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Design Part 1 – Twist: and the performance of an electric car

Power is force times speed. It's also torque times rotational speed.
SPEAKER 1: Next week– that will be week four– twist will help us find unknown forces acting on rigid bodies, a vital part of your engineers’ toolkit. But in this design evaluation, we all look at twisting effects that are important on their own. We’ll use twist to analyse the performance of an electric car to find answers to questions like– does the car have enough power to cruise at 108 kilometres an hour on a level road? Is the gearing on the car suitable for cruising at 108 kilometres per hour? What is the maximum gradient that the car can climb at 108 kilometres an hour? Can the car restart on the maximum gradient it is likely to be driven on?
We will need to relate road speed to engine rotational speed, and relate motor torque to tractive effort at the drive wheels. We will be using twist. Along the way, we’ll look at the rolling wheel. There’s rolling resistance, gravity resistance, air resistance. And there’s gearboxes and the gear ratios, efficiency, power transmitted by a shaft, and the torque and power curves for motors. To save time, we will quote the results you need. If you’d like more explanation, you can find some in “Through
Engineers’ Eyes: Expanding the Vision.” Here’s how the motor in an electric car is connected to the drive wheels. There’s no clutch or variable gears. The output from the motor is a twist, a torque. Motoring magazines often quote maximum torque as well as maximum power when they review a new car. First, we’ll see if the motor can provide enough power. The maximum power the motor can supply is shown on this graph. But that is not enough. The torque and speed of the motor have to be matched to the requirements of the car. For this, the gearing has to be right. For example, from the graph, the maximum speed of the motor is 11,000 RPM– that’s revolutions per minute.
If the speed of the car is below 108 kilometres an hour when the motor reaches 11,000 RPM, then it will never get to 108 kilometres an hour. So after we’ve decided if the car has enough power, we will check out the gearing. We’ll need to know the torque at any engine speed for this. We will get that from the graph. So start by downloading the design specification and guide. After you have worked through this, you might like to continue your exploration on your own or with colleagues. And if this topic has caught your imagination, you can analyse two very different vehicles– a bicycle and a 62 and 1/2 tonne B-double truck with a 450 kilowatt diesel engine.
Data are given at the end of the design specification and guide. So decide what you’d like to do. Maybe you could arrange to work on it with your colleagues. But first of all, download the specification and guide and start on the electric car.

Power is force times speed. It’s also torque times rotational speed.

We’ll use these equations – and others involving twist – to work out the performance of an electric car.

Talking points

  • What do you think about the future of electric cars?
  • What do you think is the significance of the power and torque curves for road transport vehicles?
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Through Engineers' Eyes - Introducing the Vision: Engineering Mechanics by Experiment, Analysis and Design

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