Design and development of compressed air powered air engine

Abstract

Air engine technology is cutting-edge research in vehicle propulsion, fuel-efficient, renewable, and reduced environmental effects technology. Non-conventional and non-hazardous green fuel-air engine technology has proven promising future in the field of propulsion technology. In the air engine, the camless mechanism has created a research scope by replacing the camfollower with a variable valve train (solenoid valve). Valvetrain timing (cranking) plays an important role, as it affects the air enter and exhaust timings and, ultimately, the quantity of supplied air. In this research work, the study of the camless (variable valve actuation system) engine was carried out to maximize efficiency by reducing the excess compressed air supply. After the compressed air supply, maximum input energy can convert to useful work if the supplied air has sufficient time for complete expansion in the cylinder. Optimum supply of compressed air possible by controlling intake duration. Keeping this intake duration control as a prime aim to enhance the engine's performance, a dedicated prototype model was prepared. Prototype model developed with a novel pneumatic circuit using two 3/2 high switching frequency directional control valve (DCV). The model was developed with a dedicated control system to control DCV based on the feedback sensors data. Electronic control system built up with open-source microcontroller to have inbuilt data acquisition system to record and control the parameters. In the first part of the work, an optimum angle method was developed. Optimum angle method experiment conducted at different intake duration at various inlet pressure and optimum intake duration derived for each operating intake pressure. The experiments were conducted with the inlet pressure range from 3.25 to 5.5 bar, and intake duration varied from 600 to 1800 crank angle (CA). Further, the mathematical relationship is formulated to find the efficiency of the air engine cycle. Experimental results reveal that the engine's average efficiency increases up to 35.67 % by closing the inlet valve at an optimum angle. Each inlet pressure has a specific optimum intake duration to have maximum efficiency and work done per cycle. A novel pneumatic circuit developed and used in the experiments reduces the movement of compressed air and friction losses. It ultimately facilitates to have higher RPM (Revolution per minute) of the crankshaft. iii In the second phase of the work, the same test bench was used to experiment with a modified algorithm. Air engine performance improves with the maximum expansion of air in the cylinder, and it reflects as near to atmospheric pressure at the exhaust port. With this as a base new algorithm was developed, which uses feedback of calculated predictive exhaust pressure. Once the predictive exhaust pressure reaches near atmospheric pressure, the controller passes the signal to control the intake duration. Thus, a new method (predictive exhaust pressure) reduces the dependency of variation in intake pressure and reduces the exhaust port's exergy losses, and increases the system's efficiency. The experiment conducted for pressure ranges from 3 to 6.5 bar. The theoretical cycle where air continuously supplies from 00 to 1800 CA is compared with the predictive exhaust pressure cycle (Case-II) results. Results show the average efficiency improvement of 11.91% in case-II compared to case-I and also found improvement in work-done and RPM of the cycle. Additionally, the above two methods of controlling intake duration have their benefits and limitation too. So for detailed analysis, both methods were compared based on all key performance parameters. Results reveal that limiting the data transfer frequency of sensors and controllers became a barrier to achieving the result equivalent of the optimum method in the predictive exhaust pressure method. Furthermore, along with intake sustain angle (intake duration), exhaust advance angle (EAA) have equal importance in the air engine cycle operation. Herewith EAA analyzed concerning the performance of the air engine system. Thus, the control system in a camless air engine increases the air engine's feasibility as one of the propulsion technologies in a city vehicle and even in the Compressed Air Energy Storage System (CAES). Keywords: Air engine, variable valve train, camless, efficiency, predictive exhaust pressure, optimum intake duration.