Catalytic AfterTreatment Systems (ATS) were designed by Dinex Ecocat for three development engines operating partially or fully on natural gas (NG). The engines were operating stoichiometric (FPT), lean (Ricardo) and Dual-fuel (Diesel-NG) modes (MAN/Idiada/TNO).
The ATS designs were based on the pre-development engine and calibration data and common design knowledge, because the engine development in many HD GAS applications was still in progress. The stoichiometric application (FPT) was best known, because these vehicles are also in commercial use. There are a lot of calibration questions in lean NG and dual-fuel applications, which might create a feedback causing later changes on ATS.
Methane (CH4) emission control is the main challenge related to NG fueled engines, even if the NG is environmentally friendly fuel with lower CO2, particulate and heavy HC emissions, in comparison to diesel. Three-way catalysts (TWCs) are applied for carbon monoxide (CO), hydrocarbon (THC, including CH4) and nitrogen oxides (NOx) removal with stoichiometric engines and Methane Oxidation Catalyst (MOC) for methane (and THC) oxidation in lean exhaust gases. SCR catalysts are applied for NOx control in lean exhaust gases. TNO dual-fuel durability studies with MOC gave feedback for design in relation of thermal and sulfur deactivation and regeneration.
Due to the high temperature nature of the lean NG combustion, compared to Diesel operation, a hybrid SCR system has been specified based on iron (Fe) and copper (Cu) catalysts. The hybrid system will consist of an upfront and a downstream SCR catalyst. It is assumed that a diesel particulate filter (DPF) is required only for NG-diesel dual-fuel applications. The DPF is also usually oxidation catalyst coated to promote passive and active DPF regeneration.
The catalysts for methane control cause a high increase on ATS costs due to large TWC and MOC (~1x engine size) and high noble metal loadings (≥ 5 g/dm3), where palladium is the main active metal to activate methane reactions. The high noble metal (palladium (Pd), rhodium (Rh), platinum (Pt)) amount in TWC or MOC will cost over 1000 € for a heavy-duty truck. The total catalyst volume is about 1 x engine volume for stoichiometric and 4-6 x engine volume for dual-fuel applications, even if a partial integration of functions is possible (Fig. 1 and 2). Stoichiometric combustion results to a smaller TWC design with stoichiometric λ control but the fuel economy is clearly better in lean combustion. The catalytic units will be integrated into muffler-silencers in the final truck design. Therefore, the engine-out methane emission control, integration of catalytic units and possible thermal management are important to keep the ATS size and cost together with fuel economy reasonable.
View the report: Exhaust system design by DINEX (Public Report)