Theoretical Investigation of using Hydrogen in SI-engine and its only Pollutant

A major portion of energy consumption is in transport sector. At present over 800 millions motor vehicles are working in the world [1]. Gasoline and diesel as two main fuels used in motor-vehicles play a significant role in pollutant formation, such as HC, CO, NOx, SO2. CO2 as the main product of fossil fuel combustion, is the biggest challenge of human being today, because of global warming effect.Using hydrogen fuel eliminates the production of HC, CO, CO2, SO2. The only pollutant of hydrogen combustion is NOx and its amount depends on the combustion temperature and hence on the hydrogen-air mixture.The idea of using hydrogen as engine fuel was first expressed by Cecil in 1820. He was the first person used hydrogen as fuel in the internal combustion engine.Research on the hydrogen engine started at the end of 1970s. This research initially was based on the practically converting the engine and vehicle to hydrogen engine and hydrogen fuelled vehicle. It is followed by studying the performance of the engine experimentally and then theoretically.In the present paper, the performance of hydrogen engine and its only pollutant is theoretically investigated. A thermodynamic based model is used. General rate equations for temperature and pressure and also two relations for cylinder volume and combustion surface area are used. The mass flow rate of inflow and outflow through valves are also considered. Four processes, (Induction, Compression, Expansion and Exhaust) are considered and the above equations for each process are simplified.The computation starts with intake process. The mass and composition of the residual gases are initially guessed and it is assumed that no chemical reaction take place during this process. The mass inflow equation has to be solved simultaneously with simplified rate equations for temperature and pressure. Composition of the mixture has to be calculated as the fresh air-fuel mixture enters into the cylinder and for that composition the gas properties required in rate equations must be calculated at any temperature during intake process.During compression process, from the inlet valve closing time till the initiation of combustion with spark plug, the mixture composition is assumed to remain constant and only simplified rate equations for temperature and pressure are to be solved. Again the required properties for solving these equations are obtained at each temperature.For combustion process, turbulent flame is assumed and the cylinder volume is divided to two zones separated by flame front whose thickness is negligible. Combustion starts from spark plug position which is at the top and centre of cylinder head (a burned zone of 1 mm diameter is initially assumed.) and propagates till the whole mixture is burned. During the combustion, pressure is uniform in the cylinder, the temperature in the burned and unburned zone are also uniform. Mixture composition in the unburned zone is assumed to be constant and in the burned zone is obtained from the chemical equilibrium. During combustion process, the contact surface area of the burned and unburned zones with the combustion chamber walls, are also considered to determine the heat transfer rate. NO formation is also considered in this process.During expansion, as the temperature and pressure fall, NO formation calculation continues.During exhaust, equation of rate of change of temperature and pressure along with mass outflow rate are solved.The computation continues till at the end of the cycle, the assumed values of mass and composition of residual gases at the start are obtained with proper approximation.In the presentation of the results; 1- Parameters pertaining to the engine condition (maximum temperature, maximum pressure and maximum rate of pressure rise), 2-Emission characteristics (nitric oxide emission), 3- Engine performance (optimum spark timing, indicated power) and 4- Efficiency characteristics (indicated thermal efficiency) are given versus equivalence ratio.From the first three curves, it is shown that the temperature, the pressure and the rate of pressure rise are high, at close to stoichiometric condition. From the curve of maximum rate of pressure rise, which is the indicative of knocking problem, the knock-limiting equivalence ratio for hydrogen engine under the specified condition is determined which is φ=.6 (by assuming that a pressure rise of 6 atmosphere per degree crank angle can cause knocking problem in the engine). So at this knock-limited equivalence ratio, the maximum temperature and maximum pressure attainable in the hydrogen engine under the specified condition can be obtained, from other two curves which are 2400 K and 5.2 MPa.From nitric oxide curve, it is shown that although NO formation is high for hydrogen engine at near stoichiometric condition due to existence of high temperature, but its maximum value occurs at equivalence ratio of φ=.8. It is also shown that at equivalence ratio lower than knock limited equivalence ratio (φ=.6) where the hydrogen engine is to be run, the NO formation is negligible.Hydrogen engine's performance is given by the curves of optimum spark timing and indicated power versus equivalence ratio. It is shown that although lower spark advance is required (because of higher flame propagation rate) near the stoichiomeric condition, but at knock free region of lower equivalence ratio higher spark advance is required. It is also shown from the indicated power curve that the power attainable from hydrogen engine is lowered because of the knocking problem.Hydrogen engine's efficiency curve indicates that, this avoidance of the knocking problem which forces the engine to be run at lean condition, and hence causes higher indicated thermal efficiency.Further, for presenting information on combustion, cylinder pressure and percent of mass burned are given in terms of the crank angle rotation, for limiting equivalence ratio of φ=0.6. The start of the combustion and the end of the combustion can be identified from the cylinder pressure versus crank angle curve, but it does not provide information about the mass fraction burned. The curve of percent of mass burned versus crank angle gives this information and shows that high percent of the mass is burnt in the later part of combustion. This is because the flame front surface area increases as the combustion proceeds.From the present study, it is concluded that, because of the high combustion rate of hydrogen-air mixture, the maximum temperature, maximum pressure and maximum rate of pressure rise are high near stoichiometric condition. It is therefore necessary to have a mixture of lower equivalence ratio than knock-limited equivalence ratio (φ=.6, under assumed condition of compression ratio 8, and rotational speed 2000 RPM) to be used in hydrogen engine. Under this lean limit condition, the nitric oxide emission as the only pollutant of the hydrogen engine is negligible, the performance and efficiency are high, but it's indicated power is low.