wickert-engines[1]

wickert-engines[1] - uild an engine iobile without .vered...

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Unformatted text preview: uild an engine iobile without .vered itself by omething (the g (because no :hat such good file the first law rm to another. ' can be used. the maximum :al bound was s interested in constraints set ‘s cycle cannot :valuating and :ssing an ideal 2 cycle. heat is lled. The gas. y reservoirs at : ideal Carnot (6.13) i on the same grees Rankine ,rres are found he conversion (6.14) quation (6.13) ance is deter- ciency can be by raising the 'actical limita— g point of the nt restrict the 1. fluid viscos— u should view nergy conver- ion (6.12) and 6.6 FIGURE 6.12 Layout of a single—cylinder internal-combustion engine. 8 E C T | 0 N G .6 Case Study 1: Internal-Combustion Engines 259 Table 6.6. The real efficiency of an engine will always be lower than Carnot‘s effi- ciency. and usually. it will be much lower. Real—efficiency values. such as those listed in Table 6.6. should be used whenever possible in calculations involving thermal and energy systems. In the remaining sections of this chapter. we turn our attention to three technologies of mechanical engineering that are based on the manipulation of heat and the effi- cient conversion of energy from one form to another: internal—combustion engines. electrical power generation. and aircraft jet engines. The internal—combustion engine is a heat engine that converts the chemical energy stored in gasoline. diesel. ethanol (derived primarily from corn). or propane into mechanical work. The heat that is generated by rapidly burning a mixture of fuel and air in the engine’s combustion chamber is transformed into rotation of the engine‘s crankshaft at a certain speed and torque. As you know. the applications of internal-combustion engines are wide and varied. and they include automobiles. motorcycles. aircraft. ships. pumps. and electrical generators. Mechanical engineers develop extensive computer models of internal-combustion engines. and before a single engine is even built. performance will be analyzed from the perspectives of fuel economy. power-to-weight ratio. noise. air pollution. and cost. In this section. we discuss some of the design. terminology. and energy principles behind four—stroke and two—stroke engines. The main elements of the single-cylinder engine (shown in Figure 0.13) are the piston. cylinder. connecting rod. and crankshaft. Those components convert the back—and-forth motion of the piston into the crankshaft‘s rotation. As fuel burns. the high pressure that develops in the cylinder pushes the piston. moves the connect ing rod. and rotates the crankshaft. The engine also contains a means for fuel and fresh air to be drawn into the cylinder. and for the exhaust gases to be vented away. We will discuss those processes separately in the context of four-stroke and two- stroke engines. Crank pin Connédm‘émd \Vr‘mp‘m / C}lrnder / Crank fl Intake val\e Rotation \_ if) ' Crankshaft \ ® \e/ / / / }/ Combustlon chamber ‘1 EE Spark plug s. Exhaust valye / Piston Piston rings 250 CHAPTER FIGURE 6.13 This four—cylinder. sixteen-valve automobile engine produces a peak power output of 100 k\\'. e 2001 General Motors Corporation L'sed with permission of G.\l Media Archnes. 6 Thermal and Energy Systems While the single-cylinder configuration may be relatiyely simple. its power output is limited by the small size. In multicylinder engines. the pistons and cylinders can be placed in the yee. in-line. or radial orientations. For instance. four-cylinder engines with the cylinders arranged in a single straight line are common in automobiles (Figure 6.13). In a \'—6 or \’-8 engine. the engine block is made short and compact by setting the cylinders into two banks of three or four cylinders each (Figures 1.9 and 6.2). The angle between the banks is usually between 60: and 90' . and in the limit of 180'. the cylinders are said to horizontally oppose one another. Engines as large as \"—125 and V—lés are found in heavy-duty trucks and luxury Vehicles. and 5-1—cylinder engines comprising six banks of nine cylinders each haye been used in some naval applications. The power that can be produced by an internal-combustion engine depends not only on the number of cylinders. but also on its throttle setting and speed. Each automotive engine. for instance. has a particular speed where the output power is the greatest. An engine that is advertised as generating 200 hp does not do so under all operating conditions. As an example. Figure 6.1—1 shows the power—speed performance curye ofa \’-6 automobile engine. This engine was tested at full throttle. and it reached peak power output near a speed of 6000 revolutions per minute. The cross section of a single-cylinder four—stroke engine is illustrated in Figure 6.15 at each major stage of its operation. This engine has two valves per cylinder: one for drawing in fresh fuel and air and one for exhausting the by-products of combustion. The mechanism that causes the valyes to open and close is an important aspect of FIGURE 6.1‘ Measured output of automobil engine as of speed. FIGURE G Major 5 four—strt cycle. FIGURE 6.14 Measured povver output of an automobiles 3.5-liter engine as a function of speed FIGURE 6.15 Major stages of the four—stroke engine‘s cycle. S E C T I 0 N 6.6 Case Study 1: |nternal»C0mbustion Engines 25l 141) 3 1203 l 100; i 801 l 60 l} 40 3 l l 20 a Power. hp l 0 ‘ 3000 2500 3000 3500 4000 4500 5000 5500 6000 6500 Engine speed. rpm this type of engine. and Figure 6.16 depicts one design for an intake or exhaust valve. The valve closes as its head contacts the polished surface of the cylinders intake or exhaust port. A specially shaped metal lobe. called a cam. rotates and controls the opening and closing motions of the valve. The cam rotates in synchronization with the crankshaft and ensures that the valve is opened and closed at precisely the correct instants relative to the combustion cycle and the pistons position within the cylinder. Four—stroke engines operate according to a continuous process called the Otto cycle that comprises four full strokes ofthe piston within the cylinder ( or equivalently. tvvo complete revolutions of the crankshaft). The engines principle of operation is named after German inventor Nicolaus Otto (1832—1891). who is recognized for having developed the first practical design for liquid—fueled piston engines. Engineers use the abbreviation TDC (which stands for “top dead center" l when referring to the Exhaust Intake Compression 252 CHAPTER 6 FIGURE 6.16 One type of cam and valve mechanism that is used in a four—stroke internal»combustion engine. Thermal and Energy Systems Cant Camshaft Spring retainer Valve stein l3 , _ IE—Milve spring V \Valve head Port ‘1 \‘al\ e seat " point where the piston is at the top of the cylinder and the connecting rod and crank are in-line with one another. Conversely. the term BDC stands for "bottom dead center." which is the orientation separated from TDC as the crankshaft is rotated by 180'. Referring to the sequence of piston positions shown in Figure 6.15. the four stages of the cycles operation proceed as follows: Intake Stroke. Just after the TDC position. the piston begins its motion down— ward in the cylinder. At this stage. the intake valve has already been opened. and the exhaust valve is closed. As the piston moves downward. the cylin— der's volume grows. As the pressure within the cylinder falls slightly below the outside atmospheric value. a fresh mixture of fuel and air is drawn into the cylinder. Once the piston nears BBC. the intake valve closes so that the cylinder is completely sealed. Compression Stroke. The piston next travels upward in the cylinder and compresses the fuel—air mixture. The ratio of volumes in the cylinder be- fore and after this stroke takes place is called the engine‘s compression ratio. Near the end of the stroke. the spark plug fires and ignites the fuel and air— now at an elevated pressure. Combustion occurs rapidly at a nearly constant volume as the piston moves from a position slightly before TDC to a posi- tion slightly after. To visualize how the pressure within the cylinder changes throughout the Otto cycle. Figure 6.17 depicts a graph that was measured on a single-cylinder engine while it was running. The peak pressure reached in this engine was about 450 psi. For each pulse of pressure in the cylinder. a portion of that rise was associated with the upward compressing motion of the piston in the cylinder. but the dominant factor was combustion following the instant labeled "ignition" in Figure 6.17. Power Stroke. With both valves remaining closed. the high-pressure gas in the cylinder forces the piston downward. The expanding gas performs work on the piston. which moves the connecting rod and rotates the crankshaft. As shown FIGURE 6.17 Measured 1 curve for a four-stroke operating z 9th rpm. S E C TI 0 N 6.6 Case Study 1: Internal-Combustion Engines 263 FIGURE ‘ One engine cycle + Measured pressure Clere for 3 Compression Exliatist Compression four—stroke engine Power Intake Power operating at 9H1! rpm. . E\ll;lll\l .4 Ignition E\liau~i \tilye closes. f \Lll\L‘ intake \alu‘ {i Intake ‘ UPC!“ “WW Intake ‘— \al\c \al\c closes i‘ Bltm glrhcx thm [I TDC BDC TDC B DC TDC Crankshaft rotation in Figure 6.1 7. the pressure in the cylinder decreases quickly during the power stroke. As the stroke nears its end. but while the pressure is still aboye the atmospheric yalue. a second cam mechanism opens the exhaust \‘alye. Some of the spent gas flows out from the cylinder through the exhaust port during this short stage (called blow—down) that occurs just before BDC. i Exhaust Stroke. After the piston passes through BDC. the cylinder still con- tains exhaust gas at approximatelyatmospheric pressure. In the final stage of the Otto cycle. the piston moyes upward toward TDC with the exhaust yalye open to force the spent gas from the cylinder. Near the end of the exhaust stroke and just before TDC is reached. the exhaust Valve closes. and the intake yalye begins to open in preparation for the next repetition of the cycle. To place this orchestration of yalye and piston motions in perspectiye for a single- cylinder engine. consider that at a speed of only 900 rpm the crankshaft completes 15 reyolutions each second and each of the four strokes occurs in only 33 ms. In fact. automobile engines often operate seyeral times faster. and the short time in— tervals between yalyes opening and closing highlight the need for accurate timing and sequencing of the four stages. In the Otto cycle. only one stroke out of every four produces power. and in that sense. the crankshaft is being driyen only 2500 of the time. Howeyer. the engine continues to rotate during the other three strokes because of the momentum stored in the engine‘s flywheel or. for a multicylinder engine. because of the power strokes that oyerlap from the other cylinders. The second common type of internal-combustion engine operates on the two—stroke cycle. which was invented in 1880 by British engineer Dugald Clerk (1854—1932). 264 FIGURE 6.18 Cross section of a two-stroke i internal—combustion engine using crankcase compression. CHAPTER 6 Thermal and Energy Systems Spark plug C imbuition chamber a ‘ ‘ \ I, Cylinder Exhaust port Transfer port Intake port / Piston ( gll‘ll\\lltll / COIUICkUH‘. lOtl Crankcase Figure (3.18 depicts the cross section of an engine that operates on this cycle In contrast to its four—stroke cousin. this engine has no valves. and hence. there is no need for springs. camshafts. cams. or other elements of a valvetrain. Instead. a two- stroke engine has a passageway called the transfer port that connects the crankcase and cylinder. The engine operates on the principle that fresh fuel and air can flow from the crankcase. through the transfer port. and into the cylinder. As it moves within the cylinder. the piston itself acts as the valve by uncovering or sealing the exhaust port. intake port. and transfer port in the proper sequence. This type of engine operates by using the principle of crankcase compression. A two-stroke engine completes a full cycle for each revolution of the crankshaft. and power is therefore produced during every other stroke of the piston. As shown by the sequence of events in Figure 6.19. the Clerk cycle operates as follows: Downstroke. The piston begins near TDC with a compressed mixture of fuel and air in the combustion chamber. After the spark plug fires. the piston is driven downward in its power stroke. and torque and rotation are transferred to the crankshaft. When the piston has moved only partially down the cylinder. the exhaust port labeled in Figure 6.18 is uncovered. Because the gas in the cylinder is still at a relatively high pressure. it begins to vent outward through the newly opened port. As discussed next for the upstroke stage. a fresh charge of fuel and air is already waiting in the crankcase. and it will be transferred into the cylinder for use in the engine‘s next cycle. The piston continues downward. and as the volume within the crankcase decreases. the pressure grows in the fuel—air mixture that is stored there. Eventually. the transfer port becomes uncovered as the upper edge of the piston moves past it. and the fuel and air within the crankcase flow through the transfer port to fill the cylinder. During this process. the piston continues to block the intake port. tft. .vn iel S E CT | 0 N 6 . 6 Case Study 1: lnternal»Combustion Engines FIGURE 6.19 Sequence of stages in the operation of a two-stroke internal—combustion engine. Exhaust ‘ ._— H Transfer i—‘ \ H l x '/ (2 (D L'pstroke. Once the piston passes the BDC position. most of the exhaust gas has been expelled from the cylinder. As the piston continues moving upward. the transfer port and exhaust port are both covered. and the pressure in the crankcase decreases because its volume is expanding. The intake port becomes uncovered as the lower edge of the piston moves past it. and fresh fuel and air flow into and fill the crankcase. 'Ihat mixture will be stored for use during the engine‘s next combustion cycle. Slightly before the piston reaches TDC. the spark plug fires. and the power cycle begins anew. ur—stroke engines each have their own advantages. Relative to Two—stroke and fo e simpler. lighter. and less ex— their four-stroke counterparts. two—stroke engines ar pensive. Whenever possible. mechanical engineers attempt to keep matters simple. and because two-stroke engines have few moving parts. there is very little to go wrong with them. On the other hand. the intake. compression. power. and exhaust stages ell separated from one another as they are in a ngine. the spent exhaust and fresh fuel-air gases mix unavoidably as the new charge flows from the crankcase into the cylinder. For that reason. a portion of unburned fuel is present in a two-stroke engine‘s exhaust. and that leakage contributes both to environmental pollution and reduced fuel econ— omy. In addition. because the crankcase is used to store fuel and air between cycles. it cannot be used as an oil sump as is the case in a four—stroke engine. Lubrication in a two-stroke engine is instead provided by oil that has been premixed with the fuel. a factor that further contributes to its emissions and environmental problems. in a two-stroke engine are not as w four-stroke engine. In a two—stroke e ...
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This note was uploaded on 10/18/2008 for the course MECHENG 102 taught by Professor Okamura during the Spring '07 term at Johns Hopkins.

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wickert-engines[1] - uild an engine iobile without .vered...

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