of the expansive force as possible and of scavenging the  cylinder as completely and rapidly as possible. The valve is  opened considerably before BDC on the power stroke (on  some engines at 50° and 75° before BDC) while there is still  some pressure in the cylinder. This timing is used so that the  pressure can force the gases out of the exhaust port as soon  as possible. This process frees the cylinder of waste heat  after the desired expansion has been obtained and avoids  overheating the cylinder and the piston. Thorough scavenging  is very important, because any exhaust products remaining  in the cylinder dilute the incoming fuel/air charge at the start  of the next cycle. As the piston travels through BDC at the completion of the  power stroke and starts upward on the exhaust stroke, it  begins to push the burned exhaust gases out the exhaust port.  The speed of the exhaust gases leaving the cylinder creates  a low pressure in the cylinder. This low or reduced pressure  speeds the flow of the fresh fuel/air charge into the cylinder  as the intake valve is beginning to open. The intake valve  opening is timed to occur at 8° to 55° before TDC on the  exhaust stroke on various engines. The two-stroke-cycle engine has re-emerged being used  in ultra-light, light sport, and many experimental aircraft.  As the name implies, two-stroke cycle engines require  only one upstroke and one down stroke of the piston to  complete the required series of events in the cylinder. Thus,  the engine completes the operating cycle in one revolution  of the crankshaft. The intake and exhaust functions are  accomplished during the same stroke. These engines can  be either air or water cooled and generally require a gear  reduction housing between the engine and propeller. The rotary cycle has a three-sided rotor that turns inside an  elliptical housing, completing three of the four cycles for each  revolution. These engines can be single rotor or multi rotor  and can be air or water cooled. They are used mostly with  experimental and light aircraft. Vibration characteristics are  also very low for this type of engine. The diesel cycle depends on high compression pressures to  provide for the ignition of the fuel/air charge in the cylinder.  As air is drawn in the cylinder, it is compressed by a piston  and, at maximum pressure, fuel is sprayed in the cylinder. At  this point, the high pressure and temperature in the cylinder  causes the fuel to burn increasing the internal pressure of the  cylinder. This drives the piston down, turning or driving the  crankshaft. Water and air cooled engines that can operate on  Work is performed not only when a force is applied for lifting;  force may be applied in any direction. If a 100-lb weight is  dragged along the ground, a force is still being applied to  perform work, although the direction of the resulting motion  As stated above, work is the product of force and distance,  and power is work per unit of time. Consequently, if a  33,000-lb weight is lifted through a vertical distance of 1 foot  in 1 minute, the power expended is 33,000 ft-lb per minute,  or exactly 1 hp. The common unit of mechanical power is the horsepower  (hp). Late in the 18$^{th}$ century, James Watt, the inventor of the  steam engine, found that an English workhorse could work at  the rate of 550 ft-lb per second, or 33,000 ft-lb per minute, for  a reasonable length of time. From his observations came the  unit of horsepower, which is the standard unit of mechanical  power in the English system of measurement. To calculate the  hp rating of an engine, divide the power developed in ft-lb per  minute by 33,000, or the power in ft-lb per second by 550. Work is measured by several standards. The most common  unit is called foot-pound (ft-lb). If a one-pound mass is raised  one foot, one ft-lb of work has been performed. The greater  the mass is and/or the greater the distance is, the greater the  work performed. A physicist defines work as force times distance. Work done  by a force acting on a body is equal to the magnitude of the  force multiplied by the distance through which the force acts. All aircraft engines are rated according to their ability  to do work and produce power. This section presents an  explanation of work and power and how they are calculated.  Also discussed are the various efficiencies that govern the  power output of a reciprocating engine. JET A fuel (kerosene) use a version of the diesel cycle. There  are many types of diesel cycles in use including two-stroke  and four-stroke diesels. Reciprocating Engine Power and  Efficiencies Exhaust Stroke Work Horsepower Two-Stroke Cycle Rotary Cycle Diesel Cycle Work (W) = Force (F) × Distance (D) One hp = ft-lb per min 33,000 ft-lb per sec 550 or 1-28