Experiment Study on Influence of Compression Ratio onto Performance and Emission of Compressed Natural Gas Retrofit Engine

Transcription

1 Experiment Study on Influence of Compression Ratio onto Performance and Emission of Compressed Natural Gas Retrofit Engine Chaiyot Damrongkijkosol 1 and Somrat Kerdsuwan 2 Department of Mechanical Engineering, Faculty of Engineering, The Sirindhorn International Thai-German Graduate School of Engineering (TGGS), King Mongkut s Institute of Technology North Bangkok 1518 Pibulsongkram Road, Bangsue, Bangkok 10800, Tel ext chaiyot_vee@hotmail.com 1, srk@kmitnb.ac.th 2 Abstract Nowadays, with the energy crisis all over the world and conventional diesel as well as gasoline engines produce several kinds of pollutants, Thai Government promotes to use the clean alternative fuel likes Compressed Natural Gas (CNG) for solving the air pollution and economic problem since the price of natural gas is cheaper than conventional fuels. However, there is no specific engine designed properly for using CNG. Therefore, the fastest way to use CNG is by converting or retrofitting the existed engines. There are well-known systems for converting engine to use CNG such as Bi-fuel system for gasoline engine or Dual fuel system for diesel engine. However, this research used the conventional diesel engine and retrofitted it to the CNG dedicated engine. The dedicated engine was tested at various compression ratios by changing the pistons (from 9, 9.5, 10 up to 10.5) since this parameter can affect on the performance and emissions. The engine was testing at 1,000-4,000 rpm, wide open throttle (WOT) and Maximum Brake Torque (MBT) at every compression ratio. Moreover, Electronic Control Unit (ECU) was set to control the engine s equivalent ratio approximately to 1. Results of study showed that increasing the compression ratio gave more power output and power derived from CNG dedicated engine was higher than conventional diesel engine. The specific fuel consumption of CNG dedicated engine was also higher than conventional diesel engine. Effects of compression ratio variation on pollutants emission (HC, NO x and CO) were also examined. CNG dedicated engine is demonstrated that the existing diesel engine can be retrofitted and switches to use CNG as alternative fuel which results on money saving, more power output and clean emission. Keyword: Compressed Natural gas (CNG), Compression Ratio, Emissions, Dedicated engine 1. Introduction As Compressed Natural Gas (CNG) is the clean alternative fuel than conventional fossil fuel, Thai government promotes to use CNG for reducing the air pollution and reducing the amount of imported conventional fuel from aboard. There are several methods to convert or retrofit the engine to use with CNG. The dedicated engine is the one choice for retrofit the engine to use the CNG, however, by doing this, engine s piston has to be modified to have lower compression ratio since the CNG engine has to run under spark ignition system which has the lower compression ratio. Therefore, when the compression ratio has changed, this parameter can affect to the performance and emissions. Moreover, the actual engine has many effects when changing the compression ratio such as combustion rate and stability, heat transfer, and friction. Also, over the load and speed range, the relative impact that these processes have on power and efficiency varies. Therefore, only the testing on real engine can be shown the real effect of compression ratio. Hence, this research aims to test the dedicated CNG engine for studying the effect of compression ratio on its performance and emissions. 2. Theory 2.1 Natural gas composition and properties Generally, natural gas is one of the hydrocarbon families, made up of carbon and hydrogen atoms. There are different compounds in natural gas such as methane, ethane, and propane as well as other non-hydrocarbon compounds such as carbon dioxide and nitrogen. The natural gas used in this research is assumed to consist of mainly methane, ethane and propane. Their respective compositions percentage of the typical natural gas found is shown in Table 1.

2 Table 1 Composition of Natural Gas [5] Composition % (vol) Methane (CH 4 ) 89.4 Ethane (C 2 H 6 ) 8.6 Propane (C 3 H 8 ) 1.39 i-buthane (C 4 H 10 ) 0.25 N-Buthane (C 4 H 10 ) 0.32 Nitrogen (N 2 ) 0.04 Moreover, the important mean properties such as boiling point, octane number, density have to be known. The boiling point of the gas is very low therefore the natural gas is in the gas phase at the atmospheric pressure. The density of natural gas is lower than air density therefore, when gas leaks, it will escape to the upper atmosphere, no accumulation of the gas like the LPG which may cause a fire since LPG is heavier than air. For the octane number, the natural gas has the higher octane number which is around 120 or higher hence it is suitable for using with the vehicle gas fuel in the spark ignition system engine. For the auto ignition temperature, it is the lowest temperature that the fuel can ignite itself through heat only without the spark or flame. The high ignition temperature of natural gas means it is more difficult to ignite by compression ignition only, consequently, it is needed to have the flame or spark plug for helping ignition. The flammability limit range is the concentration of natural gas in air to cause an explosion. This should be between the lower explosive limit (LEL) of 5% to the higher explosive limit (HEL) of 15%. If the concentration of natural gas is more or less than this range, an explosion would not occur. This will certainly reduce the risk of explosion of CNG in air due to leaking because it can only burn in air when the concentration of CNG is high. The typical value of the natural gas properties is shown in the table 2. Table 2 Properties of natural gas [3] Property Natural Gas Diesel Boiling range Pa) Density (kg/m 3 ) Octane Number 120+ n/a Autoignition Temperature (k) Flammability Limits LEL % Range UEL % 15 5 Heating Value Low (MJ/kg) High Conversion of Diesel Engine There are two methods of converting diesel engine to utilize natural gas in diesel engine. The first method uses both the natural gas and diesel so called the Dual fuel system. Another is the dedicated engine. It is use with fully natural gas, no more diesel need for this engine CNG Dedicated Diesel engine for this type of conversion used 100% CNG to operate. Therefore, only CNG is drawn into the combustion chamber to mix with air and hence the name, natural gas dedicated. This type of conversion is basically meant for heavy duty vehicles like busses and trucks that have a lot of problems with the large cylinder unit displacement. Conversion to CNG dedicated engine needs greater modification than the dual fuel conversion especially on the combustion chamber design and other components also. Basically, there are three main modifications such as the gas supply system, the ignition system and the electronic control unit (ECU) to arrange the two subsystems. The high compression ratio of the original diesel engine needs to be reduced to match the spark ignition system requirement since the spark ignition system cannot operate at the high compression ratio because of the knocking phenomena Dual fuel system This system does not need to install the spark ignition system since the CNG is just the helper of the ignition. In this system, the mixer and some control components such as venturimeter have to be installed. The dual fuel system engine still uses the compression ignition system. Diesel is still the main fuel for combustion while CNG will be mix with the air and drawn through air inlet manifold to the cylinder in the suction stroke. Therefore diesel fuel pump is adjusted and fitted to the tune control system, usually an actuator reduce the diesel supply at idle rate and during the dual fuel operation. This mechanism will be supported by the linkage between the pedal and both the fuel pump and mixer. For the amount of CNG, CNG-air mixer consists of the venturimeter and the metering device are fitted to the air manifold to control the gas mixture. 2.3 Effect of Compression Ratio Performance Compression ratio (CR) is the ratio of the total volume of the combustion chamber when the piston is at the bottom dead center (BDC) to the total volume of the combustion chamber when piston is at the top dead center (TDC). Theoretically,

3 increasing the compression ratio (CR) of an engine can improve the overall efficiency of the engine by producing more power output. The ideal cycle analysis for SI combustion chamber show that indicated fuel conversion efficiency increased continuously with compression ratio according to Equation 1. η = 1-1 γ 1 r c However, the actual engine has many effects when changing the compression ratio such as combustion rate and stability, heat transfer, and friction. Also over the load and speed range, the relative impact that these processes have on power and efficiency varies. Therefore, only the testing on real engine can be shown that what is an effect of compression ratio in engine. However, knocking is a limitation for increasing the compression ratio. For spark ignition engine, fuel should have a low ignition performance therefore the mixture residual do not self ignite before flame front, ignited by the spark plug reaches them (normal combustion). Self-ignition can cause the sharp pressure increase and a gas pressure vibration in the cylinder (knocking). Natural Gas has octane number of 120 which is higher than conventional gasoline therefore the compression ratio can be used higher than gasoline. While the fuel consumption, it has the trend that with increasing compression ratio the fuel consumption will be lower Emissions From engine, the exhaust gas contains the several kinds of emissions such as NO X, Hydrocarbon (HC) and CO which compression ratio may affect the amount of the emissions from the other side effects Nitrogen Oxides The two main nitrogen oxides emitted from combustion engines are nitric oxide (NO) and nitrogen dioxide (NO 2 ). Most nitrogen oxides from combustion engines 90 percent are nitric oxide. This gas is formed from nitrogen and free oxygen at high temperature. The rate of formation is a function of oxygen availability, and is exponentially dependent on the temperature. Most nitrogen oxide emissions form early in the combustion process, when the piston is near the top of its stroke (top dead center) and temperature are highest Hydrocarbon (1) Hydrocarbon emissions result from elements of the air-fuel mixture that have not finished burning at the time the exhaust valve opens. Hydrocarbon emissions are composed of unburned fuel and products of partial combustion, such as ethylene and formaldehyde. Hydrocarbon sources include crevice volumes, such as the space between the piston and cylinder wall above the piston ring, and the quenched layer immediately next to the combustion chamber walls. Unburned mixture is forced into these crevices during compression and combustion, and emerges late in the expansion and during the exhaust stroke Carbon monoxide Carbon monoxide emissions are caused by the combustion of rich mixtures, where the air-fuel ratio, λ is less than 1.0. Since spark-ignition engines often operate close to stoichiometric at part load and fuel rich at full load. In such mixtures, there is insufficient oxygen to convert all of carbon to carbon dioxide. A small amount of carbon monoxide is also emitted under lean conditions. 3. Methodology 3.1 Engine Description Diesel Engine was fully retrofitted to CNG dedicated engine which is a mixer type and close loop control. The specifications of engine are listed in Table 3. By retrofitting engine, some components had to modify such as fuel supply system, CR and spark system. Table 3 Natural Gas Retrofit Engine Specification Item Natural Gas Retrofit Engine Type 4-cylinder 4 stroke Displacement 2197 cc Bore (mm) 87 Stroke (mm) 92.4 Compression ratio Varied from 9, 9.5, 10, 10.5 Fuel Supply System Gas Fuel Mixer Spark System Electric discharge Because of retrofitting diesel engine (Compression ignition engine) was change to the spark ignition engine (SI engine) therefore the compression ratio should be modify to give the appropriate compression ratio for spark ignition system also. The proper CR for SI is around To obtain the design CR,

4 the diesel engine pistons were modified from the CR 22 to 9, 9.5, 10 and 10.5 for testing at various compression ratios. In case of fuel supply system, fuel pump and nozzles were replaced by gas fuel mixture. Retrofitted engine uses the mixer for mix up the air and fuel like a carburetor in gasoline engine. In addition, the ECU had been added to control the amount of the gas by get the signal from sensors such as rpm sensor, throttle position sensor and the oxygen sensor. Also for CNG it needs to have a vaporizer for reducing the CNG pressure before entering the gas fuel mixer. Moreover, the electric discharge was used instead of auto ignition of diesel engine. It needs to have 4 spark plugs and one distributor for the complete system. However, the distributor cannot control the ignition timing for giving the maximum brake torque (MBT) because of using vacuum and mechanical advance. Therefore, finally the electronic control for ignition timing was replaced the old type distributor by using the gear and crank position sensor for sending the signal to ignition coils at right time. 3.2 Experiment Setup For the engine testing, CNG dedicated engine was tested by testing at 4 compression ratio pistons as 9, 9.5, 10, The engine testing schematic is shown in figure Engine 2. Dynamometer 3. Exhaust gas analyzer 4. Dynamometer controller 5. Cooling tower 6. Duty Monitor 7. Pressure Reducer 8. ECU Figure 1: Schematic diagram of engine testing After setup and connect all equipment together, the engine was tested at MBT, wide open throttle (WOT) and the equivalent ratio controlled by ECU approximate at 1. Engine was test at compression ratio 9, 9.5, 10, 10.5 and each compression ratio the engine was tested at 1,000-4,000 rpm. In this experiment, the results of testing which are performance and emissions of the engine were recorded as the raw data before analysis. For the performance, power and torque were measured using the dynamometer controller. Moreover, the amounts of the exhaust gas, NO X, CO 2, CO, HC, were recorded using the exhaust gas analyzer. 4. Results 4.1 Performance Torque For the result of engine testing, torque of the dedicated engine is much more than the original diesel engine. Moreover, the trend of engine torque is increasing when compression ratio increases. The result of engine torque is shown in figure 2 which compare with all 4 compression ratios of CNG dedicated vs original diesel engine. It is clearly that the torque of CNG dedicated is higher than original diesel engine. However, torque at the compression ratio of 10.5 seems to have a little bit lower than compression ratio of 10 because at CR of 10.5, the knocking has observed, consequently, caused in reducing in torque of the engine Power For the result of engine power, dedicated engine gives higher power than original diesel engine. Also, the trend of engine power increases when compression ratio increases. The result of engine power is shown in figure 3 comparing with all 4 compression ratios of CNG dedicated engine vs original diesel engine. It is clearly that the power of CNG dedicated engine is higher than original diesel engine Specific fuel consumption According to figure 4, it seems that the specific fuel consumption reduces while increasing compression ratio. Also the trend of the specific fuel consumption is similar to the theory of fuel consumption trend. Anyway, at compression ratio of 10.5, the fuel consumption is not followed the theory. It might be caused from unstable combustion such as knocking which made the reduction in the combustion efficiency and hence reduction in the proportion of energy derived from the fuel. 4.2 Emissions NO X According to figure 5, for the different of compression ratios, NO X increases to the maximum value at compression ratio of 10. This is due to the thermal NO which increases where temperature inside combustion chamber increases. However, NO x at compression ratio of 10.5 is lower than that at 10. This was

5 reported earlier that at CR of 10.5, combustion instability happened. This should cause the incomplete combustion which will lower the combustion temperature, hence NO x deceases Hydrocarbon Figure 6 shows the concentration of HC from the exhaust gas of engine. It seems that HC increases with compression ratio. As the HC is caused from the incomplete combustion (unburned hydrocarbon) and also include the crevice volume (the spaces between the piston and cylinder wall above the piston ring). Unburned mixture is forced into these crevices during compression and combustion, and emerges late in the expansion and during the exhaust stroke. Therefore, when increasing the compression ratio, the pressure inside will increase and then the unburned mixture might be able to be forced more into the same crevices Carbon Monoxide The concentration of CO with various compression ratio is shown in figure 7. The air/fuel (A/F) ratio is the most important factor which affects to the concentration of the CO. The variation of the concentration of CO, as shown in figure, might be caused from the variation of A/F, which is controlled by Electronic Control Unit (ECU). Therefore, this situation might be happened from the ECU control of A/F which is not good enough to control the ratio at the constant value. Figure 2: Engine torque Figure 3: Engine power

6 Figure 4: Specific fuel consumption ppm Figure 5: Concentration of NO X ppm Figure 6: Concentration of Hydrocarbon

7 CO Figure 7: Concentration of CO 5. Conclusion 5.1 For the result of engine testing, performance of the CNG dedicated engine is much more than original diesel engine. Moreover, the trend of engine power is increasing when compression ratio increases. 5.2 For the fuel consumption, the CNG dedicated has higher specific fuel consumption than original diesel engine. The diesel engine has the lowest specific fuel consumption around 136 g/kw.h but CNG dedicated engine has the lowest specific fuel consumption at CR:9.5 around 155 g/kw.h. 5.3 Other results for emissions, the CNG dedicated engine was measured the exhaust gas emissions such as Hydrocarbon (HC), NO X and CO. The compression ratio also has the effects on the exhaust gas of the engine. For NO X and HC, its have the same trend that, when increase the compression ratio, the amount of emissions increase and reach to the maximum value, then decrease. For results of CO, it has no certainly trend relative to compression ratio because it might be caused from ECU which cannot control the equivalent ratio correctly at 1, therefore the air/fuel ratio will be able to the main effect to the amount of CO. 5.4 CNG dedicated engine is demonstrated that the existing diesel engine can be retrofitted and switches to use CNG as alternative fuel which results on money saving, more power output and clean emission. Acknowledgements The authors would like to express their grateful to Power Lab of Kiemyung University, Daegu, Korea for the facilities support. Also, The Sirindhorn International Thai-German Graduate School of Engineering (TGGS) of King Mongkut s Institute of Technology North Bangkok for financial support. Reference [1] Ju-Hee Lee., Seung-Yup Baek., Jin-Han Ma., Gyeung-Ho Choi., A study on Exhaust Performance of the Diesel engine based gas engine s. [2] Asif Faiz., Christopher S. Weaver, Michael P. Walsh: Air Pollution from Motor Vehicles. [3] LIM Pei Li., The Effect of Compression Ratio on the CNG-Diesel Engine. [4] John B. Heywood: Internal Combustion Engine Fundamental. International Edition., McGraw-Hill, Singapore. [5]