Engine Classifications
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Thursday, September 10, 2009
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Automotive engines are classified in many different ways.All automotive engines are internal combustion type.
Internal combustion engines can be classified according to:
- Number of Cylinders
- Arrangements of Cylinders
- Arrangements of Valves
- Type of Cooling
- Number of strokes per cycle (two or four)
- type of fuel used
- Method of ignition (spark or compression)
- Firing Order
- Reciprocating or rotary
Number And Arrangement of Cylinders
American passengers car engines have four,six,or eight cylinders.Foreign made cars offer a great variety , including three, four,five,six,eight and twelve cylinders.Engines with for,five,six and eight cylinders are described and illustrated in the next few sections.Cylinders can be arranged in several ways.
- In a row
- In two rows or banks set at an angle (V type)
- In two rows opposing each other (flat or pancake)
- Like spokes on a wheel (Radial airplane type)
FOUR CYLINDER ENGINE
The cylinders of a for cylinder engine can be arranged in any of three ways, in line, V or opposed.In the V type, the cylinders are in two banks or rows of two cylinders each.The two rows are set at an angle to each other.In the opposed type the cylinders are in two banks of two cylinders each,set opposite each other.
The four cylinder engine has become increasingly popular in recent years.A basic reason is the trend toward small light weight,fuel efficient cars.This trend has been caused by the oil shortage and government regulations regarding car size and fuel mileage requirements.
The small four cylinder engine does not give the car acceleration it has with a bigger engine.However turbocharging the engine can improve this.Turbocharging forces more air into the engine cylinders so that more fuel can be burned>as a result, more power is produced.There are various type of four cylinder engines.
How the Engine Works
The basic piston engine is a metal block containing a series of chambers. The upper engine block is usually an iron or aluminum alloy casting, consisting of outer walls that form hollow jackets around the cylinder walls. The lower block, which provides a number of rigid mounting points for the bearings that hold the crankshaft in place, is known as the crankcase. The hollow jackets of the upper block add rigidity to the engine and contain the liquid coolant that carries heat away from the cylinders and other engine parts.
An air-cooled engine block consists of a crankcase that provides a rigid mounting for the crankshaft and has studs to hold the cylinders in place. The cylinders are individual, single-wall castings, finned for cooling, and they are usually bolted to the crankcase, rather than cast integrally with the block.
In a water-cooled engine, only the cylinder head is bolted to the block (usually on top). The water pump is mounted directly to the block.
The crankshaft is a long iron or steel shaft (and sometimes aluminum in more high-tech or high performance applications) mounted rigidly at a number of points in the bottom of the crankcase. The crankshaft is free to turn and contains several counterweighted crankpins (one centered under each cylinder) that are offset several inches from the center of the crankshaft and turn in a circle as the crankshaft turns. Pistons are connected to the crankpins by steel connecting rods. The rods connect the pistons at their upper ends with the crankpins at their lower ends. Circular rings seal the small space between the pistons and wall of the cylinders.
When the crankshaft spins, the pistons move up and down in the cylinders, varying the volume of each cylinder, depending on the position of the piston. At least two openings in each cylinder head (above the cylinders) allow the intake of the air/fuel mixture and the exhaust of burned gasses. After intake, the pistons compress the fuel mixture at the top of the cylinder, the fuel is ignited, and, as the pistons are forced downward by the expansion of burning fuel, the connecting rods convert the up and down motion of the pistons into rotary (turning) motion of the crankshaft. A round flywheel at the rear of the crankshaft provides a large, stable mass to smooth out the rotation.
The cylinder heads form tight covers for the tops of the cylinders and contain chambers into which the fuel mixture is forced as it is compressed by the pistons reaching the upper limit of their travel. Each combustion chamber contains at least one intake valve, one exhaust valve, and one spark plug per cylinder (depending on the design). The tips of the spark plugs protrude into the combustion chambers.
The valve in each opening of the cylinder head is opened and closed by the action of the camshaft. The camshaft is driven by the crankshaft through a gear, chain, or belt at 1/2 crankshaft speed (the camshaft gear is twice the size of the crankshaft gear). The valves are operated either through rocker arms and pushrods (overhead valve and some overhead cam engines) or directly by the camshaft using cam followers which usually contain shims for adjustment (overhead cam engine).
Lubricating oil is stored in a pan at the bottom of the engine and is force-fed to all parts of the engine by a gear-type pump, driven from the crankshaft. The oil lubricates the entire engine and seals the piston rings, giving good compression.
- IN LINE ENGINES
The four cylinder engine is used widely in the modern,small fuel efficient cars.Many four cylinder in line and V type engines mount transversely in the car.With this arrangement the engine drives the front wheels.
- V-4 ENGINES
The V4 engine has two rows of two cylinders each set at an angle or a V to each other.The crankshaft has only two cranks.Connecting rod from opposing cylinders in the two rows are attached to the same crankpin.therefore each crankpin has two connecting rods attached it.
This type of engine is difficult to balance with counterweights on the crankshaft.The engine is balanced by a balance shaft that turns in s direction opposite to crankshaft.
FIVE CYLINDER ENGINE
Some five cylinder automotive engines are being built.Mercedes produces a five cylinder diesel engine.Volkswagen has a five cylinder in line spark ignition engine for a front drive car.Note the axle coming out the side of the transmission.The engine is mounted ahead of frontwheel drive axles.
SIX CYLINDER IN LINE ENGINE
Most six cylinder engine are line (straight six) although there are V-6 and flat six engines.This compares to the four cylinder engines, which can also be in line, V type or flat.
The valves are overhead and the camshaft is in the cylinder block.The crankshaft is supported by seven main bearings.This engine is also known as a slant six because the cylinders are slanted to one side.This makes additional room vertically so the hood line can be lowered.This engine is made with either a cast iron aluminum alloy cylinder block.
V6 ENGINES
A V6 engine has two rows of three cylinders each set at an angle to from a V.A V 6 engine mounted transversely between the front wheels.It also shows the transmission and axles to the front wheels.This is a spark ignition engine, with the camshaft in the cylinder block.
V-8 ENGINES
In the v-8 engine cylinders are arranged in the two rows, or banks set at a an angle to each to form a V.This engine is like two four cylinder in line engines mounted on a single crankcase and using one crankshaft.The crankshaft has four cranks.Connecting rod from opposing cylinders in the two banks are attached to a single crank pin .Two rods are attached to each crankpin.Two pistons are connected to each crankpin.The crankshaft usually is supported by five main bearings.
Cutaway of the V-8 Engine
This diagram shows the flow of fuel and exhaust within a V8 engine. It shows the timing chain (driven by the crankshaft) drives the camshaft, which opens the valves. Fuel enters the cylinders via the intake manifold. The spark-caused explosions force the pistons down. Rotation of the crank forces the pistons back up, which expels the exhaust.
Cylinder
A cylinder is a round hole through the block, bored to receive a piston. All automobile engines, whether water-cooled or air-cooled, four cycle or two cycle, have more than one cylinder. These multiple cylinders are arranged in-line, opposed, or in a V. Engines for other purposes, such as aviation, are arranged in other assorted forms.
The diameter of the cylinder is called the "bore" while its height is called its "stroke." The "displacement" of an engine is actually a reflection of the total amount of volume of the engine's cylinders, and nothing to do with the actual size of the engine itself (although the two are highly correlated). The displacement is simply the bore multiplied by the stroke of a single cylinder, multiplied by the total number of cylinders in the engine. Muscle car engine displacements were usually measured in cubic inches, while modern vehicle's are expressed in terms of liters. Roughly 61 cubic inches equals a liter of displacement. Therefore, an engine with 350 cubic inches of displacement would be the equivalent of 5.7 liters.
The Piston, Rings, and Wrist Pin
The piston converts the potential energy of the fuel, into the kinetic energy that turns the crankshaft. The piston is a cylindrical shaped hollow part that moves up and down inside the engine's cylinder. It has grooves around its perimeter near the top where rings are placed. The piston fits snugly in the cylinder. The piston rings are used to ensure a snug "air tight" fit.
The piston requires four strokes (two up and two down) to do its job. The first is the intake stroke. This is a downward stroke to fill the cylinder with a fuel and air mixture. The second is an upward stroke to compress the mixture. Right before the piston reaches its maximum height in the cylinder, the spark plug fires and ignites the fuel. This action causes the piston to make its third stroke (downward). The third stroke is the power stroke; it is this stroke that powers the engine. On the fourth stroke, the burned gases are sent out through the exhaust system.
The wrist pin connects the piston to the connecting rod. The connecting rod comes up through the bottom of the piston. The wrist pin is inserted into a hole (about half way up) that goes through the side of the piston, where it is attached to the connecting rod.
Pistons are made of aluminum, because it is light and a good heat conductor. Pistons perform several functions. Pistons transmit the driving force of combustion to the crankshaft. This causes the crankshaft to rotate. The piston also acts as a moveable gas-tight plug that keeps the combustion in the cylinder. The piston acts as a bearing for the small end of the connecting-rod. Its toughest job isto get rid of some of the heat from combustion, and send it elsewhere.
The piston head or "crown" is the top surface against which the explosive force is exerted. It may be flat, concave, convex or any one of a great variety of shapes to promote turbulence or help control combustion. In some, a narrow groove is cut into the piston above the top ring to serve as a "heat dam" to reduce the amount of heat reaching the top ring.
Cam Shafts
For an engine to make more power, it has to take in more air. In most four stroke engines, the air must enter the combustion chamber through the valves. The camshaft controls the opening and closing of the valves by regulating the time that the valve is opened and closed, and how much the valve is opened by. An easy solution to have more power, would be to alter the characteristcs of the camshaft so that it either keeps the valves open for a longer period of time, or lift the valve higher off it's seat so that more air can pass into the combustion chamber. It all sounds very easy, but once again, there's more to it than meets the eye. Like most engine mods, this one is also a compromise.
In the perfect engine, the inlet valve will open when the piston is at TDC (top dead center), and as it travels down the bore, it will suck in a full charge equal to it's displacement. The exhaust valve would open at BDC (bottom dead center), and the full displacement of spent gasses would be pumped out of the engine - the perfect engine running at 100% volumetric efficiency. In practice, the stresses on the valvetrain would just be too much for the materials to handle. To lift a valve of say 50g some 10mm off it's seat in less than a millisecond (at 6000rpm) without it bouncing or doing anything untoward in the next 100,000 miles of it's life, simply doesn't work with the materials in use today. So, the manufacturers used their multi-million dollar research budgets to come up with a simple solution.
The piston travels rather slowly at TDC compared to the middle of the stroke - there's not much of the pumping action being done in the 10 or 20 degrees around TDC. So, they start to open the valve gently while the piston is still on it's way up on the exhaust stroke. Although this creates valve "overlap" (time in which both the intake and the exhaust valves are open), it does allow the engine to breathe better and create more power.
When the time that the inlet valve stays open is made longer, the overlap starts to become a problem at low engine speed. The exhaust gasses get pumped into the inlet tracts, substantially diluting the incoming charge and causing the engine to run very poor. That's why an engine with a wild camshaft runs uneven at idle - it's choking in it's own exhaust gasses. However, when the engine speed goes up, the exhaust gasses pick up momentum, and during the overlap period, the departing exhaust charge creates a partial vacuum behind it, sucking in more of the fresh intake charge.
This leads us to two important conclusions:
Firstly, the wilder the camshaft, the less power the engine will make at low rpm. Such wild engines will normally not have enough power at regular "civilized" driving speeds to pull the skin off a rotten banana. To pull away from a stop, you will have to rev it up to come "on the cam", or stall the engine at every attempt at a civilized getaway. Secondly, the engine will only produce more power at the very top of it's rev range. These are important points to consider when choosing a racy camshaft for your engine. Are you willing to sacrifice low speed drivability in exchange for more top end power? It's up to you to decide.
No, we are not against performance camshafts. We have owned several "hairy" cammed cars, and want to point out the facts to you so you won't end up wit a car you hate. Driving such a car to work every day soon starts to get on one's nerves. And if you transport passengers in your vehicle, be warned : they are usually not very sympathetic towards the neck-wrenching style of driving that such a vehicle demands to keep it "on the boil". If you do decide to go with a hairy cam, there are a few things you can do to slightly alleviate the associated low speed problems.
1. A good free-flow extractor exhaust with long primary pipes tuned to low engine speed optimisation can make the engine come on the cam a little sooner. The long 4-into-1 systems seem to be able to "pull the engine on the cam" a little sooner than the regular banana style 4-into-2-into-1 systems.
2. Long ramstacks on the intake. A ram stack are those shiny flared tubes you often see on the carburettors of high-performance engines. These artificially create a longer intake path for the air, allowing it to build up some momentum. They also have an added benefit that they can allow up to 8% more flow into the carb when compared to the usually blunt ending of the carb mouth.
3. Proper gas-flowing of the cylinder head. A lot of cylinder heads out there flow more air in the wrong direction than they can flow in the right direction. Most people who gasflow cylinder heads don't even realize that they are making it easier for the gasses to also flow well in the wrong direction! Remember that the main problem is that the exhaust gasses flow into the intake port during the increased overlap period. We can put you in touch with people who can do special things to a cylinder head so that it is difficult for the exhaust gasses to pop out through the intake port in the camshafts' overlap period. There's a whole science behind optimising the head to make it "cam-friendly", and usually there is a substantial improvement in the low speed range if the cylinder head is flowed properly, by a person who knows what directional flowing is about. Note that it is easy - even for experienced "port grinders" - to completely ruin the reverse-flow characteristics of your cylinder head.
4. Match the engine controls to the camshaft. The different profile of the camshaft plays havoc with the fuel injection's standard factory mapping. The ignition timing and mixture requirements of the engine is vastly different to that of a standard engine. The way we would recommend to do this, is to fit a UNICHIP. The engine can be run on a loading type dynamometer, and the engine management system can be reprofiled to match the specific engine's state of tune. The unichip is perfect for modified engines, because of it's ability to be reprogrammed whenever needed, i.e. if you decide to make more mod's, you simply have the unichip reprogrammed to match your new requirements. You don't have to throw it away like a conventional, old style "chip".
Serpentine Belts
A recent development is the serpentine belt, so named because they wind around all of the pulleys driven by the crankshaft pulley. This design saves space, but if it breaks, everything it drives comes to a stop.
Timing Chain/belt
The automobile engine uses a metal timing chain, or a flexible toothed timing belt to rotate the camshaft. The timing chain/belt is driven by the crankshaft. The timing chain, or timing belt is used to "time" the opening and closing of the valves. The camshaft rotates once for every two rotations of the crankshaft.
The Cylinder Head
The cylinder head is the metal part of the engine that encloses and covers the cylinders. Bolted on to the top of the block, the cylinder head contains combustion chambers, water jackets and valves (in overhead-valve engines). The head gasket seals the passages within the head-block connection, and seals the cylinders as well.
Push Rods
Push Rods attach the valve lifter to the rocker arm. Through their centers, oil is pumped to lubricate the valves and rocker arms.
Flywheel
The flywheel is a fairly large wheel that is connected to the crankshaft. It provides the momentum to keep the crankshaft turning without the application of power. It does this by storing some of the energy generated during the power stroke. Then it uses some of this energy to drive the crankshaft, connecting rods and pistons during the three idle strokes of the 4-stroke cycle. This makes for a smooth engine speed. The flywheel forms one surface of the clutch and is the base for the ring gear.
Harmonic Balancer (Vibration Damper)
The harmonic balancer, or vibration damper, is a device connected to the crankshaft to lessen the torsional vibration. When the cylinders fire, power gets transmitted through the crankshaft. The front of the crankshaft takes the brunt of this power, so it often moves before the rear of the crankshaft. This causes a twisting motion. Then, when the power is removed from the front, the halfway twisted shaft unwinds and snaps back in the opposite direction. Although this unwinding process is quite small, it causes "torsional vibration." To prevent this vibration, a harmonic balancer is attached to the front part of the crankshaft that's causing all the trouble. The balancer is made of two pieces connected by rubber plugs, spring loaded friction discs, or both.
When the power from the cylinder hits the front of the crankshaft, it tries to twist the heavy part of the damper, but ends up twisting the rubber or discs connecting the two parts of the damper. The front of the crank can't speed up as much with the damper attached; the force is used to twist the rubber and speed up the damper wheel. This keeps the crankshaft operation calm.
Crankshaft
The crankshaft converts the up and down (reciprocating) motion of the pistons into a turning (rotary) motion. It provides the turning motion for the wheels. The crankshaft is usually either alloy steel or cast iron. The crankshaft is connected to the pistons by the connecting-rods.
Some parts of the shaft do not move up and down; they rotate in the stationary main bearings. These parts are known as journals. There are usually three journals in a four cylinder engine.
Main Bearings
The crankshaft is held in place by a series of main bearings. The largest number of main bearings a crankshaft can have is one more than the number of cylinders, but it can have one less bearing than the number of cylinders.
Not only do the bearings support the crankshaft, but one bearing must control the forward-backward movement of the crankshaft. This bearing rubs against a ground surface of the main journal, and is called the "thrust bearing."
Connecting Rod
The connecting rod links the piston to the crankshaft. The upper end has a hole in it for the piston wrist pin and the lower end (big end) attaches to the crankshaft. Connecting rods are usually made of alloy steel, although some are made of aluminum.
Connecting Rod Bearings
Connecting rod bearings are inserts that fit into the connecting rod's lower end and ride on the journals of the crankshaft.
Factory RPM Range
Note the reference to factory RPM range. This is an extremely important concept, and must be clearly understood before starting your improvement project. The factory engines were designed and built to run in a specific RPM range. Their parts were of sufficient quality to run almost indefinitely if the RPM limits were observed. The engines developed maximum power throughout the intended range with the heads, manifolds, cams, and manifolds that were installed. For example, most standard production cars used a large two barrel carb., an #066 cam (also called a #4 in the earlier versions), which is 204 degrees intake duration at .050, and ordinary heads with press-in studs, but having very good low and mid-lift air flow. This combination provides extremely strong low and mid range torque which is exactly what the larger cars with high gears need for good throttle feel and quick response. This type of engine doesn't develop high horsepower because it will not run much past 4600-4800 RPM and can't breath enough air at high RPM, but it does develop excellent torque from idle up, and essentially the same total amount of torque as the highest HP engines of the same displacement. The Ram Air IV engine was designed to run to a higher RPM of about 5900. This required more air flow into the engine at higher RPM—thus, the higher flowing heads were incorporated. A longer duration cam was needed to give the cylinders time to fill at the higher RPM. The longer duration cam causes the intake valve to close later in the intake cycle, and this in turn, required more compression. The longer duration cam kills the low RPM power while hopefully extending the upper RPM power. With very poor low end power, a lower rear end gear was needed to provide some semblance of low speed performance. As the engine was so weak at low RPM, power steering and air conditioning were not available, and the engine was available only in the lightest body style vehicles. The result was a higher RPM engine with excellent power from about 3000 to 6000 RPM. This is great for a lighter weight car with a 4-speed, or an automatic with a loose converter for drag racing but it would be a dog in a normal weight street vehicle that needs to be driven from stop light to stop light.
So what is the answer for real performance increase? First, determine what RPM range you actually need and intend to use. If you plan to drive the car for some normal transportation, any idle speed over about 650 RPM will be a constant pain with stock converters. If you want good power and throttle response from idle to 3000 RPM (about 70-75 MPH in high gear), don't install a cam with more than about 210-215 degrees intake duration as measured at .050 lift. Similarly, don't install a single plane manifold or a carb larger than 750 CFM (except for an 800 Q-Jet) on this type of vehicle. Be wary of the "Performer RPM" manifold, even though it is a dual plane. It definitely degrades low end power, and only begins to help at around 5400 RPM and up. Remember that low-end power is relative to the size/torque of an engine, and that a 455 will have relatively good low-end with an "RPM" but it will still lose power from idle to about 2000 with it! By staying in the factory intended RPM range, your rods, rod bolts, pistons crank, and oil pump are totally satisfactory for any performance use (assuming they are in normal factory condition). The heads, regardless of type, should have first quality valve guides, a valve seat preparation that optimizes low lift air flow, and matching valves. The exhaust seats do not need to be hardened, because you will never load the engine hard enough for a long enough period of time to damage the seats. Even if you somehow manage to do so, this is not a catastrophic event, and the seats could be changed later if needed.
After you have determined what RPM range you expect to use, plan accordingly If you will run higher RPM than your present engine was designed for, consider what changes will be needed. If you are thinking of building a race engine, you may need special rods, forged and/or lightened pistons, vastly improved air flow through your heads, a poorer idling cam, higher performance manifold and headers. If you are thinking an engine for race and street, all the stock internal parts are totally satisfactory. "Hotter" ignition systems or components will not improve performance over properly operating factory systems. The factory Q-Jet manifold and carb are adequate and actually superior to any aftermarket setups you can buy. The factory ignition points type or HEI, will easily do the job, although the points system must be properly adjusted and maintained. Stock exhaust or Ram Air type manifolds will work fine, and headers with 1-5/8" or 1-3/4" primary tubes can be used if you want to put up with the hassle of leakage, additional noise, poor ground clearance, difficult installation and high maintenance.
There are various methods of increasing engine RPM capability. However, increased RPM does not automatically improve acceleration. Each vehicle has unique gearing, weight, and engine power range. For optimum acceleration, the engine should be operated such that it stays in its fattest power band through each gear. For example, if the engine makes good power from 3200 to 5000, it makes no sense to shift at 5500 because you not only lose acceleration from the 5000 to 5500 range, but when you shift to the next gear, the engine will only drop to about 3500, thus losing the power from 3000 to 3500. Regardless of your engine characteristics, you must try shifting at various RPM points to find the best overall point for your combination.
VARIABLE-DISPLACEMENT V-8 ENGINES
This engine has electronic controls that selectively cut out two or four cylinders at a time.The number of cylinders the controls cut out depends on the power requirements.When the engine is idling or cruising at a steady speed on a level highway the controls cut out four of the cylinders.Only four cylinders are needed to provide sufficient power for these operating conditions.
However when the car encounters a hill, additional power is needed and electronic controls put additional cylinders to work.The same thing happens when the driver 'steps on the gas' to increase car speed, as, for example,passing another car.Additional cylinders go to work.The number of cylinders that go back to work depends on the amount of additional power that is needed.The arrangement saves fuel without sacrifice of performance,according to the manufacture.
TWELVE AND SIXTEEN CYLINDER ENGINES
Twelve and sixteen cylinder engines have been used in passenger cars,buses,trucks and industrial plants.The cylinders are mostly in two banks (V type or pancake type).Sometimes they are in three banks ( W type) or four banks (X type).The pancake engine is similar to a V engine, but the two rows are flat and opposing.The cylinders work to the same crankshaft.The only passenger cars now being made with a twelve cylinder engine are the Ferrati, the Jaguar, and the Maserati.
Twelve Cylinder Engine
The V-12 powering the new 760Li is a new design with 6.0 liters of displacement, dual overhead camshafts (per cylinder bank) and 4 valves per cylinder. Designated the N73, it is also related to the N62 V-8 engine that powers the two 745 models.
The V-12 configuration has long been recognized as an ultimate power plant concept, suited for top-class vehicles.
SIXTEEN CYLINDER ENGINE
long is no larger than a conventional V12 unit, and due to its lightweight construction weighs only
about 400 kilos. Its compact dimensions are due to the unique arrangement of its cylinder banks in
a W configuration. Two VR8 blocks, each with a fifteen degree bank angle, are joined in the crankcase
to form one engine. Both eight cylinders are set at an angle of ninety degrees to each other and are
aspirated by a total of four exhaust gas turbochargers. The engine delivers 1001 HP at 6,000 r.p.m.
and provides a maximum torque of 1250 Newtonmetres at between 2,200 and 5,500 r.p.m.
Pushroad valve train
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There are several other valve train arrangements.The most popular arrangement for many years uses a pushrod.For this reason, the engine is often called a pushrod engine.The arrangement has the camshaft in the cylinder block.Each part is described below.
- The valve lifter is around cylinder that rides on the cam.The cam rotates under it.
- The pushrod is a long rod that goes from the valve lifter up to one end of rocker arm.
- The rocker arm is pivoted at about its middle.The push rod pushes aganist one end of the rocker arm.The other end of the rocker armrests on the valve stem.
- The valve spring a coil spring that fits between the cylinder head and a retainer on the stem end of the valve.
Valve Action
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There are usually two valves per cylinder.The valves are opened and closed by the action of the valve train.One of the simplest valve trains shown below.
It has a camshaft mounted on the top of the cylinder head.The camshaft is driven from crankshaft by a belt with teeth on its inside.The teeth match the teeth on the sprockets on the crankshaft and camshaft.
the camshaft has two cams for each cylinder,one for the intake valve and one for exhaust valve.The cam is a round collar with a high spot, or lobe.
The top of the valve stem is covered with a small hollow cylinder called a bucket valve tappet.It is called a bucket valve tappet because it is shaped like an upside-down bucket.When the camshaft and cam rotate,the lobe comes around and pushes the valve tappet and valve stem,down.The valve opens.When the lobe passes out from valve up so that the valve closes.
Engine with the camshaft on the cylinder head are called overhead-camshaft engines.They are more responsive than other type of valve train.This is because there are fewer parts in the valve tarin
The Valves
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Most engine have two holes,or ports in the enclosed upper end of cylinder.One of the two ports is the intake port.It allows the mixture of gasoline vapour and air to enter the cylinder.The other is the exhaust from,or leave the cylinder.
These two parts are open only part of the time.The rest of the time they are closed off by the intake and exhaust valves.The valves are metal plugs that fit the round holes or ports.When a valve moves up into its ports,it seals off its port is open.The valves are operated by several engine parts that make up the vale train.The valve train causes the valves to open and close.
Piston And Piston Rings
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Pistons are made of aluminum alloy,which is a mixture of aluminum and other metals.Automotive pistons weigh about 1 pound (0.454 kg).They are a sliding fit in the cylinders.This means the pistons can slide up and down in the cylinders.Therefore the pistons are slightly smaller in diameter than the cylinders.
Because the pistons are slightly smaller,there is a gap or clearance, between the piston ans cylinder.This gap must be filled.Otherwise,some of the compressed air fuel mixture would leak out through the clearance.Also when combustion took place,much of the pressure gas would leak out.These leaks would greatly reduce the efficiency of the engine.Much of the power would be lost.
A piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Thermal conductivity is the ability of a material to conduct and transfer heat. Aluminum expands when heated, and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise.
Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings. The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.
A piston pin bore is a through hole in the side of the piston perpendicular to piston travel that receives the piston pin. A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston. The skirt of a piston is the portion of the piston closest to the crankshaft that helps align the piston as it moves in the cylinder bore. Some skirts have profiles cut into them to reduce piston mass and to provide clearance for the rotating crankshaft counterweights.
A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. A piston ring is an expandable split ring used to provide a seal between the piston an the cylinder wall. Piston rings are commonly made from cast iron. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces. Piston rings seal the combustion chamber, conduct heat from the piston to the cylinder wall, and return oil to the crankcase. Piston ring size and configuration vary depending on engine design and cylinder material.
Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring. A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. When the air-fuel mixture is ignited, pressure from combustion gases is applied to the piston head, forcing the piston toward the crankshaft. The pressurized gases travel through the gap between the cylinder wall and the piston and into the piston ring groove. Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. Pressure applied to the piston ring is approximately proportional to the combustion gas pressure.
A wiper ring is the piston ring with a tapered face located in the ring groove between the compression ring and the oil ring. The wiper ring is used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by the compression ring are stopped by the wiper ring.
An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to wipe excess oil from the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block. Two-stroke cycle engines do not require oil rings because lubrication is supplied by mixing oil in the gasoline, and an oil reservoir is not required.
Piston rings seal the combustion chamber, transferring heat to the cylinder wall and controlling oil consumption. A piston ring seals the combustion chamber through inherent and applied pressure. Inherent pressure is the internal spring force that expands a piston ring based on the design and properties of the material used. Inherent pressure requires a significant force needed to compress a piston ring to a smaller diameter. Inherent pressure is determined by the uncompressed or free piston ring gap. Free piston ring gap is the distance between the two ends of a piston ring in an uncompressed state. Typically, the greater the free piston ring gap, the more force the piston ring applies when compressed in the cylinder bore.
A piston ring must provide a predictable and positive radial fit between the cylinder wall and the running surface of the piston ring for an efficient seal. The radial fit is achieved by the inherent pressure of the piston ring. The piston ring must also maintain a seal on the piston ring lands.
In addition to inherent pressure, a piston ring seals the combustion chamber through applied pressure. Applied pressure is pressure applied from combustion gases to the piston ring, causing it to expand. Some piston rings have a chamfered edge opposite the running surface. This chamfered edge causes the piston ring to twist when not affected by combustion gas pressures.
Another piston ring design consideration is cylinder wall contact pressure. This pressure is usually dependent on the elasticity of the piston ring material, free piston ring gap, and exposure to combustion gases. All piston rings used by Briggs & Stratton engines are made of cast iron. Cast iron easily conforms to the cylinder wall. In addition, cast iron is easily coated with other materials to enhance its durability. Care must be exercised when handling piston rings, as cast iron is easily distorted. Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring.
Compression Ring
The compression ring is the top or closest ring to combustion gases and is exposed to the greatest amount of chemical corrosion and the highest operating temperature. The compression ring transfers 70% of the combustion chamber heat from the piston to the cylinder wall. Most Briggs & Stratton engines use either taper-faced or barrel-faced compression rings. A taper faced compression ring is a piston ring that has approximately a 1° taper angle on the running surface. This taper provides a mild wiping action to prevent any excess oil from reaching the combustion chamber.
A barrel faced compression ring is a piston ring that has a curved running surface to provide consistent lubrication of the piston ring and cylinder wall. This also provides a wedge effect to optimize oil distribution throughout the full stroke of the piston. In addition, the curved running surface reduced the possibility of an oil film breakdown due to excess pressure at the ring edge or excessive piston tilt during operation.
Wiper Ring
The wiper ring, sometimes called the scraper ring, Napier ring, or back-up compression ring, is the next ring away from the cylinder head on the piston. The wiper ring provides a consistent thickness of oil film to lubricate the running surface of the compression ring. Most wiper rings in Briggs & Stratton engines have a taper angle face. The tapered angle is positioned toward the oil reservoir and provides a wiping action as the piston moves toward the crankshaft.
The taper angle provides contact that routes excess oil on the cylinder wall to the oil ring for return to the oil reservoir. A wiper ring incorrectly installed with the tapered angle closest to the compression ring results in excessive oil consumption. This is caused by the wiper ring wiping excess oil toward the combustion chamber.
Oil Ring
An oil ring includes two thin rails or running surfaces. Holes or slots cut into the radial center of the ring allow the flow of excess oil back to the oil reservoir. Oil rings are commonly one piece, incorporating all of these features. Some on-piece oil rings utilize a spring expander to apply additional radial pressure to the piston ring. This increases the unit (measured amount of force and running surface size) pressure applied at the cylinder wall.
The oil ring has the highest inherent pressure of the three rings on the piston. Some Briggs & Stratton engines use a tree-piece oil ring consisting of two rails and an expander. The oil rings are located on each side of the expander. The expander usually contains multiple slots or windows to return oil to the piston ring groove. The oil ring uses inherent piston ring pressure, expander pressure, and the high unit pressure provided by the small running surface of the thin rails.
The piston acts as the movable end of the combustion chamber and must withstand pressure fluctuations, thermal stress, and mechanical load. Piston material and design contribute to the overall durability and performance of an engine. Most pistons are made from die- or gravity-cast aluminum alloy. Cast aluminum alloy is lightweight and has good structural integrity and low manufacturing costs. The light weight of aluminum reduces the overall mass and force necessary to initiate and maintain acceleration of the piston. This allows the piston to utilize more of the force produced by combustion to power the application. Piston designs are based on benefits and compromises for optimum overall engine performance
Automobile Engines
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Wednesday, September 9, 2009
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Internal combustion gasoline engines run on a mixture of gasoline and air. The ideal mixture is 14.7 parts of air to one part of gasoline (by weight.) Since gas weighs much more than air, we are talking about a whole lot of air and a tiny bit of gas. One part of gas that is completely vaporized into 14.7 parts of air can produce tremendous power when ignited inside an engine.
Let's see how the modern engine uses that energy to make the wheels turn.
Air enters the engine through the air cleaner and proceeds to the throttle plate. You control the amount of air that passes through the throttle plate and into the engine with the gas pedal. It is then distributed through a series of passages called the intake manifold, to each cylinder. At some point after the air cleaner, depending on the engine, fuel is added to the air-stream by either a fuel injection system or, in older vehicles, by the carburetor.
Once the fuel is vaporized into the air stream, the mixture is drawn into each cylinder as that cylinder begins its intake stroke. When the piston reaches the bottom of the cylinder, the intake valve closes and the piston begins moving up in the cylinder compressing the charge. When the piston reaches the top, the spark plug ignites the fuel-air mixture causing a powerful expansion of the gas, which pushes the piston back down with great force against the crankshaft, just like a bicycle rider pushing against the pedals to make the bike go.
Let's take a closer look at this process.
Engine Types
The majority of engines in motor vehicles today are four-stroke, spark-ignition internal combustion engines. The exceptions like the diesel and rotary engines will not be covered in this article.
There are several engine types which are identified by the number of cylinders and the way the cylinders are laid out. Motor vehicles will have from 3 to 12 cylinders which are arranged in the engine block in several configurations. The most popular of them are shown on the left. In-line engines have their cylinders arranged in a row. 3, 4, 5 and 6 cylinder engines commonly use this arrangement. The "V" arrangement uses two banks of cylinders side-by-side and is commonly used in V-6, V-8, V-10 and V-12 configurations. Flat engines use two opposing banks of cylinders and are less common than the other two designs. They are used in engines from Subaru and Porsche in 4 and 6 cylinder arrangements as well as in the old VW beetles with 4 cylinders. Flat engines are also used in some Ferraris with 12 cylinders
Most engine blocks are made of cast iron or cast aluminum..
The crankshaft is located below the cylinders on an in-line engine, at the base of the V on a V-type engine and between the cylinder banks on a flat engine. As the pistons move up and down, they turn the crankshaft just like your legs pump up and down to turn the crank that is connected to the pedals of a bicycle.
A cylinder head is bolted to the top of each bank of cylinders to seal the individual cylinders and contain the combustion process that takes place inside the cylinder. Most cylinder heads are made of cast aluminum or cast iron. The cylinder head contains at least one intake valve and one exhaust valve for each cylinder. This allows the air-fuel mixture to enter the cylinder and the burned exhaust gas to exit the cylinder. Engines have at least two valves per cylinder, one intake valve and one exhaust valve. Many newer engines are using multiple intake and exhaust valves per cylinder for increased engine power and efficiency. These engines are sometimes named for the number of valves that they have such as "24 Valve V6" which indicates a V-6 engine with four valves per cylinder. Modern engine designs can use anywhere from 2 to 5 valves per cylinder.
The valves are opened and closed by means of a camshaft. A camshaft is a rotating shaft that has individual lobes for each valve. The lobe is a "bump" on one side of the shaft that pushes against a valve lifter moving it up and down. When the lobe pushes against the lifter, the lifter in turn pushes the valve open. When the lobe rotates away from the lifter, the valve is closed by a spring that is attached to the valve. A common configuration is to have one camshaft located in the engine block with the lifters connecting to the valves through a series of linkages. The camshaft must be synchronized with the crankshaft so that the camshaft makes one revolution for every two revolutions of the crankshaft. In most engines, this is done by a "Timing Chain" (similar to a bicycle chain) that connects the camshaft with the crankshaft. Newer engines have the camshaft located in the cylinder head directly over the valves. This design is more efficient but it is more costly to manufacture and requires multiple camshafts on Flat and V-type engines. It also requires much longer timing chains or timing belts which are prone to wear. Some engines have two camshafts on each head, one for the intake valves and one for the exhaust valves. These engines are called Double Overhead Camshaft (D.O.H.C.) Engines while the other type is called Single Overhead Camshaft (S.O.H.C.) Engines. Engines with the camshaft in the block are called Overhead Valve (O.H.V) Engines.
Now when you see "DOHC 24 Valve V6", you'll know what it means.
Since the same process occurs in each cylinder, we will take a look at one cylinder to see how the four stroke process works.
The four strokes are Intake, Compression, Power and Exhaust. The piston travels down on the Intake stroke, up on the Compression stroke, down on the Power stroke and up on the Exhaust stroke.
Intake
As the piston starts down on the Intake stroke, the intake valve opens and the fuel-air mixture is drawn into the cylinder (similar to drawing back the plunger on a hypodermic needle to allow fluid to be drawn into the chamber.)
When the piston reaches the bottom of the intake stroke, the intake valve closes, trapping the air-fuel mixture in the cylinder.
Compression
The piston moves up and compresses the trapped air fuel mixture that was brought in by the intake stroke. The amount that the mixture is compressed is determined by the compression ratio of the engine. The compression ratio on the average engine is in the range of 8:1 to 10:1.
This means that when the piston reaches the top of the cylinder, the air-fuel mixture is squeezed to about one tenth of its original volume.
Power
The spark plug fires, igniting the compressed air-fuel mixture which produces a powerful expansion of the vapor. The combustion process pushes the piston down the cylinder with great force turning the crankshaft to provide the power to propel the vehicle. Each piston fires at a different time, determined by the engine firing order. By the time the crankshaft completes two revolutions, each cylinder in the engine will have gone through one power stroke.
Exhaust
With the piston at the bottom of the cylinder, the exhaust valve opens to allow the burned exhaust gas to be expelled to the exhaust system. Since the cylinder contains so much pressure, when the valve opens, the gas is expelled with a violent force (that is why a vehicle without a muffler sounds so loud.) The piston travels up to the top of the cylinder pushing all the exhaust out before closing the exhaust valve in preparation for starting the four stroke process over again.
Oiling System
Oil is the life-blood of the engine. An engine running without oil will last about as long as a human without blood. Oil is pumped under pressure to all the moving parts of the engine by an oil pump. The oil pump is mounted at the bottom of the engine in the oil pan and is connected by a gear to either the crankshaft or the camshaft. This way, when the engine is turning, the oil pump is pumping. There is an oil pressure sensor near the oil pump that monitors pressure and sends this information to a warning light or a gauge on the dashboard. When you turn the ignition key on, but before you start the car, the oil light should light, indicating that there is no oil pressure yet, but also letting you know that the warning system is working. As soon as you start cranking the engine to start it, the light should go out indicating that there is oil pressure.
Engine Cooling
Internal combustion engines must maintain a stable operating temperature, not too hot and not too cold. With the massive amounts of heat that is generated from the combustion process, if the engine did not have a method for cooling itself, it would quickly self-destruct. Major engine parts can warp causing oil and water leaks and the oil will boil and become useless.
While some engines are air-cooled, the vast majority of engines are liquid cooled. The water pump circulates coolant throughout the engine, hitting the hot areas around the cylinders and heads and then sends the hot coolant to the radiator to be cooled off.
Engine Balance
Flywheel A 4 cylinder engine produces a power stroke every half crankshaft revolution, an 8 cylinder, every quarter revolution. This means that a V8 will be smother running than a 4. To keep the combustion pulses from generating a vibration, a flywheel is attached to the back of the crankshaft. The flywheel is a disk that is about 12 to 15 inches in diameter. On a standard transmission car, the flywheel is a heavy iron disk that doubles as part of the clutch system. On automatic equipped vehicles, the flywheel is a stamped steel plate that mounts the heavy torque converter. The flywheel uses inertia to smooth out the normal engine pulses.
Balance Shaft Some engines have an inherent rocking motion that produces an annoying vibration while running. To combat this, engineers employ one or more balance shafts. A balance shaft is a heavy shaft that runs through the engine parallel to the crankshaft. This shaft has large weights that, while spinning, offset the rocking motion of the engine by creating an opposite rocking motion of their own.