Mechanical Gearbox
Discussion
They mostly have single speed mechanical gearboxes (reduciton gears), yes. Multi-speed gearboxes are rare but not unheard of, the Porsche Taycan has one.
They generally don't need as many gears as ICE cars because electric motors have a much broader power curve with no minimum rev limit, and their effiiciency doesn't vary much with RPM. Plus they're much quieter and hence more refined at high revs and, by and large, tend to have low top speeds for range reasons.
Also, variable speed gearboxes sap an awful lot of power which would negatively impact range.
They generally don't need as many gears as ICE cars because electric motors have a much broader power curve with no minimum rev limit, and their effiiciency doesn't vary much with RPM. Plus they're much quieter and hence more refined at high revs and, by and large, tend to have low top speeds for range reasons.
Also, variable speed gearboxes sap an awful lot of power which would negatively impact range.
Edited by kambites on Thursday 14th May 15:33
Another interesting trick is that on 4wd Teslas, the front and rear motors are different, and are optimised for different speed ranges. When cruising at a steady speed, the car will be FWD or RWD depending on which motor is more efficient at that speed. This makes the dual motor Teslas have a slightly longer range than the single motor RWD-only ones, even with the same battery capacity. Thus optimising the engine rpm for different speed ranges without needing a gearbox.
Ice engines need them because they typically operate over a limited rpm range (1000-6000) and have limited torque for some of that.
Electric motors have a much wider rpm range (0-20000 or so) and decent torque at most ( not all torque all time as often said but still).
So you can effectively run them at what would be 3rd/4th gear ratio in a normal car or something and drop the complexity weight,power loss and cost of a multi ration gearbox etc.
Electric motors have a much wider rpm range (0-20000 or so) and decent torque at most ( not all torque all time as often said but still).
So you can effectively run them at what would be 3rd/4th gear ratio in a normal car or something and drop the complexity weight,power loss and cost of a multi ration gearbox etc.
samoht said:
Another interesting trick is that on 4wd Teslas, the front and rear motors are different, and are optimised for different speed ranges. When cruising at a steady speed, the car will be FWD or RWD depending on which motor is more efficient at that speed. This makes the dual motor Teslas have a slightly longer range than the single motor RWD-only ones, even with the same battery capacity. Thus optimising the engine rpm for different speed ranges without needing a gearbox.
Not quite right. In a dual motor Model 3, for instance, it will always be RWD only when cruising. The front motor is utilised only under hard acceleration, cornering or when the rear loses traction.One or two things don't seem to make sense to me:
If you tried to drive an ICE car around all day in top gear the fuel consumption would be awful, the engine would be straining and you'd have to have your foot on the floor trying to pull away/accelerate.
Now to pull away from a standing start with an electric motor will draw a huge amount of current from the battery in the same way as the ICE engine will use loads of petrol, so why pull away in top gear?
Why not look after your battery in the same way as you look after your petrol consumption; and run an electric motor through a series of gears lightening the current load and therefore increasing battery longevity (and let's face facts, making it a lot more fun to drive)?
If you tried to drive an ICE car around all day in top gear the fuel consumption would be awful, the engine would be straining and you'd have to have your foot on the floor trying to pull away/accelerate.
Now to pull away from a standing start with an electric motor will draw a huge amount of current from the battery in the same way as the ICE engine will use loads of petrol, so why pull away in top gear?
Why not look after your battery in the same way as you look after your petrol consumption; and run an electric motor through a series of gears lightening the current load and therefore increasing battery longevity (and let's face facts, making it a lot more fun to drive)?
Coby1 said:
One or two things don't seem to make sense to me:
If you tried to drive an ICE car around all day in top gear the fuel consumption would be awful, the engine would be straining and you'd have to have your foot on the floor trying to pull away/accelerate.
Now to pull away from a standing start with an electric motor will draw a huge amount of current from the battery in the same way as the ICE engine will use loads of petrol, so why pull away in top gear?
Why not look after your battery in the same way as you look after your petrol consumption; and run an electric motor through a series of gears lightening the current load and therefore increasing battery longevity (and let's face facts, making it a lot more fun to drive)?
Forget about engines, electric motors are totally different when it comes to losses!If you tried to drive an ICE car around all day in top gear the fuel consumption would be awful, the engine would be straining and you'd have to have your foot on the floor trying to pull away/accelerate.
Now to pull away from a standing start with an electric motor will draw a huge amount of current from the battery in the same way as the ICE engine will use loads of petrol, so why pull away in top gear?
Why not look after your battery in the same way as you look after your petrol consumption; and run an electric motor through a series of gears lightening the current load and therefore increasing battery longevity (and let's face facts, making it a lot more fun to drive)?
An electric motor has broadly 3 major loss mechanisms:
1) Mechanical friction: this is VERY low, literally just the loss from a pair of ball bearings (lets say 100 watts at max rpm) and the windage from the air being pushed around by the rotor, even smaller, typically just 50 watts at peak speed
2) Iron losses: As the rotor spins the magnetic field cuts through the iron of the stator. As iron has a magnetic hysterisis, that induces currents and causes a small amount of drag. This could be up to about 1kW at peak rpm
3) Copper losses: These are directly proportional to stator current, which is proportional to motor torque (not torque, not Power!). As the current circulates through the copper of the stator windings, the resistance of those windings results in a voltage and a loss. However, because the stator of a properly designed eMachine is mostly reactive rather than resistive (ie it has a much higher inductance than resistance), these losses are again, not really that big.
What this means is broadly, and within a certain number of limiting factors, an eMachine really doesn't particularly care how fast it is spinning round. Yes, as machine speed increases losses rise, but they are so small as to be really pretty irrelevant. For example, at a 80mph cruise, with your eMachine at say 75% of it's peak speed, machine losses will 1 to 2 kW at most, whereas the road load at that speed will be in the order of 50kW, ie the motor is 96 to 98% efficient
at low speed, there are really no significant losses, so as long as you can meet the peak tractive effort values (for an pull away on max grade at max GVW) and provided your motor has enough speed range to cover the speeds at which you want to drive, then really, a gearbox is not required, and is not a significant benefit. (yes it's a benefit, but a small one, think 1 to 2 % increase in range / reduction in consumption at most)
Coby1 said:
Anyone selling a battery powered, electric motor driven vehicle with a manually operated five speed gearbox would even have petrol heads queuing up to have a go in it.
I know I would!
It would be pointless. You change gear to keep the engine in the sweet-spot for your desired needs (acceleration, cruising, braking) etc. I know I would!
You'd have to apply a completely artificial power band to the electric motor to replicate such a need. It would be faker than Golf R cabin noise.
I'm still not convinced.
Surely the starting point for any form of power replacing the ICE is just to take away the pistons, crankshaft etc. and replace them with (in our case) an electric motor, take away the fuel tank and replace it with batteries; simple.
There is no need to rotate an electric motor at 20 000 rpm, you can make an electric motor do what you want it to do.
Simply give it enough in the way of power to produce a similar affect to an ICE and see how far you can get with the batteries when you spread that power through a five speed gearbox.
Why the need to completely 'reinvent the wheel' here?
Surely the starting point for any form of power replacing the ICE is just to take away the pistons, crankshaft etc. and replace them with (in our case) an electric motor, take away the fuel tank and replace it with batteries; simple.
There is no need to rotate an electric motor at 20 000 rpm, you can make an electric motor do what you want it to do.
Simply give it enough in the way of power to produce a similar affect to an ICE and see how far you can get with the batteries when you spread that power through a five speed gearbox.
Why the need to completely 'reinvent the wheel' here?
Coby1 said:
I'm still not convinced.
Surely the starting point for any form of power replacing the ICE is just to take away the pistons, crankshaft etc. and replace them with (in our case) an electric motor, take away the fuel tank and replace it with batteries; simple.
Anyone, with any common sense, takes away the gearbox too. Because it's completely redundant. Usually the bottom half of ratios are only good for spinning the wheels.Surely the starting point for any form of power replacing the ICE is just to take away the pistons, crankshaft etc. and replace them with (in our case) an electric motor, take away the fuel tank and replace it with batteries; simple.
And most people, with common sense, realise that fuel tanks are rarely regular geometric shapes. Whilst battery packs most commonly are regular geometric blocks.
Coby1 said:
I'm still not convinced.
so basically, it's you, vs, well, everyone else.Being "convinced" has nothing to do with it. You either understand the physics or you don't, and i think it's quite clear you don't........
I've explained up there^^ why eMachines don't really care about how fast they are spinning with respect to losses, and remember, they are pure rotating machines with a high level of balance, and not RECIPROCATING machines like ICE's where the out-of-balance forces grow as rotational speed increases.
A modern eMachine can sit, on your desk, doing 20,000 rpm and not go anywhere even when not bolted to anything. So you have a machine capable of driving a passenger car without a gearbox, why would you add a gearbox?
Evanivitch said:
Coby1 said:
Anyone selling a battery powered, electric motor driven vehicle with a manually operated five speed gearbox would even have petrol heads queuing up to have a go in it.
I know I would!
It would be pointless. You change gear to keep the engine in the sweet-spot for your desired needs (acceleration, cruising, braking) etc. I know I would!
You'd have to apply a completely artificial power band to the electric motor to replicate such a need. It would be faker than Golf R cabin noise.
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