RECENT IMPROVEMENTS IN PISTON ENGINE PERFORMANCE FOR A NEW GENERATION OF AIRCRAFT PROPULSION SYSTEMS

Recent developments in piston engine technology have increased performance in a very significant way. In this paper a new generation of piston engines developed by our design team due to fly within the second decade of the century are introduced and are compared to up-to-date turboprops and piston engines. At first a new generation of spark ignition engines is introduced. These engines, fuelled by standard automotive petrol, are available in aspirated, supercharged and turbocharged versions up to 600 HP (V12 supercharged). In this case power to weight ratio may reach 0.45 kg/kW (0.30 kg/HP). The weak point of the spark ignition engines is the specific fuel consumption always above 270 gr/kWh (195 gr/HPh). Diesel turbocharged/turbo compound engines, fuelled by jet fuels, have different performance. They can develop power up to 4000 CV (hybrid version – take off), the weight to power ratio is slightly worse than spark ignition engines, with figures about 0,5 kg/kW (0.4 kg/HP) up to 0.21 kg/kW (0.16 kg/HP-VD007 hybrid version) with much better specific fuel consumption 190 gr/kWh (140 gr/HPh). The weak point of the common rail engine is the high pressure direct injection fuel pump that, in some cases, may cause reliability problems and in all cases the common rail systems have durability problems. A solution is the electronically controlled injector-pump technology with low friction treatments in the most stressed component. This approach is very expensive and may be adopted only in very large engines, mainly of the hybrid type. This latter diesel – turbine hybrid power units- combines the advantages of the turboprop/turbofan/turboshaft and piston turbo-diesel-prop / turbo-diesel-fan/turbodiesel engines. Hybrid engines can be used for executive airplanes with fans installed (for example the Citation X) or on the traditional turboprop aircraft (for example the C130J or the ATR 42/72). These large diesel-turbine hybrid engines may compete with up-to-date turboprops and turbofans. Turboprops and turbofans have usually better maximum thrust (up to 40%). However diesel and hybrid diesel-turbine engine have a much better efficiency (up to 40%) with the fuel consumption strictly proportional to output power. In diesel powered aircrafts APUs can be avoided. The much better fuel performance compensates the lack of thrust; the true problem is the reliability of the hybrid system, that is much more complicated than the traditional turbine-based engines. Data are available from the automotive field and indicate an average reliability of 90% in 3000 h. This figure is insufficient for aircraft, however automotive engine are not designed for aircraft usage, improvement in the reliability field are easily obtainable by using off-the-shelf aeronautical components and a proper quality strategy