## 1 Introduction  
Over the last decades, the legislated maximum levels of emissions from engines have been tightened more and more. Worldwide, in 2014, transport as a whole was responsible for 23% of total CO\({}_{2}\) emissions from fuel combustion and road transport was responsible for 20% [1]. The vast majority of heavy-duty vehicles (HDVs) in Europe are powered by diesel engines and, although they account only for 4% of the vehicle fleet, they have been identified as important sources of both pollutant and greenhouse gas (GHG) emissions. Exhaust gases from diesel engines include carbon monoxide(CO), particulate matter (PMx), nitrogen oxides (NOx) and sulfur oxides (SOx) [2]. NOx contributes to serious environmental problems, such as acid rain and photochemical smog [3] and human health problems, especially respiratory disorders [4, 5]. With the aim of reducing this contribution, the EU has adopted the strategy of supporting a mix of technologies, currently at high levels of maturity, depending on the transport modes and the travel range. The main response of the industry to the introduction of more stringent emission standards was the widespread use of selective catalytic reduction (SCR) technology for reducing NOx emissions at the tailpipe combined with diesel particle filters (DPF) for PM reduction and diesel oxidation catalysts (DOC) for the oxidation of incomplete combustion products [6]. The urea-based selective catalytic reduction (urea-SCR) technology, a lean NOx after-treatment technology, has been widely used owing to its impressive NOx reduction efficiency and compliance with newly rigorous specifications [7, 8]. In the SCR technique, a urea-water solution (UWS, 32.5% aqueous solution) is injected into hot exhaust gas using different strategies according to the manufacturer of the vehicle. Afterevaporation of water from UWS, the urea decomposes and gives out ammonia which is the reducing agent for the reduction of NOx. Reduction of NOx takes place when ammonia reacts with NOx over catalyst through different catalytic reactions. However, exhaust temperature plays an important role in catalyst performance. It tends to be rather low in real driving cycles, which causes low catalyst efficiency [9]. In [10; 11; 12] authors registered the exhaust temperatures during NEDCs and WLTPs from cold and hot conditions. During the cold NEDC, the inlet temperature was below 130 \({}^{\circ}\)C in the majority of the first 400 s, and temperatures greater than 180 \({}^{\circ}\)C were achieved only in a small portion of the hot driving cycle. This behavior strongly affects the conversion efficiency of SCR devices because a minimum substrate temperature of 180 \({}^{\circ}\)C is required to reach the SCR light-off. As a result, higher activity of SCR catalysts at low temperatures is mandated to effectively reduce real-world NOx emission while fuel economy requirement must be simultaneously met. A lot of effort has also been made, but the main obstacle is that during low-speed urban driving and cold-starts, HDV's engines only generate exhaust gas temperatures below 200 \({}^{\circ}\)C, which is not enough for the conventional urea-SCR based systems to reach the SCR light-off [13]. This issue is of great concern in combustion vehicles and several strategies for solving it are currently under development. These can be divided into: (1) engine control and (2) catalyst control. Engine control strategies are focused on increasing the exhaust gas temperatures as fast as possible. Several ways are currently under study, such us retarding the ignition timing, increasing the idling speed, bypassing the turbocharger for creating a fuel rich mixture, alternating rich and fuel lean conditions, injecting more fuel at starting conditions in dual fuel vehicles, etc. [9; 14; 15; 16; 17]. However, all these engine control techniques tend to strongly increase the fuel consumption and, therefore, reduce the net vehicle efficiency. [PERSON] et al.