Depending on the engine manufacturer, all of these  arrangements can be designed to utilize spark or compression  ignition and operate on either a two- or four-stroke cycle.  In a two-stroke engine, the conversion of chemical energy  into mechanical energy occurs over a two-stroke operating  cycle. The intake, compression, power, and exhaust processes  occur in only two strokes of the piston rather than the more  common four strokes. Because a two-stroke engine has  a power stroke upon each revolution of the crankshaft, it  typically has higher power-to-weight ratio than a comparable  four-stroke engine. Due to the inherent inefficiency and  disproportionate emissions of the earliest designs, use of the  two-stroke engine has been limited in aviation.  Recent advances in material and engine design have  reduced many of the negative characteristics associated  with two-stroke engines. Modern two-stroke engines often  use conventional oil sumps, oil pumps, and full pressure  fed lubrication systems. The use of direct fuel injection  and pressurized air, characteristic of advanced compression  ignition engines, make two-stroke compression ignition  engines a viable alternative to the more common four-stroke  spark ignition designs.  [Figure 7-3]  Spark ignition four-stroke engines remain the most common  design used in GA today.  [Figure 7-4]  The main parts of a  spark ignition reciprocating engine include the cylinders,  crankcase, and accessory housing. The intake/exhaust valves,  spark plugs, and pistons are located in the cylinders. The  crankshaft and connecting rods are located in the crankcase.  The magnetos are normally located on the engine accessory  housing.  In a four-stroke engine, the conversion of chemical energy into  mechanical energy occurs over a four-stroke operating cycle.  The intake, compression, power, and exhaust processes occur  in four separate strokes of the piston in the following order.  1. The intake stroke begins as the piston starts its  downward travel. When this happens, the intake  valve opens and the fuel-air mixture is drawn into the  cylinder.  2.  The compression stroke begins when the intake valve  closes, and the piston starts moving back to the top of  the cylinder. This phase of the cycle is used to obtain  a much greater power output from the fuel-air mixture  once it is ignited.  3.  The power stroke begins when the fuel-air mixture is  ignited. This causes a tremendous pressure increase  in the cylinder and forces the piston downward away  from the cylinder head, creating the power that turns  the crankshaft.  Figure 7-3.  Two-stroke compression ignition.  7-3  Figure 7-4.  Main components of a spark ignition reciprocating  engine.  Exhaust valve  Intake valve  Spark plug Piston  Crankshaft Connecting rod  Crankcase  Cylinder  Exhaust valve  Fuel injector  Piston  Forced air  2. Power stroke  1. Intake/compression  and exhaust 