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Auxiliary Power Unit

The Auxiliary Power Unit (APU) is the fourth column in the group of independent and mobile energy sources. In case of over-budget power demand or a longer period of heavy weather (no sunlight for days), the APU will supply the batteries with power, generated by a small internal combustion engine.

[APU Test Setup]

The system itself is pretty simple: A very small 4-stroke gas engine with 25ccm and about 0.7kW/1HP output power drives a brushless motor that is acting as a generator and converts mechanical energy from the motor's shaft into electricity by rotating strong neodymium magnets around coils. At the time being the APU is fueled by gasoline but the final goal should be a complete conversion to a mini wood-gas block cogeneration plant, combusting biogas to get 350W peak power output and warm water as well. And of course, it's a nice hacking platform for 4-stroke engine designs, to see which fuel solutions might also be usable.

Design goals of this setup

  • No more than 5Kg weight
  • No more than 250ml gasoline per hour
  • As quiet as possible
  • Possibility to use woodgas instead of gasoline as fuel

Fortunately, others were already looking for a similar solution and documented their results free and publicly-accessible, which resulted in an enormous amount of feedback and successful follow up builds. It's the perfect example for the speed, individualism and reach of an uncontrolled, naturally established peer2peer product life-cycle, that we may begin to see more and more often.

Honda GX25

For this prototype, a Honda GX25 engine was chosen, although a Subaru/Robin EH025 should be comparable and easily adaptable. The major difference between these two engines is the way the valves are controlled:

Honda uses a timing belt where Subaru implemented a gear/shaft based system. Since there was no opportunity to test both systems next to each other, there was no way of telling, whether the timing belt is actually less noisy than the gear/shaft based system, but in general, a belt should be quieter.

If you are having trouble getting the GX25 engine for a good price, start thinking out of the box: These engines, when bought as a single unit, seemed nowhere available for under 300EUR, but you might want to look for Honda UMS425 motor trimmers - which basically have the exact same engine, this one used to be an UMS425 too :)


[Original Honda GX25] [Honda GX25 3D cut][Honda GX25 design]

Attribute Value Power/Torque
Size L: 192mm W: 221mm H: 230mm
Dry Weight 2.78kg
Type 4-stroke, overhead camshaft, single cylinder
Displacement 25cm³
Bore x Stroke 35 x 26mm
Cooling Forced Air
Ignition Transistorized Magneto
PTO Shaft Rotation counterclockwise
Spark Plug CM5H/CMR5H (NGK) - Gap: 0.60-0.70mm
Fuel Unleaded >= 86 octane / Wood-Gas Option?
Oil SAE 10W30 SJ(General Purpose) 80-100ml


[Honda GX25 carburetor filtered-air intake side] [Honda GX25 carburetor filtered-air intake side 2] [Honda GX25 carburetor engine-side]

Original Tank/Carburetor connection:

  • Transparent to angled brass connector
  • Black to straight connector (further back)

[Honda GX25 carburetor mounted w/o air-filter] [Honda GX25 w/o carburetor showing paper gasket] [Honda GX25 w/o carburetor and gasket]


[First APU test-run]

During the first run on the test-rig it was clear, that the Honda GX25 still is too noisy. So the first modification efforts will focus on the development of a new muffler, a new air-intake/filtering system and a fitting case, with a dampening rig for the GX25 and additional dampening material on the outer walls.

Removed Parts

These are the unwanted parts, that are already ripped off the Honda GX25, to reduce weight as well as to remove parts which didn't perform well or could easily cause vibrations and other stuff that generally won't fit into the new case.

If you are in need of any of these parts, just drop a note, it's very unlikely that these will ever be in the somewhat more final design, so these resources are free to share.


[Honda GX25 exhaust outlet] [Honda GX25 exhaust flat] [Honda GX25 muffler side]

Drawing Part Size
Honda GX25 exhaust outlet Outlet inner 11mm
Outlet outer 13mm
Threads M5
Thread Depth 12mm
Distance between M5 Threads 30mm

The original Honda GX25 “Tin-Can” muffler is already removed and will be replaced by a new 3 chamber muffler for better sound suppression. The goal is to reduce the sound emissions of the APU by building a muffler specific for this engine size and to prove DIYability of the concept. Not to mention, getting some sound suppression and hard soldering skills for free on the way.

Muffler designs

Having more muffler volume seems to increase the dampening effect but the room available for the muffler within the case is constrained to 6cm max. in diameter, the height may vary in between 18-22cm. This lead to the following muffler dimensions, in order to get a muffler volume of almost 600ccm, still fitting in the case and having a 1:23.75 displacement to muffler volume ratio. There seems to be a commonly referred butter zone for this ratio in the range of 1:13 to 1:24, so this design may just work pretty decent:

Volume formula Calculation
<x>V = \pi * r{2} * h</x> <x>\pi * 3{2} * 21 = 593,76ccm</x>

Simple 3 chamber muffler

[3-chamber muffler]

Optimum volume of muffler Engine displacement * 13-24
Chamber volume ratio (in to out) 4:2:3
Volume ca. 565ccm
Length 20cm
Diameter 6cm
Appr. Weight 500g
1 110mm x 60mm x 0,6mm (LxDxS)
1 90mm x 60mm x 0,6mm (LxDxS)
More complex muffler design

[3-chamber muffler new design in progress]

Volume ca. 594ccm
Length 21cm
Diameter 6cm
Appr. Weight 500g
1 210mm x 60mm x 0,6mm (LxDxS)
1 90mm x 60mm x 0,6mm (LxDxS)


Torcman TM430-30

Torcman manufactures two of their brushless outrunners to be used as a generator, specifically tailored for the GX25 and the GX35. They also offer a complete, almost-ready-to-use-kit for both engines, in case you don't want to hack your own. The following configuration represents the latest specs of a brushless outrunner, to match the optimum torque and RPM of the driving engine to maximize output efficiency.


[Torcman TM430-30 Drawings] [ TM430-30 mounted on GX25 ]

Attribute Value
Type Brushless outrunner
KV 166 RPM/V
Coils 12
Windings 32 per coil
Copper 0.8mm
Magnets 14
Dimensions 63.5×52 mm (LxD)
Shaft 6mm

Mounting Plate

Combined mounting plate to attach the Torcman to a GX25 or GX35

[ ] []

4 Hex cylinder head screws, M6, 20mm thread length
4 Distance bolts, M6, 7.5mm height, 12mm dia.
4 Washers, M6, 12mm
4 Springs, M6
4 screws, M3


  • TM430-30 4000/166 = 24V
  • TM430-30 ⇒ 42V (Star = Delta * sqrt3 (1.7))
  • Frequency: rounds per second x 3 Phases x 7 “virtual gear rate” (x2 after rectification)


Honda GX25 at idle speed in Y configuation:

  • 7V per Phase
  • 311 Hz Phase Frequency
  • 18V in Y configuration after Si-6b Bridge Rectifier


The frame for the casing is made out of type 5 extruded aluminum profiles (20x20mm):

  • 4x 250mm Profile 5 (20x20mm)
  • 9x 210mm Profile 5 (20x20mm)
  • 8x Profile 5 3-edge connectors

Noise suppression


On-Demand Y/Delta Switch

Calculations Y:

<x>ULine = \sqrt 3 UPhase</x>

<x>ILine = IPhase</x>

Calculations Delta:

<x>ULine = UPhase</x>

<x>ILine = \sqrt 3 IPhase</x>


As soon as the basic mechanics are working and the sound suppression/boxing is done, development focus will shift to the Generator Control Unit.


  • Operating hour counter
  • Power Measurement
  • RPM Measurement
  • Power Control (Servo)





The choke originally was just a small plastic lever (grey), covering the hole leading from the air-filter box to the carburetor inlet (you can see it in the removed parts picture). This plastic lever has a small hole in the middle (1.5mm diameter) providing only minimal air for the combustion. This will have to be automated, if an electric starter can be realized somehow.