Evaluation of Various Rich Combustion Techniques for Diesel Engines Using Modeling

Considering future emission legislation for HD diesel engines it is apparent that it will be probably necessary to employ A/T devices to achieve them. The main problem concerns the simultaneous control of both NOx and particulate emissions at an acceptable fuel penalty. Concerning particulate matter the use of particulate traps is considered to be a proven technology while for NOx emission control; various solutions exist mainly being the use of SCR catalysts or LNT devices. But LNT traps require periodical regeneration, which is accomplished by generating reducing agents i.e. CO and H₂. The present investigation focuses on the regeneration of LNT devices through the engine operating cycle. This can be achieved using two techniques, additional injection of fuel at the exhaust manifold (external measures) or operation at low lambda values in the range of 1.0 or lower (internal measures). In the present work an investigation is conducted concerning the use of internal measures i.e. rich diesel engine combustion spikes to produce the required reducing agents. Namely, there exist the following possibilities for generating a rich diesel combustion spike: intake air throttling or W/G, EGR or late injection of fuel. All techniques are expected to have a negative impact on both engine bsfc and soot emissions. Furthermore the rich combustion spike is expected to result to an increase of exhaust gas temperature for which a limit is set before the turbine for turbocharged HD diesel engines. The investigation is conducted using a simulation model appropriately modified to account for rich diesel combustion. Using the model the impact of the rich combustion spike on engine performance and emissions is examined for each of the aforementioned techniques. Special emphasis is given to the effect of rich combustion on the combustion rate and the formation history of pollutants inside the combustion chamber. Furthermore results are produced concerning the distribution of temperature, equivalence ratio and species concentration inside the fuel jet and their evolution with engine crank angle. This information reveals the actual effect of each rich combustion technology on the combustion and pollutants formation mechanism. From the comparative evaluation, derived results can be utilized to select the most appropriate technology for rich diesel combustion and estimate the maximum allowable reduction for lambda in order to avoid excessive soot and exhaust gas temperature values.