In today’s world, everyone talks about how the internal combustion engine’s days are numbered thanks to advances in electric vehicle technology as a whole. However, what if, instead of eliminating the internal combustion design altogether, alternative fuels were utilized to reduce the chief complaints the environmentalists have — emissions?

Alternative fuels in performance applications aren’t a new idea by any means. In fact, Roush Performance has campaigned a pair of propane-powered Mustangs in the NMRA for over a decade. Also, using hydrogen as a fuel is nothing new, as manufacturers have been experimenting with (and now using) hydrogen fuel-cell vehicles for decades. However, using hydrogen as a fuel in an internal combustion engine is a tricky proposition, which Toyota has been working on for a number of years.

The idea behind using hydrogen as a fuel stems from the fact that when hydrogen is mixed with oxygen and ignited, the resultant compound is simple water. While the catalytic reaction in a fuel cell really is that clean and efficient, combusting hydrogen, creates a number of additional reactions and challenges to be considered. Exploring and explaining those factors in his usual level of detail is Jason Fenske of Engineering Explained.

Burning Hydrogen

First, we need to clarify one thing going forward, hydrogen combustion is fundamentally different than a hydrogen fuel cell. A hydrogen fuel cell creates electricity which drives an electric motor, which moves the car. hydrogen combustion maintains the standard four-stage Otto cycle of intake, compression, power, and exhaust, using hydrogen instead of gasoline or diesel fuel.

Speaking specifically of Toyota’s latest prototype (an inline-three-cylinder engine from the GR Yaris converted to burn hydrogen and all the hydrogen fuel systems fitted into a GR Yaris racecar, that recently entered a 24-hour endurance race), it is a traditional direct-injected, spark-ignited system. In theory, the products of pure hydrogen being burned with pure oxygen should be nothing more than water. Fenske points out, though, that the real world is far from perfect.

After assuming an incredibly thirsty one-quart-per-month oil ingestion habit, Fenske calculated the CO₂ emissions of a hydrogen combustion engine to be 0.26-percent (a quarter of a percent) of a traditional gasoline engine’s CO₂ emissions.

“Of course, air has nitrogen in it, and because your combustion temperatures are high, you get nitrogen oxides (NOx) forming. The other pollutant you see produced is CO₂.” Fenske explains. Now, if you did well in chemistry at all, you’re going to be wondering where on earth the carbon is coming from to create carbon dioxide. Well, Fenske has the answer.

“Your engine oil is about 80-percent carbon, and small amounts of it can get into the intake stream through the PCV system or as blowby. Then, it is burned during combustion,” Fenske explains. Further, he then calculates what kind of CO₂ emissions you might actually see from a hydrogen combustion engine in the real world, using the old project-car standard of burning one quart of oil per 3,000 miles.

“For every pound of carbon you burn, you get about 3.7 pounds of CO₂ produced. If we multiply that out, assuming burning one quart of oil every 3,000 miles — which is a lot — we get just 6.2 pounds of CO₂ per 3,000 miles,” says Fenske. He further calculates out that over a 200,000-mile vehicle life, the engine will only produce 413 pounds of CO₂ compared to a 200,000-mile production of 160,000 pounds of CO₂ with a traditional gasoline engine. “From a CO₂ standpoint, hydrogen combustion is great,” he says.

Other Methods of Combustion

Since the whole purpose of using an alternative fuel is to reduce emissions, the NOx emissions need to be addressed. Since the standard direct injection and spark ignition method of combustion is the highest NOx producer, Fenske points out that other methods of combustion, which produce lower levels of NOx, have been explored as well.

“First is HCCI or Homogenous Charge Compression Ignition. Actually, hydrogen diffuses really quickly, so creating that homogenous charge is relatively easy to do. The rest of it is pretty difficult to do with hydrogen,” explains Fenske. He points out that essentially diesel ignition of the hydrogen is both clean and relatively thermally efficient. However, the downsides to hydrogen HCCI are essentially deal-breakers. It’s very difficult to control, has a very low power output compared to a gasoline engine, and would require astronomical compression ratios for cold starts (in the neighborhood of 42.0:1).

There are two alternative configurations to combat NOx emissions: Homogenous Charge Compression Ignition (think “diesel”) and spark-ignited port injection. However, while both greatly reduce NOx levels, neither one is a practical solution due to the inherent difficulties each poses.

The other explored option is port-injection and spark-ignition. Fenske explains that with the port injection option, there is virtually no NOx emission when you operate at the stoichiometric ratio with a three-way catalytic converter. However, there are considerably more downsides as the process is less efficient than HCCI, pre-ignition is a problem, since hydrogen is so easily combustible, and possibly the largest downside, is backfiring through the intake.

“Hydrogen has a very small quench distance, meaning hydrogen combustion can travel through a very small crevice. So if the intake valve is even slightly open, combustion can travel back into the intake past the valve. The solution to that is direct injection, so you don’t have to worry about fuel in the intake like you do with port injection,” Fenske says.

The Real Drawbacks

While Fenske does a great job of espousing the positives of hydrogen combustion, he does an equally great job of explaining why hydrogen combustion is really impractical in an automotive application, at this current stage of the game. “There are many ways to produce hydrogen fuel. Many of them end up being worse from an emissions standpoint than just using gasoline or diesel in the first place,” says Fenske of Hydrogen’s production methods.

The next drawback to hydrogen combustion in its current state is its inefficiency compared to equivalent gasoline engines. “You’re talking about maybe a 25-percent efficiency at the wheels with hydrogen combustion,” says Fenske. “A fuel-cell is a much, much more efficient way of moving a vehicle using hydrogen. You might have something like 50-percent efficiency at the wheels [with a fuel-cell].”

Fenske also points out that Toyota found out just how difficult it is to covert a gasoline-powered internal combustion engine to run on hydrogen. “It’s not something you just grab off the shelf and put some hydrogen injectors on, and everything works. It’s very difficult to do that conversion.”

Ultimately, direct injection is the direction Toyota took in designing this first prototype. Hopefully, they found the data they gathered worthwhile because if technology can be advanced and methods refined further, there might be hope for internal combustion engines, yet.

Finally, we come to fuel storage. “Hydrogen is stored at 10,000psi to increase the energy density,” explains Fenske. “At that level, it has an energy density of 1.3 kilowatt-hours per liter. 10-percent ethanol gasoline has an energy density of 9 kilowatt-hours per liter. That’s seven times as much fuel required to produce the same amount of energy. That will lead to incredibly large, heavy fuel tanks.” Fenske also points out, that just with the efficiency difference between hydrogen combustion and hydrogen fuel cells, you would require double the tank space to get the same range, which becomes incredibly impractical.

Fenske does a really great job getting into the nitty-gritty of hydrogen combustion in the video, so we highly suggest you watch the whole thing. While the conclusion here is that hydrogen combustion isn’t really a solution given the current level of technology in the automotive world, it makes us very happy that there are large OEMs devoting considerable money and resources into keeping combustion engines alive in this increasingly unfriendly landscape for piston pounders.