Diesel fueled, compression ignition engines generally have higher efficiency, increased durability, and more torque than their gasoline powered spark ignition counterparts. This has led to their use in most heavy duty vehicles for transportation and construction, and an increasing presence in passenger vehicles. However, the emissions of diesel engines contribute to pollution problems, especially in urban areas. Of highest concern are the particulate matter (PM) and oxides of nitrogen (NOx) produced. Particulate soot has known carcinogens while ozone production from NOx and hydrocarbons is a respiratory irritant. In addition, the production of carbon dioxide (CO2) is also gaining importance because of its potential role in climate change. It is desirable to reduce the emissions without dramatically increasing overall cost. One approach to improvement is through modification of the fuel source. Diesel fuel must meet a large set of parameters; including viscosity, cetane number, and sulfur content. Engines have evolved to perform optimally on traditional petroleum-based fuels that meet those standards. Alternative forms of diesel fuel can meet or exceed those standards and differ from petrodiesel in other ways that result in improved performance. The development of new engines and emission systems, as well as the retrofit of these long-lived vehicles, must take into account the properties of the fuel.
In this study we evaluate the emissions of several types of diesel fuel and the effectiveness of catalysts to eliminate those species. We utilize a single cylinder, air-cooled, compression ignition, naturally aspirated, 5kW engine-generator with computer controlled load. The effect of several additives on emissions from the generator has previously been compared to chassis dynamometer testing of full size trucks to find a constant load condition which correlates to full scale transient heavy duty chassis dynamometer testing [An, H., Muncrief, R., Harold, M., Ismail, H., “Benchtop Engine System for Screening of Diesel Fuel and Additives for NOx Reduction,” presented at SAE 2010 World Congress & Exhibition, April 2010, doi:10.4271/2010-01-1293.]. The benchtop system gives results faster and at less expense compared to full scale dynamometer testing. The experimental setup also includes the ability to switch between fuels without engine stopping, giving direct comparative results.
Several alternative diesel fuels, including Fischer-Tropsch fuel and biodiesel made from various feedstocks (soy, canola, and tallow), are compared to petrodiesel in terms of the emissions and fuel consumption. The petroleum-based fuel used as a baseline is Texas Low Emission Diesel (TxLED), which is the diesel fuel used in a 110 county region of Texas in order to reduce NOx emissions. Diesel fuel sold in this region must contain less than 10 percent by volume aromatic hydrocarbons and have a cetane number of 48 or greater. Alternative diesel fuel formulations can be considered TxLED if the Texas Commission on Environmental Quality (TCEQ) determines that they achieve similar or better reductions in NOx, PM, and hydrocarbons. The emission of NOx from the bench scale engine is determined by chemiluminescent NOx analyzer and also Fourier Transform Infrared Spectrometry (FTIR). Compared to the baseline fuel, NOx was found to decrease for Fisher-Tropsch diesel and biodiesel made from canola and tallow feedstocks, while an increase was found for biodiesel made from soy feedstocks. The chemical structure of the fuels, as well as parameters such as density, heat of combustion, and viscosity can help explain the differing performance.
We also will present the speciation of hydrocarbon emissions in the raw exhaust, including species like ethylene, propylene, formaldehyde, acetaldehyde, and 1,3- butadiene. Determining the spread of hydrocarbon species present in real exhaust is important because it can affect the catalytic activity of emissions control systems. Aftertreatment systems are often tested using simulated exhaust, which may contain only one hydrocarbon species. The benchtop engine setup gives the ability to test catalyst samples under real exhaust conditions and compare the efficiency to results obtained under simulated exhaust conditions.