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The effect of high compression ratio on cycle-to-cycle variability was studied using two different combustion chamber shapes, with natural gas operation. A single cylinder Ricardo E6 test engine was run over a range of operating conditions and compression ratios. The different combustion chamber shapes were achieved by using different pistons. At each operating condition detailed cylinder pressure data were recorded for two hundred consecutive cycles allowing for a detailed study of this chaotic phenomenon. The effect of compression ratio on cyclic variation was contradictory for the two combustion chambers, and it is concluded that the real effect was not compression ratio but combustion chamber shape, since the combustion chamber geometry was changed with altering compression ratio for both combustion chambers.

The use of compressed natural gas (CNG) as a fuel for heavy transport engines is still in the development stage, however, experience has shown promising results in certain applications. While most of the recent work done in New Zealand on CNG use has been centered around non-turbocharged, high speed, spark ignition (SI) conversions, there is still likely to be a role for dual fuel engines in some applications. This paper presents a review of New Zealand experiences with the use of CNG as a fuel in heavy transport diesel engines. Both dual fuel and 100% CNG fuelling (dedicated spark ignition conversions) are discussed. The advantages and disadvantages of each are reviewed in terms of vehicle performance, fuel costs and operational experiences. Brief reviews of experimental results and discussions of technical aspects of conversions, experience in fleet use, economic and operational aspects, current research and computer modelling of fleet performance are also included.

Two heavy duty turbocharged and aftercooled diesel engines have been converted to spark ignition (SI) for natural gas fuelling. One engine features two valves per cylinder, high primary swirl and a compact combustion chamber. The other engine has four valves per cylinder and a more open combustion chamber shape. Both engines were characterized on a dynamometer and subsequently put into service where one of the engines has been monitored extensively. When a diesel engine is converted to SI operation, there is an involved process which must be carried out to insure reliable and efficient operation. Many aspects of the conversion process are limited by the original configuration of the diesel engine. Some of these aspects, such as combustion chamber shape, are shown to be important to the optimization of an SI converted engine and are highlighted in the comparison of these two engines.