Teilprojekt C02 Kubach/Koch

Teilprojekt C02 Kubach/Koch

Spray-Wall-Interaction in the cylinder of a highly charged internal combustion engine with direct injection

Motivation

The simultaneous reduction of fuel consumption and emissions is a challenge for engine developers. Downsized internal combustion engines and the use of fuels produced from renewable energy (E-Fuels) are a path leading to a solution. These downsized engines are characterized by a smaller combustion chamber while retaining their power output. The engines are charged to compensate for the smaller combustion chamber. Therefore the engines operate at higher loads which results in higher efficiency. Direct injection and high load lead to large quantities of fuel which have to be injected into the combustion chamber. Due to small combustion chambers and the injection of large quantities, the spray-wall-interaction is increased. This interaction can cause so called secondary droplets in the gas mixture and can lead to self-ignition.

Objectives

Different effects causing pre-ignition are discussed in the literature. It was shown, that shedding oil at the cylinder liner and the top land has an influence on this phenomenon. But fundamental knowledge of the shedding mechanism of oil by interactions of fuel-droplets with the liner is missing. The theory of the absorption of fuel in oil and so caused decreasing viscosity, which leads to higher rates of pre-ignition, was investigated in targeted experiments on modern engines and could not be confirmed. Because of this contradiction and the lack of knowledge of the fundamental mechanism of pre-ignition a detailed investigation of the interaction of fuel-droplets and the oil film on the liner will be in focus of this subproject.

Previous Findings

Spray-wall interaction and deposit formation in SI-engines was investigated. Systematic analysis of the influence of operating conditions on the topographic surfaces of deposits and their impact on the wall-proximity flow was done.

 Left: Experimental Setup - Rapid compression machine. Right: Coupling of a laser beam into a internal combustion engine.
Left: Experimental Setup – Rapid compression machine. Right: Coupling of a laser beam into a internal combustion engine.

Further investigations on the influence of the deposits on the heat transfer showed the higher effect of the increasing layer thickness and hence the thermal insulation than the manipulation of the near-wall-flow by different surface structures.

Picture 2: Topography of the surface of a cylinder head inlet with a thermocouple in the center before engine operation (1). Surface topography after 8 h running of the engine and deposit at 2000 rpm and IMEP = 4 bar (2). REM-detection of the surface structure (3 and 4). Comparison of the measured layer thickness after 8 h engine operation (green line) with the calculated thickness based on Hopwood and the two-layer-method (5).
Picture 2: Topography of the surface of a cylinder head inlet with a thermocouple in the center before engine operation (1). Surface topography after 8 h running of the engine and deposit at 2000 rpm and IMEP = 4 bar (2). REM-detection of the surface structure (3 and 4). Comparison of the measured layer thickness after 8 h engine operation (green line) with the calculated thickness based on Hopwood and the two-layer-method (5).

Studies showed that interaction of fuel-droplets and oil films in SI-engines has an impact on pre-ignition. The impacts of the changing oil viscosity, droplet size and the thickness of the oil films on the probability of pre-ignition were investigated. But the relation between the decreasing oil viscosity and the rate of pre-ignition seems to be significantly lower than the velocity and size of the fuel-droplet and the oil film thickness.

Approach

The examination of combustion processes and interaction of multi-phase flows requires different accessibility for optical measurement methods. For this subproject and the different condition for the investigations four test-benches will be used:

Injection-pressure-chamber: The chamber has a volume of 6.2 liter and a maximum inside pressure of 75 bar. The inert atmosphere and operating pressure is generated and regulated by a continuous flow of nitrogen. A heating cartridge adjusts the temperature up to 450°C. Four of five ports of the chamber can be used for the injection and optical access.

Thermodynamic single-cylinder engine: A thermodynamic single-cylinder engine is used to investigate theinsulating effect of deposits under realistic conditions. The measurements will be performed using fast surface temperature sensors and endoscopic access for optical analysis.

Oil-layer-pressure-chamber: In this chamber, similar to an Injection-pressure-chamber (see above), it is possible to simulate pressure and temperature operating conditions like in turbo charged engines at full load. There are four ports at a 90° angle horizontal to each other for optical access. A special device generates oil films with a thickness of <20 µm to investigate the interaction between fuel droplets and oil films.

Ignition-chamber: The cylindrical chamber is 134mm long and has a diameter of 28 mm. The chamber has one port for optical measurements and can be heated up to 500°C. The maximum inside pressure is 350 bar. It is possible to analyze the combustion of oil droplets in a hot gas atmosphere thermodynamically and optically with pressure sensors and high-speed-cameras.

Current Work

Injectors need to get adapted for the investigations with E-Fuels like DMC and MeFo. The spray-behavior is characterized in an injection-pressure-chamber. Mie-scattering and the PDA-method is used for the detection of the spray dispersion and the size of the droplets. An adaption to the pressure-chamber for the injector was designed and a fuel-system for small injection quantities is mounted. The spay-behavior of given injectors are used to select an optimized injector regarding flow rate, targeting and droplet-size. This injector is used in a single-cylinder-engine to analyze the spray dispersion and the buildup of deposits in permanent operation experiments.

Cooperations

One target of this subproject is to extend the knowledge of deposit formation by use of E-fuels. The gained measurement-data and information are shared with A01 for generic experiments and B01 for simulation models. Investigations of the interaction of fuel-droplets and the oil film and the so caused drop separation is done in close exchange on experimental level with C01 and with A04 regarding measurement-systems and interpretation of findings. Together with A05 we are going to determine the thickness of the oil layers. The detection of the splash-drops and interpretation of the measurements is supported by A02, which also provides the simulation model for the splash-limits. The measurement results are shared also with B08 for comparison with the results of numerical simulations.

Selected Publications

[E1] Weidenlener, A., Kubach, H., Pfeil, J., Koch, T.: The Influence of Operating Conditions on Combustion Chamber Deposit Surface Structure. COMODIA 2017, Okayama, Japan, July 25-28 (2017).

[E2] Kubach, H., Weidenlener, A., Pfeil, J., Koch, T., Kittel, H., Roisman, I.V., Tropea, C.: Investigations on the Influence of Fuel Oil Film Interaction on Pre-ignition Events in Highly Boosted DI Gasoline Engines. SAE 2018-01-1454 (2018).

[E3] Weidenlener, A., Kubach, H., Pfeil, J., Koch, T.: The Influence of Operating Conditions on Combustion Chamber Deposit Surface Structure, Deposit Thickness and Thermal Properties. Automotive and Engine Technology (2018) (accepted).

[E4] Forooghi, P., Weidenlener, A., Magagnato, F., Böhm, B., Kubach, H., Koch, T., Frohnapfel, B.: DNS of momentum and heat transfer over rough surfaces based on realistic combustion chamber deposit geometries. Int. J. Heat Fluid Flow 69, 83–94 (2018).