“While use cases for battery electric and fuel cell electric powertrains are promising, the pairing of green hydrogen in the proven technology of internal combustion engines provides an important complement to future zero emissions solutions.”¹

So states Srikanth Padmanabhan, president of engine business, in a recent update on Cummins’ hydrogen internal combustion (H2-ICE) project. Hydrogen is attractive as a fuel due to its high-energy density. Burning one kilogram of hydrogen generates almost as much heat as burning three kilograms of gasoline. Hydrogen is non-toxic, not a direct greenhouse gas (though there can be secondary effects) and doesn’t contaminate groundwater in the event of spills or leaks. The National Resources Defense Council,² among others, has concluded that hydrogen as a fuel is at least as safe, and in many ways safer, than gasoline or diesel fuel.

As with any alternate fuel, design changes are required to optimize an internal combustion engine to run on hydrogen. Some of these changes have significant tribological consequences. For example, when hydrogen is burned in a stochiometric mixture with air, the combustion temperature is so high as to produce a significant amount of oxides of nitrogen (NO_x), which are regulated in most countries. These pollutants can be controlled in several ways, including by catalytic after-treatment. However, the preferred solution is to not produce NO_x in the first place. This is usually accomplished by increasing the amount of air in the mixture. This is called “running lean” and can reduce the peak temperature during combustion to the extent that NO_x are reduced to insignificant levels.

The product of hydrogen combustion is water. The consequence of running lean, hence cooler, means the engine takes longer to warm up, and the peak oil and component temperature are lower. If these temperatures remain below 100 C for a significant time, liquid water can condense on the surfaces of the combustion chamber. I personally have seen red rust on the cylinders in an engine run on hydrogen. The consequence of this oxidation is rapid wear of the cylinder wall, resulting in loss of sealing by the piston rings, which increases blow-by and reduces engine power.

Even the normal amount of blow-by is a major consideration for hydrogen-fueled engines. If the oil is running cool, the water will condense into and dilute the engine oil. Significant amount of water in the oil can have drastic consequences for all the lubricated components, altering the viscosity of the oil and increasing the chances of corrosion on these surfaces.

The point isn’t that hydrogen internal combustion is a bad idea. On the contrary, I have advocated for this precisely as an interim step between fossil fuels and fuel cells as a means of developing the hydrogen infrastructure until fuel cells become economically viable. The automotive tribologist will play a critical role in the success of this strategy, through such steps as oil formulation to deal with increased water dilution, and material strategies to manage corrosion. Once again, it’s a great time to be an automotive tribologist!

REFERENCES
1. Click here.
2. Click here.