Spark Plugs (1): The Basic ~ Cars and Bikes
Cars and Bikes

Wednesday, 2 April 2008

Spark Plugs (1): The Basic

“Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many people confused.”

I quote the above sentence from the famous spark plugs manufacture NGK site.

The spark plug has three primary functions:

  • Ignite air/fuel mixture
  • Transfer heat from the combustion chamber
  • And transmit the electrical energy that converts fuel into working energy.

It is important to note that spark plugs never create heat, only remove it. The temperature at the end of the armature has to be kept low enough to prevent pre-ignition, but at the same time high enough to keep from fouling. The plug also works as a heat exchanger by transporting thermal energy from the combustion chamber to the cooling system. The term heat range is the plug’s ability to dissipate heat from the tip, but has nothing to do with the actual voltage transfer.

The plug’s efficiency is also determined by variety of factors such as insulator tip length, the absorption and transfer of combustion heat, the gas volume surrounding the insulator tip, and the construction/materials used in the center electrode and insulator. Regardless of spark plug preference, the difference in the heat range is the measured ability to remove 70 to 100 degrees from the combustion chamber. The longer the spark plug tip, the longer the travel of heat before it is absorbed by the cylinder head, retaining more heat equaling a hotter plug. Conversely, a shorter tip equals a colder one.

Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. Conversely, if the tip temperature exceeds 850 degrees, the ceramic material around its tip will blister, causing the electrode to melt. The temperature range between 450 and 850 degrees is loosely defined as the spark plug cleaning region. At these temperatures, the accumulated deposits burn away from the center isolator. In addition, this region has no bearing on spark quality or its intensity.

Spark Plugs Selection and Installation

  • Select a plug heat range that operates near the center of the normal operation range.
  • Check with the owner’s manual for recommended gap settings.
  • Always use a wire-type feeler gauge to check the gap
  • If adjustment is necessary, use a gapping tool to carefully bend the armature. Never hit, pry or use pliers against the center insulator or armature to widen the gap.
  • When installing spark plugs, clean both sets of threads, and always use torque wrench at the correct setting.

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