(a) Technical Field
The present invention relates to a control method for improving fuel efficiency of a hybrid electric vehicle. More particularly, the present invention relates to a method for controlling a fuel cut operation to be performed in a wider transmission region by a reaction control of a motor, thus improving fuel efficiency of a hybrid electric vehicle.
(b) Background Art
Intensive research has been underway to provide a high efficiency hybrid electric vehicle (HEV) by improving fuel efficiency and emission performance.
One of the various techniques for improving fuel efficiency and emission performance is to provide an HEV with idle stop system, in which the engine of the vehicle is stopped while the engine is idling as the vehicle is stopped.
Another technique for improving fuel efficiency and emission performance is to increase fuel cut operation. A fuel cut mode saves energy by stopping fuel injection when a vehicle running at a high speed is decelerated.
However, it has a problem of the fuel cut range being restricted. More specifically, a sudden change in engine torque can occur when the engine is restarted as the fuel is injected. When a torque is applied at a low speed, for example, at a low gear 1 or 2, the vehicle may be shaken due to the sudden change in engine torque.
For example, when the engine speed is lowered below about 1500 RPM, a fuel in (fuel re-injection) is performed. Although it is desirable to increase the fuel cut period for improvement of fuel efficiency, impact is brought about at the moment when the fuel cut operation is cancelled (i.e., rewetting is started). To improve fuel efficiency, it is necessary to prolong the fuel cut period at a lower speed and RPM.
In the event that a fuel cut operation is performed for an ordinary vehicle, the fuel cut operation is cancelled at about 1500 RPM to revive the engine. Converting the RPM into a vehicle speed, the fuel cut operation is cancelled at about 60 km/h (and 1500 RPM) in case of a vehicle not exceeding 2000 cc, although it may vary according to a gear ratio of the vehicle.
Accordingly, if the vehicle is running at 60 km/h or higher for a predetermined period of time, the fuel cut effecting condition is permitted. The fuel cut operation continues during deceleration period and is cancelled when a preset condition is met.
In detail, the fuel cut cancellation is made when it reaches a predetermined RPM with respect to coolant temperature, and a corrected RPM is considered thereto. Moreover, in the case where an air conditioner is turned on, an additional RPM is considered and, in the case where the air conditioner is turned off, the fuel cut cancellation is made at about 60 km/h and 1500 RPM.
HEVs developed so far reach about 1500 RPM at 60 km/h and the vehicle speed is decreased while maintaining about 1200 RPM up to about 15 km/h. With the current fuel cut logic for gasoline vehicles, it is thus impossible to improve fuel efficiency of HEVs. Moreover, a torque of about 15 to 25 Nm is suddenly applied during the fuel cut cancellation, which results in a serious impact that a driver can feel when the gear ratio is low.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.