August 27, 2007 General Motors has eliminated the spark from the engine cycle in a bold and challenging effort to produce a powerplant that delivers the fuel efficiency of a diesel with the low emissions of a petrol motor. The company has released two concept cars that use Homogenous Charge Compression Ignition (HCCI) technology to burn leaner and cleaner at lower temperatures and deliver up to 15% higher fuel economy.
‘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.
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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.
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.