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

This article explains why, when compared to other EVs, the criteria have been chosen for this prototype.

The Toyota Prius

The Prius is highly representative of EV Hybrids. As it is a large vehicle, seating 4 to 5 passengers, weighing approx 1300kg (without passengers), its best fuel economy figure is around 67mpg. Unfortunately, in cold weather, the fuel economy drops to 33mpg. Its Drag Coefficient is around 0.26, and the front surface area is about 2.1sqm. The combination of these factors means that the Prius is estimated to require about 12kW to sustain 60mph, hence the poor fuel economy figure.

The GM EV1

The General Motors EV1 was famous for having a stunningly good drag coefficent of 0.19, for its fuel economy of around 100mpg, and for being recalled and destroyed because of concerns over servicing and supply of parts. It used a Gas Turbine Power Plant. The weight of this vehicle was, again, like the Prius, around 1300kg, with a front surface area around the same. The increased fuel economy compared to the Prius can be directly attributed to the reduced Drag Coefficient.

The Nissan Leaf

The Top Gear review of the Nissan Leaf and its similar counterpart says it all about this vehicle, but it's worth reiterating: the Leaf is limited by whether access to a charging station can be obtained. In the Top Gear review, the vehicle warned that the battery was low when there was only 10 miles left, and it also kindly advised that the nearest public "Charging Station" was 35 miles away. The presenters found a nearby University, asked some students to run a power cable out the window of the building, and went to find a hotel for the night.

Amazingly, the Leaf's Front Surface area is larger, its weight heavier and its Drag Coefficient is also higher than both the GM EV1 and the Prius. Overall, it is difficult to see why this vehicle would be purchased at all.

The Chevrolet Volt

The Volt is a Parallel Hybrid, like the Prius. Its statistics again are virtually identical, leaving its fuel economy stuck in the same range. The vehicle is touted to be good, but the figure is based on nightly recharging of the on-board battery pack to avoid fuel consumption when driving.

The Renault Twizy

The Twizy is a stunning departure from conventional vehicle design. Two versions are planned: Category L6e and Category L7e (4kW and 15kW respectively) so that in many countries in Europe, a Driving License is not required. The weight is 400kg (including a leased 7kW Lithium Battery Pack, providing a 100km range), and the occupants sit one behind the other, like in the Carver. The width is therefore only 1.2 metres, but analysis of photos of the vehicle show that the main front area is only about 0.75 metres wide. The Frontal Surface Area is therefore going to be tiny: around 1.1 sqm.

So with some quick estimates (Cd of 0.28, Area of 1.1sqm), the vehicle can be shown to only require about 6kW to sustain 60mph. This is consistent with the vehicle's reported range of 100km (60 miles) using its 7kW battery pack. The only down-sides of this vehicle are, therefore, that it can only carry two people, it has no doors, and its range is limited.

However, if this vehicle was converted to a Series-Hybrid EV, with an on-board 6kW efficient Diesel Generator (about 0.28 litres per Kilowatt per hour), it would require about .43 gallons per hour to continuously replace that 7kW. Thus, its fuel economy figure would, to sustain that 60mph, be around 140mpg. Not bad, except you'd get freezing cold in winter, and soaked by spray every time a lorry went past, if it was raining - just like on a Motorbike.

The Volkswagon XL1

The XL1's fuel economy figure is just stunning: 313mpg. This figure has been achieved by creating a vehicle that is lower in height than most sports cars, a weight of only 800kg, and a Drag Coefficient of 0.186. To put its Aerodynamic figures into perspective, it's equivalent to driving a vehicle that is only 0.3sqm, front-facing (that's only 21in by 21in).

Unfortunately, Volvo had to use a Magnesium Alloy to get the weight down on the wheels, and they also invented a special kind of Resin-transfer mould system to create the Carbon Fibre bodywork. Whilst this isn't cheating, it means that there will be a delay before or if this vehicle reaches production.

Also, one of the disadvantages of the heavier weight is that when a hill is encountered, the performance obviously suffers far more than with a Category L7e 350kg vehicle that's half the weight of the XL1.

It definitely has to be said though that the XL1 is a step in the right direction, and it shows why a reduced Frontal Surface Area and Drag Coefficient are so critical. Thanks to Aerodynamic losses being a cubic Law, even compared to the EV1 the small reduction results very very quickly in huge savings.

The Opel RAK e

The Opel RAK e is a superb 2-seater experimental vehicle which can travel 61 miles on a 5kWh battery. Fuel economy equivalence (0.6 litres) is therefore just over 450mpg, assuming speeds of 60mph can be sustained for the entire 61 miles. Its top speed is 75mph, which can be reached in under 13 seconds.

The vehicle has superb aerodynamics, thanks in part to its one-behind-one seating arrangement, low height (1.2 metres) and reduced front surface area. The weight is only 380kg, including batteries.

Yet, once again, the vehicle - with its superb fuel economy and despite the backing and resources of Opel (General Motors), is let down by its limited range. If this vehicle had even a tiny on-board Generator, its battery pack size could be reduced and its range extended to well over 1,000 miles.

The Gordon Murray T27 Design

Everything about the Gordon Murray T27 design says that it should be a runaway success. Fuel economy figures are reported, from a London to Brighton competition, to be equivalent to 350mpg. Front impact crash-testing passed with flying colours. Gordon Murray Design Factory Optimisation requires factory space of only 25% when compared to current manufacturing. The dimensions are only 4ft by 8ft, and yet it holds 3 people. It contains a Zytek high-efficiency variable speed transmission that allows the Electric Motor to operate at its most efficient pretty much at all times. Its top speed is 65mph.

The problem is: yet again, this vehicle, which is in every other way deserves to be a roaring success, has its opportunities for success completely destroyed when compared to standard ICE vehicles, because it is, yet again, a "Pure Electric" vehicle with a massive on-board high-explosive Lithium battery which, despite being of considerable size, still only manages to propel the vehicle for a maximum of 100 miles.

If however this vehicle had even a tiny on-board Diesel Generator, not only would its efficiency be improved even further (thanks to only requiring a much smaller battery pack) but also the vehicle could easily have a 1,000 mile range.

The Riversimple Urban Car

This list would not be complete without the Urban, because it represents a departure in so many ways from the traditional Vehicle Model, yet looks to all intents and purposes like a SuperMini. The Urban has a 6kW Hydrogen Fuel Cell, Ultracapacitors instead of batteries, has 4 Direct-Drive Wheel Hub Motors, and weighs only 350kg. Thanks to the efficiency savings from the Direct-Drive wheels, the fuel economy equivalent is a whopping 300mpg.

Unfortunately, there's a catch. several. The first is that the limit on Category L7E is 15kW, meaning that each Direct-Drive wheel is limited to about 3.75kW. When faced with a 25% Gradient, a 350kg vehicle with 200kg of passengers on-board would need, assuming a 12in wheel radius, over 400Nm of Torque just to sustain progress up the hill, overcoming gravity alone, excluding rolling resistance losses and wind resistance.

You know what's coming next, but it has to be spelled out: 3.75kW Electric Hub Wheel Motors that can deliver over 100Nm of Torque, each, do not exist. This is why, unfortunately, the standard ICE Drivetrain was first developed, so that better acceleration can be achieved, as well as allowing vehicles to get up hills.

The second catch is the supply of Hydrogen. Until Hydrogen becomes readily available, Riversimple's Leasing arrangement with its customers includes delivery of Hydrogen Cylinders to your door. The Urban is in the right direction, but is, sadly, a step too far, which is a pity because it's the sort of concept that deserves to succeed.

The Aixam MEGA City Car

The Aixam vehicles are Category L7e and L6e (350kg), and have plastic bodywork. This means that, unlike Carbon Fibre it's cheap to mass-produce, and unlike Fibreglass, it's easy to mass-produce. Aixam actually voluntarily submit their Quadbike-classed cars for Crash Testing (which is not required, for Category L7e), and they pass with flying colours.

The EV Version of the Aixam Category L6e vehicles are noteworthy, when compared to the Riversimple, because they are restricted to 40mph. The only reason for this is because they do not have a gearbox, but instead have a single 48V motor that drives a rear axle with a standard differential. The price is reduced by using Exide 30Ah Lead-Acid batteries instead of Lithium, but the penalty for doing so is significantly increased weight.

By keeping the power to 4kW, the fuel economy equivalent if this vehicle had an on-board efficient Diesel Generator, would work out to an amazing 165mpg! Additional savings could then be had by reducing the battery pack (because the on-board Generator would constantly recharge it). The only down-side would then be that the vehicle's top speed, thanks to the lack of a gearbox, would be only 40mph.

The Aptera 2e

The Aptera 2e is specifically worth mentioning because of its stunning nature-inspired design which results in an amazing drag coefficient figure of 0.15. There are however a couple of problems with the Aptera 2e. The first is that its frontal surface area is 2.4sqm, meaning that losses to drag are about the same as an EV1. With a reduced weight (850kg) compared to the EV1's 1350kg, it's still good, but it's felt that somehow Aptera missed out on a trick, here.

The more fundamental issue with the Aptera 2e is the outrunner wheels. Any lorry, bus or large pickup truck driver would glance at the Aptera 2e in traffic, see a reduced-width vehicle and immediately smash into the wheels. The vehicle looks stunning and unusual, but it raises concerns over safety just on the psychology of reassuring other drivers that the vehicle occupies the space that it actually occupies.

The Ultra-Efficient Hybrid

By going through the list of options and available vehicles, it should be clear that the "ideal" vehicle would be a combination of the best features of "All of the Above":

The target of around 5kW and yet still achieving 50 to 60mph has to be met, one way or the other. Also, the vehicle has to have dramatically reduced weight in order to not consume large amounts of power to accelerate or get up hills.

So, with the materials chosen, and the chassis for the prototype being the Aixam's City Car, it all comes down to the aerodynamics of the bodywork. Even if the existing bodywork on the existing Aixam "MEGA" car was left completely as-is, the drivetrain replaced with a 2-speed gearbox and a Diesel Generator installed in the back, a fuel economy figure of 100 to 120mpg would be perfectly reasonable to expect. However, that figure is not 150 to 200mpg or greater. In other words, it's not a challenge!

The goal is, therefore, to create a bodywork design that looks stunning, leaves room for three people, yet has a Drag Coefficient below the quite reasonable target of 0.25, has a much reduced Frontal Surface Area of 1.35 sqm, and reassures other drivers by giving it the visual appearance of completely occupying the entire space covered by its wheelbase.

This is why the bodywork for the Ultra-efficient Hybrid has been designed as it has. From the top, it can be seen that the rear passenger's position in the middle allows the rear styling to approach that ideal "Teardrop" shape. From the front, it can be seen that the top half of the vehicle is a much reduced width, allowing just enough room for the two front occupants heads and arms, whilst the lower half encloses the wheels and directs the airflow over and around.

Then, additionally, the front and rear scoops have several purposes. The primary safety purpose is to reassure other drivers as to where the wheels are. The secondary purpose is to provide a place to put front and rear lights that can be inset into an aerodynamically-sculpted recess. The tertiary purpose is to direct airflow. Windtunnel tests show that the airflow over the rear roof and rear windowed section is caught by the scoops, resulting in a Bernouilli effect that sucks air in at the front of the scoops, just behind the doors (the critical widest part of the vehicle). The front scoops are more complex to explain, but they help guide airflow sideways over the bonnet and between the front wheels.

So thanks to the choice of materials and parts, there's nothing, technically, to stop this vehicle concept from going straight into mass-production. The cost will be reduced, thanks to no longer needing an ultra-expensive battery pack. The key is in proving that the bodywork will deliver the goods.