1. Field of the Invention
The present invention relates to a system and a method for controlling exhaust gas particulate emissions from a compression ignition internal combustion engine.
2. Background Art
Internal combustion engines, and in particular, compression ignition (or diesel) engines have a wide variety of applications including passenger vehicles, marine vessels, earth-moving and construction equipment, stationary generators, and on-highway trucks, among others. Diesel particulate trap or filter (DPFs) have been introduced into the exhaust systems for compression ignition internal combustion engines to reduce particulate emissions (PMs) that are emitted by the engine. Improved systems and methods of reducing PMs are desirable for compression ignition internal combustion engines to meet increasingly stringent emissions standards.
A DPF is an emission control device placed in the exhaust stream to reduce PMs, which are primarily carbon particles or soot. Performance of the engine in terms of drivability and fuel economy is related, in part, to pressure drop across the DPF and backpressure generated due to the DPF. As particulates accumulate in the DPF, an increasing restriction to exhaust flow results. Pressure drop across the DPF typically increases as soot accumulates in the DPF. The increasing restriction causes a gradual increase in exhaust (or engine) backpressure (i.e., the pressure within the exhaust upstream of the DPF) causing reduced engine performance. When the DPF is not properly maintained or when engine conditions prevent the DPF from being regenerated, the exhaust backpressure can increase to a point where engine component life can be jeopardized.
Further, exhaust gas recirculation (EGR) systems have been introduced into internal combustion engines. EGR systems recirculate exhaust into the intake air stream of the engine, thereby reducing oxides of nitrogen (NOx) emissions that are formed when temperatures in the combustion chamber of the engine become elevated to a temperature higher than is desirable. Compression ignition engines are often turbocharged. The turbocharger may be implemented as a variable geometry device (VGT, also called variable turbine geometry (VTG)). The VGT has movable turbine vanes (or blades) that pivot to adjust turbocharger generated intake air boost pressure in response to engine speed and load. Cross-sectional changes are made by resetting the turbine blades (e.g., smaller contact surface at low speeds, larger contact surface at high speeds). VTG turbochargers may be particularly efficient at partial load and generally reduce or eliminate “turbo lag” thereby improving drivability. VTG turbochargers can increase effective engine power, increase throttle response and can also have a beneficial effect on particulate emissions.
Compression ignition engines that implement an EGR approach to reducing NOx emissions typically also utilize a VGT to aid in the generation of proper backpressure levels to operate the EGR system. The set of vanes in the VGT closes to generate the backpressure to provide EGR operation. Air mass flow through the engine typically increases non-linearly in response to VGT vane closure. The non-linear air mass flow through the engine can have a “bell-shaped” curve. As such, air mass flow can start to decrease beyond a given level of VGT vane closure because the VGT turbine efficiency starts to decrease and backpressure starts to increase. As a result, soot accumulation in the DPF is accentuated due to the decrease in air flow. A spiraling of black smoke from the engine exhaust and failure of the DPF can result.
Thus, there exists a need and an opportunity for an improved system and an improved method for controlling exhaust gas particulate emissions from a compression ignition internal combustion engine.