Not too far in the future from today, two chemical reactions may be transforming the automotive industry. At the center of it, is an element that we are very familiar with, called hydrogen or H2. Automotive engineers are considering using H2 as a combustion fuel in engines to replace diesel/gasoline and this article is about that. The beauty of this fuel is that it is sustainable, which means there is no waste. H2 on combustion produces water, which can be converted back into H2 if needed.
Speaking of electrolysis, fuel cells are gaining popularity for sustainable mobility. There seems to be however a great market for the use of H2 in internal combustion engines (or ICE) because they require about the same assembly lines and facilities in the production process as conventional fossil fuel. Economies of scale can be achieved with relative ease[1].
Any change from the norm comes with a cost. In this case, a drive to sustainable fuel comes with a cost. This is where regulations play a critical important role. From this standpoint, things look promising globally. Countries like the USA, India, Australia, China, Japan, Korea, and the European Union already have regulations in place to enable H2-based mobility by 2025 [2]. H2 production and sourcing are also an important piece of the puzzle, but we will talk about that later.
What is an H2 ICE? I try to explore the technology by breaking it down into steps. In this article, I want to focus on H2 from tank to exhaust. I will talk about fuel storage, combustion and then the exhaust chemistry.
Fuel storage:
Hydrogen is stored in three methods in today’s day and age:
1. Compressed H2 – Gas tanks that store H2 at 3600 – 10,000 psi pressure.
2. Cryogenic H2 – Low temperature (-253o C) tanks
3. H2 slurries – Relatively safer H2 compounds (Lithium hydride LiH or Magnesium Hydride MgH2) that can be stored at room temperature.
Option 1, compressed gas may be the most practical of the three, and comparatively the simplest [3][4]. You just have to be comfortable carrying a highly flammable gas in a pressurized enclosure, not at all similar to the concept of a bomb, while being mobile on, some busy and some not, roads. To put things in perspective, the atmosphere is about 15 psi at sea level. So, H2 in an H2 tank stored at 10,000 psi is about ‘666’ times the atmospheric pressure at sea level. Coincidence.
Combustion:
The energy content of hydrogen and diesel are as shown below. HHV is the higher heating value and LHV is the lower heating value and these are terms to define the energy content of a substance.
| H2 | Diesel |
LHV | 120 MJ/kg | 42 MJ/kg |
HHV | 142 MJ/kg | 46 MJ/kg |
Ratio (HHV/LHV) | 1.18:1 | 1.09:1 |
LHV is just HHV without the latent heat of vaporization. H2 has a much higher LHV per kg, indicating a much higher energy density than diesel. With a higher HHV to LHV ratio, H2 can produce more relative work at the pistons inside the engine that drives the vehicle. In other words, more energy is converted to power than is used to convert water into water vapour [6].
H2 has a wide range of flammability. This allows for H2 to run lean mixtures. The air/fuel ratio of H2 is about 34:1 as compared to 14.5:1 for a diesel engine. It has a low ignition energy which means the amount of energy required to ignite H2 is low. Hot spots can hence become a problem at the right temperature. However, hydrogen has a high auto-ignition temperature of 500 degrees C as compared to diesel (256 degrees C), which also allows for a higher compression ratio (high temperature = high pressure) which in turn allows for a higher thermal efficiency. With high diffusivity and high flame speed of the H2 fuel, the quality of combustion is also great. H2 however has a small quenching distance, this can cause backfires and fuel leaks beyond the combustion regions inside the combustion chamber [7].
Exhaust:
About 15% of the exhaust from a H2 ICE is water. This is a problem for the after-treatment system. While the amount of N2 produced from the exhaust is about the same, the high amount of water vapour can be absorbed on the active sites by hydrogen bonds because they have lower bonding energy than ammonia. Water can hence inhibit the NOx conversion rate[8].
NOx conversion rate depends on the water content and the operating temperature. Typically, an H2 ICE engine operates at 260-400 0C and a diesel ICE operates at a little higher temperature of 295-440 0C. The NOx conversion efficiency plateaus at 90% after 295 0C regardless of the amount of H2O. At 2600C however, at a completely dry state (no water) the NOx conversion is about 85%. At the same temperature, the NOx conversion is about 70% if the water content rises to 10% - 15%. Hence, for about throughout the operation of the diesel engine, the NOx conversion rate is about 90% consistently, but for H2 ICE, at lower temperatures (260-295 0C) the NOx conversion rate is between 70 - 85%. Hence engine is better run at temperatures > 295 0C for high (90%) NOx conversion rates, and since diesel engines naturally operate at that temperature, low NOx conversion rates are an issue as they are in H2 ICE. Typically, a Diesel engine produces 11% H2O and a Hydrogen engine produces 15% H2O. Both these have a similar locus for the NOx conversion rate[8].
Conclusion:
While H2 ICE is the closest technology to diesel of all the other clean energy sources, it is still a lot more involved for a 1000-word essay. I intend to cover other aspects of it in other articles. H2 is a great combustion fuel. It has high energy density, allows for high compression ratios without auto igniting worries and offers ability higher efficiency. Although its brake thermal efficiency is about the same as a modern diesel engine. The thorn to this rose is the NOx production which doesn’t allow the H2 engine to be greenhouse gas free although it is carbon free.
H2 is the most abundant chemical in our environment, which does not mean it can be pulled from the air as easily as picking grapes. It is an involving process that requires energy during the conversions. The push to use H2 in automobiles is because it is “sustainable”. Sustainable because H2 can be returned to the exact state from the exhaust water, as the state it began with before engine intake. We often ignore that there is a bunch of energy loss involved in the process. H2 ICE may not be the best energy source for automotive use considering safety, but any development in the technology only takes us closer to its appropriate use if it isn’t in automotive.
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