by Jim Kerr
Ford Escape HEV featuring PowerSmart transmission jointly devleoped by Ford Motor Company and Aisin AW – Click image to enlarge
The search continues for new technologies that offer better fuel economy, lower emissions, and economical manufacturing while still providing the performance and convenience of our present vehicles. One of these technologies is hybrid powertrains. Honda has the Insight. Toyota has the Prius. Now Ford is getting ready to enter the hybrid vehicle market, and rather than build a passenger car, they are going after bigger game: the SUV market.
Expected in 2003, Ford is designing an optional hybrid powertrain for the Escape, a small but versatile SUV. It will feature a new PowerSmartT transaxle that combines a 65 kW traction motor, a 28 kW generator, and a planetary gearset to connect the engine with the drive wheels. Both the electric motor and generator can be used to provide power to the wheels or generate electricity for the battery pack. In addition, the hybrid Escape will also use a modified Zetec inline 4-cylinder engine. An electronic controller will continuously adjust the contribution of all of these for optimum fuel efficiency.
Ford engineers have designed the powertrain to operate in four modes. At a stop, the gasoline engine is turned off to save fuel. The electric motor in the transaxle, and generator if needed, is used to accelerate the vehicle from a stop. If more acceleration is needed the gasoline engine can be started instantly and combined with the electric motor for power equivalent to a 200 horsepower V6 engine.
Ford HEV PowerSmart transaxle
Click image to enlarge
Only full hybrids, such as the Escape, have the ability to operate in electric-only mode. At speeds under 50 kph, and if the battery has sufficient charge, the motive power can be supplied by the transaxle’s electric motors. If the battery becomes low on charge, the gasoline engine will start and charge the battery. This electric vehicle never needs to be plugged in.
At higher speeds, the gasoline engine is used, where it runs most efficiently. The electric motor can be added for more power if needed, and the generator can charge the battery pack if it is low on charge.
The fourth mode of operation is referred to as regenerative braking. Both the accelerator and brake pedal are fully electronic controls, commonly called “drive-by-wire”. During braking, the energy of the slowing vehicle is fed back into the transaxle. The electric motor and generator use this energy to charge the battery pack. A lot of research has gone into efficient regenerative braking. The system on the Ford Escape is the subject of 51 patents! The more conventional mechanical brake system will provide additional stopping force as required.
Escape HEV engine with PowerSmart transaxle
Click image to enlarge
The Zetec gasoline engine is also designed for economy, using the Atkinson Cycle for its operation. Sometimes called a “five-stroke” cycle, the Atkinson cycle uses a normal intake stroke, but as the compression stroke is about to start, the intake valve is left open to reduce pumping losses. The open intake valve allows a “back-flow stroke” of air from the cylinder into the intake manifold. As the piston moves up the cylinder, the intake valve closes and the compression stroke begins. A 12.3 to 1 compression ratio ensures there is sufficient cylinder pressure for good performance even though much of the cylinder’s charge is pushed back into the intake. The power stroke begins as the air/fuel mixture is ignited by the sparkplug, and the cycle is completed at the piston forces the exhaust gases out the exhaust valve on the exhaust stroke.
The Atkinson cycle is up to 10% more efficient than a conventional four-stroke gasoline engine, but it does reduce low-end engine torque. That is where the electric motor comes in to play. Electric motors excel at low rpm torque, so the hybrid PowerSmartT transaxle makes up for the low speed losses of the Atkinson Cycle gasoline engine.
Ford has predicted that by 2010, there will still be 90% of the vehicles on the road utilising internal combustion engines. Increasing fuel economy in Ford SUV’s by adding a PowerSmartT transaxle will make these vehicles still attractive as fuel prices increase in the years ahead.