Thursday, March 26, 2015
Base Of Engine Operation
Most of today’s automobiles and light trucks are powered by a spark-ignited four stroke reciprocating
engine. The first engine of this type was built in 1876 by Nicolaus Otto in Germany. Thus, it was
named the Otto-cycle engine. Compared to previous internal combustion engine designs using the
same amount of fuel, Otto’s four stroke engine weighed less, ran much faster, and required less cylinder displacement to produce the same horsepower. A few years later, this engine design powered a motorcycle and then a horseless carriage. Other engine designs in limited use in modern autos include the rotary (Wankel), two stroke, and compression ignition (diesel) engines. In a spark-ignited internal combustion engine, a precise mixture of air and fuel is compressed in a cylinder. The fuel must be of a type that vaporizes
easily (such as gasoline, methanol, or ethanol) or a flammable gas (such as propane or natural gas). When the compressed air-fuel mixture is burned, it
pushes a piston down in a cylinder. This action turns a crankshaft, which powers the car
engine. The first engine of this type was built in 1876 by Nicolaus Otto in Germany. Thus, it was
named the Otto-cycle engine. Compared to previous internal combustion engine designs using the
same amount of fuel, Otto’s four stroke engine weighed less, ran much faster, and required less cylinder displacement to produce the same horsepower. A few years later, this engine design powered a motorcycle and then a horseless carriage. Other engine designs in limited use in modern autos include the rotary (Wankel), two stroke, and compression ignition (diesel) engines. In a spark-ignited internal combustion engine, a precise mixture of air and fuel is compressed in a cylinder. The fuel must be of a type that vaporizes
easily (such as gasoline, methanol, or ethanol) or a flammable gas (such as propane or natural gas). When the compressed air-fuel mixture is burned, it
pushes a piston down in a cylinder. This action turns a crankshaft, which powers the car
SIMPLE ENGINE
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| FIGURE 1.1 A piston forces a crankshaft to turn |
A simple reciprocating engine has a cylinder, a piston, a connecting rod, and a crankshaft. The cylinder can be compared to a cannon and the round piston can be compared to a cannonball. The end of the cylinder is sealed with a cylinder head. The piston, which is sealed to the cylinder wall by piston rings, is connected to the crankshaft by a connecting rod and a piston pin (also called a wrist pin). This arrangement allows the piston to return to the top of the cylinder, making continuous rotary motion of the crankshaft possible. Because of the powerful impulses on the piston as the fuel is burned in the cylinder, a heavy flywheel is
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| FIGURE 1.2 A flywheel is installed at the end of the crankshaft. This is a Buick opposed engine from the early 1900s. |
bolted to the rear of the crankshaft (Figure 1.2).
The weight of the flywheel blends the power impulses together into one continuous motion of the crankshaft.
The cylinder head has one combustion chamber for each cylinder (Figure 1.3). An intake valve port allows
The cylinder head has one combustion chamber for each cylinder (Figure 1.3). An intake valve port allows
a mixture of air and fuel to flow into the cylinder, and an exhaust valve port allows the
burned gases to flow out.
burned gases to flow out.
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| FIGURE 1.3 Valves seal off the valve ports |
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| FIGURE 1.4 A head gasket seals the head to the block |
Each port is sealed off by a poppet style valve. The head is sealed to the cylinder
block with a head gasket (Figure 1.4). The opening of the valves is controlled by the camshaft
(Figure 1.5).
block with a head gasket (Figure 1.4). The opening of the valves is controlled by the camshaft
(Figure 1.5).
A stroke is the movement of the piston from TDC (top dead center) to BDC (bottom dead center), or from BDC to TDC. There are four strokes in one four stroke cycle of the engine. They are called the intake stroke, compression stroke, power stroke, and exhaust stroke.
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| FIGURE 1.5 The opening of the valves is controlled by the camshaft |
• Intake Stroke. Gasoline will not burn unless it is mixed with the correct amount of air. It is very
explosive when 1 part is mixed with about 15 parts of air. Shortly before the piston reaches TDC, the intake valve begins to open. As the crankshaft turns, it pulls the rod and piston down in the cylinder toward BDC (Figure 1.6).
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| FIGURE 1.6 The air-fuel mixture is drawn into the cylinder. |
This action creates a low-pressure void that is filled by atmospheric air pressure and fuel
through the open intake valve. About 10,000 gallons of air is drawn in for every 1 gallon of fuel
supplied by the fuel system. The ideal mixture (called stoichiometric) for the combined purposes
of engine performance, emission control, and fuel economy is about 14.7:1 (at sea level).
Older vehicles had carburetors and newer vehicles manufactured since the mid-1980s have
fuel injection systems with computer controls. The computer monitors the oxygen content in
the vehicle’s exhaust and then adjusts the fuel supply to provide the correct amount of fuel
and air for each intake stroke. As the crankshaft continues to turn, the piston
begins to move back up in the cylinder and the intake valve closes.
• Compression Stroke. The piston moves up in the
cylinder, compressing the air-fuel mixture ( Figure 1.7). If you light a puddle of gasoline on
fire in open air it does not produce power. If it is confined in a cylinder, however, usable power
can be produced.
through the open intake valve. About 10,000 gallons of air is drawn in for every 1 gallon of fuel
supplied by the fuel system. The ideal mixture (called stoichiometric) for the combined purposes
of engine performance, emission control, and fuel economy is about 14.7:1 (at sea level).
Older vehicles had carburetors and newer vehicles manufactured since the mid-1980s have
fuel injection systems with computer controls. The computer monitors the oxygen content in
the vehicle’s exhaust and then adjusts the fuel supply to provide the correct amount of fuel
and air for each intake stroke. As the crankshaft continues to turn, the piston
begins to move back up in the cylinder and the intake valve closes.
• Compression Stroke. The piston moves up in the
cylinder, compressing the air-fuel mixture ( Figure 1.7). If you light a puddle of gasoline on
fire in open air it does not produce power. If it is confined in a cylinder, however, usable power
can be produced.
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| FIGURE 1.7 The air-fuel mixture is compressed. |
Compressing the mixture of air and fuel into a smaller area makes it easier to burn. The compression stroke begins at BDC after the intake stroke is completed. As the piston moves toward TDC, both of the valves are closed as the mixture is compressed to about 1⁄8 of the volume it occupied when the piston was at BDC. In this case, the compression ratio is said to be 8:1 (Figure 1.8).
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| FIGURE 1.8 Compression ratio is a comparison of the volume of the air space above the piston at BDC and at TDC. In this example the compression ratio is 8:1. |
If the mixture is compressed to 1⁄12 its original volume, the compression ratio is then 12:1.
• Power Stroke. As the piston approaches TDC on its compression stroke, the compressed air-fuel
mixture becomes very explosive (Figure 1.9).
mixture becomes very explosive (Figure 1.9).
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| FIGURE 1.9 The air-fuel mixture heats up as it is compressed |
When the ignition system generates a spark at the spark plug, the fuel ignites. The air-fuel mixture burns, but it must not explode. As the
mixture burns it expands, forcing the piston to move down in the cylinder until it reaches BDC
( Figure 1.10). The action of the piston turns the crankshaft to power the car. The power stroke is
sometimes called the expansion stroke. Some leakage of gases past the rings occurs
during the power stroke. This leakage, called blowby, causes pressure in the crankcase
( Figure 1.11).
• Exhaust Stroke. As the piston nears BDC on the power stroke, the exhaust valve opens, allowing
the spent gases to escape. Because the burning gases are still expanding, they are forced out
mixture burns it expands, forcing the piston to move down in the cylinder until it reaches BDC
( Figure 1.10). The action of the piston turns the crankshaft to power the car. The power stroke is
sometimes called the expansion stroke. Some leakage of gases past the rings occurs
during the power stroke. This leakage, called blowby, causes pressure in the crankcase
( Figure 1.11).
• Exhaust Stroke. As the piston nears BDC on the power stroke, the exhaust valve opens, allowing
the spent gases to escape. Because the burning gases are still expanding, they are forced out
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| FIGURE 1.11 Leakage of gases past the ring is known as blowby. |
through the open exhaust valve. As the crankshaft continues to turn past BDC, the piston
moves up in the cylinder, helping to push the remaining exhaust gases out through the open
exhaust valve. A few degrees after the piston passes TDC, the exhaust valve closes.
The entire four stroke cycle repeats itself, starting again as the piston moves down on the
intake stroke. The four stroke cycle is considerably more complicated
than this simple explanation. When the engine is running, the timing of the opening and closing of the valves actually determines when each stroke effectively begins.
CYLINDER ARRANGEMENT
moves up in the cylinder, helping to push the remaining exhaust gases out through the open
exhaust valve. A few degrees after the piston passes TDC, the exhaust valve closes.
The entire four stroke cycle repeats itself, starting again as the piston moves down on the
intake stroke. The four stroke cycle is considerably more complicated
than this simple explanation. When the engine is running, the timing of the opening and closing of the valves actually determines when each stroke effectively begins.
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| FIGURE 1.12 The exhaust valve opens and exhaust gases escape as the piston comes up. |
Automobile and light truck engines have three, four, five, six, eight, or more cylinders. One complete four stroke cycle requires the crankshaft to rotate two times. Two 360° crankshaft revolutions means the crankshaft travels a total of 720° to complete one cycle. During these two
revolutions, each cylinder’s intake and exhaust valves open once. The valves are opened by the camshaft, commonly called the “cam,” which is
considered the “heart” of the engine. The cam has lobes that are off-center and push against the
valvetrain parts, causing the valves to open at precise times (Figure 1.16).
The camshaft controls the rate at which the engine breathes. Its design can be for best operation
at maximum power and high speed, or for fuel economy and best low-speed operation. A production engine is an engine produced at the factory. Production engines are a compromise between
these two concerns, and this is the reason many latemodel
vehicles use variable valve timing. Chapter 10 deals with different “cam grinds” and variable
valve timing in detail. Camshafts can be located either in the block (see Figure 1.16a) or in an overhead cam cylinder
head (see Figure 1.16b). One or more camshafts are driven via crankshaft rotation using one of several combinations, including gears or sprockets and chains or belts. The crank must turn twice for every
one turn of the cam, so there are half as many teeth on the crank drive as there are on the cam drive
(Figure 1.17).
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| FIGURE 1.13 Cylinder arrangements |
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| FIGURE 1.14 A radial engine from a vintage airplane |
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| a |
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| FIGURE 1.15 (a) Section view of an in-line six cylinder engine. (b) Section view of a V6 engine. |
VALVETRAIN
One complete four stroke cycle requires the crankshaft to rotate two times. Two 360° crankshaft
revolutions means the crankshaft travels a total of 720° to complete one cycle. During these two revolutions, each cylinder’s intake and exhaust valves open once. The valves are opened by the
camshaft, commonly called the “cam,” which is considered the “heart” of the engine. The cam has
lobes that are off-center and push against the valvetrain parts, causing the valves to open at precise times (Figure 1.16).
One complete four stroke cycle requires the crankshaft to rotate two times. Two 360° crankshaft
revolutions means the crankshaft travels a total of 720° to complete one cycle. During these two revolutions, each cylinder’s intake and exhaust valves open once. The valves are opened by the
camshaft, commonly called the “cam,” which is considered the “heart” of the engine. The cam has
lobes that are off-center and push against the valvetrain parts, causing the valves to open at precise times (Figure 1.16).
The camshaft controls the rate at which the engine breathes. Its design can be for best operation at maximum power and high speed, or for fuel economy and best low-speed operation. A production engine is an engine produced at the factory. Production engines are a compromise between these two concerns, and this is the reason many latemodel vehicles use variable valve timing. Chapter 10 deals with different “cam grinds” and variable valve timing in detail.
Camshafts can be located either in the block (see Figure 1.16a) or in an overhead cam cylinder head (see Figure 1.16b). One or more camshafts are driven via crankshaft rotation using one of several combinations, including gears or sprockets and chains or belts. The crank must turn twice for every one turn of the cam, so there are half as many teeth on the crank drive as there are on the cam drive (Figure 1.17).
CYLINDER BLOCK
The cylinder block is an intricate casting that includes oil galleries as well as jackets for coolant, which are commonly called water jackets. Cylinder blocks are made of cast iron or aluminum, cast into a sand mold called a core. Many engine blocks today are made of aluminum, cast around iron cylinder bore liners called sleeves (Figure 1.18).
This allows for the weight savings provided by aluminum, coupled with the durability and trueness of cast iron in the cylinder bore area. Some aluminum blocks do not have iron sleeves because aluminum cylinder wall surfaces can be made very hard.
There are different casting processes for engine parts. In the sand casting process, the core is suspended in a container with a liner that will provide the shape for the outside surface of the engine block
(Figure 1.19). The core is supported at several points around the outside of the core box, which leaves core holes in the finished block. The sand casting process uses binders to hold the grains of sand together.
(Figure 1.19). The core is supported at several points around the outside of the core box, which leaves core holes in the finished block. The sand casting process uses binders to hold the grains of sand together.
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| FIGURE 1.16 A cam lobe forces the valve open. (a) The cam-in-block design uses pushrods to open valves. (b) In the overhead cam design, the camshaft is located in the cylinder head.
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| FIGURE 1.18 An aluminum block with cast iron sleeves |
When molten iron or aluminum is poured into the core box, the heat of the casting process cooks the sand. When the casting cools, the sand breaks up. The casting is shaken out and any remaining sand is washed away through the core holes, leaving the finished casting. In another casting process, lost foam casting
(LFC), foam is “lost” or burned up during the pour. General Motors first experimented with LFC in 1982
and since then has refined the process for use in casting blocks, heads, and crankshafts. Saturn used
this process since its beginning in 1990.
With conventional sand casting, oil galleries must be machined in the block casting. With LFC, the oil galleries and coolant passages can be cast into the part. Foam also provides a more accurate casting compared to sand casting. The completed casting is smoother in appearance and there are no parting lines. More intricate castings are possible because the pattern does not need to be removed as was the case with sand castings.
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| FIGURE 1.19 Sand casting cores. |
The LFC pattern is made of expendable polystyrene beads, otherwise known as Styrofoam. Patterns are made by injecting the beads into a die and then superheating them with steam to bond them together and form the finished mold. The foam pattern is coated with a refractory coating, which smoothes the surface of the pattern.
Gates and risers are attached to the pattern to allow for the pouring and venting of the molten metal. Unlike the conventional sand casting process, which uses binders to hold the sand together, LFC uses dry, unbonded sand that is poured around and into the internal passages in the pattern. The sand is vibrated and compacted to thoroughly fill the voids around the pattern. During the pour, the molten metal replaces the pattern as it vaporizes.
When the finished casting has cooled and become solid, the unbonded sand is dumped out. It can be reused, unlike conventional casting sand, which requires disposal. The core holes are closed off with core plugs (Figure 1.20). Core plugs are usually made of steel or brass, although rubber and copper expandable plugs are available, too. Brass core plugs are superior because they do not rust. Brass plugs are not
used in new cars because of their extra cost and because new engines are filled with coolant, which
prevents rust. Core plugs are sometimes referred to as expansion plugs, welsh plugs, freeze plugs, or soft plugs.
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| FIGURE 1.20 Core plugs. |
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