Road trials have begun on a double-decker bus retrofitted with a hybrid powertrain that cuts fuel costs by 40%. Jonathan Newell travelled to Austin, Texas to get the story.
With the Ultra Low Emission Zone (ULEZ) ruling scheduled to be applied to central London within the next five years, the pressure is on for bus operators to ensure their equipment complies with ULEZ requirements = or risk being banned from operating within the zone.
After 7 September 2020, all double-deckers operating in central London will need to have hybrid power trains. Five years is a short amount of time to re-equip the entire London double decker bus fleet, and would require a staggering investment. However, one UK company has been developing a retrofit power train that costs a fraction of the price of a new bus.
Around three-quarters of all the non-hybrid double decker buses in London are from just two manufacturers and all have similar powertrains – comprising a diesel engine, gearbox and transmission for driving the rear wheels.
Retrofitting the hybrid power train involves removing all of this equipment and replacing it with a new engine, generator, battery pack, electric motor and single speed gearbox which drives the wheels.
To adapt the system for different bus configurations, the powertrain is modular and has further sub-modules and bracketing options to enable it to be fitted to all the existing bus models that are operated in London.
Not only does the use of a retrofit hybrid unit enable the bus operator to comply with ULEZ requirements, it also promises to have a significant impact on cost. Toby Schulz of Vantage Power explained the economic advantage of retrofitting: “Buses aren’t like cars. If you need to replace an engine in an older car, it makes no economic sense, it’s cheaper to just replace the car. However, the body and chassis of a bus retain their value and so engine replacement is viable even in very old equipment.”
The hybrid power train project for replacing diesel bus engines in existing fleets has already achieved recognition through its success in the “Engineering Impact Awards” arranged by National Instruments and announced at NI Week which took place in the company’s home town of Austin in Texas.
Vantage Power won the first prize in the Transportation category for applications using NI hardware and software for the design, test and control of transport systems. Schulz was there with Balazs Pal to receive the award.
Without a retrofit service, Schulz says that the only options for London buses would be to put them out to pasture on routes outside London or scrap them. Either option would result in the complete replacement of the bus at significant cost.
With the cost of new hybrid buses currently being between 50 and 80% higher than their non-hybrid equivalents, the potential for large savings are high since using a retrofit powertrain can be as low as a quarter the cost of buying a replacement vehicle.
The hybrid power train for London’s bus fleet was specifically designed for the kind of motion profile of city buses with frequent stops, braking and acceleration manoeuvres. Vantage Power had the objective of achieving the smoothest and most efficient use of the power available in all modes of operation. This required detailed and constant system monitoring and analysis, so that power train optimisation could be adjusted as necessary and the correct feedback given to the driver.
To achieve this, Vantage Power employed the CompactRIO controller from National Instruments. This combines the advantages of a processor for network communications, data logging and control with a user programmable FPGA (Field Programmable Gate Array). This forms the backbone of the hybrid powertrain’s control system in which highly critical analogue inputs, such as accelerator pedal pressure, can be processed and translated into commands for controlling such functions as power delivery.
The CompactRIO controller receives inputs from nearly 5000 variables produced by the vehicle and this data is able to be transmitted to cloud servers to create an “internet of buses” allowing fleet operators to monitor vehicle condition and performance and manage their deployment.
The conversion of inputs such as pedal position and fuel level is an important feature in the delivery of smooth power for acceleration or for controlling deceleration. The electric drive motor in the power train also acts as an “engine brake”, delivering smooth deceleration whilst also regenerating the kinetic energy of the vehicle and storing it as electrical energy within the battery.
The sensor inputs from driver controls, fuel levels, battery energy levels and speed are all processed by the CompactRIO controller. It assesses the ability of the vehicle to respond and to set power delivery levels and the correct split between the diesel engine and the battery – all in real time – so that it is transparent to the driver. He/she can then obtain exactly the right delivery of power when needed, without the engine stalling or any lags between pedal depression and power train response.
According to Schulz, the response from drivers to the new hybrid system has been good. There are no gears and the power is delivered constantly and uniformly whilst accelerating. With no lurching, gear changes or peaks and troughs in engine torque, the driving experience is very different from that which is provided by the usual mechanical powertrain fitted to double decker buses.
Commenting on the driver experience, Schulz said, “The drivers involved in the testing of the first retrofit hybrid bus said it was like driving a completely new vehicle, rather than one that had had the engine replaced.”
“When the vehicle is at a bus stop, there’s no engine idling so it’s completely silent and when the driver touches the accelerator pedal to move off, the power is instantly there and the engine fires up if it’s needed. However, it’s often the case that stored electrical energy alone is needed and the diesel engine remains off after moving away from the bus stop until the extra power is needed to regenerate electrical energy”.
Ready to roll
With five years still remaining for London buses to get upgraded before having to meet the ULEZ requirements, Vantage Power seems to have got its timing right in making the technology available.
After six months of rigorous stationary and track testing, the test bus has received homologation and was declared fit for use on public roads. A second prototype bus has now joined the first and they are being tested on a London bus route as part of a standard fleet of public service vehicles.
Throughout this testing phase, the fuel consumption, performance and general operation of the powertrain is being carefully monitored and software updates are being applied to tune the system to a level which will be suitable for general release, taking the powertrain one step further to being able to be installed in London’s fleets of double-deckers.