you can see that the ratios I posted are very close to your ratios as well.
So, the point is, with 6 speeds its easy to keep the car in the HP range to have max acceleration potential at any speed. (proportional to the power, as acceleration is)
again, the Tesla has a broad HP curve, which eliminates the need many gears. 2 gears would keep it on par with most cars... 1 gear as it has now however, loses a lot of acceleration perforamance after 80mph.

 

[SteamKing]

most 6 speeds look like this:
8:1 1st
7:1 2nd
6.:1 3rd
5.:1 4th
4:1 5th
3:1 6th
this is a 80-75% drop (apprx) per gear.

It's not clear for what sort of gearbox these ratios are intended. For automotive use, the fourth or fifth gear is typically a 1:1 ratio, with the higher gears (5th or 6th) typically being overdrives (that is, the gear ratio is less than 1). Of course, the final drive ratio in the differential also must be factored into give the true ratio between engine speed and wheel speed.

 

[zanick]

It's not clear for what sort of gearbox these ratios are intended. For automotive use, the fourth or fifth gear is typically a 1:1 ratio, with the higher gears (5th or 6th) typically being overdrives (that is, the gear ratio is less than 1). Of course, the final drive ratio in the differential also must be factored into give the true ratio between engine speed and wheel speed.

Its very clear... I've given you the overall ratios. you can make up any combination of rear end to transmission as you would like and it doesn't matter if 4th gear is 1:1 or 2:1, or .5 :1.
actually, Porsche has a lot of cars with 1:1 happening in 4th... as in my race car. :) you might want to talk to porsche about that. :)

so, fine, let's use your ratios.. it doesn't matter . I just picked some general close ratio numbers. your very nice Porsche street ratios are pretty close, at .57, .73, .and the rest are over .8 (post shift) RPM values vs redline

again, ratios do not matter.. the point is the gears keep the engine in the max HP range which is essential for maximzing acceleration at any speed.
the tesla falls on its face after 80mph after a gallant effort up to 80mph to have on par acceleration to any like 420hp car (actually, more like 475hp as folks are measuring 420hp at the driven wheels) after that speed, the car is anemic vs any car 300hp and even less!

 

[mheslep]

the tesla falls on its face after 80mph after a gallant effort up to 80mph to have on par acceleration to any like 420hp car (actually, more like 475hp as folks are measuring 420hp at the driven wheels) after that speed, the car is anemic vs any car 300hp and even less!

I suspect that's an intentional design tradeoff, as Tesla could have remedied that by one more gear but at a cost of some loss in drive train efficiency, more mass, more maintenance, etc. For cars not targeted for the like of the German Autobahn, what's the point of 150 mph+ top ends that will likely enable spectacular crashes reported in the newspapers?

 

[zanick]

I suspect that's an intentional design tradeoff, as Tesla could have remedied that by one more gear but at a cost of some loss in drive train efficiency, more mass, more maintenance, etc. For cars not targeted for the like of the German Autobahn, what's the point of 150 mph+ top ends that will likely enable spectacular crashes reported in the newspapers?

its cost savings and maintenance. more weight for a transmission , etc.
But, what's the point? why does any car go over 80mph. trust me, if Porsches had a limit of 100mph, no one would buy them.
the point again, is, the car is anemic past 80mph without a 2nd gear... its a trade off for the dependability. maybe folks are realizing that they don't need to go over 80mph.

Here is the revised dyno sheet with a similar hp gas engine with ratios like you have on the car.