NOx and PM emissions reduction on an automotive HSDI Diesel engine with water-in-diesel emulsion and EGR: An experimental study Alain Maiboom ⇑ , Xavier Tauzia Laboratoire de Mécanique des Fluides, UMR CNRS 6598, Internal Combustion Engine Team, Ecole Centrale de Nantes, BP 92101, 44321 Nantes Cedex 3, France article info Article history: Received 7 June 2010 Received in revised form 9 June 2011 Accepted 10 June 2011 Available online 29 June 2011 Keywords: Automotive Diesel engine Water-in-diesel emulsion Exhaust gas recirculation Combustion Heat release abstract Automotive Diesel engines exhaust emissions must constantly be reduced to comply with more and more stringent regulations, all over the world. The introduction of water in the combustion chamber is already used on some large marine diesel engines to cut down NOx emission. In this paper an experimental study is conducted on a modern automotive 1.5 l HSDI Diesel engine while injecting a water-in-diesel emulsion (WDE) with a volumetric water-to-fuel ratio of 25.6%. Four injection strategies are considered with and without pilot injection, with two levels of injection pressure. First, the injection of WDE is compared to diesel-fuel in terms of combustion and NOx and PM emissions without using exhaust gas recirculation (EGR). Depending on the WDE fuelling rate and injection strategy (with or without a pilot injection before main injection), NOx emissions are most often reduced (of up to 50%), and PM emission are most often decreased as well (the maximum relative reduction being 94%). The combustion is largely affected by the injection of WDE as compared with pure diesel-fuel, the main observations being an increased of the ignition delay and an improved mixing-process between the fuel and the surrounding gases. After that, the use of WDE in parallel with EGR (with various EGR rates) is tested with the aim at improving the NOx–PM trade-off (reduction of NOx emission at a given PM emission level or reduction of PM emission at a given NOx emission level). The results show that this method is an effective way for NOx and PM emission reduction on an automotive Diesel engine. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In light of the current requirements as regards the reduction of pollutant emissions of automotive Diesel engines such like EURO 6 in Europe, manufacturers have to develop new in-cylinder strate- gies and/or aftertreatment devices [1]. With the upcoming pollu- tant regulations, NOx emission will become particularly critical on automotive Diesel engines. As regards the in-cylinder strategies aiming at reducing NOx emission, exhaust gas recirculation (EGR) into the engine intake is the most used and studied technology. The decrease of NOx emission with EGR is the result of complex and sometimes oppo- site phenomena occurring during combustion [2–15]. The main effect is the decrease of local temperatures in the combustion chamber, in particular those corresponding to zones where NO is produced (on the lean side of the diffusion flame during fuel injec- tion [16] and in the combustion products after the end of injec- tion). The main drawback of EGR is the increase of PM emission in the classical high temperature diesel combustion (HTC) and the need to increase boost pressure at middle and high loads when using EGR to maintain the air–fuel ratio (AFR) at a suitable level [5,6]. Another in-cylinder strategy to reduce local temperatures and consequently the NO production rate is the injection of water (WI), either into the engine inlet [17–25], directly in the combus- tion chamber [26–33], or in emulsion with the fuel [18,19,21–23, 34–56]. One advantage of WI as compared with EGR is the possible reduction of NOx emission either at low loads and high loads with- out a substantial increase in PM emission. The probably easiest way to inject water in the engine is inlet WI [17–25]. This technique has been used on some large marine Diesel engines [18] and various strategies to inject water in the inlet air are presented in the literature. The main drawback of inlet WI is that a water mass of about 60–65% of the fuel is needed to achieve a 50% NOx reduction, and is very high (up to four times the amount of fuel mass) if trying to drastically reduce NOx emission [25]. Different strategies have been also proposed to inject water directly into the combustion chamber, with the aim at reducing NOx emission while limiting the water quantity as compared with inlet WI. One advantage of direct WI as compared with water-in- diesel emulsion is the possibility to change the water-to-fuel ratio, while varying engine parameters (speed and load) or during engine warm-up (cold start) [28–30]. 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.06.014 ⇑ Corresponding author. Tel.: +33 2 40 37 68 80; fax: +33 2 40 37 25 56. E-mail address: alain.maiboom@ec-nantes.fr (A. Maiboom). Fuel 90 (2011) 3179–3192 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel