Hybrid BMW X3 Compact SUV

Until recently, BMW’s commitment to hybrids could be described as minimal at best. The company has been working on hybrid-electric drive since the early ‘90s, but early prototypes weren’t up to the performance standards that BMW so famously upholds. Last year, BMW hinted it was getting closer by integrating an electric motor into an X5 SUV, a move that improved low-end torque and fuel economy. Still, BMW didn’t make a big fuss about the program, especially in comparison to the company’s headline-grabbing work in hydrogen internal combustion.

But things are changing. Hybrid sales have taken off and industry watchdogs predict a significant expansion in hybrid market share over the next decade. In other words, car buyers are asking for more fuel efficient cars...now. Automakers, not wanting to get left behind, prompted an industry-wide scramble that saw the formation of multiple unlikely alliances aimed at quickly developing hybrid technology. It was against this context that, earlier this year, BMW announced it was joining the partnership previously established between DaimlerChrysler and General Motors to develop a common hybrid drive system. And in case there was any doubt as to BMW’s intentions, at this year’s Frankfurt Motor Show BMW unveiled a vehicle that places the company firmly on the hybrid bandwagon: the X3 EfficientDynamics, a hybrid concept of BMW’s popular compact SUV.

As they say, better late than never. However, the X3 EfficientDynamics, with its promise of performance worthy of the BMW badge, is worth the wait. To meet this standard with a hybrid, BMW created what it calls the “Active Transmission.” The Active Transmission is the same size as the company’s standard automatic transmission, yet houses six driving gears, the electric motor, two clutches, and all the controls and power electronics required to operate the electric-side of the hybrid equation. In this setup, the electric motor is housed directly on the input shaft leading from the engine to the transmission, with the first clutch connecting the engine to the motor and the second connecting the motor to the transmission. The result is superior performance – 0-60 mph in approximately 6.7 seconds and a top speed of 146 mph – both better than the stock X3 with a concurrent 20 percent reduction in fuel consumption.

BMW chose the Active Transmission configuration for a number of reasons. First, it allows the vehicle to use the usual transmission ratios. Second, it renders a torque converter or lock-up clutch obsolete with its ability to power the vehicle from a standstill. Third, its size makes it possible to do a direct swap with BMW’s conventional automatic transmission and thus theoretically allow compatibility with a wide range of vehicles. Perhaps most importantly to BMW, the Active Transmission takes advantage of an electric motor’s inherent low-speed torque to give the X3 EfficientDynamics what BMW describes as the muscle of a diesel with the responsiveness of a gasoline engine. Quick acceleration from a standstill will run the engine in concert with the motor, giving a combined torque rating of 442 lbs-ft at 1,500 rpm that exceeds BMW’s own three-liter diesel engine.

A more moderate take-off, with the engine-motor clutch open and the motor-transmission clutch engaged, is motivated entirely by the 295 lbs-ft, 82 hp maximum output of the electric motor. Once the vehicle accelerates to a predetermined speed, the first clutch gently engages and the electric motor functions as the starter, firing the engine to life. The electric motor will continue to support the engine up to a speed of approximately 3,000 rpm. And since the electric motor is positioned upstream of the transmission, it is able to cut in and boost the engine in any of the six gears whenever the engine is running at low speeds. These bursts of electrical power are short; according to BMW engineers, most cases only require the electric motor to run for about three seconds to provide a significant boost and substantial fuel savings.

These generally short operating periods prompted BMW to go with a less conventional approach to the high-voltage energy storage required to power the electric motor. BMW claims its double-layer capacitors, referred to by the company as “Super Caps,” are better suited to the fast charging and discharging processes of the X3 EfficientDynamics than the electrochemical batteries most commonly used by hybrids today. In terms of power density – the power available for a given weight – a Super Cap offers 15 kilowatts per kilogram (kW/kg) compared to the 1.3 kW/kg provided by a nickel-metal-hydride (NiMH) battery. In this concept car, the Super Caps get highly visible placement in the side-sills. It’s not just for looks; placing them here saves space and ensures a low center of gravity for better handling.

The downside to capacitors is their lower energy density, the total electrical energy stored for a given weight. BMW chalks this up to a mere theoretical drawback because most of today’s hybrid batteries are not used to their full capacity for the sake of battery life. In practice, a Super Cap’s roughly 4 watt-hour per kilogram (W-hr/kg) energy density is comparable to the 5 W-hr/kg of a typical NiMH battery. For BMW, the advantages more than compensate. The capacity is sufficient to drive the vehicle under electric power alone, while the internal resistance, which is virtually zero compared to that of batteries, means less energy is lost during charging. This is especially advantageous for stop-and-go driving, when many charge-discharge cycles may occur in a short time.

Fuel consumption tends to skyrocket during these periods of continuously changing engine loads. This is where hybrid systems shine, and the X3 EfficientDynamics is no exception. Energy that is normally lost to braking and deceleration is captured when the clutch between the engine and the electric motor opens up, allowing the kinetic energy of the vehicle to spin the motor and generate electrical energy. The transition is seamless. Press the gas pedal and the Super Caps power the electric motor until they are discharged; press the brake pedal and the Super Caps are quickly recharged.

Like most hybrids, the engine remains the primary motivational force in the X3 EfficientDynamics. Fortunately, BMW’s traditional inline six-cylinder in this application is at its most efficient yet. The engine does entirely without V-belts as all accessories are driven by their own electric motors. But the primary modification is the addition of a direct-injection system called High Precision Injection. Also known as jet-guided direct-injection, the conical configuration of the injection jet improves upon previous direct-injection methods by affording more efficient combustion over a wider operating range. The system allows the engine to run on a lean mixture – that is, an elevated air-to-fuel ratio – all the way from idle speed to high revs. BMW expects this to improve fuel economy by up to 10 percent in the European cycle, and anywhere from five to 15 percent under normal driving conditions.

High Precision Injection couldn’t be more important. Like Valvetronic, which reduced pumping losses by eliminating the need for a throttle butterfly valve, High Precision Injection can improve fuel economy in every BMW gasoline engine. In fact, it is the main feature in BMW’s Technology Package that the company plans to implement across its lineup. Other fuel-saving technologies in the package include an intelligent alternator that is able to recover brake heat energy and reroute it as electrical energy to the battery, reducing loads on the engine, and an automatic engine start/stop system that reduces fuel lost during idling.

When can we expect to see the Technology Package in showrooms? BMW hasn’t established a timeline yet, but the company has said it will be available on numerous models well before the actual introduction of the self-commitment made by ACEA, the European Association of Automobile Manufacturers, to reduce CO2 emissions to 140 grams per kilometer by the year 2008. In other words, very soon. High Precision Injection in particular will be introduced in European markets first, followed by the rest of the world as low-sulfur fuel – which doesn’t harm the system’s NOx catalysts – is made more widely available.

BMW’s hybrid production plans are even harder to pin down. The company began as an engine manufacturer, and its philosophy today fits the historical mold: the evolution of the internal combustion engine remains the primary objective of BMW’s engineers. However, the rise of hybrid technology indicates engine technology is not sufficient to keep up with today’s demands for high efficiency. The X3 EfficientDynamics is an acknowledgment of this point and a different way of thinking for an engine-minded company. But this concept is also proof that BMW can build a hybrid that meets its lofty performance standards. While we don’t know yet what form a production BMW hybrid will take, you can bet it will bear out this concept in its simultaneous pursuit of both fuel efficiency and driving dynamics.