Drivetrain for Ultra-Efficient Affordable Category L7E Hybrid Electric Vehicle: 200mpg+

This article is here to help evaluate different drivetrain options for an Ultra-efficient affordable Hybrid Electric Vehicle. With a hybrid vehicle, not only is an Electric Motor required but also there must be an on-board method by which "fuel" is turned into "motion" by some means. Here are the options:

The Parallel Hybrid

The parallel hybrid involves making it possible to have the "fueled" engine work in tandem with the "electric" engine. On mechanics alone, this option is instantly ruled out for a simple easy-to-manufacture affordable vehicle. That leaves "Series" Hybrid, which is where the "fueled" engine creates "electricity", which then goes into the same circuit that powers the Electric Motor.

Direct Drive Hub Motors

Some simple calculations show that Direct-Drive Hub Motors are impractical, and some simple research shows that the cost of the Hub Motors and the associated Motor Controllers (2of or 4of) are prohibitively expensive. To demonstrate the impracticality of Direct-Drive Hub Motors, take a weight of 550kg (350kg vehicle with 200kg passengers), then, assuming a wheel radius of 13in (0.3m), calculate the torque required to overcome gravity on a 25% gradient. The maths is: 0.25 * 550 * 9.81 * 0.3 which works out to be 404Nm of torque. Then, look up the specifications on a 4kW Direct Drive Hub Motor such as from the peak torque on the "high geared" version (which can only do 40mph) is 70Nm. It is therefore impossible for a Category L7E (15kW max limit) vehicle to get up a 25% Gradient, and, even if it could, it would be prohibitively expensive.

Rear Wheel Drive, Rear Engine

Given that the use of Hub Motors is ruled out, a standard gearbox is required. Rear Engine Gearboxes with built-in Differentials (aka Transaxles) are quite wide: a minimum of 12in. However, the positioning of the Electric Motor will typically extend out to one side. In a standard vehicle, that would be fine: there is plenty of room under the bonnet. However, the bodywork is, in order to attain greater aerodynamic efficiency, narrowing down to only around 18in directly behind the rear passenger, reducing to 12in or less right at the back.

Likewise, for a transverse Transaxle (such as that on the VW Classic or the Citroen 2CV), the motor sticks out too far: the rear passenger seat would have to be moved forward (or removed). Conclusion: there simply isn't room for a Rear Transaxle, and still keep to the ultra-efficient criteria and also three people in the car.

Rear Wheel Drive, Front Engine

This option actually makes sense. Place in a standard rear axle with a Differential, have a driveshaft running to the front. Use a standard inline gearbox, positioned under the bonnet, and have the small (8in by 8in) 15kW electric motor right at the front, dead centre.

It sounds great, until you encounter the problem of where to put the on-board Generator. At the minimum, this will be about a 50cm cube, possibly slightly smaller: perhaps 50cm x 30cm x 40cm. The best arrangement would be to place it under the seat of the rear passenger - perhaps placing the Generator portion under the seat and then having the Combustion Engine behind. Unfortunately, this is exactly where the driveshaft is: underneath the rear passenger's seat.

So, again, this option has to be ruled out, because the ultra-efficient criteria would have to be dropped.

Front Wheel Drive, Front (transverse) Engine

The Citroen 2CV Gearbox is about the only example of this type of arrangement, and it is only practical with a very small engine, otherwise the engine sits too far forward of the wheels. Fortunately, the ideal 15kW motor is only 8in by 8in. There is then room to place the Generator under the rear passenger's seat.

The only reason, therefore, to rule out this option, is that examples of transverse Front Wheel Drive drivetrains are very very rare. Citroen 2CVs ceased production in 1988. It would therefore be necessary to design a new transverse FWD Gearbox before production could begin (the original Citroen 2CV gearbox was not synchromesh, and had a habit of unwinding its reverse gear cog).

Front Wheel Drive, Motorcycle Gearbox

Due to the small size constraints, which make 2CV-esque Gearboxes ideal, it is worthwhile considering a smaller gearbox, such as that designed for a Motorcycle. Unfortunately, however, most Motorcycle Gearboxes are very narrow gear ratios: 2.71 for bottom and 0.8 for top is considered to be "wide". The main reasons for such narrow gear ratios is presumed to be because of the reduced weight of motorcycles, and also that the engines run at a far higher optimum range of RPM than cars. Brushless DC Electric Motors operate best around 3,000 to 3,500 RPM.

Front Wheel Drive, Front (inline) Engine

This is about the last option left! Examples of the type of gearbox which may suit is that of the Fiat Seicento, or the Suziki Alto, or the Suzuki Swift (aka "Geo Metro" in the U.S). The dimensions of the F8B Gearbox are 330mm by 420mm by 333.5mm (13in x 17in x 13in appx). The engine and clutch would sit to the right of the gearbox's built-in differential, whilst the gears would be to the left. Here is another example, with perhaps the most helpful diagram being this one from the Service Manual which helps to show that it is the forward length which is 17in, and the width which is 13in.

Knowing that the width of the gearbox is 13in, it would be possible to place an 8in BLDC Motor and the clutch on the other side. It can be seen, therefore, that the width of the gearbox plus engine would be approximately 24in (2ft) wide, and that the height would be only 13in. There would also be room for the two front occupants to have their feet either side of the differential.

These characteristics are important, because the width of the vehicle at the front has been narrowed significantly, to present a smaller front surface area, to sculpt the airflow to flow round the front wheels, as well as to push it up and to the sides once past the wheels.

Also, running the Gear Ratios through the Simulator, it turns out that the gear ratios are ideal, and that the Final ratio of 4.35 on the differential is also good. Additionally, the peak torque (60Nm) and maximum power (30kW) match with the preferred Electric Motor as well - the Kelly Controls Mars 0913, which has a rated torque of 26Nm and a peak power of 30kW.


Going through all the options, the only one that makes sense, despite reservations over Drivetrain Efficiency, is the FWD option with an inline engine. Direct-Drive Motors do not work: Electric Motors are only efficient at high RPMs - steep gradients would kill a Direct Drive motor or require 4x the ratings and 4x the cost. Rear Wheel Drive options are out due to the lack of space.

By a happy and fortunate coincidence, the Front-Wheel-Drive Suzuki Swift gearbox fits on all counts, including size as well as ratios. It will fit under the bonnet, leave just enough room for passengers, and matches the Electric Motor. Also, as it is a mass-produced part from a mass-produced vehicle, it is easy to get hold of and also highly competitively priced. Ideal on all counts.