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Previous work has shown that a novel partially stratified-charge approach can be effective in extending the lean limit of combustion and reducing the exhaust emissions from natural gas-fueled engines. The new technique provides a relatively rich mixture in the vicinity of the spark plug, while maintaining an ultra-lean homogeneous charge in the main chamber area. In order to provide the near-stoichiometric mixture near the spark gap, a small quantity of "pilot" fuel is injected through the spark plug just prior to ignition. It has been found that for most operating conditions the required pilot fuel quantity is less than 5% of the total fuel charge. This paper also reports the results of some recent one-dimensional computer modelling in which the partially stratified-charge technique has been investigated over a range of air-fuel ratios.

In this paper a study of the squish-generated charge motion in the combustion chamber of a natural gas engine is reported. A combination of both numerical simulations and actual engine tests was used to correlate the turbulence level at the spark plug location with performance and cylinder pressure data for three different chamber configurations. The higher-turbulence combustion chamber showed an average 1.5% reduction in brake specific fuel consumption in comparison with the lower turbulence level combustion chambers. The emission levels from the high-turbulence case were, however, generally higher than those from the lower-turbulence combustion chambers.

A combustion chamber designed to increase the burning rate of lean air-fuel mixtures is described. The chamber utilizes squish motion to generate a series of jets which significantly increase the turbulence levels in the chamber during the early phase of combustion. The fuel economy and exhaust emissions resulting from the new chamber design are compared to a conventional bowl-in- piston design over a wide range of air-fuel ratios. The new chamber design results in an increase in the combustion rate as shown by a reduction in the MBT spark advance during lean operation. The faster burning rate leads to a reduction in the brake specific fuel economy of some 5 percent compared to the conventional chamber design. Emissions measurements show an extension of the lean limit of operation and a reduction in hydrocarbon emissions during lean operation. The extended lean limit also enables reduced emissions of nitrogen oxide compounds to be achieved.

An experimental investigation of the effects of squish motion on turbulence generation, burn-rate and engine performance has been undertaken in a Ricardo Hydra single-cylinder research engine. An experimental combustion chamber design incorporating a number of squish generated jets was used to enhance the turbulent motion in the chamber during combustion. Flow-field measurements were made with a hot-wire anemometer in order to characterize the turbulence field. Measurements of brake thermal efficiency and mass burn-rate showed that enhanced turbulence generated by combustion chamber geometry is effective in improving performance under lean operating conditions. A single squish generated jet motion directed towards the spark plug resulted in the highest thermal efficiency during high speed lean operation, while burn-rate analysis indicated that squish action was most effective during the main burn period.

A new combustion chamber design is proposed in which it is possible to control the scale and intensity of turbulence generated just prior to ignition. A single cylinder engine has been fitted with the new chamber, and measurements of the turbulence field with a hot-wire anemometer are presented. The chamber design has been compared to a conventional bowl-in-piston design under both motoring and fired operation. Hotwire measurements showed an increase in turbulence intensity of 50% and a reduction in the length scale of turbulence compared to the conventional chamber. Cylinder pressure measurements indicated that the mass-burn rate is increased with the new chamber, particularly during the early stage of combustion. During operation at 1140 rpm with the new chamber, peak cylinder pressure was 4% higher and occurred 3° earlier than for the conventional chamber.

An experimental evaluation has been made of the power output, specific fuel consumption and thermal efficiency of a modern 4 cylinder spark ignition engine operating on gasoline and natural gas. Tests have been conducted at various speeds, spark advance angles and air-fuel ratios, all at wide open throttle. Performance on gasoline and natural gas has been compared and optimum spark advance determined for natural gas operation. With natural gas operation, brake power was found to decrease by between 11.3% and 16.6% compared to gasoline operation. Brake specific fuel consumption was found to decrease with natural gas operation, although this is due to the higher calorific value of natural gas. In terms of brake thermal efficiency, operation with natural gas was found to be less efficient than with gasoline. This is due to the lower flame speed of natural gas and the proportionally higher friction losses at the reduced power levels.