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Hydrogen Internal Combustion Engine

From: Philip Brochu

The Hydrogen Internal Combustion Engine, otherwise known as HICE, is an engine that uses hydrogen as the combustion fuel source instead of modern gasoline and offers a bridge or transgression between gasoline engines and future hydrogen fuel cell vehicles.^[2] Hydrogen is one of the vital elements that combine into producing water. Hydrogen offers an environmentally friendly fuel source that has the potential to reduce carbon emissions as well as reduction in the dependence of imported oil.

Properties offered by Hydrogen

Available Lean Mixes of Fuel

A beneficial characteristic offered by hydrogen is it's "range of flammability."^[1] Hydrogen's approximate flammability range is 4-74% which can be compared to the percentage of the volume in an air/gasoline proportion 1.4-7.4%.^[3] This larger range offer the potential lean mixes of fuel. Hydrogen engines will theoretically be able to run on less than the stoichiometric ratio ideal amount which will improve the fuel economy.

Low Density

Hydrogen has a relatively low density compared to that of other fuel sources. The benefit of having a low density fuel source is the amount that can be contained. The hydrogen can be pressured into a liquid and a large portion can be stores for longer travel distances.^[1] Furthermore, this can also attribute to running the engine on a lean mix. It is important to note though that the energy produced during the combustion phase will be significantly less than that of a normal combustion engine and lack of power could be a potential issue.^[3]

Ignition Energy

The necessary energy required to set flame to hydrogen is one tenth compared to that or gasoline based energy. This increases the probability for an instant ignition on lean mixtures. However, data presented from Kenneth Gillingham in his report "Hydrogen Internal Combustion Engines" states that due to the low ignition energy "the danger of hot gases or hot spots on the cylinder igniting the fuel, leading to issues with premature ignition and flashback."^[3]

Hydrogen Air to Fuel Ratio's

Hydrogen internal combustion engines will follow an air to fuel ratio shown below:

Air/Fuel Ratio: '2H₂ + O₂ = 2H₂O'
- Moles of H₂ for complete combustion = 2 moles
- Moles of O₂ for complete combustion = 1 mole
- Moles of N in air = Moles of O₂ x (79% N₂ in air/ 21% O₂ in air)= 3.762 moles N₂
- Calculating moles of air = moles of O₂ + moles of N₂ = 4.762 moles of air
Weight of O₂,Hydrogen gas, N₂, and Air
- Weight of O₂ = 1 mole of O₂ x 32g/mol = 32g
- Weight of N₂ = 3.762 moles of N₂ x 28 g/mol = 105.33 g
- Weight of Air = weight O₂ + weight of N₂ = 32+105.33 = 137.33 g
- Weight of H₂ = 2 moles of H₂ x 2 g/mol = 4g
Air/Fuel Ratio based on Mass = mass of air/mass of fuel:
- 137.33g/4g = 34.33:1
Air/Fuel Ratio based on Volume = volume (moles) of air/volume (moles of fuel)
- 4.762/2 = 2.4:1
Percent of H₂ (displaced)
- H₂/(volume air + volume of H₂) = 29.6%

As shown above the air to fuel ration for the complete combustion of hydrogen in air is approximately 34:1 by mass. This fuel ratio is significantly larger than that of a gasoline engine's air to fuel ratio which is proven to be approximately 14.7:1.^[3] This is primarily due to the property of hydrogen, mainly how hydrogen displaces around 30% of the combustion chamber where gasoline displaces 1-2%. The benefit of a larger displacement is primarily for a more uniform explosion. Furthermore, the power output compared to a gasoline engine can be anywhere from 85% (focusing on the lean mixes of fuels) to 120% with the use of high pressure injection technology.^[3] Furthermore, the air to fuel ratios may range from 34:1 to 180:1 depending on the amount of power required.

The controversy of Hydrogen in an Internal Combustion Engine

Engine Backfire/Susceptibility of hydrogen to surface ignition

The first issue to be discussed when creating a hydrogen engine is the low ignition energy. Due to the low ignition energy, hydrogen engines can combust lean mixtures with certainty; however, the due to the low temperature needed spontaneous or unexpected ignition can occur due to hydrogen igniting on remaining hot spots on the cylinder.^[3] This can create problems such as hasty ignition as well as uncertain flashbacks. Furthermore, the wide range of flammability means relatively any fuel source can be ignited by hot spots.

Reduced Engine Power

The power output will heavily depend on the air to fuel ratio as well as the fuel injection. The stoichiometric air to fuel ratio determined above is 34:1. This ratio will have less energy content compared to gasoline. However, engine power can vary depending on the following injection methods. If the hydrogen engine used the carburetor and port injection method the resulting power will be 85% of that of gasoline engines.^[3] The engine however could be fitted with direct injection and run approximately equal to that of gasoline engines. It is important to note however, this will increase nitrogen oxide emissions due to higher pressure and temperature.^[3] The final method would be a high pressure direct injection which is determined to offer 120% power compared to gasoline engines, but the emissions would be high in nitrogen oxides.^[3]

High Nitrogen Oxide Emissions

The bases behind the hydrogen internal combustion engine is to produce water as the exhaust. As shown in the hydrogen air fuel ratio pure water is what would be released as the exhaust when the hydrogen and oxygen combust.^[4] However, the combustion of hydrogen with air can also produce oxides of nitrogen altering the initial equation into the following form:

H₂ + O₂ + N₂ = H₂O + N₂ + NO_x

These nitrogen oxides are created due to the high temperatures within the combustion chamber and the amount of NO_x gases formed will depend on the air to fuel ratio, the engine speed, and the ignition timing.^[4] There have been ideas of using a thermal dilution in order to reduce these nitrogen oxides.

Weighing the Pro's and Con's of Hydrogen Engines

The main goal of hydrogen engines is to create an engine that emits less emissions while at the same time offering a step between gasoline engines and fuel cell technology. Further studies have shown that the stoichiometric ratio would not be ideal for hydrogen engines due to the amount of nitrogen oxides produced. The ideal purpose would be to use twice as much air for complete combustion (lean ratio). Furthermore, the engine size would be similar, but with 30-50% decrease in power.^[3] Therefore, hydrogen engines will have to be larger to offer more power and be equipped with turbochargers or superchargers to offer more power.

References

1.College of the Desert (2001, December). Hydrogen in the Internal Combustion Engine. Retrieved from http://infohouse.p2ric.org/ref/45/44490.pdf.

2.Hydrogen Energy (2016). Hydrogen Production. U.S. department of energy hydrogen and fuel cell program. Retrieved from http://www.hydrogen.energy.gov/production.html.

3.Gillingham, K (January, 2007). Hydrogen internal combustion engine vehicles: a prudent intermediate step or a step in the wrong direction?. Standford University. Retrieved from http://www.yale/edu/gillingham/hydrogenICE.pdf.

4. Clean Air Strategic Alliance (1990). Nitrogen Oxides (NOx). Retrieved from http://dwb.unl.edu/teacher/nsf/c09/c09links/www.casahome.org/nitrogen.htm.