At the 2013 SAE World Congress, Lotus Engineering will be showing a cutaway version of its latest series hybrid electric sports car demonstrator. The Evora 414E uses a series hybrid-electric drivetrain, with electrical energy provided to the battery from Lotus’s range-extender engine, which is capable of delivering 35 kW of continuous electrical power. The motors, inverters, and battery system were specifically designed for the 414E vehicle program.
The range-extending APU is a three-cylinder, normally aspirated gasoline engine producing 35 kW (47 hp) at 3500 rpm. Directly coupled to the engine’s crankshaft is an electric generator. At the other end of the driveline are two traction motors, each having peak 150-kW power, 500-N·m (369-lb·ft) torque, and 8000-rpm speed.
Specific technology innovations on the 414E include a multimode virtual seven-speed shift scheme with regenerative braking control, torque-vectoring stability control, and unique energy and power management optimization schemes.
Since electric motors have consistent torque output across their operating range, most electric vehicles operate through a single fixed-ratio transmission. Lotus engineers believe that driver involvement and vehicle dynamic agility suffer as a result compared to conventional ICE vehicles because drivers can’t enhance cornering behavior through throttle modulation.
The 414E solution, for which Lotus has filed for a patent, enables “gearshift events” via steering wheel paddles to provide increased EV system functionality through Normal EV plus three alternative drive modes: Simulation, Sport, and City. Advantages include enhanced driver involvement, perceived performance feel improvements, and increased energy recuperated during regenerative braking.
Lotus engineers believe that torque vectoring is particularly suited to electrified vehicles and has the potential to significantly reduce conflict between and enhance both stability and response. Their solution is to independently control driving and braking torques at each wheel with independently controlled inboard motors for improved vehicle yaw response. They say handling is greatly enhanced without compromising ride quality, stability control can respond faster than with conventional methods, and the suspension setup can be designed for lower rolling resistance.
The demonstrator’s series-hybrid powertrain architecture allows for some interesting optimization in response to electrical vehicle power demands. The aim of the 414E energy management system (EMS) is to determine and apply the best ratio of engine-to-battery power to minimize fuel consumption.
The load-following strategy runs the APU at a power level equal to the instantaneous powertrain demand. The battery charges gradually as regenerative braking energy is accumulated until an upper threshold state-of-charge (SoC) is reached, at which point the APU powers off and the battery is used down to a lower threshold.
The SoC trace has a more gentle flowing nature than typical in the industry and shows smaller swings in SoC with time, causing less battery degradation—an important consideration since one of the most significant perceived “issues” with electrified vehicles among consumers is battery lifetime. Performance of the adaptive EMS, in terms of fuel consumption and related CO2 emissions, is said to be impressive compared to performance of stop-start and load-following methods.
You can read more about our earlier drive of the car and our interview with its Chief Engineer at http://www.sae.org/mags/aei/11531.