here's a short summary of my jumbled thoughts:
Originally Posted by dirtbike.off-road.com
HOT PLUGS VS COLD PLUGS?
Just about everyone has heard that plugs have heat ratings, and are called ?hot" or ?cold" types. Some people think that means the plug either makes the engine run hot or cold, which is completely wrong.
The amount of heat a plug brings into an engine due to the electrical sparks is so small it's insignificant. The amount of heat a plug removes from an engine is also small.
Those ratings - "hot" and "cold" - mean simply this: a plug becomes very hot in an engine, so we say it's a hot-running plug, or just hot. Another plug doesn't reach such a high temperature, so we say it's a cold-running plug, or just cold.
The firing end of a hot plug reaches a higher temperature than a cold plug, in the same engine, under identical operating conditions. The metal shell of each plug will operate at very nearly the same temperature as the metal of the head itself, because the plugs are screwed into the head and there is a good path for heat flow between shell and head.
Heat gathered by the insulator nose tends to accumulate there, because the insulator material is not a very good conductor of heat. The path for heat flow is away from the insulator nose. Heat has to flow upwards along the nose until it reaches the place where the insulator is in mechanical contact with the shell.
From there, heat can flow into the shell, then to the metal of the engine.
If the path for heat flow is short and relatively easy, more heat will escape and the nose of the plug will tend to run cooler.
If the path for the heat flow is long and difficult, more heat will remain at the tip of the insulator and the plug will run hotter. The main difference between hot-running and cold-running plugs is the length of the insulator nose.
Those two electrodes also get hot. The center electrode gets hotter than the side electrode, because the center one loses heat to an insulator, whereas the side electrode loses heat directly to the metal shell of the plug.
Spark plugs are manufactured with different heat ratings, from very cold to very hot, so a suitable plug can be found for your engine, depending on what you need, based on your riding or driving conditions. Plugs with the same diameter and reach will have different lengths of the insulator nose section and different type numbers to indicate which runs hot and which runs colder.
These plugs are mechanically interchangeable, but will run at different operating temperatures in the same engine. Part of the tuning problem is to find a plug that survives in an engine.
WHAT HAPPENS WHEN THE PLUG IS TOO HOT?
It's easy to overheat a plug in an engine. A mixture that's too lean will do it, because the gasoline drawn into the firing chamber has a cooling effect. If there is not enough gasoline, there is not enough cooling.
Working the engine hard at slow speeds (such as running uphill in a deep sandwash) will do it, because the engine is generating a lot of heat but airflow for cooling is reduced by the slow speed of travel.
Using the wrong spark plug will also do it also. If you are not using the plug specified in your owner's - manual, you might have problems.
When a spark plug gets too hot, the insulator may boil and bubble On examination, it will be plain that it has been too hot. Also, the metal electrodes may melt away and disappear. If any of these bad things happen, that's good.
The worst result of a too-hot plug that fails-to destroy itself, is when it destroys the engine instead; this is called pre-ignition. If the tip of the plug becomes hot enough to ignite the fresh mixture being drawn into the cylinder, then the incoming mixture will start to burn without waiting for the spark to happen.
Ignition due to any hot spot in the cylinder begins before the proper time for ignition, so it is called pre-ignition. Anything in the combustion chamber which gets hot enough can cause pre-ignition, but typically the end of the spark plug is the cause. When the mixture is firing sooner than it should, that's like advancing the spark too much, and no matter what causes it, early ignition makes engines heat up, causing pre-ignition. Eventually, something melts, which comes under the heading of a bad thing.
WHAT HAPPENS WHEN THE PLUG IS TOO COLD?
You can install a plug which is too cold-running for the engine. Or you can have an engine that's running way too cool, such as riding ten miles downhill on a frosty morning in the mountains.
Whatever the cause, if the nose of the plug is not hot enough, it will gradually accumulate deposits, known as fouling. During normal engine operation, residue from the combustion process hits the insulator nose. This may include carbon, unburned fuel and oil, and chemical additives present in both fuel and oil.
If the insulator nose and electrodes are hot enough, the combustion deposits will be continuously burned off by the heat of the plug. The ideal situation is to have the deposits burned off as fast as they accumulate, so the insulator nose stays fairly clean and free of deposits.
If the deposits accumulate on the plug because it is not reaching a high enough temperature to burn them off, the gradual accumulation will eventually short out, or foul, the plug. The fouling is electrically conductive and forms a path along the insulator, which connects the center electrode to the metal shell of the plug.
The spark current will flow along the fouling path and will not jump across the spark gap . The engine stops running, but all you have lost is a spark plug. Fouling due to a too-cold plug is much better than pre-ignition due to a too-hot plug.
Originally Posted by dirtbike.off-road.comHOT PLUGS VS COLD PLUGS?
Just about everyone has heard that plugs have heat ratings, and are called ?hot" or ?cold" types. Some people think that means the plug either makes the engine run hot or cold, which is completely wrong.
The amount of heat a plug brings into an engine due to the electrical sparks is so small it's insignificant. The amount of heat a plug removes from an engine is also small.
Those ratings - "hot" and "cold" - mean simply this: a plug becomes very hot in an engine, so we say it's a hot-running plug, or just hot. Another plug doesn't reach such a high temperature, so we say it's a cold-running plug, or just cold.
The firing end of a hot plug reaches a higher temperature than a cold plug, in the same engine, under identical operating conditions. The metal shell of each plug will operate at very nearly the same temperature as the metal of the head itself, because the plugs are screwed into the head and there is a good path for heat flow between shell and head.
Heat gathered by the insulator nose tends to accumulate there, because the insulator material is not a very good conductor of heat. The path for heat flow is away from the insulator nose. Heat has to flow upwards along the nose until it reaches the place where the insulator is in mechanical contact with the shell.
From there, heat can flow into the shell, then to the metal of the engine.
If the path for heat flow is short and relatively easy, more heat will escape and the nose of the plug will tend to run cooler.
If the path for the heat flow is long and difficult, more heat will remain at the tip of the insulator and the plug will run hotter. The main difference between hot-running and cold-running plugs is the length of the insulator nose.
Those two electrodes also get hot. The center electrode gets hotter than the side electrode, because the center one loses heat to an insulator, whereas the side electrode loses heat directly to the metal shell of the plug.
Spark plugs are manufactured with different heat ratings, from very cold to very hot, so a suitable plug can be found for your engine, depending on what you need, based on your riding or driving conditions. Plugs with the same diameter and reach will have different lengths of the insulator nose section and different type numbers to indicate which runs hot and which runs colder.
These plugs are mechanically interchangeable, but will run at different operating temperatures in the same engine. Part of the tuning problem is to find a plug that survives in an engine.
WHAT HAPPENS WHEN THE PLUG IS TOO HOT?
It's easy to overheat a plug in an engine. A mixture that's too lean will do it, because the gasoline drawn into the firing chamber has a cooling effect. If there is not enough gasoline, there is not enough cooling.
Working the engine hard at slow speeds (such as running uphill in a deep sandwash) will do it, because the engine is generating a lot of heat but airflow for cooling is reduced by the slow speed of travel.
Using the wrong spark plug will also do it also. If you are not using the plug specified in your owner's - manual, you might have problems.
When a spark plug gets too hot, the insulator may boil and bubble On examination, it will be plain that it has been too hot. Also, the metal electrodes may melt away and disappear. If any of these bad things happen, that's good.
The worst result of a too-hot plug that fails-to destroy itself, is when it destroys the engine instead; this is called pre-ignition. If the tip of the plug becomes hot enough to ignite the fresh mixture being drawn into the cylinder, then the incoming mixture will start to burn without waiting for the spark to happen.
Ignition due to any hot spot in the cylinder begins before the proper time for ignition, so it is called pre-ignition. Anything in the combustion chamber which gets hot enough can cause pre-ignition, but typically the end of the spark plug is the cause. When the mixture is firing sooner than it should, that's like advancing the spark too much, and no matter what causes it, early ignition makes engines heat up, causing pre-ignition. Eventually, something melts, which comes under the heading of a bad thing.
WHAT HAPPENS WHEN THE PLUG IS TOO COLD?
You can install a plug which is too cold-running for the engine. Or you can have an engine that's running way too cool, such as riding ten miles downhill on a frosty morning in the mountains.
Whatever the cause, if the nose of the plug is not hot enough, it will gradually accumulate deposits, known as fouling. During normal engine operation, residue from the combustion process hits the insulator nose. This may include carbon, unburned fuel and oil, and chemical additives present in both fuel and oil.
If the insulator nose and electrodes are hot enough, the combustion deposits will be continuously burned off by the heat of the plug. The ideal situation is to have the deposits burned off as fast as they accumulate, so the insulator nose stays fairly clean and free of deposits.
If the deposits accumulate on the plug because it is not reaching a high enough temperature to burn them off, the gradual accumulation will eventually short out, or foul, the plug. The fouling is electrically conductive and forms a path along the insulator, which connects the center electrode to the metal shell of the plug.
The spark current will flow along the fouling path and will not jump across the spark gap . The engine stops running, but all you have lost is a spark plug. Fouling due to a too-cold plug is much better than pre-ignition due to a too-hot plug.
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