Search Results

An important source for soot formation during the combustion of diesel engines with direct injection is the interaction of liquid fuel or a very rich air/fuel-mixture with the flame. This effect appears especially in modern direct injection engines where the injection is often split in a pre- and a main injection due to noise reasons. After the ignition of the pre-injected fuel a part of the main injection can interact with the flame still in liquid phase as the fuel is injected straight towards the already burning cylinder areas. This leads to high amounts of soot. The injection strategy for this experimental study overcomes this problem by separating the injections spatially and therefore on the one hand reduces the soot formation during the early stages of the combustion and on the other hand increases the soot oxidation later during the combustion. In particular an injection configuration is used which gives the degree of freedom to modify the injection in the described manner.

The quality of air-/fuel-mixture is of prime importance for cycle fluctuations of combustion. Investigations of mixture formation and conditions in SI engines have been subject of intensive research since many years. The scope of this work was to investigate crank angle resolved determination of qualitative and quantitative mixture conditions inside the combustion chamber in dependence on various engine operating conditions. For this experimental investigation a time resolved Gas Sampling Valve (GSV) was combined with a flame ionisation detector (FID), a CO2-analyzer and a mass spectrometer. The GSV also enables the determination of residual gas concentration. Measurements on a DI gasoline engine show influences of air-/fuel-mixture in dependence on various engine operating conditions when the engine runs in charge stratification mode. Moreover, experimental results of local mixture composi-tion are compared with fuel distribution, calculated from CFD-codes.

To operate gasoline direct injection engines at part load and in stratified mode the mixture formation has to fulfil several requirements. The complexity of this process requires - regarding a suitable mixture transportation and vaporisation of the fuel - an adjusted design of the combustion chamber and the intake ports to reliably place an ignitable mixture at ignition timing near the spark plug at any speed and load. Due to the inhomogeneous mixture distribution during stratified operation, the first combustion period is very sensitive to cycle-to-cycle variations. A reproducible mixture movement with high kinetic energy is necessary for stable engine operation with low fluctuations in the combustion process. Because of the high relevance of these facts, the effects of an adjustable air guiding system in the inlet manifold on in-cylinder flow, ignition and combustion using optical measurement techniques were investigated.

The objective of this study is to achieve a general understanding of gas exchange phenomena to develop a model for predicting the residual gas content. The knowledge of the cylinder-charge composition is important for the thermodynamic analysis of the combustion process of IC engines. Therefore, the amount of fresh air and fuel as well as the residual gas fraction has to be known. The residual gas mass strongly depends on valve train parameters and operating conditions. In this study, the residual gas fraction has been determined by using in-cylinder gas sampling from the combustion chamber of a 4-stroke SI engine. The gas sampling valve was flush-mounted to the combustion chamber walls. The gas samples were taken after the gas exchange and analysed for its CO2 concentration. In combination with the analysis of the exhaust gas composition, the calculation of the residual gas fraction is possible.

The loss of momentum of the gas-core inside inlet manifolds of four-stroke engines is characterized by loss coefficients. Usually these coefficients are obtained by experimental investigations of the flow through cylinder heads under steady-state conditions. The dynamic behavior of the gas motion under real conditions due to acceleration and vibration of the gas-core as well as the influence of the gas motion due to the exhaust can not be described by these coefficients. Therefore a basic investigation of the unsteady flow under real engine conditions has been performed. The aim was to develop a simple method to characterize the inlet flow behavior under real conditions and to define a dynamic loss coefficient. The mass flow rate was determined by time resolved pressure data inside the suction pipe and a simple numerical calculation method considering unsteady flow conditions. The verification of calculated flow velocities was performed by using Particle-Image-Velocimetry.

Knowledge of flame propagation enables a more detailed description of the combustion processes as well as assessing the influences of engine operating conditions and design parameters, particularly in view of efficiency improvements and pollutant reduction. Flame propagation in a single-cylinder spark-ignition engine is studied by means of a measuring technique using lightconductors coupled with photo-multipliers. The propagation process is monitored through a large number of optical probes arranged in a matrix in the combustion chamber. The spatial flame contour, the flame volume, and the flame front velocity as a function of time are furnished from these measurements. The investigations show the formation of quench zones ("flame quenching") not penetrated by the flame directly above the piston towards the end of the expansion phase at extremely lean operating conditions.

This paper presents investigations on the combustion process in a single cylinder SI engine with direct injection. Different nozzle types are examined i.e. hollow cone nozzles and hole type nozzles both with different geometry of the injected spray. These nozzle types have been compared in view of their suitability of creating a homogeneous as well as a stratified mixture in the combustion chamber. To create a homogenous mixture, the fuel was injected during the intake stroke. In order to examine the homogeneity of the mixture in the case of direct injection, the engine was driven with mixture formation generated through intake port injection. The comparison of the direct injection method with the intake port injection for homogenous mixture formation has shown only small differences in engine behavior. To create a stratified mixture in the combustion chamber, the fuel was injected at the end of the compression stroke.

In this paper optical investigations of a gasoline direct injection engine with narrow spacing arrangement of spark plug and injector are presented. For the combustion analysis spectroscopy techniques based on the fiber technique are used. With this measurement technique information about soot formation and temperature progression in the combustion chamber is obtained. Furthermore a validation of numerical simulation of the stratified combustion with data obtained experimentally, is performed and discussed.

The complexity of the mixture formation in direct injection engines requires - according to a suitable mixture transportation and vaporization of the fuel - detailed knowledge of the in-cylinder processes to reliably place an ignitable mixture at ignition timing near the spark plug for any speed and load. Two different optical measurement techniques were adapted to a single cylinder engine and the spray propagation was observed from the start of injection until ignition. 3-pentanone tracer-LIF signals (laser-induced fluorescence) and CN spark emission signals were detected simultaneously in order to get information about the local equivalence ratio at the spark plug and compare the two methods. While there is a good correlation for homogeneous operating conditions of the engine, the results diverge in the stratified mode.