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 injection nozzles are located in the liner wall at the opposite side of the exhaust port, with the axes oriented towards the piston at the BDC (Bottom Dead Center). The injectors are positioned above all of the cylinder ports in order to have an injection timing independent from the ports timing. The only constraint to the injection timing is related to the piston motion, with great benefits in terms of short-circuit reduction in all of the engine operating conditions.Numerical and experimental activity was carried out in order to identify the best injector configuration and to assess the system performance. The results of the preliminary 3-D CFD study are here reported. The best positioning of the injectors was determined at the operating condition of maximum power by means of a sensitivity analysis of the circumferential position and spray orientation. The performance comparison was evaluated in terms of fuel short-circuit and spray vaporization. The optimized configuration was also verified at lower revolution speeds and partial loads showing a great potential for all of the working conditions. The experiments on a real prototype on the test bench confirmed the predictions showing significant improvements in comparison with the original configuration.

Two-stroke engines complete the process cycle in one crankshaft revolution: the scavenging process takes place when the piston is close to the bottom dead center, with the opportunity to open and close the cylinder ports by means of the piston motion, greatly reducing the number of moving parts. This solution however, typically used in small engines, imposes a symmetrical timing with respect to the bottom dead center, leading to a lower scavenging efficiency than a four-stroke engine.Except for the short rpm range of dynamic tuning, two-stroke engines are affected by the short-circuit of fresh air-fuel mixture during the scavenging process: this phenomenon results in a fuel loss, subsequent lower torque and higher specific consumption, and also in an inevitable increase in pollutant emissions.This paper presents one possible mechanical solution to reduce the short-circuit in the whole rpm engine range, to keep the typical advantages of two-stroke engines (simple construction, high specific power and working regularity for a single cylinder engine of a given displacement) and, at the same time, to avoid the usual problems of the two-stroke cycle.An asymmetric timing of the exhaust port is certainly a benefit, and for this reason an innovative design solution was conceived: a rotating valve, directly driven from the crankshaft, was positioned in correspondence with the exhaust port. During every cycle, this valve prevents the leakage of the fresh charge from the exhaust port in the last phase of the scavenging process. At the same time, thanks to its particular geometry, it allows the exhaust flow during the discharge. In other words, the valve converts the typical fluid dynamic effect of the outlet overpressure wave into a mechanical system for all rpm range and not only for the tuning speed.The benefits of this solution were analyzed both in terms of global performance with a 1-D simulation code, and of fluid dynamics behavior of the system through 3-D CFD simulations. The main results, presented in this paper, show significant improvements when compared to analogous traditional two-stroke engines.

The Formula SAE is an international competition for vehicle fully designed and built by students from worldwide Universities. The engine and vehicle design in the Formula SAE competition has to comply with a strict regulation. Regarding the engine intake line an air restrictor of circular cross-section no greater than 20 mm must be fitted between the throttle valve and the engine inlet. The aim of the throat is to limit the engine air flow rate as it strongly influences the volumetric efficiency and then the maximum power. The present paper is focused on the design of the engine intake system of the Firenze Race Team vehicle in order to optimize its performance in terms of both the maximum power and the drivability of the vehicle. One of the typical solutions for limiting the air restrictor influence consists of a plenum chamber placed along the intake line downstream of the restrictor. However the plenum involves also a delay in the engine response during the transient phases. The greater is the plenum, the lower are the power losses but the greater is the engine response delay. Taking advantage of a calibrated 1D model of the engine and a simplified vehicle model, the authors numerically analyzed an innovative solution that is constituted by a variable length duct inside the plenum. When the duct is at the maximum extension, the plenum is excluded from the intake line improving the engine response time. The optimization of the plenum volume and the definition of a preliminary control logic of the innovative system were done in order to obtain the maximum advantages in terms of both performance and engine drivability

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 injection nozzles are located in the liner wall at the opposite side of the exhaust port, with the axes oriented towards the piston at the BDC (Bottom Dead Center). The injectors are positioned above all of the cylinder ports in order to have an injection timing independent from the ports timing. The only constraint to the injection timing is related to the piston motion, with great benefits in terms of short-circuit reduction in all of the engine operating conditions.Numerical and experimental activity was carried out in order to identify the best injector configuration and to assess the system performance. The results of the experimental study are reported here. Standard low cost components from the automotive market were used, without a significant increase of weight. The benefits of the innovative two-stroke LPDI engine were evaluated at the test bench and compared with the performance of the configurations equipped with the carburetor and the indirect injection system.Once the position of the injectors inside the cylinder was defined, the development of the system at the test bench was focused primarily on the definition of the best injection timing, also taking advantage of the numerical results of the CFD analysis.The experimental setup and the fine-tuning methodology are discussed in detail. The goal was to reach the best thermodynamic efficiency and the lowest pollutant emissions of the LPDI system while ensuring the same engine power with respect to the other two configurations, without increasing the manufacturing costs.

Recent innovations in engine control and diagnostics are providing room for development of innovative combustion approaches (e.g., low-temperature combustion) able to minimize the creation of pollutants. To ensure the constant fulfillment of the prescribed thermodynamic conditions, however, a fast real-time monitoring of the in-cylinder pressure is needed. To this end, dynamic pressure sensors, flush-mounted on the cylinder head, are commonly used. With this approach, the measurement accuracy is high, but the durability is limited by the harsh working conditions. The installation on the cylinder head is also complex. The development of robust and effective indirect measurement systems could then represent the enabler of a further development of this technology. In the present study, an innovative methodology to measure the in-cylinder pressure has been conceived and extensively tested on a four-stroke single-cylinder engine. The proposed approach is based on the analysis of the mechanical stress on the engine studs by means of a piezoelectric strain washer. This solution allows the user for a rapid and cost-effective sensor installation, described in the paper along with the signal post-processing techniques. Results showed good accuracy and robustness of the methodology, making the results of practical use for engine control.

Real-time estimation of the in-cylinder pressure of combustion engines is crucial to detect failures and improve the performance of the engine control system. A new estimation scheme is proposed based on the Extended Kalman Filter, which exploits measurements of the engine rotational speed provided by a standard phonic wheel sensor. The main novelty lies in a parameterization of the combustion pressure, which is generated by averaging experimental data collected in different operating points. The proposed approach is validated on real data from a turbocharged compression ignition engine, including both nominal and off-nominal working conditions. The experimental results show that the proposed technique accurately reconstructs the pressure profile, featuring a fit performance index exceeding 90% most of the time. Moreover, it can track changes in the engine operating conditions as well as detect the presence of cylinder-to-cylinder variations.