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Q: How Does Ignition Timing Work?

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How does ignition timing work?

Ignition timing is basically a coordinated dance between the ignition coil creating the spark and the rotational speed of the engine, which we call RPM (revolutions per minute). In order for the engine to produce optimum power with the least amount of fuel used, the air/fuel mixture must be ignited at just the right time in the combustion cycle of the engine.

The basic combustion cycle of your average reciprocating, internal combustion engine has four strokes:

  1. Intake stroke
  2. Compression stroke
  3. Power stroke 4) Exhaust stroke

The intake stroke draws atmospheric air, and fuel, into the cylinders of the engine where it gets ignited and burned.

During the compression stroke, the air/fuel mixture is compressed into a smaller space that we call the combustion chamber where it becomes more concentrated and volatile.

Somewhere near the end of the compression stroke, before the piston reaches top dead center (BTDC), the ignition coil sends a high voltage spark to the spark plug. The amount of voltage supplied to the spark plugs can range anywhere from 5000 volts to 40,000 volts, or more. When the spark occurs, the air/fuel mix is ignited and burns rapidly. The burning air and fuel create a lot of instantaneous heat (over 2000 ℉), which causes the nitrogen contained in the air charge to expand rapidly. The rapid expansion of the nitrogen creates an immediate increase of pressure inside the cylinder, which in turn pushes the piston down in the cylinder and forces the crankshaft to rotate. The more force we make, the faster the engine runs.

To obtain the most efficient power from this combustion process, all air and fuel must be completely burned by the time the engine crankshaft reaches the point of rotation of about 20 degrees after top dead center (ATDC). This means that we need to determine when to create the spark that starts the combustion process.

Here’s where things get interesting.

Because fuel burns at a relatively constant rate, we will have to start the combustion process sooner, or later, in relationship to the speed of the crankshaft. For example, let’s take a V-8 engine that is running at an idle speed of 700 rpm. To have all air and fuel burned by the time the crankshaft reaches 20 °ATDC, we must create the ignition spark somewhere around 10 °BTDC. When we accelerate, the engine spins faster which reduces the amount of time for the combustion process to occur. To counter this increase of crankshaft speed and to restore the lost combustion time, we advance the spark to an earlier point in the crankshaft’s rotation.

There is a wrinkle in this spark issue, however. As the engine accelerates, it draws more air and fuel into the cylinders. This increases the pressure inside the combustion chamber of the cylinder, which makes it harder for the spark to start the burning process. If we kept the spark voltage constant while increasing engine speed, we would need to advance the spark timing to a point that is just not feasible for combustion to occur. Theoretically, if we increased the engine speed from 700 rpm to 3600 rpm, we would need to advance the spark timing from 10 °BTDC to around 150 °BTDC. The engine simply will not run with this much timing advance. We need to be between 32 - 35 degrees of advance for optimum engine operation at 3600 rpm. How this is accomplished is by increasing the amount of spark energy from the ignition coil to the spark plugs. The increased spark voltage not only helps us to start the combustion process, it also helps to burn the air/fuel mixture more rapidly.

The amount of ignition timing that we can use is also affected by the fuel that we use. If we intend to make more power in the engine, we need to use a higher octane fuel. Higher octane ratings for gasoline simply mean that the fuel is more resistant to pre-ignition. Use of a higher octane fuel allows us to increase spark timing in the engine, which creates more power.

Ignition timing in computer-controlled vehicles is completely non-adjustable. Most older vehicles, prior to the 1985 model year, had to ability to adjust the spark timing because they used a distributor.

If your engine is not running up to par or you are experiencing a loss of power or fuel economy, feel free to contact a professional technician who can adress any problems and adjust your ignition timing accordingly.

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