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Investigations on the NOx reduction for natural-gas-fueled spark-ignition engines with lean-burn techniques revealed that NO2 is less sensitive to reduction measures than NO. For NO concentrations over 1000 ppm, the NO2/NO ratio is generally less than 0.1. For NO concentrations below 50 ppm, the NO2/NO ratio begins to exceed 1.0. The literature on burner and flow-reactor tests explains this phenomenon via the oxidation of NO with hydroperoxyl radicals through reactions very sensitive to component concentrations. Since NO is the precursor of NO2, measures to further lower the NO production will ultimately result in a lower NO2 emission. This has been demonstrated experimentally with an engine.

A special combustion chamber is described which burns a small, representative, fraction of the intake mixture of natural-gas-fuelled spark-ignition engines. The combustion end products are led to a lambda sensor which, consequently, will not be deteriorated by lubricating-oil additives and will not be hampered in its operation by unburnt methane. That leads to a largely extended life of the lambda sensor. The paper discusses the construction and control of the special chamber and ends with a discussion on the representativeness of the measured oxygen concentration for the air-to-fuel ratio.

For the determination of the isentropic efficiency of turbochargers, the use of the real properties of the medium gives a more accurate efficiency than the use of the idealized, constant, properties. A serious source of errors in turbine-efficiency determination can be the heat loss to the surroundings. A method is described to overcome this problem. The effectiveness of a turbocharger can be easily expressed in the ratio of the boost pressure and the pressure in the exhaust manifold.

The possiblity of using an elevated intake-manifold temperature for lean-burn, turbocharged spark-ignition engines fueled by natural gas has been investigated. Experiments have shown that the power capacity, shaft efficiency and NOx emissions could be maintained at the rated level if an increase in intake-manifold temperature was accompanied by an increase in air-to-fuel ratio. A higher air-to-fuel ratio is allowed with hotter mixtures since both the lean misfire limit and the volumetric efficiency appear to increase with increasing mixture temperature. An intake-manifold temperature at the level of the jacket-water temperature of the engine has the advantages of an easier control of the mixture temperature, a better energy utilisation in cogeneration systems and a better part-load efficiency of the engine.

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.

The performance of lean-burn stationary engines fueled by natural gas has been investigated. Relationships have been established between design parameters and process conditions on the one hand and power capacity, specific fuel consumption and emissions on the other. The paper shows that limitations for the NOx emissions restrict the developments in cylinder load and shaft efficiency. Opportunities exist for decreasing the CO and HC emissions.

A carburettor system has been designed for lean-burn heavy-duty engines to use natural gas as a fuel. The system is applied in a program of converting existing diesel buses to spark-ignition operation on natural gas. The system provides adequate air-to-fuel-ratio control, mixture homogeneity and step response. Its simplicity and ruggedness are the keys to good reliability and repeatability.

Performance of carburettors for gaseous fuels is not always satisfactory. Major difficulties are related to air-to-fuel-ratio control and to homogeneity. In that context, four carburettors, each based on a different principle, have been investigated. A major conclusion is that an integrated system is required for proper air-to-fuel-ratio control. Additionally, for sufficient homogeneity an even initial distribution of fuel over the air stream, followed by turbulence-induced mixing in a special section, is necessary.

Three-way catalytic converters proved to have a long-term capacity to effectively reduce the concentration of unwanted compounds in the exhaust gas of engines fueled by natural gas, provided a narrow air-to-fuel-ratio window is maintained. The exhaust-gas sensors hitherto incorporated in the required feedback-control system have an undesirable transfer function and suffer from ageing. This paper therefore advocates the development of an appropriate sensor for carbon monoxide and/or hydrogen to perfect the cleaning system.

The Wobbe-index is introduced as an important gas quality criterion when interchanging gaseous fuels for engines. Changes in fuel gas composition appear not to induce noticeable changes in air to fuel ratio and combustion velocity when the Wobbe-index remains the same. This implies that no readjustment of ignition timing and air to fuel ratio settings is required, then. The volumetric energy content, the explosion limits and the knock resistance of a mixture can vary to a moderate extent when the Wobbe-index is constant and the gas composition varies.

Application of apparent heat release analysis appears to be very convenient in finding the optimum ignition timing (MBT) of reciprocating engines. The crank angle where apparently 50% of heat has been released was called the combustion phasing angle. The combustion phasing angle present at MBT ignition timing has been defined the optimum combustion phasing angle. The value of the optimum combustion phasing angle appears to be close between 7 to 8 deg after Top Dead Centre, independent of process conditions. This knowledge makes it easy to readjust ignition timing of engines when fuel quality or air to fuel ratio vary, or when design changes have been carried out.

A knock severity index has been proposed which is based on the pressure jump during knock and on the fraction of mixture charge involved. The index can be applied to compare the knock severity in different engines under various conditions without the restrictions imposed by earlier methods of classification. To determine the knock severity, the mixture properties as well as the relevant cylinder process have to be known. The cylinder process can be measured by the application of digital equipment.