Running (small) engines on biogas.

The following is extracted from the Anaerobic Digestion Discussion List Archives at, hosted by REPP, and is provided as information only.

No recommendations should be taken solely from this record and the Authors disclaim all responsibility and liability for any actions taken on the basis of this information.

Initial Request

Hi all,

I am Andre Daud from Indonesia. Currently we (I and my colleagues) are
working to encourage the use of biogas, especially to smallholder dairy
cattle farmer. There is no problem so far, but it seems some of the farmers
were more interested to the use of biogas as fuel for generator due to poor
electricity infrastructure in village areas. We, together with them, tried
to modify common genset (gasoline fuel, 900-1200 watts output) to work on
biogas. But we have not fully successful because our limited knowledge in
engineering. Oh ya.. maybe we have enough background on animal-related
knowledge but not engineering. Our main problem here is to determine how
best to modify carburettor (mixing air and biogas) and how to determine the
effect of engine compression. Perhaps this mailing list could give us best
references or solution to our problem.

Thank you very much.

Bandung, Indonesia.



Biogas is a very appropriate technology for rural electricity. Can I ask 900-1200 watts if you are trying to run a 2-cycle engine? These have a couple of important draw-backs, 2-cycle engines are lubricated by mixing with the fuel. With biogas, some other kind of lubrication system I am not aware of would be necessary.

A small 4-cycle engine---possibly out of an old motor scooter or something? is ideal. You can advance the fuel/air mixture and timing to find the "sweet spot" for ignition. Also need to keep enough weight on the gas bag to produce sufficient fuel pressure. There's a number of different ways you can modify the carburettor for the biogas fuel line, but I've seen people just take the rubber fuel line and stick it down the carburettor throat---the four cycle has a separate value for exhaust and will keep fuel from flowing in during the power and exhaust strokes.

Here's a little 1 kW biogas generator made in China. They're about $450 US dollars.,0,387,17593,500,375,ea14a534.jpg

Note the mechanical fuel/air mixture advance. It appears to be a 4-cycle, note the oil filler cap at the bottom.

If you have a use for the waste heat, a steam boiler and turbine are the simplest and most powerful prime mover for a generator---90% of the electricity in the world is produced with steam---however, the trade-off is even low-pressure steam can be very dangerous.

Warren Weisman, USA

Apologies, I don't know if they're available elsewhere, but in the US we have 2-cycle lawnmower engines that one does not need to use fuel mix with and that oil themselves. If that is the case---as in the picture of the Chinese generator it has an oil filler cap---it would work with biogas (only much louder than a 4-cycle engine and not last as long).

In fact, it would have the advantage of magneto ignition, which is more reliable.


G'day All,

While I do believe in the KISS principle and really like Albert Einstein's
quotation "Things should be as simple as possible, but no simpler!" I think
the "hose down the carburettor" is "too simple".

The problem arises when the engine cuts out (granted this should not happen
on a generator, but we have to allow for it). Biogas will still be flowing
so there is a growing risk (likelihood!) of an explosion at restarting. The
solution is a solenoid held open by generator output, or maybe a valve
opened by engine rotation - you may need a starting arrangement as well.
There is also a possibility of trouble with the simple hose during starting,
so be careful.

Happy digesting,

Mr. Paul Harris,

Andre, if you want to dual fuel in the simplest way, do not modify your
carburettor at all. Instead remove the air cleaner off the top of it and
insert a tee made from a smooth bore copper pipe fitting. You will need to make flanges at either end to fit the carburettor and the air cleaner. Then to the outlet on the side of the tee you place a butterfly valve of an old carburettor, or easier but not so nice looking a gate or globe type valve and then continue the pipe work onto your gas supply.
You start the engine on petrol and then turn off the tap on the petrol fuel
line and when the engine begins to falter as it runs out of petrol you
gently open the gas valve until the engine picks up power again.
With a little bit of practice you will get the engine running on gas and
still kind of controlled by the governor and throttle valve on the
carburettor. Now this set up will only work with a constant load on the
generator. If people are switching electrical load off and on the engine
finds it hard to follow the load. And, running uncleaned biogas through
a carburettor sized throttle you will get a lot less power than you would
on petrol. However, for demonstration purposes you can show how a duel
fuel conversion works without too much sweat. Ken C.


To add to Paul's mention of the critical importance of safety, please
let me add a point about efficiency. Small engine/generators will
convert biogas to electricity at direct efficiencies which are rather
poorer than larger gensets. Thus, you might expect 20-25% conversion
efficiency with smaller generators vs. perhaps 35% with larger
generators. While either figure may seem quite similar, 20% vs. 35% is a
loss of nearly half.

But the more important fact is that the picture of the generator
provided that started this thread was of an air-cooled genset, which
means that the energy wasted as heat is expressed as diffuse hot air.

Where one is working with a water-cooled genset, /system /efficiencies
might reach 80%, because a very good portion of the 65% of the energy in
the biogas which is not provided back as electricity is poured into the
cooling water, and can therefore be far more easily used to heat the
digester. By contrast, hot air cannot easily be gathered and used to
heat the digester.

Thus, when using a small genset with poor direct conversion efficiency
which is also air-cooled, one tends to lose twice.

David William House

David (Paul, actually) I too am an Einstein Fan! My latest one is
"The only thing more dangerous than Ignorance is Arrogance!"
You are talking about small petrol engines which normally have very
simple filter type air cleaners. If you replace those with an oil bath air
cleaner and pipe in the gas underneath that, you have a low pressure non
return valve to atmosphere. And you can balance the suction on the gas
line against the several Inches water gauge of suction needed to work an
oil bath filter. Larger engines, particularly diesels, normally have them
Ken C.

G'day All,

Ken may be talking about larger diesel engines, but the "oil bath" air
cleaners used on tractors around the 1960s pass air over the oil surface
(to reduce pressure drop) and through a mesh/wire wool filter rather than
through the oil as Ken indicates. This "over oil" type would not stop gas
flow, so still be careful.

If you don't like Wikipedia
<> try "Tractors and their
Power Units" by Barger et al, 2nd Ed, John Wiley & Sons (1963),pp 193-194.

Happy digesting,

Mr. Paul Harris,

Paul et al Hi! Brought up on a N.Z farm in the 50s and 60s and drove more than a few tractors in me time!!!
The Air intake is a tube down the middle and the coarse wire net was around
the outside. The mark to fill the oil was just above the bottom of the
central tube. When the engine started, the air being sucked in down that
central tube splattered the oil up into the wire gauze around it and kept it wet and sticky and washed down so that all the dust ended up as sludge in the oil bath itself which could be easily removed.

When the engine is not running the oil covers the end of the inlet tube,
and if you try to push gas in below the air cleaner you have to push
all the oil in the wider bath back up that central tube, several inches of
it before you could start to push bubbles of gas back up through the column
of oil the that gas pressure itself was holding up!
According to Wikipedia "Lighter and smaller particles are trapped by the
filtration media in the insert, which is wetted by oil droplets aspirated
there into by normal airflow." And I trust that that is how I described
it! But they missed out on the increased pressure required for reverse
flow!! Ken C.

(I suspect that there may be several different designs of oil bath air cleaner – if you are using Ken’s suggestion make sure the air has to pass through the actual oil and that the intake line will hold a back pressure. Paul Harris)


Hi all,

Thanks for advices. If I am correct, I don't need to make any major changes
in carburettor. We have tried it. It seems that there should be no air flow
at once we start to ignite the engine. It works. One more question: we don’t have a chance to use iron sponge (to scrub water contents?), so what will happen to the genset? Or maybe, because of that, our genset sometimes go unstable (not stationary) and knocking hard.

I agreed with Mr. Warren. We don’t have abundant source of technology,
especially biogas. Our main source of biogas technology is just from old
book published by GTZ around 1980-90. I am sure that the biogas tech has
changed dramatically over ten years. FYI: we don’t do this for making money, we just jump to the remote area, hope we could do something to share with people there, especially to smallholder cattle or dairy farmer. As you
aware, along with global trade, it seems there is no more incentive to small farmer to keep farming (everything imported was cheaper!). Maybe biogas can be their main reason to stay keeping cattle in future.

Bandung, Indonesia.

For Further Information

“Engines for Biogas”
"The Complete Biogas Handbook" ||

Compiled/Edited by Paul Harris –

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