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GM’s eAssist seeks to provide a cost-effective solution for improving fuel efficiency and driving dynamics
15 December 2011
|The components of the Malibu Eco eAssist system. The 15 kW motor generator is only slightly larger than current alternators, which, for this segment, would be about 2 kW. Click to enlarge.|
General Motors is exploring the use of its light electrification system eAssist as a cost-effective means to improve both vehicle efficiency as well as driving dynamics. Initially introduced in the Buick line, eAssist has now made the jump to the Chevrolet line with its application in the new Malibu Eco (earlier post)—which will be the only version of the new Malibu initially available.
The eAssist applied in the Malibu, while not a “second-generation” of the eAssist in the sense of a new design, represents a substantive rebuild with improved components over earlier efforts, according to Daryl Wilson, eAssist Lead Development Engineer. Wilson and other members of the Malibu engineering team presented at a Malibu Eco media event.
The eAssist system consists of:
A 32-cell, 115V, 0.5-kW Li-ion battery pack (15 kW peak power) using cylindrical power-optimized cells from Hitachi. The power electronics and the pack are packaged in the “eAssist power pack” which includes the batteries (2 parallel 16-cell modules) and the power electronics, both of which are air-cooled.
- An electric motor-generator that provides 15 kW of maximum generating power @ 1570-3180 rpm; 110 lb-ft (150 N·m) maximum electric motor torque (cranking); 79 lb-ft (107 N·m) maximum electric motor torque @ 1,000 rpm (electric assist); and 15 hp (11.2 kW) @ 1,000-2,200 rpm maximum electric motor power (electric assist). The motor generator is liquid-cooled.
The system’s electric motor-generator is mounted to an Ecotec 2.4L four-cylinder engine, in place of the alternator, to provide both motor assist and electric-generating functions through a revised engine belt-drive system.
The key to the eAssist system is the ability to capture energy that would otherwise be lost and utilize that energy to optimize the efficiency of the vehicle in many different ways.—Daryl Wilson
The business key for GM on this particular system was to define the performance parameters in such a way as to deliver benefits, but without encroaching into higher cost component requirements.
As an example, Wilson said, the eAssist system captures up to 15 kW of power during deceleration or braking. A system that captured more than 15 kW from brake regen would have required a more complex, costlier braking system as well.
eAssist supports the Malibu Eco in four main ways, Wilson said:
Auto Stop. The stop/start function enabled by eAssist is seamlessly integrated into the Malibu, with very smooth engine stops and restarts. The only indication for a driver that the engine has come back on is the engine noise, and the indicators on the information screen.
Up to 15 hp of electric assist. GM uses that power to put the engine into a more efficient operating position in most driving and to support a lower final drive ratio allowing the engine to run at low rpm at a high load point. (As an example, on a test drive of a Malibu Eco during the media event, the engine was running at about 1500 rpm at about 60 mph on level terrain.)
While it’s possible to configure any car to run like that, Wilson said, you give up on driving dynamics. The extra power from eAssist offsets those losses. The new Malibu Eco is faster than the current Malibu, and the reserve acceleration form eAssist provides ample support for acceleration, grades and overtaking.
There is, however, no electric “creep” function with eAssist; i.e., when in traffic, and the driver lifts his or her foot from the brake, but does not press the accelerator, the engine still kicks in to power the creep. The reason for that, Wilson pointed out, is that because the eAssist system is, by its design, connected to the crankshaft, it would have to drag the engine along with it when creeping. Given that, it is more efficient to have the engine come back on.
Stored energy in the battery pack offsets use of fuel otherwise consumed to run an alternator for charging for the radio, fans, lights, etc.
Very aggressive use of deceleration fuel cutoff.
The 2013 Malibu Eco achieves 12% greater highway fuel economy than current models equipped with the 2.4L engine; GM estimates fuel economy of 25 mpg US city, 37 mpg US highway, 29 mpg US combined (9.4, 6.4 and 8.1 liters per 100 km, respectively).
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