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A visualization study using shadowgraphy was performed in an optically-accessible, cylindrical constant-volume combustion chamber to identify the mechanism of flow/flame interaction in spark-ignited, lean propane-air mixtures. The effect of the flow on flame initiation and propagation was examined by varying the pre-ignition mean flow and turbulence within a range typical of modern four-valve spark-ignition (SI) engines, as well as the spark plug orientation relative to the mean flow. The initial flame development was quantified in terms of 2-D images which provided information about the projected flame area and the displacement of the flame center as a function of flow conditions, time from the spark initiation and spark plug orientation. The results showed that high mean flow velocities and turbulence levels can shorten combustion duration in lean mixtures and that the positioning of the ground electrode can have an important effect on the initial kernel formation.

The ability of certain induction systems to enhance turbulence levels at the time of ignition, through formation of long-lived tumbling vortices on the plane of the valve and cylinder axes, has been investigated in a two-valve spark-ignition engine by rotating the intake port at 90° and 45° to the orientation of production directed ports. Detailed measurements of the three velocity components, obtained by laser velocimetry, revealed that the 90° port generated a pure tumble motion, with a maximum tumbling vortex ratio of 1.5 at 295°CA, zero swirl, and 42% turbulence enhancement relative to the standard configuration, while the 45° port gave rise to a combined tumble/swirl structure with a maximum tumbling vortex ratio of 0.5 at 285°CA, swirl ratio of 1.0 at TDC, and turbulence enhancement of 24%. The implications of the two types of flow structures for combustion are discussed.

The spatial and temporal characteristics of a diesel spray injected into the atmosphere through a multi-hole nozzle used in small DI Diesel engines have been investigated by laser techniques as a function of pump speed and load. The results showed that spray tip penetration and velocity depend on injection frequency rather than injected volume and the spray is asymmetric during the early and main part of the injection period. In the time/space domain different structures have been identified within the injection period, with the early injection period characterized by a well atomized cloud of droplets, the main period by the spray head and a dense core and the late injection period by the disintegrating dense core and the spray tail. IN DIRECT-INJECTION DIESEL ENGINES for passenger cars, fuel is injected through multi-hole nozzles at high pressure to promote mixing with the rapidly swirling air inside the combustion chamber.

Ensemble-averaged and in-cycle axial and swirl velocities have been measured by laser Doppler anemometry in the three-dimensional flow field of a four-stroke model engine motored at 200 rpm with a compression ratio of 6.7 and various cylinder head and piston geometries. The inlet configurations comprised an axisymmetric port with a shrouded valve and an off-centre port with two valve and swirl generating vane geometries. The piston configurations comprised flat, cylindrical and re-entrant axisymmetric piston-bowls. The results indicate that with the off-centre port a complex vortical flow pattern is generated during induction, which later either collapses in the absence of induction swirl or is transformed into a single rotating vortex in the transverse plane when swirl is present. The axisymmetric port with the shrouded valve gives rise to a double vortex structure and higher turbulence levels at TDC of compression compared to the off-centre port.

The Rayleigh light scattering technique has been used to quantify the mean and fluctuating concentration of a passive scalar used to simulate fuel injection in a reciprocating, two-stroke model engine motored at 200 rpm in the absence of compression. The transient concentration field, which results from injection of Freon-12 vapour through the centre of an axisymmetrically located permanently open valve, has been investigated for injection timings of 40 deg. before and at top-dead-centre as a function of spatial position and crank angle. The purpose-built Rayleigh system, with gated digital data acquisition and software dust particle filtering, was first evaluated in a Freon-12 free jet by comparing results to those obtained with a sampling probe.

The effect of inlet port design on swirl generation has been investigated for four helical ports from production, prototype and research Dl diesel engines by analyzing experimentally measured steady flow velocity distributions at the inlet valve curtain area and comparing their swirl characteristics in terms of the calculated in-cylinder angular momentum components and swirl ratio under operating conditions.

The origin and development of swirl center precession in engine flows has been investigated in a steady flow rig, with and without a porous plate simulating a stationary piston, and in a model engine motored at 200rpm; swirl, in all cases, was generated by means of 60° vanes located in the axisymmetric inlet port. The swirl center performs a helical motion that originates as an instability in the forced-vortex core from its interaction with the axial flow at a free stagnation point and develops in the engine from the piston towards the cylinder head; an opposite trend has been observed in the steady flow case with the open-ended cylinder. In the ensemble-averaged measurements, swirl center precession has been identified by the increased tangential velocity fluctuations around the off-centre zero swirl velocity.

A comprehensive set of computational and experimental results for high- pressure diesel sprays are presented and discussed. The test cases investigated include injection of diesel into air under both atmospheric and high pressure/temperature chamber conditions, injection against pressurized and cross-flowing CF6 simulating respectively the density and flow conditions of a diesel engine at the time of injection, as well as injection into the piston bowl of both research and production turbocharged high-speed DI diesel engines. A variety of high-pressure injection systems and injector nozzles have been used including mechanical and electronic high-pressure pumps as well as common-rail systems connected to nozzles incorporating a varying number of holes with diameters ranging from conventional to micro-size.

An optical single-cylinder four-valve high speed DI Diesel engine equipped with a high-pressure electronic fuel injection system has been used to obtain information about the spray development, combustion and exhaust emissions (NOx and smoke levels) for a range of operating conditions corresponding to engine speeds between 600 and 1800 rpm, injection pressures up to 1200 bars and fuel injection quantities from idle to full load. Two six-hole vertical mini-sac type injection nozzles with different hole sizes have been employed in order to investigate the effect of nozzle hole diameter on spray formation, combustion and exhaust emissions. Parallel to the experimental programme, a computational investigation of the fuel flow distribution inside the injection system and of the subsequent spray characteristics has been performed in order to assist in the interpretation of the results.

The spatial and temporal characteristics of the non-evaporating diesel sprays injected into the atmosphere through two pump-pipe-nozzle systems used in small DI diesel engines have been investigated by laser-single-beam deflection and phase-Doppler anemometry (PDA). The injectors used for these tests comprised a single-spring and a prototype two-spring multihole-type nozzle. The results provided quantitative information about the effect that the second spring exerts on injection duration and spray characteristics, i.e. it increases injection duration and, at the same time, improves fuel atomisation during the main injection period.

Multi-dimensional modelling of the flow and combustion promises to become a useful optimisation tool for IC engine design. Currently, the total simulation time for an engine cycle is measured in weeks to months, thus preventing the routine use of CFD in the design process. Here, we shall describe three tools aimed at reducing the simulation time to less than a week. The rapid template-based mesher produces the computational mesh within 1-2 days. The parallel flow solver STAR-CD performs the flow simulation on a similar time-scale. The package is completed with COVISEMP, a parallel post-processor which allows real-time interaction with the data.

In this study three-dimensional CFD calculations of the gas motion and spray characteristics of a small (1.9l), high-speed direct-injection Diesel engine are presented and evaluated. The calculations were performed using the SPEED code, developed within the European IDEA-EFFECT project: it uses fully implicit finite volume methodology in conjunction with an unstructured mesh to represent the full complexities of the engine geometry and solve the equations governing the gas motion, fuel spray evolution and subsequent fuel/air mixing. Submodels for particular aspects of these processes developed by various partners in the project are incorporated. The accuracy of the predictions is assessed through comparisons with detailed LDA measurements of the velocity field during the induction and compression strokes up to the time of ignition, as well as with quantitative measurements of the spray penetration and local droplet velocities. Moderately good agreement is obtained.

The objective of this paper is to present a numerical simulation method for the calculation of an unsteady, one-dimensional flow and heat transfer in the branched intake manifolds of multi-cylinder engines. The method operates on the one-dimensional differential conservation equations for a variable-area duct network with friction and heat transfer at the walls. The latter processes are represented by appropriate drag and heat transfer coefficient correlations, as also are the losses which occur at junctions and other geometrical irregularities. The equations are solved by a time-marching finite-volume method, on a computational mesh in which the velocities are located between the pressures which drive them.

The numerical methodology used to predict the flow inside pressure-swirl atomizers used with gasoline direct injection engines and the subsequent spray development is presented. Validation of the two-phase CFD models used takes place against film thickness measurements obtained from high resolution CCD-based images taken inside the discharge hole of a pressure swirl atomizer modified to incorporate a transparent hole extension. The transient evolution of the film thickness and its mean axial and swirl velocity components as it emerges from the nozzle hole is then used as input to a spray CFD model predicting the development of both non-evaporating and evaporating sprays under a variety of back pressure and temperature conditions. Model predictions are compared with phase Doppler anemometry measurements of the temporal and spatial variation of the droplet size and velocity as well as CCD spray images.

A new simulation approach to the modeling of the whole fuel injection process within a common-rail fuel injection system for direct-injection gasoline engines, including the pressure-swirl atomizer and the conical hollow-cone spray formed at the nozzle exit, is presented. The flow development in the common-rail fuel injection system is simulated using an 1-D model which accounts for the wave dynamics within the system and predicts the actual injection pressure and injection rate throughout the nozzle. The details of the flow inside its various flow passages and the discharge hole of the pressure-swirl atomizer are investigated using a two-phase CFD model which calculates the location of the liquid-gas interface using the VOF method and estimates the transient formation of the liquid film developing on the walls of the discharge hole due to the centrifugal forces acting on the swirling fluid.

A mathematical model of formation and transport of liquid films, incorporating a droplet-wall impaction model and exchange mechanisms with the gas-phase, has been developed and incorporated into the STAR-CD computational fluid dynamics code. It has been applied to a test case representation of the multi-point fuel injection in four stroke SI engines. The results indicate that the major features of droplet impaction and film development are reproduced by the model. The qualitative agreement with data in the region of spray impaction is good.

The diesel engine is the most efficient user of fossil fuels for vehicle propulsion and seems to best fulfill the requirements of the future. It is for this reason that Volkswagen has initiated a very broad research programme for diesels. The purpose of this paper is to build a bridge between fundamental research and technical developments which could allow evaluation of the prospects of direct- injection diesels as powerplants of choice for passenger cars in the turn of the century. The current knowledge on mixture formation, combustion and pollutant formation in diesel engines is presented and discussed with special emphasis given to the concept of the direct-injection diesel engine.

A laser-induced fluorescence (LIF) system has been developed to obtain measurements of the instantaneous lubricant film thickness in the piston-cylinder assembly of a firing single-cylinder, direct-injection diesel engine. Measurements were made at top-dead-centre (TDC), mid-stroke and bottom-dead-centre (BDC) position by means of three fibre optic probes inserted into the cylinder liner and mounted flush with its surface. Following extensive repeatability tests, the cycle-averaged lubricant film thickness was estimated for different multi-grade oils as a function of engine speed, load and temperature. The results quantified the dependence of the film thickness ahead, under and behind the piston rings on oil chemistry and viscometric properties, thus confirming the important role of the LIF technique in the development and formulation of new engine oils.

Multidimensional calculations and laser Doppler anemometry measurements are presented of the air flow in a research diesel engine motored at 900 rpm with a compression ratio of ∼8.5. The engine comprised the cylinder head of a Ford 2.5L high speed direct-injection diesel mounted on a single cylinder Fetter engine modified to provide optical access for LDA measurements in a toroidal piston-bowl. The accuracy of the predictions is assessed against ensemble-averaged velocity data and found to be sufficient to allow better understanding of the flow in production engine geometries under realistic operating conditions.

This paper presents a mathematical model for the prediction of the dynamic characteristics of wall films formed by impinging sprays. The model takes into account the impingement pressure due to bombardment of impinging droplets, tangential momentum transfer resulting from oblique droplet impingement on the film surface, and the gas shear force at the film surface. The general transport equations of mass, momentum and energy for wall film flows are established in the boundary-layer framework. It is shown that this set of equations can be substantially simplified if local equilibrium occurs and a dimensional analysis is performed to identify the conditions for the applicability of the local equilibrium model. Solution of the full film equations is obtained by an efficient hybrid integral/numerical method, which allows numerical calculations to be performed in a two-dimensional framework. An implicit finite volume scheme is employed for this purpose.