Hybrid vehicles are old hat, and nearly always relegated to a niche market – currently about 3 percent of U.S. car sales. Electric vehicles are sexy, but it’ll be a long wait until they’re an economically sound choice for the average Joe. As the country’s corporate average fuel economy (CAFE) standards rise, what are car manufacturers to do?
The industry’s latest answer – and hot topic – is the stop-start method. Basically, this involves turning off an engine when the car isn’t moving. This approach saves fuel, but it doesn’t add the peppy responsiveness to cars that hybrids and electrics enjoy from the massive low-end torque of their electric motors. Nevertheless, most major auto OEMs (original equipment manufacturers) have several start-stop vehicles in the works, if not already in their fleets, as it is a relatively inexpensive way to improve fuel economy by a few percentage points.
The costs to add stop-start to a vehicle are relatively minor: an increase in battery size to handle peripheral loads while the engine is off (lights, radio, A/C and power steering), an increase in alternator size to charge larger batteries faster once the engine is back on, and improvements to the starter system to make the restart and acceleration of the vehicle as seamless as possible. Auto OEMs may love stop-start for its relatively inexpensive boost to their CAFE numbers, but how do they get consumers excited enough for broad market adoption of a system that, at best, has no noticeable impact on the driving performance of a car, and at worst, leaves the car feeling sluggish or non-responsive when the driver accelerates from a stoplight?
The next logical step is to add intelligence to a stop-start’s already upsized alternator and make the vehicle a true microhybrid. Typical car alternators are rated for about 80 amps or about 1 kilowatt (kW) of power-run peripheral loads and recharge batteries. In stop-start systems, it is common to see the alternator increased to 2kW or 3kW to handle larger loads. This might not sound like much, compared to the full engine power of 160 horsepower or 120kW. But, compared to the engine power available during initial acceleration of only about 40 to 60kW, such an upsized alternator can represent more than 5 percent of the available power. A microhybrid with an intelligent alternator could keep the alternator off during acceleration, providing a perceivable performance boost. Better yet, if that alternator is engaged primarily during deceleration, the system can provide a significant boost to the car’s fuel economy.
The only example of such a system in the market today is Mazda’s i-ELOOP system, which claims to achieve up to a 10 percent improvement in fuel economy in urban drive cycles. This is impressive when you consider that Mazda states that the i-ELOOP energy storage system only has capacity for seven seconds of braking energy. For comparison, most hybrid vehicles on the market today only achieve a 25 to 30 percent improvement in fuel economy (according to a 2015 report from the International Council on Clean Transportation), and they require substantial costs and more than 1kWh or 30 seconds of braking energy storage to reach that level.
However, there’s no reason to scoff at claims that substantial fuel saving can be achieved with minimal energy storage. Back in 2008, the National Renewable Energy Laboratory (NREL) demonstrated a hybrid vehicle achieving the same 25 percent fuel economy improvement with 35 watt-hours (Wh) of ultracapacitor energy storage as the vehicle did with its originally designed 1.6 kilowatt-hour (kWh) NiMH battery. As auto manufacturers start to think beyond stop-start, the next economical gains may not come from bigger batteries or fancy electric-drive trains, but rather from small tweaks to vehicles’ alternators and 12V power systems.