Next Generation Engine Start/Stop Systems: “Free-Wheeling” 2011-01-0712

Engine Start/Stop systems reduce CO₂ emissions by turning off the combustion engine at vehicle standstill. This avoids the injection of fuel that would otherwise be needed simply to overcome internal combustion engine losses. As a next development step, engine losses at higher vehicle speeds are to be addressed.

During deceleration, state-of-the-art engine technology turns off fuel injection as soon as the driver releases the gas pedal, thus the combustion engine is motored by the vehicle. The engine's drag torque could be desired by the driver, e.g., as a brake assist during downhill driving. However, quite frequently the driver wishes to coast at almost constant speed. Similar to Start/Stop operation, in such situations fuel is injected to simply overcome the combustion engine's drag torque. An operation mode referred to as "Free-Wheeling" reduces CO₂ emissions under such coasting conditions by disconnecting the combustion engine from the powertrain and by turning it off. Free-Wheeling can be considered as a next logical step of engine Start/Stop systems, with additional requirements for the vehicle system.

This paper quantifies the CO₂ potential of Free-Wheeling for various driving conditions and for common emission test cycles. Vehicle simulations as well as vehicle measurements have been carried out in order to derive fundamental requirements for the vehicle system, the engine, the starter system, auxiliaries, electric power net and transmission.