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The increase of performance has always been a key topic of the research activities on the internal combustion engines. Nowadays this is even truer as the performance is strictly correlated to the pollutant emissions. In this sense, an interesting approach could be the improvement of the effectiveness of engine control system and optimize the combustion process. To pursue this goal it would be very important to know the in-cylinder pressure during engine operation. The measurement of this quantity is performed generally with a pressure sensor flush mounted on the cylinder head. The measurement is very accurate, but the severe ambient conditions strongly limit the lifetime of these sensors, which, therefore, are not well suited to act as a feedback to the control system of on-road engines. Even though several approaches to measure indirectly the in-cylinder pressure have been developed, their diffusion is still hampered by reliability and sturdiness problems.

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.

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.

The main drawback of an in-cylinder Low Pressure Direct Injection (LPDI) in a two-stroke engine is the difficulty of achieving a satisfactory vaporization level in low load conditions. The liquid droplets are characterized by large diameters and, when the temperature level and the velocity of the scavenging flow field are low, the time needed for the droplet vaporization and the homogenization with fresh air becomes too long to guarantee a suitable mixture formation. A transfer port injection allows a higher flexibility, due to the possibility of performing a mixed injection either directly in the cylinder or indirectly in the crank case, depending on the load request or engine speed. Also, an even lower injection pressure can be adopted with respect to an in-cylinder LPDI injection, which is relevant in case of lightweight and low power applications. On the other hand, the time available for the direct in-cylinder injection is limited to the scavenge phase.

Two-stroke (2S) engines still play a key role in the global internal combustion engine (ICE) market when high power density, low production costs, and limited size and weight are required. However, they suffer from low efficiency and high levels of pollutant emissions, both linked to the short circuit of fuel and lubricating oil. Low- and high-pressure direct injection systems have proved to be effective in the reduction of fuel short circuiting, thus decreasing unburnt hydrocarbons and improving engine efficiency. However, the narrow time window available for fuel to be injected and homogenized with air, limited to few crank-angles, leads to insufficiently homogenized fuel-air mixtures and, as a consequence, to incomplete combustions. The use of prechambers can be a well-suited solution to avoid these issues.

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.

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.