Materials Issues for Use of

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Chapter 9


Materials Issues for Use of
Hydrogen in Internal
Combustion Engines (ICE)

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

Four
-
stroke cycle (or Otto cycle)

1. Intake

2. Compression

3. Power

4. 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
o
C.
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
o
C, but
decarburization can occur at temperature as low as
800
o
C.


Exhaust valves have the highest operating temperature of components
in an internal combustion engine, and they typically operate at
maximum of 790

o
C
, while intake valves have a maximum operating
temperature of 540

o
C
.


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 H
2

will be exposed to
not only H
2

but also H
2
O at temperatures of 80 to 120
o
C.


Aluminum alloys can be totally immune to H
2

embtittlement and
H
2
-
induced crack growth if the natural Al
2
O
3

oxide is intact.


However, there are processes that can disrupt this film, and it is
know that aluminum alloys will absorb H
2

when exposed to H
2
O
vapor at 70
o
C.


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
o
C 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
o
C of about 10
-
17

cm
2
/sec for Al.

References


Materials for the Hydrogen Economy, Jones, R. H. and Thomas, G.
J., ed., CRC Press, Boca Raton, 2008.


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


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


Hydrogen Use in Internal Combustion Engines, Hydrogen Fuel Cell
Engines and Related Technologies, College of Desert, 2001.