On stationary natural gas engines, or any combustion engine for that matter, the ignition system is a critical part of engine operation. Slight issues with the ignition can have dramatic impacts on performance, emissions, and maintenance. For this reason, it is important to always make sure all parts of your ignition system are properly set up and maintained. Some of the important setup and maintenance concerns for different parts of the ignition system are discussed below.
The “Timing” of the ignition refers to when the spark plug fires for each cylinder. Timing is measured in degrees before top dead center (BTDC), which is when the cylinder volume is at its smallest. For example, if the timing is at 20 degrees, then every spark plug will fire when the crankshaft is 20 degrees before that cylinder is at full compression.
Having proper (or improper) timing can effect engine performance in many ways. As the timing is farther from the ideal point, energy and fuel will be wasted. Additionally, the emissions will suffer as the fuel is less fully burned. In some cases having the ignition too advanced will also cause engine knock, which is when multiple flame fronts exist in the cylinder at the same time, which can damage the engine over time. This and other timing issues can reduce the life of some engine components such as spark plugs and cylinder heads.
Timing on an engine might be fixed for all conditions for old ignitions, or might change with variations to speed and load. Some more advanced digital ignition systems, such as EMIT’s, will also change timing with elevation, fuel type, and other conditions. Having a timing map that best matches an engine’s operating conditions is ideal, because as conditions change compensations in timing will keep the fuel use more efficient and the emissions reduced.
Regardless of ignition type, it is a good idea to often check the timing of the engine with a timing light to make sure the actual timing matches what is expected. It only takes a few minutes and can prevent a lot of the shortfalls listed above associated with improper timing, so it is well worth the time.
Spark plugs obviously play an important role in ignition performance as well. The following items are worth some special attention.
- Resistor vs. non-resistor spark plugs– “Resistor” spark plugs, often marked with an “R” or “Q”, have an internal resistor that helps cut down on electrical noise. For any setup with digital electronics on the skid, resistor type spark plugs should always be used.
- Spark plug gap– Engine and ignition manufacturers will have a recommended gap for the spark plugs. The gap can be checked quickly and easily with a spark plug gap tool. It is important to always check the gaps of the plugs when installing new ones. If the gap is incorrect it will affect the spark energy needed to spark, which can cause misfires or poor ignition of the fuel mixture. If the gaps are not all the same, it will make the combustion on the cylinders uneven, which over time will affect wear on various components. It is well worth a few minutes to make sure the gaps are the same!
- Spark plug temperature– Each plug should have a temperature rating which should match the engine manufacturer’s recommendations. If the plug runs too hot, it might auto-ignite the fuel which will be damaging to the engine. If it runs too cold, it might not combust the fuel mixture well.
- Tightening the plug– Using a torque wrench set to the proper torque for the plug will prevent over-tightening the plug during installation. If a plug is over-tightened it can break the plug seal and affect compression.
- Noting plug wear– When changing plugs, it is a good idea to look at the plugs being removed. They should all have a relatively similar amount of electrode worn away evenly. If there is any sign of physical damage or burn marks, it can signify detonation. If one plug is worn less than it should be, the cylinder might be misfiring outside the cylinder. If there are deposits on the plug, the engine might be running too rich or have poor quality fuel.
There are two main types of ignition wiring – primary and secondary. “Primary” wiring is the wiring from the ignition module to the coil. “Secondary” wiring is from the coils (or distributor) to the spark plugs.
For the ignition primary wiring, it is important to make sure that all wires are well secured and insulated. Since a lot of electrical noise is emitted from the ignition wiring, it is best to route these wires away from other sensor wires to keep electrical noise out of other systems on the engine.
Always inspect secondary wiring for any breaks or exposed wire. If wire is exposed, the high voltage of the spark could jump out of the secondary wiring to the engine block, instead of across the plug gap. Because of the high pressures in an engine cylinder, the amount of voltage needed to break a spark plug gap in the cylinder (called firing line voltage) is very high. Any possible alternate route for the voltage to escape to the engine block will be taken if it is possible, which is why the wiring insulation has to be in good shape.
Additionally, the plug boots on the secondary wiring should have dielectric grease applied, which will prevent sparks from escaping through the boot and also prevents corrosion.
By better understanding ignition fundamentals, properly following manufacturer recommendations, and using best practices for ignition operation and maintenance, you can save fuel cost, reduce emissions, and increase service life on your engine.
For help in maximizing your engine performance and emissions with the latest in engine controls, contact EMIT Technologies at 307-673-0883.
Thomas Peterson, Embedded Systems Engineer 6/14/2016