BMW has introduced its latest supercap hybrid concept vehicle (earlier post) at the Frankfurt IAA: the Concept X3 EfficientDynamics. The X3 concept is another performance hybrid SUV. Combining a next-generation direct-injection inline six cylinder engine with an electric motor and supercapacitor energy storage allows the SUV to accelerate from 0–100km/h in about 6.7 seconds. Fuel consumption compared to a conventional X3 is reduced by approximately 20% to an estimated 9.4 l/100km (25 mpg US) from the current 11.8 l/100km (20 mpg US). The engine uses BMW’s just-announced High Precision Injection, its new spray-guided lean direct injection technology, which itself offers an increase in efficiency and higher torque and power over the current generation. All support systems—steering servo pump, the brake servo, air conditioning compressor—are electric. The Active Transmission used in the X3 has the same dimensions as the conventional automatic transmission. The Active Transmission comprises the six driving gears, the electric motor together with two clutches, and the car’s complete control and power electronics. To achieve the goal of housing the additional drive system within the space available for the hydraulic torque converter and the converter lock-up clutch, BMW engineers developed an extremely compact, weight-optimized motor housed on the input shaft leading into the transmission. Designed for maximum sustained output of 30 kW (41 hp), the motor can offer bursts of up to 60 kW (82 hp) for acceleration support. The motor delivers 400 Nm of torque. The electric motor on the Active Transmission assumes the space and function of both the torque converter and a lock-up clutch in a conventional automatic transmission, supplemented in its operation by two oil-bath clutch systems. The first set of clutch plates connects the combustion engine with the electric motor, the second links the electric motor with the transmission. Again, the objective is to fit the entire configuration including the power electronics into the transmission housing in terms of both length and diameter, thus being relatively easy to fit in lieu of a conventional BMW automatic transmission. Upon startup acceleration, the first clutch between the combustion engine and the electric motor is open in the Active Transmission, while the second clutch links the motor and the transmission unit. Once the car has started to move, the first clutch gently engages and the combustion engine will fire. At this point the electric motor also acts as the starter for the combustion engine. At low speeds (below 1,500 rpm), the engine and motor combine to deliver up to 600 Nm of torque. The motor continues to provide burst support to the combustion engine up to a speed of approximately 3,000 rpm. At that point, it deactivates, or is switched to its recharge function. The hybrid transmission servers as a booster in each of the six transmission gears, applying the electric motor power only briefly when accelerating. BMW estimates that in most cases the electric motor is required for only about three seconds to provide significant boost and save a substantial amount of fuel at the same time. As a result of these generally short operating periods of the electric motor, the supercapacitor energy storage system is required to provide only a small amount of energy at a time. The supercaps are re-charged via regenerative braking immediately after energy has been withdrawn—i.e., prior to the next burst. A supercapacitor offers specific power density of approximately 15 kW/kilogram, compared to about 1.3 kW/kg in the case of a nickel/metal hydride battery. The disadvantage of supercaps versus the battery is the far lower energy density—but given the burst-mode design of the BMW hybrid, supercaps fit the bill quite nicely. The supercaps in the X3 Concept Car measure approximately 50 millimeters (approximately 2 inches) in diameter and generate an overall capacity of 190 kW. BMW first showed a supercap hybrid concept based on the X5 in 2003. In that application the hybrid system—which predated what BMW now calls its Active Transmission—delivered a 15% improvement in fuel economy. BMW just recently has joined GM and DaimlerChrysler in work on the two-mode hybrid powertrain. (Earlier post.) More to come.
