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.