the difference being that an aircraft engine is opti- mized for reliability with dual ignition often installed  for each cylinder. Two-stroke engines are popular be- cause they have fewer components than four-stroke  engines which makes them less expensive to manu- facture, and lighter, thus increasing their power-to- weight ratio. This chapter covers the engines found on most pow- ered parachutes and includes the exhaust, ignition,  fuel, lubrication, cooling, propeller, gearbox, induc- tion, charging, and fuel systems. Reciprocating en- gine operating theory is covered, for both two-stroke  and four-stroke engines. The powered parachute engine and propeller, often  referred to as a powerplant, work in combination to  produce thrust. The powerplant propels the powered  parachute and charges the electrical system that sup- ports PPC operation. Two-stroke engines require that oil be mixed into the  fuel to lubricate the engine, instead of being held in a  sump and having a separate recirculating system like  a four-stroke engine. Details on two-stroke oil mixing  are covered later under the “Lubrication” section. The engine is one of the key components of a powered  parachute and should be maintained according to both  the engine and airframe manufacturer recommenda- tions. Preflight information, along with maintenance  schedules and procedures, can be found in the  Pilot’s  Operating Handbook  (POH) and/or maintenance ref- erences from the manufacturers. One stroke as the piston moves up is  intake  and  com- pression , the second stroke as the piston moves down  is  power and  exhaust . The two-stroke engine per- forms the same functions as a four-stroke engine in  half the strokes.  Engine inspections and maintenance must be per- formed and documented in a logbook. You should  review this logbook before flying an unfamiliar pow - ered parachute. A wide range of valve systems are found on two  cycle engines, for the purpose of opening and clos- ing ports in the cylinder to let fuel in and exhaust out  at the proper time, similar to the intake and exhaust  valves on a four-stroke engine.  One-way pressure  valves , called spring, reed, or poppet valves, open  when the pressure drops within the crankcase, pull- ing the fuel from the carburetor into the crankcase. [Figure 4-1] Mechanical rotary valves  are driven off the engine,  rotate to provide an opening at the precise time, and  can be on the intake and exhaust ports. [Figure 4-2] Most powered parachutes are designed with recipro- cating engines. Two common means of classifying  reciprocating engines are: Piston porting does not use any valves. The fuel inlet  port is opened and closed by the piston position as  it moves up and down in the cylinder. This is called  a “piston ported inlet” and will be used in the Two- Stroke Process description that follows. [Figure 4-3] Refer to Chapter 5 of the  Pilot’s Handbook of Aero- nautical Knowledge for a comprehensive review of  how reciprocating four-stroke engines operate. Two-stroke engines are commonly used in powered  parachutes. Two-stroke aviation engines evolved  from two-stroke snowmobile and watercraft engines,  The two-stroke process begins with the fuel enter- ing the engine and concludes as it exits as exhaust. [Figure 4-3] Two-Stroke Process Two-Stroke Engines Reciprocating Engines 1. By the number of piston strokes needed to  complete a cycle: two-stroke or four-stroke; and 2. By the method of cooling: liquid or air-cooled. 4-1