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The intake and exhaust lines provide the main abatement of the acoustic emissions of an Internal Combustion Engine (ICE). Many different numerical approaches can be used to evaluate the acoustic attenuation, which is commonly expressed by the Transmission Loss. One-dimensional (1D) and three-dimensional (3D) simulations are conventionally carried out only considering the acoustic domain of the muffler or of the air-box. The walls of the acoustic filter are considered fully rigid and the interaction between the acoustic waves and the structure is consequently negligible. Moreover, the effect of the manufacturing characteristics and the attenuation of the acoustic waves due to the fluid viscous-thermal effects are also commonly disregarded in the numerical analysis of the filters. In addition, the presence of a catalytic converter or a filter cartridge may have an influence on the numerical results.

In an Internal Combustion Engine, the design of the intake system is a very critical aspect since it affects both the engine power output and noise emissions at the intake side. In particular, downsized VVA engines typically produce higher gas-dynamic noise levels since, due to the intake line de-throttling at part-load, a less effective attenuation of the pressure waves is realized. In this work, the acoustic performance of the intake air filter of a commercial VVA engine is numerically and experimentally analyzed. In particular, a FEM model of the system is realized in order to compute the Transmission Loss (TL) parameter of the base device. The numerical analysis accounts of fluid-structure interaction, which gives the possibility to determine the effect of structure participation on the TL profile. Contemporarily, the experimental tests are performed on an acoustic test bench based on the multi-microphone technique for the evaluation of the acoustic parameters.

The acoustic performance of mufflers with single-inlet and single-outlet are well described using Insertion Loss (IL) and Transmission Loss (TL). These parameters represent the acoustic damping on the engine emission and on the incident pressure wave respectively. However, for mufflers with multi-inlet these parameters depend also on the sources characteristics, as consequence their use is quite difficult. In the present work the acoustic performance of a double-inlet and single-outlet muffler are experimentally evaluated in terms of reflection and transmission coefficients of each port of the muffler itself. These coefficients are used to evaluate the Insertion Loss of the manifold muffler taking into account specific sources on the inlets. The characteristic coefficients are also used to predict the acoustic emission of the manifold muffler using a known engine source on the two inlets.

In recent years, the motorcycle muffler design is moving to dissipative silencer architectures. Due to the increased of restrictions on noise emissions, both dissipative and coupled reactive-dissipative mufflers have substituted the most widely used reactive silencers. This led to higher noise efficiency of the muffler and size reduction. A dissipative muffler is composed by a perforated pipe that crosses a cavity volume filled by a fibrous porous material. The acoustic performance of this kind of muffler are strictly dependent on the porosity of the perforated pipe and the flow resistivity characteristic of the porous material. However, while the acoustic performance of a reactive muffler is almost independent from the presence of a mean flow for typical Mach numbers of exhaust gases, in a dissipative muffler the acoustic behaviour is strictly linked to the mass flow rate intensity.

In current automotive research, increasing attention is being paid to the design of mufflers due to the lower noise levels which have been established by the acoustic international standards. The traditional design approaches are no longer sufficient to meet the standards and more refined techniques are necessary. Within this context, a specific test rig was built at the Energy Engineering Department of the University of Florence to analyze the acoustic characteristics of both industrial mufflers and simplified models. In particular, the latter is commonly used to investigate in detail the physical phenomena connected to the acoustic response of these disposals and to calibrate numerical models. The test rig operates at ambient condition with no flow.

Increasing interest is being paid to noise pollution of internal combustion engines and as a result, recent international standards imposed more severe limitations to acoustic emissions on engine manufacturers. In particular, the noise coming from gas-dynamic interactions has an important influence in determining the final noise level of the engine; as a consequence, the muffler design is currently being considered as one of the most important research threads for engine companies. Within this context, the 1D approach to numerical simulations, which has been successfully applied by industrial designers to the fluid-dynamic design of the engine, is considered to be inaccurate in the evaluation of the acoustic behavior of the muffler for medium-high frequencies. On the other hand, an extension of the applicability of these codes in the medium-high frequencies would be desirable.

Modern injection systems are characterized by low cost, light weight and diversified components based on a mature technology. In addition, the constant growth of computational resources allows an in-depth understanding and control of the injection process. In this scenario, increasing interest is presently being paid to understand if an application of such technologies to small two-stroke engines could lead to a return to popularity in place of the more widespread use of the four-stroke engine. Indeed, the possibility of achieving a drastic reduction of both specific fuel consumption and pollutant emissions would completely reverse the future prospect of the two-stroke engine. The authors in previous studies developed a low pressure direct injection (LPDI) system for a 300 cm3 two-stroke engine that was ensuring a performance consistent with a standard four-stroke engine of similar size.

The small two-stroke engine represents a strategic typology of propulsion system for applications in which lightweight and high power density are required. However, the conventional two-stroke engine will not be compliant with forthcoming legislations about pollutant emissions and new solutions, such as electrification, are seriously taken into account by industry to overcome the two-stroke engine drawbacks. In this scenario, a promising way to allow the two-stroke engine to be competitive is represented by the use of direct injection systems, in order to overcome the long-standing issue of short circuiting fuel. The authors in previous studies developed a low-pressure direct injection (LPDI) system for a 300 cm3 two-stroke engine that was ensuring the same power output of the engine in carbureted configuration and raw pollutant emissions consistent with a four-stroke engine of similar performance.

In the last years, the engineering in the automotive industry is revolutionized by the continuous research of solutions for the reduction of consumptions and pollutant emissions. On this topic maximum attention is paid by both the legislative bodies and the costumers. The more and more severe limitations in pollutant and CO2 emissions imposed by international standards and the increasing price of the fuel force the automotive research to more efficient and ecological engines. Commonly the standard approach for the definition of the engine parameters at the beginning of the design process is based on the wide-open throttle condition although, both in homologation cycles and in the daily usage of the scooters, the engines work mainly at partial load where the efficiency dramatically decreases. This aspect has recently become strongly relevant also for two wheeled vehicles especially for urban purpose.

An approach for turbocharging automotive engines to reach targeted performance was developed in which the environmental and economic aspects during the turbocharger-engine matching process were considered. Three numerical assessment levels based on output performance, exhaust emissions and techno-economic metrics are established to support users during the decision-making of adequate turbochargers that meets targeted data in terms of boosting and emissions. Satisfactory improvements are measured from a 1.5L, three-cylinders, turbocharged Diesel engine, in terms of brake specific fuel consumption, thermal efficiency and NOx concentrations of about 1.73% (decrease in fuel consumption of around 2.22ml/s), 1.76%, and 4.53% (correspond to a diminution of around 217.54ppm), respectively, at the engine’s extreme conditions (full load and rated power).

In order to ensure a high level of performance and to comply with more severe limitations in term of fuel consumption and emissions reduction, a continuous supervision of the engine operating conditions, by monitoring several parameters, is needed. The growing use of turbocharger (TC) in ICE for automotive and industrial applications suggests to use the TC speed as a possible feedback of engine operating condition. Indeed, the turbocharger behavior is connected to the thermo-dynamic and fluid-dynamic conditions at the engine cylinder exit: this feature suggests that the turbocharger speed could give useful information about the engine cycle. In previous studies, a preliminary investigation of the relationship between the engine performance and the turbocharger speed of a four-stroke multi-cylinder turbo-diesel engine was carried out by varying the operating conditions of the engine such as fuel mass flow rate, EGR rate and back pressure at the turbine outlet.

The purpose of this work is to perform an analysis on the modifications necessary to convert a four-stroke engine into a non-conventional two-stroke engine. The first aim of this work is to reach the theoretical advantages of the two stroke engine (high torque values at lower rpm and working regularity) and, at the same time, to avoid the usual problems of the two stroke cycle (short-circuit of fresh air-fuel mixture and consequently pollutant emissions and high specific fuel consumption). The target is to develop a small engine with innovative solutions that allows to obtain high performance coupled with good mechanical and thermodynamic efficiency. The starting base engine is a 125cc four-stroke two-valves scooter engine equipped with a volumetric compressor. The idea is to convert it from four to two stroke cycle, using head valves and adding scavenge ports in the cylinder.

Cycle-to-cycle variation is one of the main factors for high fuel consumption and emissions of a two-stroke engine during the low-load and low-speed running. The increase of residual gas ratio due to the lower delivered amount of fresh scavenging air leads to a lower flame front speed and, therefore, to a slow combustion or even misfiring. The consequence is a very high level of unburnt hydrocarbons, since a large amount of fuel does not take part in the combustion process. The use of a direct injection system allows a more flexible management of the injection of fuel over subsequent engine cycles. Under a low-load condition, the low request in terms of brake mean effective pressure (BMEP) can be achieved by performing a load control based on an intermittent injection, thus reducing the need for intake throttling and avoiding the loss of fresh fuel resulting from cycles without combustion.

High specific fuel consumption and pollutant emissions are the main drawbacks of the small crankcase-scavenged two-stroke engine. The symmetrical port timing combined with a carburetor or an indirect injection system leads to a lower scavenging efficiency than a four-stroke engine and to the short-circuit of fresh air-fuel mixture. The use of fuel supply systems as the indirect injection and the carburetor is the standard solution for small two-stroke engine equipment, due to the necessity of reducing the complexity, weight, overall dimensions and costs. This paper presents the results of a detailed study on the application of an innovative Low Pressure Direct Injection system (LPDI) on an existing 300 cm3 cylinder formerly equipped with a carburetor. The proposed solution is characterized by two injectors working at 5 bar of injection pressure.

High specific fuel consumption and pollutant emissions are the main drawbacks of the small crankcase-scavenged two-stroke engine. The symmetrical port timing combined with a carburetor or an indirect injection system leads to a lower scavenging efficiency than a four-stroke engine and to the short-circuit of fresh air-fuel mixture. The use of fuel supply systems as the indirect injection and the carburetor is the standard solution for small two-stroke engine equipment, due to the necessity of reducing the complexity, weight, overall dimensions and costs. This paper presents the results of a detailed study on the application of an innovative Low Pressure Direct Injection system (LPDI) on an existing 300 cm3 cylinder formerly equipped with a carburetor. The proposed solution is characterized by two injectors working at 5 bar of injection pressure.

The paper investigates the influence of the fuel injection pressure on a small two-stroke engine with low pressure direct injection (LPDI). The authors in previous studies showed the benefits of the LPDI system in reducing the fuel short circuit, both from an experimental and numerical point of view. As a direct consequence, both the specific fuel consumption and the pollutant emissions were notably reduced, reaching the typical performance of a standard four-stroke engine of comparable size. The main drawback of the system is the limited time at disposal for delivering the fuel with difficulties in achieving a satisfactory air-fuel mixing and homogenization as well as fuel vaporization. In order to overcome the aforementioned issues, a detailed numerical analysis is carried out by performing a wide set of CFD simulations to properly investigate and understand the many complex phenomena occurring during the interaction between the injected fuel and the fresh scavenging air.