November 3rd, 2005

A Gasoline-Powered HCCI Hybrid?

HCCI (homogeneous charge compression ignition) has always been regarded as a strategy for diesels as the very definition of a diesel engine is the use of compression ignition to ignite the air–fuel charge. HCCI combustion offers the advantage of greatly reduced emissions, especially the reduced emission of NOx (oxides of nitrogen), as the use of large quantities of EGR in most HCCI designs has the tendency to lower combustion temperatures to the point below which NOx is no longer produced.

NOx is generally considered public enemy number one when discussing vehicle emissions as it is a precursor of so many harmful atmospheric products including ground level ozone, smog, and acid rain. NOx is produced in internal combustion engines when the air induced into the engine is combined with pressure and heat. Nitrogen liberated from the fuel molecules also combines to form NOx. The nitrogen combines with oxygen to form various compounds of nitric oxides, such as NO (nitric oxide), NO2 (nitrogen dioxide), and N2O (nitrous oxide).

Because the development of an economical and reliable HCCI engine could result in vehicles with virtually no NOx production, a great deal of research is under way to perfect HCCI.

NOx concentrations in europe

This satellite image shows the concentration of nitrogen dioxide over Europe. Photo courtesy of Wikipedia

The U.S. Department of Energy recently announced a $13 million-program to achieve a workable HCCI engine, while the Robert Bosch Corporation and Stanford University have also announced a joint effort to develop the extremely sophisticated engine controls and sensor technology that HCCI requires.

HCCI development to date requires the use of multiple operating modes during the three conventional phases of engine operation, these being idle, part load, and full load operation. HCCI is usually implemented during part load operation only (this is the operating mode during which EGR is used in conventional vehicles).

HCCI Diagram

This graphic demonstrates the difference between HCCI and the two conventional methods of combustion. Graphic courtesy of the Robert Bosch Corporation

Recently engineers have been able to use HCCI as an operating mode during idle as well, but the technology for successful use of HCCI at full load has yet to be developed. Of course, since the formation of NOx is greatest during full load, this is the mode that would see the greatest benefit from HCCI use. The Bosch–Stanford venture will work toward development of engine controls that can enable HCCI technology during all phases of engine operation.

Engineers at General Motors are working toward the development of an HCCI engine that could utilize gasoline as a fuel and combust the gasoline without flame. Achieving the goal of “Flameless Combustion Ignition” could mean a light-duty engine fueled by gasoline, which could achieve a 20 percent gain in fuel efficiency with greatly reduced emissions, especially the emission of NOx. If a HCCI gasoline/flex fuel engine (20 percent more efficient than current engines) was to be coupled to a hybrid electric drive, which already provides an efficiency gain of more than 20 percent, the resulting vehicle would be far more fuel efficient and emission free than conventional vehicles, without requiring changes in fueling infrastructure. In fact, because the engine management system of an HCCI engine could easily adapt to various fuel blends, HCCI could actually promote the greater use and acceptance of renewable fuels such as E-85 and biodiesel!

The parallel development of HCCI engines using diesel/biodiesel fuel may produce diesel/hybrid vehicles with even greater fuel efficiency while producing virtually no NOx.

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