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What is hydrogen?   Why is hydrogen?  In History  In Chemistry  The Atom
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Flame speed is one of the fundemental characteristics in combustion process. It identifies the relative motion of flame front respect the unburt mixture.The high flame speed of hydrogen provides efficient cycle similar to constant volume process. On the other hand, higher thermal and mechanical loads along with increased combustion temperatures are the reasons of thermal
losses, combustion noise and nitric oxide emissions.
 

The wide flammability limit of hydrogen in air provides lean operation which brings low NOx emission along with higher thermal efficiencies. Diffusion coefficient of hydrogen which means better homogeneous charge also affect lean operation positively.
 

High quenching distanceprovides the flame front to penetrate into smallest crevice without being quenched.With respect to this characteristic, hydrogen flame can reach the unburnt mixtures than flames of fossil fuels.
 

Low ignition energy of hydrogen causes preignition and backfiring of hydrogen engines using externally formed hydrogen-air mixtures.The reason is probably hot oil deposits on other hotspots in cylinder. Although,several approches have been applied in order to solve this problem, None of them could not provide satisfactory results. Cooled exhaust gas recirculation, cold gaseous hydrogen, water injection, four-valve schemes are among those methods.
 

Energy density is related to storage. As a gas, hydrogen has a very low energy density by volume. This causes large fuel tank size even with high pressure storage and short vehicle range. For example, to store the equivalent of only five gallons of gasoline in compressed hydrogen requires a heavy tank at least the size of a 55 gallon drum. Besides, the low energy density of gaseous hydrogen causes a 20 % power reduction compared to gasoline. Because a stoichometric hydrogen-air mixture contains 20 % less energy than same volume of a gasoline-air mixture. Exhaust heat is not sufficient for turbocharging application, but supercharger with some efficiency loss can be used in order to compansate power loss. The energy density by volume of liquid hydrogen is also low (one fourth that of gasoline). Energy density by mass of hydrogen is 3 times that of gasoline which means that liquid hydrogen system will not suffer a weight penalty compared to gasoline.
 

Direct high pressure injection is the most efficient application in IC engines. High pressure liquid which is pressurized by a high pressure pump is converted to cold high pressure gas by using a heat exchanger. Injection is provided after the closing of valves which is similar to diesel engine. Ignition is done with a spark or glow plug. Preignition decreased by using this application. Since hydrogen does not displace air in the cylinder, it provides better volumetric efficiency which means higher
power.