Southwest Research Institute® (SwRI®) News
Printer Friendly VersionSwRI technology reduces diesel emissions by more than 98 percent
Development of the innovative compression ignition engine system continues
San Antonio -- November 24, 1997 -- An innovative compression ignition engine system is being developed at Southwest Research Institute® (SwRI®) that reduces engine-out oxides of nitrogen (NOx) emissions by more than 98 percent. Engineers in the Engine and Vehicle Research Division are working to convert a multi-cylinder diesel engine to work with this system -- known as homogeneous charge compression ignition (HCCI).
"Emissions from engines have been significantly reduced in the past two decades, but more emissions reductions are necessary," says Bill Gray, a research engineer in the Engine Research Department.
Emissions from diesel engines contribute to ground-level ozone formation which can cause respiratory problems. As a result, the Environmental Protection Agency will continue to tighten emissions standards for stationary and mobile diesel engines between 1999 and 2008. These pending emission standards will require a greater than 50 percent reduction in NOx emissions from today's controlled levels.
"In HCCI combustion, the air and fuel are mixed so that each droplet of fuel is surrounded by more than twice the amount of air necessary for combustion," Gray said. "As this mixture is compressed in the cylinder, the air temperature rises until it spontaneously ignites the fuel droplets." Combustion is then initiated at many locations in the evenly distributed mixture. This combustion takes place with an over abundance of fresh air and occurs at a much lower temperature. Consequently, NOx emissions are reduced by 98 percent.
In spark ignition engines, a spark plug ignites a mixture of fuel and air, creating high temperatures and resulting in high NOx emissions. In conventional diesel engines, air is drawn into the cylinder and compressed. The start of combustion is controlled by the injection of fuel into the hot, compressed air. This system creates a flame pattern that produces two zones -- a high-temperature combustion zone that yields NOx emissions and a fuel-rich zone with particulate emissions.
"HCCI is a definite milestone in emissions reduction in internal combustion engines," says Gray. "It would take approximately 50 HCCI engines to produce the same amount of NOx emissions as one conventional diesel engine operating at the same power levels."
SwRI engineers have overcome the difficulty of preparing a mixture that self-ignites at the right time. If ignition occurs too early during compression, the combustion attempts to drive the piston against the engine's direction of rotation. If ignition occurs too late, heat escapes through the exhaust before it becomes beneficial to the piston. SwRI engineers are able to prepare the mixture so that ignition occurs at the correct time. It was originally anticipated that this combustion system was only suitable for stationary engines that operate at constant speed. However, SwRI engineers have achieved flexibility of control with the HCCI system, allowing it to operate over a wide range of operating conditions such as those found in a car or truck. The Institute's internal research will continue to address issues of configuration and operating characteristics such as start of reaction timing and intake air temperature.
"We hope to implement HCCI technology on a prototype
engine within a year," Gray said.
For more information about homogeneous charge compression, contact Deborah Deffenbaugh, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-2046, Fax (210) 522-3547.
