Efficiency

One of the major factors that affects the performance of an electric vehicle is the efficiency of the total system. The total system efficiency (Ntotal) is the product of the efficiency of each component:

Ntotal = Nbattery x Ncontroller x Nwiring x Nmotor x Ntransmission x Ndrive

In addition, the other factors that affect the total performance of the vehicle include the aerodynamic efficiency of the vehicle, the weight of the vehicle and the rolling resistance of the tyres on the ground.

Typical values for these would be:

Nbattery = 0.6 (ie 60%)

Ncontroller = 0.95 (ie 95%)

Nwiring = 0.95 (ie 95%)

Nmotor = 0.8 (ie 80%)

Ntransmission = 0.95 (ie 95%)

Ndrive = 0.95 (ie 95%)

Putting these values into the equation we get:

Ntotal = Nbattery x Ncontroller x Nwiring x Nmotor x Ntransmission x Ndrive
Ntotal = 0.6 x 0.95 x 0.95 x 0.8 x 0.95 x 0.95
Ntotal = 0.4

or 40% efficient. This compares with about 25% efficiency for a petrol car and 35% for a diesel car. It can be seen from these example values that the main losses come from the battery - 40% of the energy put into it is lost - and the motor - it loses 20% of the energy put into it. Now there is not much that can be done about the battery losses - although many scientists are working on this problem - but there is a lot that can be done with motors. In fact, the most efficient motors are over 95% efficient - but these types are difficult to find and often expensive.

Now, where do all these losses come from?

The battery efficiency can be defined in two basic ways:

• The amount of energy that can be drawn out of the battery divided by the amount of energy needed to charge the battery.
• The amount of energy actually drawn out of the battery divided by the total amount of energy stored in the battery.

Why are there two? Well it depends on the circumstances. The first one is relevant when you are charging the battery from a limited supply of electricity - for example from on-board solar panels. You may only have a limited number and limited amount of energy and so you want to make sure that as much as possible of this is used and that may mean that you need very efficient batteries and charging electronics. As the Murdoch University eV Challenge is not a 'Solar' Challenge and teams can use whatever they like for charging, this factor is not a real issue here.

The second one is however. In order to go the furthest, you need to use what you have the most effectively. Basically, a battery is not 100% efficient. When getting the energy out, some is always lost and the faster it's taken out, the more is lost. Have a look at the Battery Page for more information about this.

The controller is also important - its no use if a lot of the energy is being wasted between the battery and the controller. Many old types of controllers used 'linear' switching for example bipolar transistors or even worse, big resisters. They simply caused a voltage drop across the circuit and this reduced the current going to the motor and provided some sort of speed control. However what really happened was that the energy draw from the battery was always constant and some was simply diverted away from the motor and wasted as heat.

Another type of controller - and one that can be very efficient - is like that used on the Stealth II (have a look at the Controller Page). The problem with this is that speed control is very difficult and there are large stresses imposed on the drive components when the switch is engaged.

A fully electronic controller using MOSFET or other similar electronic devices, can not only be very efficient - 95% or more - but also can provide excellent speed control.

Wiring losses are also important and this is a result of Ohm's Law which states that a voltage loss occurs when a current flows through a resistance. This voltage drop results in an energy loss - in other words a drop in efficiency. This can be partially overcome in a number of ways and which includes:

• having thick wires (which have lower resistance) to carry the current with only minimal loss;
• by using as short wires as possible which minimises resistance;
• by using a higher battery/motor voltage - which, according to Ohm's Law ,means that the current flowing is lower which then means there is less voltage drop;

These are all important and significant in the 'real' world as well as in high-tech research. For example, the electricity transmission lines from a power station usually carry the energy through thick aluminium cables at very high voltages to minimise losses. Another example is in the Darwin to Adelaide Solar Car Race in which many of the vehicles used high voltages - often over 100 volts - and used SILVER wires in both the motors and the cables. Silver has even lower electrical resistance per weight than either copper or aluminium.

The motor efficiency is one of the very important factors and the difference between a 'bad' motor and a 'good' motor may be 40 or 50% - in other words an efficient motor may provide 40 - 50% more range than an inefficient motor. have a look at the Motors Page for more information.

Many motors run at speeds far too fast for electric vehicles and so need gears to reduce the speed. Losses occur in these which also reduces the range. The way around this is to either have an efficient gear system - the simple chain and sprocket is actually one of the most efficient types of systems - or to do away with them all-together and use a motor that spins at the right speed in the first place.

The final drive is also important and losses can easily be 5 - 10% here. Once again, chains and sprockets or good cogs are quite OK, however direct drive using hub-motors are the most efficient way to go.