Braking System for a Full Electric Vehicle with Regenerative Braking 2010-01-1680

Tata Motors Limited plan to launch a range of full electric vehicles (FEVs) to the European market. Regenerative braking is advantageous in maximising range between recharging, but presents challenges of acceptable performance, weight, cost and the ‘blending’ of regenerative braking with friction braking.

Control systems for regenerative braking have been developed by manufacturers to enable recuperation of kinetic energy which would otherwise be converted to heat and wasted through the use of friction brakes. This paper presents the approach taken by Tata Motors Ltd. to optimise the design and operation of a regenerative braking system to maximise range and energy efficiency. The Tata Ace EV is a Class N1 light commercial FEV with drive to the rear wheels only. This presents the challenge of harvesting energy from the axle which contributes a varying amount of the vehicle braking effort depending upon load. It is essential to maintain stability during braking, so premature rear-wheel-lock must be avoided at all times, particularly in low adhesion conditions. Road vehicles must be capable of high decelerations which cannot currently be achieved with conventional regenerative braking alone. Multi-mode braking enables higher decelerations to be generated by combining regenerative and friction braking. This in turn requires careful design of the control architecture to create a conventional brake pedal ‘feel’ such that the blend between regenerative and friction braking is imperceptible to the driver.

Energy recuperation is strongly influenced by vehicle usage. A vehicle travelling at constant speed on motorways may produce very little reusable braking energy because brake applications are infrequent. A vehicle travelling downhill may produce excessive braking energy which may have to be dumped. It has been suggested ^{[ 1 ]} that vehicle usage which promotes the repeated recuperation of small amounts of energy (e.g. the New European Drive Cycle test schedule, Figure 1 ) makes the best use of a regenerative braking system to maximise range (distance between recharges). Energy recovered through regenerative braking is fed to a storage device and it is the ability of this device to receive and release energy which determines the power capability of the regenerative braking system.

The Tata Ace vehicle powertrain system has been modelled in full with an accelerator pedal based regenerative braking system. Proportional regenerative braking to replicate engine braking on overrun has been applied when the accelerator pedal is released, with results suggesting that high levels of regenerative braking on the accelerator pedal can be detrimental to energy efficiency.