High-Mileage, Low Emissions

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This produces power similar to today's conventional petrol engines, but uses less fuel to do it.

Heat is a necessary enabler for the HCCI process so a traditional spark ignition is used when the engine is started cold to generate heat within the cylinders and quickly heat up the exhaust catalyst and enable HCCI operation. During HCCI mode, the mixture's dilution is comparatively lean, meaning there is a larger percentage of air in the mixture. The lean operation of HCCI helps the engine approach the efficiency of a diesel but it requires only a conventional automotive exhaust after-treatment. Diesel engines require more elaborate and more expensive after-treatment to reduce emissions.

HCCI builds on the integration of other advanced engine technologies -- some of which are already in production and can be adapted to existing petrol engines. The cylinder compression ratio is similar to a conventional direct injection petrol engine and is compatible with all commercially available petrol and bioethanol E85 fuels.

The prototype vehicles

GM has demonstrated the adaptation of the HCCI technology in driveable concept vehicles based on conventional, production-based products like the Vauxhall Vectra and Saturn Aura. The Vectra has a manual transmission, whilst the Aura has an automatic gearbox.

Both vehicles are powered by a 2.2-litre Ecotec engine (180 hp and 170 lb.ft of torque or 230 Nm) that features a central direct injection system, with variable valve lift on both the intake and exhaust sides, dual electric camshaft phasers and individual cylinder pressure transducers to control the combustion as well as deliver a smooth transition between combustion modes.

A sophisticated controller, using cylinder pressure sensors and GM-developed control algorithms, manages the HCCI combustion process, as well as the transition between HCCI combustion and conventional spark-ignition combustion. The transition between the combustion processes is notable in the demonstration prototypes, but production versions are intended to deliver an imperceptible transition while driving, similar to the deactivation performance of GM's Active Fuel Management system.

Currently, the GM demonstration prototypes can operate on HCCI up to approximately 55 mph, transitioning to spark ignition at higher vehicle speeds and during heavy engine load. An extended range for HCCI operation is intended as further refinements to the control system and engine hardware are made.

"Perhaps the biggest challenge of HCCI is controlling the combustion process," said Prof. Dr. Uwe Grebe, executive director for GM Powertrain Advanced Engineering. "With spark ignition, you can adjust the timing and intensity of the spark, but with HCCI's flameless combustion, you need to change the mixture composition and temperature in a complex and timely manner to achieve comparable performance."

GM's global HCCI team will continue to refine the technology in the wide range of driving conditions experienced around the globe, from extreme heat and cold to the thin air effects of driving at high altitude.

"Although our development costs for HCCI have been substantial, we have made tremendous strides in bringing this most awaited combustion technology out of the lab and onto the test track with the Vauxhall Vectra and Saturn Aura concept vehicles. Additional development costs, including research and testing programmes are required to make the technology ready for the great variety of driving conditions that customers experience," said Prof. Grebe.

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