Electric DIY Kitcar

[machin]

Was just stumbling around the Web and found these electric propulsion packages from UQM:-

http://www.uqm.com/propulsion_specs.php

And that got me wondering what sort of performance could be achieved in a light-weight kitcar...

I've looked at the Powerphase 125 which gives peak power of 167bp -quite typical of a middle of the range internal combustion engine in a kitcar... The advantage of this and the Powerphase 145 (194bhp) is that they rev to 8000rpm, so can use the kitcar's standard final drive ratio and still have a decent top speed.

Motor weight = 41kg
controller weight = 16kg

The main problem with electric propulsion is the battery weight, however for the branch of motorsport I'm interested in (UK Sprint/hillclimb events) the runs are short; typically about 60 seconds.... That means that the battery size can be kept to an absolute minimum. With current lithium-ion battery power densities you're looking at a battery pack weight of about 40-50kg...

50+41+16 = 107kg about comparable to 2 litre internal combustion engine and ancillaries. But it gets better... the Electric motor has much higher mid-range power, so it can deliver excellent performance without the need for a gearbox...

Seems to fit into the car rather nicely:-



And has pretty good acceleration performance (compared to the same car with a typical 164bhp internal combustion engine):-

[andylaurence]

You don't need 40-50kg of lithium. Hobby King sell 37v 450A packs for under £100 and 11 of those will suffice for the UQM 125 (looks like a shade over 400A peak). That's about 16kg of lithium.

I don't think 107kg is comparable to a 2 litre engine with ancillaries though. I've never weighed one, but internet sources suggest half that again. Of course, there's also the weight loss associated with the exhaust, fuel tank, radiator and other associated gubbins.

I'm sprinting a 460kg car in Sports Libre and considering adding a motor alongside the engine in my car. I'm looking for more modest gains (40hp) and less weight gain (25kg or so) in an effort to get more power without compromising reliability.

[flynfrog]

You still need a larger pack if you want a decent discharge rate. I was doing some rough calcs to convert my shifter kart to electric in the end the gas engine wins big time.

Check out http://dpcars.net/ he converted one of his cars to electric to even get close to the car in gas form takes a rather large motor and pack even to run a few laps.

[bill shoe]

A bit off topic here, but let me give a shout-out to an overlooked benefit of your electric motor idea-- no shifting. This allows full-time, uncompromised left-foot braking. Remember, great heel-and-toeing is just an awkward countermeasure to shifting. Once you have an equal number of feet and pedals the driving will improve in subtle but important ways. The biggest benefits come from braking and cornering, not acceleration.

[andylaurence]

The pack I linked to has a 90C peak discharge rate. That's how it pushes out 450A, which is as much as the motor needs to make 300Nm and the 125KW peak power. I can tell you now that 125KW in a <450kg car can be competitive in Sports Libre. What's important here is that sprints last no more than two minutes, so you can live with a 30C average discharge rate, provided you can re-charge between runs and you're not double-driving.

One pedal per foot is possible without going electric. I have a sequential 'box (GSXR1000) and there are several auto-blipping devices that do the heel and toe for you - just bang the lever forwards.

[machin]

I don't think 107kg is comparable to a 2 litre engine with ancillaries though. I've never weighed one, but internet sources suggest half that again. Of course, there's also the weight loss associated with the exhaust, fuel tank, radiator and other associated gubbins.

Those 2 litre Duratecs are surprisingly light compared to something like an older Zetec.... And that UQM speed controller is water cooled, so you'll still need a radiator of sorts.. albeit a lot smaller, however the VW polo radiator I have in my Westy is so light I thought the box was empty when it arrived!

I mentioned the gearbox, but didn't put a weight to it in my first post; again another big chunk of weight gone. I think we've both reached the same conclusion -compared to a car-based engine + ancillaries a motor and its associated battery and parts (if specced for hillclimbs/sprints) should be lighter. I think it would be at a bit of a disadvantage against a motorbike derived engine though....

Problem at the moment is the cost of the parts; they're not sold in large enough parts yet to bring the costs down.... Otherwise I have a nice Westfield chassis to drop the kit into!

We've probably been at the same venue at some point, -the hillclimb and sprint world being rather small....

[andylaurence]

I found this site that suggests the Duratec is quite lightweight. I stand very corrected.

Of course, there's water cooling of the controller, but the radiator will be much smaller than for an engine. Your Polo radiator might be lightweight, but how heavy is it when it's full of coolant?

The loss of a gearbox is not a good one. You're a man of calculations - check out what difference a six-speed gearbox makes to acceleration, even with the extra 45kg. Having 2.5x the wheel torque in first means 2.5x the acceleration (or wheelspin).

Compared to a car engine, I think electric wins. Compared to a bike engine, even a standard one, I think it's slightly heavier for the same power. However, the spread of power over the rev range may make all the difference. I'm proposing a combination of the two - augmenting the power of a bike engine with some electric assistance.

Using a DC motor instead of a brushless one means that you can use a contactor on full throttle instead of a controller. It's both lighter and has no requirement for cooling. The down side is that the power is either on or off, which isn't a problem when you're talking about 20% more power in a straight line. I'm also thinking of attaching it on the engine side of the gearbox, which means you benefit from gearing. I've done a little simulation and I think it's worthwhile - I can give you some raw data if you want to run your own simulations. Also, being smaller motors, they're more popular, which means they're also cheaper. The whole kit comes in at a lower cost than a cheap turbo kit.

I drive a bright orange ADR in sprints and can often be seen spectating at hillclimbs. I'm always up for a chat, especially about electric things.

[machin]

That set up u have planned sounds pretty good... What sort of total package cost are u looking at?

With regards to the UQM package I agree an extra gear or two would be beneficial... But 6 on a sprint course I don't think will be worth it... The UQM power output is flat from about half revs upwards (or falling torque curve for the torque fans!), and being grip limited at low speeds means more than a few gears and you're just increasing rotational inertia for no wheel-torque gain... Especially on some tighter uk sprint hillclimb venues where top speeds are, relatively, low.

Obviously in your planned application, with a peaky motorbike engine the more gears the better (to a point!).

[Richard]

Sounds like you are planning on an EV competing against IC in the same Westfield class?

[andylaurence]

Motor: 11kg £1000
Lithium Pack: 3.6kg £220
Contactors: 0.9kg £220

Then there's the hardware to hook it up and the cabling, which would probably end up being the same again in weight and cost. The gain is about 20% more torque across the rev range, which gives my 999cc about the same grunt as a 'Busa. There's no specific rules on electric power in the Blue Book, so I'll be in the same class, although I am in <1800cc at most events, so I would still be in the same class with a turbo. Obviously, ModProd and roadgoing classes might be a different story!

I know what you mean about limiting the number of gears. For limited horsepower, there's little point on a hillclimb where speeds are predominantly under 100mph, but average speeds on a sprint tend to be much higher. Having said that, I did Crystal Palace this year and didn't get out of 2nd gear. In fact, I did the practice runs entirely in first!

I've been running a long final drive this year and using just the first 4 gears, but I did a bit of maths to see what'd happen if I geared to use all six and, despite the time taken to change gear, the difference is huge. There's a write-up in the next Driving Mirror.