Abstract
The scope of the study is the analysis of the influence of in-cylinder gas density and injection pressure on combustion process and pollutant emissions, focusing on the NOx and smoke emissions, as these are the most restrictive ones in upcoming regulations.
Bearing this objective in mind, a parametric study was carried out in a single cylinder direct-injection Diesel engine with 1.8 litre displacement and equipped with a common rail injection system. This engine is representative of current European heavy-duty turbocharged six-cylinder diesel engines employed for road transportation.
Tests were performed at three engine speed values, which are significant in present and future emission homologation tests. In order to modify in-cylinder gas density, intake pressure was changed, while keeping intake temperature constant. Thus, a density range at the end of compression between 28 and 46 kg/m3 was explored at every engine speed.
Besides, fuel/air ratio was kept constant during the tests by adjusting the injected fuel mass according to the induced air mass. For that purpose, two different injection strategies were dealt with:
- In the first one total fuel mass was varied by increasing or decreasing injection duration at constant injection pressure.
- For the second one, injection duration was kept constant, and injection pressure was adjusted to deliver the required fuel mass.
In the analysis of the combustion process, a purpose developed diagnostic code was employed. Such model calculates heat released rate and other combustion parameters, using as input data the records of instantaneous in-cylinder pressure and the thermodynamic properties of the mass inside the cylinder. This information, together with fuel injection rate measurements and performance and pollutant emissions was used to analyse the combustion process.
Results obtained have helped understand the influences of both basic parametric variables on the different combustion stages (delay time, premixed and diffusion stage). Brake specific fuel consumption, NOx and soot exhaust emissions have been analysed according to the information obtained. From those results and for the ranges explored, it seems that injection pressure has a more dominant role than air density on combustion, performance and pollutant emission.