Ignition

Ignition

The ignition system used on OBD II equipped vehicles needs to reliably provide a properly timed spark, igniting the compressed fuel and air mixture in all the engine’s cylinders whenever it’s running. A hot spark of adequate duration that completely burns all the fuel in a cylinder is critical to the proper and safe operation of the catalytic converter. If incomplete combustion of the fuel in a cylinder occurs, the remaining fuel passes into the converter and continues to burn there. This event is called a misfire. Misfires can be caused by one of three things: fuel mixture problems, compression problems or ignition faults. 

When an engine misfires, the catalytic converter has little tolerance to properly handle the extra unburned fuel and some of it passes out the exhaust as pollutants. This is a best case scenario. If the catalytic converter continues to be exposed to unburned or incompletely burned fuel, the converter can overheat and possibly melt, reducing its ability to reduce harmful exhaust emissions. To reduce the chances of misfires by ignition related issues vehicle manufacturers have introduced a variety of more reliable ignition systems than the traditional coil and distributor systems found on pre-OBD II vehicles. Distributorless Ignition Systems (DIS) have been around for more than two decades, but in recent years the trend has been to multi-coil systems such as Coil-On-Plug (COP) or Coil-Per-Cylinder (CPC) ignition systems, and Coil-Near-Plug (CNP) ignition systems. Placing individual ignition coils near or directly over each spark plug eliminates the need for long, problem prone high voltage spark plug cables.

From a reliability perspective, having a separate coil for each cylinder gives each coil more time to recharge between cylinder firings. This provides more saturation time for a hotter spark, especially at higher rpm when firing times are greatly reduced. The result is fewer misfires, cleaner combustion and better fuel economy. Harnessing the computer horsepower available on OBD II systems the PCM now controls all aspects of time and creating the high voltage pulse to the sparkplug through a few engine mounted sensors.

Using the ignition system sensor the PCM can detect a misfire on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor. A single misfire will cause a subtle change in the speed of the crank. The PCM tracks each and every misfire, counting them up and averaging them over time to determine if the rate of misfire is abnormal and high enough to cause catalytic converter damage or excessive tailpipe emissions. Let’s take a deeper look at some of the ignition components and their functions.

Spark Plug

The ignition system sends a voltage pulse to the spark plug in each cylinder when the piston is at optimal position in the compression stroke. The tip of each spark plug contains a gap that the voltage must jump across in order to reach ground. The electricity must be at a very high voltage in order to travel across the gap and create a good spark. On OBD II equipped vehicles voltage at the spark plug can range from 8,000 to over 50,000 volts. This high voltage produces a hotter and longer duration spark in the cylinder to completely combust all the fuel. A hotter spark also makes spark plugs more resistant to fouling and allows for service intervals up to 100,000 miles.

Any fault that prevents the spark from jumping the electrode gap on a spark plug will result in a misfire. This can be caused by excessive resistance, shorts or opens in the spark plug itself (such as a cracked insulator), a carbon tracked spark plug or a fouled spark plug. When diagnosing a misfire DTC for a specific cylinder, always remove and inspect the spark plug. It can save you a lot of time if the plug is fouled, carbon tracked or damaged. Oily deposits on the plug indicate a mechanical problem is the cause of the misfire.

Coil(s)

Ignition coils provide the high voltage needed to fire the spark plugs. The ignition coil serves as a high voltage transformer; it steps up the ignition system's primary voltage from 12 volts up to thousands of volts.

Most OBD II equipped vehicles rely on multi-coil systems such as Coil-On-Plug (COP) or Coil-Per-Cylinder (CPC) ignition systems, and Coil-Near-Plug (CNP) ignition systems. Placing individual ignition coils near or directly over each spark plug eliminates potential misfire problems caused by burned, chafed or loose cables, and reduces resistance along the path between the coil and plug. Consequently, each coil can be smaller, lighter and use less energy to fire its spark plug.

There are several common multi-coil ignition systems designed to fit different engine configurations. On many dual overhead camshaft engines the coils are mounted directly over the spark plugs. Many of these are the thin coils that extend down into recessed wells in the engine's valve covers. In other applications the individual coils are mounted in a cassette or carrier that positions the coils over the spark plugs. On some V8s, a Coil-Near-Plug (CNP) setup is used because the spark plugs protrude from the side of the cylinder head and there isn't room to mount a coil on the end of each plug. Here, the individual coils are mounted on the valve cover and attached to the plugs by short plug wires.

In most of the older DIS ignition systems, an electronic module was part of the coil pack assembly and controlled the switching of the coils on and off. On most of the newer systems, the switching function is handled by the powertrain control module, though there may some additional electronics and diodes built into the top of each coil.

Crankshaft Position Sensor

The crankshaft position sensor does two things: it monitors engine rpm and helps the computer determine relative position of the crankshaft so the PCM can control spark timing and fuel delivery in the proper sequence. On some engines, an additional camshaft position sensor is used to provide additional input to the PCM about valve timing.

Knock Sensor

The knock sensor creates a voltage signal based on the vibrations caused by detonation. The computer uses this signal to retard timing when spark knock occurs. The knock sensor is typically located in the lower engine block, cylinder head or intake manifold.

A knock sensor detects the sharp noise made by a gasoline engine when its fuel air mixture is being ignited in one or more cylinders prematurely. This is called detonation. On OBD II equipped vehicles the PCM uses the signal from the knock to help optimize ignition timing resulting in better performance and greater fuel efficiency. The PCM will set a DTC if problems are detected in the knock sensor circuit.

Powertrain Control Module

The PCM receives a basic timing signal from the crankshaft position sensor and sometimes a camshaft position sensor to determine engine speed, firing order and timing. It then looks at inputs from the throttle position sensor, airflow sensor, coolant sensor, MAP sensor and even the transmission to determine how much timing advance is needed in each cylinder. Most OBDII PCMs no longer need an ignition module to trigger the spark through the ignition coil(s); the ignition transistor is part of the PCM or directly controlled by the PCM.

Most of today's multi-coil ignition systems are capable of making timing adjustments between cylinder firings which makes these systems very responsive and quick to adapt to changing engine loads and driving conditions. On some systems, individual cylinders can have their own individual timing so that timing can be as aggressive as possible per cylinder without fuel detonation, resulting in better performance and greater fuel efficiency.

Front Oxygen Sensor

The oxygen sensor provides real time information about the air/fuel mixture. The PCM uses this to constantly re-adjust and fine tune the air/fuel ratio. This reduces emissions and optimizes fuel efficiency and performance. A faulty oxygen sensor will typically make an engine run rich, use more fuel and pollute.