Assembly Of Piston And Connecting Rodin Four Stroke Engine

Thus, in the ideal Diesel cycle, efficiency is a function of the compression ratio and the expansion ratio. For example, with k held constant at a value of 1.4, a compression ratio of 16:1, with a cut-off ratio of 2:1, would yield the following ideal thermal efficiency:

Actual Cycle Performance

There are dramatic differences between ideal cycle efficiencies and those achieved in practice. In addition to the fact that engines do not actually operate on these ideal cycles, there are several causes of irreversibility that occur in practical applications. Following are some of the reasons for the differences in ideal and actual cycle performance:

1. The specific heat content of the actual gases increases with a rise in temperature.

2. Dissociation of combustion products occurs.

3. Combustion may not be complete due to incomplete air-fuel mixing and/or lack of sufficient oxygen.

4. Combustion does not occur instantaneously, so constant volume combustion is an ideal representation of the actual combustion event. In the actual combustion process, combustion commences prior to TDC and continues after TDC. Because combustion continues after TDC when the cylinder volume is much greater than the clearance volume, actual peak-pressure value is lower than ideal, resulting in less expansion. After peak pressure, expansion stroke pressure is greater than ideal because less work has been extracted from the cylinder gases than in the ideal cycle.

5. During the inlet and exhaust strokes, there is a pressure drop through the valves and a certain amount of work is required to charge the cylinder with air and exhaust the combustion products.

6. Actual compression ratios are less than the nominal value because of late intake valve or port closing.

7. Exhaust blowdown losses occur in actual cycles because the exhaust valve is opened well before BDC to reduce pressure during the first part of the exhaust stroke in four-stroke-cycle engines and to allow time for scavenging in two-stroke-cycle engines. The gas pressure at the end of the power (expansion) stroke is, therefore, reduced, resulting in a decrease in expansion stroke work transfer.

8. There is considerable heat transfer between the burned gases in the cylinder and the cylinder walls. This heat transfer causes the gas pressure in the actual cycle to be less than ideal as volume increases.

9. Cylinder gas leakage occurs. As cylinder pressure increases, gas flows into numerous crevices, such as the region between the piston rings. Some of the gas flows into the crankcase and is lost from the cycle. Some of the gas flow returns, but has been cooled by heat transfer in the crevices. Both effects reduce cylinder pressure and, therefore, expansion work relative to ideal-cycle values.

10. Engine aspiration effectiveness is less than ideal because the cylinders are not completely filled with fresh air with each intake stroke and exhaust gases are not completely removed with each exhaust stroke. The faster the engine runs, the less time there is to fill the cylinder on each intake stroke and, therefore, the engine has less ability to rid exhaust gases and take in fresh air.

11. There is irreversibility associated with all real processes (e.g., pressure and temperature changes).

In practice, thermal fuel efficiency of an engine is the degree to which the engine is successful in converting the energy of the fuel into usable mechanical energy. It is that part of the heat energy in the cylinder that forces the pistons to move, resulting in crankshaft rotation.

Compression ratio is still a leading indicator of engine thermal efficiency as well as power density. Thermal efficiency improves with higher compression ratios because the combustion gases are able to expand further in the expansion or power stroke, allowing more heat energy to be transformed into mechanical energy. Despite all of the practical limitations, reciprocating engines generally offer the highest simple-cycle thermal fuel efficiency of all prime movers of comparable capacity. Thermal fuel efficiencies typically range from 28 to 46% (LHV basis), with large capacity Diesel engines at the highest end of the efficiency range.

Comparison of Otto- and Diesel-Cycle Engines

Both spark-ignited, Otto-cycle engines and self-ignited, Diesel-cycle engines are available as two-stroke and four-stroke designs and with natural or charged aspiration. In the Otto-type engine combustion process cycle, the engine compresses an air-fuel mixture in a cylinder. The mixture is generally ignited in the cylinder by a spark at or near the completion of the compression stroke. In the Diesel cycle, air only (not an air-fuel mixture) is compressed in the compression stroke. The temperature of the air within the cylinder rises to the auto-ignition temperature of the Diesel fuel prior to the end of the compression stroke. Ignition occurs when the fuel is injected, at high-pressure, into the cylinder starting at or near TDC and continues during most of the combustion (power) stroke.

From the ideal cycle P-V diagrams shown above, it can be seen that the isentropic compression and expansion ratios are equal in the Otto cycle, but compression ratio is greater than the expansion ratio in the Diesel cycle. From the T-s diagrams, it can also be seen that the ideal Otto cycle has higher efficiency and higher work area than the Diesel cycle. Less heat is rejected with the same amount of heat input and the same compression ratio. For the same amount of heat rejected, then, the Otto cycle would produce more work. In the example calculations provided above, the ideal thermal efficiency for an Otto cycle with a compression ratio of 11:1 was essentially the same as that of a Diesel cycle with a compression ratio of 16:1.

Thus, in theory, the Otto cycle is more efficient than the Diesel cycle for engines operating with the same compression ratio. However, in practice, Diesel engines achieve significantly greater thermal efficiencies than Otto engines due to the ability to operate with higher compression ratios and at higher peak pressures.

State-of-the-art, large capacity, low-speed Otto-cycle engines may achieve a thermal fuel efficiency of about 41% on a LHV basis (this corresponds to about 37% on a HHV

basis with natural gas). A Diesel-cycle engine of similar capacity and operating speed may achieve a thermal fuel efficiency of about 46% on a LHV basis (or 44% on a HHV basis using Diesel oil).

With an Otto engine, a combustible air-fuel mixture is compressed and heated in the engine's cylinders. Fuel is mixed with the air prior to the compression stroke of the cycle. Ignition occurs due to an externally timed electrical spark. Since the fuel-air mixture is compressed, it is necessary to use a volatile gaseous or readily vaporized fuel that can be distributed uniformly into the incoming air at relatively low pressure. This fuel-air mixture can prematurely ignite during the compression stroke if the cylinder pressure and/or temperature become too high. Compression ratios and mean effective pressures of Otto-cycle engines are, therefore, limited by two types of detonation considerations: surface ignition and combustion knock.

The problems of surface ignition and knock generally do not exist in the Diesel engine, because air only is compressed during the compression stroke. Since there is no fuel present during the compression stroke, much higher compression ratios can effectively be achieved. Whereas the maximum compression ratios of Otto engines are limited to about 12.5:1 or 13:1, compression ratios for Diesel engines may exceed 20:1.

Another difference is that Otto-cycle engines are governed by throttling the charge while Diesel engine are governed by varying the amount of fuel injected into the cylinders. In Otto-cycle engines, a throttle is required for part-load operation. Throttling increases pumping work under partial loads and, therefore, decreases efficiency. Diesel engines do not require a throttle and, therefore, achieve better part-load efficiencies than do Otto-cycle engines.

The typical thermal fuel efficiency advantage of Diesel engines versus spark-ignited Otto-cycle engines will range from 10 to 20%. Otto-cycle engines will typically have 20 to 25% lower power output and correspondingly greater heat rejection than the same size Diesel engine at the same operating conditions.

Despite the simple-cycle thermal fuel efficiency disadvantage, Otto-cycle engines offer several advantages with respect to Diesel engines. The higher compression ratio and combustion pressure in Diesel engines produce increased engine stress, necessitating a heavier and sturdier design. The abuses of oil contamination and soot deposits from combustion also increase maintenance requirements and costs.

Of great significance in the current energy market is the need to control environmentally harmful air emissions. Otto-cycle engines operating on fuels such as natural gas produce lower emission rates in many regulated pollutants than do Diesel-cycle engines operating on liquid fuels. This environmental efficiency advantage often translates into capital and operating cost advantages and often allows natural gas-fired Otto-cycle engines to be more easily permitted for stationary applications.

In applications that employ heat recovery (cogenera-tion cycles) in facilities with sufficiently large heat requirements, the higher rate of heat rejection of the Otto-cycle engine compensates to some extent for the lower simple-cycle thermal fuel efficiency compared with Diesel-cycle engines. Since more heat is rejected from the simple-cycle, more heat is available to be recovered. Also, due to the composition of the exhaust gases, heat recovery systems can often extract more heat from Otto-cycle engine exhaust by cooling the exhaust gases to a lower final exit temperature than can be practically achieved with liquid fuel-fired Diesel engine exhaust.

A critical parameter in exhaust gas heat recovery system design is the exit temperature of the exhaust gas as it enters the stack. The limiting factor is the temperature at which the heat exchanger surfaces reach a point where combustion product acids and water precipitate onto the metal surfaces, causing corrosion. The acid dew point is a function of the amount of sulfur in the fuel and, subsequently, in the exhaust gas. Exhaust streams from natural gas firing, therefore, may be cooled lower than exhaust from sulfur-bearing liquid fuels.

The combined impact of higher heat rejection rate and the lower heat recovery system exit temperature can result in a significant heat recovery efficiency advantage for gas-fired Otto engines versus oil-fired Diesel engines. Therefore, with cogeneration-type applications, overall thermal efficiency may be fairly similar, though the constituent components of power output and recovered heat will be different in each case.

Dual-Fuel Engines

Dual-fuel engines are Diesel-type engines that are capable of operating on natural gas and other gaseous fuels, as well as on liquid fuels. The approach can provide fuel source and price flexibility, as well as some of the advantages of both alternatives.

All current designs feature combustion initiated by self-ignited pilot oil, with remaining combustion energy provided by natural gas. Thermal fuel efficiencies well in excess of 40% (LHV basis) have been achieved while operating predominantly on natural gas, with NOx emission levels of 1 gram/bhp-h (1.3 gram/kWh) or less.

There are two general, currently available dual-fuel engine design types. One type features direct-injection of highly compressed gas into the cylinder for Diesel cycle-type combustion. This type allows for up to 95% gas firing at full load, with 5% pilot oil. The second type locates the gas valve in the intake manifold with the air-gas mixture being compressed and then ignited by the compression-ignited pilot oil. This type allows for up to 99% gas firing at full load with 1% pilot oil.

The ability to vary the amount of gas used allows for the use of maximum gas based on fuel pricing and/or air emissions control strategies. The ability to purchase natural gas on an interruptible (non-firm) basis, based on the availability of an alternative fuel, generally results in a much lower gas price. Operation with maximum use of natural gas may also allow for flexibility in meeting air emissions requirements. For example, some NOx emissions control regulations require lower NOx emissions levels in the summer months when ground-level ozone problems are more severe. The ability to operate almost exclusively on natural gas during the summer months can enable the engine to meet strict permitting requirements, while still maintaining the ability to operate on liquid fuel during other months of the year.

Engine Components and Operation

The four-stroke combustion process cycle produces power in the following manner. The piston compresses air (compression-ignited Diesel-cycle engines) or an air-fuel mixture (spark-ignited Otto-cycle engine), fuel is burned within the cylinder, expanding combustion gases exert force on the piston and this force is transferred, through a connecting rod, to produce rotation in a crankshaft. Following is a discussion of how reciprocating engines operate, with a focus on several basic engine components and design features.

Engine Frame

The engine frame structure includes all fixed parts that hold the engine together. Its function is to support and align the moving parts, while resisting the forces imposed by the engine's operation. It also supports auxiliaries and provides jackets and passages for cooling water, a sump for lubricating oil, and a protective enclosure for all of these parts.

For stationary engines, which rest on a substantial foundation, two-piece construction is common. The lower section, or bedplate, forms a base, supports the main bearings, encloses the lower part of the crankcase, and forms a sump for lubricating oil. The upper section, or center-frame, includes the upper part of the crankcase and the cylinder block in which the cylinders are supported. Automotive type engines have a one-piece cylinder block and are typically constructed of cast iron. Figure 9-4 shows the center-frame of a large capacity 14-cylinder, V-type, four-stroke-cycle engine.

Fairbanks Morse Locomotives

Fig. 9-4 Center-Frame for Large Capacity 14-Cylinder, V-Type Engine. Source: Fairbanks Morse Engine Div.

Fig. 9-5 Frame Box for Large Capacity Two-Stroke-Cycle, In-Line Diesel Engine. Source: MAN B&W

Fig. 9-6 Bedplate for Large Capacity Two-Stroke-Cycle, In-Line, Diesel Engine. Source: MAN B&W

Fig. 9-4 Center-Frame for Large Capacity 14-Cylinder, V-Type Engine. Source: Fairbanks Morse Engine Div.

Figures 9-5 and 9-6 show the frame box and bedplate for a large capacity 6-cylinder, in-line two-stroke-cycle Diesel engine. The frame box is of welded construction featuring a hinged door for access to crankcase components. The bedplate is built-up of longitudinal side girders and welded cross girders with cast steel bearing supports. Stay bolts connect the bedplate, the frame box, and the cylinder frame to form a rigid unit.

Fig. 9-6 Bedplate for Large Capacity Two-Stroke-Cycle, In-Line, Diesel Engine. Source: MAN B&W

Cylinders and Pistons

Cylinders are chambers located inside the engine in which air or an air-fuel mixture is compressed, the fuel is ignited, and the power is produced. Figure 9-7 shows a single engine cylinder liner and water jacket. Liners are referred to as wet or dry, depending on whether the sleeve is in direct contact with the cooling water. The cylinder liners are inserted in the large circular holes in the cylinder block.

The cylinder head (or heads) forms the top or lid to seal the cylinders. Cylinder heads are typically made of cast iron or aluminum and must be strong and rigid to distribute the gas forces acting on the head as uniformly as possible through the engine block. The cylinder head contains the spark plug or fuel injector and, in overhead valve

Fig. 9-5 Frame Box for Large Capacity Two-Stroke-Cycle, In-Line Diesel Engine. Source: MAN B&W

Fig. 9-7 Engine Cylinder Liner and Water Jacket.

Source: Fairbanks Morse Engine Div.

Fig. 9-7 Engine Cylinder Liner and Water Jacket.

Source: Fairbanks Morse Engine Div.

engines, parts of the valve mechanisms.

Figure 9-8 is a cross-section illustration of a cylinder head for a four-stroke-cycle Diesel engine. The cylinder head is designed to withstand operation with combustion pressures of up to 2,610 psi (180 bar). The fuel injector is flanked by the valve assemblies on either side. As shown in Figure 9-9, studs at the top of the frame are used to fasten the cylinder head to the frame. The cylinder head closes the top end of the cylinder so as to make a confined space in which to compress the air or air-fuel mixture and to confine the gases while they are burning and expanding.

angle, and the radial arrangement. An additional longstanding design is the opposed piston type, which features two pistons facing each other in the same cylinder and two crankshafts. A given engine model will feature numerous sub-models that are differentiated by the number of cylinders, i.e., I6, I8, V8, V12, V16, etc.

The piston slides up and down within the cylinder and serves to seal the cylinder, compress the air charge or air-fuel mixture charge, resist the pressure of the gases while they are burning and expanding, and transmit the combustion-generated gas pressure to the crank pin via

Fig. 9-8 Cross-Sectional Illustration of Four-Stroke-Cycle Diesel Engine Cylinder Head. Source: MAN B&W

Multi-cylinder engines can include in-line, V, flat, and radial cylinder arrangements. In-line arrangements are the simplest. If the engine has more than eight cylinders, it becomes difficult to make a sufficiently rigid frame and crankshaft with an in-line arrangement. Also, the engine becomes quite long and takes up considerable space. The V-type arrangement, with two connecting rods attached to each crankpin, reduces length and makes the frame and crankshaft stiffer. Typically, the angle between the two banks of cylinders in a V-type arrangement is between 40 and 75 degrees.

Variations on the V-type arrangement are the flat arrangement, in which the banks are on a 180 degree

Fig. 9-9 Studs at Top of Frame Used to Fasten Cylinder Head to Frame of Very Large Capacity V-Type Engine. Source: Fairbanks Morse Engine Div.

the connecting rod. Pistons may be made of cast iron, aluminum, steel, or a combination. Pistons are cooled by circulating lubricating oil (or, in some cases, cooling water) through the cavities or spaces in the piston — the method varying with different designs. Oil spray is also a common cooling method.

The piston is fitted with rings, which ride in grooves cut in the piston head to seal against gas leakage and control oil flow. The compression rings make the piston and the cylinder walls airtight by sealing the space between the piston and the liner. Oil rings, which are located below the compression rings, prevent surplus oil from being carried up into the combustion chamber where it would burn incompletely and form carbon. They are designed to scrape off, on the down-stroke, most of the lubricating oil splashed into the cylinder and return it to the crankcase and ride over the remaining oil film on the way up. The crankcase must be ventilated to remove gases that blow by the piston rings to prevent pressure build-up.

Figure 9-10 shows a piston made of high-tensile steel, with piston pin and rings, and Figure 9-11 illustrates a piston designed for a four-stroke-cycle Diesel engine. This composite piston consists of a forged steel crown, designed to withstand typical firing pressures of about 2,600 psi (180 bar), and a nodular cast piston skirt. This piston design features three compression rings and one oil scraper ring.

The piston pin, or wrist pin, is the link between the connecting rod and the piston. The skirt is the portion of the piston that extends below the piston pin and serves as a guide for the piston and connecting rod. A sometimes-used alternative design to a piston pin is a circular ball joint that allows the piston to rotate.

Fig. 9-10 Piston with Piston Pin and Rings. Source: Wartsila Diesel

Cylinder displacement is the product of stroke (piston travel from BDC to TDC) and the cross-sectional area of the cylinder. The diameter of an engine cylinder is called the bore. Total engine displacement is equal to the displacement of one cylinder times the number of cylinders in the engine. It is calculated as follows:

Fig. 9-11 Diesel Engine Piston Illustration. Source: MAN B&W

Therefore, a 16-cylinder engine with a 13.5 in. bore and a 16.5 in. stroke would have a displacement of:

In SI terms, the bore of this 16-cylinder engine would be 343 mm and the stroke would be about 419 mm. The displacement would be:

(n171.52) x (419.5) x (16) = 619,458 cm3 or 619 liters

Figure 9-12 illustrates the measurements of bore and stroke, as well as engine displacement and crank angle. The crank angle is commonly used to refer to the crank and piston position with respect to TDC and BDC. As shown, at TDC, the crank angle is 0 degrees and at BDC, it is 180 degrees. Engine events are commonly described with respect to crank angle. The timing of fuel injection, for example, is commonly expressed as occurring at a certain number of degrees of crank angle before or after TDC.

Figures 9-13 and 9-14 are cross-sectional illustrations of large capacity, four-stroke-cycle Diesel engines. Both use the same cylinder cover, piston, cylinder liner, and connecting rod, and produce a cylinder output of about 1,300 hp (975 kW). Figure 9-13 features a V-type frame

Stroke Cast Piston Clearance

Fig. 9-12 Illustration Showing Engine Cylinder Geometry.

Fig. 9-13 Cross-Sectional Illustration of Four-Stroke-Cycle, V-type Engine. Source: MAN B&W

Fig. 9-12 Illustration Showing Engine Cylinder Geometry.

design and Figure 9-14 features an in-line frame design. In each engine, the cylinder bore is 18.9 in. (480 mm) and the piston stroke is 23.6 in. (600 mm). Mean piston speed varies from 32.8 to 29.5 ft/s (10.0 to 9.0 m/s), depending on engine rotational speed.

Converting Piston Reciprocating Motion to Shaft Circular Motion

To produce usable engine power output, the force of the reciprocating motion of the piston must be converted to the force of rotational motion of a shaft. A connecting rod (Figure 9-15) is a bar, or strut, with a bearing at each end. Its purpose is to transmit force in either direction between the piston and the crank on the crankshaft of an engine.

The reciprocating motion of the piston and its connecting rod is thus converted into rotating motion of the crankshaft, which is used to drive the load (i.e., generator, compressor, pump, etc.), as well as various engine components. Figure 9-16 illustrates a piston and connecting rod assembly attached to the crankpin on a crankshaft. The

Fig. 9-14 Cross-Sectional Illustration of Four-Stroke-Cycle In-Line Engine. Source: MAN B&W

crankshaft, which is usually a steel forging, is made of a series of cranks. In the case of in-line engines, there is one crank for each cylinder. With V-type engines, each crank

Fig. 9-15 Connecting Rod. Source: Wartsila Diesel

Connecting Rod Attached The Piston
Fig. 9-16 Piston and Connecting Rod Assembly Attached to the Crankpin on a Crankshaft. Source: Waukesha Engine Div.

Fig. 9-15 Connecting Rod. Source: Wartsila Diesel will serve a pair of cylinders.

The crankshaft is made up of a series of bearing surfaces called journals. Figures 9-17 and 9-18 show crankshafts of four-stroke-cycle engines. The crankshaft is housed within the crankcase and is supported in the cylinder block by means of bearings at each of the main bearing journals. Attached to the crankshaft in most engines is a heavy wheel or disc known as a flywheel. The flywheel helps to ensure that the crankshaft turns smoothly by evening out the power pulses from each cylinder.

Work and Power Expressions

There are several ways to refer to the work done by an engine. Engine power, or the rate at which work is done, is expressed as hp or kW and thus is a function of mean effective pressure (mep), the displacement of the engine, and the speed of the engine. The following are several concepts and definitions common to discussions of reciprocating engine power and work. They build the basic concepts presented in Chapter 2.

Engine power (P) is measured by the product of a force (F) and the rate at which it moves, or the distance through which that force travels per unit of time. It can be expressed as:

Fig. 9-17 Crankshaft of Four-Stroke-Cycle Engine. Source: Fairbanks Morse Engine Div.

Fig. 9-18 Crankshaft for Very Large Capacity Reciprocating Engine. Source: Fairbanks Morse Engine Div.

Fig. 9-18 Crankshaft for Very Large Capacity Reciprocating Engine. Source: Fairbanks Morse Engine Div.

Where:

r = Effective length of a brake lever N = Crankshaft rotational speed

In English system units, 1 hp is defined as the power needed to raise 550 lbm through a height of 1 ft in 1 second (550 ft-lbf/sec). This is equivalent to 33,000 ft-lbf/min or 745.701 (746) watts. Thus, when power is expressed in hp, force in lbf, length in ft, and crankshaft rotational speed in rpm, Equation 9-17 becomes:

33,000

5,252

In SI units, when power is expressed in kW, force in Newton (N), length in meters (m), and crankshaft rotational speed in rpm, while noting that 1 kW is equivalent to 60,000 N-m/min, Equation 9-17 becomes:

60,000

9,549

Mean effective pressure (mep) is the work produced by a cycle divided by the volume swept out by the piston in the working stroke and is expressed as:

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