Abstract
In order to maximise engine heat efficiency an engines charge flow must be properly designed -especially its swirl and tumble ratio. A two-stroke compression-ignition opposed piston engine reacts to engine swirl differently compared to a standard automotive engine with axially symmetric combustion chamber. In order to facilitate direct fuel injection, high-pressure injectors must be positioned from the side of combustion chamber. Depending on the combustion chamber geometry the swirling gases impact greatly how the injection stream is formed. If the deformation is too high the high temperature combustion gases can hit the piston surface or get into gaps between the pistons. This greatly affects the heat lost to the pistons and raises their local temperature. More atomised injection stream is more prone to swirling gas flow due to its reduced droplet size and momentum. The paper presents simulation results and analyses for different intake process induced swirl ratios and different types of combustion chambers in an experimental aviation opposed piston engine.
Highlights
The compression ignition engines are more thermodynamically efficient than the SI engines, even though the theoretical thermodynamic cycle seems less efficient at first glance
This paper presents mainly the impact of varying swirl ratio on combustion process
The results show that the heat flux can vary vastly with the change of swirl ratio, injection parameters and combustion chamber geometry
Summary
The compression ignition engines are more thermodynamically efficient than the SI engines, even though the theoretical thermodynamic cycle seems less efficient at first glance (heat capacity is higher for the constant pressure heat addition). Higher efficiency is achieved because of the lean combustion and higher compression ratio – not feasible in SI engines due to knock combustion phenomena. Lean combustion in an CI engine ensures better isolation of hot combustion gases from cold combustion chamber walls. This is the reason why proper injection, intake, exhaust and combustion process design is important for the heat loses. The smaller the combustion chamber active area the smaller the heat flux will be. This is why it is favourable to make more spherical combustion chamber as sphere has the best volume to surface ratio
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