Aviation 100LL (100 octane, low-lead) gasoline is produced in relatively small quantities, contains lead, is expensive and difficult to obtain outside the developed world. Jet fuel is widely available, cheaper and has higher energy density than aviation gasoline. The turboprop or turbojet engine solutions with weight and reliability advantages over reciprocating engines, however, are too expensive for much of general aviation and the fuel burn is higher. For these reasons, considerable effort has been expended to develop jet fuel burning aviation diesel engines.
Propeller speeds in general aviation engines are generally below 2700 rpm. The standard spark ignition general aviation engine is direct drive and has a large displacement in order to develop the necessary power at propeller speeds. Because of the added mass needed to constrain the increase in pressures of diesel engines, much of the development for general aviation has focused on geared designs to produce smaller engines running at higher rpm to generate the necessary power at propeller speeds while meeting the weight requirements for general aviation.
Despite work in this area spanning several decades, there is an impasse in the development of geared aviation diesel engines with greater than 4-cylinders and 180 hp. A major reason for this is torsional vibration. Simulation studies show that 4-stroke, 6 and 8-cylinder diesel engines with standard gear reduction exhibit torsional stresses much greater than 4-cylinder engines or any of the other 4-stroke engines spanning 4 to12 cylinders. Six-cylinder, 4-stroke engines with power transmission through the camshaft or a dedicated internal driveshaft, however, have the lowest torsional stresses of 4-stroke engines with either engine configuration spanning 4 to 12 cylinders. The increase in power train length with power transmission through the cam drive shaft results in a decrease in stiffness of the powertrain. This in concert with a 4-S, 6-cylinder engine configuration results in a salutary combination of first mode natural frequency and excitatory harmonics that is not seen in any of the other engine configurations. The 4-S, 6-cylinder cam drive shaft engine may represent the best geared design to achieve a 224 kW (300 hp) class diesel engine for general aviation.