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Cold spark plugs versus hot spark plugs
2022-01-14
Cold spark plugs versus hot spark plugs
“Cold” spark plugs normally have a short heat flow path. This results in a very quick rate of heat transfer. Additionally, the short insulator nose found on cold spark plugs has a small surface area, which does not allow for a massive amount of heat absorption.
On the other hand, “hot” spark plugs feature a longer insulator nose as well as a longer heat transfer path. This results in a much slower rate of heat transfer to the surrounding cylinder head (and consequently, the water jacket).
The heat range of the spark plug must be carefully selected in order to create optimal thermal performance. If the heat range is not correct, you can expect serious trouble. Typically, the appropriate firing end temperature is (approximately) 900-1,450 degrees. Below 900 degrees, carbon fouling is possible. Above it, overheating becomes an issue.
Spark plug voltage rise
In terms of operation, the spark plug is connected to the high voltage generated by an ignition coil (by way of a conventional distributor or by way of an electronic means). As electricity flows from the coil, a voltage difference develops between the center electrode and ground electrode on the spark plug.
Because of the spark plug “gap,” coupled with the air/fuel mixture (which acts as an insulator) within the gap, the spark plug cannot immediately fire.
As the voltage rise increases to approximately 20,000 volts, the gap within the spark plug can be “breached” and it fires. With a spark plug removed from the cylinder head and properly grounded to fire, you can hear a definitive click. If conditions are dark enough, you can see the spark.
The click you hear is essentially a miniature clap of thunder, and the spark you observe is similar to a miniature form of lightning.
Within the combustion chamber, the intense heat created by the spark plug creates a small fireball within the gap. The fireball or combustion “kernel” expands and the cylinder (at least in theory) experiences complete combustion.
