Article pubs.acs.org/est
Reductions in Particulate and NOx Emissions by Diesel Engine Parameter Adjustments with HVO Fuel Matti Happonen,*,† Juha Heikkila,̈ † Timo Murtonen,‡ Kalle Lehto,¶ Teemu Sarjovaara,¶ Martti Larmi,¶ Jorma Keskinen,† and Annele Virtanen† †
Aerosol Physics Laboratory, Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland ¶ Department of Energy Technology, Aalto University, P.O. Box 14300, FI-00076 Aalto, Finland § Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, FIN-70211 Kuopio, Finland ‡
ABSTRACT: Hydrotreated vegetable oil (HVO) diesel fuel is a promising biofuel candidate that can complement or substitute traditional diesel fuel in engines. It has been already reported that by changing the fuel from conventional EN590 diesel to HVO decreases exhaust emissions. However, as the fuels have certain chemical and physical differences, it is clear that the full advantage of HVO cannot be realized unless the engine is optimized for the new fuel. In this article, we studied how much exhaust emissions can be reduced by adjusting engine parameters for HVO. The results indicate that, with all the studied loads (50%, 75%, and 100%), particulate mass and NOx can both be reduced over 25% by engine parameter adjustments. Further, the emission reduction was even higher when the target for adjusting engine parameters was to exclusively reduce either particulates or NOx. In addition to particulate mass, different indicators of particulate emissions were also compared. These indicators included filter smoke number (FSN), total particle number, total particle surface area, and geometric mean diameter of the emitted particle size distribution. As a result of this comparison, a linear correlation between FSN and total particulate surface area at low FSN region was found.
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compounds.6,9 The major differences of HVO fuels compared to fossil diesel are their higher cetane number and lower density. The use of F-T and HVO diesels has been shown to decrease all regulated emissions.5,6,10 Most of the studies that compare the emissions with biofuels to the emissions with fossil fuel content only to change the fuel and measure the emissions. However, as the compositions of the fuels are different, the engine optimized to conventional EN590 fossil fuel is not able to take full advantage of the studied biofuel. Some recent biodiesel studies, e.g., refs 11 and 12, and some F-T studies, e.g., refs 13 and 14, have accounted for this fact mainly by adjusting engine parameters for the studied fuel. In the case of HVO, only a few studies10,15 have adjusted engine parameters to better suit the HVO fuel. However, no HVO studies have been reported where more than two engine parameters have been simultaneously balanced to optimize the engine. If these biofuels are to become more generally used in vehicles, the full potential of these fuels to reduce emissions should be studied.
INTRODUCTION Due to dwindling oil reserves and increasing demand of fuel, the period of cheap oil is coming to an end. Consequently, unconventional sources of fuel need to be developed in order to meet the demand. One approach to ease the problem is reducing oil dependency of vehicles by using biofuels. The use of biofuels has also potential to reduce greenhouse gas emissions as well as regulated exhaust emissions. The environmental impacts of different biofuels have been reviewed by Sunde et al.1 Increasing the share of renewable energy overall and, specifically, including the transport sector has been incorporated to EU strategy2 and the usage of biofuels has also been encouraged by regulations in the U.S.3 The biofuels suitable for use in vehicles with a diesel engine are mainly biodiesels consisting of fatty acid methyl esters (FAME),4 Fischer−Tropsch fuels (F-T) produced from biomass,5 and fuels consisting of hydrotreated vegetable oils (HVO).6,7 Of these, FAME biodiesels are perhaps most extensively studied. The use of FAME diesel has been shown to reduce particulate, hydrocarbon (HC), and carbon monoxide (CO) emissions but slightly increase NOx emission.4,8 Most of these changes occur mainly due to the fact that FAME fuel contains significant amount of oxygen.8 F-T and HVO fuels have very similar composition.6 They are composed of paraffinic hydrocarbons and contain essentially no aromatic © 2012 American Chemical Society
Received: Revised: Accepted: Published: 6198
February 3, 2012 April 26, 2012 May 8, 2012 May 8, 2012 dx.doi.org/10.1021/es300447t | Environ. Sci. Technol. 2012, 46, 6198−6204
Environmental Science & Technology
Article
fuel used in this study. It is notable that the HVO fuel actually meets the EN590 standard in all respects except in density.9 The paraffinic nature of the fuel has not been observed to influence detrimentally in particle oxidation in existing aftertreatment devices.21,22 Engine and Studied Running Conditions. The engine used in this study was a single-cylinder research engine based on a commercial 6-cylinder off-road engine. The commercial engine fulfilled EU 97/68/EC Stage III A and EPA 40 CFR 89 Tier 3 emission standards. The details of the engine and the control system have been published earlier.15 However, it is worth mentioning also here that exhaust gas recirculation (EGR) was simulated using neat nitrogen in the charge air. The EGR percentage was defined as the percentage of added nitrogen in the total inlet air mass flow. Even though the simulated and real EGR are not fully comparable in respects of EGR percentages, the trends in exhaust emissions are similar. Using the trends reported elsewhere by Lehto et al.15 as a starting point, different combinations of advanced intake valve closing (IVC), exhaust gas recirculation (EGR) percentage, injection pressure (Pinj), and start-of-injection timing (SOI) were studied on 50%, 75%, and 100% loads. When IVC was advanced, it was accompanied with an increase in charge air pressure in order to keep the intake air mass flow constant. Three different engine conditions were chosen for each load. These were low-NOx conditions (LN), low-smoke conditions (LS), and conditions where both NOx and smoke are relatively low (LNLS). The details of the chosen engine conditions for each load can be found in Table 2. The engine loads
The interest to reduce other than total CO2 emissions arise largely from the fact that emissions have been linked to adverse health effects. Especially fine particle emissions have been shown to have a correlation with mortality and reduced life expectancy.16,17 Until recently, the legislation has addressed the issue by increasingly limiting total particulate mass. This has caused that reaching the particulate mass limit has become an ongoing goal in automotive industry. In the newest Euro 5b and Euro 6 legislation for light duty vehicles, the manufacturers have to reach a limit for total particle number emission as well. However, to better link the emissions to the resulting health effects, also more elaborate metrics to measure nanoparticles have been proposed. It has been proposed that the metric to measure emissions should be linked to the mechanics by which these emissions influence health.18 For example, the mass of volatile particles is a proper metric since their biological effect is dependent on the amount of hazardous material that is dissolved in the lung fluid. A significant part of the adverse health effects caused by nonvolatile soot, on the other hand, is associated with soot particles providing a surface on which reactions generating reactive oxygen species occur in lungs. Thus, accessible deposited surface area is one possible measure of biological effects of nonvolatile particles. The relevance of surface area regarding health effects of nanoparticles has been proposed also elsewhere.19,20 In this study, engine parameters of a research engine are adjusted to take better advantage of HVO fuel to reduce exhaust emissions. The exhaust emissions of each chosen engine condition are studied with three different loads. The chosen conditions target to either reduce NOx or particles extensively or both together as much as possible. Particulate emissions of the engine are measured with different methods and different indicators describing particulate emissions are compared. Additional focus is given to the relation of filter smoke number (FSN) to other indicators of particles.
Table 2. Engine Load, Power, Efficiency (Eff.), IVC, SOI, EGR, and Pinj of the Measured Engine Conditionsa load (%) 50
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EXPERIMENTAL SECTION Fuel. The studied HVO fuel was first presented by Rantanen et al.9 The fuel is fully paraffinic, and, thus, it contains no aromatics, sulfur, or oxygen. The properties of the studied HVO fuel and, for comparison, typical properties of fossil EN590 fuel are shown in Table 1. The HVO fuel was the only
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Table 1. Properties of the Studied HVO Fuel and, for Comparison, Typical Values for EN590 (Summer Grade) Diesel Fuel As Presented in Kuronen et al.6 fuel property
HVO
EN590
density at +15 °C (kg/m3) viscosity at +40 °C (mm2/s) lower heating value (MJ/kg) CFPP (°C) cetane number sulfur content (mg/kg) aromatics (wt-%) distillation (°C) 5 vol-% 50 vol-% 90 vol-% 95 vol-% final boiling point
780 3.0 44.0 −15 88
