Truck & Off-Highway Engineering - December 2021 - 4

TECHNICAL INNOVATIONS
" We're seeing extremely low NOx levels, essentially
close to where a NOx analyzer would not even be
able to measure it. "
Mike Bunce, head of research for Mahle
Powertrain US.
will effectively be burnt there before a
knocking event can occur. " This allows
the ignition of fuel/air mixtures that are
typically not very ignitable, " Bunce explained.
" So, you can ignite mixtures that
have really heavy dilution - standard
dilution or EGR [exhaust gas recirculation]
dilution beyond what a typical
spark plug would be able to handle.
" You can also ignite fuels that are
problematic. Some fuels don't ignite very
well, generally. Some fuels maybe ignite
too well, and you lack a degree of controllability.
Here a pre-chamber jet igniter
gives you that controllability back
and allows you to ignite these mixtures
that are typically not very ignitable. "
Using the MJI system enables the
dilute hydrogen fuel/air mixture that
brings the hydrogen pre-ignition under
control. Bunce said that dilute fuel/air
mixtures usually lead to substantial increases
in thermal efficiency. This can
be in the 15 to 25% region for gasoline
or natural gas engines but has yet to be
quantified for hydrogen-fueled engines.
In fact, improving thermal efficiency
is not the objective of using MJI with
hydrogen. " Lowering the compression
ratio is of course going to lower your
ability to achieve the target power and
torque, " Bunce said. " So, the use of the
active pre-chamber here is meant to try
to get that compression ratio back up
and allow you to get the power and the
torque that you want. "
" In terms of development, the good
4 December 2021
thing about pre-chambers, both active
and passive, is that the hardware is very
simple, " Bunce continued. " If you think
about an active pre-chamber, components
include an off-the-shelf spark
plug and we use a direct fuel injector
for the pre-chamber. That's a bit of a
custom job, but there's nothing exciting
about the underlying technology - it's
just a solenoid DI injector. "
The rapid combustion triggered by the
MJI generates very high cylinder pressures,
which are similar to those found in
a diesel engine. This ensures that diesels
are particularly suitable for conversion to
this type of hydrogen combustion.
Overcoming unique challenges
Bunce sounds a note of caution regarding
hydrogen and steel. " Hydrogen is
not particularly compatible with some
steels; it tends to cause embrittlement
because it's such a small molecule and
can really dig itself into surface-level
marks, " he explained. " So, you have to
pay close attention to material compatibility
for everything the hydrogen is
going to touch. "
Fuel/air mixture " blow-by, " where the
mixture is forced past the piston rings
and into the crankcase, is a common issue
with piston engines but can pose a particular
problem with hydrogen engines.
Because of the tendency to pre-ignite, it
could lead to ignition in the crankcase
with devastating effects. The Mahle solution
is to design the piston rings to prevent
blow-by and introduce an electric
pumping system to evacuate the crankcase
very quickly, routing the gases back
to the intake side of the engine.
The tendency for pre-ignition also
means that hotspots in the combustion
chamber need to be eliminated. " You
need to take great care to make sure that
you get really homogenous temperatures
across the piston crown, so you don't
have individual hotspots, " Bunce said.
" That's what our component group really
focuses on, and we're developing really
good solutions in that respect. "
Hydrogen can combust with high levels
of air dilution even with a conventional
spark plug. " What we found in our work
is that even operating with that really
high level of air dilution, it's possibly not
lean enough to be able to significantly
reduce the pre-ignition problem, " Bunce
said. " In order to solve pre-ignition, it's
going to need reduced combustion temperatures
and that will reduce the surface
temperatures. Air dilution is the best way
Current fuel cells tend to be quite sensitive to impurities in the air. For off-highway applications,
hydrogen-fueled ICEs could be either a bridge to fuel cells or a more permanent replacement for
fossil-fueled engines.
TRUCK & OFF-HIGHWAY ENGINEERING
FROM TOP: MAHLE POWERTRAIN; LIEBHERR

Truck & Off-Highway Engineering - December 2021

Table of Contents for the Digital Edition of Truck & Off-Highway Engineering - December 2021

Truck & Off-Highway Engineering - December 2021 - CVR4
Truck & Off-Highway Engineering - December 2021 - CVRA
Truck & Off-Highway Engineering - December 2021 - CVRB
Truck & Off-Highway Engineering - December 2021 - CVR1
Truck & Off-Highway Engineering - December 2021 - CVR2
Truck & Off-Highway Engineering - December 2021 - 1
Truck & Off-Highway Engineering - December 2021 - 2
Truck & Off-Highway Engineering - December 2021 - 3
Truck & Off-Highway Engineering - December 2021 - 4
Truck & Off-Highway Engineering - December 2021 - 5
Truck & Off-Highway Engineering - December 2021 - 6
Truck & Off-Highway Engineering - December 2021 - 7
Truck & Off-Highway Engineering - December 2021 - 8
Truck & Off-Highway Engineering - December 2021 - 9
Truck & Off-Highway Engineering - December 2021 - 10
Truck & Off-Highway Engineering - December 2021 - 11
Truck & Off-Highway Engineering - December 2021 - 12
Truck & Off-Highway Engineering - December 2021 - 13
Truck & Off-Highway Engineering - December 2021 - 14
Truck & Off-Highway Engineering - December 2021 - 15
Truck & Off-Highway Engineering - December 2021 - 16
Truck & Off-Highway Engineering - December 2021 - 17
Truck & Off-Highway Engineering - December 2021 - 18
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Truck & Off-Highway Engineering - December 2021 - 24
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Truck & Off-Highway Engineering - December 2021 - 26
Truck & Off-Highway Engineering - December 2021 - CVR3
Truck & Off-Highway Engineering - December 2021 - CVR4
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