Environ. Sci. Technol. 2005, 39, 1887-1892
Influence of Diesel Engine Combustion Parameters on Primary Soot Particle Diameter URS MATHIS, MARTIN MOHR,* AND RALF KAEGI EMPA, Swiss Federal Laboratories for Materials Testing and Research, U ¨ berlandstrasse 129, CH-8600 Du ¨ bendorf, Switzerland ANDREA BERTOLA AND KONSTANTINOS BOULOUCHOS ETH, Swiss Federal Institute of Technology, Aerothermochemistry and Combustion Systems Laboratory, CH-8092 Zu ¨ rich, Switzerland
Effects of engine operating parameters and fuel composition on both primary soot particle diameter and particle number size distribution in the exhaust of a directinjected heavy-duty diesel engine were studied in detail. An electrostatic sampler was developed to deposit particles directly on transmission electron microscopy (TEM) grids. Using TEM, the projected area equivalent diameter of primary soot particles was determined. A scanning mobility particle sizer (SMPS) was used for the measurement of the particle number size distribution. Variations in the main engine operating parameters (fuel injection system, air management, and fuel properties) were made to investigate soot formation and oxidation processes. Primary soot particle diameters determined by TEM measurements ranged from 17.5 to 32.5 nm for the diesel fuel and from 24.1 to 27.2 nm for the water-diesel emulsion fuel depending on the engine settings. For constant fuel energy flow rate, the primary particle size from the water-diesel emulsion fuel was slightly larger than that from the diesel fuel. A reduction in primary soot particle diameter was registered when increasing the fuel injection pressure (IP) or advancing the start of injection (SOI). Larger primary soot particle diameters were measured while the engine was operating with exhaust gas recirculation (EGR). Heat release rate analysis of the combustion process revealed that the primary soot particle diameter decreased when the maximum flame temperature increased for the diesel fuel.
Introduction Emissions of particulate matter (PM) and nitrogen oxides (NOx) are of growing concern because of their effect on human health and climate. Diesel engines are significant sources of both pollutants, and these emissions must therefore be significantly reduced. Great research effort is currently focused on advanced emission controls to achieve near-zero diesel emission levels in the future. In-cylinder emission control technology and combustion optimization are key points for compliance with future emission standards, particularly for heavy-duty diesel engines. In addition to * Corresponding author phone: +41-1-823-4190; fax: +41-1-8234041; e-mail:
[email protected]. 10.1021/es049578p CCC: $30.25 Published on Web 02/04/2005
2005 American Chemical Society
minimized engine-out emissions, exhaust gas after-treatment systems such as diesel particle filter and chemical reduction of NOx will be necessary to meet the U.S. Federal (EPA) 2007 emission standards for heavy-duty diesel engines. The combination of exhaust gas recirculation (EGR) with highpressure fuel injection is an efficient strategy for the simultaneous reduction of PM and NOx emissions (1-4). Moreover, modification of fuel composition, such as making use of water-diesel emulsion fuel, offers the possibility of overcoming the NOx-PM tradeoff without fuel consumption penalty (2, 5-10). When using water-diesel emulsion fuel, the reduction of NOx is due to decreased peak flame temperatures during combustion (lower thermal NO formation rate) (9, 11, 12). The reduction of PM emissions can be attributed to improved mixing of fuel with air (from microexplosions of water droplets in the fuel) (13, 14) and to the presence of additional OH radicals in the combustion region from partial water dissociation (14-16). Our study is focused on changes of soot nanostructure caused by varying combustion parameters. Several studies have investigated the morphology of soot particles from combustion processes using electron microscopy (TEM) (17-20). Diesel engine soot is composed of agglomerated carbonaceous primary particles (21-26). Solid carbon is formed during combustion in locally fuel-rich regions, and substances such as hydrocarbons can be adsorbed or condensed on the surface of soot particles (27). The soot particle nanostructure is highly dependent on synthesis conditions such as temperature, time, and fuel properties (28). The diameter of primary soot particles determined by TEM analysis has been reported to be 22.6 ( 6.0 nm for diesel soot in a smog chamber (29). Few recent studies have investigated the dependence of primary soot particle size on engine operating parameters. Good agreement was found for primary soot particle size distribution determined by laser-induced incandescence and TEM (30). TEM measurements have revealed an increasing diameter of primary soot particles from 12.9 to 21.7 nm for a heavy-duty diesel engine due to increased engine load from 25% to 75% (31). Primary soot particle diameters obtained from TEM analysis have been investigated for light-duty (common-rail, direct-injection) and heavy-duty (singlecylinder, direct-injection) diesel engines (32-34). The primary soot particle diameter ranged from 19.4 to 32.5 nm for the light-duty engine and from 28.5 to 34.4 nm for the heavyduty engine. The primary soot particle size decreased with engine load and increased with EGR rate. This behavior was explained by increased soot oxidation due to higher combustion temperature. The limited knowledge of the influence of engine operating parameters on soot particles motivated us to investigate the primary particle soot diameters in detail using TEM. Variations in the main engine operating parameters related to fuel injection system (injection pressure, IP, and start of injection, SOI), to air management (exhaust gas recirculation rate, EGR), and to fuel properties (low-sulfur diesel and water-diesel emulsion fuels) were made in a systematic way at constant engine speed and at different engine loads. This allowed us to vary widely the conditions in which soot particles are formed and oxidized in the combustion chamber of a diesel engine.
Experimental Methods Engine Parameters and Fuels. The investigation was carried out on a four-cylinder direct-injected diesel engine with a total displacement of 6.64 L (bore 122 mm, stroke 142 mm, VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (A) Setup of the complete particle sampling system. (B) Determination of the projected area equivalent diameter of soot particles. Positioning of the TEM grid on the electrostatic filter holder (C) and on the gravimetric filter (D).
TABLE 1. Engine Operating Conditions engine operating condition
engine speed (rpm)
engine load (%)
low engine load high engine load
1250
25
1250
75
a
engine torque (N m)
intake temperature (°C)
injection pressure (IP) (bar)
start of injection (SOI) (°CA)
4.97
260
27
500-1550
8 BTDC to 4 ATDC
14.91
780
37
800-1550
4 BTDC to 4 ATDC
BMEPa (bar)
Engine brake mean effective pressure.
compression ratio 17.2). The engine features turbocharger, intake air-cooling, high-pressure common rail fuel injection, and exhaust gas recirculation (2). Measurements were performed in 2 of the 13 modes of the European SteadyState Cycle (ESC) at constant engine speed and different engine load (see Table 1). In addition, engine parameters and fuel composition were varied to investigate the effects on nanostructure and on number size distribution of the soot particles. The injection system allowed us to vary independently the parameters injection pressure (IP) from 500 to 1600 bar and start of injection (SOI) from 8° crank angle degrees before top dead center (°CA BTDC) to 4° crank angle degrees after top dead center (°CA ATDC). The engine tests were conducted with a conventional diesel fuel and a commercial water-diesel emulsion fuel with 21% water by mass (2). Both fuels had a low sulfur content of about 15 ppm. In the water-diesel emulsion fuel, water droplets with a mean size of about 8 µm are dispersed in the continuous phase of diesel fuel. Due to the different energy content of the fuels (the lower heating value of the water-diesel emulsion is about 20% lower than that of the diesel), the IP was increased for the tests with the water-diesel emulsion 1888
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fuel to keep the same duration of injection as with the diesel fuel. With this injection strategy, the injected energy flow rate was kept constant (2, 9). Particle Sampling. The complete setup for the characterization of the PM emissions is given in Figure 1A. Particle number size distribution measurements with high stability and repeatability were achieved with a dilution scheme that avoids nucleation (35). The exhaust gas was diluted with a two-stage dilution unit, and the transfer line from the exhaust to the first diluter was heated at 170 °C. The first diluter (ejector pump, Dekati Ltd.) and the dilution air were heated at 150 °C. The second unheated diluter (ejector pump, Dekati Ltd.) was placed downstream of the first diluter and was operated at room temperature. The dilution ratios of the first and second diluter were kept constant at 10 and 13, respectively. The dilution air for both diluters was purified via an ultrafine filter to remove ambient particles. Primary Soot Particle Diameter Measured by Transmission Electron Microscopy (TEM). A schematic layout of the particle sampling setup for TEM analysis is given in Figure 1C and D. To sample soot particles, TEM grids (copper, 200 mesh, and lacey carbon film) were used. The grid was placed
on an electrostatic filter holder (see Figure 1C; prototype, EMPA). The particles were positively charged by the corona charger unit of an electrical low-pressure impactor (ELPI, Dekati Ltd.), and then deposited on the TEM grid that was connected to the negative high-voltage source of -14 kV (prototype, EMPA). Particle TEM samples were also conducted with gravimetric filters (see Figure 1D; prototype, EMPA) (36). TEM grids were taped on 47 mm diameter Nucleopore polycarbonate filters. Glass fiber filters were used to support the Nucleopore filters. The transmission electron microscope (CM 30, Philips, source: LaB6) was operated at an accelerating voltage of 200 kV in bright field mode. For the determination of the projected area equivalent diameter of primary soot particles, at least two agglomerated soot particles were selected. The primary soot particle diameter was measured manually from the selected agglomerates (see Figure 1B). The calculations of the average and standard deviation of the primary soot particle diameters were based on at least 19 measurements. The diameter of primary soot particles collected with the two sampling methods mentioned above did not show a significant difference. Particle Number Size Distribution Measured by Scanning Mobility Particle Analyzer (SMPS). A commercial SMPS consisting of a differential mobility analyzer (DMA, model 3071, TSI Inc.) and a condensation particle counter (CPC, model 3022, TSI Inc.) was used to determine the particle number size distribution. The inlet flow was set to 0.6 L min-1, and the sheath air flow was set to 6 L min-1. Therefore, the analyzed particle size ranged from 10 to 445 nm. The data presented in this paper are the mean values of at least three consecutive SMPS scans.
Results It has to be noted that our evaluation of engine operating combustion parameters may be specific to modern directinjected diesel engines with high-pressure fuel injection system. Therefore, one should be cautious applying these results to other engines. Start of Injection (SOI). Effects of SOI on primary soot particle diameter and particle number size distribution were investigated at low and high engine loads, while all other engine parameters were kept constant (see Figure 2). The primary soot particle diameter decreased with advancing SOI. This effect was most pronounced for low engine load. The primary soot particle diameter was lowered from 25.9 to 17.5 nm when advancing the SOI from 4° crank angle after top dead center (°CA ATDC) to 4° crank angle before top dead center (°CA BTDC). For high engine load, the primary soot particle diameter decreased from 28.4 to 21.6 nm when the SOI was advanced from 4 °CA ATDC to 3 °CA BTDC. The effect of SOI changes on agglomerated particles had little influence on particle number size distribution at low engine load, but a distinct reduction in particle number concentration was observed when advancing the SOI at high engine load. At the same time, the mode diameter of the particle number size distribution was reduced in the latter case from 62 to 53 nm. Injection Pressure (IP). Effects of IP on primary soot particle diameter and particle number size distribution are shown for both investigated engine loads in Figure 3. The most pronounced effect of IP was found at low engine load. A reduction from 32.5 to 29.6 nm and 17.5 nm was found by increasing the IP from 500 to 1100 bar and 1400 bar, respectively. However, at high engine load, an increase in IP from 800 to 1400 bar had only a small effect on primary soot particle diameter, which decreased from 21.4 to 20.4 nm. In line with the primary soot particle diameter, a reduction in particle number and mode of particle distribution was
FIGURE 2. Variation of start of injection (SOI) at constant injection pressure (IP ) 1400 bar) for the two engine loads with the diesel fuel. (A) Primary soot particle diameter (TEM) as a function of SOI at low engine load. (B) Primary soot particle diameter (TEM) as a function of SOI at high engine load. (C) Particulate number size distributions (SMPS) for low and high engine load.
FIGURE 3. Variation of injection pressure (IP) at constant start of injection (SOI) for the two engine loads with the diesel fuel (advanced SOI at IP ) 500 bar and low engine load). (A) Primary soot particle diameter (TEM) as a function of IP at low engine load. (B) Primary soot particle diameter (TEM) as a function of IP at high engine load. (C) Particulate number size distributions (SMPS). observed with increasing IP (see Figure 3). For an increase in IP from 500 to 1400 bar at low engine load, the particle numbers were reduced by about a factor of 10. The mode VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Variation of exhaust gas recirculation rate (EGR) at constant injection pressure (IP) and start of injection (SOI) with the diesel fuel at low engine load. (A) Primary soot particle diameter (TEM) as a function of EGR rate. (B) Particulate number size distributions (SMPS). diameter of the particle number size distribution was reduced from 73 to 68 nm and 48 nm by increasing IP from 500 to 1100 bar and 1400 bar, respectively. While the effect of IP on primary soot particle diameter was quite small at high engine load, particle concentration and mode diameter changed significantly. An increase in the IP from 800 to 1400 bar led to a reduction in particle concentration by a factor of about 2.5 and a decrease in the mode diameter from 77 to 55 nm. Exhaust Gas Recirculation (EGR). The recirculation of exhaust gases in the cylinder is a very effective method for the reduction of NOx emissions. Due to the experience that the EGR rate affects particulate emissions (37), a high EGR rate was chosen. Figure 4 shows by way of example the effect of EGR on the size-resolved particulate emissions. A significant increase in the primary soot particle diameter from 17.5 to 28.9 nm was detected when increasing the EGR rate from 0% to 40.5%. According to the SMPS data, the particle number concentration was increased by a factor of about 40, and the mode diameter increased from 46 to 70 nm. Water-Diesel Emulsion Fuel. Changes in IP and SOI hardly affected the primary soot particle diameter when water-diesel emulsion fuel was used (see Figure 5). The primary soot particle diameter increased slightly from 26.4 to 27.2 nm with an increase in IP from 600 to 1550 bar at comparable SOI (4 °CA BTDC to 3 °CA BTDC). These IP settings correspond to the same injected energy flow rate at 500 and 1500 bar for the diesel fuel. Advancing the SOI from 3 °CA BTDC to 8 °CA BTDC at a constant IP of 600 bar led to slightly smaller primary soot particle diameters (from 26.4 to 25.9 nm). For comparable fuel injection settings, the water-diesel emulsion fuel showed much lower soot particle concentrations in the engine exhaust than the diesel fuel. However, nucleation mode particles occurred at low concentration of soot particles. A growing nucleation mode was registered at low engine load with high IP and moderate IP and advanced SOI as shown in Figure 5. Increasing nucleation mode particles with decreasing accumulation mode particles have also been reported in other studies (27, 37). While for the diesel fuel a reduction of the number of particles in the accumulation mode went in line with a shift of the mode diameter toward smaller values (see Figures 2 and 3), this 1890
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FIGURE 5. Variation of start of injection (SOI) for two different injection pressure (IP) settings at low engine load with the waterdiesel emulsion fuel (21% water). (A) Primary soot particle diameter (TEM) as a function of SOI at low and high IP. (B) Particulate number size distributions (SMPS). was not a general observation for the water-diesel emulsion fuel. As shown in Figure 5 for the water-diesel emulsion fuel, the mode diameter of the accumulation mode increased slightly (55 to 62 nm) when the SOI was advanced by 5 °CA and the IP was kept constant. The accumulation mode almost disappeared at extremely high IP (1550 bar at low engine load).
Discussion Because the formation and oxidation of soot particles is highly dependent on the temperature in the combustion chamber, accurate heat release rate analysis was performed and the temperature in distinct zones of the combustion chamber was computed (2). The change in adiabatic flame temperature during the combustion process is plotted in Figure 6 for three distinct operating conditions. Increasing IP leads to higher burn rate, increased proportion of homogeneous combustion, and shorter duration of combustion. Therefore, the maximum adiabatic flame temperature is higher and its drop during the expansion stroke is faster. For constant injected energy flow rate (IP 800 bar for the diesel fuel and 930 bar for the water-diesel emulsion fuel), the variation in adiabatic flame temperature of the water-diesel emulsion fuel with time was similar to that of the diesel fuel but at a lower level. A reduction in the primary soot particle diameter due to higher soot oxidation rate is expected to correlate with temperature and time available for the oxidation process. The TEM measurements revealed that the primary soot particle diameter with the diesel fuel decreases when SOI is advanced and IP is increased (see Figure 7, top panel). For both changes, higher maximum adiabatic flame temperatures were registered, but the time available for soot oxidation during the expansion stroke was shorter. Temperature thus has a greater effect than time on the size of the primary soot particles. Improved mixture formation and higher proportion of homogeneous combustion lead to a reduction in both primary soot particle diameter and total number of particle concentration. The dependence of primary soot particle diameter on maximum adiabatic flame temperature for a variation in the
FIGURE 6. Change in the computed adiabatic flame temperature during the combustion process; variation of injection pressure (IP) with the diesel fuel and comparison of the diesel fuel with the water-diesel emulsion fuel at constant fuel energy flow rate.
FIGURE 7. Primary soot particle diameter (TEM) as a function of maximum adiabatic flame temperature. (A) Variation of start of injection (SOI) or injection pressure (IP) at low and high engine load. (B) Variation of exhaust gas recirculation (EGR) rate constant SOI and IP at low engine load. (C) The diesel and water-diesel emulsion fuels at constant SOI. The IP settings correspond to the same injected energy flow rate for both fuels. The direction of the arrow indicates the change from the diesel fuel to the waterdiesel emulsion fuel. EGR rate is illustrated in Figure 7 (middle panel). The soot oxidation was reduced at increased EGR rate because both oxygen availability and temperature were lowered in the combustion chamber. Larger primary soot particle diameters and higher number concentration of particles were observed at higher EGR rates.
The effect of temperature on primary soot particle diameter can also be illustrated by comparing measurements with the diesel fuel and the water-diesel emulsion fuel (see Figure 7, bottom panel). The water-diesel emulsion fuel showed larger primary soot particle diameters as well as lower maximum temperatures for high IP (1400/1550 bar) during combustion at constant fuel energy flow rate for both fuels. Because the changes in primary soot particle diameter for variations of IP or SOI were also smaller with the waterdiesel emulsion fuel than with the diesel fuel at comparable settings, other parameters beside the temperature in the combustion process must play an important role in affecting the particle nanostructure. Increased soot oxidation by enhanced mixing of fuel with air due to microexplosions (higher availability of oxygen) and additional OH radicals from water may have a major effect on soot primary particle diameter. The fact that the number concentration of soot particles was lower, even if the maximum temperatures were lower, may also be attributed to increased mixing of fuel and higher concentration of OH radicals. The SMPS measurements revealed that a number reduction of soot particles for the diesel fuel was accompanied by a smaller mode diameter of the soot particle number size distribution and a smaller primary soot particle diameter. While for the water-diesel emulsion fuel a lower number of soot particles also correlated with smaller primary soot diameter, a different trend was found for accumulation mode diameter. The mode diameter was indifferent or increased when the particle number concentration of soot was reduced. However, reduced particle number concentration of soot was correlated with increasing particle concentration of nucleation mode particles. Therefore, size growth of the accumulation mode particles can be explained by condensation of volatile compounds involved in the formation of nucleation mode particles on soot particles. The primary soot particle diameter determined by TEM was not affected because these volatile compounds were possibly desorbed by the ultrahigh vacuum during TEM measurements. Our investigation confirmed that particle emissions from engines fueled with diesel are optimized when high IP and advanced SOI are applied. The use of water-diesel emulsion fuel led to an additional reduction in soot particles. However, high concentrations of nucleation mode particles occurred under engine operating conditions optimized for low particle emissions.
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Received for review March 18, 2004. Revised manuscript received October 4, 2004. Accepted December 15, 2004. ES049578P
