Absorption-type refrigerating machines operating on ammonia and water and equipped for
automatic thermo-siphon circulation can be fuelled with biogas.
Since biogas is only the refrigerator’s external source of heat, just the burner itself has to be
modified. Whenever a refrigerator is converted for operating on biogas, care must be taken
to ensure that all safety features (safety pilot) function properly. Remote ignition via a
piezoelectric element substantially increases the ease of operation.
For 100 liters refrigeration volume, about 2000 l of biogas per day, depending on outside
temperatures, must be assumed. A larger household refrigerator consumes about 3000 l per
If the output of a biogas system is to be used for fueling engines, the plant must produce at
least 10 m3 biogas per day. For example, to generate 1 kWh electricity with a generator,
about 1 m3 biogas is required. Small-scale systems are therefore unsuitable as energy
suppliers for engines.
Types of engines
The following types of engines are, in principle, well-suited for operating on biogas:
• Four-stroke diesel engines
• Four-stroke spark-ignition engines
• Converting diesel engines
• Converting spark-ignition engines
Four-stroke diesel engines:
A diesel engine draws air and compresses it at a ratio of 17:1 under a pressure of
approximately 30-40 bar and a temperature of about 700°C. The injected fuel charge ignites
itself. Power output is controlled by varying the injected amount of fuel, i.e. the air intake
remains constant (so-called mixture control).
Four-stroke spark-ignition engines:
A spark-ignition engine (gasoline engine) draws a mixture of fuel (gasoline or gas) and the
required amount of combustion air. The charge is ignited by a spark plug at a comparably
low compression ratio of between 8:1 and 12:1. Power control is effected by varying the
mixture intake via a throttle (so-called charge control).
Four-stroke diesel and spark-ignition engines are available in standard versions with power
ratings ranging from 1 kW to more than 100 kW. Less suitable for biogas fueling are:
• loop-scavenging 2-stroke engines in which lubrication is achieved by adding oil to the
liquid fuel, and
• large, slow-running (less than 1000 r.p.m) engines that are not built in large series,
since they are accordingly expensive and require complicated control equipment.
Biogas engines are generally suitable for powering vehicles like tractors and light-duty trucks
(pickups, vans). The fuel is contained in 200-bar steel cylinders (e.g. welding-gas cylinders).
The technical, safety and energetic cost of gas compression, storage and filling is substantial
enough to hinder large-scale application.