Tech Talk: BAE Systems advertising feature

BAE Systems’ first Series-ER extended range drivetrain from its Series Electric Hybrid product range, and featuring a 32kwh Li-ion battery pack coupled with a Cummins diesel ISB engine, is rolling out on an Alexander Dennis bus in the UK in August.

This latest version of the BAE Systems electric driveline family was launched in 2018, alongside the latest generation of the Series-E hybrid and the proven Series-EV full-electric drivetrain. Deliveries of buses powered by the Series-EV drivetrain will be delivered this year to France, the Netherlands and Germany, adding to the 100 or so already in service.

In all, BAE Systems has an installed base of some 10,000 fully or partially electric drivetrains in service across four continents, including over 4,500 in Europe (among them 1,500 in the UK), with over 5,500 in North America. The series hybrid takes up the lion’s share of those. Originally launched in 2000, the Series-E is BAE Systems’ third generation product, following the first-generation TB-08 propulsion system working alongside a conventional belt driven alternator and the second-generation HDS-100 with 600V to 28V electronic alternator allowing engine stop-start. The first and second generation drivetrains were supplied between 2010-2016 to London, Paris and many other cities in mainland Europe. What’s new is that the latest, third generation Series-E  variant comes with a choice of energy storage from BAE Systems: either a 1kWh ultracapacitor (which carries much less charge, but withstands many more charging cycles for arrive and go functionality) or 32kWh battery (allowing extended zero emission operation for ultra-low and zero emission zones).

Series-E and Series-ER products are both series electric hybrids: they convert energy generated by the engine into electrical power using a generator bolted to the engine flywheel. Two integrated starter generators (ISG)  models are standard: the ISG100 and ISG200 units are most common in Europe; the ISG300 unit is fitted in North America, where higher speed limits prevail. The power generated is fed to traction motors mounted on drive axle wheels to propel the vehicle (they come in 120kW continuous power direct drive, or 160kW or 180kW alternating current traction motors. The linchpin between generator, motor and battery is the propulsion control system (PCS), that converts between three-phase AC generated by the generator to DC (which is how the batteries store charge) and back into AC to drive the motor (or vice versa). The ISG spins the engine up eliminating the need for a starter motor. Finally, three accessory packs provide charge for vehicle power; either 15kW DC output for a 24V vehicle system; or with the addition of a single 30kW AC output for three-phase variable-frequency systems such as air compressors; or with the addition of a further 60kW AC output to support vehicle air conditioning systems.

LOTS OF OPTIONS

The family of BAE Systems drivetrain systems largely shares a common family of components. In the case of hybrid or full electric vehicles, the way in which those components are controlled and configured determines whether the driving mode is suited towards engine-on, engine-off or full-zero emission operations.

The primary benefit of an extended range hybrid bus is that unlike a battery electric vehicle it effectively is not limited in its daily operating range, while still being able to deliver significant zero emission operating distances. With its 32kwh Li-ion battery pack a Series-ER powered vehicle can operate up to 40% of its daily duty cycle with its engine turned off. The combination of hybrid technology with GPS geolocation means that this emission-free driving can be delivered where air quality concerns are at their highest.

The fact remains that even with the highly impressive performance of battery technology today, battery electric buses still have to compromise between range and passenger capacity (weight). On top of that there is the obvious cost and infrastructure challenge of implementing a fleet of battery electric vehicles. It is clear that extended range hybrid offers a low risk, high performing compromise solution to delivering zero emission bus operations.

Extended range hybrid buses do however face a significant technical challenge: ensuring that extended periods of driving with the engine turned off does not have a negative impact on emissions. Stop-start systems that turn off the engine when the vehicle is stationary to reduce fuel consumption and prevent emissions are a common feature of passenger cars. But internal combustion engines don’t just produce torque; they also generate heat, which is essential for the proper functioning of catalysts in the emissions aftertreatment system. Turning off the engine cools down the aftertreatment, reducing its effectiveness. For this reason, BAE Systems carefully monitor and control the propulsion solution during extended “engine off” events and engine-restarts to ensure that the after-treatment system still functions as required.

In addition to providing the drivetrain BAE Systems also provide support to OEMs and operators in determining factors such as energy storage.

BATTERY BASICS

In considering battery size, BAE Systems principal systems engineer Mike Suranyi distinguishes between two types of electrical loads experienced by the battery. There are propulsion loads – such as vehicle weight or rolling friction – and they are computed in terms of a kWh/km figure. This is the operational world of the drivetrain components supplied by BAE Systems. And then there are hotel loads – created by systems for cabin heating, cooling, or accessories such as air compressors to power the doors. Their demand is fixed, so they are calculated per unit time, kWh/hr, or just kW.

Depending on the bus route’s average speed and terrain, the proportion of total energy demand from propulsion loads and from hotel loads can vary. On a distance-travelled basis, a bus at low speed through city traffic with the air conditioning on maximum output would experience much higher hotel loads than one travelling at a higher speed through rural areas on a cool day. Hotel loads can become significant with respect to propulsion loads in particular in extreme hot and cold climates. Suranyi points out that this variation complicates power demand calculations. 

Hilly terrain poses an interesting challenge for full-electric drivetrains. A bus needs more energy to travel up a grade; which in turn requires larger capacity batteries. The regenerative braking system – where the traction motors functions as an endurance braking system – is able to recover up to 80% of the energy originally consumed.

Operational life of the battery on a full-electric or hybrid bus with extended EV operation is dependent on factors such as the number of electric only events per day and the distance and duration of those events. The depth of discharge due to those events is dependent on the capacity of the battery fitted. Batteries need to be sized according to the duty cycle required.

Such is the state of current drivetrain technology; BAE Systems has technology available today that can provide varying levels of electrification to meet operator needs. Operators need to embrace the technology, plan for the appropriate infrastructure, and potential modify their routes to fit the bus.

BOX: RELIABILITY AND SERVICE

A hybrid or full-electric drivetrain is no less reliable than a conventional one, claims Suranyi. Its components are designed for a 12-year life operating based upon a typical London duty cycle (averaging about 40,000mi/year). Most OEMs using the BAE Systems propulsion systems have their own teams of mobile service technicians, trained by BAE to provide service and repair on their installed components. BAE retains a team of service technicians to provide more in depth diagnosis and repair where required.