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The simulation activity for the piston bowl development in a low compression ratio (CR) high-performance diesel engine is described, starting from the calibration of a 3-D CFD commercial code by pressure-based combustion diagnostics data. Calibration was made for the baseline engine built by GMPT-E, matching experimental pressure traces and heat release rates derived from these through the diagnostic tool. Measured pollutant emissions were also applied for calibration at this stage. The engine was susceptible to modifications, according to the outcomes of combined simulation and experimental investigations. The validated CFD model was used for the screening of three new piston bowls featuring a reduced compression ratio. The 3-D code has been integrated with a robustly calibrated 1-D hydraulic model for the injection system simulation and with a 1-D fluidynamic tool for modeling engine flow processes external to the cylinder to provide quite accurate boundary conditions.

The development trends of advanced automobile engines towards high power-to-volume and power-to-mass ratios are partially in contradiction with the requirements regarding drastically reduced fuel consumption and pollutant emission. The development way of the engine between customer acceptance and limitations by law is mainly determined by the optimization of scavenging, mixture formation and combustion characteristics, as functional base for the engine design. The paper presents a new direct injection concept and its optimization correlated with the scavenging process. The process simulation - as a base for the engine development - was carried out using concomitantly two CFD codes - FIRE and VECTIS. The main optimization parameters were the combustion chamber design, the injector location, the spray characteristics, the spark location, the injection timing and duration.

The injection rate modulation and the spray characteristics are determining factors for the quality of mixture formation when applying GDI. Their variation with load and speed is a basic criterion for the adaptability of a type of injection system to an engine with known requirements. The increased interest for the utilization of regenerative fuels - such as methanol obtained from biomass - as well as the success of previous utilization scenarios of variable gasoline/methanol mixture using manifold injection formed the base of the present analysis: the paper describes the results concerning injection performances and spray characteristics when using gasoline/methanol mixtures with different ratios in a direct injection system with high pressure modulation. The results are compared for different parameters of the injection systems as follows: injection volume, injector opening pressure, needle lift, pintle/seat geometry.

The fuel direct injection in SI engines is demonstrating a remarkable potential regarding the reduction of consumption and pollutant emission. Nevertheless, the management of the mixture formation “in-cylinder” - in conditions of a short duration and of a complex fluid dynamic configuration imposes both an accurate modeling and an exact control of the process. The problem gains on complexity when considering the use of alternative fuels which becomes more and more a subject of actuality. The paper presents a comparative analysis of mixture formation process and engine performances, when applying direct injection of gasoline, respectively of ethanol in a four-stroke single cylinder SI engine. The modulation of the injection rate shape is the result of a fuel high pressure wave, generated in a pressure pulse direct injection system.

The spark ignition engines offer a remarkable potential of improvement – from performances to consumption and pollution – which begins to be better exploited than in the past. This survival process is imposed by the increasing demand for performances, by the drastical limitation of pollutant emission, as well as by noticeable competitors such as the fuel cell. The improvement base for SI engines consists in the accurate control and adaptation of the thermodynamic process stages – scavenging, mixture formation and combustion – for every load, speed and surrounding conditions. The classical process itself offers a substantial improvement reserve when applying a load and speed adapted process management. The higher level of improvement is given by internal mixture formation using direct injection, by fully variable valve control as well as by generally higher load operation using super- or turbocharging. All these elements can be combined in an advantageous down sizing concept.

The paper presents a GDI concept applied to a four-stroke four-valve single cylinder engine in base of the original “crevice-like” combustion chamber of the basic engine with external mixture formation. For such application both, an adapted shape of injection rate as well as a good correlation of location and timing between injection and spark is requested. The shape of injection rate is adapted in base of a pressure pulse injection system. The paper presents the special features of the system conceived for this aim, as well as the results for different locations of injector and spark plug. The best results in terms of bmep, bsfc and pollutant emissions are obtained with a twin spark configuration. The mixture formation and combustion particularities of this concept are analyzed going from the experimental results at the engine test bench for full and part-load in condition of widely unthrottled operation.

The spray characteristics are determining factors for the quality of mixture formation respectively for the combustion, when applying GDI. Their variation with load and speed is a basic criterion for the adaptability of an injection system type to an engine with known requirements. CFD models of the fluid flow dynamics, mixture formation and combustion are a determining condition for such adaptation. The paper presents the development results of a GDI four-stroke, four-valve, single cylinder engine. The pressure pulse injection system involved in this application is analyzed and presented from the fluid-dynamic behavior up to the obtained injection spray characteristics. The mixture formation and combustion processes are simulated for different load and speed values, respectively for favorable combinations of parameters, such as the injection system configuration, the opening pressure of the applied mechanical injector and the injection duration.

The actual trends in development and series application of mixture formation techniques for SI engines converge irrevocably to a process after scavenging, by direct injection, the reason being the higher thermal efficiency in a wide operation range of the engine, leading to substantially lower bsfc and pollutant emission. After numerous and successful research projects of direct injection for two-stroke engines, the most of series applications are being introduced for four-stroke automotive engines. A main reason for this profitable way consists in the better fluid dynamic conditions and in the longer time for mixture formation in the case of the four-stroke process.

The paper presents the results of a down-sizing concept implicating gasoline direct injection, which is applied to a four-stroke four-valve SI engine with a displacement of 500 ccm per cylinder. The typical features of a down sized engine such as a high level of engine speed, high power density at low fuel consumption and a low level of pollutant emission form the main targets of this study. Numerical models of the process stages have been developed in 1D and 3D CFD codes. The accurateness of the models has been proved using experimental results. The main work consisted on the application of a direct injection system to the engine. The compact engine design and the high compression ratio have been maintained resulting in a combustion chamber design without any cavities or bowls. To obtain accurate results, the simulation work has been carried out using two different CFD-codes (FIRE and VECTIS); the results have been analyzed and compared.

In order to develop modern two-stroke engines with low fuel consumption, respectively with low exhaust emissions, two alternative development areas - the mixture formation and the scavenging system - have been correlated. For a satisfying mixture formation without fuel losses by scavenging, the direct injection seems to be one of the best solution for the high speed two-stroke engine of the future. On the other hand the modern development of two-stroke scavenging systems shows a large field of application and improvement methods of cross and loop scavenging [1]. Based on the specific optimisation factors of the injection system, respectively of the scavenging system, the aim off this common work of the Universities of Pisa and Zwickau is to correlate both the optimisation fields in an advantageous mixture formation process.

This paper describes the operation and performance of two direct fuel injection strategies, namely a ram-tuned injection system (single fluid) and an air-assisted injection system (twin fluid). The injection systems are tested on a 500 cm3 single-cylinder cross scavenged two-stroke cycle engine during part load operation at 1600 rev/min. Particle sizing tests are presented for each injection system, in conjunction with specific emissions and performance results for firing engine tests.

World-wide attention to environmental issues in recent years has resulted in a greater demand for cleaner engines, especially with regards to the two-stroke. Considering the techniques for reduction of exhaust emissions the direct injection of fuel into the combustion chamber adapted for a loop scavenged cylinder seems to be an advantageous method. This paper describes the application of advanced experimental and computational techniques to evaluate mixture formation produced on a commercial engine by means of a direct fuel injection strategy, namely a ram-tuned injection system. The injection system data are experimental while air flow and fuel air mix for the direct injection engine are calculated using a turbulent model of the three dimensional code FLUENT. Extension of a first work in this field is presented. In particular two possible strategies to simulate direct injection are tested. The influence of different boundary conditions on the scavenging process was examined too.

The balance of the exchange between internal combustion engines and the environment regarding both consumption and emissions depends principally on the type of working substance, respectively on their management during the process within the engine. Regarding the process management, the internal mixture formation by direct injection has a better potential for reduction of both consumption and pollutant emission than external mixture formation, by carburetor or manifold injection. This fact results from the possibilities of control extended up to the start of combustion and is well demonstrated by recent developments. On the other hand, the limited availability of fossil resources, as well as the stringent requirement of a partial CO2 re-circulation in the atmosphere imposes an increased use of alternative fuels. The paper presents the performances obtained with a direct injection system with high pressure modulation when using different gasoline/methanol ratios.

The advantages of internal mixture formation regarding the important reduction of fuel consumption, respectively of the pollutant emission determined in the last years an intensive development of direct injection concepts. Superimposing the potential of direct injection with the advantages of ethanol concerning the availability and partial CO2 re-circulation, there appears to be an advantageous scenario for the future of SI engines. The paper presents a direct injection system with high pressure modulation which is able to operate with different ratios of gasoline/ethanol mixture. The paper presents furthermore a concept for modeling and optimization of the mixture formation process in the case of gasoline / ethanol direct injection.

The future development of two-stroke engines will be conditioned by the drastic reduction of pollutant emission, especially of hydrocarbon. This goal is not achievable only by scavenging improvement, rather a new quality of mixture formation using direct injection is imposed. However, the internal mixture formation in a large range of speed and load, considering the scavenge flow particularities of two-stroke engines as well, appears as an extremely complex process. Thereby a numerical simulation is in this case very effective for the adaptation of a direct injection method at the engine. The paper presents a concept for modeling and optimization of the mixture formation process within a high-speed two-stroke engine with liquid fuel injection system. The injection system generates a pressure pulse which is not dependent on the engine speed.

The paper presents a concept for modeling and optimization of the mixture formation process during gasoline direct injection, using a high-pressure single fluid injection system which allows the modulation of the injection rate independently on the engine speed. Going from this favorable premise for the adaptation of the mixture formation to various load and speed conditions, the aim of modeling is to find the optimum combination between the adaptable elements as follows: form of the fuel pressure wave, injection timing, spray form, injector location, form of the combustion chamber. Moreover, the interaction between fuel and air flow within the cylinder during the mixture formation is considered as a determining factor for the combustion process, and forms thereby an important part of the modeling.

The internal mixture formation within SI engines using fuel direct injection has a significant potential regarding the reduction of bsfc and pollutant emission. However the short time available for injection and spray distribution, as well as the complexity of the fluid dynamic conditions, amplified in a wide load and speed range, form a different base for the combustion process than using external mixture formation. The intend of the present study is to develop a method for modeling and optimization of mixture formation and combustion using a general approach for the fuel direct injection, which consist in the modulation of the injection rate, independently on the engine speed. In the first stage of modeling, the optimum combination between mixture formation elements as fuel pressure history, injection timing, spray characteristics, injector location or combustion chamber design is of great importance, forming the conditions for the subsequent combustion process.

The main development targets for future motorcycle and small vehicle propulsion units are the reduction of dimensions, weight, fuel consumption and pollutant emission for a considered power output. The paper presents a concept for the improvement of the thermodynamic process stages consisting on scavenging, mixture formation and combustion - as a main support for achieving the mentioned targets. The concept is exemplified by results in terms of compared indicator diagrams, specific fuel consumption and exhaust gas emissions - in base of numerical simulation and experimental analysis at the engine - respectively at the vehicle test bench.