A spark-ignition engine can run on biogas if it has a gas mixer instead of a carburettor and gas that is scrubbed of hydrogen sulfide and drained of water, but it takes 1.3-1.8 cubic metres of biogas to replace a litre of gasoline, and a 25% efficient engine yields only 1.3-1.7 kWh per cubic metre. A family digester’s day of gas (1.5-2.0 m³) therefore runs a 3 kW generator for under an hour, so engines need larger plants than household cooking does.
Part of Biogas Digesters
Using biogas as engine fuel means feeding the gas to a stationary spark-ignition engine in place of gasoline, with the changes that cleaner, wetter, lower-energy gas requires.
How Much Engine A Cubic Metre Runs
- Gasoline holds about 32 MJ per litre (recalled); biogas 18-25 MJ/m³. One cubic metre equals 18 ÷ 32 = 0.56 to 25 ÷ 32 = 0.78 litres of gasoline, in energy terms; or, the other way, a litre of gasoline equals 32 ÷ 25 = 1.3 to 32 ÷ 18 = 1.8 m³ of biogas.
- Electrical output at an assumed 25% efficiency: 18 MJ/m³ × 0.25 = 4.5 MJ = 1.25 kWh, and 25 × 0.25 = 6.25 MJ = 1.74 kWh, so 1.3-1.7 kWh per m³ (3.6 MJ per kWh).
- A 3 kW generator at 25% burns 12 kW of fuel energy = 43.2 MJ an hour = 43.2 ÷ 25 = 1.7 m³ to 43.2 ÷ 18 = 2.4 m³ an hour. A family plant’s 1.5-2.0 m³ day runs it for 0.83-0.87 hours, about 2.5 kWh.
Biogas also delivers less power than gasoline from the same engine, because the gas fills part of the cylinder instead of air, and it burns slowly; expect lower power and more spark advance (recalled, with no corpus figure).
What The Engine Needs
- A gas mixer (venturi) in place of the carburettor, with gas supplied at the pressure it is designed for.
- Clean, dry gas. Hydrogen sulfide dissolves in the oil and corrodes the engine, and water condenses in the intake; scrub (Piping & Valves) and trap the water.
- A gas buffer so the engine gets steady pressure (Gas Holder Design).
- Frequent oil changes while the gas is dirty (recalled).
- Diesel engines cannot burn biogas alone; they need a pilot of diesel or biodiesel to ignite it (dual-fuel; the pilot share is not in the corpus). Biodiesel is another route (Biodiesel Production).
The corpus’s guides to engines as power sources: Converting a Vehicle Engine to Stationary Use, Repurposing Engines & Motors, and Internal Combustion.
Method
Step 1: Confirm the plant makes enough: gas needed = engine power ÷ 0.25 ÷ 6.25 kWh/m³ or the worked figure above, per hour.
Step 2: Fit the mixer and a throttle control, a flashback seal and a gas holder near the engine.
Step 3: Start on a short gas line; adjust mixture by exhaust and running smoothness.
Step 4: Run outdoors, with exhaust vented away from people.
Step 5: Check oil colour and condition often.
Common Mistakes
- Sizing a generator from the digester’s size alone.
- Skipping the scrubber, and corroding the engine from inside.
- Running in a closed shed.
- Starting an engine near a leaking gas line.
Exhaust and gas leaks
Engine exhaust carries carbon monoxide, which kills silently in a closed room. A gas leak at a hot engine ignites. The raw gas carries lethal hydrogen sulfide, and the scrubber hides it by removing the smell. Run outdoors, with a flashback water seal, regularly leak-tested lines, and no flame to find leaks (Gas Hazards (Methane, CO)).
What’s Next
- Learn next: Digestate as Fertilizer
- Combine with: Using Biogas