[Digestion] small generators

[Jim and Amy Rankin]

Hi Waren,

I certainly agree with Leslie on most points, but believe that "co-fueling"
biogas and petrol (gasoline) is not the easiest thing to accomplish
successfully.  Unless you have some mixture control (much like an aircraft
engine) so you can actually control the gasoline content of the mixture, you
will be attempting to run the engine on a rich fuel mixture which is
wasteful/harmful to the engine.   Much better to "dual fuel" it so you may
start up on "good" fuel if the biogas is lower than normal methane content,
making for hard starting, but would switch over to the biogas by turning on
the supply and cutting off the gasoline completely once the engine is warmed
up.  This would be a small "hiccup" in the fuel supply where perhaps the
mixture would be too rich or too lean for a second or two.  If the biogas is
good, start up and run on biogas, If no biogas available, run on liquid
fuel.

Diesel/biogas a more appropriate co-fuel system since the diesel injection
pump and governor control the fuel only and there's always excess air
(oxygen) in the fuel mixture in the cylinder.  As biogas is introduced into
the intake air stream, there is plenty of oxygen to burn it efficiently, so
it makes good power which the governor senses and begins to cut back on the
diesel it is metering into the cylinders.  As the biogas is increased, the
diesel will reach a minimum flow (usually the amount used to make the engine
"idle" which may be a little too low to keep the injectors cool depending on
the design.  So there are some "watch out's" about diesel/biogas co-fueling
as well, but in general it is pretty simple.

I expect any engine capable of running on gasoline will operate successfully
on biogas without increased compression ratio, subject to the derating
Leslie mentioned.  It won't be as efficient as a high compression engine,
but probably will far outlast an engine which was designed for a finite
(usually short) life as a low compression engine but was modified to
increase the compression ratio, thereby shortening it's service life even
more.

The diesels you mentioned that were totally converted to spark ignition
probably had lowered compression ratio to avoid preignition, though some
diesel designs aren't that high compression to begin with.  The more
sturdily built diesel engines, generally designed for a long service life
and relatively easy overhaul when necessary are a great platform to start
from.


> Electronic ignition circuits are generally better for biogas as the spark
> is fatter and poor quality biogas can be difficult to ignite.
>
> The system is used on diesel engines and the diesel fuel is the igniter.
> I have seen several 1 megawatt units started on diesel and the biogas
> manually increased until the engines are at full output and 5% diesel
> consumption.  If you can live with using a small amount of puchased fuel
> then this is a good way to go for small units.   You could make your own
> biodiesel and then extend the running time for each gallon by 10x with a
> biogas amendment.  The second fuel is also useful if your biogas plant
> runs low.   The end user always demands reliable service of the system.
>
> Old slow running Lister engines are great for this kind of work.  Does
> anyone remember the work of Fry in South Africa in the 1970s - look up his
> approach.
>
> Slow running engines give the flame time to propagate across the face of
> the piston but require heavy flywheels.
>
> I hope this is helpful.
>
> Best Regards
> Les Gornall
>
>
>
>> Date: Sun, 30 Mar 2008 10:13:33 -0700
>> From: weiswar at yahoo.com
>> To: digestion at listserv.repp.org
>> Subject: [Digestion] small generators
>>
>>
>> Is anyone familiar with an "off the shelf"
>> spark-ignited four cycle generator that could be
>> converted to biogas smaller than the Honda EU1000 ?
>> Possibly something in the 250-500 watt range?
>>
>> Not sure the Honda has high enough compression, as
>> over 8:1 is necessary for biogas---though, maybe Honda
>> makes some different piston shapes to improve this? I
>> noticed the Chinese are taking old diesel rigs and
>> punching spark plug holes in them to run biogas, so a
>> tiny diesel generator might work, too?
>>
>> Thanks,
>>
>> Warren Weisman
>> USA
>>
>>
>>
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