HYDROGEN INTERNAL COMBUSTION ENGINE -PROPERTIES, LIMITATIONS AND MODIFICATIONS
HYDROGEN INTERNAL COMBUSTION ENGINE
Hydrogen can be used as the primary fuel in an internal combustion engine or in a fuel cell. A hydrogen internal combustion engine is similar to that of a gasoline engine, where hydrogen combusts with oxygen in the air and produces expanding hot gases that directly move the physical parts of an engine. The only emissions are water vapour and insignificant amounts of nitrous oxides. The efficiency is small, around 20%.
PROPERTIES OF HYDROGEN THAT CAN BE EXPLOITED IN HYDROGEN INTERNAL COMBUSTION ENGINE
Wide range of flammability: Hydrogen has a wide flammability range in comparison with all other fuels. As a result, hydrogen can be combusted in an internal combustion engine over a wide range of fuel-air mixtures. A significant advantage of this is that hydrogen can run on a lean mixture.
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
Unfortunately, the low ignition energy means that hot gases and hot spots on the cylinder can serve as sources of ignition, creating problems of premature ignition. Preventing this is one of the challenges associated with running an engine on hydrogen. The wide flammability range of hydrogen means that almost any mixture can be ignited by a hot spot.
Small quenching distance: 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. The smaller quenching distance can also increase the tendency for backfire since the flame from a hydrogen-air mixture more readily passes a nearly closed intake valve, than a hydrocarbon-air flame.
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.
Very 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 density of a hydrogen-air mixture, and hence the power output, is reduced.
EMISSIONS FROM HYDROGEN INTERNAL COMBUSTION ENGINE
The combustion of hydrogen with oxygen produces water as its only product:
2H2 + O2 = 2H2O
The combustion of hydrogen with air however can also produce oxides of nitrogen (NOx):
H2 + O2 + N2 = H2O + N2 + NOx
The oxides of nitrogen are created due to the high temperatures generated within the combustion chamber during combustion. This high temperature causes some of the nitrogen in the air to combine with the oxygen in the air.
Depending on the condition of the engine (burning of oil) and the operating strategy used (a rich versus lean air/fuel ratio), a hydrogen engine can produce from almost zero emissions (as low as a few ppm) to high NOx and significant carbon monoxide emissions.
LIMITATIONS WITH HYDROGEN INTERNAL COMBUSTION ENGINE
The following is a list of some limitations associated with hydrogen internal combustion engine.
Hydrogen as a compressed gas has merely around 5% of the energy of gasoline of the same volume. This is a major shortcoming particularly for transport applications. Engines fuelled with hydrogen suffer from reduced power output, due mainly to the very low heating value of hydrogen on volume basis and resorting to lean mixture operation.
There are potential operational problems associated with the uncontrolled pre-ignition and backfiring into the intake manifold of hydrogen engines.
The high burning rates of hydrogen produce high pressures and temperatures during combustion in engines when operating near stoichiometric mixtures. This may lead to high exhaust emissions of oxides of nitrogen.
Hydrogen engine operation may be associated with increased noise and vibrations due mainly to the high rates of pressure rise resulting from fast burning.
Great care is needed to avoid materials compatibility problems with hydrogen applications in engines. In certain applications, such as in very cold climates, the exhaust emission of steam can be an undesirable feature leading to poor visibility.
Hydrogen requires a very low ignition energy, which leads to uncontrolled pre-ignition problems.
A hydrogen engine needs to be some 40–60% larger in size than for gasoline operation for the same power output. This could impose some reduction to engine speed, increased mechanical losses.
ENGINE MODIFICATION FOR HYDROGEN INTERNAL COMBUSTION ENGINE
Pre-ignition and Backfire
Hydrogen burns quickly and has a low ignition temperature. The hot spots in the cylinder may cause the fuel to be ignited before the intake valve closes. It may also cause backfire, pre-ignition, or knock. These problems are particularly more with high fuel-air mixtures. Uncontrolled pre-ignition resists the upward compression stroke of the piston, thereby reducing power. Backfire remedies include: timed port injection, delayed injection to make sure the fuel detonates only after the intake valve is closed; water injection, 1.75 water to hydrogen, by weight. An appropriately designed timed manifold injection system can overcome the problems of backfiring in a hydrogen engine.
Keeping the air and fuel separate until combustion is an important strategy for controlling the difficulties arising from the fast-burning of hydrogen. The low flammability limits and low energy required for ignition of hydrogen causes pre-ignition and backfire when using hydrogen fuel. Pre-ignition occurs when a fuel-air mixture ignites in the combustion chamber before the intake valve closes. Pre-ignition can cause backfire when ignited fuel-air mixture explodes back into the intake system. It is most present at higher loads and at higher fuel-air mixtures near open throttle.
Pre-ignition is not a necessary precursor to backfiring and probably not occurs under normal circumstances at moderate compression. Because of the low volumetric energy content of hydrogen, higher compression ratios or higher fuel delivery pressures are needed to avoid reduced power. Some spark ignition engines compresses the fuel-air mixture before being inducted into the cylinder. Direct fuel injection involves mixing the fuel with air inside the combustion chamber. Until then fuel and air are kept separate. If the fuel and air are mixed before entering the combustion chamber; the arrangement is called external mixing. A carburettor usually accomplishes this.
Internal combustion engines waste about two-thirds of the combustion energy as heat. Adding water to hydrocarbon fuels allows the heat of combustion to combine the oxygen in the water with unburned carbon in the exhaust. This produces a combination of hydrogen and carbon monoxide. The hydrogen then burns, creating additional power. The induction of water vapour into the cylinder reduces the combustion temperature of nitrous oxide formation. Water induction is an effective means of controlling nitrous oxide without loss of power, efficiency, or exhaust temperature. The effectiveness of water induction increases with rpm.
LIST OF RECENT HYDROGEN INTERNAL COMBUSTION VEHICLES
2002 – BMW 750hl
2007 – BMW H2R speed record car – ICE-liquid hydrogen
2001 – Ford P2000 concept car using the Zetec 2.0L engine. (Note: Ford had several concept vehicles that used the P2000 designation.)
2006 – F-250 Super Chief a “Tri-Flex” engine concept pickup
2006 – Ford E-450 H2ICE Shuttle Bus a 12 passenger shuttle bus with a supercharged V10 fueled by compressed hydrogen
1991 – Mazda HR-X hydrogen Wankel rotary
1993 – Mazda HR-X2 hydrogen Wankel rotary
1993 – Mazda MX-5 Miata hydrogen Wankel rotary
1995 – Mazda Capella Cargo, first public street test of the hydrogen Wankel rotary engine
2003 – Mazda RX-8 Hydrogen RE hydrogen-gasoline hybrid Wankel rotary
2007 – Premacy Hydrogen RE Hybrid