It's time to nerd out with our favorite explainer-in-chief, Jason Fenske, of Engineering Explained to learn about the differences between gasoline- and hydrogen-powered engines.
Jason describes eight key differences between a gasoline and hydrogen engine and the advantages and disadvantages of each. They are the combustion process itself, air-to-fuel ratio, ignition energy, flame velocity, auto-ignition temperature, diffusivity, quenching distance, and density.
We'll get the big one out of the way first. No, hydrogen is not a totally clean form of fuel to power an engine. The combustion process, theoretically, should only leave only water as a byproduct. However, the high temperatures of combustion create the pollutant NOx, which every automaker on earth is working to eliminate. The good thing is no C02 is created. Hydrogen fuel cells, on the other hand, offer a solution to that problem.
Hydrogen does have a big advantage when it comes to the air/fuel ratio, flame velocity, and diffusivity. A hydrogen engine can run an extremely lean air/fuel ratio, as high as 180:1. For comparison, the very efficient Mazda Skyactiv-X gasoline engine runs a 37:1 ratio, and that's a huge milestone for mass-production gas engines, which tend to run about a 14:1 ratio. Hydrogen can run much leaner and that can make it extremely efficient. At its highest ratios, however, it's hard to make a lot of power.
Flame velocity is also an advantage for hydrogen. Greater flame velocity within the combustion cylinder creates a better ratio of air that can be burned quicker. Engineers always like to see fuel used to extract the most work. Ask the Mercedes-AMG Formula One team.
Hydrogen's automatic ignition temperatures are also much higher, around 500 degrees Celsius versus about 230 Celsius for gasoline. This means hydrogen has a higher octane, which allows the engine to run at higher compression ratios.
Diffusivity is how quickly the fuel disperses, and in this case, it's a good thing hydrogen wants to spread out more quickly. Again, it helps with a more-efficient combustion process. It also helps create a very uniform mixture of air and fuel.
Hydrogen's quenching distance—how far from the cylinder wall it gets before the flame extinguishes—works similarly. It helps spread out that fuel to burn better, but it might also create a backfire because the hydrogen can fit through cracks, like between a valve and the combustion chamber, more easily.
If hydrogen is so great in these areas, why is gasoline our default fuel? For one, gasoline is far easier to store. Hydrogen is incredibly dense in terms of energy to volume. That means it takes up far more space in a combustion chamber (30% to 1-2%), which reduces its power output compared to the more power-dense gasoline. It takes more hydrogen to do a similar amount of work as gas. A hydrogen fuel tank also requires more space, so it might take up valuable trunk and passenger space to move a car the same distance as a gas-powered car.
Check out Jason's full explanation above. It's 14 minutes of valuable educational content.