|The FHI Turbo Parallel Hybrid Powertrain|
Fuji Heavy Industries’ (FHI) new Subaru hybrid (earlier post) will use a thin 10-kW motor in combination with the 2.0-liter 16-valve twin-scroll turbo engine currently used in the Legacy to create the Turbo Parallel Hybrid (TPH) powertrain.
The combination of the motor generator and Subaru’s Boxer turbo engine, which is adopting the Miller cycle in this application, creates a motor-assist hybrid (similar in concept to the first generation of Honda IMA) that FHI hopes will provide the performance of the current 2.0 Legacy GT (turbo) while delivering a 20% improvement in fuel consumption.
Compared to the SSHEV (Sequential Series Hybrid) system that FHI had previously developed and shown in 2003, the TPH uses a much smaller, compact motor and a smaller battery—also reducing the system cost.
A Boxer engine is horizontally opposed engine—i.e., the pistons lie in a horizontal plane, with pairs of cylinders on the left and the right, as opposed to engines where all pistons are inline, or positioned in a V.
The Boxer design ideally provides perfect balance because each piston’s movement is exactly counterbalanced by the corresponding piston movement of the opposite side. Subaru uses Boxer engines in all its cars.
The basis for the TPH engine is the turbo EJ20 currently being used in the Legacy. This engine comes equipped with Active Valve Control System (AVCS), but the TPH engine will specially modify the valve timing to implement the Miller cycle.
The Miller cycle, developed in the 1940s (by Ralph Miller), is a modification of the basic Otto four-stroke combustion cycle. Later adapted for use in cars—notably by Mazda in the Millenia—the Miller cycle is similar in approach to the Atkinson cycle engine used by Ford in the hybrid Escape and Mariner, with one major difference.
In both Atkinson and Miller cycles, the engine leaves the intake valves open during the beginning of the compression stroke. This pushes part of the charge back out the normally closed valve.
The late closing of the intake valve eliminates the substantial amount of energy normally required to overcome friction (as well as pumping losses) in the process of completing a normal compression stroke.
Put another way, the benefit of this is that the compression stroke effectively becomes shorter than the expansion stroke. The compression work starts when the valve is closed, so the piston gets all the compression for a percentage of the normal work. The result is increased engine efficiency, at around 10%–15%, although with a loss of power.
In the prototypic Miller configuration, however, a supercharger over-feeds the cylinder to compensate for the loss or blowout of charge.
Subaru, however, is not using a belt-driven supercharger with its TPH-Miller cycle engine, but rather a turbocharger, with which it has years of experience in its car lines. Because the turbine’s output decreases as a result of the Miller cycle, the TPH turbine has been designed to provide higher flow than normal.
As shown in Volkswagen’s design work on its dual-charged TSI engines (earlier post), turbochargers are good at higher speeds, but lack in boost at low-engine speed ranges. Without the supercharger to provide low-speed boost, the Miller engine would deliver relatively poor low-end torque.
To provide that low-speed boost, Subaru is turning instead to the electric motor in its hybrid powertrain.
The 10-kW electric motor provides 150 Nm of torque. (Subaru is proud of its higher torque spec. Compare that to the 14-kW motor providing 135 Nm of torque in the Honda Accord hybrid with a larger 3.0-liter engine. The about-to-be-released 2006 Civic hybrid, with a smaller 1.3-liter engine, uses a new 15-kW motor delivering 103 Nm of torque.)
Combining the Miller cycle and TPH systems will result in performance that matches the current 2.0 GT-turbo specs in the Legacy, while providing an approximate 20% improvement in fuel economy, according to FHI.
The current non-hybrid 2.0 turbo Legacy (sold in Japan, the US only offers the 2.5-liter version) delivers 191 kW (256 hp) and 343 Nm of torque. On a combined Japanese cycle, it consumes 7.69 liters/100km, or about 30.6 mpg US.
Applying the 20% hybrid factor to those numbers would yield comparable performance, but at 6.15 liters/100km—or 38.3 mpg US.
|Rough Comparison of Hybrid Powertrains|
|2006 Civic Hybrid||2005 Accord Hybrid||2007 Subaru Legacy Hybrid1|
|1 Based on preliminary targets|
2 Honda estimates of new 2006 system
3 Japanese 10*15 cycle
|Engine displacement||1.3 liters||3.0 liters||2.0 liters|
|Elec. Motor Output||15 kW||14 kW||10 kW|
|Motor Torque||103 Nm||135 Nm||150 Nm|
|Hybrid Powertrain Power||82 kW (110 hp)||190 kW (255 hp)||191 kW (256 hp)|
|Hybrid Powertrain Torque||166 Nm||314 Nm||343 Nm|
|Fuel consumption||4.7 l/100km2||7.4 l/100km||6.15 l/100km3|
|Fuel economy||50 mpg US2||32 mpg US||38.3 mpg US3|
If—and of course, that’s a big if—Subaru is able to deliver on that goal of maintaining current 2.0 GT performance while delivering a 20% improvement in fuel economy, the Legacy hybrid will outperform the Accord hybrid, while using a smaller engine and consuming less fuel.