Toyota has developed a gasoline- engine- regenerative- electric- power hybrid, which will power a production Corolla-size passenger car in Japan by the end of the year.

Toyota (www.toyota.com) is vigorously pursuing its own "Eco" movement. A spokesperson says there are many routes to the top of the Eco hill, and Toyota's climbers have just started up their respective paths, all confident theirs is the most promising. These include the recently launched RAV-4 EV with advanced nickel/metal-hydride battery pack, the D4 direct-injection gasoline engine, the soon-to-be-announced IC/electric hybrids, and, further in the future, a fuel-cell development—not to mention natural gas vehicles. The spokesperson, well versed in the subject, adds that it will be another 15 years before it is determined which of these systems might be mainstream powertrains.

Battery pack fits neatly behind the rear seat. A tape recorder is placed to give size reference.

For now, Toyota's executive vice president Akihiro Wada—in charge of engineering—states that THS (short for Toyota Hybrid System) is one of the best solutions. Toyota will install this system in a new compact sedan specifically designed for the application, which itself will be a "sedan revolution," according to the spokesperson.

Dr. Takehisa Yaegashi, senior staff engineer, heads the company's electric hybrid group that was involved in Toyota's LEV and ULEV programs, and leads the THS project. Of the three objectives in its development, the foremost concern, says Yaegashi, was a drastic reduction of CO2 emissions, a product of combustion which can only be reduced by improving fuel economy. The THS, currently undergoing testing, has attained a 28 km/L economy on Japan's 10/15-mode urban test cycle, which is twice that of a typical small car powered by a gasoline 1.5-L engine. For example, Toyota cites a certified 13.8 km/L economy with its latest car, the Raum.

Nickel/metal-hydride battery pack housing cylindrical batteries.

Secondly, the THS achieves a marked reduction in the emissions of three pollutants, NOx, CO, and HCs, to a tenth of Japan's regulated level, or about equal to California's LEV standards. Yaegashi is confident it could readily be brought up to the ULEV level with some refinement. Thirdly, the THS vehicle must have comparable performance and practicality, including ease of maintenance. This vehicle has slightly superior mid-speed passing performance to a 1.5-L automatic Corolla, according to Toyota. This editor took a heavier testbed car to a comfortable 120-km/h cruise with the generator almost paying back what the motor had used on reaching that speed (20 km/h above Japan's maximum speed limit).

Electric hybrids are nothing new at Toyota, says Yaegashi; some 30 years ago an S800 two-seat sports car was converted to a gas-turbine hybrid test bed. It was a series-hybrid type whose engine was used to generate electricity to power the propulsion motor. In fact, Toyota will soon launch a commercial vehicle employing a series-hybrid drive with an internal combustion (IC) engine. The passenger-car THS, on the other hand, is a parallel type in which the IC engine and electric motor alternately or jointly provide propulsion.

Inverter.

Many components of the powertrain are of Toyota's own design and manufacture, including the ac permanent synchronous propulsion/regenerative motor, permanent synchronous generator/starter, inverter, and IGBT elements.

Further, the gasoline engine is specific to the hybrid application. It is a dual-overhead camshaft, 16-valve, 1.5-L inline four-cylinder unit with aluminum cylinder head and block. It is a long stroke engine with a unique bore pitch (distance between the bore centers) for the sake of compactness. The engine operates in a relatively narrow band, its upper limit being 4000 rpm, with emphasis on fuel economy, and its components are optimized in size (e.g., its thinner forged crankshaft, and smaller journal and pin diameters). The engine employs the Atkinson/Miller high-expansion principle by means of late intake-valve closing. While the engine's mechanical compression ratio is about 9:1, its virtual compression ratio (expansion ratio) is as high as 14:1, without inducing harmful detonation. The Atkinson/Miller cycle's known quality is a marked reduction in pumping losses.

The DOHC 1.5-liter gasoline engine operates in an Atkinson/Miller cycle with late intake-valve closing. The engine is fitted with Toyota's VVT-i continuously variable intake-valve-timing system.

The engine also employs Toyota's VVT-I continuously variable intake-valve timing, which alters timing by 40¡, and a slanted squish area in the compact pentroof-shaped combustion chamber that promotes rapid flame propagation.

The slant squish chamber, by the way, is adopted in another of the latest Toyota engines, the type-1GZ-FE V12. The intake manifold is relatively short, with no variable length device; that suits the engine's relatively narrow operating range. The engine operates at a stoichiometric air/fuel ratio.

The engine is mounted transversely, canted rearward with the exhaust manifold on the rear side, and an underfloor catalytic converter is placed in close proximity to help quick catalyst activation on start-up. The catalyst has a special additive to cope with excess oxygen that passes through it when the engine is momentarily stopped, such as during start-up, low-speed deceleration, at very low speeds, and at idle. The exhaust system employs two oxygen sensors.

The engine has a relatively short, U-shaped intake manifold, indicating its operation in a relatively narrow and most efficient zone.

The generator/starter, power-split device, propulsion motor/regenerator, and sprocket chain to the final drive are housed within a common aluminum casing whose length is about the same as that of a conventional four-speed automatic transmission. The powertrain has a mass about 10 kg greater than a conventional 1.5-L engine/automatic transmission combination, according to Dr. Yaegashi.

The 1995 Tokyo Motor Show concept vehicle Prius, the forerunner to the THS, used a belt/pulley-type continuously variable transmission. In the THS, this was replaced by a clever planetary-gear-type power-split device which also acts as an electric CVT. The planetary geartrain's planet-gear carrier is driven by the engine, the sun-gear connected to the generator/starter and the ring-gear to the propulsion motor's output shaft. Electronically controlled and the two electric components electrically actuated, the device performs operating mode changes and provides a continuously variable transmission function. The propulsion motor and invertor are liquid cooled by a dedicated cooling system.

The nickel/metal-hydride battery was jointly developed by Toyota and its supplier, Panasonic. It is a high-output type with cylindrical cells and reduced internal resistance. Its output per unit volume is about 3.5 times higher than Toyota's own EV high-density variety, its volume is about one ninth of the EV package, and its voltage (300 V) is higher than the EV's. The battery pack fits neatly behind the rear seat, leaving plenty of luggage volume behind it.

The THS operates in the following modes:

1. Startup, low-speed, and low-speed deceleration: propulsion motor only with the engine stopped.
2. Normal operation: engine's output is divided by the power-split device, one part driving the vehicle and the other driving the generator whose electric output is supplied to the propulsion motor assisting the engine. The split ratio is controlled to optimize efficiency.
3. Full-throttle operation: electricity supplied by the battery adds to the power.
4. Deceleration: wheels drive the propulsion motor, generating electricity which is stored in the battery.
5. Recharging battery: battery level is constantly monitored and maintained by bringing in the engine-driven generator.
6. When the vehicle stops, the engine is automatically stopped.

Cutaway model shows, from left: engine, generator/starter, and reduction gear/chain drive to the final drive and motor.

The engine is run when the air conditioning is on. The brake system's hydraulics are modulated so that the propulsion motor's regenerative capability is optimally utilized. Toyota attributes 80% of the efficiency gain to the powertrain and 20% to the recovery of energy by regenerative braking.

The production vehicle should gain about 100 kg over a comparable small car, according to vehicle chief engineer Takeshi Uchiyamada. The vehicle will also be equipped with electric power steering—also of Toyota's in-house design and manufacture.

By Jack Yamaguchi