O-rings for hydrogen combustion engines

Where are O-rings located in a hydrogen engine?

Most O-rings are located in the fuel supply and in the transition between the hydrogen installation and the engine. From the tank, hydrogen passes through shut-off valves, safety components and pressure regulation to the rail. The gas is then introduced into the intake tract or directly into the cylinder via injectors. Every transition can be a sealing point.

O-rings may also be needed for cooling circuits, the air side, throttle body, MAP and IAT sensors. These are not always in direct contact with pure hydrogen, but they do count in the total engine package. Especially with port fuel injection, the intake tract may locally contain an H2-air mixture.

Why hydrogen places different demands

Hydrogen is not automatically chemically aggressive for many elastomers. The problem is more often physical: the gas easily penetrates polymers, can diffuse through the material and can cause damage during rapid depressurization. Think of swelling, blistering, cavities, small cracks or permanent loss of sealing force.

In addition, hydrogen has little lubricating effect. In dynamic or semi-dynamic components, such as injector and valve interfaces, wear can therefore start to play a role more quickly. An O-ring that performs perfectly well in a conventional fuel rail is therefore not automatically suitable for H2 service.

The safety margin is also smaller. Small leakage paths are more relevant because hydrogen escapes quickly, accumulates at high points and has low ignition energy. For seals, this means: do not only look at the material, but also at groove design, surface finish, squeeze, extrusion gap, hardness, backup rings and the complete test procedure.

Port fuel injection or direct injection

The difference between port fuel injection and direct injection strongly determines where O-rings are loaded.

With port fuel injection, hydrogen is introduced into the intake tract. The pressures are usually lower and the components are more often located in cooler zones. This often makes elastomer O-rings in injector and manifold positions more realistic, provided the compound is suitable for hydrogen, temperature, air contact and any traces of oil or condensate. At the same time, there is more overlap with the intake system. Sensors, throttle body and manifold seals may therefore indirectly come into contact with an H2-air mixture.

With direct injection, the load shifts to the injector, rail and cylinder head interface. The pressure is higher, the temperature load is more severe and the requirements for leak-tightness are stricter. In these zones, standard O-rings are not always enough. Depending on the construction, you are more likely to end up with harder H2-qualified compounds, PTFE or PEEK backup rings, spring-loaded PTFE seals or metallic seals.

Material selection for H2 engine seals

There is no universal best O-ring material for hydrogen internal combustion engines. The right choice depends on the exact position, pressure, temperature, type of movement, co-media and desired service life.

HNBR is interesting when mechanical strength, wear resistance and extrusion resistance matter. This can make HNBR suitable for semi-dynamic or vibration-sensitive zones, but only with H2 cycling and decompression testing.

FKM is often a logical candidate for dry fuel zones with higher temperature or possible traces of oil and hydrocarbons. Please note: FKM (Viton®) is not automatically RGD-resistant. The compound must be tested for hydrogen, pressure changes and the correct temperature.

EPDM is strong on the water, steam, glycol and coolant side. For certain static H2 positions, EPDM can be interesting, but not as a standard choice in zones with oil or fuel-like contaminants.

FFKM is a premium option for highly critical static seals where heat, chemical load or low compression set is the decisive factor. The price is higher, so we prefer to use FFKM specifically in the few positions where it really solves something.

NBR can be useful in less critical oil- or fuel-related peripheral zones, but is usually not our first choice for primary H2 barriers in hot or rapidly decompressing sections.

PTFE and PEEK are mainly seen as backup rings, seat materials or part of an engineered seal. PTFE has low permeation and broad chemical resistance, but lacks elastic recovery. PEEK helps with extrusion resistance and structural support.

Because we mainly supply O-rings, we always start with the question of whether a classic O-ring is still technically the best choice here. In many engine-side interfaces, it is: rail connections, sensor bosses, housing covers, regulator covers and service ports can often be designed well with an O-ring. With internal injector valve seals, high stroke frequencies or very low leakage requirements, a different geometry may be better. In that case, it is more honest not to force an O-ring, but to translate the function into a suitable seal with backup, PTFE, PEEK or metal.

For the procurement phase, it is wise not only to request the inner diameter and cord thickness. Also ask for hardness, compound code, cure system, available H2 data, RGD results, compression set, temperature limit and any compatibility with oil, coolant or condensate. Those details determine whether the same O-ring size becomes a reliable seal or a risk on the test bench.

Design and validation

A suitable compound is only the beginning. For O-rings in hydrogen internal combustion engines, the groove must be assessed just as seriously as the material. ISO 3601 is a logical starting point for dimensions, tolerances and housing design. Application-specific choices then follow: static or dynamic, radial or axial, single-sided or alternating pressure, with or without a backup ring, and how much swelling or thermal expansion the groove can still accommodate.

For critical H2 positions, we recommend a step-by-step test setup. First, you screen the material for permeation, compression set and compatibility with dry hydrogen and co-media such as oil, coolant, moisture and condensate. Then you test the actual groove geometry with helium or bubble leak tests. Pressure cycling, RGD/AED tests and temperature cycles then follow. Only after that can subassemblies such as injectors, rails, pressure regulators, valves and sensor bosses be assessed for service life, vibration and hot soak.

This prevents an O-ring from appearing suitable on paper, but failing in the real engine due to installation, microleakage, pressure pulsation or loss of sealing force.

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