Insights on strategies for particle emissions control in HD SI Natural Gas engines in view of EURO VII limits

Engines fed by gaseous fuels, such as Natural Gas (NG), are proving to be an alternative to the traditional gasoline and Diesel engines. Considering the more and more restrictive regulations on pollutant emissions and the global direction towards decarbonization, Natural Gas could represent a midterm alternative solution. Methane is the primary component of NG and it has the highest H/C ratio among the hydrocarbons, providing lower carbon based emissions. In Heavy Duty sector, historically represented by Diesel vehicles, NG engines show comparable performance and benefits in terms of emissions [1]. The upcoming EURO VII regulation for gas engines is pointing the attention to the sub- 23 nm solid Particle Number (PN) emissions which can represent a critical aspect, even for this kind of engine [2]. The authors evaluated the potentialities offered by three different strategies in the abatement of PN emissions, mainly focusing on sub-23 nm particles control. An extensive experimental campaign has been carried out on a test bench Euro VI NG HD Spark Ignition engine. It is recognized that the main source of PN emissions from gaseous fuels engine is linked to the lube oil consumption [3] and its introduction into the combustion chamber. In this sense, the effect of improved rings pack design was studied, comparing two configurations of the same engine characterized by different oil rings pack. The upgrading of the rings pack reduces the oil deposits in the crevices and in the ring seats thus lowering the oil access to the combustion chamber. The results demonstrate a significant abatement (95%) of engine out PN emissions with the new designed rings pack. Since oil consumption is also influenced by the oil quality [4], the impact of specific lubricant properties has been studied. Typical automotive lubricants consist of base oil (roughly 75-90% by mass) and an additive package; the ash content is the incombustible component of the oil. The effects of different base oils and ash contents on PN emissions have been evaluated. The experimental tests have been performed on the engine with upgraded rings pack. The results show that the improvement of oil quality determines a further PN reduction up to 70%. Differently from the previous described approaches, the last strategy is not headed to the control of particle formation processes, but towards the abatement of PN at the exhaust through the addition of a CNG Particulate Filter (CPF) coupled with a Three Way Catalyst (TWC). Such technology is derived from the well consolidated DPF for Diesel engines. The potentiality of the filter has been studied when applied to a NG engine, characterized by different working conditions and significative low level of emitted particle. The study also provides insights future optimization of the system. The CPF has been tested on the engine with improved rings pack. The abatement efficiency has been calculated through a comparison of the PN emitted with and without the CPF. Although the CPF technology is still under development and optimization, the results are very encouraging and show its high potential to achieve an important PN decrease from a NG engine of roughly 85% A recap of the strategies tested and the achieved results is presented in Figure 1. Fig. 1. Technologies and abatement efficiencies on particle emissions The analyzed methods can lead to a proper sub-23 particles control toward the future regulation limits.