The higher the number that determines the thermal value of the candle, the higher its resistance to glow ignition and the less their resistance to pollution.
The lower the number that determines the thermal value of the candle, the lower their resistance to glow ignition and the higher their resistance to pollution. At a high thermal value, a candle conducts heat better, and at a low one, it is much worse. When used in the same engine and under the same engine load conditions, spark plugs with:
A) very high thermal value are the cause of the appearance of sediment on them. This is the result of the spark plugs not reaching their self-cleaning temperature as so much heat passes through them to the walls of the head, i.e. spark plug is very cold.
b) very low gross values are the cause of spontaneous ignition. This is the result of the plug reaching a temperature well above 850°C. since very little heat passes through it to the walls of the head, i.e. the candle is very hot.
To give an idea of what temperatures we are talking about, below are the temperatures surrounding the candles during engine operation.
- End of intake stroke 60°C
- Ignition 350°С
- Maximum values 3000°С
- End of stroke 1100°C
During idling or at partial engine loads (for example, stopping at an intersection or driving in a traffic jam). The temperature of the candle drops even below 350°C. Therefore, after working in such conditions, the candle is covered with a black precipitate. It is the greater, the more oil gets under the piston.
The number that determines the thermal value is contained in the spark plug code.
Bosch type spark plug
Example: WR 7 D C R (1) (2) (3) (4) (5) (6)
(1)
- W = M 14x1.25 thread, flat fit, wrench 21
- F = M 14x1.25 thread, flat fit, wrench 16
- M = thread M 18x1.5, flat fit, wrench 25
- I = thread M 14x1.25, cone fit, wrench 21
(2)
- R = with anti-interference resistor.
(3)
A number that defines the thermal quantity. The scale of thermal quantities is from 06 ("cold") up to 13 ("warm"). The number 7 corresponds to the old thermal value 175 (old designation), a: 6-20 5-225 etc.
(4)
- A and B = thread length 12.7 mm;
- C, D, DT and L = thread length 19 mm.
(5)
Stop center electrode material Cp-Ni
- C = Ni-Cu combined center electrode;
- S = Center electrode silver;
- P = Central electrode platinum;
- O = Standard spark plug with reinforced center electrode.
(6)
- R = 1 ohm fouling resistor, central copper or silver with increased thermal conductivity and therefore thermal load.
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