A petrol engine converted to producer gas needs a mixer, about 10-15° more spark advance because the gas burns slowly, and, if possible, a compression ratio raised from about 8:1 to 10-12:1, which gains only about 7-12% in the ideal cycle and therefore recovers only a small part of the 40-60% power loss. Raising compression by milling the head removes about 2.7-4.5 mm on a typical 2-litre four-cylinder and risks valve contact, and it makes the engine knock on petrol.
Part of Engine Connection
The changes to an engine’s ignition timing, compression ratio and intake that suit it to producer gas, and what each one gains and risks.
What The Parent Recommends
For spark-ignition engines the parent asks for three changes: a mixer valve, ignition advance of 10-15° because producer gas burns more slowly than petrol vapour, and a compression ratio raised from about 8:1 to 10-12:1 for better efficiency on a low-energy gas. For diesels it keeps the diesel injection as a pilot (Engine Connection).
What Compression Is Worth
The ideal Otto-cycle efficiency is 1 − r^(1−γ), where r is the compression ratio and γ the heat-capacity ratio of the charge. For a lean-ish mixture take γ = 1.35 (an assumption).
- r = 8: 8^(−0.35) = 0.483, efficiency 0.517.
- r = 10: 0.447, efficiency 0.553 (7% better than r = 8).
- r = 11: 0.432, efficiency 0.568 (10% better).
- r = 12: 0.419, efficiency 0.581 (12% better).
So the gain is 7-12% in the ideal cycle and real engines gain less. Against a power loss of 40-60% (Power Output Derating), the modification is worth doing only if the head can be changed safely, and it will not make the engine match its petrol output. The fraction recovered in a real engine is not sourced here; measure it.
How Much Metal Comes Off
Take a 2-litre four-cylinder with 500 cc per cylinder, a bore of 86 mm (area π/4 × 8.6² = 58.1 cm²) and a compression ratio r = (Vc + 500) ÷ Vc.
- r = 8: Vc = 500 ÷ 7 = 71.4 cc.
- r = 10: Vc = 500 ÷ 9 = 55.6 cc, so remove 15.8 cc, which is 15.8 ÷ 58.1 = 0.27 cm = 2.7 mm of chamber depth.
- r = 12: Vc = 500 ÷ 11 = 45.5 cc, so remove 25.9 cc, which is 4.5 mm.
These dimensions are a model of a plain flat-top engine, not a measured head. A real chamber is shaped, so measure the chamber volume by filling it. Removing material from the head or block face also brings the valves closer to the piston, shifts cam timing on belt-driven engines and may break valve clearance, so check piston-to-valve clearance with clay before running.
Ignition Timing
Producer gas burns more slowly than petrol vapour, so the spark must fire earlier for the burn to finish at the right point in the stroke. The parent gives 10-15° advance. The corpus internal-combustion article uses the same range. On a distributor engine rotate the distributor body slightly against the rotor direction, as that article describes, and check with a timing light.
The catch is the petrol start: with raised compression and the extra advance, the engine knocks on petrol, since the setting suits the gas and not the petrol. Start and warm up on petrol at light load, or use a manual advance control so the advance can be reduced for petrol and increased for gas.
Method
- Fit the mixer and gas line first and prove the gas (Mixing Valve & Air-Fuel Ratio).
- Set the timing by 10-15° more advance, and listen for knock on petrol.
- Raise compression only if it can be done with a measured chamber and valve clearance.
- Keep the engine’s cooling in good order: a slow-burning mixture heats the exhaust valves.
Common Mistakes
- Milling for compression without checking valves. Contact destroys the engine.
- Leaving the advance fixed. The petrol start knocks, or the gas runs retarded.
- Expecting full power back. The energy density of the mixture caps it.
Safety
A modified engine is still a producer-gas engine
The gas it runs on is about 18-22% carbon monoxide by volume. Never test, tune or run it on gas indoors, in a garage, in a shed or in an enclosed yard, and run it only with its exhaust well clear of people. Backfires in the intake can ignite the gas and air in the line. See Carbon Monoxide Risks.
What’s Next
- Learn next: Power Output Derating — what is left after the modifications
- Combine with: Internal Combustion — the wider engine background