21 December, 2017
This year has seen a number of governments around the world make momentous announcements to ban on the sale of new petrol and diesel cars. India, France, Britain and Norway have all formally set out a timeline for the shift, and Germany and China are not expected to be too far behind with their own plans.
Aside from the environmental benefits of these decisions, this dramatic shift away from ‘conventional’ vehicles means it is an exciting time to be an engineer. The obvious knock-on effect of these announcements is a likely acceleration the growth of the electric vehicle market. Indeed at least eight countries have already set sales targets for electric cars.
Compared with ‘conventional’ cars, powered by an internal combustion engine (ICE), electric vehicles (EV) provide a different set of engineering challenges. Optimising the efficiency, range and performance of electric vehicles is a major undertaking that will force a raft of new innovations, particularly from a thermal management perspective.
In an ICE powered vehicle, the main thermal challenge is removing heat from the engine. However, while there can be thermal complications with conventional engines – as anyone who has experienced a steaming radiator can tell you – typically mechanical parts are far less sensitive to temperature than the electronics and batteries involved in electric vehicles.
The lithium-ion batteries used in EV’s are much more sensitive to temperature than conventional fuels, where temperature would only become a concern at around -20°C (for a diesel car). The chemical reactions within the EV batteries slow down below 0°C, reducing performance and range. Above 30°C the performance of the battery deteriorates exponentially to the point that irreversible damage can occur above 40°C.
As such, achieving the range and reliability drivers expect will require warming of the batteries on some days when the temperature drops, and cooling below the ambient on hotter days.
Whether controlling the charging of the batteries or in transmitting power from the batteries to the motor – power electronics components are also thermally sensitive and elevated temperatures can lead to thermal runaway. If the operating temperature of a device is not managed correctly the operation of the device might change, leading to a rise in temperature. 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.
Whereas an ICE is typically mounted in the engine bay with easy access to airflow at the front of the vehicle, electric motors tend to be axle-mounted. This makes them significantly more complicated to cool.
Looking to the future
Overcoming these challenges necessitates the use of thermal simulation. Simulation enables a virtual model to be tested in the harshest environments without the time and cost of building and testing prototypes. This means that thermal issues can be identified, solved and then the solution optimised in a matter of hours or days as opposed to weeks or months.
However, when it comes to electric vehicles, simulation itself presents another set of challenges. Automotive electronics are regularly produced in unconventional shapes in order to fit into the available space within a curved body shell or dashboard. This means an engineer needs a thermal simulation tool which can import and solve complex CAD geometry; something legacy simulation tools often struggle with.
As we move towards 2040, these issues will become increasingly urgent and important. Automotive engineers are going to be faced with a wide-range of design challenges, and the thermal management of the electronics components of electric vehicles will be one of the most complex issues to overcome. Engineers need to make sure they have the right tools for the job at their disposal.
By: Matt Evans, Product Engineer