technologies of electric and hybrid vehicles

Part two: In the second part of his look into EV and hybrid technology Peter of Tech-Club examines the functional specifics of electric vehicles

By Peter Coombes |

Published:  16 January, 2018

In the first article in this series, published in the November issue, we looked at some of the issues relating to the batteries used in electric and hybrid vehicles. As brief summary: Modern lithium based batteries typically store four times more energy than a traditional lead-acid battery of the same weight.  

To be able to store the same amount of energy as a full tank of petrol or diesel fuel, lithium batteries would however typically have to be 100 times heavier than a tank of fuel. So the question is: how can an electric vehicle compete against a conventional vehicle if the battery pack has to be so heavy.


Benefits of electric vehicles
Now, let’s look at some of the benefits of electric vehicles: The electric motors are able to convert up to 90% or more of the supplied electrical energy into mechanical energy to propel the vehicle, whereas an internal combustion engine and transmission assembly might only convert as little as 20% of the energy that is stored in the petrol or diesel fuel. An internal combustion engine unfortunately converts a lot of energy into heat during combustion, and energy is also used up overcoming internal friction and heat. Additionally, the gearbox and many auxiliary devices driven by the engine also absorb energy due to friction. However, electric motors do not waste energy to produce combustion heat, and they have very few moving parts. Therefore energy or power losses due to heat and friction are relatively low; and there is usually no requirement for a multi-speed gearbox, so again, power or energy losses are also significantly reduced.  


Power and torque
Another advantage of the electric motor is the way that it delivers power and torque. An electric motor fitted to a vehicle will obviously need sufficient power (horsepower, PS or Kilowatts) to enable the vehicle to achieve the required speed; Assuming that the speed is similar to an equivalent size of vehicle powered by an internal combustion engine, then the electric motor would need to be able to produce similar power to the engine. However, the electric motor will then usually be able to produce much higher levels of torque than the equivalent internal combustion engine. For virtually all types of electric motors used in modern electric cars, the motor produces maximum torque (or very close to maximum) at zero RPM; and the torque remains high throughout much of the motor speed range, which can reach 10,000 RPM or much more for many motors. However, the torque of even the latest petrol or diesel engines generally doesn’t become useful until the engine speed reaches at around 1,200 or 1,500 RPM; and the torque will typically peak at around 3,000 to 3,500 RPM, the torque then progressively reduces as the engine speed rises to its maximum of approximately 6,000 or 7,000 RPM.   

As well as producing instant torque at zero RPM, an electric motor will usually produce greater torque than a petrol/diesel engine used in an equivalent size and type of vehicle. Therefore, unlike a petrol or diesel engine that needs low gears to mechanically multiply the engine torque (to enable the vehicle to accelerate from rest), the high torque electric motor can operate using a single gear.

Electric motor torque will eventually reduce at higher motor speeds, but the drop off in torque is often intentionally restricted. An electric motor is able to produce high levels of torque when high levels of current (high amperage) are allowed to pass from the battery to the motor; but the high currents create heat that can damage electronic control systems and wiring. Therefore, to prevent component overheating, the current can be limited by the control system, which will then provide an artificial limit to torque and power. However, one extreme example of where high current is delivered for just short periods is on the high performance Tesla Model S. The Tesla electronic control system allows the current to briefly rise to levels that are reputed to be as high as 1,800 amps; and this enables the electric motor to produce exceptionally high torque and provide the car with acceleration times of zero to 60 mph in less than three seconds.


Cooling
Even though the current can be limited by the electronic control systems, the reality is that the current flowing through the control systems can still cause heat when the electric motor is propelling the vehicle under higher loads (such as during acceleration or propelling the vehicle at high speeds). Therefore many electric vehicles make use of cooling systems to help reduce the heat, with air cooling systems being fitted to many vehicles. But liquid cooling systems are increasingly being used to cool the electronic control systems as well as for cooling the electric motors and batteries.  


Coming up
So far, we have identified that batteries can deliver more than sufficient current to enable the electric motors to then deliver good power and high levels of torque, which enables electric vehicles to accelerate at similar or better levels than a petrol or diesel fuelled vehicle. And in much the same way that fitting a small fuel tank to a vehicle won’t reduce the performance but it will reduce the range, a small lightweight battery pack will still allow an electric vehicle to achieve good performance, but the driving range will be limited.

With electric vehicles therefore, the big disadvantage is that the battery packs currently need to be heavy and relatively large to be able to provide long range driving without battery re-charging; and this is covered in the next article in the February issue.


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