A sportscar concept CAD model

[marcush.]

+1

reflecting exactly my thoughts.

[Edis]

If you use two electric machines for the front axle you still need some sort of mechanical coupling in between them to be able to control the torque distribution and to take advantage of the full power offered by the machines.

There are plenty of vehicles that use independant motors for each wheel... torque biasing is simply part of the speed control system, and doesn't need to be linked mechanically.

Such a setup is quite rare and require some very oversized motors in order to be able to transfer a significant part of the total power to a single wheel.

I would start with a mechanical 4WD system

Remember the Power split, even in the case when you have full KERS boost, is still far in favour of the rear wheels in this concept, where the engine, and hence most of the weight is, so traction will not be a problem... in this case there really is no need to use mechanical 4WD, this just adds weight and cost, -although I agree plenty of rally machinery has this capability, so it is a readily available technology and in a bigger, more powerful vehicle it would have its advantages. There's a further disadvantage of mechanical 4WD if it were applied to my vehicle; routing the tranmission from front to rear. What's not immediately obvious from the screenshots is the narrow cockpit area... the occupants sit literally shoulder to shoulder to reduce frontal area... the result is that there is no room for a transmission tunnel between the seats.

I suspect that a mechanical drive will both be cheaper and lighter. That will also sort all the torque transfer issues issues and only require one motorgenerator, and like usual with these, larger means more efficient, lower cost/kW and a higher power to weight ratio.

The tranmission can still be at the rear of the engine away from the cabin, so you only need some room for the shaft that powers the front diff. Most cars already have such a space simply because it provides some extra stiffness to the chassi and/or because it's used for exhaust/coolant plumbing.

In certain load points it can also use power from the engine to charge the energy storage unit.

This is possible with a front mounted power unit that isn't connected to the ICE inside the car... the engine and MGU(s) are effectively connected via the road... so the KERS system harvests energy (i.e Brakes the front wheels) a small amount when the ICE isn't being fully utilised to charge the KERS unit, this is what I call "Sport" mode -when the car tries to keep the KERS system fully charged by utilising unused engine power (e.g during steady peed driving), even though it means burning more fuel to do so... although as you say, and I said in an earlier post, it'll be relatively efficient because the engine will be under higher load and hence BSFC is low.

While possible, it also means that there are additional losses as seen from the engine-battery-wheels perspective.

[

So for racing use, I would pick a flywheel, and for road use I would go with batteries

I agree... I should have written it in my first post, but the ethos for the car is to cover a stretch of European B-roads (read "twisty"), at the same pace as one of today's supercars, but with far less fuel consumption. I'm less concerned with in-city driving, and therefore I'm not concerned with the flywheel's self-discharge problem that clearly is a major advantage to battery storage for stop-start driving. With this ethos in mind I would stick with the flywheel storage method; its almost perfect for the high charge rate, short storage time, high discharge rate, that this application requires.

The flywheels I've seen haven't really impressed, and they are worse off in energy density and the power density isn't just that good. The Williams flywheel used by Porsche provides 120 kW with a weight of 48 kg. That's just 2.5 kW/kg. Some of the latest batteries I've seen have offers up to 5.3 kW/kg. Sure, this is probably measured on the cell itself, while Porsche measures their output at the motor, but even if we assume a 20% loss between the battery and the motor output shaft, and some additional material required for packaging and cooling the battery, the battery can still challenge the flywheel in terms of power to weight. The energy density should also be quite a lot better, even if the expected life probably is shorter. The battery is also significantly better in terms of packaging.

[machin]

and require some very oversized motors in order to be able to transfer a significant part of the total power to a single wheel.

Hmmm maybe... although typical maximum torque biasing is 4:1 (or 80%) for a motorsport LSD, so assuming the "normal" output is 40bhp per wheel, that means 64bhp in the peak 80:20 cornering case, which for a short time an electric motor designed for 40bhp continuous output could provide, so I wouldn't see the need for full 100% torque biasing...

I suspect that a mechanical drive will both be cheaper and lighter.

I just don't see the need for fully transferring a lot of the power to the front... The car has a projected static weight balance of 40:60 front to rear, add in weight transfer under acceleration and there would be very little benefit of full mechanical 4WD... so it would just add weight....and ruin cabin layout...

Most cars already have such a space simply because it provides some extra stiffness to the chassi and/or because it's used for exhaust/coolant plumbing.

If you check out the pictures you can see the radiators are side-mounted... so no coolant pipes, oh and the engine's mid-mounted, so no exhaust pipes either... The Lotus chassis has large side frames so doesn't need a central tunnel for stiffness.... assuming the gearchange linkage runs between the seats a drive shaft would just push the seats outward too much and ruin the concept... especially for almost no gain in performance (see paragraph above); the net gain would be negative.

The flywheels I've seen haven't really impressed,

The jury is definitely out on which is the best way for energy storage at the moment, which makes the whole KERS thing so interesting (and why F1 NEEDS to re-introduce it!). Personally, for this application (short time storage, high charge/discharge rates) I prefer flywheels, but I do agree there are some advantages to battery storage which should get even better with further battery development....

[machin]

and require some very oversized motors in order to be able to transfer a significant part of the total power to a single wheel.

Hmmm maybe... although typical maximum torque biasing is 4:1 (or 80%) for a motorsport LSD, so assuming the "normal" output is 40bhp per wheel, that means 64bhp in the peak 80:20 cornering case, which for a short time an electric motor designed for 40bhp continuous output could provide, so I wouldn't see the need for full 100% torque biasing...

I suspect that a mechanical drive will both be cheaper and lighter.

I just don't see the need for fully transferring a lot of the power to the front... The car has a projected static weight balance of 40:60 front to rear, add in weight transfer under acceleration and there would be very little benefit of full mechanical 4WD... so it would just add weight....and ruin cabin layout... Full 4WD is good where you have poor grip surfaces, a front mounted engine, or a lot of power for the vehicles's weight... none of which apply here...

Most cars already have such a space simply because it provides some extra stiffness to the chassi and/or because it's used for exhaust/coolant plumbing.

If you check out the pictures you can see the radiators are side-mounted... so no coolant pipes, oh and the engine's mid-mounted, so no exhaust pipes either... The Lotus chassis has large side frames so doesn't need a central tunnel for stiffness.... assuming the gearchange linkage runs between the seats a drive shaft would just push the seats outward too much and ruin the "narrow cockpit" concept... especially for almost no gain in performance (see paragraph above); the net gain would be negative.

The flywheels I've seen haven't really impressed,

The jury is definitely out on which is the best way for energy storage at the moment, which makes the whole KERS thing so interesting (and why F1 NEEDS to re-introduce it!). Personally, for this application (short time storage, high charge/discharge rates) I prefer flywheels, but I do agree there are some advantages to battery storage which should get even better with further battery development....

[Edis]

and require some very oversized motors in order to be able to transfer a significant part of the total power to a single wheel.

Hmmm maybe... although typical maximum torque biasing is 4:1 (or 80%) for a motorsport LSD, so assuming the "normal" output is 40bhp per wheel, that means 64bhp in the peak 80:20 cornering case, which for a short time an electric motor designed for 40bhp continuous output could provide, so I wouldn't see the need for full 100% torque biasing...

Yes, but then again, had you used a single motor you could have used an even lower continuous power rating. The situation also gets worse the more driven wheels you got.

I suspect that a mechanical drive will both be cheaper and lighter.

I just don't see the need for fully transferring a lot of the power to the front... The car has a projected static weight balance of 40:60 front to rear, add in weight transfer under acceleration and there would be very little benefit of full mechanical 4WD... so it would just add weight....and ruin cabin layout... Full 4WD is good where you have poor grip surfaces, a front mounted engine, or a lot of power for the vehicles's weight... none of which apply here...

Audi ran four wheel drive in their supertouring cars in the nineties. These had only around 300 hp, a weight soemwhat over one ton I believe, and probably a close to 50:50 weight distribution. Aside from killing the competition in rain, they provided very good acceleration out of particulary slow turn, in the start of the race and also had a very stable behaviour during braking where the drivetrain also prevented lock up of individual wheels. So, I don't think the positive effect of four wheel drive should be underestimated even on low power high grip applications, and Audi only ran a passive system. With an active system you can probably do much better. Something that I see as a big plus is that it also do a lot for the performance on real roads where the tarmac doesn't quite have the grip as it do on racing tracks and where racing slicks aren't used. Under such conditions many high performance cars have trouble reaching their claimed performance.

I also suspect it can be done for 50 kg or less. The motor package you talk about, including energy storage will probably add 150 kg.

Most cars already have such a space simply because it provides some extra stiffness to the chassi and/or because it's used for exhaust/coolant plumbing.

If you check out the pictures you can see the radiators are side-mounted... so no coolant pipes, oh and the engine's mid-mounted, so no exhaust pipes either... The Lotus chassis has large side frames so doesn't need a central tunnel for stiffness.... assuming the gearchange linkage runs between the seats a drive shaft would just push the seats outward too much and ruin the "narrow cockpit" concept... especially for almost no gain in performance (see paragraph above); the net gain would be negative.

I don't think it will ruin the narrow cockpit concept. The shaft itself can be made quite thin, and being low down in the car, there is still going to be some space availible between the seats.

Infact, with batteries as the choice of storage, I would probably place the batteries under the chassi and left some room in the middle for the shaft.

By the way, how did you plan to cool the front electric motors? And what about the power electronics? In Porsches hybrid using the Williams flywheel, the flywheel takes up the passenger space, and it's cooled by separate radiators using oil for the flywheel.

[machin]

you could have used an even lower continuous power rating. The situation also gets worse the more driven wheels you got.

Very true... so the weight comparison that needs to be done is one big motor with active diff and control, versus two small motors and control unit... it'd be a close run thing as the active diff wouldn't be an insignificant weight... to benefit it would need to be lighter than the difference between one big motor and two small motors... it'd be a close run thing and would depend very much on the detail design of the diff and the motors. I'd leave that detailed design to a sub-supplier

Audi ran four wheel drive in their supertouring cars in the nineties.

The example you've given is a front engined installation... and I agreed that in this instance compared to a front-engine, front wheel drive application there are definitely benefits. Against a similarly powered, mid engined application there wouldn't be the same benefit.

I also suspect it can be done for 50 kg or less. The motor package you talk about, including energy storage will probably add 150 kg.

150kg? You've quoted the Porshe-Flybrid system here I guess... that's a 160BHP approx 960kJ storage system.... The F1 KERS systems weigh between 25-30kg for 80BHP(60kW) with 400kJ of storage. OK, so two small motors (or a front mech diff) would weigh a bit more, but considering that normal driving doesn't result in the big stopping events of an F1 race you probably wouldn't need 400kJ storage so the weight would be no where near 150kg...

Also your reply implies you wouldn't have a KERS system? That's missing the whole point of the car... the idea is to hav a 2 seat sportscar that can cover a european B-road in the same time as a current supercar whilst using a lot less fuel... to have the same performance without KERS you'd need a 300-340BHP ICE... whereas this car has a 260BHP ICE and an 80BHP "free" (apart from the energy required to move the additional system weight) boost from the KERS system. instantly you'd be buring 30% more fuel (assuming the engines were of similar efficiency).

The shaft itself can be made quite thin

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Its true that you only need a thin amount of material to transmit power... the problem is if you have a long thin prop shaft it will be very prone to whirling... that's why prop shafts are thin-walled wide diameter to increase the natural frequency and push the whirling frequency outside the operating range. Of course, you could add mid-span bearings, but that adds to the space required right where you don't want it; between the seats. Also the packaging in the engine bay becomes very tricky... the engine is mounted very close to the seats... right where the prop shaft would come through between the seats if you had one, so you'd need to off-set the engine... too many compromises for very little, if any, performance gain in my opinion.

I would probably place the batteries under the chassi and left some room in the middle for the shaft.

If there's room under your chassis then you obviously haven't mounted the chassis low enough!!!! In a passenger car you can do this easily as you're not too bothered by the vertical CG....

By the way, how did you plan to cool the front electric motors? And what about the power electronics?

You can see from the model that the KERS system is mounted up front (the 30kg KERS weight up front wouldn't be problem), so it'd simply be fed with air from the front-centre of the car, the outlet would be at the base of the windscreen.