BMW Introduces Intelligent Alternator Control with Regenerative Braking; Reduces Fuel Consumption by About 4%
At the Paris Auto Show, BMW introduced a system it calls Intelligent Alternator Control (IAC) to generate electric power for a car’s on-board network exclusively in overrun and during braking—IAC thus also incorporates a system for regenerating brake energy.
The system is part of a larger BMW initiative to improve the overall efficiency of a vehicle by decreasing ancillary loads on the engine and recuperating more of the waste heat energy. (BMW is currently involved in one of several projects tackling the development of a thermoelectric waste heat recovery system targeted to deliver a 10% improvement in fuel economy. More on this below.)
Only about 25–30% of the energy contained in fuel is actually used for driving the vehicle. Most of the energy consumed is still converted into heat, although the fuel burnt also serves to generate electrical energy for the on-board network.
|Electric power requirements for mid-size and luxury cars. Source: DOE|
The on-board demand for power is also steadily increasing, even without factoring in traction support. Air conditioning, telecommunications, entertainment, as well as new components for enhanced safety and driving dynamics such as suspension management, Active Steering, engine management, and ABS all require electric power.
A mid-size car currently has an electric power requirement of about 3.5kW for all its systems. (See chart at right.) Hence, the generation of electricity for the car’s on-board network consumes an increasing share of the power generated by the engine.
With the IAC system, the alternator will operate primarily when the engine has no need for power—i.e., in overrun or during braking. The alternator remains passive while the car is under power, with needed electrical power provided by the battery.
The alternator becomes active when the engine switches to overrun or if the battery charge is insufficient.
For regenerative braking, a power converter fitted directly within the brake system converts the energy generated upon application of the brakes into electric power.
The battery is charged to only about 80% of its capacity whenever the engine is pulling the vehicle, always maintaining an adequate reserve for the consumption of energy at a standstill and for starting the vehicle. A higher charge level is generated only when the vehicle is in overrun or upon application of the brakes, that is in phases with a better energy balance.
With the number of charge cycles increasing thanks to these specific control functions, BMW combines Intelligent Alternator Control with AGM (absorbant glass mat) batteries able to handle a higher load than conventional lead/acid batteries.
BMW found that the on-demand generation of electrical energy helps to reduce fuel consumption in the EU homologation test by approximately 4%. The driver also has access to more engine power for acceleration and dynamic driving.
|BMW’s electric water pump.|
IAC is one effort to generate electrical energy in the car more efficiently, and use that energy for a wider range of purposes and functions. BMW is now using new electrical coolant pumps in its straight-six engines. The electrical pumps operating exactly—and only—when required, meaning that they develop their maximum output and performance only at high and very high speeds.
They remain passive immediately after the engine has been started, ensuring in this way that the engine is warmed up more quickly. This alone helps to reduce fuel consumption in the EU homologation test by approximately 2%.
|The waste heat recovery project envisions a heat exchanger in the exhaust line (middle of car) that then feeds a working fluid to the thermoelectric generator (TGM). Click to enlarge.|
Thermoelectric Waste Heat Recovery. BMW is a member of one of four teams engaged in a DOE-sponsored project on thermoelectric waste heat recovery. (Earlier post.) BMW’s team is lead by BSST, with Visteon, Marlow, Purdue, UC Santa Cruz, NREL, Teledyne and JPL also as contributors.
Another team is lead by GM and GE, with University of Michigan, University of South Florida, Oak Ridge National Laboratory, and RTI International as members.
The BMW team is using a 2006 BMW 530i as the target platform. The 3.0-liter engine is of the newest BMW generation, with characteristics representative of engines in the 2010 to 2015 timeframe.
The project, which began in 2005, has four phases. The teams are now in phase 2, in which they are building the subsystem elements, testing them independently, and then updating their system model.
Next year will see the integration and operation of the components as a system. Subsequent to that will be vehicle integration and testing at NREL.