Truck & Off-Highway Engineering - December 2021 - 3

TECHNICAL
INNOVATIONS
SAE INTERNATIONAL
BOARD OF DIRECTORS
Jeff Hemphill
President
Todd Zarfos
2020 President
Srinivasa (Sri) Srinath, Ph.D
2022 President Elect
Susan Ying, Ph.D
Vice President - Aerospace
Ken Washington, Ph.D
Vice President - Automotive
Michael Weinert
Vice President -
Commercial Vehicle
Andrew Jeffers
Treasurer
David L. Schutt, Ph.D
Chief Executive Officer
Gregory L. Bradley, Esq.
Secretary
Pascal Joly
Jeff Varick
Rhonda Walthall
Joan Wills
SAE International Sections
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technical or geographic area. The purpose
of local Sections is to meet the technical,
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SAE Members in a given area. For more
information, please visit sae.org/sections
or contact SAE Member Relations Specialist
Abby Hartman at abby.hartman@sae.org.
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Collegiate Chapters
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SAE Member Relations Specialist Abby
Hartman at abby.hartman@sae.org.
While there is limited scope for internal-combustion
engines (ICE) operating on conventional
diesel, gasoline and natural gas in a " Net
Zero " world, there could be a place for hydrogen-fueled
ICEs, either as an interim power
source while hydrogen fuel cells are fully developed,
or as a power source where it would be
difficult to electrify in the given location.
Hydrogen, however, has properties that
make it challenging as a straight replacement
for diesel or gasoline. The specialist engineering
consultancy Mahle has been working with
Liebherr Machines Bulle SA on resolving the
issues surrounding hydrogen-fueled engines.
" The challenge has been to get it to run with
stable combustion without resorting to reductions
in compression ratios to avoid engine
knock and pre-ignition. Our common work with
Liebherr suggests we have the answer, " Mike
Bunce, head of research for Mahle Powertrain
US, told Truck & Off-Highway Engineering.
The key appears to be Mahle's active prechamber
technology, Mahle Jet Ignition (MJI),
which was originally developed for gasoline
applications. Bunce explained the principle:
" There is still a conventional spark plug, but
here initiating combustion in a very small volume
that for our applications is typically 5 cm3
TRUCK & OFF-HIGHWAY ENGINEERING
Adaptation of Mahle's
active pre-chamber
in Liebherr's H966
and H964 engines
has demonstrated
that this technology
extends the stable
dilution limit of the
engine well beyond
the capability of
traditional ignition
systems, with
much more rapid
and complete
combustion.
or less - the size of a thimble. Inside the prechamber
is a very small mixture of fuel and air
for ignition, so you create very small combustion
events inside that pre-chamber.
" The pre-chamber has a nozzle with a few
orifices in it. The products that were formed as
a result of that very small combustion event
will shoot out very quickly from the nozzle orifices.
If you have a six-orifice nozzle, for instance,
there will be six jets coming out of it.
These jets are very hot and very reactive, because
they have combusted, so they then initiate
combustion inside the main cylinder. "
Instead of just the single point ignition around
the spark plug, there may be six distinct ignition
sites around the main combustion chamber, assuming
the pre-chamber is fitted with a six-hole
nozzle. " Each flame that is formed in that ignition
site doesn't have to travel as far to consume
fuel, resulting in very, very fast burning in the
main chamber, " Bunce said.
Preventing pre-ignition
Hydrogen has a tendency to pre-ignite, causing
the knocking that is familiar in a gasoline engine
with over-advanced ignition timing or too lean a
mixture. By directing jets from the nozzle to
sites where knocking is likely to occur, the fuel
December 2021 3
PROPULSION
Mahle, Liebherr develop active pre-chamber for hydrogen ICE
LIEBHERR
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Truck & Off-Highway Engineering - December 2021

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