Ignition Timing & Knock Control

Concepts

Ignition

For more information on how fuel is delivered, see our documentation page on Fueling & Injection

Ignition, Flame, and Degrees BTDC

Gasoline engines operate on the principle of fuel combustion. Typically in a direct injection system, first, fuel is directly injected into a cylinder some time before the spark is ignited. Additionally, it is compressed during the compression stroke as the piston travels upwards towards top dead center (TDC). Relative to the top dead center position, and with its timing precisely controlled by the tuner's map, the spark plug is electrically ignited by a signal sent from the ECU to start the process of combustion.

Once combustion is started ideally with the spark plug, a flame front propagates evenly throughout the cylinder at a specific speed relative to factors such as the fuel sprayed and the air-fuel ratio. Due to the flame's propagation speed and a goal of complete combustion, the ignition of the spark plug should be timed early enough in the piston's movement that the flame can fully propagate within the available time window up to the end of the exhaust stroke, helping to ensure that the air-fuel mixture is entirely combusted before the gasses are exhausted. Additionally, the timing of the flame propagation can be precisely controlled by the spark's timing to exact an idealized amount of force on the piston as it travels downwards during the power stroke, encouraging efficient energy extraction from the fuel-air mixture.

To control this ignition timing, the ECU has a series of maps that define when the ignition spark should take place relative to the top dead center position of the piston as it ends the compression stroke. Typically, and in the Subaru DI logic, this timing is measured in degrees before the top dead center position, or BTDC. In this case, the degrees in the measurement relates to the degrees of crankshaft rotation of the engine, with a 4 stroke engine (such as the FA24) taking two full revolutions (or 720 degrees) for all 4 strokes to take place. Here is a reference of BTDC values and the timing they are associated with relative to cylinder events:

Extreme values such as 90.00°, and 180.00°, negative or otherwise, are only provided as an example reference to cylinder events; these values are entirely unrealistic for engine operation, and should not be used in real-world tuning.

Degrees Before Top Dead Center (BTDC)	Relative to TDC	Event
180.00°	Before	End of intake stroke, beginning of compression stroke.
90.00°	Before	Half-way through compression stroke, piston travels upwards towards the valves.
0.00°	TDC	Top dead center, power stroke begins.
-90.00°	After	Half-way through power stroke, piston travels downwards towards the crank-shaft.
-180.00°	After	End of power stroke, beginning of exhaust stroke.

Knock, Detonation, Pinging, etc.

In the ideal scenario, the ignition of the air-fuel mixture in a cylinder will result in a single, evenly propagating flame front that expands outward from the spark plug at a predictable and controlled rate of expansion. This controlled expansion of the flame front results in a linear, even pressure across the cylinder walls and the piston's surface. However, it is unfortunately possible for the air-fuel mixture to spontaneously ignite in various conditions, causing more than one flame front to propagate throughout the cylinder's volume in an unpredictable manner.

This spontaneous ignition of the air-fuel mixture is referred to as knocking due to the (sometimes audible) sound that is produced by multiple flame fronts colliding and creating extremely high pressure spikes inside the cylinder. These collisions can directly cause damage to the cylinder walls, valves, head surface, and piston surface. Additionally, the unpredictable pressures generated by knocking can exact chaotic forces on the piston surface, suddenly forcing its connecting rod into the crankshaft, possibly damaging the rod bearing.

Knocking can be caused by several factors, often compounding together simultaneously:

1. Faulty spark plugs or fuel injectors.
2. An air-fuel mixture that is too lean or too rich for proper combustion with cylinder conditions.
3. Low fuel octane, the measurable capability of a fuel to resist detonation. Fuels such as 93 or E85 have a higher octane rating, but it is possible for fuel to lose octane after sitting for a long period of time. Fuel octane can be affected by any contaminants such as oil or coolant mixing with the air and fuel in a cylinder.
4. A compression ratio incompatible with cylinder conditions and the commanded ignition timing. Compression ratios can be increased by carbon deposits (buildup) inside a cylinder.
5. Excessive temperatures inside the cylinder, including air temperature and hot spots within the cylinder surfaces. Carbon deposits can also be sources of these hot spots.