For a given spark plug installed in a rating engine, by gradually increasing the amount of supercharging and adjusting the fuel mixture strength to give optimum temperature at each setting, the plug experiences higher and higher temperatures until it begins to run into pre-ignition (indicated by a rapid rise in the measured plug temperature). As pre-ignition occurs, the fuel supply instantly is cut off, preventing uncontrolled tem- perature rise and possible damage to the engine. When stable operation is obtained (34 mb of supercharge boost below the pre-ignition point for three minutes), the torque is measured, allowing an IMEP value to be calculated according to Eq. 6.4. At any fixed set of engine conditions, there is a definite boost-IMEP relationship, which is a straight-line function (Figure 6.3).
Spark Plugs 105
Figure 6.3 Engine boost against output for opti-
40 60 80 100 120 mum plug temperature at
~ o o o t ~ o c h e s ~ a c u r y ~ 30" spark advance.
Various IMEP values are required because of the different demands on engine perfor- mance. For example, a racing car must run at high temperatures for maximum efficiency and power output over a relatively long period. The best spark plug for this environment would be one that could dissipate heat rapidly to the engine mass. For a hot engine, a plug is needed that remains as cold as is necessary to prevent pre-ignition. Therefore, a cold plug is required (Figure 6.2).
The IMEP value of a cold plug would be relatively high. However, if the same plug were used on a family sedan that is used on short urban journeys and thus never fully warms up, combustion deposits soon would build up and lead to misfiring. In this situ- ation, a plug with a relatively low IMEP value is required that does not dissipate the heat as readily and that has an operating temperature that will be sufficiently high to burn off the combustion deposits. Thus, a cold engine calls for a hot plug (Figure 6.4).
Other terms that are used are "hard," which is the equivalent of cold, and "soft," which is the equivalent of hot.
Figure 6.4 Hot plug heat fro w.
106 An Introduction t o Engine Testing and Development
However, the much shorter heat path of the cold plug can cause problems in that there is much less area of the ceramic insulator exposed to the cylinder. Under certain engine conditions, such as cold start with full enrichment, carbon combustion deposits build up on the nose of the spark plug, offering a leakage path to earth for the current. This leads rapidly to a situation wherein the spark plug ceases to function, known as cold fouling.
Substitution by a normal spark plug of lower IMEP rating, thus providing a longer nose to overcome this problem, may be unsatisfactory due to the reduction in maximum safe operating temperatures. However, the spark plug designer and the engine development engineer have one or two options available to satisfy the provision of a greater surface area of the insulator nose while maintaining the IMEP rating.
Recent years have seen the adoption of center electrodes containing a core of copper.
These electrodes are more thermally conductive than the typical nickel-alloy types and enable a longer electrode and ceramic nose to be employed. This solution elegantly achieves the aims discussed previously, that is, larger surface area for the same heat rating. An alternative means of increasing the heat removal rate is to eliminate the air gap between the ceramic nose and the center electrode by filling the space with a refrac- tory cement material (Figure 6.5). This normally is achieved by the application of a vacuum to the tip of the spark plug, thus allowing the re-establishment of atmospheric pressure to force the cement into the evacuated space. Air can be an effective thermal insulator, and its replacement by the solid cement enables the heat to transfer from the ceramic insulator much more efficiently by conduction into the center electrode along its entire length and hence be more readily dissipated.
Figure 6.5 Improvement in heat transfer rates by vacuum cementing.
Because most of the heat conduction takes place by way of the electrodes, the use of materials that are more efficient in this respect obviously must be considered. How- ever, the employment of, say, nickel in place of nickel alloy increases heat dissipation, and hence IMEP, but at the expense of electrode durability. This is due to more rapid chemical, electrical, and mechanical erosion of the softer material, despite its higher temperature tolerance.
One problem of which the designer must be aware in producing a very cold plug is a possible change in the site from which pre-ignition may occur. Normally, initiation of pre-ignition will take place from the overheated ceramic nose, which is unable to dissipate
Spark Plugs 107 the heat rapidly enough, provided there are no incandescent sharp burrs or a glowing
protrusion of combustion deposits present to pre-empt such occurrence. As the IMEP value is increased, the removal of heat from the nose becomes more efficient, lessens the chance for overheating of the ceramic body to occur, and shifts the emphasis to the side electrode, which then becomes the prime site for pre-ignition. The plug then is said to rate off the side wire instead of the nose. Attempts to control this phenomenon center on improving the heat removal rate of the side electrode either by a change of material, as already discussed, or by cutting back the side wire to provide a shorter path.
The material changes within the ceramic insulator itself can have an effect. The almost universal use of aluminum in this application is the result of historical changes that are admirably chronicled. (See J.V.B. Robinson, History of the Sparking Plug. Motor Management, 197 1 , Vols. 6 and 7, and J.S. Owens et al., Development of the Ceramic Insulator,for Sparking Plugs, American Ceramic Society, 1977, Vol. 56.) The various manufacturers employ compositions ranging from approximately 88 to 95% alumi- num. Changes in aluminum content toward the lower end of this range are attractive to manufacturers for several reasons. The major benefit is the large savings in energy usage arising from the lower sintering temperature, which is brought about by the replacement of aluminum with less refractory additives. Additional savings come from increased kiln and kiln furniture life at the lower firing temperatures, as well as reduced material costs. However, the design engineer then must accommodate altered material properties, the most important of which, from his point of view, is the reduced thermal conductivity-because the alumina content decreases in the annular cross-sectional area of the ceramic nose to retain the heat transfer ability. Such thickening of the nose has the effect of aiding anti-fouling characteristics of the design by increasing the surface area. Two or more of these changes can be combined by the spark plug development engineer to achieve a degree of fine-tuning of the rating values.
Changes in spark plugs have been caused by the demands for longer life and improved ignitability in today's lean-burning modern engines. It is critical that each cylinder fires to prevent unburned fuel from reaching the catalytic converter and causing dam- age. Variations in burn rates on a cycle-to-cycle basis are critical when new emission regulations are reviewed; a few parts in one million of trace emission gases can be the difference between a regulatory pass or fail.
With engine manufacturers striving toward lean-burn engines to improve fuel con- sumption, the possibility of misfire becomes a problem. In addition, manufacturers are striving to produce an engine in which the spark plugs will never need to be changed.
A high voltage with large gaps appears to be the answer.
It has been proven that a thin electrode will improve ignitability; however, a thin electrode made from nickel would erode quickly and fail. Therefore, spark plug manufacturers have necessarily turned to precious metals to withstand the harsh environment.
The introduction of precious metals such as platinum (Figure 6.6) and iridium give spark plugs the added benefit of long life. Note that spark plugs historically have been regarded as consumable, with a limited service life, and as such required to be changed at regular intervals. Some NGK spark plugs were designed with a patented V-groove for improving ignitability in nickel alloy electrodes. This style of spark plug incorporates a 90" V-groove in the center electrode and ensures that sparking occurs at the periphery of the electrode, thus enhancing ignitability. The introduction of 42-volt ignition systems means that even higher spark plug voltages can be applied, leading to larger plug gaps (up to 3 mm) that have extended life.
108 An Introduction t o Enaine Testina and Develo~ment
Figure 6.6 Platinum- tipped electrodes.
As stated, the use of precious metals in spark plugs has increased their service life. The use of iridium spark plugs is only starting with Japanese car manufacturers, who are finding them ideal for very low emission engines.