The demands on the performance of spark plugs are increasing because of developments in lean-burn engines leading to adverse conditions for ignition and because of the desire for a longer life of the plugs. For improving spark plugs and ignition systems, more insight into the relationship between gap wear and spark-plug design, ignition-system characteristics and in-cylinder conditions is required. For that purpose, an instrument has been developed to measure on-line the gap size of spark plugs. This monitor can also be used for on-condition maintenance indicating that a necessary replacement of the spark plugs will be near. The monitor, called SPECTOR, receives a signal from an inductive coil around the high-voltage cable which, under given conditions, is representative for the gap size.SPARK PLUGS are commonly used to initiate the combustion process in natural-gas-fuelled engines. Natural gas is a suitable fuel for spark- ignition engines because of its high knock resistance and wide ignition range. Its low cetane number and low density hamper its use as a single diesel fuel, although dual-fuel and pilot-injection engines running on natural gas, where injected diesel oil is the ignition source, are common practice. The natural-gas-fuelled engines in the expanding market of combined-heat-and-power units are of the spark-ignition type, for emission reasons.Units for the combined generation of electric power and heat are attractive because of their total energy efficiency of close to 90%. However, the units driven by reciprocating engines have to compete economically and ecologically with conventional power plants and with new developments such as fuel cells. The desire for higher shaft efficiencies and higher power densities shift developments towards a higher cylinder load (bmep) by turbocharging and towards a higher compression ratio. These developments are necessarily accompanied by further leaning of the fuel-air mixture, because otherwise the peak temperatures in the cylinder process would increase and result in higher NOx emissions. The current generation of open- chamber lean-burn gas engines operates on an air-to-fuel ratio, lambda, between 1.55 and 1.65 (1)*. Stoichiometrically operating engines of industrial quality cannot attain the current performance of lean-burn engines, as yet.A leaner mixture requires more energy for ignition than a mixture closer to stoichiometric. Moreover, it is more difficult to establish an electric current in the gap of a spark plug if the density of the medium between the electrodes is higher. Thus, all new developments in engine performance are putting an increased demand on the performance of ignition systems. This tends to shorten the life of spark plugs. However, the necessities of low maintenance costs and a high reliability require the life of spark plugs to be extended to beyond a few thousand operating hours at an engine speed between 1000/min and 1800/min. Currently, the spark plugs determine the maintenance interval.To be able to find ways to improve spark-plug life, it is necessary to establish a relationship between the lifetime, the design of the plug, the operating conditions in the engine and the properties of the ignition system. This is a difficult task, considering the many different engines, operating conditions and spark-plug types. The literature and our practical experience thus far give insufficient answers to these problems. This paper first gives an introduction into the nature of spark-plug wear, followed by a discussion of a literature study on the ignition process. The major part of this paper is used to describe a special on-line monitor developed to measure indirectly the gap width of a spark plug. This work was part of a program of Gasunie Research to improve engines fuelled by natural gas. The monitor can be used as a tool for on-condition maintenance, indicating spark-plug wear. Next to that, it can be used to study the wear rate of spark plugs in relationship with operating conditions and engine design.