If last weeks column was anything to follow, EV engines are much more interesting than they seem. Once people start thinking about them, they can have the same fascination that engines have enjoyed since they were invented.

Electric machines (the correct name for motor-generators) in EVs today take various forms electrically, but what they have in common is that they are all radial flow machines. Flow is the scientific name for the direction taken by magnetic fields but, in simple terms, it means that they are cylindrical in shape with a rotating rotor inside a cylindrical stator.

There is, however, another type of electric machine that is emerging, and it is one that some automobile manufacturers are looking for intensively, especially to integrate with a combustion engine to hybridize it, or to nail into a small engine to make an extension autonomy generator. It is the axial flow motor of the biscuit can.

If the cookie can sounds derogatory, its not (and come on, who doesnt like a Hobnob?). Its the simplest way to describe the shape: short in length and big in diameter. This is useful because it means that in some applications they can use space more efficiently than a radial flow motor, such as when placed between a motor and a gearbox on a transverse motor. They have other important advantages, such as the ability to generate more torque than an equivalent radial flow.

While the rotor (which rotates) of a conventional radial flow machine is relatively small in diameter because it is housed within the stator (which remains static), the rotor and stator of the axial flow machine are like two large dinner plates facing each other. Therefore, the magnets on both plates, whose fields interact to generate torque and rotate the rotor, are much further away from the motor drive shaft. Because of this, the force they produce has more influence on the main shaft. This means more torque, or the same torque for less energy consumption.

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