Broad Range of Flammability Hydrogen has an extensive combustible range compare to other fuelsuch as gasoline, diesel and natural gas 1.
Therefore, highly lean or richmixture of hydrogen-air can be burn. Greater fuel economy achieved if hydrogeninternal combustion engine run at lean operation 2. The quantity of pollutant such as nitrogenoxide lowered in exhaust due to lower combustion temperature. However, amountof hydrogen injected into the engine should have the perfect limit when theengine runs in the lean condition. Engine power output decrease significantlycaused by depletion in the volumetric heating value of the air/fuel mixturewhen lean condition 3. Minimum ignition energy Hydrogen has extremely low ignition energy.
Combustion energy for hydrogen is about oneorder magnitude smaller than needed for petroleum fuel. The advantages ofhaving least ignition energy to allow hydrogen engine to burn lean mixture andsecure quick ignition 3. However, there is a drawback of having lowerignition energy.
There is a hugepossibility of the hydrogen-air mixturecan be ignited by different source rather than spark plug such as hot pots 4. Short QuenchingDistance Quenchingdistance of hydrogen is shorter compared to petroleum fuel. Consequently,hydrogen fires move nearer to the cylinder wall than gasoline before hydrogenquench. Quenching the flame will be difficult if the quenching distance isextremely smaller. This may raise the tendency to backfire because thehydrogen-air blend easily reaches a nearly closed intake valve than anisooctane-air flame. 5. High AutoignitionTemperature Hydrogen has arelatively great auto-ignition temperature.
This has a significant impact on a hydrogen-air mixture is compressed.The compression ratio of an engine is determined by the autoignitiontemperature of the fuel.The temperature rise is shown by the equation: T2=T1( ) ?-1 where: V1/V2 = thecompression ratio T1 =absolute initial temperature T2 =absolute final temperature ? = ratio of specific heats From the aboveequation, we know that compression ratio relies on T2 which istemperature during compression. This T2is restricted by auto-ignition temperature of the fuel to limit air-fuelmixture to ignited automatically (knocking) before spark plug sparks theair-fuel mixture. An engine can reach great thermal efficiency with highcompression ratio when the auto-ignition temperature fo of fuel is high 6. High flame speed, high diffusivity and lowdensity Hydrogen internalcombustion engine able to achieve similarity to the thermodynamically idealengine cycle because of the high flame velocity. Nevertheless, the flame speedreduces significantly when the engine operates in the lean condition 7.
Hydrogen capable of diffuse in air rapidly than hydrocarbon fuel such asiso-octane and natural gas. This is due to high diffusivity of hydrogen. Thisenables a uniform mixture of the air-hydrogen mixture. Hydrogen will notpollute the environment if any leakage happens. However, there are problems inhydrogen internal combustion engine due to very low density. Hydrogen storagerequired a very large volume of sufficient driving range and diminish poweroutput due to low energy density.