Enrico Mattarelli

CFD Analyses on 2-Stroke High Speed Diesel Engines

In recent years, interest has been growing in the 2-Stroke Diesel cycle, coupled to high speed engines. One of the most promising applications is on light aircraft piston engines, typically designed to provide a top brake power of 100-200 HP with a relatively low weight. The main advantage yielded by the 2-Stroke cycle is the possibility to achieve high power density at low crankshaft speed, allowing the propeller to be directly coupled to the engine, without a reduction drive. Furthermore, Diesel combustion is a good match for supercharging and it is expected to provide a superior fuel efficiency, in comparison to S.I. engines. However, the coupling of 2-Stroke cycle and Diesel combustion on small bore, high speed engines is quite complex, requiring a suitable support from CFD simulation. In this paper, a customized version of the KIVA-3v code (a CFD program for multidimensional analyses) has been used to address ports and combustion chamber design of a new project (a 3-cylinder 1.8L engine, with a power rating up to 150 HP). Multidimensional calculations have been supported by 1D engine cycle analyses, using GT-Power. Two types of combustion-scavenging system have been considered, both of them featuring direct injection: a configuration with exhaust poppet valves and another one with piston controlled ports. A development of both projects has been performed through a coupled 1d-3d computational approach. A first set of KIVA calculations has been performed, in order to characterize the scavenging and the port flow patterns of both configurations, considering three different operating conditions, representative an aircraft engine. Then, several combustion simulations have been run, for defining two chambers able to match the project goals (high fuel efficiency, limited in-cylinder peak-pressure). For the two best configurations, the most interesting calculation results are presented in the paper.

1D Engine Simulation of a Small HSDI Diesel Engine Applying a Predictive Combustion Model

The paper analyzes the operations of a small high speed direct injection (HSDI) turbocharged diesel engine by means of a parallel experimental and computational investigation. As far as the numerical approach is concerned, an in-house 1D research code for the simulation of the whole engine system has been enhanced by the introduction of a multizone quasi-dimensional combustion model, tailored for multijet direct injection diesel engines. This model takes into account the most relevant issues of the combustion process: spray development, air-fuel mixing, ignition, and formation of the main pollutant species (nitrogen oxide and particulate). The prediction of the spray basic patterns requires previous knowledge of the fuel injection rate. Since the direct measure of this quantity at each operating condition is not a very practical proceeding, an empirical model has been developed in order to provide reasonably accurate injection laws from a few experimental characteristic curves. The results of the simulation at full load are compared to experiments, showing a good agreement on brake performance and emissions. Furthermore, the combustion model tuned at full load has been applied to the analysis of some operating conditions at partial load, without any change to the calibration parameters. Still, the numerical simulation provided results that qualitatively agree with experiments.

Optimization of a Supercharged Single Cylinder Engine for a Formula SAE Racing Car

The paper reviews the development and optimization of a SI high performance engine, to be used in Formula SAE/Student competitions. The base engine is a single cylinder Yamaha 660cc motorcycle unit, rated at about 48 HP at 6000rpm. Besides the reduction of engine capacity to 600cc and the mounting of the required restrictor, mechanical supercharging has been adopted in order to boost performance. The fluid-dynamic optimization of the engine system has been performed by means of 1D-CFD simulation, coupled to a single-objective genetic algorithm, developed by the authors. The optimization results have been compared to the ones obtained by a well known commercial optimization software, finding a good agreement. Experiments at the brake dynamometer have been carried out, in order to support engine modeling and to demonstrate the reliability of the optimization process.

Virtual design of a novel two-stroke high-speed direct-injection diesel engine

Abstract The paper reviews the virtual design process of an automotive high-speed direct-injection (HSDI) two-stroke diesel engine, developed at the Department of Mechanical and Civil Engineering (DIMeC) of the University of Modena and Reggio Emilia. The new concept of the engine is the tumble-supported and spray-controlled combustion system, which enables the adoption of loop scavenging without valves, and the use of a very simple and compact combustion chamber, carved in the engine head. The concept has been applied to a three-cylinder engine, with a capacity of 1050 cm3, supercharged by means of a Roots compressor and a variable geometry turbocharger. Some alternative configurations have been defined. Integrated one-dimensional and multidimensional computational fluid dynamics (CFD) simulations have been performed in order to optimize the main engine parameters, as well as to predict brake performance and emissions, in comparison with a reference four-stroke automobile diesel engine. Simulation results demonstrate the potential of the concept, which may be applied to develop a new generation of ultra-compact and clean automotive diesel engines.

Development and calibration of an enhanced quasi-dimensional combustion model for HSDI diesel engines

The paper describes the development and validation of a quasi-dimensional combustion model, applicable to any type of high-speed direct-injection (HSDI) diesel engines. In this model, the fundamental in-cylinder processes are taken into account, including turbulence, fuel injection, spray dynamics, ignition, and combustion. In comparison with similar models presented in the literature, a more physical description of average in-cylinder turbulence properties and their interaction with spray dynamics is introduced, as well as a detailed modelling of fuel jet wall impingement. Some experimental measures available in the literature and three-dimensional computational fluid dynamics simulations were considered to calibrate the modelling parameters. These improved sub-models make results accuracy less dependent on the calibration carried out on each engine, so that the same parameter setting can be successfully applied to different combustion chamber configurations. The model was first applied to a small HSDI turbocharged diesel engine. The specific calibration was supported by both experimental and simulation results, the latest being obtained from the three-dimensional computational fluid dynamics analyses. Then, a different diesel engine was simulated, adopting the same set-up of the model parameters. For both engines, the comparison between experiments and simulation showed a very good agreement in terms of in-cylinder pressures and heat release rates, as well as of average in-cylinder turbulence properties. It is worth mentioning that the two engines had a quite different unit displacement, i.e. 312 and 697 cm3, respectively. As a conclusion, this model was demonstrated to be a reliable tool for addressing the optimization of the main engine design parameters, such as injection rates and timings, combustion chamber base geometry, and so forth.

The Modular Engine Concept: a Cost Effective Way to Reduce Pollutant Emissions and Fuel Consuption

A promising technique to enhance fuel efficiency of large capacity S.I. engines is the de-activation of some cylinders at partial load, through the cut-out of fuel metering and a specific control of the airflow. Thanks to the ensuing reduction of throttling losses (the active cylinders operate at a much higher load), fuel consumption can be reduced, without any negative perception from the driver. Such a technique has been already applied successfully on some production engines, at the cost of some additional complication on the valve-train system. The application analyzed in this study is a little bit different, being aimed to reduce both fuel consumption and emissions, with a minimum impact on engine design. Larger fuel savings may be obtained by coupling the cylinder de-activation with VVT. However, the most important advantage of the modular engine concept proposed in this paper is in terms of emissions: this study demonstrates that the light-off time of the catalysts may be strongly reduced, and a further improvement is obtained by doubling the effective surface of the catalytic bed. The study has been carried out on a conventional SI 4.2L V8 engine. The first step of the analysis has been the experimental validation of a 1D-CFD model of the engine, achieved with a very good accuracy at both full and partial load. Then, the engine has been simulated on a grid of 15 operating points, representing the usage in the New European Driving cycle. The following configurations have been analyzed and compared to the base engine: 4 active cylinders, 3 active cylinders; 4 active cylinders and optimization of valve timings; 3 active cylinders and optimization of valve timings.

Numerical Analysis of Swirl Control Strategies in a Four Valve HSDI Diesel Engine

Recent four value HSDI Diesel engines are able to control the swirl intensity, in order to enhance the in-cylinder flow field at partial load without decreasing breathing capabilities at full load. Making reference to a current production engine, the purpose of this paper is to envestiage the influence of port design and flow-control strategies on both engine permeability and in-cylinder flow field. Using previously validated models, 3-D CFD simulations of the intake and compression strokes are performed in order to predict the in-cylinder flow patterns originated by the different configurations. The comparison between the two configurations in terms of airflow at full load indicates that Geometry 2 can trap 3.03% more air than Geometry 1, while the swirl intensity at IVC is reduced (−30%). The closure of one intake valve (the left one) is very effective to enhance the swirl intensity at partial load: the Swirl Ratio at IVC passes from 0.7 to 2.6 for Geometry 1, while for Geometry 2 it varies from 0.4 to 2.9.Copyright

MotoGP 2007: Criteria for Engine Optimization

The paper proposes some design criteria for the MotoGP engines, complying with the FIM 2007 Technical Regulations. Five configurations have been considered: engines with three cylinders in line and four cylinders in line, and three V engines with four, five, and six cylinders. All the analyzed solutions have been optimized from a fluid-dynamic point of view by means of one dimensional engine cycle simulations. Then, the engines are compared in terms of full load performance at steady conditions. Finally the influence of engine performance, along with operation regularity and motorbike weight, is assessed by means of a lap time simulator, developed by the author on the base of real data. The best configurations turned out to be the four-cylinder engines, while three-cylinder and five-cylinder engines are quite penalizing. The key of the four-cylinder engines success is their good breathing capability and mechanical efficiency at high speed, yielding an optimum power-to-weight ratio, associated with a good engine regularity, i.e., a smooth response to throttle angle variations.

Advances in The Design of Two-Stroke, High Speed, Compression Ignition Engines

An interesting concept in order to meet the conflicting requirements mentioned above is the 2-Stroke cycle combined to Compression Ignition. Such a concept is widely applied to large bore engines, on steady or naval power-plants, where the advantages versus the 4-Stroke cycle in terms of power density and fuel conversion efficiency (in some cases higher than 50% [1]) are well known. In fact, the double cycle frequency allows the de‐ signer to either downsize (i.e. reduce the displacement, for a given power target) or “down-speed” (i.e. reduce engine speed, for a given power target) the 2-stroke engine. Furthermore, mechanical efficiency can be strongly improved, for 2 reasons: i) the gas ex‐ change process can be completed with piston controlled ports, without the losses associ‐ ated to a valve-train; ii) the mechanical power lost in one cycle is about halved, in comparison to a 4-Stroke engine of same design and size, while the indicated power can be the same: as a result, the weight of mechanical losses is lower.

Combustion optimization of a marine di diesel engine

Enhanced calibration strategies and innovative engine combustion technologies are required to meet the new limits on exhaust gas emissions enforced in the field of marine propulsion and on-board energy production. The goal of the paper is to optimize the control parameters of a 4.2 dm3 unit displacement marine DI Diesel engine, in order to enhance the efficiency of the combustion system and reduce engine out emissions. The investigation is carried out by means of experimental tests and CFD simulations. For a better control of the testing conditions, the experimental activity is performed on a single cylinder prototype, while the engine test bench is specifically designed to simulate different levels of boosting. The numerical investigations are carried out using a set of different CFD tools: GT-Power for the engine cycle analysis, STAR-CD for the study of the in-cylinder flow, and a customized version of the KIVA-3V code for combustion. All the models are calibrated through the above mentioned experimental campaign. Then, CFD simulations are applied to optimize the injection parameters and to explore the potential of the Miller combustion concept. It is found that the reduction of the charge temperature, ensuing the adoption of an early intake valve closing strategy, strongly affects combustion. With a proper valve actuation strategy, an increase of boost pressure and an optimized injection advance, a 40% reduction of NOx emissions can be obtained, along with a significant reduction of in-cylinder peak pressure, without penalizing fuel efficiency.