Truck & Off-Highway Engineering - December 2024 - 12

DEMOING CLEAN HYDROGEN
COMBUSTION
The basis for the SwRI
H2 demonstrator truck
is a Cummins X15N engine.
SwRI's Ryan Williams (left) said that the project's aggressive
timeline - about 18 months from start to finish - didn't allow
enough time to fully redesign the cylinder head to consider
direct injection.
the air-fuel ratio before you start to get into unstable combustion. "
" One of the big challenges for on-road hydrogen combustion is transient
response, " Williams noted. " Hydrogen is great for stationary generation,
but when you start putting it in on-road application, things get
more difficult. With hydrogen, we can go well beyond that to about
two and a half Lambda. There's a sharp curve for engine-out NOx and
around two [Lambda] is where that curve gets steep. If we can stay
above two Lambda, then we can keep the engine-out NOx very low.
We just have to make sure that our Lambda stays within that window. "
Williams continued, " If you go too rich during conditions such as
throttle tip in, then two things happen. First, you get a NOx spike
because you're on that steep part of the curve on the NOx trade-off
already, and then your temperature starts to spike at the same time.
That's where supercharging the engine has been really beneficial because
at tip in we can get Lambda on demand. "
SuperTurbo provided one of its combination supercharger/turbocharger
units for the demonstration truck. " What the SuperTurbo unit
does is decouple the turbine from the compressor, which enables us
to control the compressor relative to the turbine, " Williams explained.
" We can actually pull power off of the crank via the pulley on the
front of the engine. So that power can be delivered straight to the
compressor to supercharge the engine.
" Then on the back end, when we've got excess energy going
12 December 2024
through the turbine, we can actually pull that back
through the transmission and put it back into the engine
to turbo compound, " he continued. " One of the
concerns about supercharging has always been that
it's a parasitic loss on the engine. But with the turbo
compounding over the full cycle of the engine, it kind
of balances itself out because you're pulling out just
as much energy as we're putting in. "
Williams also discussed the injection system that the
consortium used for the demonstration truck. The lowpressure
port injection system operates between 10
and 15 bar (145 and 218 psi). " The reason for choosing
port injection was component availability and overall
complexity, " he said. " To do direct injection right, you
have to design the cylinder head around it. "
Williams continued, " Hydrogen is a little counterintuitive
because it's so diffusive. You would think that you
inject it and it's just going to mix perfectly and be great.
Well, it doesn't really work like that because when you
inject it into that air in the cylinder, the air just acts like a
brick wall. So, most of these engines are designed with
a lot of swirl for that's beneficial for diesel combustion.
" If you have a flat cylinder head and a tangential port,
that makes the air swirl around the outside. But, when
you go to direct injection, you inject the fuel right in the
middle and all of your air just swirls around it and it never
mixes. So if you wanted to go that route, you'd have
TRUCK & OFF-HIGHWAY ENGINEERING
FROM LEFT: CUMMINS; SAE/MATT WOLFE
Truck & Off-Highway Engineering - December 2024

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