We continue to see problems surrounding the use of spark plugs in Rotax engines. Many of the rules which we have used in the past for typical aviation type spark plugs, used on air cooled engines, no longer apply to the automotive type spark plugs used in a Rotax engine. As with most technical subjects, an underlying understanding of the theory and physics involved is essential to our ability to make good judgments about the use and operation of spark plugs. So let’s start with the basics.
The spark plugs used in the Rotax engines are specific to each type of engine. Figure: 1. The most prolific of the Rotax engines is the 912S 100 hp and it uses the DCPR8E. Figure: 2. Using this plug let’s look at the part number designation and what each one of the numbers and letters indicate for the design of the spark plug.
Spark Plug Size: The (DC) in the part number is the thread diameter and pitch. Looking at the NGK part numbering chart, it indicates that this is a 12 mm diameter spark plug with a 1.25 mm pitch on the threads and uses a 16mm wrench on the hex portion of the spark plug.
Reach: The Last Letter in the part number (E) indicates that this plug has a 19 mm thread reach. This is measured from the base of the plug, above the gasket, to the last thread.
Shape: The (P) in the part number indicates this plug has a projected center electrode insulator. The projected center electrode insulator is what you would normally recognize as a typical spark plug and is of course the most common type.
Construction: And the (R) in the part number indicates this is a resistor type spark plug. When a spark jumps the gap on a spark plug, it creates a high frequency burst of energy. This creates radio frequency interference or (RFI) which can generate significant interference with your radios and other electronic equipment. Placing a resistor within the spark plug significantly reduces this RFI. Figure: 3.
Heat Range: The (8) in the part number is an indicator of the heat range. The heat range of the spark plug is designated by the ability of the spark plug to dissipate heat that is absorbed from the combustion chamber. The heat within the insulator nose is transferred into the body of the spark plug and out into the cylinder head which is cooled by air or by water/antifreeze.
A hot spark plug is just as it sounds. It is designed to hold more heat at the insulators nose in order to burn off oil or carbon deposits. A hot spark plug is designed with a longer ceramic nose and a much smaller area of the ceramic insulator in contact with the spark plug body. This slows down the transfer of heat from the ceramic insulator into the cylinder head. This spark plug design is useful on engines that are designed with low compression ratios or engines that typically operate at a lower manifold pressure continuously. Insulator temperatures less than 450°C / 842°F will not burn off carbon deposits. Because carbon is a good conductor of electricity, this accumulation of carbon deposits on the insulator nose will form an electrical path to ground which can cause the spark plug to misfire. This same condition can occur on or aircraft that spends a considerable amount of time at idle or taxiing. One of the purposes for an engine run-up, just before takeoff, is to ensure that we have not fouled a spark plug. We also have the added benefit of burning off any carbon accumulations on the spark plug insulator during the run-up procedure. Figure: 4
A cold spark plug on the other hand is designed to transfer heat more efficiently from the center electrode and ceramic core into the cylinder head. The ceramic insulator on a cold plug has a larger surface area in contact with the spark plug body when compared to a hot plug. On the high-performance engines and engines with higher compression ratios the temperatures and pressures within the combustion chamber are more significant. Looking at the spark plug application chart Figure: 1, you can see that the 80 hp Rotax 912 uses a spark plug with a heat range (7), while the 100 hp Rotax 912S and IS use a spark plug with a heat range (8), and the turbocharged 914 uses a heat range (9). The engine manufacturer typically selects a plug with a heat range corresponding to the design of the engine. The more aggressive the performance, the cooler the spark plug requirement in order to keep the insulator and center electrode from becoming too hot. A spark plug that runs too hot may ignite the fuel on the next compression stroke before the ignition spark. This is known as pre-ignition. Pre-ignition can in turn lead to higher cylinder pressures and temperatures commonly resulting in detonation, a loss of power, and serious damage to the engine. Temperatures in excess of 870°C / 1598°F can lead to pre-ignition. Insulators approaching these temperatures and can be identified by the insulator being blistered or white in color. The NGK spark plug provides a design that is less susceptible to fouling by incorporating a longer ceramic nose which allows the ceramic to maintain a high enough temperature at low power settings while simultaneously using a copper core in the center electrode of the spark plug which transfers heat very efficiently out of the spark plug nose raising the pre-ignition rating. This provides for a very broad thermal range particularly suited for Rotax powered aircraft applications.
Rotax is gone one step further, and they have done so for a very good reason. Unlike a passenger car, an aircraft engine operates at very high power settings continuously. It becomes very important to be able to transfer the heat out of the spark plug and into the cylinder head. In order to significantly improve this heat transfer rate, Rotax adopts the use of a heat transfer paste on the spark plug threads of the 9 series engines. If we look at a cross-section view of the spark plug threads installed into the cylinder head and zoom in on a microscopic level, we can start to see how we could significantly improve the heat transfer rate. Figure: 6.
When we begin to torque the spark plug into the cylinder head, the top of the spark plug threads make contact with the bottom of the threads on the cylinder head. This pressure on one surface of the spark plug thread naturally pulls the other side of the spark plug thread away from the opposing surface of the spark plug thread in the cylinder head leaving a small gap on the opposite side of the thread. The air gap between these two contact faces acts as a very efficient insulator and restricting the heat transfer across these two surfaces. By applying a small amount of heat transfer paste to the threads of the spark plug, we can bridge this gap and significantly improve the heat transfer from the spark plug to the cylinder head. The heat transfer paste that is used is very similar to that used in the computer industry for transferring heat from a computer chip into a heat sink. The Rotax part number for this heat transfer paste is 897-186 and it is shown in the parts catalog as Wacker P-12. Figure: 5.
There are also several after-market brands of heat transfer paste sold by California Power Systems, Lockwood Aviation, and Leading Edge Airfoils. These are the three primary companies offering Rotax parts and supplies for sale in the United States. These companies generally do a good job at setting the viability of after-market products used on the Rotax engine. Be cautioned, however, since there are many other heat transfer paste products that are not equivalent to the P-12 and won’t provide the same kind of heat transfer performance required by the Rotax engine. One of the most common indicators that the heat transfer paste is inadequate is that during the removal of the spark plug the heat transfer paste appears dry and powdery on the spark plug threads. A high quality heat transfer paste will remain sticky and gooey throughout its service life.
This efficient transfer of heat from the spark plug insulator into the cylinder head is precisely the reason that spark plug torque is so very important. A spark plug that is under torqued does not make good contact with the cylinder head. This allows heat to build up within the center electrode and ceramic core creating the dilemma of pre-ignition just as if we had a spark plug with improper heat range rating installed pleading once again to pre-ignition/detonation and subsequent failure of the engine.
Over torque of the spark plug can damage the threads in the cylinder head. This can in turn reduce the ability of the spark plug to transfer heat into the cylinder head. Both Rotax and NGK recommend against the use of an anti-seize compound on the spark plug threads. There are several reasons for this. First, the anti-seize compound significantly reduces the friction and over torquing the spark plug becomes very likely. Second, the anti-seize compound is typically an electrical conductor, which, if it propagates to the electrodes, can short out the spark plug causing ignition failure. NGK manufacturers its spark plugs with a corrosion resistant coating that is designed to be installed into the cylinder head without any thread lubrication. This is the recommended procedure for the Rotax 2 stroke engines.
In part one of this article, we’ve talked primarily about the theoretical aspects of the spark plug in the Rotax engine. As you can see, Rotax is put a lot of thought into the selection of the spark plugs which they use in each of their particular engines. Keep in mind that these spark plugs are selected for a multitude of reasons, and they have an extremely good performance and safety record in the Rotax engine. If you find yourself thinking about using a different spark plug, other than that which is recommended by Rotax, you might want to rethink your thought process. If you’re using the spark plugs recommended by Rotax and your engine is still not running correctly. It’s probably not the spark plugs that are the cause. In part two of this article we will take a look at the more practical aspects of inspection, installation, and maintenance of the spark plugs installed in the Rotax engine.