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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.

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.

The potential of direct injection in spark-ignition engines regarding the significant reduction of fuel consumption and pollutant emission as well as advantages in the field of torque characteristic and acceleration behavior induced a remarkable development of GDI engines for automobile applications. The mentioned advantages are of special interest for motorcycle four-stroke engines as well. However, in compact cylinder units, respectively in a wide speed range as for motorcycle engines, the adaptation of direct injection gains on complexity. The paper presents the development results of a GDI four-stroke four-valve single cylinder motorcycle engine with 125 cc. The pressure pulse direct injection system developed for this application is presented from the fluid-dynamic behavior up to the obtained injection spray characteristics.

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.

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 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 manufacturers continually look for improved methods to design engines. Increased durability, enhanced engine performance, decreased emissions and low cost are all issues to consider during the early design stages. The purpose of this paper is to investigate the thermal flows and heat transfer phenomena occurring in the small engines and to suggest valid methods for their prediction to be used inside computer design software. A new approach to theoretically calculate the heat transfer based on the thermal vibrational convection theory is first proposed. The basic idea of this approach is that the heat transfer process can be correlated mainly with the thermal explosion and with the detonation wave produced by combustion. Later on, a simplified model based on energy balance method is investigated and its use in the engine computational software is proposed showing how it represents the best solution to develop a software procedure to simulate heat transfer.

A very stringent problem in most of European cities is the individual mobility. This problem is mainly caused by traffic jam and arising from this are two particularly interesting environmental issues: pollution and noise [1]. Use of two wheeler vehicles does not represent the final solution to these problems, nevertheless they can supply a useful aid to ease them. Recently, two stroke engines are being replaced with four stroke engines. For small capacity engines this means a true reduction in exhaust emissions, especially unburned hydrocarbons (HC), but, on the other hand it involves a performance reduction, particularly for sudden accelerations, which constitute an essential requirement for these vehicles [2, 3, 4, 5]. Hybridisation can help to fill the gap between two stroke engines and cleaner, but less performing four stroke engines [6]. At the same time, it can help to lower fuel consumption, by means of a reduction in the revolution speed [2, 5].

One of the characteristic features of two-stroke cycle engines is to have the cylinder filling phase with fresh charge over-lapped to the exhaust phase. This situation leads to an inevitable loss of fresh mixture through the exhaust port. The first part of this work deals with the possible methods of evaluating the percentage of fresh charge so wasted. The validity of the simplifying hypothe sis is verified by analysis both of gas composition inside the cylinder as a function of crankangle, carried out by electrovalve sampling, and hydrocarbon composition in the exhaust gases carried out by gaschromatogra-Phy. A calculation method for evaluating the short-circuiting loss in S.I. two-stroke engines fed with air only, with subsequent gasoline injection, is then described. In fact, this system seems to be a valid solution for the future development of this type of engine.