Retrofit For The Future

Jon Lawson

Advances in motor, drive and battery technology are making electrification increasingly attractive for commercial vehicles. Jon Lawson speaks to one manufacturer to find out how it’s done

Considering the enormous amount of energy it takes to manufacture a vehicle, it makes good environmental sense to replace fossil fuel drivetrains with their electrical equivalents where possible, rather than just buying a whole new vehicle. And with the spiraling cost of diesel, it makes good economic sense too.

Equipmake and partners TfL and Metroline are currently testing a prototype Routemaster which has had just such a mechanical makeover. For Equipmake, the project started during the first lockdown in 2020.

Chief Development Engineer Jonathan Stevens takes up story: “The process begins with a strip down of the bus. Removed parts are weighed and the location noted so if possible the standard weight distribution can be adhered to, however the over riding requirement is vehicle and axle weight. In this particular bus we uprated the front axle as packaging prevented the old weight distribution being matched. We make extensive use of scanning, which we sub-contract. We often don’t have access to vehicle CAD and even when we do there is often variations in real life with buses.

“Once we have the scanned data we identify the load-bearing structures. Battery packs can weigh up to 500kg, therefore ascertaining the sections of key load-bearing structures and simulating in FEA is critical. In the electrical and systems domain we must identify what systems we can communicate with, what we need to replace and what we can modify. In all situations we add a powertrain controller. This needs to interface with the driver controls, in some instances we need to update controls such as the throttle pedal.”

The modelling is a crucial part of the process. Equipmake uses Matlab Simulink for simulation, typically modelled against known routes or industry standard duty cycles.

The parts that are removed are checked and serviceable items returned to the operator. Stevens notes, “As the converted fleet increases in numbers this can be a useful spares resource for the standard fleet.”

The Complete Package

Stevens continues, “The batteries on the Routemaster are particularly complicated from a packaging perspective. There are 5 separate packs in a variety of challenging shapes located under stairs, above wheel arches and so on. Typically Equipmake would use our standard battery pack. Fortunately we are used to dealing with multiple packs and they are connected to a Power Distribution Unit (PDU) of Equipmake’s own design. This incorporates the controller and distributes the high voltage to the traction motor and auxiliary systems.

“Equipmake do not build batteries from cell level, but work with modules that in recent years have reached very competitive prices. This allows us to have flexibility of pack shape, but with a relatively short development time. We must however take care of crash loads and UNECE Regulation 100, which includes external conditions such as surviving a fire for a period of time.”

The drivetrain consists of Equipmake’s HTM-3500 motor. This is a 3,500RPM, 3,500Nm 400kW unit. In bus applications the peak power is not required, but double decker buses such as the Routemaster require the torque for fully-laden hill climbing. This motor can also be stacked in tandem to give 7,000Nm, such as required for fire truck applications. Stevens explains, “The installation of the motor is often one of the simpler parts of the conversion. We position it where the gearbox output would typically be and drive the differential directly with what is often a custom-length driveshaft. After removing the engine and gearbox there is often space left over for batteries or other systems. This was certainly the case with the Routemaster.” The motor is liquid cooled and weighs around 195kg. Due to its relatively flat shape it is often mistakenly thought to be axial flux, but it is a radial IPM (Interior Permanent Magnet) machine.

Testing Times

The bus is now undergoing real-world testing. Stevens notes, “We had a trial day in London prior to the unveiling at Euro Bus 2022 and it is now back in London for a 6-month trial. We have the ability to receive all vehicle, battery and drivetrain parameters over a third party telemetry system. Clearly our main focus is on miles/kWh, but we can also read faults and remotely diagnose any issues we may have during the trial.”

It’s not the only prototype bus the company has. Stevens continues, “We are also trialing a retrofit Optare Versa in York, and we have another bus on trail in Buenos Aries on a new-build Agrale chassis. This trail is going well and the long term plan with overseas customers is to ship batteries and drivetrains for local assembly. We have a number of other buses currently undergoing the design process detailed above. Retrofit in the UK has become extremely popular as operators realise the cost and supply lead time issues with replacing their fleets with new models. This is also a benefit to the country as a whole as it should accelerate the phasing out of the diesel fleet.

“An interesting observation for the retrofit market is how attractive lower duty cycle routes are. These could include school run coaches or city tour operators. These vehicles tend to do lower daily mileages and therefore have smaller battery requirements – the key cost driver for a conversion. The option not to buy new electric makes this an attractive proposition for older vehicles.”

Space Race

Equipmake also supplies aerospace parts and is currently involved in an interesting collaboration with Gilmour Space, an Australian manufacturer. Launch is scheduled for later this year. Stevens explains, “We have designed a motor to pump fuel in a hybrid rocket engine and an inverter to drive it. The brief was for a 350kW motor at 15,000RPM. We responded with a variation on Equipmake’s spoke motor technology, this gives good performance for mass of magnet. We mated this to an unconventionally laid out Silicone Carbide inverter.”

When considering the challenge, compared to supplying parts for terrestrial vehicles, he points out, “The main differences to our other systems is that for the motor we have to keep rocket fuel out of the electrical motor space, this means it has to resist internal pressure from inert gasses supplied from the rocket. These vent slowly in to the fuel through a labyrinth seal. The bigger challenge was with the inverter. Reducing atmospheric pressure means the distance high voltage electricity can jump increases – this would mean designing a larger, heavier inverter to meet these requirements. Fully encapsulating the internal electronics is also an option, but this too is heavy. The solution we opted for was to radically change the shape of the inverter to a cylinder in order to resist the pressure loads of maintaining internal pressure at sea level. This had the added benefit of hugely reducing the mass. However, this was at the expense of electrical design complexity.”

The testing phase for parts going into space takes on a whole new level of importance. The motors and inverters were first tested on the Equipmake dynamometer to measure and tune the control software. Gilmour Space then took over, firstly pumping water and then moving on to the fuel itself. The impeller design is unique to Gilmour and the motor and inverter have been used to pump fuel for the full flight duration during a static fire test. Stevens continues, “Vibration testing is obviously an important part of many product development cycles. In this application not only did we have to pass a design verification test, but each system will go through an acceptance vibration test.

“We see aerospace becoming increasingly important. We are working with three companies across eVTOL, fixed wing and space launch systems. Equipmake is looking to become an approved production organisation for aerospace components. We have noticed an increasing demand for aerospace projects, and we are currently working for eVTOL, fixed wing and space launch system customers.”