Why are electric trucks much more efficient than diesels?

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An argument between Tesla and Daimler started after Tesla announced the launch of their electric semi-truck with a quoted range that when most people including Martin Daum who is the head of Trucks at Daimler did the maths seemed impossible, based on the known fuel consumption of a diesel truck turned into battery capacity. Even when allowing for a much higher level of efficiency from an electric motor rather than a diesel engine.

Martin Daum of Daimler
Martin Daum of Daimler

The Issue

It seemed that Tesla might have been guilty of overselling the capability of their new product and people were quick to call them out on it. Currently, Tesla has not yet specified the exact battery capacity of the truck, however, the predicted size is thought to be somewhere between 500kWh and 1MWh, which will still be a big battery.

In an announcement from Tesla in early 2018, they stated the range that was being achieved in testing was actually better than they had initially expected. Therefore, as we know Tesla is not breaking any laws of physics, what is it that the established industry is missing?

What Makes the Tesla Truck So Efficient?

The new Tesla truck has a highly optimised and integrated driveline. Whilst many electric truck projects have simply converted a diesel truck by taking out the diesel engine and replacing it with an electric motor.  The Tesla semi uses a powertrain that is quite different. Tesla has eliminated many of the components found in a heavy duty truck driveline and is using a clever system with a motor for each driven wheel, the motors are mounted inboard on the live axle, with each motor having its own reduction gearbox and a short drive shaft out to the wheel. This system eliminates the heavy differentials that this kind of truck is normally fitted with, replacing it with a system that provides controllable torque to each wheel.

The axle mounted motors and gearboxes may not be practical for mass production due to the additional mass on the unsprung part of the suspension and the high shock loads on the motors and gearboxes, meaning Tesla may have to go to an independent suspension system with the motors and gearboxes rigidly mounted to the truck chassis to package this in, but the benefits of controllable torque delivery to each wheel would still make this worthwhile.

This highly unconventional drive system has also allowed Tesla to create a large volume to the package with the battery in the frame of the truck underneath the cab. However, it is important to note that Tesla is not the first to use this sort of configuration. They could have taken the ‘easy’ route for their first project with a single electric motor driving a relatively standard driven axle system, but they have decided to do as they have always done, and go all in on their truck design to challenge on a number of fronts.

The electric powertrain and neat packaging have enabled Tesla to vastly improve aerodynamics compared to a traditional internal combustion engine (ICE) truck, there are several reasons for this. Comparing a Tesla truck to a more conventional tractor unit, it becomes fairly obvious that the Tesla will slip through the air more easily. However, there is more to it than simply the pretty streamlined shape. First of all, the requirement for air flow for cooling systems is radically reduced by the electric powertrain compared to a diesel truck. The diesel truck has to deal with at best a 50% efficient engine, which means that 50% of the energy from the fuel is being lost as heat in the cooling system and out of the exhausts. Normal trucks have large cooling packs and cooling fans which really hamper the aerodynamics. Air is drawn in through the cooling pack and over the engine, it comes out behind the cab and underneath the chassis. A close look at the Tesla semi and you can see that there is a very small air intake along the bottom of the bumper which is a fraction of the area of a conventional truck, this combined with the lower cab and more aerodynamic nose gives a huge difference to the overall package.

Tesla truck annoucement with Elon Musk
Tesla truck announcement with Elon Musk

In a normal truck, up to 50% of the fuel consumption can be in relation to overcoming aerodynamic drag, so there are huge gains to be made with more aerodynamic designs, particularly at higher speeds. Where a 2% reduction in drag results in 1% better fuel consumption, we estimate that the Tesla semi has an enormous reduction in drag compared to a conventional truck.

Even compared to other electric vehicles there are some notable features where a small change can have a large overall impact. For example, Tesla showed a new power inverter in the M3 which uses SiC switching devices. We can assume these are the probably the inverters used in the new truck. SiC has a lower switch on resistance than traditional IGBT modules, so they generate less heat which has to be disposed of through the cooling system. Going to a 4 x motor system instead of running a large single motor means the phase currents are significantly reduced, this means the I2 loses will be much lower.

Finally, the other big advantage of an electric truck is the massively reduced parasitic power consumption. All trucks have certain key ancillary systems which are normally driven off the engine, such as the engine cooling fan, coolant pump, AC compressor, steering hydraulic pump and air compressor for the brakes and suspension. The total power consumption of all these devices on a conventional truck can be more than 50kW, so together they are a really significant energy consumer. On an electric truck, there is no engine so all these devices must be electrified.

Running these ancillaries separately each with their own electric motor and electronics means they can all be precisely controlled which is very difficult in an engine driven truck. Because they can be optimised and individually controlled their average power consumption can be further reduced, meaning the peak power consumption of these systems in a truck will be less than 15kW.

So in summary, building an electric truck can be as simple as strapping a big motor and battery pack to a truck that was originally designed for a diesel engine. However, this approach will not yield the best results or take advantage of the massive system level gains in efficiency that can be made. Optimising the truck design around the EV powertrain, and also optimising the components within the powertrain presents some huge opportunities to reduce power consumption which critically results in an improved battery range and performance.AVID Learning Electric Vehicle Technology Podcast Cover Art

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