Fuel Properties and Emissions

W. Addy Majewski

Abstract: There is a clear correlation between some fuel properties and engine-out diesel emissions. However, drawing general conclusions is difficult due to such factors as inter-correlation of different fuel properties or different engine technologies. In heavy-duty engines, increasing the cetane number lowers HC, CO, and NOx emissions, while reducing fuel density lowers NOx and PM but increases HC and CO. Light-duty engines show a different fuel sensitivity than heavy-duty engines. In engines with emission aftertreatment, fuel quality has only a minor effect or no effect on tailpipe emissions.

Introduction
Heavy-Duty Diesel Engines
Light-Duty Diesel Engines
Engines with Emission Aftertreatment

Introduction

Historically, fuel properties have changed for various reasons, including crude oil prices, crude oil quality, refinery technologies, the relative demand for diesel and gasoline fuel, and changing engine and emission control technologies. Since the 1990s, environmental considerations and emission legislation have been increasingly more important in the formulation and properties of fuels. The interaction mechanisms between fuel quality, engine technologies, and emissions were investigated extensively to find the most effective approach towards low-emission diesel engines. Example research studies include the European Programme on Emissions, Fuels and Engine Technologies (EPEFE) [229][228][5708], the American Auto/Oil Air Quality Improvement Research Program (AQIRP) [230], and numerous other studies conducted by the oil and fuel industry, engines and components manufacturers, research institutes, and academia. Literature reviews have been published that provide guidance through the vast amount of research devoted to fuel quality effects on engine and vehicle emissions [227][571][5709].

This body of research informed the evolution of fuel quality standards and the adoption of certain diesel fuel specifications—most notably the maximum sulfur content—as mandatory environmental parameters, paving the way for the adoption of emission control technologies on diesel engines—first in-cylinder, such as exhaust gas recirculation (EGR), followed by emission aftertreatment, including diesel particulate filters (DPF) and selective catalytic reduction (SCR) systems for NOx control. As the emission control technology matured, it gradually became the primary driver for diesel emissions. In modern engines with aftertreatment, fuel quality has only a minor effect on tailpipe emissions [5705]. The focus of contemporary fuel research has shifted to engine efficiency and performance, synergies between fuels and combustion systems, and synthetic alternative fuels—as evidenced, for instance, by the US DOE Co-Optima project [5707].

Despite the wealth of experimental data, the effects of fuel properties on engine-out emissions have never been fully understood. Several factors and uncertainties make the interpretation of data challenging:

Inter-correlation of fuel properties—Diesel fuel properties that influence emissions are usually inter-correlated. An example of this is density, aromatics content, and cetane number. Diesel fuel blending streams that contain high levels of aromatics are high in density and also have low cetane numbers. Therefore, to study the effect of a specific fuel property on emissions, care must be taken to decouple the change in this particular property from changes in other properties of the test fuel. If a number of fuel properties are changed simultaneously, it is not possible to ascribe any emission changes to a change in one property.
Fuel additives—Changes in fuel properties are often induced using fuel additives, such as using a cetane improver to increase cetane numbers. It is uncertain whether natural and additive-induced changes in fuel properties have the same effect on emissions.
Engine technology—Diesel engine technology has evolved differently around the world. In the 1990s, heavy-duty engines in the United States had large displacements and already featured a high degree of electronic control. In Europe, mechanical engine control still dominated, engines were more highly rated, and had a smaller displacement. In Japan, large displacement, naturally aspirated engines dominated the market. These different engine technologies show somewhat different emission sensitivities to fuel quality.
Emission test cycles—Different emission tests present another potential complication in the analysis of different sets of data. However, the EPEFE study found that the effects of fuel quality on emissions from heavy-duty engines were generally similar in the US and EU results, despite different test cycles (FTP and R-49, respectively) [228]. This suggests that extrapolation of fuel effects over different tests may be possible—at least in some cases.

Substantial differences in fuel quality effects on emissions have been found between heavy-duty and light-duty diesel engines [231]. Apparently, results from heavy-duty engine studies cannot be extrapolated to light-duty engines, or vice versa, and the two engine classes should be discussed separately.

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