84 Diesel Engine Technology Copyright Goodheart-Willcox Co., Inc. Specific Gravity Specific gravity compares the weight of a material with the weight of an equal volume of water. Specific gravity ratings are used when grading fuels and lubri- cating oils. Mean Effective Pressure Mean effective pressure (MEP) is the difference between the compression pressure and the expansion pressure. On naturally aspirated engines, the MEP is between 95–140 psi (667–965 kPa). On a turbocharged engine, MEP can range from 150–300 psi (1035–2068 kPa). Horsepower and Torque Engines are rated according to horsepower and torque. The speed and fuel setting of an engine will deter- mine its actual rated horsepower setting. Horsepower is the ability to maintain a load at a desirable or fixed speed. It is a measure of how fast work can be done. Horsepower is produced as a direct result of combustion in the cyl- inder. Combustion pressures force the piston down the cylinder, producing work. The amount of work produced on the power stroke can be determined using the fol- lowing formula: work = force × distance where: force = pressure acting on the piston in psi distance = length of piston stroke The more fuel released into the cylinder during a given period of time, the greater the amount of horsepower produced. Horsepower The term horsepower was first used by the inventor James Watt. He tried to compare the power produced by a steam engine with the power of a horse. Watt determined that a strong horse could pull a 200 lb load of ore 165 ft straight up a mine shaft in one minute. This equals 33,000 ft/lb of work every minute—the modern equivalent of one horsepower. 1 hp = 33,000 ft/lb minute The metric equivalent for horsepower is the watt (W). This unit of measure is more familiar as a unit of electric power. Horsepower can also be measured in kilogram- meters per second. One horsepower equals 746 watts or 76 kg-m/sec. Not all horsepower measurements are the same. The term rated horsepower refers to the horsepower that the engine will produce at its maximum full-load, governed speed in a particular application. Indicated horsepower (ihp) is the power transmitted to the pistons from the expanding combustion gases. Indicated horsepower is not measured with a test instrument it is mathematically calculated. Indicated horsepower does not take into account frictional power losses of moving parts or the power losses caused by accessory items, such as water pumps, blowers, oil pumps, fuel injection pumps, and cooling fans. The formula for calculating engine indicated horse- power is: ihp = P×L×A×N×K 33,000 where: P = mean indicated pressure in psi L = stroke of the piston in feet A = net piston areas in square inches N = number of power strokes per cylinder per minute K = number of cylinders in engine Brake horsepower (bhp) is the actual usable horse- power produced by the engine. The first step in deter- mining brake horsepower is to connect a dynamometer to the engine’s output shaft and measure the torque gener- ated. The following formula is then used to calculate the brake horsepower. brake horsepower = torque × rpm 5252 Because brake horsepower accounts for power losses due to operating friction and accessory items, it is normally between 10% and 20% less than indicated horsepower for the same engine. Mechanical efficiency is the difference between indi- cated horsepower and brake horsepower. Typical mechan- ical efficiency in a four-cycle engine is between 80% and 90%. This is slightly lower than the mechanical efficiency of a comparable two-cycle engine. Torque Torque is the rotational or twisting force around a fixed point. Torque is calculated by multiplying the applied force by its distance from the centerline of rotation. A simple example of this principle is demonstrated whenever a torque wrench is used to tighten a bolt. If a force of 20 lb is applied perpendicular to the centerline of the bolt’s rota- tion at a distance of 1 ft from the centerline, 20 ft-lb of torque is generated. The torque wrench acts as a lever to apply this force. If the lever is fastened to a rotating shaft, it is called a crank—hence the term crankshaft. In a diesel engine, the length of the lever or crank is the distance between the centerline of the connecting rod journal on the crankshaft and the centerline of the main bearing journal on the crankshaft. The longer the crank, the greater the amount of torque generated. The amount of time that force is applied