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Hydrogen Application in an Internal Combustion Engine.
In the future, hydrogen fuel cells are expected to power automobiles as a practical replacement for the internal combustion engine. But researches are now made on the plausibility of hydrogen powering an existing internal combustion engine. Hydrogen fuel cell engines are proven to be more efficient but several advantages are presented by the use of an internal combustion engine both economically and technically. One is that retooling for production costs are much lower and there are several instances where a hydrogen fuel cell engine is not practical like in cold weather applications. Another is that fuel cell engines are still fragile and costly to produce. Currently the Hydrogen Internal Combustion Engine (HICE) is still under development and there are still problems regarding hydrogen application in the engine.
Challenges with Hydrogen Use
Unlike petroleum, hydrogen as a combustive fuel presents the following properties; wide range of flammability, low ignition energy, small quenching distance, high auto ignition temperature, high flame speed, high diffusivity and very low density. This must be taken in consideration in the case of its application in an internal combustion engine.
The wide range of flammability of hydrogen brings an advantage in its use with the engine. It could combust on low air to fuel ratio, also known as lean mixture. This brings a greater fuel economy and combustion reaction is more complete. It should be noted that hot gases and hot spots within the engine could uncontrollably ignite hydrogen due to its low ignition energy. Preventing this from occurring is just one of the challenges in developing the HICE. What's more, due to the small quenching distance of hydrogen, the flame travels closer to the cylinder wall and the flame could pass through a nearly close valve and there is a greater chance for backfiring. Hydrogen also had a high auto ignition temperature and this could raise problems when it is applied in a Diesel engine where the compression temperature could be insufficient to ignite hydrogen. But since hydrogen had a high flame speed, hydrogen engines can more closely approach the thermodynamically ideal engine cycle. Also hydrogen could disperse in the air easily due to its high diffusivity. One advantage of this is that it facilitates the formation of a uniform mixture of fuel and air at the same time if hydrogen leaks develop, hydrogen disperses rapidly, and thus unsafe condition can be minimized. But two problems are presented regarding the low density of hydrogen. One is that a very large volume is necessary to store enough hydrogen to give an adequate running time for the engine and the low energy density could reduce the power output.
Modifications taken for the Internal Combustion Engine
Several problems regarding hydrogen use are still yet to be solved. But for the problems regarding pre-ignitions, which is the uncontrollable ignition oh hydrogen with the engine, a modified fuel delivery system was suggested. Hydrogen fuel delivery system can be broken down to three main types; central injection, port injection and direct injection. Central injection is fuel delivery through the carburetor; port injection is fuel injection into the intake manifold and the direct injection systems is done into the combustion cylinder during compression stroke. Pre-ignition conditions can also be curbed using thermal dilution techniques such as exhaust gas recirculation or water injection. Engines are also redesigned to effectively control pre-ignition, particularly for the combustion chamber and the cooling system where the combustion chamber are disk shaped to reduce turbulence and the cooling system to provide a more uniform flow. As for the ignition system in the case of a gasoline engine, the use of waste spark was eliminated as it is a source of pre-ignition and spark plugs should have a cold rating and have non platinum tips. The engine is also properly ventilated to prevent the water vapor, which is the exhaust of hydrogen from condensing within the crankcase. Lastly some engine components materials are replaced to prevent hydrogen embrittlement, or the cracking of metal due to hydrogen exposure.
Due to some problems that are still unresolved regarding HICE, the technology is yet to be applied. Future plans include further study in the design improvements. Nevertheless several companies like Mazda are now developing their own HICE seeing its potential advantage towards