Improvements of Vehicle Fuel Economy Using Mechanical Regenerative Braking

Alberto Boretti
2010 SAE Technical Paper Series   unpublished
Improvements of fuel economy of passenger cars and light and heavy duty trucks are being considered using a flywheel energy storage system concept to reduce the amount of mechanical energy produced by the thermal engine recovering the vehicle kinetic energy during braking and then assisting torque requirements. The mechanical system has an overall efficiency over a full regenerative cycle of about 70%, about twice the efficiency of battery-based hybrids rated at about 36%. The technology may
more » ... e technology may improve the vehicle fuel economy and hence reduced CO 2 emissions by more than 30% over driving cycles characterized by frequent engine start/stop, and vehicle acceleration, brief cruising, deceleration and stop. KERS FUNDAMENTALS It is a fundamental of physics that transforming energy from one form to another inevitably introduces significant losses. This explains why the efficiency of battery-based hybrids is so low for a regenerative braking cycle. When a battery is involved, there are four efficiency reducing transformations in each regenerative braking cycle. (1) Kinetic energy is transformed into electrical energy in a motor/generator, (2) the electrical energy is transformed into chemical energy as the battery charges up, (3) the battery discharges transforming chemical into electrical energy, (4) the electrical energy passes into the motor/generator acting as a motor and is transformed once more into kinetic energy. The four energy transformations reduce the overall level of efficiency. If the motor/generator operates at 80% efficiency under peak load, in and out, and the battery charges and discharges at 75% efficiency at high power, the overall efficiency over a full regenerative cycle is only 36%. The ideal solution is to avoid all four of the efficiency reducing transformations from one form of energy to another by keeping the vehicle's energy in the same form as when the vehicle starts braking when the vehicle is back up to speed. This can be done using high-speed flywheels, popular in space and uninterruptible power supplies for computer systems, but novel in ground vehicles. For the space and computer applications, high-speed motor/generators are used to add and remove energy from the flywheels. In ground vehicles, more efficient mechanical, geared systems are preferred. A mechanically driven flywheel system has losses, due to friction in bearings and windage effects, which make it less efficient than a battery-based system in storing energy for long times. Over the much shorter periods required in cut-and-thrust traffic, a mechanically driven flywheel is much more effective, providing an overall efficiency over a full regenerative cycle of more than 70%, almost twice the value of battery-based hybrids. Almost every vehicle with a manual transmission is already fitted with a flywheel to smooth the flow of power from the engine and to provide a small store of energy to help prevent stalling on launch. Toy cars use a small flywheel geared up to spin fast enough to provide spectacular scale performance. The geared high-speed flywheel concept is now applied to full-sized cars, trucks and buses. The result is a dramatic improvement in fuel economy, at lower cost, without sacrificing acceleration. Subject of the paper is to compute through vehicle simulations the improvements in fuel efficiency over a driving cycle recovering the braking energy with a mechanically driven flywheel to stop the thermal engine at idle, during braking and during accelerations when energy is available in the flywheel therefore reducing the supply of fuel energy to power the vehicle. The fuel economy can be increased by reducing the amount of mechanical energy to be provided by the thermal engine recovering the braking energy and shutting down the engine during decelerations, at rest and during the portion of the acceleration following a deceleration that can be covered by the energy stored. Considering the theoretical advantages of storing braking mechanical energy with a much more efficient, simple and lighter mechanical device, and the recent improvements in kinetic energy recovery systems (KERS) for F1 applications [1 to 11], improvements in fuel economy are being considered using a KERS to recover the braking energy and to buffer the thermal engine.
doi:10.4271/2010-01-1683 fatcat:hoedinefjve7vbpf2voifnkgjm