smart Micro-Hybrid Going Into Production
11 July 2007
|smart fortwo mhd.|
With the weekend’s Live Earth concerts as a backdrop, smart announced that it will begin production of a micro-hybrid variant of the smart fortwo. (Earlier post.) The smart micro-hybrid will be available as a coupé and a cabrio in all three lines: pure, pulse and passion.
The 52 kW (70 hp) smart fortwo micro-hybrid drive (mhd) uses a belt-driven starter generator from Valeo (StARS 137) to provide stop-start functionality. To ensure a low-slip and durable connection of the crank assembly and the starter generator, both components have been given wider belt pulleys, as was the water pump. A six-rib poly-V-belt from Gates Corporation transmits the power.
In conjunction with slightly modified gear ratios, the micro-hybrid system provides a fuel saving of approximately 8% in the New European Driving Cycle (NEDC). Standard fuel consumption of the 1.0-liter gasoline engine is reduced by approximately 0.4 liters per 100 kilometers—from 4.7 liters to around 4.3 liters (54.7 mpg US).
Depending on traffic conditions, this can lead to a fuel saving of approximately 13%. There is also a corresponding reduction in CO2 emissions from 112 grams to approximately 103 g/km.
Belt tension is particularly important because of the changing loads in start/stop operation. A coaxial spring-and-shock absorber unit that is hinged to the starter generator is supported by the engine block. The starter generator is pivoted so that it can apply the tension force to the belt drive. This ensures that both the belt section pulled by the combustion engine when it is running and the complementary belt section pulled by the starter during the start are able to reliably transmit the torque needed.
The activities of the system are controlled by a separate control unit with integrated power electronics that is installed behind the battery recess. This communicates with the vehicle’s other control units via CAN databus. A three-phase cable transmits the generator power of up to 120 ampere. The power electronics regulate the power of both the starter and the generator.
An AGM lead-acid battery stores the energy for the on-board electrics. Its physical properties make it more resistant to varying loaded and unloaded conditions (more cycle resistant) than conventional lead-acid batteries with sulphuric acid electrolyte.
The power electronics of the belt-driven starter generator switch off the combustion engine at a speed of below 8 km/h when the driver presses the brake pedal, signaling a stop. The engine starts as soon as the driver releases the brake pedal again. This guarantees an immediate response. The start/stop function can be deactivated if required with a switch on the center console until the next starting procedure (ignition off/ignition on).
smart also showed an all-electric version of the fortwo. A 30 kW/40 hp magnetic motor runs at the rear of the smart fortwo electric drive that is powered by a Zebra sodium-nickel-chloride battery housed in the underbody.
NEDC power consumption is 12 kWh per 100 kilometers. When charged, the 30 kW/41 bhp two-door car can travel around 115 kilometers (72 miles). An 80% charge takes four hours at a 230-volt outlet; a full charge takes 8 hours. The battery has a cycle life of at least 1,000, and lasts for up to 10 years.
smart created the electric drive in cooperation with Zytek, a British company that focuses on developing hybrid and electric drives. (Earlier post.) The company currently has the smart ev in a commercial trial for the UK market. (Earlier post.)
(A hat-tip to Mark!)
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