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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.

A typical issue of the two-stroke engine in monitoring the combustion process is to measure the actual burning mixture with a conventional 02-sensor placed in the exhaust duct. In fact, the short circuit of fresh charge affects the correct acquisition of the residual oxygen associated to the completeness of the combustion process, leading to the overestimation of the trapped air-fuel ratio. In a conventional homogenously scavenged two-stroke engine, a possible solution to the aforementioned issue is the direct measurement of the mass flow rate of both the intake fresh air and the fuel delivered by the fuel supply system. This methodology cannot be applied to 2S direct injection engine because air and fuel are not premixed. The paper shows the application of a methodology for the evaluation of the trapped air-fuel ratio of the mixture inside the combustion chamber in a small two-stroke low pressure direct injection (LPDI) engine.