Energy & Fuels 2006, 20, 1015-1023
1015
Optimization of Exhaust Emissions of a Diesel Engine Fuelled with Biodiesel D. Y. C. Leung,*,† Y. Luo,† and T. L. Chan‡ Department of Mechanical Engineering, the UniVersity of Hong Kong, Hong Kong, and Research Centre for Combustion and Pollution Control, Department of Mechanical Engineering, The Hong Kong Polytechnic UniVersity, Hong Kong ReceiVed NoVember 19, 2005. ReVised Manuscript ReceiVed February 16, 2006
The performance of biodiesel in a single-cylinder diesel engine was studied and optimized by varying the engine settings, including the injection timing, injection pressure, and fuel pump plunger diameter. The engine emissions were found to be lowered for particulate matters (PM) and hydrocarbon (HC) with the use of biodiesel, but an obvious increase in the oxides of nitrogen (NOx) was observed, particularly at high engine loadings. The results revealed that individual adjustment of the above-mentioned parameters could not acquire a good balance between PM and NOx emissions. On the other hand, multiparameter engine adjustment with the consideration of their cross-interactive effects can keep the benefit of reducing PM and HC without increasing NOx emission and sacrificing fuel combustion efficiency.
1. Introduction Today, air pollutants emitted from diesel vehicles, such as oxides of nitrogen (NOx) and particulate matter (PM), have created serious air pollution problems in major cities around the world. It is well understood that PM emitted from vehicles contains many carcinogenic compounds and increases the risk of human cancer. NOx is an irritating air pollutant and contributes to the formation of tropospheric ozone and enhances photochemical smog formation in the atmosphere. Regulations and control measures aimed at lowering exhaust emissions from truck and bus diesel engines have been adopted worldwide in an effort to improve air quality in cities. However, the degree of improvement seems to be not very satisfactory, mostly because of the difficulties in removing NOx and the compromise between PM and NOx emissions. Vegetable oil methyl ester, or biodiesel, is of interest to auto manufacturers and drivers because it is a renewable fuel source that produces less PM, carbon monoxide (CO), hydrocarbons (HC), and other gaseous pollutants than conventional diesel fuel.1-3 However, despite its potential benefits of lowering these pollutants, experiments indicated that the NOx emission will be increased with the use of biodiesel. Last et al.4 showed that the HC, CO, and PM emissions with 10-100% biodiesel blends were decreased by at least 28, 6.9, and 24.1% respectively, while * Corresponding author. Fax: 852 2858 5415. E-mail:
[email protected]. † The University of Hong Kong. ‡ The Hong Kong Polytechnic University. (1) Cheng, W. K.; Leung, D. Y. C. Dynamometer testing of in-service vehicles using biodiesel and ultralow sulphur diesel. In Air Pollution X; Brebbia, C. A., Martin-Duque, J. F., Eds.; WIT Press: Southampton, UK, 2002; pp 401-410. (2) U.S. Environmental Protection Agency. A ComprehensiVe Analysis of Biodiesel Impacts on Exhaust Emissions; EPA420-P-02-001; 2002. (3) Leung, D. Y. C. Feasibility study of using biodiesel as motor fuel in Hong Kong; Final report submitted to the Hong Kong Environmental Protection Department, Hong Kong SAR Government MV00-153; 2004. (4) Last, R. J.; Kruger, M.; Durnholz, M. Emissions and Performance Characteristics of a 4-Stroke, Direct Injected Diesel Engine Fueled with Blends of Biodiesel and Low Sulfur Diesel Fuel; SAE Paper 950054; 1995.
the NOx emission increased by 3.5-28%. Doradoa et al.5 also observed a decrease in CO and HC emissions with biodiesel produced from transesterified waste olive oil. Although NO2 was increased in the experiment, the overall NOx concentration was decreased. Schumacher et al.6 tested various soydiesel/diesel blends on a heavy duty engine and found that NOx emission increased with increasing the soydiesel proportion, while both the total HC and PM decreased. Moreover, the brake-specific fuel consumption decreased slightly. The above comparisons between diesel and biodiesel are based on the original setting of an engine using diesel fuel. The formulation of fuel composition can improve the biodiesel combustion performance and exhaust emissions.7 However, the results show that it is difficult to acquire NOx emission neutral while improving other pollutant emissions simply by fuel reformulation. Therefore, modification of engine parameters may be feasible to optimize the engine emissions due to the difference in chemical composition and combustion characteristics between diesel and biodiesel. Since the premixed combustion, injection duration, and spray quality have strong interactive effects on pollutants’ formation and fuel consumption, these factors will be discussed in the following sections to find ways of mitigating the NOx emission without trading off with other pollutant emissions. 1.1. Premixed and Diffusion Combustion. The combustion process of a direct injection (DI) diesel engine can be divided into two phases: premixed combustion and diffusion combustion. The fuel mixes with air and forms some fuel-rich combustible areas during ignition delay. Upon ignition, the premixed mixtures react rapidly. When the oxygen in these areas is exhausted, the combustion changes to diffusion mode, which (5) Doradoa, M. P.; Ballesteros, E.; Arnal, J. M.; Gomez, J.; Lo´pez, F. J. Exhaust emissions from a diesel engine fueled with transesterified waste olive oil. Fuel 2003, 82, 1311-1315. (6) Schumacher, L. G.; Borgelt, S. C.; Fosseen, D.; Goetz, W.; Hires, W. G. Heavy-Duty Engine Exhaust Emission Tests Using Methyl Ester Soybean Oil/Diesel Fuel Blends. Bioresour. Technol. 1996, 57, 31-36. (7) Chang, D. Y. Z.; Van Gerpen, J. H. Fuel Properties and Engine Performance for Biodiesel Prepared from Modified Feedstocks; SAE Paper 971684; 1997.
10.1021/ef050383s CCC: $33.50 © 2006 American Chemical Society Published on Web 03/23/2006
1016 Energy & Fuels, Vol. 20, No. 3, 2006
Leung et al.
Figure 1. Schematic diagram of the experimental setup. Table 1. Specifications of the Tested Engine displacement compression ratio max power bore × stroke injection pressure fuel injection system injection timing combustion chamber cooling
903 cm3 18:1 11 kW at 2300 rpm 100 × 115 mm 200 bar mechanical, direct injection; four holes × 0.3 mm; pump plunger diameter 8.5 mm 24° ATDC ω model water
is governed by the mixing of fuel and air. The premixed phase is much shorter than the diffusion phase. The amount of fuel consumed in the premixed combustion phase has a positive correlation with the NOx emission, while diffusion combustion correlates with the amount of PM generated. Even though the mean temperature is low, the locally rich fuel-air mixture will result in high local temperature.8 The heat released during the premixed phase will preheat the reactant in the diffusion flame, thus increasing the temperature and pressure in the cylinder. Therefore, reducing the amount of fuel consumed in the premixed combustion phase can decrease combustion temperature and ultimately reduce NOx emission. However, the more the extent of diffusion combustion, the more PM will be produced. The amount of fuel consumed in the premixed combustion phase depends on the ignition delay and fuel-air mixing speed. The ignition delay is mainly affected by the composition of fuel, pressure, and temperature of charge. Higher pressure and temperature of charge can shorten the ignition delay. Sidhu et al.9 found the biodiesel ignition delay longer than that of diesel with a reflected shock tube. However, most studies on biodiesel ignition delay showed that biodiesel has a shorter ignition delay due to the higher cetane number and a faster combustion rate in the premixed combustion stage.10 In recent years a new combustion concept, homogeneous charge compression ignition (HCCI), provided a new view to (8) Flynn, P. F.; Durret, R. P.; Hunter, G. L.; zur Loye, A. O.; Akinyemi, O. C.; Dec, J. E.; Westbrook, C. K. Diesel Combustion: An Integrated View Combining Laser Diagnostics, Chemical Kinetics, and Empirical Validation; SAE Technical Paper 1999-01-0509; 1999. (9) Sidhu, S.; Graham, J.; Striebich, R. Semi-volatile and particulate emissions from the combustion of alternative diesel fuels. Chemosphere 2001, 42, 681-690.
Table 2. Properties of the Biodiesel and Diesel Tested viscosity @40 °C (cSt) density @15 °C (kg/L) cetane number flash point (°C) sulfur (mass %)
biodiesel
diesel
4.6 0.875 51 210
