Numerical study of the mixture formation process in a four-stroke GDI engine for two-wheel applications

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Abstract

Guidelines for managing the mixture formation process in a high-performance four-stroke Gasoline Direct Injection (GDI) engine for two-wheel applications are discussed, as derived from a multidimensional modelling of the in-cylinder processes. Gasoline adduction from a multi-hole injector is simulated by resorting to a properly developed model that accounts for the dependence of the initial droplets size distribution upon injection pressure. The model portability is preliminary demonstrated by comparison with experimental measurements carried out on sprays entering a confined vessel at controlled conditions.

The simulation of different engine operating conditions highlights the capability to work under the so-called “mixed mode” boosting with spray guided mixture formation.

Introduction

The worldwide concern about the environmental issues, as well as the more and more stringent emission regulations enforced in the industrialised countries, are driving several actions in the automotive field towards the development of more efficient internal combustion engines and more economical vehicles.

Between solutions aimed at improving the performances of spark ignition (SI) engines, in particular, Gasoline Direct Injection (GDI) is one of the most pursued, especially if integrated with engine downsizing strategies and turbo-charging.

In order to understand the potential advantages of the GDI concept with respect to the mixture formation realised within the intake ducts (Port Fuel Injection (PFI)), fundamental limitations of this last are to be considered into detail. First of all the need of working with stoichiometric mixtures determines both significant pumping losses during the part-load engine operation, due to the throttle load control, and unfavourable mixture properties (low ratio of specific heats), deriving from the production of high concentrations of CO2 and H2O. Under low speed–high load conditions the knock tendency is high and forces to lower the compression ratio of the engine. High concentrations of pollutants emissions are found at the exhaust, mainly due to crevices unburned hydrocarbons (HC) and nitrogen mono-oxide (NO). This last is a consequence of the high combustion temperature typical of the stoichiometric combustion. Throttle load control and stoichiometric operation significantly increase the fuel consumption of the engine, whereas the high HC and NO emissions constitute a minor problem, due to the efficiency of the generally employed three-way catalysts mounted at the exhaust [1], [2].

GDI technologies, on the other hand, allow a more precise control of the mixture formation process and of the whole combustion event. The injection of gasoline within the cylinder enhances the engine resistance to knock, especially at high loads and low engine speeds, as a consequence of the reduction of the charge temperature due to evaporation. Most importantly, direct injection (DI) allows operating the engine under overall lean conditions by realising the stratification of the charge, namely by creating a zone with stoichiometric air-to-fuel ratio around the spark plug, and a zone with leaner conditions close to the cylinder walls. This reduces the wall heat losses, the HC and CO formation, and, at the same time, strongly increases the engine volumetric efficiency. DI also allows decreasing the need of throttling for control purposes, thus reducing the cycle pumping work with respect to the PFI technology.

Indeed, it is to be noticed that a lean engine operation is generally feasible only during low loads and speeds operation, while at higher loads, and at all loads at higher speeds, the engine better works as homogeneous/stoichiometric. This resumes the so-called “mixed mode” DI boosting, that represents, nowadays, one of the most promising solution for the development of SI engines.

Various techniques for achieving the desired charge stratification within the cylinder are possible. In the wall-guided mode the gasoline spray is directed toward the piston, which exhibits a properly shaped “nose” deflecting the mixture cloud in the vicinity of the spark plug. In the air-guided mode the mixture richer region is brought toward the ignition location by the tumble motion of the air entering from the intake ducts. Finally, the jet-guided mode typically has close spacing between the injector and the spark, with the fuel spray directed towards the ignition location. This last technique offers the greatest possibilities of extending the limits of lean operation of the engine, because it is characterised by low combustion efficiency losses and combustion phasing losses, thus resulting in a significant further improvement in the fuel economy and the noxious emissions with respect to the other two concepts [3], [4].

Several kinds of injectors suitable to be employed for jet-guided combustion have been developed. The earliest solution to reduce the rapidly changing fuel concentration gradients as the fuel passes the spark location during the injection period, hence to increase the combustion robustness, relies on the adoption of air-assisted injection systems, such as the one developed by the Orbital Engine [5]. This technology is today still applied because it offers an additional degree of freedom constituted by the direct injection of air, that allows a more effective control of the local concentration of oxygen, temperature and charge motion through the cycle [6]. Alternative solutions that meet the requirements for the development of more efficient GDI engines are the high pressure injectors: the swirl type injector generates an hollow-cone fuel spray by providing a swirl rotational motion to the fuel that gives rise to widely dispersed and well-atomized sprays at moderate injection pressures [7]; the multi-hole configuration, on the other hand, exhibits flexible spray patterns that reduce the fuel impingement on the cylinder walls and improve the spray stability (cone shape) with respect to the existing backpressure.

The described scenario clearly highlights the importance of a full control of the mixture formation process within the combustion chamber for the future development of GDI engines, to assure combustion robustness under a wide range of operating conditions. By controlling the spray orientation and fragmentation, a flexible charge stratification can be achieved, that, case by case, as the engine load and speed are changed, is able to assure an optimal combustion process development.

The present work is finalized to the assessment of a 3D numerical model for the prediction of the in-cylinder mixture formation and combustion processes of a high-performance GDI engine for two-wheel applications, equipped with a new generation multi-hole injector. The considered engine, four-stroke, four-valve, in the PFI mode, with a smaller stroke and two cylinders, is in commerce as equipping a motorcycle 1200 cc. The study is carried out in order to develop a DI one-cylinder configuration through the characterisation of the thermo-fluidynamic processes relevant to the direct injection of gasoline into the cylinder by means of a multi-hole injector produced by Bosch. After a preliminary description of the computational model for the dynamics of a gasoline spray issuing in a confined environment [8], the 3D Computational Fluid Dynamics (CFD) study of the in-cylinder processes is described. The in-cylinder cycle is analysed under operating conditions relevant to both high and moderate load. In this last case globally lean stratified operation is studied. The influence on the mixture formation and combustion processes of important parameters, as the injector orientation, the Start Of Injection (SOI), the time of spark ignition and the injection pressure are discussed.

Section snippets

Experimental characterisation of the multi-hole injector

Data relevant to a preliminary experimental characterisation of the spray dynamics from the considered six-hole GDI injector are here reported. The injector is the Bosch HDEV 5.1, with holes 0.193 mm in diameter, solenoid actuation and application between 5.13 and 17.1 g/s at the injection pressure of 10 MPa. Direction of the six jets gives the spray footprint structure a hollow-ellipsoid shape on a plane perpendicular to the spray axis. Fig. 1 represents a view of the position of the holes on the

Assessment of the spray model

The model of the gasoline spray dynamics is assessed within the AVL Fire™ code environment, as a further step following the preliminary results described in Ref. [8]. Droplets evaporation [11], turbulent dispersion, coalescence and break-up are considered. This last is simulated according to the model of Huh–Gosman [12]. Initial droplets size at the nozzle exit section (0.193 mm in geometrical diameter), is assumed according to a probabilistic log-normal distribution of given variance, σ, and

Moving mesh generation

The discretisation of the computational domain is realised by means of the pre-processing software included in the same Fire™ Graphical User Interface (GUI), called Fame Engine Plus (FEP). This allows to roughly control the cell size by locally thickening nodes where particular geometric conformations of the outer surface are present, or where intense gradients of the thermo-fluid variables are expected. An example of grid used in the range of crank angles relevant to the valves overlap,

Simulation of the in-cylinder mixture formation process

Boundary and initial conditions for the 3D simulation of the four-stroke engine cycle are derived from a preliminary computation of the whole motorbike propulsion system effected by means of a one-dimensional (1D) code. This, in fact, is used to derive the inlet total pressure and temperature and the outlet static pressure, as a function of time, to be used as conditions at the 3D domain boundaries. Inlet and outlet sections coincide with intake ducts entrance and exhaust ducts exit,

Conclusions

A 3D CFD model able to define the main guidelines for the management of the mixture formation process in a high-performance GDI engine is assessed. Simulation of the whole four-stroke engine cycle is effected by considering gasoline adduction during intake through a new-generation Bosch six-hole injector. Boundary and initial conditions for the 3D model are defined, as a function of time, on the ground of a 1D simulation of the whole propulsion system. A preliminary experimental

References (16)

  • A.C. Alkidas

    Combustion advancements in gasoline engines

    Energy Conversion and Management

    (2007)
  • J.K. Dukowicz

    A particle–fluid numerical model for liquid sprays

    Journal of Computational Physics

    (1980)
  • C. Stan

    Direct Injection Systems for Spark-Ignition and Compression–Ignition Engines

    (2000)
  • B.A. Van Der Wege, Z. Han, C.O. Iyer, R.B. Munoz, J. Yi, Development and analysis of a spray-guided DISI combustion...
  • C. Schwartz, B. Schunemann, B. Durst, J. Fischer, A. Witt, Potentials of the spray-guided BMW DI combustion system, in:...
  • G. Cathcart, D. Railton, Improving robustness of spray guided DI combustion systems: the air-assisted approach, in:...
  • Y.S. Shim et al.

    Numerical modelling of hollow-cone fuel atomisation, vaporisation and wall impingement processes

    International Journal of Automotive Technology

    (2008)
  • S. Brewster, G. Cathcart, C. Zavier, The potential of enhanced HCCI/CAI control through the application of spray guided...
There are more references available in the full text version of this article.

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