| GM has built two road-going prototypes
with homogeneous charge compression ignition (HCCI) engines. The OEM said
the technology can contribute to fuel savings of 15 per cent, compared
to a conventional port injected engine.
The prototype combines HCCI with direct injection (GDI), electric cam
phasing, variable valve lift and cylinder pressure sensing. The fuel economy
improvements seem modest – GDI alone should be 15 per cent more
efficient – but this is an early prototype.
Mercedes has also developed an HCCI engine (See page 55), but this is
only running in a laboratory. Once the technology is production-ready,
it will be able to give gasoline vehicles the economy of a diesel without
the need for expensive NOx aftertreatment.
GM powertrain chief Tom Stephens said: “We’re beginning to
see how we can make this technology real by combining HCCI with other
advanced gasoline and control technologies.”
HCCI ignites a lean mixture of fuel and air by compressing it in the cylinder.
Unlike spark ignition, which creates small hot spots in the combustion
chamber where NOx forms, HCCI ignites the mix evenly through the chamber.
Cylinder pressure sensors and GM-developed algorithms control the HCCI
combustion process and the transition between the HCCI and spark-ignition
modes. The switch between the two is noticeable in the prototypes. GM
engineers will work to remove the noise issues and to extend the HCCI
mode’s operating range.
The test cars operate on HCCI up to around 90km/h, handing over to spark
ignition at higher speeds and during heavy engine load. GM Powertrain
advanced engineering director Dr Uwe Grebe, executive director said: “With
spark ignition, you can adjust the timing and intensity of the spark,
but with HCCI, you need to change the mixture composition and temperature.
It’s far more complex.”
The vehicles, a production-based Saturn Aura and an Opel Vectra, both
run a modified version of GM’s 2.2-litre four-cylinder Ecotec engine.
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