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.

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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 :)

Specifications

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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

Carburetor

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Original Tank/Carburetor connection:

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

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Modifications

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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.

Muffler

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Drawing	Part	Size
	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

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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

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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)

Materials

V2A	1.4301	X5CrNi18-10	AISI 304	600°C
V4A	1.4571	X6CrNiMoTi17-12-2	AISI 316Ti

This seems to be the best source for inox pipe:

http://www.zimmermannschalldaempfer.de/edelstahlrohr.html

Alternatives:

Air-Filter

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.