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This paper presents an experimental study of a water injection (WI) application where water fog is added in the intake of a common rail High-Speed Direct Injection (HSDI) automobile Diesel engine in order to reduce pollutant emissions Nitrogen Oxides and Particulate Matter (NOx and PM) for future emissions standards. Also studied are the physical parameters of the engine (in-cylinder pressure, air inlet temperature, air mass flow, specific fuel consumption etc). The results are compared with those obtained with low-pressure dry Exhaust Gas Recirculation (LP EGR) on the same engine. Tests performed with the water injection system show that a much better NOx / PM trade-off (reduced NOx emission levels at constant PM emission levels) is obtained than with EGR especially at points of high engine loads. In addition, tests are performed with EGR in parallel with water injection to investigate the reduction of NOx emissions while potentially reducing water consumption.

Pollutant emissions standards (such like EURO 6 in Europe) are increasingly severe and force a search of new in-cylinder strategies and/or aftertreatment devices / schemes at a reasonable cost. On a conventional Diesel engine an excess of air is usually used to allow very high combustion efficiencies and reasonable levels of soot which can then be after-treated in a diesel particulates filter (DPF). As a consequence, NOx emissions cannot be easily after-treated (lean NOx trap (LNT) and selective catalytic reduction (SCR) are quite expensive even if effective, solutions), as a result they are generally controlled by means of internal measures such as High Pressure (HP) or Low Pressure (LP) exhaust gas recirculation (EGR). In light of ever more stringent NOx emissions regulations, NOx aftertreatment devices seem to be becoming unavoidable.

Previous experimental studies on Diesel engines have demonstrated the potential of high-pressure exhaust gas recirculation (HP EGR) as an in-cylinder NOx control method. With ever more stringent emissions standards, the use of a low pressure EGR loop (LP EGR) seems to be an interesting method to further reduce NOx emissions while maintaining PM emissions at a low level. Actually, contrary to HP EGR, the gas flow through the turbine is unchanged while varying the EGR rate. Thus, by closing the variable geometry turbine (VGT) vanes, higher boost pressure can be reached, allowing the use of high rates of supplemental EGR. Some experiments are conducted on a 2.0 l HSDI common-rail DI Diesel engine equipped with HP and LP EGR loops on a test bench under low and part load conditions, as those encountered in the European emissions test cycle for light-duty vehicles.