Since diesel engines are inherently more fuel efficient than petrol engines, they are an ideal candidate for hybridization.  Until now, car manufacturers have been reluctant to introduce a diesel hybrid because of the fact that diesels are more costly to make than their petrol-powered equivalent and hybridization adds yet another layer of cost.

Enter Peugeot.

Peugeot wasn’t satisfied with just creating the first diesel hybrid.  The 3008 HYbrid4 is also the first through-the-road (TtR) hybrid.  The company’s engineers developed a unique drivetrain layout that supports four-wheel drive.  The engine is located in its usual position under the hood and drives the front wheels.  The electric motor, however, is located under the floor near the rear axel and powers the rear wheels.

Most hybrid vehicles, such as a Toyota Prius, use a power-split parallel hybrid system. This means that the power that drives the wheels of one axle is mechanically split between the engine and the electric motor and transferred through a single transmission based on what the car’s computer determines is the most efficient method based on current conditions.

The complexity of this kind of system is one reason that hybrids are more expensive than their less green counterparts.  Peugeot’s TtR system is not only less complex but it’s significantly less expensive as well.

Click here to continue to Page 2 for complete details on the 3008 HYbrid4′s engine.

A few months ago we reviewed the world’s fastest gas-electric hybrid, the BMW ActiveHybrid X6, and that title has now passed to the BMW ActiveHybrid 7.  This may sound like an odd title for a hybrid but, as we found out in our tests, it proves one can have fun and enjoy better fuel economy at the same time.

The previous version of the BMW 7er Series (the 7er Series was all new for model year 2009) was available in a limited-edition, duel-fuel hydrogen powered configuration and the engineers at the company were hard at work to create additional, green options for their customers.

Enter the ActiveHybrid 7.  Unlike the ActiveHybrid X6, which is a full hybrid that has an electric-only means of propulsion, the ActiveHybrid 7 is a mild hybrid, an approach that adds less weight and complexity (and cost) to the vehicle.  The mild-hybrid platform was jointly developed by BMW and Mercedes-Benz as an outgrowth of a cooperation that began in 2005 and Mercedes uses the same platform in the S400 that we reviewed last month.

A mild hybrid is a gasoline-fueled vehicle equipped with an electric motor that allows the engine to be automatically shut off whenever the car is braking, coasting, or stopped. A mild hybrid also uses brake energy regeneration to recover energy that would normally be wasted as heat through the brakes.  The integrated electric motor acts as a generator when coasting or applying the brakes and feeds electric power into the battery.    As a result, they do not require the same level of battery power and also do not achieve the same increase in fuel economy as full hybrids.

The ActiveHybrid 7 has an upgraded twin-turbo direct injection V-8 engine with a three-phase synchronous electric motor, giving it a combined output of 455 hp and maximum torque of 151 pound-feet.  The electric motor is positioned between the engine and the torque converter and power is transmitted via a new eight-speed transmission.

It also has almost the same amount of luggage space as the gasoline-only version as the 120-volt battery occupies only one cubic foot.

When the driver accelerates, electricity (from the battery) is fed back into the on-board power network and provides power directly to the rear wheels, relieving the combustion engine from this task.

All this translates into 0-60 mph in just 4.7 seconds with 15% better fuel economy than the (slower) 750i.

The ActiveHybrid 7’s transmission uses start-stop technology to avoid engine idling when stopped, although other systems such as air conditioning remain operational as power is supplied through the lithium-ion battery.  Similar to the ActiveHybrid X6, it is equipped with light-alloy Aero wheels that reduce drag.

The car’s Central Display (see video below) can provide a detailed real-time view of the interaction between the V-8 engine and the electric motor including the current charge level of the battery and the real-time flow of energy.  A bar graph that shows the current efficiency over the past fifteen minutes is also available.

In terms of safety, the high-voltage system is fully insulated and the battery is protected by a steel housing.  In the event of a collision, the entire high-voltage system is immediately switched off.

After all this, you may wonder how the ActiveHybrid 7 drives.  I spent a week with the ActiveHybrid 7, driving it on twisty mountain roads, highways, and city streets, and I was hard-pressed to tell the difference between it and the V-12 powered 760Li when hitting the accelerator.  The start-stop feature is reasonably inconspicuous although a quick move from brake to gas can result in a bit of a jerk.  Thanks to the tiny battery and mild hybrid system, the 7er’s weight distribution was preserved, which bodes well for handling (this is one of the reasons BMW uses different hybrid systems in the X6 versus the 7er).

The firm and responsive ride was adjustable via the four driver-selectable settings in the Driving Dynamics Control system.  Set for sport, the ActiveHybrid 7 attacked twisties with a vengeance.  On some pot-holed New York City streets, I didn’t notice the bumps go by.

The fuel economy I got during that week, however, served as an excellent reminder that I was, indeed, in a hybrid vehicle.  It consistently got 3 mpg more than the 750i in city driving, averaging 20 mpg.  My overall fuel economy for the week was 24.5 mpg.

Best of all, the ActiveHybrid 7 qualifies for a $900 IRS Alternative Motor Vehicle Credit.

The Toyota Prius, the world’s first mass produced gasoline-electric hybrid automobile, was first introduced to the domestic Japanese market in 1997 and launched worldwide in 2001.  Buyers in the United States have comprised more than half of the more than 1.5 million Prius (the plural of Prius is Prius according to Toyota) sold worldwide thus far.

The all-new 2010 Prius is the third generation of this eco-trendsetter.  It’s a bit sleeker and sportier than its predecessors and has a larger engine and revised electric-assist steering system that result in even better fuel economy and greatly improved handling (earlier models gave the driver the unsettling feeling of playing a video game with a joystick instead of steering an automobile).

Indeed, it’s clear that Toyota is aiming to give the car mass appeal from the option list alone, which now includes radar cruise control, the automated self-parking system (Toyota calls it the Intelligent Parking Assistant), heated seats, and Bluetooth with support for audio streaming.   Safety features abound, including a lane-departure warning system that nudges straying drivers back into the correct lane.  One option not in our test car but worthy of note is the solar-powered ventilation system that provides ventilation to keep the car cooler when parked.   On the minus side, Toyota doesn’t offer iPod or USB integration and the navigation system is relatively slow.

The 2010 Prius is available in four trim levels. The Prius II($22,800)  includes keyless entry and ignition, cruise control, a six-speaker sound system, a tilt-and-telescoping steering wheel, and a hybrid display. The Prius III ($23,750) adds Bluetooth connectivity and an upgraded sound system. The Prius IV ($26,550) further adds leather upholstery, heated front seats, an auto-dimming mirror, and driver-adjustable lumbar support. At the top  of the lineup is the Prius V ($28,020), which also gets 17-inch alloy wheels, fog lamps, and LED headlights.

Driving a Toyota Prius continues to be unlike driving almost any other car.  With an EPA rating of 50 mpg (combined), it is the most fuel-efficient car available in the United States.

The Prius starts out under electric power, silently moving ahead at low speeds, and the gasoline engine turns itself on when needed, a transition which is extraordinarily seamless.  The 1.8-liter four-cylinder engine provides power in conjunction with a pair of electric motors (one for propulsion, the other an electrical generator) through a planetary-type continuously-variable transmission, providing 134 horsepower in total.

Thanks to the electric motor and application of sound deadening material, the interior is virtually silent.

The Prius driver has a choice of three modes in addition to normal: EV (battery power for up to half a mile), Eco (sluggish but very economical), and Power (the accelerator pedal’s sensitivity is increased and it provides decent acceleration).  I selected the driving mode based on the current driving scenario.  EV is good for local trips as driving is limited to 25 mph (40 km/h) and the battery must be at least half charged.  Power mode is absolutely necessary for merging into highway traffic and passing.  Eco of course is where the Prius and its miserly fuel usage shine.

There is no conventional dashboard behind the steering wheel. Instead the driver has to look to the digital display just below the center of the windshield, which combines the speedometer, fuel consumption, and the fuel gauge with an energy monitor, fuel consumption history, hybrid system information, and additional fuel economy information.   The display is not only off-center but competes with the road for the driver’s attention, despite a touch sensitive display for its steering wheel controls, which Toyota says is designed to help keep the driver’s eyes on the road.

Seats were reasonably comfortable and interior space is more than sufficient.    Cargo space has been increased to 21.6 cubic feet and the split rear seats fold down.

For the center stack, Toyota has adopted the floating console that Volvo has been using for the past few years. It took me a while to realize that the switch for the heated seats was (inconveniently) placed in the open space below the console.  It also took me a while to get used to the gear shift lever mounted on the center console.

All in all, the 2010 Prius provides better fuel economy and a greatly improved driving experience compared to its predecessors, as well as more interior space and more cargo room.

[Editor's note: this review was completed before the unintended acceleration problem surfaced.  The jury is still out on the root cause of the problem.]