Why thermal management is crucial to the design of electric vehicles

Author : Matt Evans, Product Engineer at 6SigmaET

06 April 2018

Following the UK government’s (and many other countries') announcement to ban the sale of new petrol and diesel cars from 2040, there is likely to be a seismic shift in the automotive industry, as lithium-ion battery-powered cars are likely to become the de facto standard for road cars. Accordingly, 6SigmaET look at the importance of thermal management in EVs...

Electric vehicles (EVs) present a considerable engineering challenge compared to internal combustion engine (ICE) powered cars. Designers of EVs need to provide drivers with equivalent – if not better – performance in terms of range, reliability and overall driving experience.

At the same time, EVs must still provide the creature comforts such as sat nav, entertainment and climate control found in modern ICE vehicles. Meeting these challenges demands innovative solutions – especially with regard to the thermal management of the lithium-ion batteries and other key components. 

In a ‘conventional’ ICE car, the main focus of thermal management is the removal of heat from the engine. Although crucial for reliability, there are still generally quite generous tolerances in terms of mechanical component operating temperatures, as compared to the electronic components in EVs.

Automotive power electronics – liquid-cooled IGBT
Automotive power electronics – liquid-cooled IGBT

In order to achieve the range and reliability that drivers expect, Li-ion batteries need to be kept within a strict, narrow window of operating temperature. Below 0°C, the chemical reactions within the battery slow down, reducing performance and range. But above 30°C, battery performance deteriorates exponentially, up to the point that irreversible damage can occur above 40°C.

However, while the optimal temperature range for the batteries typically lies between 20°C and 30°C, environmental temperatures can vary far more. This means that on some days, batteries may need to be warmed – and on others, cooled below ambient.

Automotive lighting unit – LED
Automotive lighting unit – LED

EVs also present a number of other equally important thermal management issues. For example, the power electronics components of EVs need to be managed carefully in order to avoid ‘thermal runaway’ caused by high temperatures. This is where an elevated ambient temperature can change the operation of the power electronics device, leading to a further temperature rise. As the junction temperature increases, the on-resistance of transistors increases, which in turn creates more heating of the junction. This creates a positive feedback loop in which the electronics may ultimately burn out.

The electric motors used to power EVs present engineers with a further set of challenges. Axle-mounted electric motors are far more complicated to cool than an ICE, positioned in the engine bay, with easy access to airflow at the front of the vehicle.

As a result of these challenges, thermal simulation is critical to the design of EVs. It is simply not practical or affordable to optimise each individual part or system through prototypes alone. Simulation is the only means by which engineers can test a system in a full range of operating environments, identify thermal issues, and then design and optimise a solution. And all in a matter of hours or days, as opposed to weeks or months.

Motor control board
Motor control board

Given the importance of simulation in the design of EVs, engineers need the right tools to be able to perform a full, detailed analysis. In particular, engineers need tools that can import, handle and solve the complex geometries involved in automotive design, where electronics are designed to fit into the available space within curved body shells or dashboards – something that legacy simulation tools often struggle with.

As we get closer to the 2040 deadline, other design issues that we can’t currently foresee are likely to crop up as well, and these will only increase the need for thermal simulation. This is almost certain to introduce a completely new set of thermal performance parameters and variables that design engineers will need to account for. As other technologies are introduced over the coming years to enhance EV performance, engineers must make extensive use of simulation to ensure that the future delivers the experience drivers expect, without compromise.

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