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Energy & Fuels 2005, 19, 1468-1472
Experimental Investigation of the Effects of Water Addition on the Exhaust Emissions of a Naturally Aspirated, Liquefied-Petroleum-Gas-Fueled Engine Hakan O ¨ zcan and M. S. So¨ylemez* Department of Mechanical Engineering, Faculty of Engineering, University of Gaziantep, 27310 Gaziantep, Turkey Received June 25, 2004. Revised Manuscript Received March 8, 2005
The purpose of this study is to evaluate the effect of water addition on combustion in a conventional SI engine. The manifold induction method is used for water addition in this study. The exhaust emissions, ignition timing, and exhaust temperature values were measured for different equivalence ratio values by using a naturally aspirated liquefied-petroleum-gas-fueled spark ignition, four-cylinder engine. The water induction is accomplished over a wide range of water to fuel mass ratios of 0.2-0.5. The results showed that water addition worked as a cooling mechanism for the fuel-air charge and slowing the burning rates, yielding a reduction of the peak combustion temperature, which in turn provides a 35% reduction in peak NOx emissions without any significant change in CO and HC emissions. In addition, greater ignition advance is obtained.
1. Introduction The concept of water addition as a supplement to the internal combustion engine has been around for over 50 years.1 It is a well-known fact that water does not burn but it is excellent at absorbing heat due to water having a high specific heat capacity and latent heat of evaporation. The latent heat of evaporation of water is 2256 kJ/kg, which is approximately 6 times greater than that for gasoline under standard atmospheric pressure and temperature. Water addition, as a separate liquid or emulsion with fuel for automobile engines, has been investigated and reported in published papers extensively. These investigations are generally related to water effects on engine performance, knock, and emissions. Lestz et al.2 showed that the NOx concentration decreased while HC and CO emissions increased with water injection for a diesel engine. Harrington3 tested a single-cylinder engine, and concluded that engines could be calibrated to operate with small amounts of water to gasoline and by doing this knock could be suppressed, hydrocarbon emissions would slightly increase, NOx emissions would decrease, CO would not change significantly, and fuel and so energy consumption would be increased. Peters and Stebar4 investigated the effect of water-gasoline fuels on spark ignition engine emissions and performance. They showed that * To whom correspondence should be addressed. Phone: +90-342360-1200. Fax: +90-342-360-1100. E-mail address:
[email protected]. (1) Obert, E. F. Detonation and Internal Coolants. SAE Q. Trans. 1948, 52-59. (2) Lestz, S. J.; Melton, R. B., Jr.; Rambie, E. J. Feasibility of Cooling Diesel Engines by Introducing Water Into the Combustion Chamber. SAE Trans. 1975, 606-619, Document Number 750129. (3) Harrington, J. A. Water Addition to GasolinesEffect on Combustion, Emissions, Performance, and Knock; SAE Paper No. 820314; Society of Automotive Engineers: Warrendale, PA, 1982.
40% of water addition by weight to the fuel produced about a 40% drop in the peak nitric oxide emission level. Conversely, direct manifold water injection in amounts equal to the fuel flow caused about a 50% increase in HC emissions. In addition, their results showed that the effect of water addition on carbon monoxide emissions was small. Nicholls et al.5 reported that dramatic reductions of about 50% occurred in nitric oxide emissions by the effect of water addition at a water/fuel mass ratio of unity. Several different methods of water addition have been developed for diesel engines.6,7 These studies have shown that further reduction of harmful emissions is still possible. For example, Kahketsu et al.6 investigated the direct injection of water with a new designed stratified fuel-water injection system. They applied this new system to an automotive diesel engine. Their results showed that NOx emissions were reduced by 50%. Kegl and Pehan7 discussed some aspects of injecting or adding water either into the intake air or to the fuel of a diesel engine to reduce harmful emissions. Several authors studied the performances of wateremulsified diesel and water-emulsified gasoline engines. Park et al.8 showed that water-emulsified diesel reduced NOx and smoke at the same brake-specific fuel consumption at high speeds after their experiments. Tsu(4) Peters, B. D.; Stebar, R. F. Water-Gasoline Fuels-Their Effect on Spark-Ignition Engine Emissions and Performance; SAE Paper No. 760547; Society of Automotive Engineers: Warrendale, PA, 1976. (5) Nicholls, J. E.; El-Messiri, I. A.; Newhall, H. K. Inlet Manifold Water Injection for Control of Nitrogen Oxides-Theory and Experiments. SAE Trans. 1969, 780; Document Number 690018. (6) Kohketsu, S.; Mori, K.; Sakai, K.; Nakagawa, H. Reduction of Exhaust Emission with New Water Injection System in a Diesel Engine; SAE Paper No. 960033; Society of Automotive Engineers: Warrendale, PA, 1996. (7) Kegl, B.; Pehan, S. Reduction of Diesel Engine Emissions by Water Injection; SAE Paper No. 2001-01-3259; Society of Automotive Engineers: Warrendale, PA, 2001.
10.1021/ef049850g CCC: $30.25 © 2005 American Chemical Society Published on Web 04/15/2005
Water Addition Effects on an LPG-Fueled Engine
kahara and Yoshimoto9 reported the reduction of NOx with emulsified diesel fuel for a low compression ratio diesel engine. Abu-Zaid10 investigated the effect of water emulsification on the performance and exhaust gas temperature of a single-cylinder diesel engine. His results indicate that the exhaust gas temperature decreases as the percentage of water in the emulsion increases. The engine tests do not yield homogeneous results, owing to various typologies of the engine and different fuels used for the experiments. LPG (liquefied petroleum gas) has been widely used in automobiles recently as an alternative to gasoline due to its comparably lower price. In addition, it is wellknown that experts set their hopes and expectations on this fuel because of its sufficient reserves in the world. When environmental effects of LPG are taken into consideration compared with those of liquid fuels, significant improvements in exhaust emissions can be achieved.11 LPG-fueled spark ignition engines produce virtually zero emissions of particulate matter, very little carbon monoxide, and moderate hydrocarbon emissions.12 A major disadvantage of the LPG is the NOx emission is greater than that for liquid fuels.13 In this work, an experimental investigation was performed on a four-cylinder conventional spark ignition engine using LPG fuel. The objective of the study is to investigate the effect of water addition on the level of exhaust emission and temperatures for a typical LPGfueled engine at different fuel/air equivalence ratios and to investigate the occurrence of knock and misfiring under different water/fuel mass ratios. The results obtained in this study are compared with the general results of exhaust gas recycling (EGR) in the literature.
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Figure 1. Schematic of the engine test bed arrangement. Table 1. Test Engine Specifications
2. Experimental Apparatus and Setup Figure 1 shows a schematic diagram of the engine and test setup that was used for the experiments. The principal specifications of the water-cooled, four-cylinder SI engine of the setup are listed in Table 1. LPG may be in the form of propane (C3H8), butane (C4H10), or a mixture of both. An ordinary LPG fuel (containing 30% propane and 70% butane) was used as the test fuel. The cylinder pressure was measured with a water-cooled piezoelectric pressure transducer. The output signal of the pressure transducer was amplified using a charge amplifier. This amplified signal was transmitted to the data acquisition system and/or displayed on a DSO (digital storage oscilloscope). Two magnetic proximity pickups were used to generate 0.25° (degrees of crank angle) incremental timing information, (8) Park, J. W.; Huh, K. Y.; Lee, J. H. Reduction of NOX, Smoke, BSFC with Optimal Injection Timing and Emulsion Ratio of Wateremulsified Diesel. Prog. Inst. Mech. Eng. 2001, 215, Part D. (9) Tsukahara, M.; Yoshimoto, Y. Reduction of NOX, Smoke, BSFC and Maximum Combustion Pressure by Low Compression Ratios in Diesel Engine Fuelled by Emulsified Fuel; SAE Paper No. 920464; Society of Automotive Engineers: Warrendale, PA, 1992. (10) Abu-Zaid, M. Performance of Single Cylinder, Direct Injection Diesel Engine Using Water Fuel Emulsions. Energy Convers. Manage. 2004, 45, 697-705. (11) Bayraktar, H.; Durgun, O. Investigating the effects of LPG on spark ignition engine combustion and performance. Energy Convers. Manage., in press. (12) Bass, E.; Bailey, B.; Jaeger, S. LPG conversion and HC emissions speciation of a light-duty vehicle; SAE Paper No. 932745; Society of Automotive Engineers: Warrendale, PA, 1993. (13) Murillo, S.; Miguez, J. L.; Porteiro, J.; Gonzalez, L. M.; Granada, E.; Moran, J. C. LPG: Pollutant emission and performance enhancement for spark-ignition four strokes outboard engines. Appl. Therm. Eng., in press.
fuel swept volume compression ratio maximum torque maximum power
LPG 1297 cm3 7.8/1 12.5 kg m (DIN) at 3000 rpm 70 bHP (DIN) at 5500 rpm
faithfully track the crankshaft position, and trigger the cylinder pressure recording. The cylinder pressures vs crank angle were monitored on a DSO for observing any engine instability at the beginning of each test. In addition, cylinder pressures were monitored to detect the onset of knock and measured as a function of the crank angle. The ignition timing was varied over a wide range of water/fuel mass and equivalence ratios. The ignition timing was monitored and measured as a function of the crank angle by capacitive coupling pickups with the high-voltage pulse associated with the spark event. The airflow meter was used to determine the airflow rate through the engine. Airflow was measured by using a precision long-radius flow nozzle inserted into a pulse-damping drum. All air which entered the engine was drawn through the nozzle. The airflow rate was calculated directly by using a data conversion formulation program and corrected for temperature (measured with a thermometer in Kelvin) and pressure (measured with a barometer in bars) in the test conditions to obtain the actual airflow rate value during the experiments. The discharge air temperature was measured by a T-type thermocouple, and two precision pressure transducers were used to measure the differential and absolute pressures. The concentrations of NOx, HC, and CO in the exhaust gas were measured by the electrochemical sensors for the detection of toxic gases at the ppm (parts per million) level except for carbon monoxide, which was represented as a percentage of volume (vol %). Overall, electrochemical sensors offered very good performance for the routine monitoring of toxic gases and
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oxygen. Measured data for emissions were sent into a computer by using the computer’s serial port with a processor. The condensation trap that was installed in the measurement line removed the water/moisture. It was in-line with the sampling hose and placed before the filter. This trap collected the water and needed to be emptied after each set of experiments. The engine torque was measured by a dynamometer. The instantaneous engine speed was measured by an inductive pickup clamp. The exhaust gas temperature was measured using a K-type thermocouple located downstream of the exhaust port. The temperatures at selected points on the engine block were measured by a T-type thermocouple. These temperatures were used to detect the steady-state conditions at the start of the experiments. Two electronic scales, which were interconnected to the load cells, were employed to weigh the fuel and water consumption during the engine operation continuously. Liquid water was introduced into the intake manifold upstream of the intake valve using a capillary tube with a 0.35 mm bore diameter. Control over the water flow rate to the engine was obtained by varying the water supply pressure and/or by needle valve adjustments. This valve was adjusted for different engine loads by means of a stepper-motor-controlled mechanism. A computer drove this stepper motor. The water addition rate was controllable either automatically by the same computer, via the help of proper software, or manually. In addition, acceleration of the test engine was controlled with a linkage, actuated by a stepper motor. This stepper motor was also driven with the same computer through its parallel port and via software similar to that of the water addition mechanism. All these variables were monitored and measured by using three personal computers. Analog voltages representing temperatures, airflow, and engine torque were conducted through an analog-to-digital converter. Proper software was used to store the acquired data and for their analysis. A multiport serial card was used with its software for continuous measurements of fuel and water consumption rates, rpm (revolutions per minute), and different gas compositions in the exhaust. This software included the correction for exhaust emission readings. A high-speed data acquisition system was used to display and store the cylinder pressures, crankshaft positions, and spark event data. Complex software was used for the display and analysis of the acquired data. The engine data were collected at 2000 rpm, which was selected as a moderate engine speed for internal combustion engines. The flow rate of fuel was adjusted for each different fuel/air equivalence ratio. At each equivalence ratio and water to fuel mass ratio, the ignition timing was set at maximum brake torque (MBT). At each operating point, once every 5 s during a 4 min period, about 48 values of speed, torque, water and fuel consumption rate, temperature, and exhaust emission were recorded continuously. These measurements were conducted under steady-state conditions, that is, when the change of temperatures over the surface of the engine block was measured as close to zero.
3. Experimental Errors Table 2 lists the accuracy and resolution of the instrumentation used in detail. A calibration check of the devices was made two times, before and after each successive test. It should be noted that all data collected by the data acquisition systems used in this experimental study are subject to small errors. These errors are on the order of 0.02% for a 12-bit system and 0.001% for a 16-bit system and are considered to be negligibly small when compared to the other sources of error. 4. Results and Discussion The effects of manifold water induction on an LPGfueled SI engine on knock and misfiring limits and
Table 2. Accuracy of Instrumentation no.
measured value
accuracy
resolution
1 2 3 4 5 6 7 8 9 10 11
fuel flow rate air flow rate water flow rate temp exhaust gas temp torque speed ignition timing [NOx] [CO] [HC]
1% 0.75% 1% 0.75%, (0.1 °Ca 0.75%, (2.2 °Ca 0.25%, 0.1 N ma (1 rpm 0.5% 1%, 5 ppma 1% (12 ppm
0.02 g/s 0.01 g/s 0.02 g/s 0.1 °C 1 °C 0.01 N m 1 rpm 1° 1 ppm 0.01% 1 ppm
a
Whichever is greater.
Figure 2. Effect of water addition on NOx emissions as a function of the fuel/air (F/A) equivalence ratio.
exhaust temperature and emissions were searched at different fuel/air equivalence ratios. Results show that the water induction affects the information gathered such as knock, misfiring, and exhaust emission and temperature during the tests. Production of NOx depends on the fuel/air equivalence ratio, maximum cycle temperature, and burning rate. Figure 2 shows the NOx emission as a function of the fuel/air equivalence ratio for different water to fuel mass ratios. Results indicate that the peak NOx emissions occur at slightly lean conditions, where the combustion temperature is high and there is excessive oxygen to react with the nitrogen as a result of the tendency of dissociation. Figure 2 also indicates that the water injection reduces the NOx emission in the lean region having a local maximum between equivalence ratios of 0.9 and 1.0. Because the combustion process is closer to a stoichiometric ratio and produces a higher flame temperature, the NOx emission is increased, particularly by the increase of thermal nitrogen oxide. Experimental results also show that a maximum 35% reduction in the NOx emission level was achieved with water injection. The reduction of the NOx emission level is evident in the lean mixture. The drop in temperature and reduction of the combustion rate with water addition are the main reasons for the NOx reduction. A basic thermodynamic analysis was performed for this purpose. The effect of 0.5 g of water addition/g of fuel yields a lower adiabatic flame temperature at a level of approximately 150 K for a stoichiometric fuel/air mixture. It can be readily shown that, for the Zeldovich mechanism of a stoichiometric mixture, a 100 K drop in the combustion temperature would cause a 33% reduction in the NO production rate. This result agreed with the measured trends in NOx.
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Figure 3. Effect of water addition on HC emissions as a function of the equivalence ratio. Figure 4. Effect of water addition on the CO concentration.
Several experimental results in the literature14-16 showed that EGR lowers the NOx concentration in the exhaust gas since the recirculated exhaust gas is mixed with the fresh fuel-air mixture to dilute the concentration of fresh fuel-air charge. The new mixture has a reduced oxygen concentration due to a higher mean specific heat and higher temperature compared to the air alone. This is like the effect of water induction. Water is added into the intake port and will produce a reduction in the peak flame temperature similar to that of EGR, lowering the thermal NOx content. With EGR, a large reduction in NOx emissions was obtained with a significant increase in HC emissions. Substantial reductions in NOx concentrations are achieved with 10-25% EGR.14 This result agrees with that of Woo et al.,15 who worked on a modified commercial heavy-duty diesel engine using LPG as a fuel. Figure 3 shows the HC emission as a function of the fuel/air equivalence ratio and water to fuel mass ratio. HC emissions are not strongly affected near the stoichiometric mixture with an increased water injection rate. The HC emission slightly increases in the lean and rich regions as the water to fuel mass ratio increases. The higher heat of vaporization of water reduced the temperature of the mixture. In addition, water addition decreases the burning rate of the fuel-air mixture. The lower mixture temperature, combustion chamber deposits, and longer burning period could have contributed to higher HC emissions from the engine. EGR increases the HC emissions. The increase of HC emissions may reach over 60% at higher rates of EGR.14 Woo et al.15 also concluded in their experimental work that the NOx decrease caused HC to increase and the increase rate in HC emissions was more moderate than the rate of NOx reduction. Results in Figure 4 explain the variation of the CO concentrations as a function of the fuel/air equivalence ratio and water to fuel mass ratio. For lean mixtures, CO concentrations vary little with the equivalence ratio and increase rapidly as the fuel-air mixture becomes (14) Mozafari, A. Exhaust Gas Recirculation in Spark Ignition Engine. Adv. Heat Transfer ASME 1994, PD64 (1), 197-202. (15) Woo, Y.; Yeom, K.; Bae, C.; Oh, S.; Kang, K. Effects of Stratified EGR on the Performance of a Liquid-Phase LPG Injection Engine; SAE Paper No. 2004-01-0982; Society of Automotive Engineers: Warrendale, PA, 2004. (16) Caton, J. A. Effects of Burn Rate Parameters on Nitric Oxide Emissions for a Spark Ignition Engine: Results from a Three Zone, Thermodynamic Simulation; SAE Paper No. 2003-01-0720; Society of Automotive Engineers: Warrendale, PA, 2003.
Figure 5. Effect of water addition on the exhaust temperature.
richer than the stoichiometric level. CO concentrations in the exhaust increase as the fuel/air equivalence ratio for fuel-rich mixtures increases in the amount of excess fuel naturally. A reduction in the CO level of about 1% is observed in the rich mixture region since a higher CO concentration exists and water in combustion helps to complete the combustion process by improving the oxidation of CO. The combustion of CO occurs slowly and yields a late burning in the combustion of the fuel. The presence of water vapor in the combustion process fundamentally changes the combustion chemistry. Specifically, it does help the combustion of CO. EGR has little effect on CO emissions.14 The exhaust temperature significantly decreases for the lean mixture as presented in Figure 5. This drop in the exhaust temperature is relatively small for the rich mixture. This is because the addition of water has increased the total mass in the cylinder and water absorbs a great deal of heat due to its high specific heat similar to a supercharging and intercooling effect. Woo et al.15 show that EGR reduced the exhaust gas temperature. This observation agreed with that of Caton.16 He concluded that, as the homogeneous EGR rate increased, the exhaust temperature decreased due to a slower burning rate. Figure 6 shows the MBT ignition timing for various fuel/air equivalence ratios at different water addition levels. The MBT ignition timing for an LPG-fueled engine increases as the water addition increases. Water addition to the reactant mixture decreases the burning rate of the fuel-air mixture during combustion in a
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Figure 6. Effect of water addition on MBT timing.
Figure 8. Effect of water addition on the engine thermal efficiency.
lowered tendency of dissociation reactions as a result of a decrease in the average cylinder temperature during combustion. EGR also lowered the specific fuel consumption and decreased the heat transfer from the cylinder contents to the surrounding surface.14 The engine thermal efficiency increased due to the decrease in the brake-specific fuel consumption with EGR. 5. Conclusions Figure 7. Effect of water addition on knocking and misfire.
spark ignition engine. As a result of this, a minimum MBT is advanced. The thermodynamic heat release analysis showed that the burning period was decreased in the case of stratified EGR. The combustion speed was increased as the EGR rate was decreased. As the EGR amount was increased, the fuel-air mixture was diluted by the inert gas and the total heat capacity was increased.15 Therefore, the MBT spark advance requirement increases with the rate of EGR directly. The experimental results in Figure 7 indicate that, as the water to fuel mass ratio decreases, the knock and misfire limits occur at less advanced ignition timing since water induction leads to slowing of the burning rate and increases the ignition delay period and combustion period. In addition, the cooling effect due to evaporation of water has a significant effect on the reduction of the charge temperature in the cylinder. As a result of these, water addition decreases the knock tendency, which yields a desirable result for IC engines. The range of ignition timing between knock and misfire limits increases from 15 to 22 crank angle degrees when the water to mass fuel ratio increases from 0 to 0.5. For this study, it was observed that water input caused a slight but sustained increase (0.2-2%) in the thermal efficiency as compared to the no water addition case. Figure 8 shows the effect of water addition on the thermal efficiency of the engine for a fuel/air equivalence ratio of 0.9. The results show that, as the water addition level increases, the thermal efficiency increases. This occurs due to formation of steam as the compression stroke results in a pressure drop that causes a decrease in the compression work. At the same time, an increase in the thermal efficiency occurs as a result of the
The effect of water addition on exhaust emissions of an LPG-fueled SI engine is investigated experimentally. In the experimental measurements, water is added into the fuel-air charge at the intake manifold with the induction method, which is presumably the simplest and most effective method. The following conclusions are drawn from the experimental measurements to investigate the effect of water addition on exhaust emissions with LPG fuels: (1) The CO concentration is slightly affected by different water addition rates. This yields a small drop in the rich fuel-air mixture region. In addition, the CO concentration slightly increases with water addition in the lean fuel-air mixture region. (2) HC emissions increase with all water addition levels for all equivalence ratios. (3) The experimentally observed reduction in the NOx levels by water injection for all cases in the rich mixture is more significant than that for the lean mixture. (4) The knock and misfiring occur at more advanced ignition timing by water injection, but an excessive amount of water addition relative to the fuel-air mixture mass caused additional misfiring. (5) The exhaust gas temperature is reduced for all equivalence ratios by increasing the amount of water addition. This reduction is more significant for the lean mixture case as compared to the rich mixture case. (6) Water addition to the intake port seems to be a simpler method of EGR since injected water certainly dilutes the charge, exactly as EGR does, but due to the higher latent heat of evaporation of water, the magnitude of the effects on combustion and emissions is relatively different. Acknowledgment. This study was financially supported by the Research Fund of the University of Gaziantep in terms of a research project coded MF 01-06. EF049850G
