Reduce Fleet Downtime: Understanding CNG Ignition Systems

By Kasia McBride, Marketing Manager, NGVi

Natural gas engines differ significantly from their diesel counterparts, and technicians who maintain them must have a full understanding of their operation and specific diagnostic parameters.  Perhaps the most prominent difference these technicians must deal with is the fact that natural gas fuel ignites via spark ignition–not compression ignition, as in diesel engines. Natural gas vehicle engines use an ignition system, including spark plugs and coils, to ignite their fuel/oxygen mixture.

Obviously, the ignition system is a critical part of engine operation. Even small ignition problems can significantly impact engine performance. To avoid these problems, and unnecessary maintenance costs, technicians should always make sure all parts of the ignition system are properly installed, calibrated, and maintained. This article will discuss the factors affecting different parts of the ignition system such as plugs and coils, additional maintenance concerns and specific strategies to maximize component life.

Why Ignition Systems in Natural Gas Engines Can Be “Tricky”

There are two major engineering obstacles associated with spark plugs used in natural gas powered engines. First, natural gas has a much higher ignition temperature compared to traditional liquid fuels. Second, the spark plugs are under much higher cylinder pressures due to the related higher compression ratios. Both situations put a lot of stress on secondary ignition components.

Because natural gas is harder to ignite, the gap between the electrodes of the spark plug must be properly sized in order to help the spark jump the gap. If the gap is incorrect it will affect the energy needed to spark, which can cause misfires or poor ignition of the fuel mixture. For instance, if the gap is too wide, the spark is going to take the path of least resistance and go to the ground through another source, causing misfire. This could potentially damage coils and coil extensions.

Proper Gap is Important

To avoid this situation, engine and ignition manufacturers publish recommended gap measurements for the spark plugs. The recommendation for the gap in the spark plugs used in Cummins engines is .013 to .015 in. The recommended gap varies widely for light-duty engines due to variables, although  most gasoline engines start out with the plugs set anywhere from .035 to.045 in. or higher. For light-duty natural gas vehicles, technicians should check with the fuel system manufacturer or  upfitter for the recommended spark plug gap.

It is important to always check the gaps of the plugs when installing new ones. If the gaps are not all the same, combustion in the cylinders will be uneven or incomplete, which over time will cause undue wear on various components. Cummins recommends to visually inspect plugs to ensure that the ground electrode is parallel to the center electrode and aligned along the center line of the plug.

Because of the thin, protective coating (typically made of iridium) on the spark plugs, it is not recommended that the gap of the plug be changed. Cummins recommends to only use a wire spark plug gap tool to check the specifications. Careful visual inspection should be made as to whether the right spark plugs are used and that they have no signs of: flash-over onto the porcelain, electrode deposits, worn or missing electrodes, excessive or insufficient gap, or any signs of aluminum or other contamination.

Coils Can Be Tricky Too

In addition to the size of the gap, the voltage required to bridge it is affected by the cylinder pressure (the spark needs a lot of force to jump the gap) and the air/fuel mixture, which is not very conductive so there is a lot of resistance. This voltage is produced by the ignition coil, which produces the spark needed to create a sufficiently high temperature so that the fuel surrounding the spark will actually ignite. The coils on a Cummins engine will create a spark that is approximately 30,000°F.

Due to the high temperaturesthat can be present in the spark plug well, strain can be placed on the materials that help insulate the electromagnetic field at the coil. The potential for cracking becomes greater under conditions in which the temperature fluctuates dramatically from cold to hot. If a crack does exist in the insulating materials, the electric current will take the path of least resistance through the block, thus bypassing the spark plug completely.

Inspecting the Ignition System Helps Prevent Performance Problems

To avoid major problems with the ignition system components, it is critical, at minimum, to perform the inspection, maintenance and replacement of plugs and coils at recommended manufacturer intervals. If the engine is converted and the plug gaps aren’t changed, there should be recommendations from the vehicle upfitter regarding their correct replacement schedule. The upfitter should also provide documentation specifying the plug gaps for when plugs are replaced.

Early Replacement of Coils and Plugs

Many companies have learned that replacing spark plugs before they can fail can substantially save on the cost of replacing the coils, extensions and/or wires as well. So, in addition to the recommended intervals, preventative diagnostics and maintenance is highly advised. For instance, if the coils and plugs start to fail at 40,000 miles, it can be beneficial to change the plugs at 35,000 to 37,000 miles.

Once Again, Training is the Key

Here is the bottom line: with any new technology, sufficient technician training is a must. Traditional diesel technicians are not commonly familiar with NGV ignition systems, and even seasoned technicians should not be expected to possess the specific knowledge necessary to avoid serious economic impacts from poorly adjusted, poorly calibrated, or damaged components. By better understanding ignition system fundamentals, being able to determine the root cause of ignition problems, and being familiar with the best practices for ignition operation and maintenance, technicians can not only increase the service life of NGV engines, but also save their company thousands of dollars by reducing maintenance costs and vehicle downtime.

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