Toyota Prius: Best Engineered Car of 2001

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Closer to ZEV
Toyota engineers used current emissions regulations as well as those proposed by various markets (principally Japan, the U.S., and Europe) for their development targets. They met their goals by optimizing the noble-metal loading of the catalyst, underfloor catalyst, HCAC system, and vapor-reducing fuel-tank system. The SULEV exhaust system consists of an initial high-performance "starter" catalyst with ultra-thin walls and high cell density, close-coupled to the exhaust manifold, with O2 sensors upstream and downstream. Further downstream, the underfloor catalyst is surrounded by an outer coaxial HC adsorber cylinder.

For hybrid vehicles in which IC engines are frequently started and stopped, a key goal is how to minimize exhaust emissions when the engine is restarted, according to Abe. During a cold start, the Prius relies predominantly on its electric motor to drive the vehicle while the engine operates to warm the catalyst so it can quickly begin cleansing the exhaust. Rapid warm-up of the catalyst is achieved by adopting a stoichiometric air/fuel ratio and considerably retarding ignition timing. By reducing opportunities for unstable combustion after the engine is started, HC emissions caused by incomplete combustion are reduced.

Though not a zero-emissions vehicle (ZEV) as defined by the California Air Resources Board, the Prius, with its revised hybrid control, operates in ZEV (electric) mode about 43% of the time (vs. 30% for the previous Prius) in California's LA#4 driving mode. The maximum power requirement of 38 kW in that mode's second hill climb prevents the car from operating purely as an EV in the LA#4 test, which would require a bigger motor and greater battery energy storage, which would lead to higher cost and more mass.

The biggest concern of hybrid-vehicle developers remains the high cost of the electric components. Toyota engineers expect future breakthroughs will make hybrid system costs equivalent to a conventional IC powertrain's.

Information was provided by Shinichi Abe of Toyota Motor Corp.

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