Search Results

The present work addresses the possibility to correctly simulate Partial Homogeneous Charge Compression Ignition (PHCCI) combustion, obtained by the application of EGR up to 60% without using detailed kinetic models. In particular, the laminar and turbulent time characteristic model has been analyzed and improved. The study illustrates the prediction capabilities that can be achieved with such an approach. The paper reports the results obtained from the simulation of a single cylinder research engine and a four-cylinder diesel engine to verify the validity of the proposed method independently of engine geometry and configuration. All numerical results are compared with experimental pressure traces and rates of heat release, as well as with NOx and soot emissions over a wide range of operating conditions. With the modified characteristic time model, realistic simulations of engine combustion up to EGR values of about 60% have been obtained for both engines.

It is well established that the combustion development and the consequent pollutant emissions in a Diesel engine depend on the fuel spray characteristics. Nowadays, many new injection strategies are being tested, in order to improve the diesel combustion efficiency and so to reduce its environmental impact in terms of pollutant emissions and fuel consumption. Many works have proved that NOx, THC, CO and particulate levels emitted from a Diesel engine, as well as specific fuel consumption, depend, for a given engine speed and load, upon the parameters characterizing any particular injection strategy. Despite the extensive experimental activity analyzing the effects of the previous control parameters on the engine emission and fuel consumption levels, very few are the works dealing with the effect of the same parameters on the fuel spray, and in particular on the droplet dimension distribution. The aim of this work is therefore to study these effect.

The instantaneous power carried out by the frequency components of the in-cylinder pressure results from the rate of energy released during the whole development, and in particular the first stage, of the Diesel combustion. On the other hand, the non-linear pressure fluctuations, caused by the steeped fronted waves that develop after the mixture ignition, characterize predominantly the first stage of the combustion development in Diesel engines. These pressure waves are transmitted to the engine block and then to its surface according to the block damping characteristics. The aim of the present work is then to analyze in detail the relation existing between the block vibration signal and the in-cylinder pressure, and, in this way, to monitor continously the behaviour and the quality of the combustion. Therefore, a series of experiments has been performed on a direct injection common rail Diesel engine, varying the combustion characteristics.

The effects of exhaust gas recirculation (EGR) cooling, in combination with a partially-premixed charge obtained by a very advanced fuel injection (early injection), on the combustion of a Common-Rail Diesel engine, have been experimentally investigated. The combustion behavior, for different injection strategies, has been analyzed as well as the emission levels. Experimental tests have been carried out on a direct injection Fiat 1929 cm3 Common Rail Diesel engine. Two different injection strategies have been investigated: the first one was obtained performing pilot and main injections only, the second one adding an early injection. The effects on engine combustion of hot and water cooled EGR have been compared with the combustion behavior without EGR, also showing the effect produced by different pilot injection timings and quantities.

An existing simulation code of a COMMON RAIL Diesel fuel injector, named UNIJET, has been revised and completed, with the aim to predict injection characteristics and to investigate on the instabilities which appeared in experimental tests. Particular attention has been given to the injector control valve modeling, to understand the influence of cavitation on discharge flow characteristics. The valve residual motion, after a Pilot Injection cycle, is responsible for the clear and sudden injected quantity variations of subsequent Master injection, as the time delay between the two cycles decreases. Numerical predictions have been compared with valve lift measurements, and with experimental relation among fuel injected quantity, injection pressure and duration.

An axi-symmetric, five-hole V.C.O. nozzle mounted on an electro-injector was used to analyze the spray produced by each hole varying feeding pressure, injected quantity and timing. An advanced experimental apparatus has been used to investigate particle size and velocity of the sprays. The investigation comprized a hydraulic characterization, a photographic one with back-light technique, the analysis of the droplet size distribution through laser diffraction technique via MALVERN 2600 SERIES and the analysis of particle size and velocity using PDPA technique (AERMETRICS DSA 4000) with the extremely dense spray produced by the common-rail electronically controlled injection system. Four feeding pressures (25, 30, 90, 120 MPa) and five injected quantities (2, 4, 10, 25, 40 mm3/shot) were chosen to characterize the nozzle behavior. Sprays from different holes appeared different in shape but very similar in the droplet size distribution.

A simulation code of an innovative electro-injector for Diesel engines is presented with the preliminary analysis carried out using the code. The simulation code is based on the concentrated volume method. The energy and continuity conservation equations and dynamic equations are used for the movable parts of the system under friction. The one dimensional code simulated the propagation in the feeding pump and the control of the electro-injector. The program uses the method of characteristics to solve conservation equations, simulating the propagation in the pipe between the two chambers. To go deeply into the study of the electro-injector, main routine tests were carried out checking the exact value of diesel fuel parameters and the fuel energy losses with stationary and instationary flows. A comparison with different experimental results obtained by different types of electroinjectors, running at real conditions, has been made with good agreement.

The spray of a 5-hole, V.C.O. nozzle for D.I. diesel engines has been studied; the data obtained from each hole were compared. A high pressure injection system (Elasis patent), electronically controlled, was used. To investigate the behaviour of each hole a “cap” was used to cover the nozzle in such a way that only one hole at a time could inject in the spray chamber. The investigation consisted of a photographic characterization with back-light technique and of the droplet size distribution through laser diffraction technique via Malvern. Four pressure values (30, 60, 90, 120 MPa) and three injected flowrates (10, 25, 40 mm3/inj.) were chosen to characterize the behaviour of the nozzle. Each test was carried out with the nozzle under normal running conditions, measuring the laser signal background after each acquisition.

The study of the spray of standard, five-hole, V.C.O. nozzles for D.I. diesel engine has been carried out. Several pictures of the spray have been taken using the back-light photography technique. Injected volume, feeding pressure, injection duration, spray penetration, angle and thickness, and flash delay from the start of injection have been measured both for nozzles of two different manufacturers and for a group of four injectors before and after a fatigue test of about 1500 hours running. Measurements revealed that each spray strongly depends on the needle lift, the back-pressure and the nozzle geometry and that, for the same injector, sprays are totally different from hole to hole. The causes of the most important differences have been attributed at the different hole inclination, their asymmetric feeding conditions (due to the eccentricity between needle and nozzle) and the microdefects caused by drilling operation.

A simulation code of an electro-injector for Diesel engines is presented with the preliminary parametric analysis carried out with the code. The simulation code is based upon the concentrated volume method as for the chambers of the system. Energy and flowrate conservation equations and dynamic equations are used for the movable parts of the system under stress or friction. The magnetic force acting on the electro-injector actuator has been calculated by means of a Finite Element simulation. The one-dimentional code simulated the propagation in feeding pipes and the control of the electro-injector. The program, infact, uses the method of the characteristic equations to solve conservation equations, simulating the propagation in a pipe between two chambers. The sensitivity analysis has pointed out that the parameters which are influenced by the propagation in the pipes are: needle lift, injected flow-rate, pressure in each chamber and volume.