The present publication describes investigations on a lean burn gas engine equipped with a variable intake valve train and demonstrates how steady state engine performance can be improved in comparison to a conventional state-of-the-art application with constant Miller timing. As the knock border represents a significant limitation of the operating range of gas engines, the engine specific knock limit was derived from measurements on a single cylinder research engine and transferred to a 1D simulation model of the corresponding multicylinder engine; a large bore, two stage turbocharged gas engine in the 5 MW power range with a variable intake valve train. Special attention was given to the setup of the simulation model to improve prediction quality and reduce simulation effort. An optimal strategy using the flexibility of a variable intake valve train for engine operation is presented that is capable of accommodating fluctuating gas qualities, which are described by the methane number. The operating strategy was derived from the 1D simulation model. The better performance than with a state-of-the-art strategy will be quantified in terms of engine efficiency while knocking combustion caused by low methane numbers is prevented. Since ambient temperatures in certain regions where the engine is operated do not remain stable throughout the year and ambient pressure varies depending on sea level, these issues must also be addressed. The temperature and density of the intake air have a large influence on the performance of the turbocharging unit and thus overall engine efficiency. The simulation results show the engine’s behavior under varying ambient conditions and outline potential strategies for improvement made possible by using variable valve timing on the intake side.
The present paper describes the investigations made using the electro-hydraulic intake valve timing system VCM® on a large bore gas engine. The first section explains what challenges have to be faced when developing concepts for present and future applications of large bore gas engines. Following an introduction to the VCM® system, an outline is presented of expected opportunities for using variable intake valve timing in combination with modern turbocharging concepts. The second section describes 0D/1D engine cycle simulations that were carried out to assess the influence of variable valve timing on the intake side compared to a fixed intake valve profile, which is the current standard for large bore gas engines. As a result, first predictions can be made about the gain in engine efficiency achieved with different operating strategies. In order to assess the performance potentials of the variable valve train, extensive experimental investigations were carried out on a single cylinder research engine based on GE’s Type 6 gas engine. The investigations consisted of varying engine parameters including varying the geometric compression ratio as well as the engine boundary conditions. It will be shown how intake valve timing can be used to optimize engine efficiency by improving gas exchange. Furthermore, variable intake valve timing affects the overall system behavior, e.g. distances to the engine’s operating limits. Special attention was paid to analyzing combustion itself, which is necessary due to the strong influence that intake valve timing has on the thermodynamic states of the cylinder charge.
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