Chapter 9Chapter 9

Materials Issues for Use of Materials Issues for Use of Hydrogen in Internal Hydrogen in Internal

Combustion Engines (ICE)Combustion Engines (ICE)

two-stroke engine in operation, with a tuned pipe exhaust

Four-stroke cycle (or Otto cycle)1. Intake2. Compression3. Power4. Exhaust

The Wankel cycle. The shaft turns three times for each rotation of the rotor around the lobe and once for each orbital revolution around the eccentric shaft.

Filler neck for hydrogen of a BMW, Germany

Tank for liquid hydrogen of Linde, Germany

A hydrogen internal combustion engine vehicle (HICEV) is a type of

hydrogen vehicle using an internal combustion engine. Hydrogen internal

combustion engine vehicles are different from hydrogen fuel cell vehicles

(which use hydrogen + oxygen rather than hydrogen + air); the hydrogen

internal combustion engine is simply a modified version of the traditional

gasoline-powered internal combustion engine.

Introduction Internal combustion engines (ICEs) offer an efficient, clean, cost-

effective option for converting the chemical energy of hydrogen into

mechanical energy.

The basic of this technology exist today and could greatly accelerate

the utilization of hydrogen for transportations.

It is conceivable that ICE could be used in the long terms as well as a

transition to fuel cells. However, little is known about the durability of an

ICE burning hydrogen.

The primary components that will be exposed to hydrogen and that

could be affected by this exposure in an ICE are (1) fuel injectors, (2)

valves and valve seats, (3) pistons, (4) rings, and (5) cylinder walls.

A primary combustion product will be water vapor, and that could be an

issue for aluminum pistons, but is not expected to be an issue for the

exhaust system expect for corrosion.

Fuel Injectors

The combustion of hydrogen in an internal combustion

engine is a technology to help expand the utilization of

hydrogen fuel in the near term, before fuel cell

technology is fully developed.

In order to gain the highest efficiency, the use of direct

injection will be needed.

There are several elements to these injectors that could

experience degradation in the presence of hydrogen: (1)

injector body, (2) actuator, (3) epoxy used to encase the

actuator, and (4) electrical contacts.

Injector Body Injector bodies are made primarily form steels such as M2 (UNS T11302),

H13 (T20813), and 4140 steel (UNS G41400).

The alloy M2 is a high-carbon tool steel with a carbon concentration ranging

between 0.8 and 1.05 %, while H13 is a tool steel with a carbon concentration

of 0.3 to 0.45 % and 4140 steel is an alloy steel with a carbon concentration

of 0.4 %. M2 is a highly alloyed tool steel with about 4 % Cr, 5 % Mo, 6 % W, and 2 % V.

these elements are all carbide formers, so their combination with high carbon

results in a significant volume fraction of carbides in the microstructure. These

carbides provide wear resistance, which is needed for the pin and seat of the

injector.

H13 is a lower-alloy tool steel with approximately 1 % Si, 5 % Cr, 1 % Mo, and 1 %

V.

Alloy 4140 steel contains approximately 1 % Cr and 0.2 % Mo as the primary alloy

additions.

M2 has excellent retention to softening at temperature as high as 600 oC. This

hardness retention results from the stable carbides.

Composition and hardness are factors that directly affect the performance of

these steels in hydrogen.

Actuator Materials• Injectors may use electromagnetic or piezoelectric actuators to

provide the active fuel control.

• Some actuators for direct H injection utilize piezoelectric wafers

made of lead zirconium titanate (PZT) embedded into an epoxy

or other insulating material.

• For direct injectors, the actuator is embedded in the hydrogen

gas, which has the potential to affect performance by the

following processes:

1) change the capacitance of the PZT

2) mechanical failure or cracking of the PZT

3) separation of the PZT wafers

4) debonding of electrical connections

5) degradation of the epoxy or polymer casing materials

Hydrogen Effects on Internal Engine Components

A number of internal components, such as valves, valve

seats, cylinder walls, pistons, and rings, will be exposed

to hydrogen and water vapor.

The potential effects are of two primary types: (1)

decarburization of steels and cast iron and (2) hydrogen

embrittlement of aluminum pistons.

Water vapor could cause excessive corrosion of exhaust

systems, but this could be minimized by use of titanium.

Decarburization Effects• Decarburization occurs in steels and cast irons in

hydrogen gas by the reaction of H with C in the steel.

• The decarburization rate is primary dependent on the

diffusion rate C in the steel, but is also affected by the

carbon content of the steel, alloying elements in the

steel, such as chromium, impurities in the hydrogen,

and of course time and temperature.

• Carburization of steels, reverse of decarburization, is

usually conducted at temperature of about 900 oC, but

decarburization can occur at temperature as low as

800 oC.

Exhaust valves have the highest operating temperature of components

in an internal combustion engine, and they typically operate at

maximum of 790 oC, while intake valves have a maximum operating

temperature of 540 oC.

Light-duty intake valves are typically made from SAE 1547, which is an

iron-based alloy with 1.5 % Mn and 0.57 % C.

For higher-temperature application, the ferritic stainless steel alloy 422

is used. This alloy has about 8.5 % Cr, 3.25 % Si and 0.22 % C.

Because exhaust valves operate at higher temperatures, materials

with a higher alloy content are used. A primary alloy for exhaust valves

is 21-2N, which has 21 % Cr, 2 % N.

Other alloys used for exhaust applications, depending on the desired

operating temperatures, are 21-4N, 23-8N, Inconel 751, Pyromet 31,

and nimonic 80A.

Valves used for heavy-duty application have one of these alloys for the

valves head with a hardenable martensitic stem.

Hydrogen Embrittlement of Pistons

• Aluminum pistons in an engine that burns H2 will be exposed to

not only H2 but also H2O at temperatures of 80 to 120 oC.

• Aluminum alloys can be totally immune to H2 embtittlement and

H2-induced crack growth if the natural Al2O3 oxide is intact.

• However, there are processes that can disrupt this film, and it is

know that aluminum alloys will absorb H2 when exposed to H2O

vapor at 70 oC.

• There will also be periods when the engine is cool and

condensed water will be present so that aqueous corrosion

could occur, but this is not expected to be any different than with

an engine with cast aluminum pistons that burns gasoline.

• H is very insoluble in Al at 25 oC and 1 atm pressure, with values

ranging from 10-17 to 10-11 atom fraction.

• There are several studies that resulted in diffusion coefficient at

25 oC of about 10-17 cm2/sec for Al.

ReferencesReferences

Materials for the Hydrogen Economy, Jones, R. H. and Thomas, G. Materials for the Hydrogen Economy, Jones, R. H. and Thomas, G.

J., ed., CRC Press, Boca Raton, 2008.J., ed., CRC Press, Boca Raton, 2008.

http://en.wikipedia.org/wiki/Internal_combustion_enginehttp://en.wikipedia.org/wiki/Internal_combustion_engine

http://en.wikipedia.org/wiki/http://en.wikipedia.org/wiki/

Hydrogen_internal_combustion_engine_vehicleHydrogen_internal_combustion_engine_vehicle

Hydrogen Use in Internal Combustion Engines, Hydrogen Fuel Cell

Engines and Related Technologies, College of Desert, 2001.