Formula E

[FW17]

A F1 car currently stores about 4 MJ in its batteries, Porsche had 6 MJ in its batteries, both could recharge them within minutes.

I am not sure what is the capacity of road cars and Formula E but why are the charging rates not as fast as F1? a tesla road car needs about 6 hours while Formula E needs about an hour.

What is the difference, why cant a recharge of Formula E car be done in minutes?

Also I did not understand the 5 speed gear box from Hewland , when they could have used a Williams CVT

[autogyro]

A F1 car currently stores about 4 MJ in its batteries, Porsche had 6 MJ in its batteries, both could recharge them within minutes.

I am not sure what is the capacity of road cars and Formula E but why are the charging rates not as fast as F1? a tesla road car needs about 6 hours while Formula E needs about an hour.

What is the difference, why cant a recharge of Formula E car be done in minutes?

Also I did not understand the 5 speed gear box from Hewland , when they could have used a Williams CVT

CVT Transmissions whether Van Dorne cone or toroidal use to much energy to operate the cones or disks in high energy applications.
They are only really useful in instrumentation.
It is why Patrick Head developed the flywheel energystorage system for Porsche instead of continuing with the CVT that Coultard tested.

[Lycoming]

Well, for comparison, a tesla model S battery is 85 kWh (only scrubs buy the 60 kWh version). You know how much that is in megajoules?

306 MJ.

Naturally, it's going to take a while to charge compared to a 4 or 6 MJ battery. Also, that 6 hours is if you're charging from 120v mains. The charging time drops right down if you use a high voltage charger.

A formula E car is obviously also going to have a much, much larger battery than a hybrid system, so of course it will also take longer to charge.

To be honest I don't know why they're using a multispeed gearbox at all with an electric motor, much less a 5 speed. The model S goes up to 210 km/h with a single speed box.

Especially with an electric motor, there's no point to doing CVT. The idea of CVT, in theory, is that you tune the engine to have a narrow power band, but relatively high power within it, and instead of the engine revving up and down as vehicle speed changes, you just vary the gear ratio to compensate so engine RPM stays nearly constant.

This is completely pointless for an electric motor, whose power delivery characteristics are mostly independent of RPM.

Just do the lightest, simplest, cheapest thing and use a single speed gearbox.

[FW17]

Well, for comparison, a tesla model S battery is 85 kWh (only scrubs buy the 60 kWh version). You know how much that is in megajoules?

306 MJ.

If 4 MJ battery weighs 20 kgs(without cooling) a 306 MJ will weigh 1.5 tones. Does he tesla weigh so much?

[Lycoming]

Probably quite a bit less than that, the car alltogether is 2100 kg.

[FW17]

Probably quite a bit less than that, the car alltogether is 2100 kg.

Actually I was wrong, F1 has a capacity to discharge 4MJ per lap, what the storage capacity would be.

[andylaurence]

This is completely pointless for an electric motor, whose power delivery characteristics are mostly independent of RPM.

Is that really the case? Generally, motors have a maximum current. The torque of the motor is roughly proportional to the current. The power is the torque multiplied by the voltage. As such, the maximum torque of a motor tends to be constant, irrespective of RPM, but the power rises with RPM. That's what makes a gearbox useful on an electric motor. Electric motors are not noisy at high RPM like an ICE and nor are they less efficient, so there's no need for a gear on a road car that lets the motor run at low revs. If you only used the lowest gear possible in your car, then you'd not need most of your gears. Most cars can quite happily do 70mph in 3rd gear if you don't mind the noise.

[Andres125sx]

To be honest I don't know why they're using a multispeed gearbox at all with an electric motor, much less a 5 speed.

I´ve wondered myself that too, it looks like using a gearbox improves the range. Electric motors have the torque to use single speed box, but the efficiency is worse, and since range is their main problem right now, they chose to put a gearbox to improve range as much as possible

But I´m sure they could have used 2-3 gears, 5 looks excesive for an EV

The model S goes up to 210 km/h with a single speed box.

The other day I saw a car at the road and tought "what´s that? Maseratti?, Jaguar?... It was a Model S, much much more beautiful live than in pictures/videos. Loved it

[machin]

This is completely pointless for an electric motor, whose power delivery characteristics are mostly independent of RPM.

Is that really the case?

Indeed Andy, the annoying thing is that the "Electric motors have Power output characterictics independant of RPM" Myth is another one of those things that can be quashed with a simple bit of knowledge and a quick google search:-



As you can see Lycoming, it certainly does not have a flat power curve, and as we all (should) know; any car which operates at various road speeds and has a prime mover with a non-flat power output curve, will benefit from improved acceleration if it uses a multispeed gearbox.

[Lycoming]

Bad choice of wording on my part. Power is obviously not RPM independent.

However, note that even in the graph you have, the power output is high through most of it's RPM range, which is often not the case or gas engines. You can still get by with fewer gears.

[machin]

Bad choice of wording on my part. Power is obviously not RPM independent.

However, note that even in the graph you have, the power output is high through most of it's RPM range, which is often not the case or gas engines. You can still get by with fewer gears.

5 being fewer than, say, 8...

[Lycoming]

Most cars used to be 4. The trend towards 8 or 9 gears in passenger cars is a recent thing. Not so recent for trucks though.

Besides, the model S is a single speed box. There's a huge weight saving to be had there.

[machin]

Besides, the model S is a single speed box. There's a huge weight saving to be had there.

The weight of a 5 speed gearbox is pretty small compared to the car... (for my lightweight kitcar it was only about 5%)... for a big car fitted with a big battery pack it will be a lower %. it is more likely that they went with a single speed box for simplicity/reliability. (Either that or someone at Tesla thinks motor power is independant of RPM!)

if we had the actual power curve of the Formula E car we could easily determine the optimum number of gears, but it will be fairly safe to say that someone has looked at this and decided 5 gives the best compromise of performance vs reliability vs weight for the Formula E car.

[autogyro]

The original Tesla had a two speed gearbox.
It was removed because it was too jerky in operation and only lasted for 2000 miles.
The Hewland box in the FE is also jerky (watch the videos) and we have yet to see how reliable it is.
I consulted with Hewland about EV gearboxes before the decision to give the FE car a gearbox.
IMO no lay shaft gearbox is suitable for EV power trains.
We shall see.
My ESERU was designed for use in EVs perhaps one day I will see it in use.

[NTS]

Could anybody explain to me what the main reasons are that a normal gearbox is not suitable for an EV? Gearboxes that can handle high torque exist for heavy diesel vehicles so that's presumably not the main issue?