Investigating the effect of engine speed and flight altitude on the performance of throttle body injection (TBI) system of a two-stroke air-powered engine

Abstract

Two-stroke engines are one of the most commonly used engines for unmanned aerial vehicle (UAV) propulsion, which has benefits such as high power, low fuel consumption. Before, most of the two-stroke engines used in UAVs were the carburettor, but nowadays, carburettor engines are not suitable for UAVs because of stringent standards such as high flight endurance, tolerance of extreme environmental conditions. Also, the solution to meet these expectations is the use of electronic fuel injection systems. In this study, the performance of the fuel injection system in the throttle body injection (TBI) of the two-stroke air-powered engine with the capacity of 30 cc used in the UAV is investigated. Therefore, the performance of the TBI system and the engine is simulated using the lotus engine simulation (LES) software. The obtained results (mass flow rate of air and fuel, the pulse width of the injector) are used as the initial and boundary conditions of numerical simulation of the TBI system using computational fluid dynamics (CFD) to simulate and check the airflow and fuel injection in the TBI system. Design features pressure drop in the throttle body, turbulence, speed-pressure distribution in flow, fuel injection and droplet distribution, and mixing them with air, were analysed in the TBI system. First, the air flow is simulated in a Samorfeld rotary burner; where the k−w SST turbulence model is found suitable for airflow modelling in the throttle body. In the next step, to validate the fuel injection modelling, a non-reactive fuel spray was simulated in Sandia burner, and it is observed that the results of fuel injection modelling using the Kelvin–Helmholtz and Riley–Taylor failure model were well-precise to verify the hypothesis and models used. Then the performance of the TBI system was simulated in various operating conditions (1000, 3000, 6000 and 9000 RPM, flying heights of 0 to 20,000 feet). The results of the study of the effect of engine speed on the pressure drop and turbulence intensity of the flow turbulence showed that with the increase in engine speed from 1000 to 9000, the intensity of the turbulence of the flow in the throttle body was increased. Also, with the increase in engine speed, the power of the recycling area was increased. According to the results, the air pressure in the throttle body will decrease with increasing the height. In the study of the effect of the engine speed on the fuel injection characteristic, it was observed that in the 30 degrees throttle and the high speed due to the presence of strong recycling area, the turbulence intensity was high, and the deviation of the droplets from the direction of injection and the dispersion of droplets was also higher.