More on the smart electric plustwo
July 19, 2006 Last week we reported on the coming of the smart EV, an electric version of smart's iconic two-seater commuter. Now we can release the firstdetails of the highly integrated electric drivetrain developed by British specialist Zytek which has allowed DaimlerChrysler to introduce Europe’s first high quality, high efficiency electric commuter car. The new smart ev (electric vehicle) shown at the British International Motorshow overnight provides all the safety features and dynamic ability of the standard smart fortwo, but with zero vehicle emissions and fuel costs slashed from an already class-leading EUR0.06 per kilometre (6.3p per mile) for the petrol version to just EUR0.02 per kilometre (2.1p per mile) for the vehicle with the new electric drivetrain.
All the safety features of the smart fortwo are retained, making it the only electric vehicle with two airbags, ESP, ABS, pre-tensioned seatbelts and a sturdy safety cell. “We were asked to develop a drivetrain that was in keeping with the smart spirit of energy, quality and design excellence,” explains Zytek sales and marketing director Steve Tremble. “We believe this is the only pure electric city car that combines a high level of safety and refinement with outstanding performance and style.”
Up to two hundred electric smarts will be built by Zytek at its technical centre in Fradley, Staffordshire, for use in UK market trials. The vehicles will be leased to selected corporate customers from November.
“The major challenge for this programme was to deliver a level of refinement that matched or exceeded the standard vehicle,” explains Zytek Electric Vehicles managing director Neil Heslington. “The combination of speed, acceleration and range, along with excellent driveability and NVH (noise, vibration and harshness), all in a small package, requires some very advanced engineering."
The starting point was Zytek’s new, highly integrated electric drivetrain, which uses rare-earth magnet technology to deliver exceptional power density and efficiency. The brushless DC motor (which can deliver up to 125Nm peak torque and 55kW (74bhp) peak power in other applications) is calibrated to produce a smooth 120Nm peak torque and 30kW (40bhp) peak power in a package that weighs just 70kg including the motor, gearbox, inverter and control electronics. The standard car’s cooling system is modified to control internal temperatures, using the existing front-mounted radiator with water flow assisted by an electric pump.
The new drivetrain module fits neatly onto the existing engine mounting points and interfaces with all existing vehicle systems including ABS and ESP. It also provides the complete range of diagnostic information that would be expected from a high-quality conventional powertrain as well as its own range of diagnostic codes. Zytek believes that this is the only EV powertrain in production that delivers such a high level of integration with other vehicle systems.
Power is provided by a Zebra liquid sodium nickel chloride battery, mounted under the floor near the centre of the vehicle to help retain the smart’s nimble dynamics. Running at 180 - 310V it has a specific power density of 194W/kg, giving a range of up to 115 kilometres (72 miles) in a standard EUDC test. The high voltage charger, 12V charger, battery cooling fan and brake vacuum pump are installed on a purpose-designed ancillaries frame at the rear of the powertrain bay.
Because electric motors produce maximum torque throughout a large rev range (in this instance up to 12,000 rpm) there is no need to change gears, so a single ratio equivalent to second has been selected. To help maximise the driving range, the maximum speed is electronically limited to 112 kmh (70mph). With up to 50 percent more torque than the standard 50bhp petrol car, acceleration from low speeds is brisk.
To charge the vehicle, users simply pull up within reach of a standard 13A socket and plug the charge cable into a connector behind the fuel filler. Two and a half hours is sufficient to take the battery from 30 percent capacity to 80 percent and Zytek’s electronics ensure that even short charges can usefully extend the range without damaging the battery. The battery is also charged by a regenerative braking system, which works with the ABS and ESP systems to recover as much energy as possible whilst maintaining excellent driveability and refinement in all weather conditions.
In the cabin, a battery state of charge gauge replaces the tachometer and the gear selector has been modified to provide Drive, Neutral and Reverse. An electric element now heats the cabin air, replacing the engine hot water system.
Zytek paid particular attention to achieving a very smooth drive. “Although electric motors are capable of delivering seamless torque throughout the rev range, the way that the energy is supplied changes depending on the demand on the motor and its operating speed. In many EV applications, this creates harshness at the transition points,” explains Heslington. “Zytek’s advanced control systems allowed us to analyse these transitions and develop control strategies that make them almost imperceptible to the driver.”
Zytek has more than ten years experience designing, developing and integrating electric and hybrid drivetrains and has worked with smart on a range of projects since 1998.
The Zytek integrated drivetrain fitted to the smart ev is the first to enter production from a range that will eventually provide vehicle manufacturers with power options from 30kW up to 100kW. “We believe these are the most highly integrated EV modules available, offering direct engine replacement and considerably simplifying the engineering required to deliver safety, performance and refinement equivalent or superior to the standard vehicle,” concludes Steve Tremble. smart ev data Weight of Zytek integrated drivetrain 105kg
Max power at 278V 30kW (standard car 37kW or 45kW)
Max torque 120Nm (standard car 80Nm or 95Nm)
Operating ambient temperature -20C to +40C
Vehicle weight 1,120kg (standard petrol car 990kg)
0-30mph 6.5 seconds
Range (urban driving) 115 kilometres (72 miles)