Influence of inlet temperature and hot residual gases on the performances of a mini high speed glow plug engine

Nowadays the power supplying systems have a fundamental importance for all small and portable devices. For low power applications, there are two main ways for producing power: electrochemical batteries and mini engines. Even though in recent years many developments have been carried out in improving the design of batteries, the energy density of 1MJ/kg seems to be an asymptotic value. If the energy source is a hydrocarbon fuel, whose energy density is 46 MJ/kg, with an overall efficiency of only 2.5 % it is possible to surpass the electrochemical batteries. On the other hand, having a mini engine, as energy source, implies three main problems: vibrations, noise and emissions.

A light (230 g) model airplane engine with a displacement volume of 4.11 cm₃ and a geometrical compression ratio of 13.91 has been studied. The work carried out in this paper can be divided basically in three parts. The first part consists of characterizing the behavior of this mini engine studying the variation of: IMEP, heat released, duration of the combustion, amount of residuals and emissions using six different propellers. After this first set of measurements, the exhaust pipe has been modified in order to increase the back pressure and keeping inside the cylinder a large amount of residual gases. With the new exhaust system and with the propeller that best suited the engine, the previous parameters have been measured for checking if an increment of the hot residuals can be a benefit or not. In the third part a new inlet has been designed and mounted on the engine in order to heat up the incoming charge and checking if the increment of the temperature combined with a high back pressure is able to decrease the emissions level without penalizing the performances of the model engine.