Questions & Answers

Questions & Answers about electric vehicles

The design of every vehicle will be a compromise.  Petrol and diesel engines use a power source that has a high energy density. Batteries for electric cars have significantly less energy in the equivalent space or weight. Energy density is measured in watt hours per litre or per kilogram.

A quick inspection on Google shows these values:-

For liquid fuels:- 

                                     Diesel              10,942 Wh /per litre or   13,762 Wh/kg

                                     Gasoline            9,700 Wh /per litre or   12,200 Wh/kg

The values for batteries are:-

                                      LiFePO4                           98-116 Wh/kg

                                      Nickel Metal Hydride     60-100  Wh/kg

                                      NiCad                               35-  60  Wh/Kg

                                      Lead Acid                         25-  45  Wh/kg

The best of lithium batteries are 100 times larger than gasoline. Standard lead acid batteries are 200-300 times larger and heavier.  Moving a vehicle takes energy, which is measured in watt hours. However the efficiency of an electric car is above 80% and a normal petrol car is as low or less than 30% efficient.

Using the above figures a diesel car that does 50 miles per gallon (imp) will use 49000 watt hours which equates to 3.6 kg of fuel. An equal powered battery pack using the best batteries would weigh 112 kg. The saving grace of the battery solution is that they could last up to ten years and are cheap to recharge, but they are also currently very expensive, but these prices are coming down.

Design for an electric vehicle

The balance to be struck when thinking about electric vehicles is all about weight and performance. The lighter the vehicle the less power is required to move it; the lower the range required and the lower the speed required the smaller the energy store needed on board.

Modern cars are built to withstand the torque from heavy engines and transmissions. They have heavy suspension to cope with the weight and a strong body to provide protection and load carrying which is again heavy. All this weight means they need a high power source to move them and give the required performance. To provide only a limited range, and using the weight of batteries calculated earlier, it is clear that modern family saloon cars, which weigh up to two tonnes, are not good candidates for converting to electric drive.

Any design for an electric vehicle must take account of its main function. A car to take the kids to school and shop at the local market does not need to do 100 mph. Nor does it need to do 0 to 60 in 3 seconds.

First work out what the car is for and what range, speed and acceleration are required. Then find or design a suitable vehicle that will meet the planned use and make it as light as possible. Composites are a good idea to provide lightness and strength.

Converting an existing road vehicle is easier than starting from scratch, but there is likely to be a weight penalty to start with. However, it is surprising how much weight is lost when the unnecessary parts are removed to fit a GMS Dual Drive. Remove the engine, gearbox, differential, exhaust system, cooling system, fuel tank, and lead acid battery, and you are left with wheels, suspension and fully fitted out bodyshell. Weight distribution may become an issue since the original steering and suspension system was designed to work with the original payload.

When you have a good idea about the weight and performance of the vehicle you can work out what power will be required at the wheels to meet the performance spec. This will determine the power required from the electric motors and the gearing necessary, and the range will dictate how many batteries you need to make space for. Remember that you can have fast acceleration or high top speed, not both, unless you want to spend super car money.

If you are designing from scratch you can build for lightness. This is not a new concept. The old 1930’s Austin Seven only had a little 750cc engine and gave good performance through being light. As always it is the power to weight ratio that matters, and the lighter your vehicle the lower your power requirements and the better the range for a given battery load.

We would recommend trying to get a finished fully loaded weight of less than 1000kg. The lower the better, and if you can get to 700 kg or less then the performance possibilities increase significantly.

Typical expected performance from the GMS Dual Drive

The charts below show the likely optimum performance for vehicles of different weights fitted with a GMS Dual Drive and using different wheel sizes and gearing. For all these a drag factor of 0.38 was used with a frontal area of 1.5 sq metre and the tyres are 185x50xradius – three examples of radius are used, 12, 15 and 18 inches (0 to 31 and 62 mph acceleration times relate to 50 and 100 kph) note that although 0 to 62 times are shown in all instances this may exceed maximum speed available.

Voltage

GMS motors are designed to run at 48v, but will run at 72 v  or higher. We do not favour higher voltages since safety requirements come into play, and what really matters is the power delivered to the motor in watts which is amps x volts, so if you want a high voltage you need a lower current. Essentially this is just swings and roundabouts – there is no definitive best voltage, just cable sizes and safety issues that will influence your choice. Once the voltage is decided the size of the battery pack can be established. The voltage required will determine the number of cells to be connected in series to form a pack, and the required energy load will determine how many pack are required to be connected in parallel. The packs will be assembled by the manufacturer, but how many to use and how to connect them together is your decision.

Batteries

This is an area which is moving rapidly as manufacturers push the technology to deliver ever higher energy densities, but you get what you pay for  - the higher the energy density, the higher the price.

Charging and discharging

This is a very important area and is best left to the battery manufacturers to specify, but in simple terms some batteries need to have an electronic battery management system to ensure that all the cells in the pack behave in a similar manner and maintain the same levels of charge and discharge rates. If they get out of balance the cell could fail, and the pack could then fail. The life of the battery can also be affected by the speed of charging. In theory some batteries could be recharged in minutes, as against hours for others, but charging at the fastest rates can seriously reduce the life of the battery.

The best advice is to follow the directions of the manufacturer and use the recommended charger.

Why direct drive using the GMS Dual Drive is best

The majority of cars available today use a heavy single motor and a differential, and many also use a gearbox. Furthermore, the motors in use have often been derived from industrial applications where robustness and reliability have been the main considerations. For automotive applications a motor also needs to be light, flexible and responsive. The GMS Dual Drive has been specially designed to meet the requirement of purpose built electric cars. This unique design provides direct drive to the wheels and avoids all the additional weight and power losses incurred by using conventional gearboxes and differentials.

Why brushless ac motors

In commercial applications many DC motors require continuous cooling across the brushes in order to maintain high-energy outputs. Brushless motors can alleviate some of these problems, but normally have many components and can be expensive because of such constructions.  AC motors do not suffer to the same extent from these problems.

Motors best suited to automotive use are AC motors for many reasons, principally:

• An AC drive system is a lot better suited for hilly terrain than a DC drive system.
• An AC system is a lot gentler in its power delivery than a DC system allowing obstacles such as kerbs to be managed at slow speeds. 
• An AC drive system can move a much heavier car.
• The range is better with an AC drive system, for two reasons. First, because it uses the batteries more efficiently in most cases, and second, because of excellent regenerative braking, which acts like a generator to put energy back into the batteries.

Why liquid cooled

The gms range of products is designed to be robust and avoids the problems experienced by manufacturers using air cooled motors who have found that these motors can burn out unexpectedly when faced with unplanned changes in load, such as exceptionally high gradients off road or payloads exceeded. GMS motors can also get hotter under these circumstances, but if tolerances are passed the motor will tell you about it and shut down safely. It will never burn out. If you overload the motor enough to make it overheat, simply change the loading and wait for the motor to cool down, then set off again.

Why we do not use hub motors or pancake motors

Other electric drive solutions currently available include in-wheel technology, sometimes called hub motors, which bring their own issues and dynamics.

Some hub motor manufacturers make confident claims about the benefits of their hub motors, but evidence is sparse. We believe that the market for these kinds of motors is best kept for the applications where unsprung weight on the wheel doesn’t matter. However for many designers the idea of combining the electronics in the wheel raises serious concerns about the effect of vibration on the electronics, which is normally best avoided.

We have also looked at pancake motors that claim to be really efficient but we rejected this option since we found they are all very expensive and complex in comparison to the AC motors and some suffer from grit / water contamination through the cooling fins which leads to failure if mounted in a dirty environment.