The meredith ramjet: An efficient way to recover the heat wasted in piston engine cooling

Piston engines with power up to 1000HP (735.5 kW) are becoming popular in the aeronautical field for the high efficiency and the possibility to work with Diesel, jp4 and jp8 fuels up to altitudes up to 20,000m (65,000 ft). This paper updates a secondary advantage of piston engines: the Meredith effect. The Meredith duct is a ramjet powered by the heat wasted in cooling. In this way the efficiency of the original piston engine that can be higher than 50% which is the normal in common rail diesel engines. Even if the efficiency of this ramjet is lower than 30%, an accurate Design of this secondary engine can add a significant amount of thrust to the fan or the propeller powered by the piston engine. This effect, well known since the beginning of WWII, is being thoroughly analysed in this paper with regard of the efficiency. Previous papers introduced and a new radiator, with wing section tubes. As it will be seen the main design variable for the Meredith ramjet it the air temperature increase. This paper demonstrates that it is not convenient to increase this temperature over 200°C for aircrafts flying at about 600km/h@6000m (~330knots@20,000ft). The in-wing duct appears to be slightly better than the in-fuselage or the in-nacelle ones.