Compression ratio is a controlling factor in the maximum horsepower developed by an engine, but it is limited by present day fuel grades and the high engine speeds and manifold pressures required for takeoff. This comparison is expressed as a ratio, hence the term compression ratio. The compression ratio of an engine is a comparison of the volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke. The fuel/air charge in the cylinder can be compared to a coil spring in that the more it is compressed, the more work it is potentially capable of doing. The formula is:Ĭompression RatioAll internal combustion engines must compress the fuel/air mixture to receive a reasonable amount of work from each power stroke. The area of any circle is found by squaring the radius (r) and multiplying by π. The distance that is one-half the diameter of a circle is known as the radius. The area of a circle, as in a rectangle or triangle, must be expressed in square units. It is 3.1416 expressed to four decimal places, which is accurate enough for most computations. Pi cannot be stated exactly because it is a never-ending decimal. This number represents the ratio of the circumference to the diameter of any circle. The area of the base is the area of the cross-section of the cylinder.Īrea of a CircleTo find the area of a circle, it is necessary to use a number called pi (π). Since the volume (V) of a geometric cylinder equals the area (A) of the base multiplied by the height (h), it is expressed mathematically as: For multicylinder engines, this product is multiplied by the number of cylinders to get the total piston displacement of the engine. The piston displacement of one cylinder may be obtained by multiplying the area of the cross-section of the cylinder by the total distance the piston moves in the cylinder in one stroke. The volume displaced by the piston is known as piston displacement and is expressed in cubic inches for most American-made engines and cubic centimeters for others. When a piston moves from BDC to TDC, it displaces a specific volume. Piston DisplacementWhen other factors remain equal, the greater the piston displacement, the greater the maximum horsepower an engine is capable of developing. Since 1 hp is 33,000 ft-lb per minute, the hp expended in this case is 29,700 divided by 33,000, or 0.9 hp. The amount of work performed in the 2 minutes is 59,400 ft-lb or 29,700 ft-lb per minute. Assume that the force required is 90 lb, and the 100-lb weight is dragged 660 feet in 2 minutes. If the weight were attached to a spring scale graduated in pounds, then dragged by pulling on the scale handle, the amount of force required could be measured. The amount of this force would depend upon the roughness of the ground. 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 is approximately horizontal. Work is performed not only when a force is applied for lifting force may be applied in any direction. 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. 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.Īs stated above, work is the product of force and distance, and power is work per unit of time. From his observations came the unit of horsepower, which is the standard unit of mechanical power in the English system of measurement. Late in the 1 8th 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. HorsepowerThe common unit of mechanical power is the horsepower (hp). The greater the mass is and/or the greater the distance is, the greater the work performed. If a one-pound mass is raised one foot, one ft-lb of work has been performed. The most common unit is called foot-pound (ft-lb). 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. WorkA physicist defines work as force times distance. Also discussed are the various efficiencies that govern the power output of a reciprocating engine. This page presents an explanation of work and power and how they are calculated. All aircraft engines are rated according to their ability to do work and produce power.
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