Fuel Properties and Emissions

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Abstract: There is a clear correlation between some fuel properties and regulated diesel emissions. Drawing general conclusions is, however, difficult due to such factors as intercorrelation of different fuel properties, different engine technologies, or engine test cycles. 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 the heavy-duty engines. Sulfur increases PM in both classes of engines. Sulfur is also known to interfere with several diesel emission control strategies.

Introduction

Historically, fuel properties have been continuously changing for various reasons, including crude oil prices, crude oil quality, refinery technologies, relative demand for diesel and gasoline fuel, and changing engine technologies. In the recent years 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 need to be understood to find the most effective approach towards low-emission diesel engines. A number of research studies have been carried out to investigate the influence of fuel properties on emissions. The most comprehensive programs include the European Programme on Emissions, Fuels and Engine Technologies (EPEFE) [229] and the American Auto/Oil Air Quality Improvement Research Program (AQIRP) [230]. Numerous other studies have been conducted by the oil and engine industries, research institutes, and universities. A bibliography of publications selected for modeling of fuel emission effects in heavy-duty engines has been published by the US EPA [571].

Despite the wealth of experimental data, the influence of some of the fuel properties on emission is still not clear. The following is a list of considerations that make the interpretation of results and the comparison of data from different studies difficult:

Intercorrelation of fuel properties. The properties of diesel fuel that influence emissions are usually intercorrelated. An example of this is density, aromatics content, and cetane number. Diesel fuels blending streams that contain high levels of aromatics are high in density and also have low cetane numbers.

In order to study the effect of a specific fuel property on diesel emissions, care must be taken to decouple the change in a particular fuel property from changes in other properties of the test fuel. Some studies have not decoupled the fuel properties adequately. If a number of fuel properties are changed simultaneously it is not possible to ascribe any emission changes to a change in one property.

Engine technologies. Diesel engine technology has evolved in different directions around the world. In the 1990s, at the time when most of the above quoted studies were conducted, heavy-duty engines in the USA had large displacements and already featured a high degree of electronic control. In Europe the mechanical engine control still dominated. The engines were more highly rated and had smaller displacement. In Japan large displacement, naturally aspirated engines dominated the market. All of these different engine technologies tend to show somewhat different emission sensitivities to the fuel quality. It is also almost certain that the emission response of future engine technologies will be different from those currently in production.

The biggest difference in the fuel quality impact on emissions has been found between heavy-duty and light-duty engines [231]. Apparently, the results from heavy-duty engine studies cannot be extrapolated to the light-duty engines, or vice versa, and the two engine classes should be discussed separately.

Emission test cycles. Engines for different geographical markets are emission certified using different engine test cycles. Most research on the influence of fuel quality on emissions has focused either on US technology engines tested on the US transient FTP cycle or on EU engines tested on the ECE R-49 cycle. The EPEFE study attempted to make a comparison between these two test cycles [228]. Considering the magnitude of effects found in the study and the spread of effects across the EU fleet that was tested, the effects of fuel quality on emissions from the US and EU sets of data are generally similar. Despite the different test cycles and different rates of pollutant formation, general extrapolation of fuel effects from one data set to another appears to be possible.

Aftertreatment technologies. Meeting future emission standards may require a more extensive use of exhaust gas aftertreatment technologies, such as diesel oxidation catalysts, lean NOx catalysts, diesel particulate filters, or other techniques. The influence of fuel quality on these technologies is generally unknown. One exception is the fuel sulfur, which has been thoroughly tested for its influence on the performance of diesel catalysts.

If an efficient aftertreatment device is used, it will become the primary driver on tailpipe emissions. From the emissions point of view the properties of fuel would have only secondary importance. Thus, the primary fuel issue would be its compatibility with particular aftertreatment technologies.

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