Ford’s 3rd generation powersplit hybrid system
24 February 2012
The hybrid system in the coming 2013 MY Fusion and C-Max hybrid electric vehicles (HEVs) (earlier post) marks the third major generation of Ford’s powersplit system. The overall system sizing and all major components were optimized for class-leading efficiency, with each major subsystem and many new components being redesigned to deliver the gains, said Chuck Gray, Chief Engineer - Ford Electrification Engineering, at the SAE Hybrid Vehicle Technologies Symposium this week in San Diego.
With the Generation 3 HEV system, Ford decided to downsize the engine from a 2.5L Atkinson to a 2.0L Atkinson to improve fuel economy. This, noted Gray, results in a reduction in acceleration and vehicle speed. However, Ford was able to compensate for that reduction by increasing battery power with its new Li-ion pack (earlier post), and increased electric power through the motor and the generator to recover to nearly the same level of performance as the outgoing model, but with significantly improved fuel economy. In-house engineering was a key enabler and included all controls, the battery pack, transmission and eDrive system, Gray said.
With the final system, Ford was able to deliver 140 kW of power from a combined engine peak power of 105 kW and its new 35 kW battery. The 2012 Fusion Hybrid carries an EPA fuel economy rating of 41 mpg city, 36 mpg highway; Ford expects the upcoming 2013 Fusion Hybrid to offer about 47 mpg city, 44 mpg highway.
|Rough schematic of Ford’s powersplit architecture. Click to enlarge.|
Powersplit architecture. Ford has been working on its powersplit system for more than 8 years, and more than 10 years, counting early development, Gray said. The powersplit system consists of two electric machines connected to the engine and wheels through a single planetary gear set. Engine power is split between mechanical and electrical paths; E-CVT functionality is delivered through generator speed control.
It’s a simple mechanical arrangement, Gray said, noting that there are no clutches, and no torque converter. The e-CVTs functions to operate either power source in the optimum arrangement. The hybrid system offers four modes of operation:
- Electric drive.
- Engine drive - positive split.
- Engine drive - negative split.
- Regenerative braking.
One of the outcomes of the redesigned system is reduced engine runtime; in the third-generation system, Ford was able to reduce engine runtime by up to 30%. Additionally, the extra capability of the e-Drive increased the all-electric speed range up to 62 mph. All of that contributes to the overall fuel economy improvement, Gray noted.
Ford was also able to improve the brake specific fuel consumption of the 2.0-liter engine over the 2.5-liter engine from 0-10% in the main regions where they operate. On a plot of break specific fuel consumption vs. output power, the 2.0-liter Atkinson shows about a 3% improvement in the primary real-world operating region compared to its 2.5-liter predecessor.
With the e-Drive system—the inverter, controls and electric machines—Ford engineers were able to deliver measurable loss improvements of up to 4.5% over three different operating points, due to the attention to detail in design and the controls, Gray said.
The battery itself is another whole conundrum...various considerations to be pondered are size, location, thermal management, type of cell, type of chemistry, how control it, how much power, what’s the SOC and so on—a whole multitude of decisions.—Chuck Gray
The new battery pack, designed and built by Ford using prismatic Li-ion cells from Panasonic, has a charge and discharge power of 35 kW and nearly double the specific power per unit mass of the outgoing NiMH product (0.84 kW/kg vs. 0.47 kW/kg). The pack is cabin air cooled and packaged in a similar location to its predecessor.
Ford also redesigned the Fusion top hat with an eyer to improving aerodynamics; the 2013 Fusion Hybrid offers a 10-15% drag reduction form the outgoing product due to body design as well as the addition of features such as a full active grill.
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