Abstract
Keywords - Diesel, Emissions, EGR, Boost, Pressure
Abstract - Considering the always stringent emission limits in Europe and the US pollutant emissions and specifically NO and Soot are one of the most important problems that engineers have to face when developing heavy duty DI diesel engines. During the past decade, serious improvement has been accomplished leading to a sharp reduction of emissions from diesel engines and a parallel reduction of engine bsfc. However, despite this improvement, considering future emission limits, we still have a long way to go. The challenge that engineers and engine manufacturers have to face is that reduction of pollutants emissions must be accomplished with the least possible penalty on fuel consumption. Two are the main strategies available today for controlling pollutant emissions. The first is reduction of pollutant formation inside the combustion chamber using advanced combustion and fuel injection technologies and the second is the use of after-treatment systems. The last requires the introduction of additional equipment resulting to a more complex structure. Such systems have been recently considerably improved, yet there is a great deal to do as far as NO emission control is concerned. On the other hand, the first strategy focuses on the reduction of pollutant emissions inside the combustion chamber using advanced combustion techniques. This is the strategy considered in the present investigation for controlling NO and Soot.
For this purpose a simulation model is used to examine the possibility for controlling both pollutants and considering at the same time for the corresponding effect on bsfc. The computational study is conducted on a DI single cylinder heavy-duty research engine. It is examined the use of advanced injection timing to reduce soot emissions and improve bsfc. Consequently, this results to a considerable increase of peak combustion pressure and a sharp increase of NO emissions for all operating cases examined. The increase of NO is extremely high and for this reason it is examined the use of EGR to control it. In the present, various
EGR rates are examined ranging from 0-20% at full load and 0-25% at medium load. As revealed the use of EGR has a negative effect on bsfc and soot emissions. The positive impact of advanced injection timing on soot and bsfc is partially restricted, mainly due to the reduction of AFR when using EGR. To account for this negative effect it is considered the use of increased boost pressure to maintain the overall AFR the same as in the case with zero EGR. This has a positive effect on tailpipe soot emissions that are now less affected by EGR. On the other hand, this has a small negative effect on NO emissions which does not alter the overall affect of EGR upon it. To account properly for the additional power required to generate the increased boost pressure, we have maintained in the simulation the pressure difference between inlet and exhaust at the same level. As revealed the use of advanced injection timing accompanied with EGR and increased boost pressure is a favourable solution for controlling both NO and Soot. There exist areas where we can have a reduction of both pollutants to acceptable levels and at the same time without a fuel penalty. On the contrary, there exist cases where engine bsfc is reduced.