Water-cooled starter/generator (GM photo)
by Jacques Gordon
GM’s eAssist™ is a mild-hybrid system that was introduced on the Buick Regal in 2012 and is now available on the Buick LaCrosse and Chevrolet Impala. As on other hybrid vehicles, the eAssist system is used to assist the engine during acceleration and to provide engine stop/start functions. The difference between this and other hybrids is that the starter/generator is not part of the drivetrain; it’s mounted on the engine in place of an alternator and it adds or recovers torque through a serpentine belt connected to the engine’s crankshaft.
The engine automatically shuts off (Auto Stop) when the vehicle comes to a full stop and all the proper conditions are met (full operating temperature, transmission in Drive, etc.). The engine will restart when the brake pedal is released and the accelerator is depressed, or after two minutes. A list of Auto Stop and Auto Start parameters is available on a scan tool.
In addition to restarting the engine, the starter/generator also provides up to 15 horsepower for acceleration and climbing hills. In generator mode, it supplies up to 15 kW of 3-phase alternating current for charging the 115-volt battery pack. The control unit operates the generator during deceleration and braking, providing the same regenerative braking function as on other hybrid vehicles.
All the starter/generator’s torque is transmitted through a wide 7-ribbed belt, and the tensioner spring is quite strong (there’s a special tool for compressing that spring). The same belt also drives the air conditioner compressor. Since the water pump is chain-driven, on some vehicles this is the only belt on the engine.
There are two eAssist cooling systems, both separate from the engine’s cooling system and managed by the starter/generator control module. The starter/generator is liquid cooled and has its own radiator and electric pump. The small radiator is mounted behind the engine coolant radiator near the top. The pump is operated with a relay that controls the power side of the pump circuit. It’s a simple on/off circuit, so the pump has only one speed. The coolant temperature sensor is a familiar negative-temperature-coefficient (NTC) thermistor connected to the control module’s 5-volt reference signal, but it’s built into the starter/generator and cannot be replaced.
The lithium-ion battery pack and control modules are air cooled. A sealed cover under or behind the rear seat has an externally-mounted ‘squirrel cage’ fan that pulls cabin air through ducts, into the battery housing and past heat sinks on the battery assemblies and control modules. The air then passes through the fan and is ducted outside the vehicle. This keeps the battery pack housing under slight negative pressure to avoid forcing fumes into the vehicle if a seal fails. There are also vent tubes on either side of the housing for passive venting. The battery pack assembly contains seven NTC temperature sensors; six to measure battery temperature (two on top, four below) and one to measure cooling air inlet temperature. These sensors communicate with the battery control module, and their readings are shared with the starter/generator control module that controls cooling fan speed.
The HVAC controls have an “eco” setting that enables the Auto Stop mode more often and for a longer duration. When the A/C compressor is not running, cabin comfort can be maintained longer with the controls in recirculation mode.
The engine cooling system holds 7.5 quarts of DEX-COOL, and there’s a coolant level sensor in the bottom of the coolant reservoir. The temperature sensor is on the thermostat housing behind the engine. As noted earlier, the water pump is driven by the timing chain that also operates the balance shaft. With the right tools it can be removed without removing the timing chain cover.
Although similar to GM’s earlier Belt Alternator Starter (BAS) mild-hybrid system used on vehicles like the Saturn Vue, the eAssist system operates at much higher voltage and therefore provides more power to the engine and more powerful regenerative braking. GM claims a 25% improvement in gas mileage over the same vehicle without eAssist. Even it that figure is a best-case scenario, it’s an impressive return for the relatively modest weight and complexity of this system, and we expect to see it on several more models in the future.
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