**1. Introduction**

Conventional diesel combustion exhibits the trade-off relationship between reduction of NOx and soot emissions. The strategies for reducing soot increase NOx; for example increasing injection pressure and swirl ratio reduce soot, but increase NOx in the conventional diesel combustion. High levels of exhaust gas recirculation (EGR) also reduce NOx but increase soot. It is well known that NOx emissions are largely dependent on the equivalence ratios of air-fuel mixture. NOx emissions reach maximum value when combustion occurs near stoichiometric air-fuel ratio. However, it can be lowered when mixture is over-rich or over-lean. Generally speaking, soot emission is largely produced during the diffusion combustion, but it is very low during the premixed combustion. Therefore researchers attempt to mix fuel and air as homogeneous as possible prior to ignition to achieve the premixed combustion (Kimura et al.,1999,2001; Hasegawa & Yanagihara,2003;Takeda et al.,1996; Shimazaki et al.,1999). Studies show that homogeneous charge premixed mixture low-temperature combustion can simultaneously reduce NOx and soot emissions(Nandha & Abraham,2002;Walter & Gatellier,2002; Lewander et al.,2008;Husberg et al.,2005;Lejeune et al.,2004).

The purpose of this investigation is to develop a new low-temperature premixed combustion mode in a six-cylinder commercial vehicle diesel engine using the UMH nozzle and EGR. The UMH nozzle facilitates better mixing of fuel and air prior to ignition and the resultant realization of the premixed combustion, because of its shortened the injection duration and improved atomization compared with the conventional nozzle (Miao et al.,2009). This investigation also explores the combustion characteristics of the UMH nozzle through the experiments of selected operation conditions of 1400 r/min, 0.575MPa and 1000 r/min, 0.279 by adjusting injection timing, injection pressure and EGR rate. The results showed that NOx and soot emissions of the selected operation conditions were simultaneously largely reduced.
