The “sweet spot” between the capability and efficiency of 200-V plus systems and the low cost of 12-V stop-start in future hybrid systems will likely use a belt-driven electric machine rated at 48 V and producing 10 to 12 kW. Such systems are under development at Ford for potential future production applications, said Dr. Mazen Hammoud, Chief Engineer for Electrified Powertrain Systems.
Dr. Hammoud, an SAE Fellow, was speaking on February 11 at the 2015 SAE Hybrid and EV Technologies Symposium in Los Angeles. His technical presentation, “Mild Hybrid and Enablers for Fuel Economy Features,” highlighted the many potential benefits of 48-V integrated starter-generators (ISG), which Ford engineers see as compatible with gasoline and diesel powertrains in both passenger vehicle and light commercial truck applications.
The 48-V working voltage has been identified as the best compromise in terms of CO2 reduction, voltage level, and electric current. And according to ECE-R 100 regulations for dc voltages under 60 V, shock protection is not required for 48 V, which helps lower cost.
“It [48 V] gives a good balance—no real electric-only drive capability, but better capacity for capturing braking regen energy, up to about 60% of that available, and it’s a good stop-start enabler,” Dr. Hammoud noted. “Typically a mild HEV can provide more than 3% increased engine-off time versus a 12-V stop-start in real-world driving conditions,” he added. Typical stop-start time is 450-500 ms, he said.
48-V mild hybrids enable powertrain engineers to vary load and push the engine into more optimum BSFC points, at a cost that is approximately 25% of a high-voltage “full” hybrid, he noted. “In terms of dollars-per-percent-of-efficiency improvement, the [mild] HEV is as good as a full hybrid in certain driving cycles," such as urban and parcel delivery.
Ford has been working with Ricardo and other partners (including Controlled Power Technologies) to develop a cost-effective 48-V ISG system as part of the U.K.-based ADEPT (Advanced Diesel Electric PowerTrain) collaborative research program launched in 2013. The project has tested a Ford Focus equipped with a 48-V electrical architecture, Ricardo hybrid controller, and CPT 10-kW ISG, with an advanced lead-carbon battery pack supplied by EALABC.
According to Dr. Hammoud, his teams' 48-V developments and testing have encouraged Ford engineers to investigate a broad range of vehicle capabilities and features that potentially can be enabled by the 48-V system, including electric drive axles, torque vectoring using two e-machines which Dr. Hammoud said “are coming soon” (likely on the new Focus RS first) and electrically-driven turbochargers.
“On the Focus equipped with an e-turbo, we’ve seen a 40-50-g reduction in CO2 with no reduction in performance,” he reported. The 48-V system will retain the vehicle’s 12-V lead-acid battery to handle hotel loads due to its proven, low-cost performance, he said.
48-V systems are a development priority at a number of European OEMs and suppliers who are increasingly concerned about meeting the new, more aggressive WLTP (Worldwide harmonized Light vehicles Test Procedures), which will call for additional reductions in CO2 emissions from the current NEDC (New European Drive Cycle) test. The EU is aiming to roll-in the WLTP by 2017, and by 2020 at the latest, to coincide with the new 95 g CO2/km regulations. The new regs bring a twist: they typically reduce by half the CO2 reduction that is gained from current stop-start systems. Engineers are encouraged that the 48-V ISG systems will help them meet the new more real-world-focused test cycles.
In addition, Ford also is working within the U.S. Advanced Battery Consortium (USABC) on 48-V mild hybrid propulsion.