4.  The exhaust stroke is used to purge the cylinder of  burned gases. It begins when the exhaust valve opens,  and the piston starts to move toward the cylinder head  once again.  Even when the engine is operated at a fairly low speed,  the four-stroke cycle takes place several hundred times  each minute.  [Figure 7-5]  In a four-cylinder engine, each  cylinder operates on a different stroke. Continuous rotation  of a crankshaft is maintained by the precise timing of the  power strokes in each cylinder. Continuous operation of the  engine depends on the simultaneous function of auxiliary  systems, including the induction, ignition, fuel, oil, cooling,  and exhaust systems.  The latest advance in aircraft reciprocating engines was  pioneered in the mid-1960s by Frank Thielert, who looked  to the automotive industry for answers on how to integrate  diesel technology into an aircraft engine. The advantage  of a diesel-fueled reciprocating engine lies in the physical  similarity of diesel and kerosene. Aircraft equipped with a  diesel piston engine runs on standard aviation fuel kerosene,  which provides more independence, higher reliability, lower  consumption, and operational cost saving.  In 1999, Thielert formed Thielert Aircraft Engines (TAE)  to design, develop, certify, and manufacture a brand-new  Jet-A-burning diesel cycle engine (also known as jet-fueled  piston engine) for the GA industry. By March 2001, the first  prototype engine became the first certified diesel engine  since World War II. TAE continues to design and develop  diesel cycle engines and other engine manufacturers, such as  Société de Motorisations Aéronautiques (SMA), now offer  jet-fueled piston engines as well. TAE engines can be found  on the Diamond DA40 single and the DA42 Twin Star; the  first diesel engine to be part of the type certificate of a new  original equipment manufacturer (OEM) aircraft.  These engines have also gained a toehold in the retrofit  market with a supplemental type certificate (STC) to re- engine the Cessna 172 models and the Piper PA-28 family.  The jet-fueled piston engine’s technology has continued to  progress and a full authority digital engine control (FADEC,  discussed more fully later in the chapter) is standard on such  equipped aircraft, which minimizes complication of engine  control. By 2007, various jet-fueled piston aircraft had logged  well over 600,000 hours of service.  The propeller is a rotating airfoil, subject to induced drag,  stalls, and other aerodynamic principles that apply to any  airfoil. It provides the necessary thrust to pull, or in some  cases push, the aircraft through the air. The engine power is  used to rotate the propeller, which in turn generates thrust  very similar to the manner in which a wing produces lift.  The amount of thrust produced depends on the shape of the  airfoil, the angle of attack (AOA) of the propeller blade, and  the revolutions per minute (rpm) of the engine. The propeller  itself is twisted so the blade angle changes from hub to tip.  The greatest angle of incidence, or the highest pitch, is at the  hub while the smallest angle of incidence or smallest pitch  is at the tip.  [Figure 7-6]  The reason for the twist is to produce uniform lift from the  hub to the tip. As the blade rotates, there is a difference in  the actual speed of the various portions of the blade. The tip  of the blade travels faster than the part near the hub, because  the tip travels a greater distance than the hub in the same  length of time.  [Figure 7-7]  Changing the angle of incidence  (pitch) from the hub to the tip to correspond with the speed  produces uniform lift throughout the length of the blade. A  propeller blade designed with the same angle of incidence  Figure 7-5.  The arrows in this illustration indicate the direction of  motion of the crankshaft and piston during the four-stroke cycle.  Propeller  7-4  1. Intake  2. Compression  3. Power  4. Exhaust  Exhaust valve Intake valve  Spark plug Piston  Crankshaft  Connecting rod 