A comprehensive study is carried out in order to better understand combustion behavior in a direct injection Diesel engine working under multiple injection strategies, in particular when using post-injections. The aim of the study is to provide criteria to more easily define optimized injection strategies.
During the study two main phenomena have been observed and characterized: an acceleration of the final stage of combustion and an apparent disconnection between the combustions of the two pulses (“split flame”). Thanks to the combustion acceleration phenomenon, if the post-injection is placed near enough the main injection, the end of combustion can take place even earlier compared to the case with a single main injection. In such conditions NOx emissions increase (most likely due to a higher temperature level during the last stage of combustion), but soot and specific fuel consumption decrease (due to a faster last phase of combustion).
The second observed phenomenon is that if the post-injection is not excessively near to the main injection, the two combustions act as two independent combustions (“split flame”), thus allowing an easier interpretation of the main behaviors of the whole combustion process. As an example, the two observed phenomena help to understand why a small post-injection is an effective way to reduce soot emission with no fuel penalty.
The knowledge gained during the study is used to propose some improvements of the combustion process under certain operating conditions.