04-12-2012, 11:40 AM
HYDROGEN AS FUEL FOR IC ENGINE
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ABSTRACT
In the history of internal combustion engine development, hydrogen has been considered at several phases as a substitute to hydrocarbon-based fuels. Starting from
the 70’s, there have been several attempts to convert engines for hydrogen operation.
Together with the development in gas injector technology, it has become possible to
control precisely the injection of hydrogen for safe operation. Since the fuel cell needs
certain improvements before it is widely used in vehicles, the conventional internal combustion engine is to play an important role in the transition. This study examines the
performance characteristics and emissions of a hydrogen fueled conventional spark ignition engine. Slight modifications are made for hydrogen feeding which do not change the basic characteristics of the original engine. Comparison is made between the gasoline and hydrogen operation and engine design changes are discussed. Certain
remedies to overcome the backfire phenomena are attempted.
INDRODUCTION
The incentives for a hydrogen economy are the emissions, the potentially CO2-free use, the sustainability and the energy security. In this paper the focus is on the use of hydrogen in internal combustion engines (ICE), or more precisely, hydrogen fuelled spark ignition (SI) engines. Internal combustion engines are classified as spark ignition (SI) and compression ignition (CI) engines, depending on the combustion process initiated in the cylinder. A spark plug initiates the combustion of the fuel-air mixture in SI engines. In CI engines, fuel-air mixture is self-ignited by compression. It must be mentioned that hydrogen’s auto-ignition temperature is high (about 576), and it is impossible to bring hydrogen to its auto-ignition temperature by compression only. So,
Supportive ignition triggering devices should be used in the combustion chamber.
Many researchers have been directed their studies towards the effect of using hydrogen in internal combustion engines. Das [2, 3] evaluated the potential of using hydrogen for small horsepower SI engines and compared hydrogen fuelling with compressed natural gas (CNG). Another study dealt on certain drawbacks of hydrogen fuelled SI engines, such as high NOx emission and small power output determined the performance, emission and combustion characteristics of hydrogen fuelled SI and CI engines. Karim [4]reviewed the design features and the current operational limitations associated with the hydrogen fuelled SI engine. Li and Karim [5]investigated the onset of knock in hydrogen fuelled SI engine applications. This experimental study which has been carried out at Engines Laboratory of Dokuz Eylul University at Izmir examines the performance and emission characteristics of hydrogen fueled conventional.
HYDROGEN
Hydrogen is clean because water is the product after combustion in an internal combustion engine. However, it is important to note that nitrogen oxides [NOX] are produced as well because the high temperature in the engine makes it possible for the nitrogen in air to react with oxygen. However, the NOX produced is still less than that from petrol/diesel internal combustion engines.
Properties of Hydrogen
Hydrogen is an odorless, colorless gas. With molecular weight of 2.016, hydrogen
is the lightest element. Its density is about 14 times less than air (0.08376 kg/m3 at standard temperature and pressure). Hydrogen is liquid at temperatures below 20.3 K (at atmospheric pressure). Hydrogen has the highest energy content per unit mass of all fuels – higher heating value is 141.9 MJ/kg, almost three times higher than gasoline.
Combustive Properties of Hydrogen
Wide Range of Flammability
As can be seen the flammability limits (= possible mixture compositions for ignition and flame propagation) are very wide for hydrogen (between 4 and 75 percentage hydrogen in the mixture) compared to gasoline (between 1 and 7.6 percentage). This means that the load of the engine can be controlled by the air to fuel ratio, as for diesel engines. Nearly all the time the engine can be run with a wide open throttle, resulting in a higher efficiency
Low Ignition Energy
Hydrogen has very low ignition energy. The amount of energy needed to ignite hydrogen is about one order of magnitude less than that required for gasoline. This enables hydrogen engines to ignite lean mixtures and ensures prompt ignition.
Small Quenching Distances
Hydrogen has a small quenching distance, smaller than gasoline. Consequently, hydrogen flames travel closer to the cylinder wall than other fuels before they extinguish. Thus, it is more difficult to quench a hydrogen flame than a gasoline flame.
High Auto ignition Temperature
The temperature may not exceed hydrogen’s auto ignition temperature without causing premature ignition. Thus, the absolute final temperature limits the compression ratio. The high auto ignition temperature of hydrogen allows larger compression ratios to be used in a hydrogen engine than in a hydrocarbon engine.
High Flame Speed
Hydrogen has high flame speed at stoichiometric ratios. Under these conditions, the hydrogen flame speed is nearly an order of magnitude higher (faster) than that of gasoline. This means that hydrogen engines can more closely approach the thermodynamically ideal engine cycle. At leaner mixtures, however, the flame velocity decreases significantly.
High Diffusivity
Hydrogen has very high diffusivity. This ability to disperse in air is considerably greater than gasoline and is advantageous for two main reasons. Firstly, it facilitates the formation of a uniform mixture of fuel and air. Secondly, if a hydrogen leak develops, the hydrogen disperses rapidly. Thus, unsafe conditions can either be avoided or minimized.
Low Density
Hydrogen has very low density. This results in two problems when used in an internal combustion engine. Firstly, a very large volume is necessary to store enough hydrogen to give a vehicle an adequate driving range. Secondly, the energy den-sity of a hydrogen-air mixture, and hence the power output, is reduced.
Experimental set up and procedure
The setup consists of test banks involving water and eddy current type dynamometers, exhaust emission analyzers, fuel metering devices and support equipment. The dynamometer and supporting electrical equipment were calibrated a few days before the tests began. To avoid temperature and pressure variations as far as possible, experiments with gasoline were immediately followed by hydrogen experiments with the engine already warmed up to operating temperature. Compressed hydrogen at 200 bar from 50 l steel bottles was dropped down to 3 bar in the first stage regulator. The fuel line is a copper tube connected to a hydrogen flow meter. The second stage regulator supplies the gaseous hydrogen to the mixer according to the inlet manifold pressure.
The engine is coupled to the dynamometer with its gearbox. The 4th gear has a ratio of 1:1 so the rotational speed measured at the dynamometer is exactly the same as the engine speed. Besides the engine itself; flywheel, starting motor, alternator, fuel pump, fuel tank, dashboard assembly and exhaust assembly are mounted to the required parts and places. At the exhaust outlet, there is a standard muffler and a final silencer muffler. Exhaust temperature was measured between the two muffler positions and emission values were obtained just after the final silencer.
Description of the Test Rig
Illustrates the basic setup of the test bench. The engine is coupled with its original shaft to the dynamometer. The control panel of the dynamometer is placed at a safe distance from the setup but is easily accessible. Ambient pressure and temperature as well as engine speed and torque values are easily read from the large size gauges. Load is varied by two knobs that change the current in the stator of the eddy current dynamometer. Basically three types of loading are possible, constant speed, variable speed and a combination of these. A 3-way switch is installed on the dashboard assembly that allows immediate switching from gasoline to hydrogen. This switch controls the solenoid valves on the gasoline line and hydrogen regulator. In this way, switching between fuels is possible without stopping the engine.
Fuel Supply System
Compressed hydrogen at 200 bar is used for fuelling the engine. 50 liter steel bottles were just enough for a load session. Pairs of bottles were always kept ready since during the tune up of the regulator considerable amount of hydrogen was spent. An LPG conversion kit was used for hydrogen feeding. Since the experiments were made at partial load and the specified rated power for the regulator was 190 HP (140 kW), it compensated for the high volumetric flow needs due to hydrogen’s low density.
Figure shows the regulator installed above the radiator of the engine.
CONCLUSION
Specific features of the use of hydrogen as an engine fuel have been analyzed.
• Power and torque loss occurs at low speed hydrogen operation. At high speed hydrogen gives better performance as compare to gasoline operation.
• Similarly Thermal efficiency and Brake mean effective pressure of hydrogen is more at higher speed.
• NOx emission of hydrogen fuelled engine is about 9-10 times lower than gasoline fuelled engine.