GM investigating ultracap-based Active Energy Recovery Buffer as low-cost mechanism to improve conventional vehicle fuel economy
GM is exploring the use of an ultracapacitor-based Active Energy Recovery Buffer (AERB) scheme as a low-cost means to improve fuel economy in conventional vehicles; results of a simulation study presented at SAE 2011 World Congress showed improvement of up to 1.3% on FTP composite drive cycle for a small crossover utility vehicle with a 4-cylinder, 2.4-liter engine and a six-speed transmission.
With the AERB, the kinetic energy of the vehicle during coast down events is used to charge the ultracapacitor either directly, or through a dc-dc converter, allowing the voltage to increase up to the maximum permissible level. When the vehicle starts after a Stop event, the ultracap discharges its energy to power the accessory loads, thereby relieving the generator torque load on the engine, resulting in a decrease in fuel consumption.
Although there are a number of methods reported to use brake energy recuperation to improve fuel economy in conventional vehicles, the GM team notes, the simplest is to capture braking energy and charging the 12V battery by modifying the generator voltage command. The issue with this approach is the impact of such charge/discharge cycling on battery life and also the effect of voltage fluctuations on the lamps and other low-voltage loads.
In the study, GM considered two topologies for the system:
A simple add-on to the conventional vehicle 12V electrical system, comprising the ultracapacitor bank and the dc-dc converter connected across the dc bus. During coasting events, fuel is cut off; the generator current command is set to maximum level under the fuel cut-off condition. The generator supplies the 12V load current with the additional current charging the ultracaps.
Once capacitor voltage reaches the maximum permissible limit, the generator current is set to a nominal level to support the 12V loads. When the engine controller exits the fuel cut-off state, the generator current is set to a low value close to zero, and the ultracap supplies the 12V load through the dc-dc converter until the ultracap voltage falls to the minimum permissible value. Thereupon, the dc-dc converter is switched to standby mode, and the generator current command is adjusted to support the 12V loads.
The second topology is capable of a higher level of regeneration during braking events, but requires a redesign of the generator controller. This second topology allows the generator voltage to vary over a wider range compared to the conventional generator. This significantly increases the peak power capability of the generator at any given engine speed.
The ultracaps are connected directly across the generator output and to the input side of a unidirectional dc-dc converter, the output of which is connected to the 12V loads. This topology has the ability to capture a higher amount of regenerative energy than topology 1 because of the high power capability of the generator. The main disadvantage, says GM, is that it requires continuous operation of the dc-dc converter since the 12V load is supplied only by the converter.
The optimal size of the various components—sizing being an important factor in keeping the incremental cost low—depends on vehicle size, the drive cycle, and also the maximum current capability of the production generator in the vehicle.
The AERB system could capture the regenerative energy during coasting events without affecting the battery cycling thereby reducing warranty costs while improving fuel economy...Optimally sizing the AERB system components could result in a low-cost means to improve fuel economy in conventional vehicles.—Gopalakrishnan et al.
Suresh Gopalakrishnan, Chandra Namuduri and Michael Reynolds (2011) Ultracapacitor Based Energy Recovery Scheme for Fuel Economy Improvement in Conventional Vehicles (SAE 2011-01-0345)