The spark has to fire before the piston reaches top dead centre so that the mixture, which takes a certain time to burn, reaches its peak pressure just after the piston starts down. Ten degrees of crank angle lasts 0.56 ms at 3,000 rpm but 1.67 ms at 1,000 rpm, so the same burn time needs more advance at high speed, and slow-burning fuels need more advance still: 10 to 15 degrees extra for producer gas and 5 to 10 for ethanol.
Part of Internal Combustion
Why spark timing is set early, how advance mechanisms follow speed and load, how to find and adjust timing on a salvaged engine, and what over- and under-advance do.
The Burn Takes Time, So The Spark Must Come Early
Burning does not happen instantly. If the spark fired exactly at top dead centre the pressure would peak well after the piston had started down, and the engine would lose power and run hot. So the spark fires earlier, and the burn is timed to give its peak pressure a little after top dead centre. The burn lasts roughly the same time at any engine speed, but at high speed the crankshaft turns through more degrees in that time, so the spark must be earlier in degrees as the engine speeds up.
Fuels change the burn time. The corpus says producer gas burns more slowly than petrol vapour and wants 10 to 15 degrees more advance (Engine Modifications), and the carrier and the corpus ethanol article give 5 to 10 degrees more for ethanol (Carburetor & Injection Modifications), found by advancing a little at a time until the engine runs best without pinging.
Advance Mechanisms
- Fixed timing. Small simple engines set the spark at one angle, a compromise for all speeds.
- Centrifugal advance. Weights on the ignition shaft swing outward against springs as speed rises and turn the cam or rotor forward. It follows speed.
- Vacuum advance. A diaphragm connected to the intake advances the timing at light load, when the mixture is thin and burns slowly. It follows load.
- Manual lever. The operator sets advance by hand; this is useful when the engine runs on petrol to start and gas to run, because the corpus notes that the setting that suits the gas knocks on petrol.
The corpus shows the distributor adjustment in the carrier: on a distributor engine, rotate the distributor body slightly against the direction of rotor rotation to advance. A magneto changes timing by rotating the housing or the cam relative to the flywheel, as Magneto Ignition describes.
What You Need
- The engine’s timing marks or a way to make them: find top dead centre on the compression stroke by feeling pressure at the plug hole and standing a rod on the piston, then mark the flywheel.
- A tape or string to measure arc distance on the flywheel rim.
- A timing light, if one can be made or salvaged; the carrier mentions checking with one.
- A listening stick (a screwdriver or rod against the block) to hear knock.
Method: Set Timing On An Unmarked Engine
Step 1: Find top dead centre on the compression stroke and mark it on the flywheel and the fixed pointer.
Step 2: Convert the advance angle to distance along the flywheel rim, as in the worked example, and mark that point.
Step 3: Set the spark to fire when that mark lines up. For a points ignition, turn the engine to the mark and adjust the housing until the points are just opening.
Step 4: Start retarded and advance a little at a time under load. Stop advancing at the first knock or ping and back off a few degrees (recalled practice, flagged).
Step 5: Re-check after any change of fuel. The setting for gas or ethanol is not the setting for petrol.
Worked Example: Time Per Degree And Distance On The Rim
Time per degree. One revolution lasts 60 ÷ rpm seconds.
- At 3,000 rpm: 20 ms per revolution, so 20 ÷ 360 = 0.0556 ms per degree. Ten degrees is 0.556 ms, about 0.56 ms.
- At 1,500 rpm: 40 ms per revolution, so 10 degrees is 1.11 ms.
- At 1,000 rpm: 60 ms per revolution, so 10 degrees is 1.67 ms.
If the burn needs a fixed time, the angle needed in advance scales in proportion to speed: tripling speed from 1,000 to 3,000 rpm triples the degrees of advance for the same burn time (1.67 ms is 30 degrees at 3,000 rpm, since 1.67 ÷ 0.0556 = 30). The burn time itself is not sourced, so the example shows the scaling and not the setting. In a real engine the churning of the charge speeds the burn as the engine speeds up, so the advance needed rises less than in proportion (recalled, flagged).
Distance on the rim. For a flywheel 300 mm in diameter, the rim is π × 300 = 942.5 mm round. Ten degrees of it is 942.5 × 10 ÷ 360 = 26.2 mm. That is the distance to mark ahead of top dead centre for 10 degrees of advance.
Common Mistakes
- Over-advancing to gain power. The engine knocks and the pistons burn.
- Adjusting timing on the wrong stroke. The mark is a degree-for-degree match only on the compression stroke.
- Leaving petrol-start timing on the gas setting. It knocks and kicks back.
- Skipping the points gap check before timing. The gap changes the opening point.
Safety
Over-advanced timing makes a hand-started engine kick back
If the spark fires before top dead centre while the crank is being turned slowly by hand, the piston is driven backwards and the crank handle whips round, breaking wrists and thumbs (recalled hazard, flagged). Retard the spark for hand starting, use a crank that disengages, or use a decompressor. Keep fingers out of the flywheel plane. Knock under load destroys pistons; back off at the first sound.
What’s Next
- Learn next: Cooling and Lubrication — keeping the engine alive once it is firing correctly
- Combine with: Otto Cycle — the ratio and knock arithmetic