Lexus RX hybrid system
Lexus RX hybrid system. Click image to enlarge

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Article by Paul Williams
Photos courtesty Lexus

Photo Gallery:
2010 Lexus RX

See Also:
Test Drive: 2010 Lexus RX 450h Touring, by Jil McIntosh

There has long been an argument that the best application for hybrid technology is in heavier vehicles of the type that historically have poor fuel economy. After all, squeezing an extra 100 kilometres out of an already fuel-efficient compact car has arguably less positive environmental impact than cutting fuel consumption in a larger SUV, or a full-sized pickup truck.

The 2010 Lexus RX450h falls into the “larger SUV” category (although it’s more correctly described as a “midsize”), and is an evolution of the RX400h, first introduced in 2006. After a one-year absence from the market (2009 model year), the RX 450h returns fully redesigned, a little bigger and heavier, but with more power and better fuel economy.

Like all Toyota and Lexus hybrid models, motive power for the RX 450h is an Atkinson-cycle internal combustion engine — in this case a 245 horsepower (hp), 3.5-litre V6 — in conjunction with electric motors that increase output to 295 net hp. Fuel consumption for this 2,110 kilogram (4,810 pound) vehicle is 6.6 L/100 km city and 7.2 L/100 km highway (you read that right; this SUV gets better fuel economy in city driving). Combined fuel consumption is 6.8 L/100 km.

Lexus RX hybrid system
Lexus RX hybrid system. Click image to enlarge

Perhaps unexpectedly for such a modern vehicle, the Atkinson Cycle was invented over one-hundred years ago by a British engineer named James Atkinson. It enables the four strokes of an internal combustion engine — intake, compression, power and exhaust — to occur in one rotation of the crankshaft. This is more efficient, but produces somewhat less power than the conventional Otto Cycle engine which uses two rotations of the crankshaft for the four strokes.

The engine features an Active Control Induction System (ACIS) that divides the intake manifold into two stages in which a control valve opens and closes, effectively varying the length of the intake manifold. Depending on engine speed and throttle position, the engine is supercharged by high volumes of air/fuel that are pulled into the combustion chamber, increasing torque and power output.

The electric motors — the RX450h has three: one at the rear that drives the rear wheels, and two in the front driving the front wheels — also compensate for the reduction in power caused by the use of an Atkinson Cycle engine. They work together when the engine is idling and when starting, and also in combination with the engine when accelerating, and during normal driving.

The motors get their power from a nickel-metal-hydride battery that is smaller and lighter, compared with the outgoing model. It is continuously recharged while driving (via a generator that doubles as a “starter” for the engine) and from regenerative braking. In the latter case, the motors invert, becoming generators that also help to stop the vehicle. Lexus states that the battery is designed to last for the lifetime of the vehicle.

Coordinating the output and behaviour of the engine, the motors, the battery and the generator is an Electronic Control Unit (ECU) that is programmed to optimally balance power requirements and fuel consumption.

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