|Prototype BMW i8 at the track. Click to enlarge.|
BMW i presented a prototype of its second production vehicle, the i8 (earlier post), at a driving event at the BMW Group’s Miramas test track in France. Following the world debut of the first production model, the battery-electric urban-focused BMW i3 (earlier post), the BMW Group’s next step will be to address the sports car segment with the plug-in hybrid BMW i8. The BMW i8 will make its world debut at the Frankfurt Motor Show next month and arrive in BMW showrooms in 2014.
Calculated using the EU test cycle for plug-in hybrid vehicles, the average fuel efficiency of the BMW i8 at model launch will be less than 2.5 liters/100 km, which equates to approximately 95 miles per US gallon, with CO2 emissions of less than 59 grams per kilometer.
|Plug-in hybrid system of the i8: 3-cylinder engine in the rear, battery in the center, eDrive unit in the front. Click to enlarge.|
The BMW i8 pairs an exceptionally lightweight, aerodynamically optimized body with BMW eDrive technology; a compact, turbocharged 1.5-liter BMW TwinPower Turbo gasoline engine; and intelligent energy management. With its carbon-fiber-reinforced plastic (CFRP) passenger cell, the BMW i8 sets new standards for a plug-in hybrid vehicle in terms of low weight. It can operate solely on electric power and offers the dynamic performance of a sports car, with an expected 0 – 60 mph sprint time of less than 4.5 seconds.
Plug-in hybrid system. The plug-in hybrid system of the BMW i8 comprises a three-cylinder, 1.5-liter BMW TwinPower Turbo engine—making its debut in this model—combined with BMW eDrive technology. The BMW Group has developed not only the internal combustion engine and electric motor in-house but also the power electronics and the battery.
|Rear-mounted 3-cylinder engine. Click to enlarge.||Front-mounted eDrive. Click to enlarge.|
The rear wheels of the BMW i8 are driven by the gasoline engine via a six-speed automatic transmission. The front wheels are driven by the electric motor via an integrated two-stage automatic transmission. Combined maximum power and torque is 362 hp (266 kW) and 420 lb-ft (570 N·m).
The BMW i8 is the first BMW production model to be powered by a three-cylinder gasoline engine. This highly turbocharged unit is equipped with latest-generation BMW TwinPower Turbo technology. It is exceptionally compact and develops maximum power of 231 hp (170 kW). The resulting specific output of 154 hp (113 kW) per liter of displacement is on a par with high-performance sports car engines and is the highest of any engine produced by the BMW Group.
The new three-cylinder engine derives its typical characteristics from the BMW inline six-cylinder engines, to which it is closely related. The three-cylinder’s BMW TwinPower Turbo technology comprises a high-performance turbocharging system and direct gasoline injection with high-precision injectors positioned between the valves, along with VALVETRONIC throttle-less load control, which improves efficiency and response. Like a six-cylinder engine, the three-cylinder unit is free of first and second order inertial forces.
The low roll torque, a typical feature of a three-cylinder design, is further reduced by a balancer shaft, while a multi-stage damper integrated in the automatic transmission ensures very smooth and refined running at low rpm.
BMW TwinPower Turbo technology and low internal friction improve both fuel consumption and torque characteristics. Accelerator response is sharp and the three-cylinder unit quickly reaches its maximum torque of 236 lb-ft (320 N·m).
The BMW i8’s second power source is a hybrid synchronous electric motor specially developed and produced by the BMW Group for BMW i. The electric motor develops maximum power of 131 hp (96 kW) and produces its maximum torque of around 236 lb-ft (320 nm) from standstill. Typical of an electric motor, responsive power is instantly available when starting and this continues into the higher load ranges.
The linear power delivery, which extends right up to the high end of the rpm range, is down to a motor design principle exclusive to BMW i. BMW eDrive technology refines and improves on the principle of the permanently excited synchronous motor via a special arrangement and size of the torque-producing components. This results in a self-magnetizing effect normally confined to reluctance motors. This additional excitation ensures that the electromechanical field generated when current is applied remains stable even at high rpm.
As well as providing a power boost to assist the gasoline engine during acceleration, the electric motor can also power the vehicle by itself. Top speed in this mode is approximately 75 mph (120 km/h), with a maximum driving range of up to 22 miles (35 kilometers).
The model-specific version of the high-voltage Li-ion battery was developed and produced by the BMW Group. It has a liquid cooling system and can be recharged at a conventional household power socket, at a BMW i Wallbox or at a public charging station. In the US a full recharge takes approximately 3½ hours from a conventional 120 volt, 12 amp household circuit or approximately 1½ hours from a 220 volt Level 2 charger.
When power demands allow, the high-voltage battery is recharged by the electric motor. The high-voltage starter-generator, responsible for starting the engine, can also be used as a generator to charge the battery, the necessary power being provided by the BMW TwinPower Turbo engine. The battery can also be recharged via the electric motor during overrun.
These various processes help to prevent depletion of the BMW i8’s battery in order to maintain the electric drive power. The all-electric driving range is sufficient to cover most urban driving requirements. Out of town, the BMW i8 offers sporty performance which is also efficient due to the power-boosting support for the gasoline engine from the electric motor.
LifeDrive and light weight. The main components of the LifeDrive architecture are the aluminum Drive module, which incorporates the internal combustion engine and electric motor, the battery, the power electronics and the chassis components, along with structural and crash functions; and the Life module, which essentially comprises the 2+2-seater CFRP passenger cell. This architecture also gives the BMW i8 a very low center of gravity and a near-perfect 50:50 axle load ratio, both of which increase its agility.
With a length of 184.6 inches (4,689 mm), a width of 76.5 inches (1,942 mm) and a height of 50.9 inches (1,293 mm), the BMW i8 has the proportions of a sports car. It features a long hood, conspicuous aerodynamic features, an elongated roofline, short overhangs and a long wheelbase of 110.2 inches (2,800 mm).
The BMW i8 combines a drag coefficient (Cd value) of 0.26 with outstanding aerodynamic balance.
Use of CFRP, which also offers excellent crash performance, brings weight savings of 50% over steel and around 30% over aluminum, with equivalent or improved strength. These savings offset the additional weight of the electric motor and the high-voltage battery, giving the BMW i8 a curb weight of less than 3,285 lbs (1,490 kilograms).
The LifeDrive architecture also brings benefits in terms of weight distribution. The battery pack is situated low down in the middle of the vehicle, resulting in a low and central center of gravity, which improves safety. No other current model of a BMW Group brand has such a low center of gravity.
The front-rear axle load distribution maximizes agility with a near-perfect 50:50 axle load ratio. The compact electric motor, together with the transmission and power electronics, are situated in close proximity to the electrically powered front axle. The turbocharged gasoline engine, which is located together with its transmission in the rear of the BMW i8, likewise sends its power to the road via the shortest possible route, i.e. through the rear wheels.
The lithium-ion battery pack is centrally located in the vehicle, slightly forward of the mid-point. In terms of crash safety, this is an ideal location for the battery, which is integrated in an aluminum housing.
The doors comprise a CFRP inner structure and an aluminum outer skin. This reduces the weight of the door by 50% compared with a conventional design. A magnesium instrument panel support saves weight on two fronts—first through intelligent design, leading to around 30% weight savings compared, for example, with the BMW 6 Series. In addition, the high structural rigidity provides a strengthening effect which allows the number of components to be reduced, thereby lowering weight by a further 10%. New foam plastic technology used in the air conditioning ducts brings 60% weight savings over conventional components, while also improving acoustics due to its sound-absorbing properties.
Having the power electronics and electric motor directly connected reduces the amount of wiring required, while partial use of aluminum wiring brings further weight reductions.
Lightweight design is also a feature of the BMW i8’s chassis systems, including the wheels, where the standard-fitted 20-inch forged aluminum wheels combine aerodynamic design with weight savings. The application of the lightweight design strategy even extends to aluminum screws and bolts, which are around 45% lighter than corresponding steel components, with the same strength and functionality.
The BMW i8 is also the world’s first volume-produced vehicle to be equipped with chemically hardened glass. This technology, to date used mainly in smartphone manufacturing, results in very high strength. The partition between the passenger compartment and boot of the BMW i8 consists of two layers of chemically hardened glass, each of which is just 0.7 millimeters thick, with acoustic sheeting sandwiched between. In addition to excellent acoustic properties, a further advantage of this solution is weight savings of around 50% compared with conventional laminated glass.
Driving Experience Control and eDrive button. The BMW i8’s intelligent powertrain control system coordinates both power sources. The variable power-sharing between the internal combustion engine and the electric motor makes the driver aware of the sportiness of the BMW i8 at all times, while at the same time maximizing the energy efficiency of the overall system.
Utilizing both power sources, the 0 – 60 mph acceleration time is expected to be less than 4.5 seconds. Linear acceleration is maintained even at higher speeds since the interplay between the two power sources efficiently absorbs any power flow interruptions when shifting gears. The BMW i8 has an electronically controlled top speed of 155 mph (250 km/h), which can be reached and maintained when the vehicle operates solely on the gasoline engine.
Variable front-rear power splitting in line with changing driving conditions makes for dynamic cornering. On entering the corner, the power split is biased towards the rear wheels to improve turning precision. For more vigorous acceleration out of the corner, the powertrain controller returns to the default split as soon as the steering angle becomes smaller again.
The BMW i8 also offers the driver unusual scope to adjust the drive and suspension settings of the vehicle in order to adapt the driving experience to his or her individual preferences. As well as the electronic gear selector for the automatic transmission, the driver can also use the Driving Experience Control switch— a familiar feature of the latest BMW models—or, exclusively to the BMW i8, the eDrive button.
Using the gear selector, the driver can either select position D for automated gear selection or can switch to SPORT mode. SPORT mode offers manual gear selection and at the same time switches to very sporty drive and suspension settings.
In SPORT mode, the engine and electric motor deliver extra-sharp performance, accelerator response is faster and the power boost from the electric motor is maximized. And to keep the battery topped up, SPORT mode also activates maximum energy recuperation during overrun and braking: for this, the electric motor’s generator function, which recharges the battery using kinetic energy, switches to a more powerful setting. At the same time, gear change times are shortened and an extra-sporty setting is selected for the standard-fitted Dynamic Damper Control. Also in this mode, the programmable instrument cluster supplies further driving-related information in addition to the rev counter display.
The Driving Experience Control switch on the center console offers a choice of two settings. On starting, COMFORT mode is activated, which offers a balance between sporty performance and fuel efficiency, with unrestricted access to all convenience functions.
Alternatively, at the touch of a button, ECO PRO mode can be engaged, which, on the BMW i8 as on other models, supports an efficiency-optimized driving style. Here, the powertrain controller coordinates the cooperation between the gasoline engine and the electric motor for maximum fuel economy. On deceleration, the intelligent energy management system automatically decides, in line with the driving situation and vehicle status, whether to recuperate braking energy or to coast with the powertrain disengaged.
At the same time, ECO PRO mode also programs electrical convenience functions such as the air conditioning, seat heating and heated mirrors to operate at minimum power consumption, but without compromising safety. The maximum driving range of the BMW i8 on a full fuel tank and with a fully charged battery is more than 310 miles (500 km) in COMFORT mode. In ECO PRO mode, this can be increased by up to 20%.
The BMW i8’s ECO PRO mode can also be used during all-electric operation. The vehicle is then powered solely by the electric motor. Only if the battery charge drops below a given level, or under sudden intense throttle application such as kickdown, is the internal combustion engine automatically activated.
High-quality chassis technology, DSC and Dynamic Damper Control as standard. The high-end chassis and suspension technology of the BMW i8 is based on a double-track control arm front axle and a five-link rear axle, the aluminum components and geometry of which are specially configured for weight savings.
The electromechanical power steering offers easy maneuvering in town and typical sports car-style high-speed steering precision. Also standard is Dynamic Damper Control: the electronically operated dampers change their characteristics according to the selected driving mode to deliver the desired vehicle dynamics. The DSC (Dynamic Stability Control) stability system includes the Anti-lock Braking System (ABS), Cornering Brake Control (CBC), Dynamic Brake Control (DBC), Brake Assist, Brake Standby, Start-Off Assistant, Fading Compensation and the Brake Drying function. The push button-activated Dynamic Traction Control (DTC) system raises the DSC thresholds, allowing some controlled drive wheel slippage for easier start-off on snow or loose ground, or for extra-dynamic cornering.