Search Results

The proportion of nitrogen dioxide in the engine-out emissions of a Diesel engine is of great importance for the conversion of the total oxides of nitrogen (NOX) emissions in SCR catalysts. Particularly at lower engine loads and lower exhaust temperatures an increase of the already low NO2/NOX fraction will enhance the SCR operation significantly. For this purpose, the understanding of the NO2 formation during the Diesel combustion and expansion stroke is as substantial as being aware of the different thermodynamic impacts and engine operating parameters that affect the formation process. To determine the influences on the NO2 emission level several variation series were performed on a single-cylinder research engine. Especially the charge dilution parameters like the air-fuel ratio and the EGR rate as well as the injection parameters could be identified to be decisive for the NO2 formation.

Given the fact that, in an endeavor to achieve the goals of engineering for a trade-off between cleaning up exhaust emissions and maximizing fuel economy, two main paths are being followed in advancing and optimizing SI-engine operating strategy in the upper part-load range. On the one hand, homogenization and operation in the compression ignition mode seem to offer a promising means of minimizing NOx emission by keeping the combustion temperature below the formation borderline and accepting a high cylinder-pressure gradient to obtain benefits in fuel economy. On the other hand, there are ambitions to widen the range of stratified operation using a supercharger or turbocharger. This way, efficiency of the engine cycle can be improved by operating at a higher global air-fuel ratio and, with this, a higher polytropic exponent, thereby taking the efficiency chain to a higher level.

To this day, Diesel engines with direct injection are the most efficient internal combustion engines for passenger cars. The major challenge of these engines with a conventional Diesel combustion process is the high level of particulate matter and nitrogen oxide emissions. Diesel engines in passenger cars normally use a pilot injection strategy for NVH reasons, which influences the engine-out soot emissions negatively. The Diesel fuel of the pilot injection is still burning when the main injection takes place, so, liquid components of the main injection interact with the flame of the pilot injection. The time for mixture formation decreases and the combustion takes place under locally very rich conditions which results in high levels of soot formation. For this reason new emission level restrictions cannot be reached without modern exhaust gas aftertreatment systems, which are quite expensive and can have an impact on the gas exchange.

Engines with gasoline direct injection promise an increase in efficiency mainly due to the overall lean mixture and reduced pumping losses at part load. But the near stoichiometric combustion of the stratified mixture with high combustion temperature leads to high NOx emissions. The need for expensive lean NOx catalysts in combination with complex operation strategies may reduce the advantages in efficiency significantly. The Bowl-Prechamber-Ignition (BPI) concept with flame jet ignition was developed to ignite premixed lean mixtures in DISI engines. The mainly homogeneous lean mixture leads to low combustion temperatures and subsequently to low NOx emissions. By additional EGR a further reduction of the combustion temperature is achievable. The BPI concept is realized by a prechamber spark plug and a piston bowl. The main feature of the concept is its dual injection strategy.