The Hybrids: Toyota and Lexus
All the production hybrids we've seen to date work basically the same way: gasoline engine + electric motor = one fuel-efficient power source. From there, manufacturers have taken separate paths. Honda's "mild" hybrids rely chiefly on the gas engine, using a small electric motor/generator to supplement it. In "strong" hybrids like the Toyota Prius and Ford Escape Hybrid, a larger role is given to the electric motor, allowing it to propel the car on its own at low speeds.

As in their smaller (and lighter) brethren, the 2006 Toyota Highlander Hybrid and Lexus RX 400h feature a powerful motor/generator to provide propulsion and regenerative braking, as well as a second smaller motor/generator to start the 3.3-liter V6 gas engine, rated for 208 horsepower and 212 pound-feet of torque in this application.

Juice from the 288-volt DC nickel-metal hydride battery pack flows through a boost converter that increases voltage to 650V DC and an inverter that switches it to AC power. The main electric motor/generator produces a maximum of 167 hp and 247 lb-ft of torque from zero to 1,500 rpm. The smaller motor allows the gas engine to stay at the most ideal rpm for every situation and speed.

The path of energy can vary widely depending upon conditions and driver demand. For instance, the electric motor provides most of the momentum at lower speeds, even though the engine may be revving. Excess power from the engine is absorbed by the smaller motor/generator, then routed right back to the main motor to move the car or charge the battery pack.

Jab the throttle and the engine leaps to its peak rpm, while energy stored in the battery surges to the electric motor. Hit the brakes and the smaller motor/generator allows the engine to shut down, while the main motor scrubs off speed by acting as a generator. This energy is used to charge the battery pack.

As complicated as it may sound, the system is really quite simple. Even though Toyota and Lexus say the trucks use a Continuously Variable Transmission (CVT), there's no rotating belt like there is in a conventional CVT. Instead there are just two electric motors and a gas engine permanently linked together by a planetary gear set. Though the different modes vary greatly, the driver perceives nothing but a smooth flow of power.

A third electric motor/generator is the secret to the electronically controlled all-wheel-drive systems in the RX 400h and Highlander Hybrid 4WD-i. Note that front-wheel-drive Highlanders do not have this motor. A typical all-wheel-drive system involves individual driveshafts that connect the front and rear axles, and a center differential that varies the power split. These hybrid SUVs have neither element. Instead, the independent third electric motor/generator, rated for 68 hp and 96 lb-ft of torque, is built into a rear transaxle.

The rear electric motor only comes into play when needed, such as during hard acceleration or when starting on a slippery surface. During coasting and braking, it, too, becomes a generator. Otherwise, the Highlander Hybrid and RX 400h function as front-drive SUVs.

Diesel: Jeep Liberty CRD
The diesel-powered Jeep Liberty CRD uses a much simpler design than the hybrids, but based on our testing, it's just about as effective when it comes to fuel economy.

Diesel engines differ on a basic level from gasoline engines: they don't require spark plugs. Higher compression ratios — 17.5 to 1 in the Liberty CRD — produce higher air temperatures in the combustion chamber. When even a small amount of diesel fuel is injected, it ignites on its own accord.

Because it takes less fuel to start the combustion process, diesel-powered cars use it more sparingly than most gasoline-powered peers. Further, since diesel fuel is richer in energy than gasoline, you get more power out of each power stroke. Add a turbocharger to the equation, as in the Liberty, and it's not hard to understand why these engines produce so much low-end torque.

Since diesels operate at very high compression ratios and tend to be built with heavy-duty parts, they physically cannot spin at high rpm (the Liberty CRD is electronically limited to 4,300 rpm). The net result is a lower overall horsepower rating compared to a similar gasoline engine.

The diesel engine is about where the Liberty's fuel-sipping traits end. Power is routed through a traditional five-speed automatic transmission and an off-road-capable dual-range 4WD system. Good fuel economy is more of a fringe benefit, since the diesel's generous low-rpm torque steals the show by providing drama-free towing and hill-climbing ability. In fact, a Liberty with the turbodiesel four-cylinder can tow 5,000 pounds, the same as the V6 model. — Jeff Bryan and Erin Riches