Abstract
Exhaust gas recirculation is one of the technologies that can be used to improve the efficiency of spark-ignition engines. However, apart from fuel consumption reduction, this technology has a significant impact on exhaust gaseous emissions, inducing a significant reduction in nitrogen oxides and an increase in unburned hydrocarbons and carbon monoxide, which can affect operation of the aftertreatment system. In order to evaluate these effects, data extracted from design of experiments done on a multi-cylinder 1.3 L turbocharged spark-ignition engine with variable valve timing and low-pressure exhaust gas recirculation (EGR) are used. The test campaign covers the area of interest for the engine to be used in new-generation hybrid electric platforms. In general, external EGR provides an approximately linear decrease of nitrogen oxides and deterioration of unburned hydrocarbon emissions due to thermal and flame quenching effects. At low load, the impact on emissions is directly linked to actuation of the variable valve timing system due to the interaction of EGR with internal residuals. For the same external EGR rate, running with high valve overlap increases the amount of internal residuals trapped inside the cylinder, slowing down combustion and increasing Unburnt hydrocarbon (HC) emissions. However, low valve overlap (i.e., low internal residuals) operation implies a decrease in oxygen concentration in the exhaust line for the same air–fuel ratio inside the cylinders. At high load, interaction with the variable valve timing system is reduced, and general trends of HC increase and of oxygen and carbon monoxide decrease appear as EGR is introduced. Finally, a simple stoichiometric model evaluates the potential performance of a catalyst targeted for EGR operation. The results highlight that the decrease of nitrogen oxides and oxygen availability together with the increase of unburned hydrocarbons results in a huge reduction of the margin in oxygen availability to achieve a complete oxidation from a theoretical perspective. This implies the need to rely on the oxygen storage capability of the catalyst or the possibility to control at slightly lean conditions, taking advantage of the nitrogen oxide reduction at engine-out with EGR.
Highlights
The great majority of research regarding internal combustion engines over the last few decades has been focused on efficiency increase and pollutant emission reduction as a consequence of the progressively more stringent regulations and the need to address the problem of climate change [1]
The results showed the greater potential of exhaust gas recirculation (EGR) compared to retarded ignition timing on reducing harmful NOx and HC emissions
In order to synthesize these results, two keypoints representative of medium and low engine load operation were selected. The reason for this distinction is the different behaviors in terms of internal residuals: while in the turbocharged area the amount of internal residuals is not very sensitive to the variation of the valve timings, in throttled operation, the valve timing system (VVT) settings induce significant variation of the engine trapping ratio
Summary
The great majority of research regarding internal combustion engines over the last few decades has been focused on efficiency increase and pollutant emission reduction as a consequence of the progressively more stringent regulations and the need to address the problem of climate change [1]. In this sense, for the last decades, compression ignition engines (CI) have shown important benefits in terms of fuel consumption compared to spark-ignition (SI), whereas the control of exhaust gas emissions requires more expensive and complex aftertreatment systems [2,3].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
