Influence of Diesel Fuel Cetane Number And Aromatic Content On Engine Exhaust Emissions
Authors: Li X. , Gulder O.L. 
Published: March 2010

Exhaust emissions from a single cylinder heavy-duty diesel engine were measured to assess the effects of fuel properties on engine emissions. The AVL 8-mode steady-state simulation of the EPA transient test procedure was used in the engine tests. The results show that an increase in cetane number of the fuel reduced nitrogen oxide emissions (NOx), and possibly carbon monoxide emissions (CO). However, increasing the cetane number of the fuel caused an increase in particulate matter emissions (PM). The results also suggest that the fuel with lower total aromatic content produced slightly lower NOx and noticeably lower PM emissions. For fuels having comparable total aromatic contents, the one with a higher tri+-aromatic content produced slightly higher NOx emissions and noticeably higher PM emissions. However, the number of fuels in the fuel matrix and the fuel matrix design were not satisfactory to claim universality for the observed relationship between aromatics and engine exhaust emissions.

Introduction

It is generally agreed that exhaust emissions are affected by changes in fuel properties. However, the bulk of the emissions are dependent on the characteristics of the engine and its ability to utilize the fuel over the test cycle. Overall, the changes in emissions due to the independent changes in fuel properties appear small relative to the mean emission levels, although these changes are considered significant to achieve future emission standards. Most studies assume a priori causative relationships between primary fuel variables and certain emissions. These assumptions have not been clearly confirmed yet. The vast literature on the relationship between diesel fuel variables and exhaust emissions is full of contradicting results and claims.

In this study, the main objective is to understand the influence of fuel properties on engine emissions. The fuel properties that are under consideration at this stage of the research program are the aromatic content and the cetane number. Ultimate questions that we aim to answer are:Is the oil sand derived diesel fuel different from the conventional crude oil derived diesel fuel as far as the exhaust emissions are concerned?Do polynuclear aromatics carry the same or more weight relative to mono- and di-aromatics in terms of their contribution to emissions?

Experimental

Test Engine

The test engine used in this program was a single-cylinder Ricardo Proteus research engine. The engine was a direct injection type and had a displacement volume of 2 litres. Major engine configuration data are shown in Table 1. The performance and exhaust emission levels measured over the load and speed range were representative of current premium heavy-duty diesel engines.

The speed and the load of the test engine were controlled independently by a dynamometer and a fuel control system. The fuel injection pump governing system was disabled to allow fixed fuelling levels. Static fuel injection timing can be adjusted 30 crank angle degrees either side of the top dead centre (TDC). To simulate a turbocharger, externally compressed air, controllable for temperature and pressure, was supplied to the engine.