Search Results

Two-stroke S.I. engine survival is submitted to direct fuel injection and charge stratification. An exhaustive activity concerning a 50 cm3 two-stroke S.I. engine with liquid direct injection and charge stratification has given really satisfactory results as regards engine aptitude to operate unthrottled at every speed and load. However, unthrottled operation does not necessarily lead to the best overall result. By CFD investigation and experimental tests, this paper proves that some throttling reduces HC and NOx emissions as well as pumping loss and increases exhaust gas temperature at light loads, with evident advantage for catalytic converter efficiency.

High-pressure liquid fuel injection is a suitable means to get either stratified charge or homogeneous charge for two-stroke engines. This paper shows the development of this solution for a small 50 cm3 engine for light motorcycles. By means of computational fluid dynamics, a combustion chamber suitable for proper fuel distribution in every engine operating condition has been designed. It has been realized, and experimental results confirm its fairly satisfactory behaviour, with good fuel economy, low exhaust emissions and small cycle-to-cycle variation even at light loads. Recent CFD studies indicate how to improve engine geometry to achieve a better stratification stability at partial loads independently on engine speed.

Direct fuel injection is becoming mandatory in two-stroke S.I. engines, since it prevents one of the major problems of these engines, that is fuel loss from the exhaust port. Another important problem is combustion irregularity at light loads, due to excessive presence of residual gas in the charge, and can be solved by charge stratification. High-pressure liquid fuel injection is able to control the mixing process inside the cylinder for getting either stratified charge at partial loads or quasi-stoichiometric conditions, as it is required at full load. This paper shows the development of this solution for a small engine for moped and light scooter, using numeric and experimental tools. In order to obtain the best charge characteristics at every load and engine speed, different combustion chambers have been conceived and studied, examining the effects of combustion chamber geometry, together with injector position and injection timing

Multi-Dimensional modeling was carried out for a Mercury Marine two-stroke DISI engine. Recently developed spray, ignition, and combustion models were applied to medium load cases with an air-fuel ratio of 30:1. Three injection timings, 271, 291 and 306 ATDC were selected to investigate the effects of the injection timing on mixture formation, ignition and combustion. The results indicate that at this particular load condition, earlier injection timing allows more fuel to evaporate. However, because the fuel penetrates further toward the piston, a leaner mixture is created near the spark plug; thus, a slower ignition process with a weaker ignition kernel was found for the SOI 271 ATDC case. The measured and computed combustion results such as average in-cylinder pressure and NOx are in good agreements. The later injection case produces lower NOx emission and higher CO emission; this is due to poor mixing and is in agreement with experimental measurements.

The paper deals with experimental activity concerning ATAC, which, in two-stroke gasoline engines, helps solving the crucial problem of combustion instability at light loads. ATAC consists of employing the energy of residual gas to prime an efficient combustion. The research is aimed to give further insight into ATAC mechanism both by visualisation of the combustion process and by examination of the influence which relevant parameters like air-fuel ratio, engine speed, compression ratio, scavenging passage design have on ATAC operation. Several results have been acquired and collected hitherto. A part of them are shown and discussed in this paper.

Computations were carried out to simulate in-cylinder flow field and mixture preparation of a small port scavenged direct-injection two-stroke spark-ignition engine using a modified version of KIVA-3 code. Simulations of the interaction between air flow and fuel were performed on a commercial Piaggio (125 cc) motorcycle engine modified to operate with a hollow-cone injector located in different positions of the dome-shaped combustion chamber. The engine has a large exhaust port and five smaller transfer ports connecting the cylinder to the crankcase. The numerical grid of this complex geometry was obtained using an IBM grid generator based on the output of engine design by CATIA solution. To take into account the rapid distortion of flow, the standard k-ε turbulence model in KIVA-3 was replaced by the RNG k-ε model.

A new original air-assisted, low-pressure, pumpless fuel injection system for two-stroke, S.I. engines is presented. The system does not annul the typical simplicity and low production cost of conventional two-stroke, crankcase scavenged, S.I. engines. Indeed the injection air is supplied by the crankcase and the whole design is based on simple and low-cost solutions. With reference to this system methodologies are also discussed to improve the behaviour of combustion at low loads.

Combustion and flow were calculated in a spark-ignited two-stroke crankcase-scavenged engine using a laminar and turbulent characteristic-time combustion submodel in the three-dimensional KIVA code. Both premixed-charge and fuel-injected cases were examined. A multi-cylinder engine simulation program was used to specify initial and boundary conditions for the computation of the scavenging process. A sensitivity study was conducted using the premixed-charge engine data. The influence of different port boundary conditions on the scavenging process was examined. At high delivery ratios, the results were insensitive to variations in the scavenging flow or residual fraction details. In this case, good agreement was obtained with the experimental data using an existing combustion submodel, previously validated in a four-stroke engine study.

A modification of the computational fluid dynamics code KIVA-II is presented that allows computations to be made in complex engine geometries. An example application is given in which three versions of KIVA-II are run simultaneously. Each version considers a separate block of the computational domain, and the blocks exchange boundary condition information with each other at their common interfaces. The use of separate blocks permits the connectedness of the overall computational domain to change with time. The scavenging flow in the cylinder, transfer pipes (ports), and exhaust pipe of a ported two-stroke engine with a moving piston was modeled in this way. Results are presented for three engine designs that differ only in the angle of their boost ports. The calculated flow fields and the resulting fuel distributions are shown to be markedly different with the different geometries.