The Green Piece Column, Tuesday 16 February 2010
Anyone who hadn’t heard of regenerative braking before has surely heard the term now after a host of issues surrounding the hybrid cars produced by both Toyota (see article) and Ford (see article).
So is this technology, that’s meant to boost both fuel efficiency and performance, actually littered with technical issues that are threatening the reputation of vehicle electrification?
How does regenerative braking work
Most people know the basic purpose of regenerative braking – it captures the vehicle’s kinetic energy when you brake and stores it in the battery pack for use during acceleration. However, what is the technical process behind the theory?
Regenerative braking occurs by allowing a vehicle’s electric motor generator to spin freely, which then generates an alternating current which is rectified to direct current to charge a vehicle’s battery pack. The motor generator must be connected to the vehicle’s final drive so that the coasting motion causes it to rotate. The rate of regenerative braking is restricted by both the size of the motor-generator and the amount of charge the battery pack can accept.
With some early hybrids, regenerative braking is employed alongside a conventional hydraulic braking system – hydraulic braking was applied to slow the vehicle when the driver applies the brake pedal and this was supplemented by regenerative braking. However, this system limited the recuperation of brake energy and was deemed effectively redundant by the technology employed by most of today’s hybrid cars that combines friction braking with a control strategy that allows the vehicle to be slowed by as much regenerative braking as possible.
Friction and blended braking
In a non-hybrid vehicle with an electro-hydraulic braking system, pedal force is read by pressure sensors that transfer a signal to a brake control unit which will typically use a motor-driven hydraulic pump and control solenoids to calibrate brake force.
However, with a hybrid vehicle that blends friction and regenerative braking, the applied brake force data must be transferred to a power control unit which calculates the friction-to-regenerative ratio. With the control parameters continually changing – such as the steering angle, rate of deceleration, applied brake force and battery state of charge – and a greater number of control units contributing data, software design can be problematic.
The problem is in the software
Both Toyota and Ford have indicated that their recent issues will be resolved with software changes.
Many of the reports of irregular braking in the 2010 Toyota Prius and some 2010 Ford Fusion and Mercury Milan Hybrids seem to be focused on the activation of ABS with Prius drivers reporting an inconsistent brake pedal feel and a reduction in braking force as they transition from high-traction surfaces to low traction surfaces. Toyota has reported around 200 complaints and four incidents but has not stated whether there have been any injuries – Ford meanwhile, has not cited the number of complaints but has stated there have been no injuries. Toyota has since decided to adopt ‘full disclosure’ of complaints and issues surrounding its vehicles following criticism of its handling of the situation (see article).
How will these issues affect electrification? Our verdict
With Toyota recalling vehicles and Ford informing its customers of potential problems via mail, questions have accumulated about whether the prospects for hybrid electric vehicles and battery electric vehicles have been harmed by this negative publicity.
Certainly a recall is a bump in the road, but to TheGreenCarWebsite.co.uk it is likely to only be a short-term issue. The technology surrounding regenerative braking has been successfully used for more than a decade in the Toyota Prius, as well as in many non-Toyota models, and Toyota itself had previously earned a stellar reputation for quality.
The close scrutiny and media uproar surrounding these issues is not so much an issue about electrification as it is about a catalogue of events that have brought questions about Toyota’s quality. It is likely that it is the company’s reputation that has taken the most severe blow rather than that of hybrid cars in general – and with time, chances are that even Toyota itself will re-emerge and the problems will be a distant memory.
The need for electrification remains strong – there is an urgent need to reduce fuel consumption and CO2 emissions; oil price and supply issues continue to resonate; and, perhaps most crucially, all of the leading car manufacturers have recognised a strategic need to develop hybrid, plug-in hybrid and electric vehicles. With prices too expected to tumble over the next 10 years thanks to a reduction in lithium-ion battery costs, the road remains clear for the era of electrification to continue at full speed.
Faye Sunderland
