Propulsion research | Senior thesis
Ammonia-Fueled UAV Range Extender
I took a compact two-stroke engine from reverse engineering through a high-compression dual-spark head, CFD comparison, CNC manufacturing, and cracker-assisted ammonia bench operation.
Design to test
Project brief
- 01 Measured Stock head, interfaces, chamber, and UAV envelope
- 02 Modeled Three ignition layouts in CONVERGE CFD
- 03 Manufactured Dual-spark stainless-steel head and sensor access
- 04 Bench tested Speed and in-cylinder pressure on cracker-assisted ammonia
Thesis responsibilities
My role
I owned the cylinder-head redesign and carried it from the stock engine through CAD, analysis, manufacturing, instrumentation, and engine testing, with support from my advisor and the university laboratory.
- Measured and 3D-scanned the original GTT70 head, then rebuilt the critical geometry in SolidWorks.
- Designed the approximately 17:1 chamber, dual-spark packaging, and in-cylinder pressure-sensor access.
- Extracted the internal fluid domain and built the two-stroke CONVERGE model.
- Prepared the manufacturing geometry and engineering drawing for the CNC prototype.
- Integrated the pressure-measurement chain and ran the bench test sequence with laboratory support.
Range-extender boundary
System architecture
The engine sits in a series-hybrid architecture. It turns a generator, while the aircraft remains electrically propelled through its motor and buffer battery.
The starting point was a compact GTT70 gasoline engine already associated with a UAV engine-generator platform. My work focused on the engine, fuel path, combustion chamber, and integration envelope. The modified ammonia configuration was evaluated on the bench rather than in flight.
Ammonia can be stored as a liquid and contains no carbon in its molecule, but its low reactivity and slow flame propagation make it difficult to use in a small, fast spark-ignition engine. That combustion problem drove the head geometry and ignition study.
Mechanical design
Cylinder-head redesign
I reduced the combustion-chamber clearance volume to raise geometric compression from approximately 10:1 to approximately 17:1. The higher compression improves the end-of-compression state for a fuel that is difficult to ignite.
The redesign preserved the original cylinder interface, fastener pattern, sealing surfaces, piston clearance, and installation envelope. I packaged two CM6 spark plugs and a Kistler pressure-sensor port inside the same compact head.
I used 304 stainless steel for the test article, then checked the geometry with an initial ANSYS static analysis at the thesis design load before releasing it for machining. The material choice favored repeated assembly, thread integrity, and chemical compatibility for a laboratory prototype.
Ignition selection
CFD trade study
High-energy single spark
The simplest package, but one flame kernel has to cross the full chamber.
Glow-assisted ignition
A heated surface improves local reactivity but gives less direct timing control than a spark.
Dual spark
Two controlled kernels shorten the maximum propagation distance and increase the early flame-front area.
I extracted the engine's actual internal fluid domain and modeled piston motion, two-stroke port boundaries, local refinement, and detailed NH3/H2 chemistry in CONVERGE.
At the 3000 r/min design condition, I compared the three heads across 20%, 10%, 5%, and idealized 0% hydrogen cases. I used pressure, heat-release timing, combustion phase, CA10-90 duration, and indicated-work trends to judge the architectures.
The dual-spark head gave the most robust timing as mixture reactivity fell because it addressed flame travel directly. The CFD selected and explained the architecture; it did not substitute for the later engine test.
Physical test sequence
Bench validation
Confirmed starting, hot operation, clearances, and pressure acquisition with the modified head.
Unassisted pure ammonia without the cracker did not ignite reliably in the existing engine configuration.
Later runs operated at approximately 2000-2500 r/min without gasoline or ethanol assistance.
The measurement chain combined a Kistler 6052C in-cylinder pressure sensor, Type 5018 charge amplifier, LeCroy WaveSurfer 3054Z oscilloscope, and PCAN monitoring.
The recorded outputs were engine speed and in-cylinder pressure traces, not brake power. Generator-side coupling and alignment prevented a reliable loaded result, so no brake-power or efficiency figure was derived from the bench run.
Awards and honors
Recognition
Ranked 1st of 127
Tsinghua University Outstanding Undergraduate Thesis Award, top 3% of the Vehicle Engineering cohort.
First Prize
Tsinghua 44th Challenge Cup.
Second Prize
Tsinghua-Toyota Future Mobility Innovation Challenge.
