Turbulent Combustion Part 2

Turbulent Combustion Part 2 - Cycle to Cycle Variations...

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Unformatted text preview: Cycle to Cycle Variations Cylinder pressure development depicting cycle-to-cycle variations at an idle condition. • Cycle to cycle variations occur in all spark-ignited engines. • They are a natural result of the turbulent fluid mechanics which affects local velocity and length scales. • In addition, cycle to cycle variations can occur in residual gas mass and temperature as well as in the amount of fuel delivered each cycle. In the following discussion we will highlight the importance of reducing cycle to cycle variations as well as discuss the origins of cycle to cycle variations in some detail. Note, the constancy of the expansion stroke pressure. If amount of fuel delivered from cycle to cycle changed one would see different expansion pressure traces. Most of the cycle to cycle variations are due to velocity and scale fluctuations at the spark plug which then affect the growth of the ignition kernel. Figure CTC_2: Burned mass fraction development for flame kernels that were larger than the average flame kernel (cycle 9) taken from reference 3. Note, since the early rate at which mass is consumed is greater for cycles 7 and 19 the pressure developments for these cycles will be different than the average cycle. Cycle to Cycle Variations Causes of Cycle to Cycle Variations Causes of Cycle to Cycle Variations Causes of Cycle to Cycle Variations Schematic of flame kernel motions and growth during the early stages of the combustion process. Since the flow field in the engine cylinder is turbulent, there are normal variations in the characteristic lengths and velocities at the spark plug location. You might ask the question: Why aren’t the velocity and the important length scales the same at a given point, i.e. the spark plug location, from cycle to cycle? The reason is that the intake event is an independent starting event each cycle. Since the vorticies shed off the intake valve differently every time there is an intake event, there is no reason to expect the same velocities to occur at the same positions from cycle to cycle. Note, the vortex shedding event depends on very local instabilities in the boundary layers of the flow which can never be the same for each intake event. Causes of Cycle to Cycle Variations Schematic of flame kernel motions and growth during the early stages of the combustion process. In addition to the fluctuations in the turbulent flow, there can be small differences in the mean flows. Both of these effects can transport the spark kernel to different effective spark locations, which then result in different burntimes. Also, the mass of residual gases can change from cycle to cycle due to the change in burntime which effects the residual temperatures and hence residual mass....
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Turbulent Combustion Part 2 - Cycle to Cycle Variations...

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