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UBC Theses and Dissertations

Performance of a partially stratified-charge natural gas engine Reynolds, Conor

Abstract

The Partially Stratified-Charge (PSC) concept is a novel way to improve the combustion characteristics of an otherwise standard spark-ignition engine, operating at a lean air-fuel ratio. PSC involves providing a relatively rich air-fuel mixture in the vicinity of the spark plug, while maintaining an ultra-lean homogeneous charge in the main combustion chamber area. This is achieved by injecting a small quantity of natural gas, comprising less than 5% of the overall fuel mass, in the region of the spark plug just prior to ignition. A stable flame kernel is formed in this region, which then rapidly propagates through the combustion chamber. PSC requires no modification to basic engine design, but only to the method of introducing fuel to the combustion chamber. Natural gas has been used, both for injection through the spark-plug and for the main homogeneous fuelling, because PSC is suited to gaseous fuelling and also because natural gas is an abundant, clean burning and relatively low cost fuel. Tests have been performed using a fully instrumented single cylinder research engine. The experiments were undertaken at selected engine speeds and MBT (minimum advance for best torque) spark timing and wide-open throttle for both homogenous and PSC engine configurations. Engine load was varied without use of a throttle by leaning the air-fuel mixture; the minimum achievable load therefore occurred at the lean misfire limit. The effect of changing PSC injection timing and quantity has been investigated. The performance characteristics of PSC have been compared to the general homogeneous fuelling case. The results show that the improved engine performance with PSC, particularly during ultra-lean operation, reduced levels of both brake specific fuel consumption and exhaust emissions by approximately 8% and there was an increase in brake mean effective pressure of approximately 7%. An analysis of in-cylinder pressure data revealed that PSC reduces ignition delay and combustion duration, and increases peak cylinder pressure. An extension of the lean misfire limit was achieved due to improved combustion initiation and stability with PSC.

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