Is it possible to design an engine that uses gasoline but switches between spark ignition and diesel-style compression ignition?
The engine would combine the efficiency of a diesel engine with the power and low emissions of a gasoline unit.
A research consortium led by Robert Bosch GmbH and several partners is taking a shot at it with a $24.5 million research program that is partially funded by a $12 million grant from the U.S. Energy Department.
The partners hope to develop a commercially viable engine called a homogeneous charge compression ignition, or HCCI, engine.
Bosch estimates the engine could improve fuel economy as much as 30 percent over a conventional engine, with most of that achieved by downsizing the engine and adding a stop-start system.
The prototype engine, which has a supercharger, turbocharger, direct injection, stop-start and variable valve timing, "will be a technology platform," said Hakan Yilmaz, Bosch's director of advanced engineering for gasoline systems. "When you open the hood, you will see a research lab. But we want to show that it can be made for production."
How it works
An HCCI engine gets its efficiency by mimicking a diesel engine's combustion process.
Gasoline, fresh air and recirculated exhaust gases are fed into the combustion chamber and compressed, raising the temperature until the fuel spontaneously ignites. To get the proper air-fuel ratio, a small supercharger feeds extra fresh air into the engine.
The engine can run on a lean mix of gasoline — thus improving fuel economy — without need for the elaborate emission treatment for oxides of nitrogen required by diesel engines.
An HCCI engine stays in compression-ignition mode at cruising speeds and switches to a spark ignition at high speeds. And that's where things get tricky.
The engine needs a sophisticated control unit to switch between spark and compression ignition.
"When you make the transition to higher speeds, it becomes more and more difficult to control," says Dean Tomazic, vice president of light-duty diesel and gasoline engines for the engineering firm FEV Inc.
The goal is to create an engine that can switch between the two ignition modes without a noticeable change in torque or vibration. That sort of seamless transition "takes quite a bit of control, but it's doable," Tomazic says.
There are other problems. When the engine runs in compression ignition mode, even small changes in engine temperature can throw off the timing of the combustion process.
That's one reason why HCCI engines have not yet been commercialized.
Daimler AG, Ford Motor Co., Volkswagen AG, Honda Motor Co. and General Motors have tinkered with HCCI engines.
In 2007, Daimler developed an engine dubbed the DiesOtto, and in 2009 GM allowed journalists to test drive a Saturn Aura fitted with an HCCI-modified 2.2-liter Ecotec engine. The Aura's HCCI engine operated in compression ignition mode from idle up to 60 mph, a breakthrough for the technology.
But neither GM nor its competitors has brought this technology to market. GM worked with Bosch and others on an HCCI engine in an earlier research project at Stanford University.
Now, Bosch has teamed with Stanford, AVL, Emitech Corp. and the University of Michigan.
The researchers have set up shop in U of M's engineering lab. Amid a welter of boxes, tools, tubes, unidentifiable metal bits and construction debris, Bosch's Yilmaz offered a progress report on the project, dubbed ACCESS (for Advanced Combustion Controls — Enabling Systems and Solutions for High Efficiency Light-Duty Vehicles).
According to Yilmaz, the consortium will produce a prototype engine by starting with GM's 2.0-liter direct-injection Ecotec engine combined with a six-speed automatic transmission.
With the Ecotec engine, the consortium will try to match the power of GM's 3.6-liter V-6 engine, but with a 30 percent improvement in fuel economy.
And the researchers will try to produce two demonstration engines by mid-2014. The result will be "the ultimate gasoline engine," Yilmaz predicts. "We want to close the [fuel economy] gap between gasoline and diesel engines. We will get close."