13 replies to this topic

[Hyatt]

Lucid in their latest persentation claim the powertrain (inverter+motor+diff+gearbox) weights 73kg and provides 670hp.

This is ~3x the power to weight ration of the Taycan drivetrain. 

 

 

What do you think, this is possible?

 

The presentation can be found here: 

They talk about the motor at ~ 24:00m

[Greg Locock]

Didn't watch it by why (a) do they need a gearbox and (b) need a diff?

[Ben1445]

Didn't watch it by why (a) do they need a gearbox and (b) need a diff?

If it’s a single motor per axle, the motor will often be connected first to a reduction gearbox and then to a diff.

If you’ve got two motors on an axle you might be able to do without the diff and use torque vectoring.

Edited by Ben1445, 12 September 2020 - 21:09.

[Greg Locock]

Exactly. You save the weight of the diff. Oh, I thought they meant multiple gears, a speed reducer makes sense. Diffs are horribly inefficient. Pop quiz, does anybody know how inefficient?

[Charlieman]

Diffs are horribly inefficient. Pop quiz, does anybody know how inefficient?

I recall scary numbers for diffs used in the 1970s, when Lotus suggested that locked diffs were part of the recipe for the Lotus 78 and 79 in an attempt to mislead other teams. Might it be that oil coolers for racing transaxles are mostly diff coolers? 

[Greg Locock]

It might indeed. All numbers off the top of my head. 

 

So a hypoid diff is a gearbox, handling big torques. The two gears can only mesh perfectly at one torque, where the twist is the housing is perfectly accounted for. The power turned to heat in the diff is the product of (100%-the efficiency at that torque)*torque*rpm.

 

So, if you set your diff to be aligned at high power you minimise the worst case heat buildup - and in practice that is what is done for many higher powered cars so that the diff oil doesn't turn into chip fat at high speeds.

 

This means that when you are tootling around town your diff gears aren't meshing very well, and so your losses are high as a %age of the effective power at the wheels. Added to that is the oil drag friction in the diff. So at high torque you might see 94% efficiency. At low speed, once you account for friction, I saw one test that suggested worst case was 60%. Over  a typical fuel economy cycle it averages out at 85%

 

This is all based on one paper I read long ago, I've never seen another, which is crazy when you think about it.  A FWD diff doesn't suffer from the twisting problem to the same extent as it is isn't trying to turn the torque by 90 degrees.

Edited by Greg Locock, 13 September 2020 - 22:19.

[kikiturbo2]

You have mentioned the optimal mesh before. What does that mean in practice? I always adjusted the mesh by putting some paint and adjusting the spacers until I got the mesh point to be in the middle of the tooth... but that is usually done with little load.. should we adjust it tighter to work better at higher load.. ? Or try to load the diff somehow on the table.. and simulate the real load..

[just me again]

It might indeed. All numbers off the top of my head.

So a hypoid diff is a gearbox, handling big torques. The two gears can only mesh perfectly at one torque, where the twist is the housing is perfectly accounted for. The power turned to heat in the diff is the product of (100%-the efficiency at that torque)*torque*rpm.

So, if you set your diff to be aligned at high power you minimise the worst case heat buildup - and in practice that is what is done for many higher powered cars so that the diff oil doesn't turn into chip fat at high speeds.

This means that when you are tootling around town your diff gears aren't meshing very well, and so your losses are high as a %age of the effective power at the wheels. Added to that is the oil drag friction in the diff. So at high torque you might see 94% efficiency. At low speed, once you account for friction, I saw one test that suggested worst case was 60%. Over a typical fuel economy cycle it averages out at 85%

This is all based on one paper I read long ago, I've never seen another, which is crazy when you think about it. A FWD diff doesn't suffer from the twisting problem to the same extent as it is isn't trying to turn the torque by 90 degrees.

Would that mean that a RWD with a transverse engine and chaindrive to the diff. Would be more efficient than a normal RWD setup?

[Charlieman]

This means that when you are tootling around town your diff gears aren't meshing very well, and so your losses are high as a %age of the effective power at the wheels. Added to that is the oil drag friction in the diff. So at high torque you might see 94% efficiency. At low speed, once you account for friction, I saw one test that suggested worst case was 60%. Over  a typical fuel economy cycle it averages out at 85%

The numbers I recall from the 1970s are similar. I assumed that efficiency has improved since then, on the basis that there would be a cheaper win in transmission improvements.

 

For a racing car, low efficiency when the diff is slipping in a low or medium speed corner is less important because the engine is not running at max power (ie the car can afford to waste power). Higher efficiency makes most difference for straight line and high speed corners. Obviously, both factors matter more if the racing rules are based on fuel efficiency.

 

The factors for a road car are different and sharp corners are more prevalent than on a circuit. 1970s and 1980s RWD road cars with automatic transmission reportedly had transmission efficiency of 65% or so -- that's with a fluid torque converter, limited number of gears and hypoid gear diff. Performance improved significantly with electronic control and multispeed gearboxes in the 1990s. 

[gruntguru]

It might indeed. All numbers off the top of my head. 

 

So a hypoid diff is a gearbox, handling big torques. The two gears can only mesh perfectly at one torque, where the twist is the housing is perfectly accounted for. The power turned to heat in the diff is the product of (100%-the efficiency at that torque)*torque*rpm.

 

So, if you set your diff to be aligned at high power you minimise the worst case heat buildup - and in practice that is what is done for many higher powered cars so that the diff oil doesn't turn into chip fat at high speeds.

 

This means that when you are tootling around town your diff gears aren't meshing very well, and so your losses are high as a %age of the effective power at the wheels. Added to that is the oil drag friction in the diff. So at high torque you might see 94% efficiency. At low speed, once you account for friction, I saw one test that suggested worst case was 60%. Over  a typical fuel economy cycle it averages out at 85%

 

This is all based on one paper I read long ago, I've never seen another, which is crazy when you think about it.  A FWD diff doesn't suffer from the twisting problem to the same extent as it is isn't trying to turn the torque by 90 degrees.

I would think that at light loads, viscous friction becomes dominant. There is a lot of sliding in a hypoid gearset. Not as much as a worm but more than a bevel drive (the hypoid being a hybrid sitting somewhere between the two.)

[Ben1445]

This is all based on one paper I read long ago, I've never seen another, which is crazy when you think about it.  A FWD diff doesn't suffer from the twisting problem to the same extent as it is isn't trying to turn the torque by 90 degrees.

Neither should this be the case for an EV. Motors are typically mounted with the rotational axis aligned horizontally, including in the Lucid Air: 

 

 

In theory you could have two smaller motors and use computer controlled torque vectoring... but are you definitely going to be saving all that much weight on the diff and reduction steps? Torque vectoring would also have to be an actively controlled system with complex software - doable, but something to think about. 

[kikiturbo2]

Already being done on various EVs actually... from what I have seen they are still developing the strategy when the vehicle is sliding but getting there...

[Ben1445]

Already being done on various EVs actually... from what I have seen they are still developing the strategy when the vehicle is sliding but getting there...

I'm not saying it isn't doable, there's that Abt Sportsline modified mad thing that could do donuts on the spot around the car's central vertical axis. 

 

It's just that for a commercial product for the road where keeping costs down and safety is paramount... a tried and tested, well understood mechanical system is perhaps, at this stage, a safer/more cost effective bet than a new, still in some ways quite experimental torque vectoring active control system? 

[Greg Locock]

I should have realised the motor would be east west Ben.