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Ford Previews Next-Generation Hybrids
14 February 2008
by Jack Rosebro
Speaking at the 2008 SAE Hybrid Vehicle Technology Symposium in San Diego yesterday, Sherif Markaby, Global Core Engineer for hybrids at Ford Motor Company, provided some technical details of the coming 2009 Ford Fusion and Mercury Milan hybrids, due at the end of the year, as well as of the 2009 Escape and Mariner hybrids, which are scheduled to be released in a few months.
All four hybrids will share a new powertrain that combines a 2.5L inline four-cylinder engine with a series-parallel transaxle. While the basic layout and packaging of the powertrain and related components remains for the most part unchanged from the 2005-2008 Escape component architecture, many of those components have been redesigned for improved efficiency.
Markaby also noted that Ford has sold 70,000 Escape hybrids to date, and that some units have reached 200,000 miles in service with no service problems.
The next-generation hybrid system applied in the Fusion, for example, will provide a more than 60% improvement in city cycle fuel economy over a non-hybrid I4 engine, Markaby said, and a more than 80% improvement over a V6 on the city cycle.
Variable-Voltage Converter (VVC). The new powertrain’s inverter assembly utilizes a DC-DC buck-boost converter, which Ford refers to as a Variable-Voltage Converter, to step up the voltage potential of current from the battery pack before it is synthesized into a three-phase AC waveform to power the transaxle’s two electric motor-generators.
Every model-year 2004 and up Toyota/Lexus hybrids utilizes a buck-boost converter; however, this is the first time that the architecture has been employed in a mass-produced passenger hybrid vehicle built by another manufacturer.
Stepping up voltage allows the powertrain to produce a given output using less current, which reduces resistance losses and increases efficiency. It also allows the manufacturer to use a smaller, lighter battery: the 275V, 5.5Ah, 27 kW peak power NiMH battery pack used in the new Fusion/Milan hybrids features improved cell chemistry, and is more compact, for example, than the 2005-2008 Escape hybrid’s 330V battery pack.
One of the most important attributes of VVC, according to Markaby, is “
reducing the cost of the battery.” Previous Escape and Mariner hybrids used a stand-alone air conditioning loop to cool the battery pack during peak operating temperatures: the new powertrain uses only a forced-air ventilation system.
High-Efficiency Internal Combustion Engine (ICE). The 2.5L iVCT I-4 engine gets variable valve timing on the intake side (Intake Variable Cam Timing, iVCT), as well as a wide-band lambda sensor in place of a conventional oxygen sensor. The compression ratio in the engine is 12.3:1.
Idle-stop and start events are much quicker, and enable greater efficiency with as many as two times the starts and stops in a given driving cycle as the previous system. The quicker engine shut-downs and pre-positioning (for the next start) also reduces O2 build-up in the engine’s catalyst. Decel Fuel Shut-Off (DFSO) mode and electric-only mode have also been expanded.
High-Efficiency DC-DC Converter. The conventional DC-DC converter, which steps down system voltage to run the vehicle’s 12V accessory bus and charge the auxiliary 12V battery, has been redesigned to operate at higher temperatures. The unit remains liquid cooled, and can switch at higher frequencies.
The VVC variably boosts traction battery voltage to operate the motor and generator more efficiently. Click to enlarge.High-Efficiency Transaxle. The maximum RPM of the new transaxle’s permanent-magnet synchronous motor-generators has been increased, and a new low-drag transaxle fluid has been formulated. The higher available voltages from the variable-voltage converter allow greater torque at higher speeds. Typically, such voltages also enable a more efficient recuperation of kinetic energy via regenerative braking.
Update on the SCE PHEVs. Markaby also made mention of the plug-in hybrid Escapes that Ford is delivering to Southern California Edison (SCE) for V2G research. The PHEV Escapes use a 10kWh Li-ion battery pack, and are getting as much as 120 MPG in testing. The PHEV powertrain operates in three distinct modes: electric drive (ED) mode, blended mode (a combination of engine operation and charge-depleting electric drive), and conventional hybrid mode.