While their European counterparts are gearing up to show off a new Corsa Hybrid 'concept', flexfuel and natural gas-powered vehicles and other new engine technology at next month's Frankfurt motor show, General Motors engineers in the United States today (24 August) announced new technology that could cut fuel consumption by up to 15%.

GM calls the combustion process "homogeneous charge compression ignition (HCCI)" and claims it is the "most awaited advanced combustion technology" of the past 30 years.

The automaker is demonstrating HCCI in two driveable concept vehicles, a 2007 Saturn Aura and a GM Europe Opel Vectra.

When combined with other advanced technologies such as direct injection, electric cam phasing, variable valve lift and cylinder pressure sensing, HCCI provides up to a 15% fuel savings, while meeting current emissions standards, GM said.

"I remember debating the limits of combustion capability when I was in college," said GM powertrain and quality group vice president Tom Stephens in a statement. "HCCI 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 gasoline engine and control technologies we can deliver a good fuel savings value for consumers."

GM said that, in an integrated engine concept (the company has recently started referring to various engine technologies it is launching as 'concepts'), HCCI, along with other advanced technologies, approaches the engine efficiency benefit of a diesel, without the need for expensive lean NOx after-treatment systems.

Its 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, both with a modified 2.2L Ecotec four-cylinder engine, are said to drive like conventionally powered vehicles, but offer up to 15% improved fuel efficiency relative to a comparable port fuel-injected engine. (though this fuel efficiency improvement will vary depending on the vehicle application and the customer driving cycle).

"The driveable concept vehicles represent some of the first tangible demonstrations of HCCI technology outside of the laboratory," GM added.

"I am pleased with our engineering team's progress," said Stephens. "It is another initiative in GM's advanced propulsion technology strategy to lessen our dependence on oil.

"HCCI, direct-injection and variable valve timing and lift 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."

GM noted that the HCCI requires only conventional automotive exhaust after-treatment - ie a catalyst - and is compatible with all commercially available gasoline and E85 ethanol fuels.

How HCCI works

An HCCI engine ignites a mixture of fuel and air by compressing it in the cylinder. Unlike a spark ignition petrol or diesel unit, HCCI produces a low-temperature, flameless release of energy throughout the entire combustion chamber. All of the fuel in the chamber is burned simultaneously.

This produces power similar to today's conventional gas 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-injected petrol engine and is compatible with all commercially available petrol and E85 fuels.

The driveable Saturn Aura prototype vehicle has an automatic transmission while the Vectra, aimed at the European market, has manual transmission.
Both vehicles are powered by a 2.2-liter Ecotec engine (180hp/134kW and 170 lb-ft/230Nm of torque) with central direct-injection system, with variable valve lift on both the intake and exhaust sides, dual electric camshaft phasing 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.

The demonstration prototypes can operate on HCCI up to approximately 55 mph (about 95km/h) switching 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," noted GM powertrain advanced engineering executive director, Uwe Grebe.

"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 a wide range of driving conditions around the world, 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," Grebe said.

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," he added.