Toyota demonstrates why hydrogen combustion engines are not the answer

Toyota is one the legacy automakers still betting on hydrogen technology to save their business, despite clear evidence that battery electric vehicles are winning the race for zero emissions transport.

Alongside development of hydrogen fuel cell vehicles, Toyota has also been building liquid hydrogen fuelled engines for racing. 

Jason from the Engineering Explained YouTube channel posted an excellent video last year describing Toyota’s liquid hydrogen combustion engines.

The video starts off with the advantages of using liquid hydrogen over compressed gaseous hydrogen that is normally used with hydrogen fuel cell electric vehicles (FCEV) such as the Toyota Mirai and Hyundai Nexo.

Liquid hydrogen is more energy dense than compressed hydrogen gas, allowing roughly 50% more energy to be stored in the same volume.

Liquid hydrogen can also be stored under atmospheric pressure, removing the need for high pressure storage tanks. Apart from burning hydrogen, the actual combustion part of this prototype engine is very similar to petrol engines.

Apart from those advantages, the video details three major problems with liquid hydrogen combustion engines that are not easy to solve. Most of the problems revolve around liquid hydrogen needing to be kept extremely cold, below minus 253 degrees celsius to prevent it vaporising or boiling away.

The fuel pump in Toyota’s prototype vehicle needed to be swapped twice during a twenty four hour race and took three and a half hours each time.

The hydrogen fuel pump wears out quickly and becomes inefficient because oil lubricants cannot be used, otherwise they would contaminate the hydrogen fuel. It is also very difficult to maintain a good seal between the pump components when they are cooled to this extreme temperature.

Even though liquid hydrogen is more energy dense than compressed hydrogen gas, large fuel tanks are required for hydrogen combustion vehicles to have decent range.

The prototype Toyota Corolla was fitted with a 150 L tank and could only drive 65 km between pit stops for refuelling. To illustrate this problem clearly, Jason calculated that a formula 1 race car would need a huge 700 L hydrogen fuel tank to drive roughly the same distance as they do using petrol.

Keeping liquid hydrogen at minus 253 degrees celsius without it vaporising is also problematic for longer periods of time, which is why Toyota developed their prototype specifically for racing where fuel is burned quickly before it can be wasted.

BMW produced a hydrogen combustion vehicle called the BMW Hydrogen 7 between 2005 and 2007. During that time, BMW found it took only 10-12 days for the entire contents of the liquid hydrogen tank to vaporise with the vehicle parked, making it impractical for passenger vehicle use. 

Even if these problems with liquid hydrogen could be solved, the overall efficiency of burning hydrogen is wasteful compared with using the same electricity to charge a battery electric vehicle. Overall efficiency for hydrogen combustion engines is similar to petrol and can be as low as 20-40%, whereas electric vehicle efficiency is typically 77%. 

The push for hydrogen vehicles by legacy automakers may be analogous to people advocating for nuclear power and trying to delay the renewable energy transition. Battery electric vehicles are similar to wind and solar power in that regard, as they will remain a far cheaper and simpler solution even if the challenges with hydrogen could one day be overcome.

In related news for hydrogen, at least one fossil fuel company seems to be concluding there is no future in hydrogen for passenger vehicles. Shell recently announced the permanent closure of most of their hydrogen refuelling stations in California, citing external market factors and issues with hydrogen supply. 

See the full Engineering Explained video here: https://www.youtube.com/watch?v=DGL5g91KwLA