1. Next‐Gen APU Research at
NUI Galway
Rory MonaghanHarald BerresheimBarry Flannery
Acknowledgements
This research is supported by the Irish Research Council Enterprise
This project would not be possible without the conti‐
uing support of technical officers Bonaventure Kennedy,
Patrick Kelly, William Kelly and Aodh Dalton.
Partnership Scheme (EPSPG/2013/579).
Cooling with Heat ‐ The Adsorption Cycle Next Generation Engine ‐ The Stirling Engine
What is an APU?
Preliminary Modelling Results
Model Sensitivity Analysis Results
Table 1 Summary of key modelling results
Graph 1 Sensitivity analysis based on system‐level model. Key results are diminishing importance of COP
(yellow) and relative unimportance of Stirling engine efficiency (red).
Auxiliary power units (APUs) are small secondary engi‐
nes found on heavy trucks that operate independent‐
ly of the main engine to provide for small in‐cab elec‐
trical loads and air‐conditioning while the truck is
stationary.
APUs are most commonly used for idle‐reduction
to save fuel and increase driver comfort.
Problems With Current Technology
Diesel engines produce many unwanted emi‐
ssions such as NOx, CO and diesel particulates.
Research Questions
1. Can the proposed system meet
the performance requirements
of a heavy truck APU?
Experimental Testing
Conclusions & Future Work
Adsorption refrigeration is virtually identical to vapor
compression refrigeration except that the compress‐
ion is produced via thermal means as opposed to pis‐
ton work. By using special water adsorbent materials
like zeolite it is possible to deliver cooling at a coeffic‐
ient of performance (COP) of 0.5 using low temperat‐
ure (<100o
C) waste heat such as that found in the cooling loops of Stirling
or diesel engines.
Benefits of Adsorption Cooling
Silent
Maintenance free
Runs on waste heat
Fig. 1 Zeolite coated heat exchanger
Stirling engines operate on a closed thermodynamic cycle
meaning that they can be driven via external combustion
and so they will inherently be quiet and produce clean em‐
issions without after treatment. Additionally, the amount
of easily extractable waste heat in a Stirling engine is
twice that of a diesel engine. This heat, which would
otherwise be rejected, can instead be used to
provide useful work and therefore incre‐
ase the overall fuel utilization of an APU.
Benefits of Stirling Engines
Extremely quiet
Virtually zero‐maintainance
Clean emissions without after treatment
2. How does the proposed system compare
to conventional technology?
Proposed
System Architecture
Identifying
Novel Technologies
Fig 3. APU Sub‐System Hierarchy
Fig 2. Conventional APU Breaking down an APU into its con‐
stituent subsystems quickly reveals that
there are many alternative potential techno‐
logies and countless more combinations of each.
However, very few technologies can meet
the tough requirements of an APU such as
power density, emissions, efficiency,
maintenance and reliability.
Fig 4. Free‐Piston Stirling Engine
Fig 5. Kinematic Stirling Engine
The proposed system will be evaluated
through continued experimental testing
and with the aid of system‐level phy‐
sics based modelling tools such as
OpenModellica and Dymola.
Fig 5. Example of Modellica‐type analysis
Preliminary results shown in Table 1 indicate
that the proposed system could offer significant
benefits over conventional technology.
By coupling a free‐piston Stirling engine to a waste‐heat driven
adsorption chiller it is possible to create a highly reliable APU
that is virtually silent, maintenance free and requires no cos‐
tly exhaust after treatment. This configuration can also util‐
ize waste heat from the main truck engine while the truck
is driving. A simplified system diagram is shown below.
Fig 6. Proposed Architecture
Current APUs require regular maintenance and
explosive combustion produces considerable
noise which negatively impacts on driver comfort.
Refrigerants used for air conditioning
are environmentally damaging
and are being phased out.
Fig 7. Engine Test Bed at NUI Galway
A prototype of the Stirling‐adsorption syst‐
em has been built and is currently undergo‐
testing at NUI Galway. The novel system
will be benchmarked against a leading
conventional APU system in range of
relevant environments and conditions.
Experimental testing will be enable
validation of system level models
and identify any potential technical
issues. Physically prototyping the
system will reduce overall technol‐
ogy risk and enable further develop‐
ment of the concept.
Fig 8. Testing of
a 1 kWe Stirling
engine.