High mileage, low emissions: GM revisits sparkless compression ignition engines

‘Homogenous charge compression ignition’ technology – or HCCI for short – which General Motors debuted recently in a Vauxhall Vectra and a Saturn Aura, enables conventional petrol engines to approach diesel-like engine efficiency but does not require the same expensive exhaust after-treatment to reduce emissions that diesel engines need. Not only is the new system adaptable to today’s standard petrol engine architectures and compatible with all commercially-available petrol, it is also compatible with bioethanol E85 fuel. When combined with other advanced technologies such as direct injection, electric cam phasing, variable valve lift and cylinder pressure sensing, HCCI provides up to 15 percent fuel savings, while meeting future emissions standards.

Compression ignition is not a new idea – in fact, compression-ignited engines were in use before electronic spark ignition became popular. Compared to a spark ignition, which begins the combustion of the air/fuel mix at a specific time and location, the spontaneous combustion of a compression engine occurs at lower temperatures, leading to less NOx emissions, and at leaner air/fuel combinations, leading to greater efficiency. On the flip side, spark ignition is very controllable – the exact timing of the ignition can be controlled to the millisecond, where an HCCI engine effectively ignites whenever conditions are appropriate, making ignition timing very difficult to manage at different revs and different throttle openings.

“I remember debating this combustion capability when I was in college,” said Tom Stephens, group vice president, GM Powertrain and Quality. “It was just a dream then. Today, using math-based predictive analysis and other tools, we are beginning to see how we can make this technology real. By combining HCCI with other advanced petrol engine and control technologies we can deliver good fuel savings for consumers.”

In an integrated engine concept, HCCI along with other enabling advanced technologies approaches the engine efficiency of a diesel, but without the need for expensive lean nitrogen oxide (NOx) after-treatment systems. The engine’s efficiency comes from burning fuel at lower temperatures and reducing the heat energy lost during the combustion process. Consequently, less carbon dioxide (CO2) is released because the vehicle’s operation in HCCI mode is more efficient.

The HCCI-powered concept vehicles – a production-based Saturn Aura and the Vauxhall Vectra, both with a modified 2.2L Ecotec four-cylinder engine – drive like conventionally powered vehicles, but offer up to 15 percent improved fuel efficiency compared to a comparable port fuel injected engine (these fuel efficiency improvements will vary depending on the vehicle and the customer driving cycle). The driveable concept vehicles represent one of the first tangible demonstrations of HCCI technology outside of the laboratory.

“This is another initiative in GM’s advanced propulsion technology strategy to lessen our dependence on oil,” said Stevens. HCCI, direct-injection, variable valve timing and lift, and Active Fuel Management all help improve the fuel economy and performance of our internal combustion engines. I am confident that HCCI will one day have a place within our portfolio of future fuel saving technologies.”

Similar to a diesel engine – but without the expense

An HCCI engine ignites a mixture of fuel and air by compressing it in the cylinder. Unlike a spark ignition petrol or diesel engine, HCCI produces a low-temperature, flameless release of energy throughout the entire combustion chamber. All of the fuel in the chamber is burned simultaneously as instead of a single spark ignition point, the mix spontaneously ignites at several points at once. This produces power similar to today’s conventional petrol engines, but uses less fuel to do it.

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