Spark Plugs—I

Key To Smooth Engine Operation

Hot and Cold Plugs ? ? ?

(Courtesy: Champion Spark Plug)

Today, the term "hot and cold" is commonplace in general aviation — especially when related to engine spark plugs.

With the introduction of high compression, and high horsepower engines, a need for improved spark plugs was eminent. Spark plugs used in low compression, low horsepower engines were not compatible with the new, more sophisticated powerplants. The noncompatibility factor of existing plugs with new engines resulted in development of spark plugs capable of operating efficiently at high compression ratios and high power settings.

Many aircraft operators have come in direct or indirect contact with the term "hot and cold" during the course of conversation with other pilots or mechanics. Its meaning and relationship to engine operation was sometimes rather vague. What do we mean by "hot and cold" spark plugs? What is the relationship between an engine and spark plugs? How important is it to smooth engine operation? These are but a few questions we will try to answer in this article.

Both spark plug and engine manufacturer working together determine the proper type spark plug suitable for each engine model These plugs can be either fine wire or massive electrode type. Before being released for production, each new type plug is checked in the laboratory and under actual flight conditions. They are tested through a wide range of operating conditions and at different power settings, and only after both engine and spark plug manufacturer are completely satisfied with test data are plugs released for production. To eliminate any possibility of error in spark plug selection, both manufacturers provide spark plug charts as a guide for proper plug selection. Final authority concerning proper plugs for a specific engine is the engine manufacturer. When selecting spark plugs, be sure to also consider the spark plugs’ heat range.

Operating temperature of the spark plug insulator core nose is one factor that governs formation of troublesome combustion deposits. To help overcome this problem, selection of spark plugs with the proper heat range should be made. Spark plugs are susceptible to carbon deposits when the operating temperature of the core nose insulator is at or below 800o F, but an increase of just 100o F is sufficient to eliminate formation of these deposits. Also, lead deposits form because the bromide scavenger contained in tetraethyl lead is nonactive at low temperatures. At 900o F temperature, the bromide scavenger is fully activated, disposing of lead deposits with combustion gases during exhaust cycle. In this case, an increase of just 100o F was sufficient to make the difference between a smooth and rough running engine. To eliminate or keep this problem at a minimum, avoid prolonged idling at low RPM, avoid power-off let downs, and after flooded starts run engine at medium RPM before taxiing.

Deposits formed between 1000o F and 1300o F are low in volume and electrical conductivity and are least apt to cause spark plug fouling. This is the reason for selecting a plug that will operate within the aforementioned temperature range at all power settings.

Now let’s get back to the term "hot and cold" as related to engine spark plugs. Normally, a hot plug is used in a cold engine — low horsepower, and a cold plug in a hot engine — high horsepower. In actuality, these terms refer to the plugs ability to transfer heat from its firing end to the engine cylinder head. To avoid spark plug overheating where combustion chamber or cylinder head temperatures are relatively high, a cold plug is recommended, such as in a high compression engine. A cold running plug has the ability to transfer heat more readily. A hot running plug has a much slower rate of heat transfer and is used to avoid fouling when combustion chamber and cylinder head temperatures are relatively low.

From our discussion, it is clear to see that there is more to spark plugs than just buying a set and installing them in your engine. Be sure you know what type of spark plugs to use with your engine. Also, good spark plug service and maintenance is as important as proper plug selection. Take care in selecting and maintaining your plugs, it can result in many additional hours of smooth engine operation. Additional spark plug information is always available from the engine or spark plug manufacturers and other service organizations.

A listing of approved spark plugs in Service Instruction No. 1042 is available by writing to Product Support Department, Textron Lycoming, Williamsport, PA 17701. Additional information about spark plugs and their servicing may be obtained by writing Champion Spark Plug Division, P.O. Box 686, Liberty, SC 29657.