When General Motors unveiled the plug-in hybrid Chevrolet Volt concept at the 2007 North American International Auto Show, interest immediately focused on the electric aspect, and rightfully so. However, the one-liter, three-cylinder turbocharged internal combustion engine on the concept vehicle is interesting and unique as well. The engine is only one of the options in GM's E-Flex system, where the E stands for electric. Other options include a fuel cell and possibly even a bio-diesel depending on the region of the world. In all cases, the propulsion comes from an electric motor powered by Li-Ion batteries. The batteries represent the biggest challenge toachieving a production vehicle. If the issues can be resolved, the vehicle may be available in the 2010 timeframe.
With the internal combustion engine (ICE) configuration, the Volt gets 50 mpg by using the engine to run a generator and convert gasoline into electricity. This gives the electric vehicle an operating range of up to 640 miles, solving one of the main complaints about the last decade's electric vehicles — limited driving range. The engine provides a key and well-established building block for the system.
“The three-cylinder engine that we used in our concept vehicle is actually a production engine, the non-turbocharged, normally aspirated version of it,” said Nick Zielinski, vehicle chief engineer, Advanced System Integration, General Motors. The engine is currently manufactured in Austria and used on the Opel Corsa, a smaller vehicle than the Chevrolet Cobalt that is sold in North America. The new aspect of the engine is the turbocharger. The design team chose the small engine to provide significantly increased efficiency and because of the engine's unique role in the system. “We don't have to depend on it to provide all the performance for the vehicle, Zielinski explained. “We depend on the battery to really do that.” However a turbocharger was required to meet the vehicle's performance goals.
TURBOCHARGED WITH SPECIAL OPERATING MODES
“We went through our library of engines that we have available and the three cylinder in the normally aspirated condition did not have enough power to power the generator at its max operating point,” said Zielinski. Adding the turbocharger provides sufficient power, when it is needed, to get the maximum output from the generator. Under normal operating conditions, the load on the engine-generator set is low enough that the engine runs without the turbocharger and produces sufficient power out of the generator to keep the battery charged. In a high load condition, like heavy acceleration or climbing a steep grade, the turbocharger kicks in for increased output from the generator to keep the battery charged. The engine's role in this system dictates significantly different operating conditions than a normal ICE.
“We anticipate the engine speed will be between 1500 rpm and 3000 rpm and we are evaluating different types of control strategies,” said Zielinski. The engine's operating range provides new design options for the vehicle. Since it does not idle, the engine mounts can be designed for the best noise isolation between 1500 rpm to 3000 rpm. Zielinski noted that the entire acoustic package would be optimized for minimal noise intrusion from the engine generator.
All this is done for a portion of the system that many people may not use that frequently. “We don't' anticipate the engine generator running all the time, said Zielinski. “For most people and their normal driving, they will be in pure electric mode without having the engine-generator set running.”