Hydrogen as alternate fuels

Hydrogen as alternate fuels

Hydrogen is a long-term renewable and less-polluting fuel. In addition hydrogen is clean burning characteristics and better performance drives more interest in hydrogen fuel. When it is burnt in an internal combustion engine, the primary combustion product is water with no CO2. Although NOx emissions are formed when hydrogen is used.

Combustive Properties of Hydrogen

Wide range of flammability

Compared to nearly all other fuels, hydrogen has a wide flammability range (4-75% versus 1.4-7.6% volume in air for gasoline). This first leads to obvious concerns over the safe handling of hydrogen. But, it also implies that a wide range of fuel-air mixtures, including a lean mix of fuel to air, or, in other words, a fuel-air mix in which the amount of fuel is less than the stoichiometric, or chemically ideal, amount.

Small quenching distance

Hydrogen has a small quenching distance (0.6 mm for hydrogen versus 2.0 mm for gasoline), which refers to the distance from the internal cylinder wall where the combustion flame extinguishes. This implies that it is more difficult to quench a hydrogen flame than the flame of most other fuels, which can increase backfire since the flame from a hydrogen-air mixture more readily passes a nearly closed intake valve, than a hydrocarbon-air flame.

Flame velocity and adiabatic flame

Hydrogen burns with a high flame speed, allowing for hydrogen engines to more closely approach the thermodynamically ideal engine cycle (most efficient fuel power ratio) when the stoichiometric fuel mix is used. However, when the engine is running lean to improve fuel economy, flame speed slows significantly.

Flame velocity and adiabatic flame temperature are important properties for engine operation and control, in particular thermal efficiency, combustion stability and emissions.

Minimum ignition source energy

The minimum ignition source energy is the minimum energy required to ignite a fuel-air mix by an ignition source such as a spark discharge. For a hydrogen and air mix it is about an order of magnitude lower than that of a petrol-air mix 0.02 mJ as compared to 0.24 mJ for petrol - and is approximately constant over the range of flammability.

The low minimum ignition energy of the hydrogen-air mix means that a much lower energy spark is required for spark ignition. This means that combustion can be initiated with a simple glow plug or resistance hot-wire. It also ensures prompt ignition of the charge in the combustion chamber.

High diffusivity

Hydrogen has very high diffusivity. This ability to disperse into 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

The most important implication of hydrogen’s low density is that without significant compression or conversion of hydrogen to a liquid, a very large volume may be necessary to store enough hydrogen to provide an adequate driving range. Low density also implies that the fuel-air mixture has low energy density, which tends to reduce the power output of the engine. Thus when a hydrogen engine is run lean, issues with inadequate power may arise.

High auto-ignition temperature

The auto ignition temperature is the minimum temperature required to initiate self-sustained combustion in a combustible fuel mixture in the absence of an external ignition. For hydrogen, the auto ignition temperature is relatively high 585ºC. This makes it difficult of ignite a hydrogen–air mixture on the basis of heat alone without some additional ignition source. This temperature has important implications when a hydrogen–air mixture is compressed. In fact, the auto ignition temperature is an important factor in determining what maximum compression ratio an engine can use, since the temperature rise during compression is related to the compression ratio.

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.

Advantages of hydrogen as alternate fuel:

1.  Hydrogen produces only water after combustion.

2H₂ + O₂ = 2H₂O

2.     When hydrogen is burned, hydrogen combustion does not produce toxic products such as hydrocarbons, carbon monoxide, and oxide of sulfur, organic acids or carbon dioxides

3.     Hydrogen has some peculiar features compared to hydrocarbon fuels, the most significant being the absence of carbon.

4.     The burning velocity is so high that very rapid combustion can be achieved.

5.     The density of hydrogen is 0.0838 kg/m3, which is lighter than air that it can disperse into the atmosphere easily.

6.     Hydrogen has the highest energy to weight ratio of all fuels.

Disadvantages of hydrogen as alternate fuels:

1.     NO_x is formed as emission.

2.     Storage of hydrogen is more difficult as it leads to crack.

3.     It is not possible to achieve ignition of hydrogen by compression alone. Some

sources of ignition have to be created inside the combustion chamber to ensure ignition.

Formation of NO_x depends on the factors like

·        The air/fuel ratio

·     The engine compression ratio

·        The engine speed

·        The ignition timing

·        Whether thermal dilution is utilized