Hydrogen internal combustion engine vehicle (HCIV) at least for now is believed to be the solution to achieve carbon neutrality. Advancement in hydrogen engine means that we might see a surge in HCIV followed by the hydrogen fuel stations. A gradual transition from diesel and gasoline fuel based IC engine is expected to occur. Therefore, question arises whether current gasoline and diesel engine will run on hydrogen fuel.
In this article we will look into the combustion properties and suitability of hydrogen fuel to the gasoline and diesel engine technology.
Combustion Properties of Hydrogen Fuel
Hydrogen fuel has energy density of 142.0 MJ/kg, which makes it fuel with one of the highest energy density when compared to the Gasoline and Diesel which has energy density of 45.8 and 45.3 MJ/kg respectively.
Air-Fuel Mixture Ratio
While gasoline engine operate in a specific Air-to-Fuel ratio (AFR) or Lambda value with idea/complete combustion at 14.7 and lean as much as 16.0; hydrogen fuel can operate at higher ratios of lean mixture. Generally speaking a lean mixture increases the fuel economy and reduces the amount of pollutants. More on the hydrogen fuel mixture can be found here:
Stoichiometric ratio for hydrogen is 34.3:1 by mass and 2.4:1 by volume.
Ability to be combustible at lean mixture setting is due to its flammable nature; which means at lower ignition energy is required for hydrogen combustion. The Minimum Ignition Energy or MIE of hydrogen is around 0.019mJ whereas for gasoline this number is around 0.1mJ.
Pre-ignition and Knock
Low MIE increases the risk of pre-ignition when charge mixture comes into contact with hotspots or surfaces within the combustion chamber, valves and spark plugs. However, hydrogen has very high auto-ignition temperature 585C ; which means only compression (such as in diesel engines) is not sufficient for combustion. Furthermore, low quenching distance of hydrogen fuel increases the tendency of backfire.
Auto-ignition Temperature
Auto-ignition is a key parameter to determine compression ratio limit on engine with a specific fuel type. A comparison of auto-ignition temperature of few domestic fuels is given below; a relatively high auto-ignition temperature of hydrogen means that a high compression ratio can be achieved; on other hand it is harder for compression ignition engines such as a diesel engine to work with the hydrogen.
Gasoline | 246 |
Diesel | 210 |
Hydrogen | 585 |
Fuel Metering and Mixture Control
So far we learn about the combustion properties of hydrogen fuel; in short, hydrogen fuel is highly flammable (with low ignition energy) and short quenching distance. It has properties to sustain lean burn with high auto-ignition temperature making it suitable for high compression engine design. However, it occupies larger volume fraction in fuel-air charge mixture due to its gaseous nature.