1. Field of the Invention
The present invention relates to combustion control processes for reducing harmful emissions produced in conventional diesel internal combustion engines.
2. Background of the Invention
The continuing use of diesel engines in motor vehicles greatly adds to the atmospheric presence of harmful pollutants such as nitrogen oxides (NOx) and particulate matter (PM). Conventional diesel engines emit NOx and/or PM substantially in excess of acceptable environmental levels. Nevertheless, because of their fuel efficiency, diesel engines remain preferable to gasoline engines for many applications. Attempts to reduce NOx and PM emissions from diesel engines have therefore continued for many years.
For reasons that will be discussed hereafter, the prior art has never been successful in developing a robust diesel combustion system (i.e. providing commercially acceptable responsiveness and horsepower across diverse speed and load ranges) that is capable of maintaining engine-out emissions levels of both NOx and PM simultaneously below environmentally acceptable levels. For purposes of this invention, environmentally acceptable levels of diesel NOx emissions will be defined as 0.2 g/bhp-hr or lower, and environmentally acceptable levels of diesel PM emissions will be defined as a level that can effectively be reduced to 0.01 g/bhp-hr or lower through exhaust aftertreatment.
The challenge of trying to maintain diesel engine emission levels of both NOx and PM simultaneously below environmentally acceptable levels has been a long-standing unsolved environmental and industry problem that has never been overcome despite extensive, concerted efforts by government and industry worldwide.
With the problem of simultaneously satisfactory engine-out NOx and PM reductions unsolved by the prior art for diesel engines, the diesel industry has instead turned primarily to development of NOx and PM aftertreatments (i.e. post-engine, but before the exhaust gas is released to the atmosphere) to meet current and upcoming international PM and NOx environmental regulations. However, currently many in the diesel industry view such aftertreatment systems as expensive, as retaining issues of effectiveness and durability, and as creating size (“packaging”) concerns. As an example, an ongoing challenge for NOx traps is maintaining NOx emissions always below desired levels on vehicles with diverse duty cycles, and the effectiveness and durability of NOx adsorbers may also be undermined by factors such as high temperatures or by sulfate adsorption and desulfization due to sulfur content in diesel fuel. As a second example, to effectively maintain PM levels within environmentally acceptable emission levels, PM filter traps depend on engine-out PM emissions not being too high. In addition, PM filter traps must be regenerated (i.e. cleaned, such as by combusting the particulate matter trapped therein), with the frequency of such renewal dependent upon the amount of engine-out PM the trap is forced to catch and retain.
Because of the above-mentioned perceived problems with the expense and performance of NOx aftertreatments in particular, it is desirable to develop an alternative, namely, a commercially acceptable diesel combustion system that can meet environmentally acceptable levels of NOx emissions (i.e. 0.2 g/bhp-hr or lower) without the need for use of NOx aftertreatments, and thereby avoid such perceived additional costs, performance, durability and packaging issues that accompany use of NOx aftertreatments.
In addition, because of the above-mentioned perceived problems with inconvenience and expense in regenerating PM traps, the frequency of which is dependent upon the amount by which engine-out PM emissions exceed environmentally acceptable levels (i.e. exceed 0.01 g/bhp-hr), it is also desirable to simultaneously at least maintain engine-out PM emissions low enough for PM traps to efficiently and cost-effectively be able to keep PM emissions within environmentally acceptable levels. For purposes of this application, and for ease of measuring, a smoke content level at or below a maximum of 3 BSN (Bosch Smoke Number) engine-out (i.e. before aftertreatment), preferably with an average smoke content level below 1.5 BSN, would be considered indicative of an acceptable PM engine-out exhaust level for PM aftertreatment to efficiently and repeatedly keep PM emissions within environmentally acceptable levels.