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Since the Euro VI/6 regulation came into force in 2013/2014, most of the Diesel applications are equipped with both selective catalytic reduction (SCR) systems and Diesel particulate filters (DPF). On the one hand, SCR requires ammonia for the reduction of nitrogen oxides (NOx) created during the combustion process. An aqueous urea solution (AUS) containing 32.5% wt. urea, such as AdBlue® is injected into the hot exhaust gas upstream of the SCR catalyst to produce ammonia for NOx reduction. On the other hand, DPF demonstrates very high particle filtration efficiency, but requires to be periodically regenerated at high temperature to burn off accumulated soot. The regeneration temperature and duration can be significantly lowered by using fuel additives (fuel-borne catalyst or FBC) or by washcoating a catalyst into the DPF (catalyzed DPF or cDPF).

Diesel Exhaust Fluid (DEF) like Adblue® is a urea/water solution injected upstream from the SCR catalyst. Urea decomposes into ammonia (NH3) which acts as reducing agent in the de-NOx reaction process. However, incomplete decomposition of urea can lead to unwanted deposits formation, thereby resulting into backpressure increase, loss of NOx reduction efficiency, and durability issues. The phenomenon is aggravated at low temperatures and can lead to restriction or stop of DEF injection below certain exhaust temperatures. This paper focuses on the influence of the additivation of DEF on deposits formation in a passenger car close-coupled SCR on filter Diesel exhaust line installed in a laboratory flow bench test. The behavior of two different additivated DEF was compared to Adblue® in terms of deposits formation on the mixer and SCRF canning at different temperatures comprised between 240°C and 165°C, and different air flows.

The more and more stringent regulations on particle emissions at the vehicle tailpipe have led the car manufacturers to adopt suitable emissions control systems, like particulate filters with average filtration efficiency that can exceed 99%, including particulate mass (PM) and number (PN). However, there are still some specific operating conditions that could exhibit noticeable particle number emissions. This paper aims to identify and characterize these persistent sources of PN emissions, based on tests carried out both at the engine test bench and at the chassis dynamometer, and both for Diesel and Gasoline direct injection engines and vehicles. For Diesel engines, highest particle numbers were observed downstream of the catalyzed DPF during some operation conditions like engine warm up or filter regeneration phases. PN could be 50 times higher during the warm up phase and can reach as much as 2000 to 3000 times more during the regeneration phase compared to normal operation.

Euro VI standards for heavy duty vehicles require the use of a DPF in order to comply with the particulate matter emission limit. Although passive regeneration of soot by NO2, promoted by a DOC located upstream the DPF, is preferred, the use of an active regeneration might be required whenever the soot mass trapped in the DPF increases. Some manufacturers made the choice of having a fuel injection in the exhaust system in order to generate an exothermic reaction in the DOC that helps to regenerate the particulate filter. This dedicated circuit avoids the use of in-cylinder post-injection which may induce oil dilution by diesel. The DPF regeneration is efficient and the DOC works durably if the exhaust diesel spray is completely vaporized before entering the DOC and is thoroughly mixed with the exhaust gases. However, ensuring complete evaporation and an optimum mixture distribution in the exhaust line is challenging.

The selective catalytic reduction (SCR) based on urea water solution (UWS) is an effective way to reduce nitrogen oxides (NOx) emitted by engines. The high potential offered by this solution makes it a promising way to meet the future stringent exhaust gas standards (Euro6 and Tier2 Bin5). UWS is injected into the exhaust upstream of an SCR catalyst. The catalyst works efficiently and durably if the spray is completely vaporized and thoroughly mixed with the exhaust gases before entering. Ensuring complete vaporization and optimum mixture distribution in the exhaust line is challenging, especially for compact exhaust lines. Numerous parameters affect the degree of mixing: urea injection pressure and spray angle, internal flow field (fluid dynamics), injector location …. In order to quantify the mixture quality (vaporization, homogeneity) upstream of the SCR catalyst, it is proposed to employ non intrusive optical diagnostics techniques such as laser induced fluorescence (LIF).

Meeting the upcoming NOx emissions standards is a major challenge for the lean-burn engines, thus requiring a highly efficient exhaust gas aftertreatment. Currently, the Selective Catalytic Reduction (SCR) appears to be the most promising technology, especially when operated with two kinds of reductants: ammonia (generally derived from urea) and ethanol. In order to reach high conversion levels while avoiding the overinjection of the reductant, a very accurate model-based control assisted with at least one NOx sensor is required. This study focuses on the sensitivity of NOx sensors to the main nitrogenous species encountered: ammonia, isocyanic acid (HNCO) and hydrogen cyanide (HCN). The cross-sensitivity to ammonia is the only one to be already described in literature and already used in the urea-SCR control systems to limit the risks of ammonia-slip. However, HNCO can also be found downstream of a catalyst during urea-SCR if the urea delivery or the catalyst are deficient.

The challenge for decreasing the emissions of compression ignition engines now remains mainly on NOx control. If the Lean NOx Trap (LNT) and Selective Catalytic Reduction by Urea (Urea-SCR) are very efficient, their extra-cost and management are a major issue for the OEMs. In that context, the selective catalytic reduction by hydrocarbons (HC-SCR) appears to be an interesting alternative solution, with a more limited NOx conversion efficiency but an easier packaging (diesel fuel as a reductant) and a limited price (reasonable coating cost / no PGM). In the framework of the RedNOx project, a prototype catalyst made of 2% silver on Alumina coated on cordierite was manufactured and tested on a synthetic gas bench. In parallel, an exhaust implementation study has been led to ensure the most suited conditions for injection. Thanks to SGB and simulation results, adapted engine tests have been designed and performed.

Lean-burn combustion in SI engines can significantly reduce fuel consumption but NOx reduction becomes challenging because classic three-way catalyst (TWC) is no more efficient. Urea-SCR is then an interesting alternative solution because of its high NOx conversion efficiency without any additional fuel consumption. The coupling between two SI lean-burn engines (stratified and homogeneous combustion) and a urea-SCR catalyst was simulated on the NEDC cycle. Simulation results showed that the SCR efficiency would comply with the limits required by future Euro 5/6 regulations. Associated urea solution consumptions were estimated thanks to a simplified model. Finally, a comparison with a Diesel application was also made. It showed that the required amount of reducing agent remained significantly higher for SI lean-burn engines than for Diesel engine.

A comprehensive experimental approach has been developed for a Fe-ZSM5 micro-porous catalyst, through a collaborative project between IFP, PSA Peugeot-Citroën and the French Environment and Energy Management Agency (ADEME). Tests have first been conducted on a synthetic gas bench and yielded estimated values for the amount of NH3 stored on a catalyst sample. These data have further been compared to those obtained from an engine test bench, in running conditions representative of the entire operating range of the engine. 15 operating points have been chosen, considering the air mass flow and the exhaust temperature, and tested with different NH3/NOx ratios. Steady-state as well as transient conditions have been studied, showing the influence of three main parameters on the reductant storage characteristics: exhaust temperature, NO2/NOx ratio, and air mass flow.

The influence of the type of fuel and the feeding means to a DOC, in order to regenerate a DPF, was investigated. Diesel fuel in cylinder late post-injection was compared to the injection in the exhaust line, through an exhaust port injector, of diesel fuel, B10 (diesel fuel containing 10% of esters) and gasoline. Diesel fuel exhaust injection resulted in a deteriorated conversion efficiency, while the incorporation of esters to the diesel fuel was demonstrated to have no influence. Gasoline exhaust injection led to less HC slip than diesel fuels. Temperature dynamics resulting from injection steps showed taught that the shorter the hydrocarbons (within the tested fuels), the slower the response. These differences can be caught by simple models, leading to interesting opportunities for the model-based control of the DPF inlet temperature during active regenerations.

Within the framework of the French research program PREDIT, a study was undertaken by ADEME, IFP, LGRE, PSA Peugeot Citroën and Umicore, whose main objective was a better understanding of the NOx storage and reduction phenomena on an aged, sulfated and desulfated NOx-trap. The target of this work was to use the information on catalyst working conditions to optimize catalyst management for a gasoline direct injection engine. The catalysts were characterized on both engine and synthetic gas benches. Aging and poisoning phenomena were studied and a variety of different chemical analytical tools were used. The behavior of two different thermally aged cores was investigated under rich conditions on a synthetic gas test bench. The dependence of the NOx regeneration efficiency of the traps is reported for several operating parameters, including reductant concentrations, durations of the rich pulse and trap loadings.