Copyright Goodheart-Willcox Co., Inc. Chapter 10 ∣ Ignition Systems 213 Figure 10-16B shows how the heat generated in the insulator must travel up until it can escape through the seal, to the shell, and then to the head. The longer and thinner the insulator tip, the less efficiently it can trans- fer heat. As a result, it will run hotter. A plug with a short, heavy insulator (cold plug) carries heat well and will operate much cooler than a plug with a long, thin insu- lator (hot plug). All engine manufacturers specify heat ranges for the plugs to be used in their engines. Projected-Nose Spark Plugs One spark plug design extends the electrode tip and insulator farther down into the combustion chamber. A projected-nose spark plug places the insulator more directly into the path of the incoming air-fuel charge, which cools the tip. See Figure 10-17. The projected-nose plug, due to its long tip, tends to run hot at low engine speeds because the incoming air-fuel charge is moving slowly and provides little cooling effect. This prevents low-speed fouling and assists in producing a spark without excessive voltage from the coil. At high engine speeds, the insulator temperature is kept from exceeding accept- able limits by the rapid flow of the incoming air-fuel mixture. The washing effect of the escaping hot exhaust tends to keep the plug clean. The projected-nose plug will perform satisfactorily within acceptable temperature limits over a wider vehicle speed and load range. See Figure 10-18. Resistor Spark Plugs The spark at the plug electrodes is delivered in two stages. The voltage at the plug’s center electrode will rise rapidly until the voltage is sufficient to ionize the gap and cause the plug to fire. This is the first stage, usually called the capacitive stage. The second stage, called the inductive stage, is longer and follows the first. It is produced by the remaining residual voltage in the coil. The combustion process takes place during the first stage. The second stage causes electromagnetic interference and can cause the plug electrodes to wear. To shorten the inductive stage, resistor spark plugs are used in all late-model vehicles. Resistor spark plugs are designed with a resistor of around 10,000 ohms located in the terminal stud. Exposed tip Moving gases cool exposed tip Cylinder head Insulator Shell Gasket Goodheart-Willcox Publisher Figure 10-17. A projected-nose spark plug. Not all spark plugs use this type of nose plug. 10 20 30 40 50 60 70 80 90 100 110 120 Hot—Causes preignition Cold Plug will foul Vehicle Speed (mph) Temperature Range Regular plug Projected-nose plug 1500°F (816°C) 1000°F (538°C) 500°F (260°C) Goodheart-Willcox Publisher Figure 10-18. A performance graph showing the operating temperature range for standard plugs and projected-nose plugs at different vehicle speeds.