Understanding the power output of an electric motor

Published by on 11.13.2019 - 3 min

Their design is far removed from that of gas or diesel engines, yet electric car motors now have a power output range similar to what we are used to with combustion engines. What kind of power output is it, how is it quantified and how does it impact performance at the wheel?

What does car engine power output refer to?

In physics, power output refers to an amount of energy delivered within a given timeframe. Applied to the automotive industry, it refers to the amount of mechanical energy output generated by the motor, again within a given timeframe. It has an impact on the car’s acceleration, traction capacity (the weight that it is able to move) and its ability to climb uphill.

Whether it be a combustion engine or electric motor, its power output refers to the product of rotation speed (measured in revolutions per minute) plus torque. Expressed in Newton meters (Nm), torque refers to the motor’s pulling power.

This accounts for the fact that two motors with the same power output can behave differently and feel very different to the driver. A sports car delivers performance that can’t be compared to that of a large truck, even if they both have equally powerful engines in terms of output!

Motor New ZOE

How is the power output of an electric car motor calculated?

Manufacturers can’t just claim the power output of a motor: it’s measured during a testing process, illustrated by changes to the torque depending on rotation speed. The value used by automotive manufacturers generally refers to the maximum power output measured. It’s expressed in watts (W), and, more generally, in kilowatts (kW).

How do kilowatts (kW) relate to horsepower (PS)?

“Horsepower” historically refers to the power output of a car engine, and dates back to the late nineteenth century. It’s a way of expressing the power output in a more literal way by equating it to a workload that people can understand. Horsepower, sometimes abbreviated to PS (German for “Pferdestärke”), therefore refers to the power output generated by a horse in order to lift a 75 kg weight one meter high in one second. Under the metric system, it is equal to around 736 W.

So the power output of a car motor can be stated interchangeably in either kW or PS. The R135 motor in New ZOE, for example, generates output of 100 kW, i.e. 135 PS — hence the name! Its torque is now improved at 245 Nm, versus 225 Nm for the R110 motor launched in 2018, to make the car more dynamic in situations when acceleration is needed, like when passing or merging into highway traffic.

power of the motor

What factors determine the power output of an electric car?

The role of a motor is to create mechanical energy out of another form of energy. So its power output is derived from its maximum energy transformation capacity. In the case of an electric car, its power output depends on the size of its motor (its volume) and the wattage of the incoming current. So the battery’s storage capacity and the power electronics that drive the motor affect its power output.
Power output is also a result of yield, i.e. the quantity ratio of incoming electricity supplied to outgoing mechanical energy delivered.

The aim consists then in reducing power output losses caused by heat or friction to achieve maximum energy efficiency. This way, most of the energy stored in the battery is used to extend the vehicle’s range.
In this respect, New ZOE performs especially well. With a WLTP* range of 395 km thanks to a 52 kWh battery, it offers one of the best ratios on the electric vehicle market, all segments combined.

Power output, consumption and range

The maximum power output does not directly affect the range of an electric car, since driving style has the greatest impact on the motor’s consumption. For example, sharp acceleration will mean a spike in electricity consumption. Periods of high-speed driving also draw on the battery significantly. The higher the speed, the more energy is needed to sustain it.
Conversely, relaxed driving keeps instant consumption down and makes regenerative braking more effective. This is the principle behind eco-driving, which is one of the best ways to increase the range of an electric car.

 

*The duration and distances mentioned here are calculated from results obtained by the New ZOE during the WLTP (Worldwide Harmonized Light Vehicles Test Procedure, standardized cycle:* 57% urban driving, 25% suburban driving, 18% highway driving), which aims to represent the actual conditions of a vehicle’s use. However, they cannot foresee the type of journey after recharging. The recharging time and the recovered range also depend on the temperature, battery wear, power delivered by the terminal, driving style and level of charge.

Copyrights : Jean-Brice LEMAL, Pagecran, Renault Marketing 3D-Commerce