Climate change and urban air quality are two of the most important issues of our time. Emissions from vehicles and in particular Heavy Duty Vehicles being a significant contributor to both emissions of CO2 and exhaust pollutants leading to an epidemic of public health and environmental problems.
Many cities around the world are considering radical plans to improve their urban air quality and large fleet operators are increasingly aware of the need to move to new lower emission forms of transport. Governments are investing in and legislating for the deployment of hybrid and electric public service vehicles such as buses, taxis and refuse collection vehicles.
There have in the past been various attempts to successfully launch and commercialise electric trucks however issues with lack of funding, costs and in particular battery performance and costs have hampered success with some notable failures and cancelled programs.
In the last 2-3 years battery performance and costs have improved significantly which, combined with a much more supportive investment environment, the possibility of bans on diesel engines in some area’s and even countries is making the electrification of heavy-duty trucks and buses for urban operations a more appealing proposition.
The recent announcements from Tesla and new entrant Nikola motors look promising and have, perhaps, prompted the more established players in the industry to get in the ring and promote their own efforts. Most major truck OEM’s have development programs underway for electric drive trucks in the 7.5T to 26T chassis categories commonly used in urban distribution applications.
The recently announced Daimler Urban eTruck below (click on it to get Daimler’s promotional video), caught a lot of publicity and has been followed closely by an announcement from their rival MAN Trucks (one of the truck brands owned by Volkswagen along with Scania) of their own electric platform.
Daimler announced that their electric truck carries over the traction system used in their Hybrid buses. This is allowing Daimler to leverage on experience it has gained in the public transit sector where the markets have been driven by governmental investments and transfer this to its Truck line up. This is not the usual direction of technology transfer in the heavy-duty vehicle industry, but is refreshing to see!
The new Daimler truck has an unusual but effective powertrain configuration, it uses a ZF AVE portal axle as shown in the picture above. This approach eliminates the differential and prop shaft and utilises a dual motor drive system instead. With one asynchronous electric motor driving each wheel but mounted inboard. The portal axle has a reduction ratio meaning that higher speed and therefore smaller/lighter traction motors can be used than a direct drive concept. A portal axle is highly unusual for a truck (Unless its the legendary Daimler Unimog!). Portal axles are typically used on transit buses to allow for low/flat floors to comply with disability access regulations. It can be envisaged that using portal axles throughout a truck chassis could result in a lower chassis and driving position that would have some benefits for urban delivery operations improving both visibility and vehicle loading. This could help to solve the huge issue in many cities with heavy duty vehicles and cyclist/pedestrian injuries and fatalities, due in part to the raised driving position and poor visibility from conventional trucks.
Using multiple drive motors to power each wheel is a technique which AVID was using for electric vehicle powertrain many years ago, producing several concept vehicles using this approach, internally referred to as AVID M^3 (Multiple Motor Management) system. There are 3 main advantages to the multi-motor approach:
- The elimination of the differential saves weight and cost to partially offset the additional weight of the electric powertrain components.
- Using 2 smaller motors and controllers rather than a single large motor improves the system efficiency as the current passed through each motor is half that of the single large motor and as the winding losses in the motor are proportional to current^2. There is also an advantage in terms of improved efficiency under transient (i.e. part load) driving conditions – which is the driving condition of the urban vehicle!
- Using 2 motors allows for improved vehicle dynamics through torque vectoring which can improve traction, reduce cornering loads and reduce wheel slip. This should result in reduced tyre wear in a delivery truck application.
It is also arguably easier to design/source 2 x 100kW traction motors and power electronics than a single 200kW unit. The key disadvantage is that the system requires more design work than simply applying a single electric motor to an existing live axle arrangement. There can be an additional cost due to increased motor and inverter count, although this can be offset by the improved efficiency and reduced loses.
The MAN truck below (clicking on the link takes you to MAN’s promotional video) has taken a more conventional approach with a single traction motor connected to a conventional rear differential. The traction motor and power electronics, in this case, being a Sumo drive from Canadian company TM4.
Particularly on a large commercial vehicle, there are packaging advantages through the elimination of the prop shaft and differential which clears space for the mounting of the battery pack in between the frame. This can be seen when looking at the battery packaging between the two trucks. There is a lot more space for batteries on the Daimler which by contrast is occupied by driveline on the MAN.
Additional packaging space for the battery is important as it has allowed Daimler to package a relatively large 212 kWh battery pack into the truck chassis, with clearly plenty of space for more. Daimler is likely to be using Blended Lithium NMC chemistry cells with an energy density of around 100 Wh/kg meaning a raw battery weight of around 2,100 kg which when packaged and including electrical interconnects is closer to 2,500 kg. Combined with the weight saving of the other powertrain components this should result in a relatively small net weight increase compared to the diesel engine truck.
As can be seen in the press literature both trucks have a motor and inverter cooled by a low-temperature cooling circuit and the Daimler battery is subcooled with a chiller unit being visible on the left side of the chassis just ahead of the portal axle. A chiller is used to achieve optimal performance but does add weight and cost to the system.
In addition to the main traction systems, the trucks have electrified high voltage air compressors (trucks need compressed air for their suspension and brake systems), power steering drives and HVAC pumps. Electrification of these systems is not only necessary for an electric truck but it also allows efficiency gains to be made compared to a conventional truck. With up to a 75% reduction in power consumption over an urban driving cycle being possible when compared to conventional engine driven ancillaries.
It seems like the race for electrification of the heavy-duty sector is well underway, there may be some new competition from Tesla and others, Daimler and MAN have a pedigree and established global distribution and support network’s that have taken many years to develop. Whilst neither Daimler or MAN has chosen to develop their own electric powertrain like Tesla successfully did in their passenger cars, both choosing instead relying on third-party suppliers, they do have a lead in terms of an array of already established heavy duty truck chassis platforms and manufacturing capacity. This approach is common in conventional heavy-duty vehicles with engines, gearboxes and more being supplied and developed by a range of tier 1 suppliers and also in-house efforts by larger manufacturers. Tesla has shown that it is capable of beating the established players at their own game in the passenger car market, but it seems that it may not have the early lead in the heavy-duty market that it has managed to establish in the passenger car field. What all of these players new and old have on their side is a maturing battery market, growing public awareness of at least urban air quality issues, positive political action and the ability to deploy the required levels of investment to make these ambitious projects a commercial success. There are still challenges to overcome particularly around battery cost and weight on the vehicle side and available infrastructure to charge any quantity of these vehicles in a practical time, but the last 3 months has represented a huge leap forward in the quest for electrification in the heavy duty markets.
About the author; Ryan Maughan is the Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of electrified systems for electric, hybrid and ICE heavy duty and high-performance vehicles such as coolant pumps, cooling fans, air compressor and steering drive systems. AVID supports its customers from innovation to production with systems integration, component design, testing, and manufacture.