Lightning/Roadster comparison

[TEG]

A quick check reveals that the UK braking systems regulations are complex and confusing.

Will it pass the MOT? (umm, is the brake fluid reservoir full or not?)
"...[FONT=Verdana, Arial, Helvetica, sans-serif]Future developments
In future hydraulic brakes could become a thing of the past. With vehicles’ electrical voltage increasing, in the not too distant future we will probably see fully electrical brake operation.
Of course the problem of ‘no dismantling’ remains, and there’s also the added difficulty of testing the electronic systems, as the brakes will inevitably be computer controlled. But that’s nothing new; current systems like adaptive cruise control and traction control apply the brakes without driver intervention.
So brake testing is already inadequate because we don’t test the electronic systems currently installed in some vehicles and in the future with full computer control, diagnostic checking of braking sytstems will be an essential aspect of MOT Testing of a vehicle’s brakes. ..."

[/FONT]
The "Department for Transport" seems to have endless amendments to regulations that it is a chore to find if you comply with all of them. One example, and another...

Again, if a vehicle is NOT "mass produced" then it doesn't have to adhere to many of the regulations.

 

[doug]

Does the Lightning have some kind of hill-hold feature? They certainly must have some kind of wheel locking scheme worked out for when the car is parked on an incline. But what if I'm driving in San Francisco and stopped on a hill. Holding the car stationary with just the motors would waste a lot of energy. It's also a good way to burn out a motor.

 

[GSP]

Lithium Batteries vs. Ultracaps

Its not the matter that regen is used at all or if it works till full stop but how you store / use that regained energy.

- the key is that the energy is best stored to a cool capacitor (95% effectivity) and not converted (very uneconomical) into chemical energy - a battery cannot take that load in that short time and heats up (waste energy) instead of doing what it should do - store.

While ultracaps are efficient, they operate over a wide voltage range. This requires less efficient power electronics. Overall the efficiency is about the same as a Li-Ion battery. Also, at 95% efficiency, they don't run cool. At the power levels needed for automotive use, 5% going to heat is quite a lot. I have seen a Volvo/Mack Hybrid Dump Truck that used Ultracaps. The air filters and fans used to cool the Ultracaps looked like they flow a LOT of air.

In BEVs, like the Tesla, the large Lithium battery is sized to get as much energy as possible, to maximize range. As a result the battery has much more power capability than the Motor/Generator and Inverter can handle. So the additional power capability of Ultracaps would be wasted.

As for eliminating the friction brakes, again there is an issue with the Motor/Generator's power. In a typical ICE car, the brakes have about three times the power as the engine. This allows greater deceleration capability vs. acceleration. Racers use this to pass by late braking into a corner, and it is sometimes needed in regular traffic to avoid collisions. BEVs have motors that are comparable to ICE's in power, and when in generator mode they produce about the same level of power. This is plenty for normal stops, but not enough for a panic stop.

GSP

 

[doug]

BEVs have motors that are comparable to ICE's in power, and when in generator mode they produce about the same level of power. This is plenty for normal stops, but not enough for a panic stop.

This is an excellent point.

Back in the "old days" we had a thread from a kid that seemed to think you could stop faster by spinning the wheels backwards. I tried to find it once before but had no luck. Anyone know the one I'm talking about?

Edit: Found it: http://www.teslamotorsclub.com/electric-cars/447-stupid-question.html
Hard to believe that that was only a year ago. Feels like longer.

 

[malcolm]

Does the Lightning have some kind of hill-hold feature?

My understanding is that it will have a traditional handbrake with mechanical linkages and that there will also be a footbrake pedal in the cabin. However, they are hoping that the footbrake will be electrical only and slow the car to a stop through controlling regen.

Tesla's regen-as-engine-braking approach is a factory preset; it always behaves in a predictable manner as the speed of the car falls. So it can be designed to place known current loads back into the battery. Mechanical footbraking is there anyway and can deal with situations where the car is fully laden and isn't stopping quickly enough under regen or where the battery is close to full and can no longer receive large regen currents.

Lightning's proposed regen-as-both-engine-and-footbraking is going to be trickier to design from a battery management point of view. Sudden stops from even quite moderate speeds will deliver very high but relatively brief current surges to the battery. This is actually one of the few situations where the pointless "recharge in 10 minutes" ability of lithium titanate helps.

However, the other benefit of Lithium titanate is improved calendar life. It claims charge/discharge cycles in the thousands rather than in the hundreds which compensates for the poorer energy density. But of course, like any official figures these cycling numbers are produced under controlled charging and discharging currents - small ones:-

http://www.cea-valorisation.com/hom...licensing offer - Lithium titanate anodes.pdf

In the graph in this document on page 2 you'll see that capacity holds up very well at charge/discharge rates of C/10, but at the higher rates demanded in EVs (typically 2-4C?) there is a noticable drop-off in capacity as the cycling continues. Obviously you could just make the battery pack bigger in order to reduce the demand on each individual cell, but size, weight and cost limit this.

All EVs based on any Lithium chemistry have to face the fact that, away from the wall socket, aggressive regen and acceleration reduces battery capacity. (This may be another reason why Tesla are reluctant to implement owner regen profile selection until they have more feedback on the behaviour of the ESS)

If they do go ahead and implement regen-only braking, Lightning will be placing a greater (and less predictable) strain on their battery pack. In short, heavy use of the brake pedal will start to eat into the maximum range of the vehicle.

Incidentaly, if the Lightning's battery is too full to absorb a given regen load, there will have to be some form of electrical bypass safety system - which could be cooled capacitors, but might have to be cheaper resistors.

The other problem is that the further they depart from traditional automotive mechanical systems, the more redesign may be required to get into certain territories at a later date. Let's hope they are designing for this eventuality now. It'll be much cheaper in the long run.

Further info: Tesla Motors - think
Lithium Secondary - Rechargeable - Cells

 

[WarpedOne]

BEVs have motors that are comparable to ICE's in power, and when in generator mode they produce about the same level of power. This is plenty for normal stops, but not enough for a panic stop.

Though there is another point. Electric motors usualy have two power ratings - constant power and max i.e. short term power which can be much higher than the first. This comes from the fact that cooling an electric motor (particulary its rotor) can be very hard, so longterm power has to be adapted to cooling capabilities, while short term power is limited by motor construction only (and energy source of course).

Could this shortterm power handle a panic stop? Of course it won't handle racing conditions with lots of hard brakings but in a normal familiy car that is not intended for race track this short term power migh suffice.

That electric mini from PML has over 700HP. I bet thist is short term or max power, constant power is somewhere around 160HP.

 

[malcolm]

The amount of regen current produced from Tesla's induction motor is controlable at any speed since there are no magnets in the rotor, but Lightning have gone for permanent magnet motors.

http://www.lightningcarcompany.co.uk/files/Lightning-Spec-Card.pdf

so the faster they rotate, the more current they generate (when on regen) for a given load. Always. And under braking they'll also have to provide the car's ABS (see under Specifications / Features on the main site).

So the four motor (generator) currents would have to be sent through some sort of variable electrical load in order to provide a suitable pulsing torque to stop the wheels locking.

If you don't worry about calendar life you could dump this power at whatever current or pulse frequency into the battery. Cell Balancing and Battery Equalisation but a more sensible option might be to limit the maximum current to the battery and handle any excess seperately.

Goodness only knows how you achieve all that.

Further info:- Tesla Motors - touch

 

[malcolm]

Sunday Times article reinforces the 500 cars per year figure.

Hot wheels: Lightning GTS

Just a thought; mechanical ABS pulses the brakes, can electromagnetic ABS (for want of a better term) achieve the same effect without pulsing?

 

[GSP]

Though there is another point. Electric motors usualy have two power ratings - constant power and max i.e. short term power which can be much higher than the first. This comes from the fact that cooling an electric motor (particulary its rotor) can be very hard, so longterm power has to be adapted to cooling capabilities, while short term power is limited by motor construction only (and energy source of course).

Could this shortterm power handle a panic stop? Of course it won't handle racing conditions with lots of hard brakings but in a normal familiy car that is not intended for race track this short term power migh suffice.

That electric mini from PML has over 700HP. I bet thist is short term or max power, constant power is somewhere around 160HP.

Good point, the generator rating would not have to be continuous. However, as Malcolm has pointed out, the battery would have to charge at a very high power level, or large resistors would be required to dump the excess power to heat. Also, the inverter would have to handle the power, probably tripling the amount of expensive IGBT's and requiring a more robust cooling system.

In the end, I think Martin would say "what problem are they trying to solve. Friction brakes work fine." They are lightweight, inexpensive, *very* powerful, and safe. Avoiding this kind of distraction, and instead solving the problems that impede the market for EVs (cost/value, range) has been Tesla's great strength.

GSP

 

[WarpedOne]

In the end, I think Martin would say "what problem are they trying to solve. Friction brakes work fine." They are lightweight, inexpensive, *very* powerful, and safe. Avoiding this kind of distraction, and instead solving the problems that impede the market for EVs (cost/value, range) has been Tesla's great strength.

And I'd wholehartedly agree.

I see wheel motors having their place in all-terrain vehicles where power delivery per each wheel is very important for traction (now they have to use complicated electro-mechanical tourqe splitters - expensive stuff). They might be placed onboard though and not into wheel hubs for durability and cooling reasons.

 

[doug]

My understanding is that it will have a traditional handbrake with mechanical linkages ...

That's what I'd like to find out about. Mechanical linkages to what if there are no brake calipers?

 

[vfx]

In wheel might be better for trucks, vans and other non performance clunkers.

 

[malcolm]

The handbrake could be applied to the driveshaft rather than the wheels. Land Rover have done this.

 

[domenick]

The Lightning GT doesn't have a driveshaft. As far as the batteries accepting large amounts of current, that should be no problem since they are Altairnanos. Not sure how much stock I'd put in an article the can't get the name of the car right. GTS?
They say this chassis is from a petrol car which isn't true. The former chassis was petrol, this new one was designed to be electric.
Also they call the Tesla Roadster "essentially an electric Lotus Elise".

 

[malcolm]

D'Oh!

I give up.