OPTIMIZATION OF PRE-IGNITION STRENGTH AND NO X REDUCTION IN HYDROGEN FUELED INTERNAL COMBUSTION ENGINE

Transcription

1 OPTIMIZATION OF PRE-IGNITION STRENGTH AND NO X REDUCTION IN HYDROGEN FUELED INTERNAL COMBUSTION ENGINE Uday Pratap Singh 1, Ishan Sahu 2, Ravikant Shukla 2, Navpreet Chaddha 2 1 Assistant Professor, Noida, India, 2 Student, Noida, India, Abstract: In current scenario, excessive use of internal combustion engine is a major source of air pollution which is a main cause for global warming and threat to life. Hydrogen fuelled internal combustion engine are efficient and cleaner alternative with power and efficiency equivalent to gasoline engine but pre-ignition and backfiring occurs easily in Hydrogen Internal Combustion Engine (HICE) with manifold injection type especially at higher equivalence ratio (ɸ>0.8) backfiring and NO x emission is high whereas low equivalence ratio (ɸ<0.8) will reduce engine power output. This paper attempts to explore the investigation related to the concept of backfiring and engine performance. The functional relationship between inhibition degree of pre-ignition and ignition timing and between elimination degree of backfire and equivalence ratio has been established for quantitative analysis and research into eliminating backfiring and improving performance of HICE. This paper throws light on stable combustion in HICE which is possible with Direct Injection System which reduces backfiring and knocking and using lean mixture (0.3<ɸ<0.5) which reduces temperature below NO x generation temperature due to which exhaust was almost NO x free and supercharged lean mixture provided power equivalent to gasoline engine. Thus an effort is made to provide guidance to the technological research on HICE, which not only eliminates backfire and NO x emissions but also take into account the power output and economy. Keywords: Pre-ignition, backfire, NOx emission, equivalence ratio, Direct Injection (DI) system, lean air-fuel mixture. INTRODUCTION: In a developing country like India where major industries mainly rely on fossil fuels for transportation and to fulfil their energy requirements. Also about ton of carbon dioxide is emitted every second into the atmosphere having adverse effects on climate and is main cause for global warming. Globally, about 32% of carbon dioxide is emitted from transport vehicles and also harmful gases like nitrous oxide and carbon monoxide. This has motivated a vigorous policy debate on alternative pathways for the light duty vehicle transportation sector. Researchers, technologists and automobile manufacturers throughout the world have been putting up their best efforts on the development of engines which would be more efficient than convention ones and should run cleaner. In the search of new cleaner alternative, Hydrogen is considered as a potential carrier of energy and can be generated domestically from variety of methods including coal gasification, natural gas steam reforming, electrolysis using solar or wind generated electricity. Powering a vehicle using hydrogen fuel produces little or no tail pipe carbon dioxide emission (the 324 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha

2 combustion of hydrogen is water). This opens the possibility of running transport vehicles on energy derived from very low carbon source, alleviating one of the major stumbling blocks in the way of reducing carbon dioxide emissions and oil imports. In this paper the researches are mainly focused on hydrogen due to its clean burning characteristics. When hydrogen is burnt in the combustion chamber the exhaust emitted contains water. The only pollutant of concern is NO x which can be drastically reduced at lean operation of engine. Thus the hydrogen operated engines are intrinsically capable of providing ultimate solutions to the problems. Also it is believed that use of hydrogen as a fuel would reverse or decelerate the greenhouse phenomena. COMBUSTION PROPERTIES OF HYDROGEN: Due to certain properties, hydrogen is potential contender for fuelling the internal combustion engine. These properties contributes to its use as a combustible fuel 1. Wide Flammability Range: Hydrogen can be combusted in an internal combustion engine over a wide flammability range (flammability limit 4-75%) whereas flammability range for gasoline is less (1 7.6%) so it can be combusted over a wide range of air fuel mixtures. Any engine which runs on lean mixture generally gives better fuel economy as most of fuel is completely combusted, also less emissions of certain pollutants like nitrous oxides (NO x ) will take place as combustion temperature will be less. problems like pre-ignition and flashback (ignition after vehicle is turned off). 3. Small Quenching Distance: Hydrogen flames travels much closer to cylinder walls before extinguishing. Also due to smaller quenching distance backfiring can also increase as flames will pass very near to the intake valve. 4. High Auto-ignition Temperature: High auto-ignition temperature of hydrogen allows larger compression ratios to be used in a hydrogen engine than in a hydrocarbon engine. Higher the compression ratio higher will be thermal efficiency of the system. The temperature rise is given by following equation- T 2 = T 1 (V 1 /V 2 ) (γ-1) Where, V 1 /V 2 = compression ratio T 1 = Absolute initial temperature T 2 = Absolute final temperature γ = ratio of specific heats 5. High Flame Speed: High flame speed of hydrogen (270 cm/s) which is much higher than CNG (39 cm/s) or gasoline (30 cm/s), allowing hydrogen to reach more close to thermodynamically ideal engine cycle conditioned to stoichiometric air-fuel ratio, but to reduce NO x emission, lean mixtures are used due to which flame speed goes down significantly. *NOTE: Hydrogen disperses quickly in air due to its low density and high diffusivity, also air fuel mixture has low energy density and will evolve with no phase change as hydrogen is not liquefied at any stage. 2. Low Ignition Energy: Hydrogen LIMITATIONS OF HYDROGEN FUELLED requires significantly less energy to ignite i.e. INTERNAL COMBUSTION ENGINE [7] : 0.02 MJ than what is required by gasoline i.e. Apart from various advantages of hydrogen 0.2MJ which allows prompt ignition of lean fuelled internal combustion engine (HICE) there mixtures, but on the other hand there are chances are some limiting points which needs to be of hot spots generation on cylinder surface which considered to provide good service in the future. can serve as a source of ignition generating These limitations includes- 325 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha

3 1. Pre-ignition: For the development of operational hydrogen engine, pre-ignition or knocking is the problem which is due to hydrogen s lower ignition energy, wide flammability range and short quenching distance. The pre-ignition is a phenomena in which the fuel mixture in combustion chamber ignites before ignition by spark plug. In case of hydrogen engine, pre-ignition is caused due to hot spot in combustion chamber or by hot spot are developed near spark plug or exhaust valve or carbon deposits in crevices. It has also revealed that the pyrolysis (chemical deposition brought about by heat) of oil suspended in the combustion chamber or in the crevices just above the top piston ring, also contributes to preignition. 2. Backfire: Pre-ignition and backfire are inter-related as backfire develops if the preignition occurs near the fuel intake valve and the resultant flame travels back into the induction system. In backfiring, the flame from the combustion chamber travels back into the intake manifold. It is mainly caused due toa. Improper injection system b. Overlapping between the opening of intake and exhaust valves c. High temperature in combustion chamber d. Pre-ignition 3. Nitrous Oxide (NO x ) Emission and Performance: After many studies it is concluded that NO x emission and performance of an engine are related to each other. In hydrogen engine the only pollutant release is NO x which is formed due to presence of nitrogen in atmosphere. Factually nitrogen is inert at normal conditions due to strong covalent bond between N-N atoms dissociation of which requires high temperature which is difficult to obtain at normal conditions. During combustion of hydrogen fuel in internal combustion engine produces sufficiently high temperature enough to break N- 326 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha N bond and convert it into harmful nitrous oxide (NO x ). In order to control NO x formation, the temperature in the combustion chamber should be less then N-N dissociation temperature. This can be achieved by using lean mixture of hydrogen (ɸ<0.7). This has shown a significant decrement in the NO x emission. But due to lean mixture power output and efficiency of hydrogen engine has been decrease significantly. Decrement in brake mean effective pressure was also noticed due to low temperature which results in low brake thermal efficiency. Also there are other methods for lowering down the NO x emission like exhaust gas recirculation and water injection in the combustion chamber in order to bring its temperature below N-N bond dissociation temperature. But in these methods brake power and thermal efficiency become a matter of concern and were not equivalent to gasoline engine. FUEL DELIVERY SYSTEM: Many studies have been conducted in order to reduce preignition and backfire. Even extreme lean operation in the presence of still burning gases from the previous cycle (particularly when intake valve opens) was also observed to be the cause of backfire in several configuration. Various fuel injection systems were studied which is used in gasoline and CNG engine in order to fuel Hydrogen engine. But these fuel delivery systems like- a. Carburation b. Port Fuel Injection (PFI) were not so effective to stop backfiring and preignition. Further studies were also done on Direct Injection system which is normally used in diesel engine. The detail of this fuel delivery system is discussed below- DIRECT CYLINDER INJECTION SYSTEM [6] : Direct injection is another option for fuel delivery in an internal combustion engine. In this system the fuel is directly injected into the

4 combustion chamber during the compression stroke and the intake valve is closed after the fuel is being injected in order to avoid pre-ignition during the intake stroke. Consequently the engine cannot backfire into the intake manifold. The power output of a direct injected hydrogen engine is 20% more than of gasoline engine and 42% more than a carburetted hydrogen engine. Beside these features various test on direct cylinder injection has indicated that it is very tough for the injector to survive in the severe thermal environment of combustion chamber over a prolonged engine operation. Also due to reduced mixing time of air and hydrogen in direct injection, the air/fuel mixture is nonhomogeneous which often results in incomplete combustion. Studies have also suggested that direct injection can lead to higher NO x emissions than the non-direct injection. From the equation it is clear that the maximum temperature within engine decreases even with same energy supply. This is achieved by increasing the quantity of lean mixture for every combustion process. The decrease in maximum gas temperature for Hydrogen and Hydrocarbon fuels are significantly different from each other even if same energy is supplied. This is because hydrocarbon fuels like gasoline (C 8 H 18 ) and methane (CH 4 ) have different material properties as that of Hydrogen. The specific heat at constant volume of hydrogen is approximately kJ/kg-K at 300K which is about 6.2 times larger than gasoline (1.642kJ/kg-K) or natural gas (1.736kJ/kg-K). A thermodynamic relation between temperature, pressure and mass flow rate was established and supercharging pressure was introduced which can be understood from the given graph below. POLLUTION FREE NO x EMISSION AT HIGH POWER: The air which goes into the cylinder contains O2 and N 2 in sufficient amount. The N2 which is present in air is responsible for the formation of NO x emission but to break N-N bond, high temperature is needed which is produced during the combustion of fuel in combustion chamber. A study was done to understand the relationship between energy (heat) and temperature in order to control the formation of NO x. It was found that the gas temperature (T g ) in the combustion chamber is proportional to the supplying energy (Q), and it is inversely proportional to the mass of fresh air (m a ) and specific heat at constant volume (C v ) as given by equation (1) T g = T 1 e n-1 + Q/ (m a C v ) (1) Here, T 1 = initial compression temperature e = compression ratio n = polytrophic index 327 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha Figure-1: Decrease rate of temperature (%) vs supercharging pressure (bar) The figure 3 shows the relation between temperature and supercharging pressure to compare the low temperature combustion effect of hydrogen with those of gasoline and natural gas. In the study, the amount of energy produced

5 during natural intake condition at ɸ = 1.0 was fixed for further engine cycles. The mixture which came in the chamber became leaner and combustions (backfire and NO x emission) was checked with increasing the supercharging pressure. However, while the values of gasoline and natural gas fuels were similar to each other and the value for hydrogen were relatively lower than the others. As discussed earlier, the supplying energy was constant. There were differences in fresh air masses with respect to the mass of each fuel, depending on the change in supercharging pressure of lean mixture, however, the mass of fresh air is generally proportional to the supercharged pressure. Due to the factors above, the maximum gas temperature reduction rate of hydrogen with supercharging mixture is lower than for other fuels. For example, the maximum gas temperature reduction rate, which of hydrogen, at a supercharged pressure of 0.15 MPa is approximately 23.1% lower than that of gasoline fuel. Consequently, to lower the maximum gas temperature of hydrogen to below the thermal dissociation temperature of NOx, hydrogen requires a more lean mixture than hydrocarbon fuels, due to its small gas temperature reduction rate and high adiabatic flame temperature. The maximum gas temperature was computed from the gas pressure within the cylinder and the ideal gas equation. The maximum gas temperature value for an identical supercharged pressure decreases rapidly as the equivalence ratio decreases. However, the maximum gas temperature value for an identical equivalence ratio is almost constant, even with the increase in supercharging pressure, due to the simultaneous increase in supplying energy and mass of the Fig.5. NOx versus fuel air equivalence ratios above bmep of gasoline level under 10 ppm of NOx. [2] Fig.4. NOx versus fuel air equivalence ratios above bmep of gasoline level. [2] mixture. This trend is shown in the regions that represents the low temperature combustion effect yield gasoline-level power as well. Theoretically 328 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha

6 the generation of thermal NO x takes place due to the Zeldovich NO reaction when the flame temperature is above 1850 K. It has been reported that NO x is generated from the flame temperature of 2200 K or above from experimental results. As expected, the condition that yields a temperature below NO x generation temperature in a hydrogen engine occurs in the super lean region, regardless of the change in supercharged pressure, and the region is the region below the equivalence ratio of ɸ = more, stable combustion becomes possible with an even leaner mixture, and achieves pollutionfree NOx emission without backfire generation even above the gasoline-level power measured. The maximum gas temperature needed was according to the load reduction, which verifies that NO x can be controlled with every load. The case of 100% load of gasoline-level power in the NO x pollution-free region is mentioned above. 75% load achieves 0 ppm from respective boost pressure and equivalence ratio of 0.15 MPa and From the experiments reviewed, the ɸ = 0.3. NO x emission at 50% load was verified supercharged pressure that satisfied the region to be below 10 ppm at its maximum, and 0 ppm above was approx MPa. Figure 4 illustrates was achieved from a supercharged pressure and the NO x emission according to changes in equivalence ratio of 0.12 MPa and ɸ = 0.315, equivalence ratio and supercharged pressure, and respectively. 0 ppm of NO x was achieved even Figure 5 illustrates the regions capable of NO x with a natural intake condition (ɸ = 0.27) with emissions of less than 10 ppm. The maximum 25% load. NO x emission, 2120 ppm, was observed when the mass of fresh air was increased right before knocking occurred, at the supercharged pressure ACHIEVEMENT OF HIGH capable of producing gasoline-level power. PERFORMANCE DUE TO LEAN SUPERCHARGING: It is a well-known fact However, when the supercharged equivalence that brake thermal efficiency shows an ratio became approximately ɸ = 0.5, NO x emissions decreased to approximately 330 ppm due to the low temperature combustion effect. As illustrated in figure 5, the NO x emission stays below 10 ppm, even with the high supplying energy required by gasoline-level power, when the supercharged pressure and equivalence ratio reach 0.16 MPa and ɸ = 0.4, respectively. In addition, the NO x emission around the equivalence ratio ɸ = 0.35 was pollution-free, with 0 ppm, when the maximum gas temperature was confirmed to be below the thermal dissociation, at a supercharged pressure of 0.17 MPa. Such a result shows that NO x can be controlled to the pollution-free level, even under high power, with the applications of valve time manifold injection (TMI) and supercharging the lean mixture to a hydrogen engine. Also, when the supercharging pressure is increased even Fig. 6. BTE vs. Equivalence Ratio [2] 329 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha

7 increasing trend with increasing supercharged pressure of lean mixture, with the decrease in cooling loss due to the noticeable decrease in the combustion temperature. This occurs even when the combustion time increases with a lean mixture. This trend was observed to be the same in the regions that yield gasoline-level power. The equivalence ratio that yields the maximum thermal efficiency was around ɸ = 0.6 in a natural intake condition, and became a leaner region as the supercharged pressure increased. When the supercharging pressure reached 0.21 MPa, which was the maximum supercharged pressure under the conditions of this experiment, the equivalence ratio was around ɸ = The thermal efficiency in the region that yielded gasoline-level power was generally above 35%, and under the supercharged pressure of 0.21 MPa, it reached the maximum of 39.05%. For conventional hydrogen with SI in the natural intake condition, the increase in thermal efficiency was approximately 30% without considering supercharger contribution. With the increase in supercharging pressure for lean mixture, the total combustion period increased proportionally due to the decrease in combustion velocity, due to the use of lean mixture, even with the same supplying energy. The flame development period ratio, which represents the difference with the increasing rate of the total combustion period, is almost the same, and the rapid burning period and final burning period ratios showed a slight increase and decrease. The gas temperature of the initial combustion and cylinder wall temperature decreased due to the use of the lean mixture, and the low temperature combustion effect with the supercharging the lean mixture. the supercharged pressure, brake thermal efficiency also shows increasing trend and the engine speed is constant at 1600 rpm. Conclusion: Many researches have been conducted for different injection system for suitable combustion phenomena. Direct Injection (DI) system was reviewed as most apt injection system. An appropriately designed direct injection system can get rid of any undesirable combustion phenomena such as backfire and rapid rate of pressure rise. The maximum temperature reduction rate of hydrogen from supercharging pressure is greater than those of other fuels due to the high specific heat of hydrogen at constant volume. However, lean limit of Hydrogen is higher than any other fuel and is capable of stable combustion up to the lean region of around ɸ=0.2. The temperature in combustion chamber can be reduced to below NOx formation temperature. Almost zero NO x emission was achieved and power was equivalent to gasoline operated engine with low temperature combustion supercharging super lean mixture. Acknowledgement: The present level of accomplishment has been an outcome of many individuals efforts. I would like to give sincerer thanks to professor LM Das (senior scientist at centre of energy, IITD) for his valuable guidance and suggestions. I would also like to thank Dr. S. Rajeshra (HOD of Dept. of Mechanical Engineering, JSSATE, Noida) and Mr. Uday Pratap Singh (Assistant Professor, JSSATE, Noida) for their supervision on this project. Reference: 1. L.M. Das Hydrogen Engines: Research and Development Programmes in Indian Institute of Technology Delhi International Journal of Hydrogen Energy Vol 27, pp (2002). When the lean charge is supercharged with pressure with the help of supercharger than brake thermal efficiency is maximum between equivalence ratios of 0.2 to 0.4. With increasing 330 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha

8 2. Jongtai Lee, Kwangju Lee, Jonggoo Lee, Byunghoh Anh, High power performance with zero NOx emission in a hydrogen-fuelled spark ignition engine by valve timing and lean boosting International Journal of Hydrogen Energy, Fuel 128 (2014) Shravan K. Vudumu, Umit O. Koylu, Computational modelling, validation, and utilization for predicting the performance, combustion and emission characteristics of hydrogen IC engines Energy 36 (2011) 647e L.M. Das a, Milton Polly b, Vishal, Performance Evaluation of a Hydrogen Fuelled SI Engine Genset WHEC 16 / June 2006 Lyon France. 5. S. Verhelst, Recent progress in the use of hydrogen as a fuel for internal combustion engines International Journal of Hydrogen Energy 39 (2014) 1071e College of the Desert, Hydrogen used in Internal Combustion Engine Revision0, December Kenneth Gillingham, Hydrogen Internal Combustion Engine Vehicles: A Prudent Intermediate Step or a Step in the Wrong Direction? Stanford University Department of Management Science & Engineering Global Climate and Energy Project, January Roger Sierens, Sebastian Verhelst, Hydrogen Fuelled Internal Combustion Engines Laboratory of Transport Technology, Ghent University Sint Pietersnieuwstraat 41 B-9000 Gent, Belgium. 331 Uday Pratap Singh, Ishan Sahu, Ravikant Shukla, Navpreet Chaddha