1. Introduction
The European Union’s (EU) clean air policy is based on three main pillars: (i) ambient air quality standards set out in the ambient air quality Directives [
1]; (ii) national emission reduction targets established in the National Emission Ceilings (NEC) Directive [
2]; (iii) emission and energy efficiency standards. The annual limit value for nitrogen dioxide (NO
2) continues to be widely exceeded across Europe, even if concentration and exposure are decreasing. In 2016, 88% of concentrations exceeding this limit value were observed at traffic stations [
3]. The transport sector was the largest contributor accounting for 39% of total NO
2 emissions in the EU in 2016 [
3].
In Europe the NO
x emissions from vehicles are regulated through Euro standards: the Euro 5 limit for diesel light-duty vehicles, which was introduced in 2009, was 180 mg/km and the Euro 6, which was introduced in 2014, is 80 mg/km. The standards have to be fulfilled in the laboratory following a prescribed procedure [
4]. However, it was found that the real driving emissions are much higher than the limits [
5]. On average, Euro 5 vehicles were 4.1 times higher than the Euro 5 limit and Euro 6 vehicles were 4.5 times the Euro 6 limit [
6]. Recently a Real-Driving Emissions (RDE) procedure was introduced in the regulation for both type approval of vehicles and their In-Service Conformity (ISC) [
7]. However, a study estimated that there are still around 29 million high emitting (defined as >3 times higher than the type approval limit) Euro 5 and Euro 6 diesel passenger cars and vans on the European roads, which corresponds to about 76% of all diesel vehicles registered over the 5 years assessed (2011–2015) [
8]. A study estimated that diesel Euro 6 vehicles may contribute 49–83% of NO
x emissions from road transport in 2050 [
9]. If diesel vehicles respected their type approval limit also on the road, the impact of excess NO
x emissions could be at least halved [
10]. Another study estimated that if the on-road emissions will respect the type approval with a conformity factor of 1.5, the fraction of traffic-influenced German stations exceeding the air quality limit for annual mean NO
2 could be reduced to 8% in 2025 and 1% in 2030 from about 50% in 2015 [
11].
The European Commission launched two prizes funded by the EU’s research program Horizon 2020 to identify breakthrough ideas that could drastically reduce air pollution caused by transport. The Horizon prize for the “Engine Retrofit for Clean Air” (2016–2018) [
12] (award 1.5 million Euros) aimed at reducing the pollution produced by the existing passenger cars fleet by spurring the development of retrofit-able technology (i.e., additional devices and/or modification) applicable to diesel engines. The focus was on NO
x emissions of Euro 5 light-duty vehicles under real driving conditions, but other pollutants, such as particles, N
2O and ammonia (NH
3) were also considered. Additionally, vehicle fuel efficiency, retrofitting costs, durability, maintenance, usability, safety, drivability, and noise were taken into account in the prize criteria. The retrofit technology had to be installable on a mass production Euro 5b C-class compact car in the top C-class sales, but limited to high-volume hatchback and three volumes family car bodies. The vehicle should retain most of its payload carrying capability, but the retrofit was allowed to reduce boot volume by 20 litres. The most demanding objectives were absolute NO
x emissions below 180 mg/km, less than 10% fuel consumption increase, and maximum retrofit and consumables costs for 100,000 km of <2000 Euros.
On 16 April 2018 the European Commission awarded the Horizon Prize on “Engine Retrofit for Clean Air” at a ceremony during the Transport Research Arena (TRA) in Vienna, Austria. The 1.5 million Euro prize was awarded to the winning consortia consisting of lead company Amminex Emissions Technology, supported by the Technical University of Graz, Johnson Matthey, and International Council on Clean Transportation (ICCT) Europe. The retrofit consisted of an under-floor SCR (Selective Catalytic Reduction) for NO
x catalyst in combination with a solid ammonia based dosing system as the NO
x reductant. The technology demonstrated that effectively retrofitting diesel vehicles is feasible. The NO
x emissions of a Euro 5b retrofitted vehicle were reduced by 350–1100 mg/km (60–85%) depending on the test cycle and engine conditions (cold or hot start), resulting in NO
x emissions of the retrofitted Euro 5b vehicle around 150 mg/km [
13]. Thus, the final NO
x emissions of the retrofitted vehicle were lower than many Euro 6 vehicles on the market.
During the evaluation of the retrofit with the Euro 5b vehicle, a retrofitted Euro 6b vehicle was also provided. The objective of this paper is to present the NOx reductions of the retrofit technology on a relatively lower emitting vehicle equipped with a Lean NOx Trap (LNT) and to compare the reduction efficiency to the Euro 5b results.
2. Experiments
2.1. Vehicle
The vehicle was a Euro 6b certified Renault Megane 1.5 dCi 110, year 2015 Sport Tourer (station wagon) 1461 cm3, 81 kW, 40,000 km, 1331 kg (empty without the retrofit), with winter tires. The original exhaust configuration consisted of Exhaust Gas Recirculation (EGR), Diesel Oxidation Catalyst (DOC), Lean NOx Trap (LNT), and Diesel Particulate Filter (DPF).
2.2. Retrofit
The retrofit, BlueFitTM comprises an Ammonia Storage and Delivery System (ASDSTM), mounted in the spare wheel well, and a commercially available underfloor Cu-Zeolite SCR catalyst with Pt-containing ammonia slip coating, installed downstream of the DPF.
The ASDS
TM prototype for passenger cars consisted of two 1.2 litres AdAmmine
TM cartridges, where ammonia is absorbed in strontium chloride salt in a solid form [
14] (the commercial unit will be 4.5 litres), a start-up unit, which also contains AdAmmine
TM, but in a much smaller volume (0.5 litre) to enable fast dosing (for cold start), the dosing unit, which provides dynamic dosing of ammonia, and finally a controller with software. The dosing strategy is based on NO
x measurements upstream and downstream of the SCR (with NO
x sensors), the exhaust mass flow (via the On-Board Diagnostics (OBD) port), and the measured exhaust temperature upstream of the SCR.
In order to release ammonia, cartridges are equipped with electric heaters. During the engine cold start, most of the electrical power goes to the start-up unit to warm it up fast and enable ammonia dosing shortly after the engine start. The remaining power is then directed to one of the main cartridges. The system is ready to dose when the start-up unit has reached the target desorption pressure.
At engine start the exhaust gas temperature and therefore the SCR temperature in under-floor position is low and the light-off of the SCR catalyst becomes a limiting factor even though the start-up unit is already ready to dose. As the exhaust gas temperature increases, and the NO
x sensor upstream of the SCR catalyst reaches the light-off temperature of the catalyst (140 °C), ammonia dosing commences and the NO
x emissions decrease [
15]. For DPF equipped vehicles as the exhaust gas temperature increases, the NO to NO
2 conversion increases at the DOC in order to assist the passive DPF regeneration with NO
2 at lower temperatures. At these exhaust gas temperatures the reduction efficiency of the SCR is optimal including the effect of the fast SCR reaction involving NO and NO
2 resulting in very low NO
2 emissions. The dosing strategy of the specific retrofit does not take into account the stored NH
3 with storage model; it uses the NO
x concentration measured by the NO
x sensors upstream and downstream of the SCR to adjust the dosing. Overdosing (over-release) of NH
3 that cannot be stored can result in excess NH
3 emissions. At high speeds the catalyst reaches temperatures that can maximize the production of N
2O from NO
2 or NH
3.
The net system’s weight (without AdAmmineTM cartridges) was 10 kg. Each cartridge (main unit) weighted 3.1 kg, whereas the start-up unit 1 kg. The retrofit had a default calibration (i.e., it was not optimized for the specific vehicle and the Prize rules, as the one retrofitted in the Euro 5b vehicle). The implications will be discussed when the NOx reductions on the two vehicles will be compared.
2.3. Chassis Dynamometer Tests
The vehicle was tested at the Vehicle Emission Laboratory (VELA 2) of the European Commission Joint Research Centre (JRC), in Ispra, Italy. The test cycles were the New European Driving Cycle (NEDC), the Worldwide Harmonized Light vehicles Test Cycle (WLTC) and the Common Artemis Driving Cycle (CADC) with engine cold or hot start. The climatic test cell temperature was kept at 23–25 °C or 7 °C with relative humidity of 50%. The chassis dynamometer parameters were selected according to the rules of the prize based on the UNECE (United Nations Economic Commission for Europe) Regulation 83 [
16] roller dynamometer coefficients using the empty weight of the donor (unmodified) vehicle plus 100 kg (driver and fuel), plus the retrofit weight (for the post retrofit tests): 1470 kg, a = 7.4 N, b = 0.0502 N/(km/h)
2. The temperature range 23–25 °C was selected because it is defined in the regulation, while the 7 °C was selected in order to challenge the retrofit, as at low ambient temperatures the engine out emissions are higher and it takes longer for the retrofit to reach its optimum temperature.
It was decided to cover as many test cycles and engine conditions as possible, rather than focusing on few cases and determining with higher accuracy the performance of the retrofit. For this reason one or two repetitions were conducted. The NEDC is smoother but challenging for the aftertreatment systems to heat up, the WLTC and the CADC are considered realistic driving. In order to avoid any influence of the precedent (or pre-conditioning) cycle on the results (especially for the Euro 6 which has a LNT), only repetitions that were following identical procedures were compared. Based on this and the previous campaign with the Euro 5b vehicle, the repeatability is around 3% for CO2 and 10% for NOx.
Measurements of CO
2, CO (both with Non-Dispersive Infrared Detectors, NDIR), NO
x (Chemi-Luminiscence Detector, CLD) and total HC (Flame Ionization Detector, FID) were taken from the tailpipe, and from the diluted gas in the full dilution tunnel in real time. Measurements from bags that were filled during the test, as described in the regulation, were also taken for every test. The sample collected in the bags was analysed after each test. Although the accuracy is better than the real time measurements (one single measurement), there is no possibility to analyse the second by second behaviour of the pollutants. The gas analysers were the MEXA 7000 series from Horiba (Kyoto, Japan). The solid particle number system connected at the full dilution tunnel was an AVL (Graz, Austria) Particle Counter APC 489 [
17]. Non-regulated pollutants, including ammonia (NH
3) and nitrous oxide (N
2O) were measured with a Fourier-Transform Infrared Spectroscopy (FTIR) instrument (Sesam i60 from AVL) connected to the vehicle tailpipe, using a heated polytetrafluoroethylene sampling line (191 °C). The fuel consumption was calculated from the CO
2, CO and HC measurements as described in the regulation (carbon balance method) [
1]. The contribution of CO and HC is negligible due to their low concentration and consequently the fuel consumption can be assumed proportional to the CO
2 emissions.
More details of the experimental campaign and the test cycles and the quality assurance can be found elsewhere [
13].
2.4. On-Road Tests
The Portable Emissions Measurement System (PEMS) that was used to measure CO
2, CO and NO
x (with two separate chemical cells for NO and NO
2) during the on-road tests was the Ecostar from Sensors (MI, USA). The first route, which complied with the trip requirements defined in the RDE [
1] (
Table 1), was carried out in the morning starting with cold engine. The second route, which was carried out after a 2 hours break, was not RDE compliant, but focused on urban conditions and high altitude (1100 m) (positive altitude gain 1800 m per 100 km) (
Table 1). The vehicle’s battery was left to recharge before each test.
The car was parked indoors at a temperature of 16 °C due to convenience reasons (access to calibration cylinders, power supply). The ambient temperature during the tests was 0–10 °C and the relative humidity 65–95%. The tests were intentionally conducted in winter time in order to challenge the retrofit with low ambient temperatures under real driving. The weight of the car with the instruments, the driver and the co-driver was 1650 kg.
5. Conclusions
In this study we evaluated a retrofit technology installed on a Euro 6b diesel vehicle. The retrofit included a solid ammonia storage system that delivered gaseous ammonia to a SCR catalyst based on measurements of NOx upstream and downstream of the catalyst, exhaust gas temperature and exhaust mass flow. The tests included on-road tests and various cycles on a chassis dynamometer with cold and hot engine start and ambient temperatures of 7 °C or 23 °C.
The retrofit reduced the NOx emissions of all cycles tested from, on average, from 570 mg/km to 200 mg/km. The on-road performance was even better: the reduction was from >1100 mg/km to 220–240 mg/km. The reductions at low ambient temperatures with cold start were negligible. For an on-road test it took approximately 10 min to reach appropriate exhaust gas temperature. There was no detection of ammonia slip or formation of particles. The N2O increased on average 7 mg/km, mainly at the hot cycles. The increase of the CO2 was negligible.
A similar prototype retrofit system installed on a Euro 5b vehicle achieved better NOx reduction, but higher N2O emissions. The differences were attributed to the more aggressive NH3 release strategy, and the higher exhaust gas temperature at the retrofit due to the insulation of the Euro 5b vehicle tailpipe. Dealing with the dynamics of the NOx entering the SCR when there is an already-existing LNT is also more challenging.
The results of this study confirm that retrofitting is an option to decrease NOx emissions of Euro 6 diesel vehicles to approximately 200 mg/km without vehicle specific calibration, and without significant increase of the fuel consumption and other pollutants such as particles, NH3 and N2O. Further improvements could be achieved by improving the cold start performance (e.g., active heating of the catalyst or engine out emissions re-calibration). It is necessary to confirm the performance, durability and cost of the commercial retrofit with more vehicles and technologies.