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Engine Power Concepts

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Engine Power Concepts Engine Power Concepts Stroke C/R Stroke PRODUCT INFORMATION PRODUCT INFORMATION – ENGINE POWER CONCEPTS CONTENTS Introduction . 4 Engine Sizing . 5 Engine Power . 6-7 Brake Mean Effective Pressure . 8-9 Power Curves . 10-13 Summary . 14 3 PRODUCT INFORMATION – ENGINE POWER CONCEPTS ENGINE POWER CONCEPTS Selling engines requires knowledge of their they refer to processes or relationships which capabilities and components. Potential are actually easy to understand — most relate to customers will have questions regarding size or power. These general terms apply to all applications and engine design features. While piston engines — Cat engines as well as the some of the terms used to describe various engine in your car. engine parts or functions can sound complex, 4 PRODUCT INFORMATION – ENGINE POWER CONCEPTS ENGINE SIZING Bore refers to the inside diameter of the Displacement, or swept volume, per cylinder is cylinders in an engine. The piston is slightly the volume of air a piston displaces as it moves smaller than the bore measurement because it through one stroke. These terms are used slides in the cylinder. interchangeably. Both mean bore area times stroke. Stroke is the distance the piston travels in the cylinder. The length of the stroke is determined Bore Area = (3.14 x bore squared)/4 by the crankshaft radius also known as crank Displacement per Cylinder = Bore Area x Stroke throw (the distance from the centerline of main Engine Displacement = Displacement per bearing journal to centerline of connecting rod Cylinder x No. of Cylinders bearing journal). This movement is controlled by the shape of the crankshaft. displacement or Stroke swept volume C/R Stroke C/R If the bore diameter and stroke are in inches, the displacement will be in cubic inches. If the bore diameter and stroke are in centimeters, The connecting rod connects the crankshaft to displacement is in cubic centimeters. 100 cubic the piston. As the crankshaft rotates through centimeters is one liter. 180 degrees, the connecting rod and the piston Compression Ratio is the ratio of volume in the move from the extreme bottom position (BC) to cylinder with the piston all the way down vs. the extreme top position (TC). The stroke then is all the way up. If the minimum volume in the two times the crankshaft crank radius (C/R). The cylinder with the piston at TC is one cubic inch crank radius is also the lever arm on which the and the maximum volume with the piston at BC force from the piston acts to produce torque. is 10 cubic inches, the compression ratio is 10:1. • Automotive gasoline engines have compression ratios between 7:1 and 12:1. • Diesel engines have compression ratios between 13:1 and 24:1. Generally, larger diesel engines have the lower compression ratios. 5 PRODUCT INFORMATION – ENGINE POWER CONCEPTS ENGINE POWER Engine speed (the number of revolutions made by the crankshaft in one minute) is measured in rpm (revolutions per minute). rpm Torque Lever arm An engine producing 1000 lb-ft torque at 2000 Torque is the twist on a shaft resulting from a rpm, through transmission gearing can produce force applied perpendicular at a lever arm. Its 2000 lb-ft torque at 1000 rpm assuming no units are force (pounds) times distance from the efficiency losses through the transmission, for center of the rotating shaft (feet). Thus, 100 example. An increase in torque is achieved at the pounds applied at a lever arm of 2 feet results in expense of speed. The power in both cases is the 200 lb-ft torque. Equal torques can be produced same. To increase engine power we strive to by a large force applied at a short lever arm or increase torque (lb-ft) or speed (rpm) or both. a small force applied at a long lever arm. The torque from one-pound force applied at a 10-foot lever is the same as from a 10-pound 10 lbs force applied at a one-foot lever, etc. 1 lb 2 ft 1 ft ue rq 10 ft o 4 lb-ft T 2 lbs Torque = 10 lb-ft In an engine, pressure is applied to the top of the piston from expansion of an ignited air and fuel mixture. This pressure results in a force from the piston applied at the crank radius through the connecting rod. The resulting torque causes the crankshaft to rotate. By definition, work is force applied for a distance, or in the case of a rotational situation, work is torque applied through an angle. Power is work performed per unit of time. 6 PRODUCT INFORMATION – ENGINE POWER CONCEPTS ENGINE POWER The most common unit of engine power in the Variables influencing power rating are: U.S. is horsepower (hp). Originally, this unit • Temperature of the air was derived by what an average horse could do. Rigged up with a pulley system, an average • Temperature of the fuel horse could lift 33,000 pounds one foot off the • Barometric pressure ground in one minute. • Humidity • Heat content of the fuel The total horsepower actually developed on the pistons is called indicated horsepower. It is greater than the power measured at the engine flywheel by the horsepower required to Because power takes into account engine torque overcome frictional losses in the bearings, piston output as well as engine speed, it is a convenient rings, etc. as well as operating satellite systems unit used to compare engine size. such as fuel, oil, and water pumps. The Though horsepower is an accepted unit to rate difference between indicated horsepower and engines, each application must be considered flywheel horsepower is called friction individually. The engine ratings can be: horsepower. • Power that can be produced continuously The friction horsepower of an engine can be determined in the laboratory by motoring the • Power that can be produced for a given engine with an electric motor. In this test the time period, (generally one hour) followed engine’s fuel rack is at shut-off. The electric by an equal time period at a lower rating power required to motor the engine at any • Power that the engine can deliver for very given speed is the engine friction horsepower short times, such as five minutes at that speed. 7 PRODUCT INFORMATION – ENGINE POWER CONCEPTS BRAKE MEAN EFFECTIVE PRESSURE Indicated horsepower less frictional Another way of viewing BMEP is that it horsepower equals brake horsepower. measures how effectively an engine uses its BMEP is a value referring to the constant piston displacement to produce torque. pressure which would have to exist in a cylinder • The higher the BMEP, the greater the during its power stroke to produce the same torque per unit of displacement. horsepower at the flywheel, as actually exists. • BMEP can only be compared between 4-cycle engine to 4-cycle engine and 2-cycle engine to 2-cycle engine. • Over the years, BMEP has become known as a measure of engine life, however, it is NOT. + • BMEP gives a fair indication of mechanical pressure stresses within the engine, but in no way is o indicative of thermal loads. compression power exhaust intake – Pressure within the cylinder varies considerably. A rough indication of that pressure is shown above. You see that the pressure acting on the Example: One engine operating at the same piston varies considerably during the power speed (1800 rpm), but with varying turbocharger stroke. The mean or average pressure which boost and amount of aftercooling. would produce the same brake horsepower is the BMEP. Example Engine (1800 rpm) BMEP + pressure o compression power exhaust intake – Column 1 — shows a naturally aspirated engine producing 100 hp at a BMEP of 84 Column 2 — light turbocharging greatly increases the air inlet temperature, but raises horsepower to 134 and BMEP to 119; fuel consumption has decreased 6% to .402 lbs./bhp hr., but internal pressures have increased 44%; 8 PRODUCT INFORMATION – ENGINE POWER CONCEPTS BRAKE MEAN EFFECTIVE PRESSURE because air inlet temperature has increased 171°, Conclusion — As BMEP is increased, fuel thermal loading has increased 19%. consumption falls consistently. Mechanical loadings due to cylinder pressures increase, Column 3 — an engine with the same degree of while thermal loadings rise slightly, then start to turbocharging as in column 2, but with moderate decrease. This demonstrates that BMEP, with aftercooling; air inlet temperature decreased to little or no direct correlation to either 200° F, although horsepower and BMEP both mechanical or thermal stresses, it is not an increased; fuel consumption again decreased; indication of engine life. because turbocharging is also the same as in column 2, maximum cycle pressure is also the • Properly designed high BMEP engines may same, while cooler inlet air lowers thermal have even better life expectancy than a loading. naturally aspirated engine. Column 4 — the same degree of aftercooling as • A high BMEP engine will have better in column 3, but with a light turbocharging bhp-hr production (i.e. total amount of boost; again bhp and BMEP increased, while work performed) than its low BMEP lowering fuel consumption; the higher boost counterpart. pressure brings considerably higher maximum cycle pressures; but thermal load remains almost A modern, naturally-aspirated, heavy-duty diesel unchanged from column 3. will live 10,000 hours between overhauls. For example, a moderately blown version of the Column 5 — a very high degree of aftercooling same engine will produce 35 percent more on the same amount of turbocharging boost as power for 8500 hours before overhaul, the blown column 4; Horsepower now stands at 214 — engine, at higher BMEP, has produced nearly 15 114% more than the naturally aspirated engine; percent more bhp hours than the naturally- BMEP is nearly 100 psi higher than that of the aspirated engine, using only about 10 percent naturally aspirated engine; fuel consumption is more fuel to do it (less, per bhp-hr.).
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