UC Davis Study Finds That Ultracapacitor-based Micro-Hybrid Systems Can Deliver Substantial Fuel Economy Improvements
Using a micro-hybrid system featuring carbon/carbon ultracapacitor units as energy storage can result in significant increases in fuel economy over a baseline conventional vehicle, according to a study by Dr. Andrew Burke at UC Davis.
In a series of simulations of mid-size passenger cars using ultracapacitors in micro-hybrid, charge-sustaining hybrid, and plug-in hybrid powertrain designs, the micro-hybrids delivered improvements of about 40% on the FUDS (Federal Urban Driving Schedule) and ECE-EUD (Economic Commission for Europe-Extra Urban Driving) cycles and 20% on the Federal Highway and US06 cycles.
The ultracapacitors were used to improve fuel economy with only a minimal change in vehicle acceleration performance. The control strategy used was to operate on the electric drive when possible and to recharge the ultracapacitors when the engine was operating...this resulted in a large improvement in average engine efficiency from 19% in the ICE vehicle to 30% in the micro-hybrid even though the electric motor had a peak power of only 6 kW.—Burke, 2009.
The simulations were part of a study evaluating the present state-of-the-art of ultracapacitor technologies and their suitability for use in electric and hybrid drivelines of various types of vehicles.
All the simulations used powertrains in the same mid-sized vehicle. The engine map used in the simulations of micro-hybrid, charge-sustaining hybrid, and plug-in hybrid was for a Ford Focus 2L, 4-cylinder engine. The rated engine power was 120 kW for the conventional ICE vehicle and the micro-hybrid and 110 kW for the charge sustaining and plug-in hybrids. In addition, Burke ran simulations on an extended range electric vehicle design, and on a fuel cell vehicle design.
The simulations used a “sawtooth” control strategy to optimize the efficiency of the engine. The sawtooth strategy has essentially two modes: charge depleting (operation in the electric mode with the engine off) and recharging (engine on at relatively high power to recharge the ultracapacitor or battery).
In the charge depleting mode, system efficiency is maximized by relying on the electric drive, which is inherently efficient; in the recharge mode, the engine runs in the most efficient region (torque and speed) of the engine map. In this mode, the engine both recharges the ultracapacitors and provides power to drive the vehicle. The ultracapacitors are also recharged during regenerative braking. With these two modes, the engine can be run at its most efficient states while keeping the energy storage within a given SOC range.—Burke, 2009
The micro-hybrid used a 30 Wh, 6 kW peak power ultracapacitor unit. Burke ran additional computer simulations for higher motor power (up to 12 kW) and larger ultracapacitor energy storage (up to 50 Wh) in the micro-hybrid application, and found that the improvements in fuel economy were only marginally greater. Using a motor power of 3 kW reduced the fuel economy improvement on the FUDS by more than 50%.
The fuel economy improvements using hybrid carbon ultracapacitors was less, Burke found, because the round-trip efficiencies for the carbon/carbon units were 95-98% and those of the hybrid carbon units were 75-90% for the various driving cycles.
As noted previously, the hybrid carbon devices had higher energy density, but even though their power density for 95% efficiency was relatively high (1050 W/kg), it was not proportionally higher—that is twice as high—as the carbon/carbon devices with lower energy density. These results show clearly that it is essentially to develop high energy density ultracapacitors with proportionally higher power density; otherwise their use in vehicle applications will be compromised.—Burke, 2009.
Other findings from the simulations included:
Using the carbon/carbon ultracapacitor unit, the fuel savings for the charge-sustaining hybrid are about 45% for the FUDS and ECE-EUD cycles and about 27% for the Federal Highway and US06 cycles. The values are higher than for the micro-hybrid, Burke noted, but not by as large a factor as might be expected.
The prime advantage of the high power electric driveline in the charge sustaining hybrid is that it yields large fuel economy improvements even for high power requirement driving cycles like the US06. The fuel economy improvements using the hybrid carbon ultracapacitor unit are not much less (5-10%) than those with the carbon/carbon unit even though the round-trip efficiency of the hybrid carbon unit is only 85-90% compared to 98% for the carbon/carbon unit. Since the weight/volume of the hybrid carbon unit is relatively small—43% of that of the carbon/carbon unit, it appears that the charge sustaining hybrid application is a better one for the hybrid carbon technology than the micro-hybrid application.
The plug-in hybrid vehicle utilized a high energy density battery (200 Wh/kg) and ultracapacitors that would provide two-thirds of the power to a 70 kW electric motor.
In charge sustaining mode after the energy battery (12 kWh) has been depleted for all-electric operation, the operation of the plug-in hybrid vehicle is essentially the same as previously discussed for the charge-sustaining hybrid. Burke concludes that the hybrid carbon ultracapacitor unit would also be suitable for the plug-in hybrid.
The combined weight of the cells in the battery and hybrid carbon ultracapacitors would be 78 kg for a plug-in hybrid with an all-electric range of about 40 miles. The combined weight using the carbon/carbon ultracapacitors would be 100 kg. Using a high power lithium-ion battery with an energy density of 100 Wh/kg without ultracapacitors, the weight of the battery cells alone would be 120 kg. Hence for plug-in hybrids combining a battery with ultracapacitors is an attractive design option.
Ultracapacitors could also be used in the plug-in, series hybrids (extended range electric vehicles) if the batteries were unable to provide the peak power to the electric motor. This would be most likely, Burke wrote, if the plug-in range was relatively short and/or high energy density batteries of modest power density were being used in the vehicle. In those cases, the ultracapacitors would greatly reduce the power demands on the battery and lead to less stress on the battery and longer cycle life.
The simulation results indicate that the fuel economy of the series hybrid is slightly higher than that of the parallel charge sustaining hybrid when both are operated in the charge sustaining mode. The engine/generator was sized such that the series hybrids were full-function vehicles. Thus all the hybrid vehicles including the series hybrids in the all-electric mode have performance equivalent to the conventional ICE vehicle.
In the fuel cell vehicle, the presence of the capacitors significantly reduced the power demand on the fuel cell even with the capacitors connected in parallel. The capacitors provided most of the power when their voltage is relatively high and the fuel cell provides high power when the capacitors become significantly discharged. The fuel cell rapidly recharges the capacitors when the power demand is reduced.
If interface electronics are used to control the current from the fuel cell, it is possible to maintain the fuel cell operation at a near constant power. This is probably the preferred arrangement of fuel cell and ultracapacitors, but it is more expensive. In their present fuel cell vehicle, the Clarity, Honda utilizes a lithium-ion battery and interface electronics to load level the fuel cell.
Burke, Andrew F. (2009) Ultracapacitor Technologies and Application in Hybrid and Electric Vehicles. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-09-23