ZfrkS62
05-03-2006, 12:32 AM
****I've noticed that this forum has gotten rather cluttered in the few years it's been opened. There is alot of good information in here that is somewhat buried. But none of the information does any good if you don't know how the system is supposed to operate.
So i am putting together some posts that will hopefully help out and will eventually be accompanied with FAQs and Common Problem sections, along with a glossary of terms and components. I will also be including images and hopefully animations at some point along the line. As the title states this is going to be a work in progress, so please be patient.
I plan on putting together 3 more of these; Transmission, Chassis and Driveablility. These will be worked on after i complete this one.
Again please be patient as i am going to be updating this little by little.
__________________________________________________ __________
DISCLAIMER: I AM NOT GOING TO BE HELD RESPONSIBLE FOR ANYTHING YOU SCREW UP BY WORKING ON YOUR CAR YOURSELF. IF YOU DON'T THINK YOU CAN HANDLE A JOB, DON'T DO IT! THIS IS SUPPOSED TO BE AN INFORMATIVE GUIDE, NOT A LICENSE TO CALL YOURSELF A MECHANIC!!
Basic Theory
An engine consists of mutiple moving parts working together to create the torque, and horsepower needed to make a car move. This is achieved by igniting a mixture of air and hydrocarbons, wether it be petrol (gasoline) or diesel. the method of igniting the two is different, however the theories behind making them combustible is the same. This section will focus on gasoline engines.
First we're going to look at the basics as to how the engine operates:
An internal combustion engine is essentially an air pump. It draws air in, compresses it, and pushes it back out. Only when you add the air fuel mixture and detonate it, do you get the power that makes the engine the marvel it is. There are 4 steps in the combustion process. Intake, compression, power and exhaust. Otherwise known as Suck, Squeeze, Bang, Blow.
Intake: As the piston travels downward in the cylinder, the intake valve is open, allowing the air/fuel mixture to be drawn in. The intake valve will hang open for a few degrees as the piston starts it's upward travel in the next step. This allows the speed of the inrushing air to pack a little extra in.
Compression: When the piston begins it's upward travel, the intake valve will close, sealing the cylinder and allowing the piston to compress the mixture until reaching the point where spark plug fires, igniting the mixture. The compression of the air causes it to heat up. This makes the ignition process easier.
Power: The air/fuel mixture has ignited and the thrust produced pushes the piston back down the cylinder. As the piston reaches the bottom of it's travel, the exhaust valve will begin to open to cool the cylinder and begin venting the spent gasses.
Exhaust: The piston once again is on it's way up, this time the exhaust valve is open, which allows the piston to push out the byproducts of the combustion process. Before reaching the top of the cylinder to begin the next intake stroke, the intake valve will open. This is to take advantage of the vacuum created by the escaping gasses and pull in a little extra air. This is valve overlap and also for what is referred to as the scavenging effect.
It is essential that the opening and closing of the valves be correctly timed in order for the engine to operate at it's peak efficiency. Opening the valves too early/late will result dramatically reduced power, if the engine starts at all and possibly engine damage depending on wether or not the engine is an interference fit. The camshaft, which controls the opening and closing of the valves, is timed to the crankshaft by means of either a belt, gear or a chain, though gears are not used any longer as far as i am aware, and will be discussed in the FAQ.
Engine Construction
There are two halves to every engine. The Bottom End which consists of the Engine Block and Crankshaft assembly; and the Top End which is the Cylinder Head and valve train. We'll start with the bottom end.
The engine block is an intricately cast piece of metal, these days it's aluminum but there are still a few manufacturers using iron blocks. Machined into this is the cylinder bores, oil gallies, and water jackets. Below this, machined into the supports, is a row of main bearing journals. These are where the crankshaft is held to the block. There are main bearing caps that are bolted over these journals. These must be torqued properly in order to withstand the immense pressures imparted on the crank during combustion. In order to protect the crank journals, plain bearings are fitted between the crank and the engine block. These bearings come in halves and are replaceable.
The crankshaft is responsible for changing the reciprocating (up and down) motion of the pistons into the rotational force that is transferred through the drivetrain. There are 2 types of journals on the crankshaft. Main and Rod. The main journals are the ones that are held by the main bearing journals in the block. The Rod Journals are what the Connecting Rods are bolted to. There are bearings on the con rods as well. They are just a smaller version of the main bearings and they do the same thing; protect the crank journals. The main journals are machined inline with each other down the centerline of the crank. The rod throws, where the connecting rods fit, are made off the centerline. A counterweight is made into the crank opposite of the rod throw. A crankshaft must be perfectly balanced in order to run smoothly at high rpms. Material is drilled out of the counterweights to balance the crank. Oil passages are drilled through the crank in order to lubricate the bearings.
The connecting rods are the key to transferring the combustion force from the piston to the crankshaft. The bottom end of the rod, like the main bearing journals, come in 2 halves. The smaller half is the rod cap. This bolts to the connecting rod to secure it to the crank. Like the Mains, these also require proper torquing, otherwise catastrophic engine failure will result. (Think big plumes of thick white smoke behind the F1 car of your choice :mrgreen: )
At the top of the connecting rod, snuggled tightly in the cylinder bore, is the piston. Securing the piston to the connecting rod is a wrist pin. This makes a pivot point to allow the con. rod to change the direction it leans when it is pushing the piston up, or pulling it down the bore. The piston is not perfectly fit into the cylinder. There is a small gap between the piston and the cylinder wall. To seal the cylinder, there are a series of rings around the piston. Fitted in grooves between the piston crown and piston skirt, there are the Top Compression Ring, Second Compression Ring, and the Oil Control Rings; respectively. There are holes drilled in the oil control ring grooves to allow oil to drain back to the oil pan.
The piston rings are made larger than the cylinder bore so when installed, the spring tension pushes them out against the cylinder walls. The action of the compressed gasses tightens them out and coupled with the sealing action of the small amounts of oil that get into the pores of the cylinder walls, provide a nearly air tight seal that enables the engine to reach compression pressures up to 200 psi.
Top End Construction
The engine's cylinder head, like the block, is intricately cast. Typically these days, the cylinder head is made of aluminum, however in the past it was also made out of cast iron.
In the head, is machined the combustion chambers, valve ports, oil drains, spark plug bores and, if the engine is an overhead cam engine, the camshaft journals are also machined in the top of the head. If it is a cam in block set up, then passages for the push-rods are drilled and studs for the rocker arms are bolted in.
Valve Train
The valve train consists of the camshaft, lifters, push-rods, rocker arms, valve springs, and valves. Again, if the engine is an overhead cam design, the push-rods and lifters are omitted. You may still have the rocker arms though.
The camshaft consists of cam journals which perform the same task as the crankshaft's main journals, and cam lobes which directly, or indirectly actuate the valves. The lobe is an egg shaped finger on the camshaft. The lobe's base circle is what the lobe would look like without the lobe. Each lobe has an opening and closing ramp and flank. The nose of the cam is what holds the valve open. There are 2 factors to a camlobe. Lift and duration. Lift is the height which the lobe raises the lifter, or pushes the valve. Duration is the amount, in degrees, the crankshaft rotates while the valve is open.
The valves fit through holes drilled in the head, through the intake and exhaust runners. This is called a valve guide. Valve guides can be integral or inserts. In an aluminum head, they must be inserts. The insert style guides are replacable. It keeps the valve from moving laterally which would lead to head and or engine damage. At the top of the guide, where the valve stem comes through, is a valve seal. This keeps the oil in the top of the cylinder head from being sucked down into the combustion chamber while a vacuum is being pulled in the intake manifold under idle, and deceleration conditions.
The top of the valve stem is grooved so that the valve keepers have something to fit into. The valve keepers are held on the outside by the valve spring retainer which fits over the top of the valve spring. The valve spring surrounds the valve stem and returns the valve to the closed position as the cam lobe rotates off the valve.
Valve Seats are machined into the valve ports in the combustion chamber. These provide the seal between the valve and valve port when the valve is closed.
Lifters ride on the cam lobes in a cam-in-block type engine. The lifters are made of the same material of the cam so that neither component is more prone to wear than the other. There is a tremendous load imparted on the lifters of up to 100,000 psi. To disperse this load, the lifter is designed with a convex (protruded face) and rides off center of the cam lobe. There is a taper of .0007" to .002" across the face and cause the lifter to spin. If the lifter fails to spin, the full load will be forced on the lifter causing it to wear. This will result in edge loading due to the convex shape of the face being worn concave (inverted face) and will rapidly wear the cam lobe. Lifters come in either Hydraulic or Solid types.
The construction of the hydraulic lifter is made up of a plunger, plunger return spring and check valve assemble. The hydraulic lifter maintains a zero lash condition in which there is no clearance between the valve and the valve operating mechanisms. This eliminates the need for periodic and sometimes labor intensive valve adjustments. The way it operates is simple. When ever there is clearance in the valve train, the plunger return spring expands, moving the pushrod upward against the rocker arm to take up the slack between the rocker arm face and the valve. The expanding lifter allows engine oil in to maintain the position. Because oil is a liquid, it cannot be compressed like air. This allows the oil to act as a solid spacer. The lifter is allowed to "leak down" as engine parts expand as they heat up. This allows the lifter to expel any excess oil. There is only a .0002" clearance between the lifter body and plunger, so leak down is very small. The reason it is so tight, is because foreign particles in the oil are generally larger than .0002" so they cannot wedge themselves between the plunger and lifter body.
Roller Lifters. Unlike regular lifters, roller lifters are not allowed to spin in their bores because there is a roller that spins on top of the cam lobe. They are held by either brackets or pins. If it spins, the roller will not work and quickly wear the cam lobe. This reduces the friction on the lobe and lifter drastically. Usually by 50%. This drop in friction results in an increase in horsepower, and fuel economy. The roller camshaft is made of steel and it's lobes have a different shape than standard cams to promote faster acceleration (valve opens faster).
The push rod is a simple metal rod that fits between the lifter and rocker arm. It is generally hollowed out from tip to tip to allow pressurized oil to flow through it from a hole in the top of lifter, to the rocker arm for lubrication of the top end of the valve train.
The rocker arm is a simple lever that transfers the motion of the cam and lifter to the valve. Picture the way a seesaw works on a playground. The pushrod fits under it on one end, it is bolted to the head by means of a threaded stud. The top of the valve fits under the other side. A rocker ball pivot is fitted on the topside and acts as the fulcrum. An adjustment nut is torqued on top of the rocker pivot. This nut will stop on a shoulder made into the stud and is called a positive stop arrangement. This arrangement is used on hydraulic lifter engines and requires no adjustment. Solid lifters periodically require adjustment. Because of this there are 2 types of rocker arms. Adjustable and Non-Adjustable. Only non-adjustable rockers can be used on positive stop arrangements.
Overhead Cam Hydraulic Lash Adjusters These work the same was as the hydraulic lifter. They sit in the head and keep the lash adjusted out of the valve train by means of a rocker arm. The camshaft rides on top of the rocker and pushes it down on top of the valve.
The cylinder head is bolted to the block using bolts designed specifically to handle the pressures of the engine. Between the head and the block is a gasket. The Cylinder head gasket seals the combustion chambers, cooling jackets and oil galleys. It must withstand the heat of combustion, and the expansion/contraction of the engine as it heats up and cools down.
In order to ensure that the engine gets the air it needs to run correctly, the opening of the valves must be precise. At the end of the cam and crank is a sprocket. The cam sprocket is twice as big as the crank sprocket so that it turns half as fast. The reason for this is that because the combustion process has 4 strokes, it takes 2 revolutions of the crank to complete the process. If the cam rotated at the same speed as the crank, the combustion would never occur because the piston would be too low in the cylinder for the force to do any good, providing there was any compression in the first place. Marks on the sprockets should line up with marks on the engine block and cylinder head(s) to ensure correct timing.
Induction
In order for the fuel to ignite, air must be fed into the cylinders. So far we've covered What happens to the air once it has mixed with the fuel and compressed in the cylinder, but there is a small part that plays a vital part in the performance, and idle quality of an engine's running.
Some of the things discussed here are going to be rehashed in Fuel Systems and probably referenced in Forced Induction, when I eventually get there.
Intake Manifold and Vacuum
The intake manifold is the airflow's link between the throttle body and the intake ports in the cylinder heads. The passages that carry this air to it's destination are called runners.
Because there are 2 different types of fueling systems, carburetted/throttle body injection and Fuel Injection, intakes are set up differently.
Carburetted and Throttle Body Injection manifolds carry the air/fuel mixture to the intake valve, atomizing the fuel on the way; whereas the intake manifold of a fuel injection system carries air only since the fuel is sprayed directly on the intake valve.
There is a distinct difference in the shapes of the two types of manifolds. Fuel injection manifold have larger runners and can feature sharper bends since they only carry air. Because the runners of a carburetted system must maintain the suspension of the fuel in the air, the runners are smaller and need to be straighter to maintain the velocity of the mixture. Should the velocity drop off, the suspended fuel mist could condense, form droplets and fall out of the airflow. Though large diameter runners flow well at high engine speeds, they will not support the fuel's suspension within the air, so a smaller diameter must be used, making the carburettor's intake manifold a compromise. Smaller runners provide the flow rate needed for the average rpm range of passenger cars. There is also a relationship between diameter and length of the runners. Larger diameter runners are usually shorter, and small runners are generally longer.
Dual Plane
Carburetted V8 engines typically used Dual Plane Manifolds, meaning that one barrel of the carb would feed 4 cylinders; The outside 2 runners on it's own side, and the inner two on the opposite side. Dual Planes have smaller runners and are much better suited for low rpm use.
Single Plane
These manifolds use both barrels to feed all 8 cylinders. These runners are larger and much more suited for high rpm use. These manifolds are the form that were adapted to Fuel Injection use and are found on pretty much all V8s nowadays, with few exceptions.
Siamese Runners
Siamese runners are used on older inline engines and use one runner to feed 2 cylinders.
Multiple Runner Manifolds
Multiple valve heads sometimes use multiple intake runners to improve volumetric efficiency of the cylinder. There is a butterfly valve that is used to maintain velocity and swirl at low rpm and flow rate in the higher ranges.
Variable runner length manifolds
There are some manufacturers that are using systems to change the length of the intake runners depending on the engine's rpm range. Short runners for low rpm, and long runners for high rpm. This ensures that power is maximised through out the rpm range.
manifold vacuum
There is a difference in pressure between the 14.7 psi of atmospheric pressure, and the pressure in the intake manifold. This is called Manifold Vacuum. The reason for this is because as the piston travels down during the intake stroke, the valve is open and the air rushes in to replace the area the cylinder was taking up. With the throttle closed, this translates into vacuum. As the throttle is opened, more air is allowed in which requires more fuel. This increases engine speed and at about 2750-3000 rpm, typically the air speed now rushing into the engine is maintained by sheer inertia and vacuum drops off.
Manifold vacuum is crucial to the operation of both carbureted and injected engines. With a carb, you need the vaccum to draw the air through the barrels to mix with the fuel.
Engine Configurations
There are a few different types of engine set ups. V, Inline, W, Opposed and Rotary. These configurations, with exception of the Rotary, refer to the way the cylinders are arranged. There are also sub categories which were discussed earlier, that refer to valve and camshaft location.
Inline This is a common set up with the cylinders arranged in a straight line: o o o o Inline 4 and 6 cylinder engines are the most common examples. There used to be Inline 8 cylinder engines, but their length and weight made them incredibly inefficient. A few companies even used an inline 5 cylinder engine.
V These engines are composed of 2 banks of cylinders that when looking at it from the front, the construction resembles a V. Today, the common angle is a 90 degree angle, but this can vary greatly depending on the application. Cylinders range from 4-12, some aircraft use 16. V4 applications are generally industrial such as forklifts.
W This is almost exclusively Volkswagen's set up as far as i know, with 8 and 12 cylinder arrangements. As with the V, looking at the arrangement from the front, makes it look like a W.
Opposed This is also referred to as a Boxer engine. The engine is flat and the cylinders are set up opposite from each other. Porsche and Subaru favor these engines heavily.
Rotary The rotary engine is pretty much related to Mazda, and does not use pistons, or valves. Instead, there is a triangle shaped rotor set inside an oblong engine block. There is a port for the intake, a spark plug and an exhaust port. The crankshaft is actually an eccentric shaft which moves the rotor. As it spins in the oblong block, the space between the rotor and block shrinks and expands. As the rotor tip, which has a special seal to ensure compression, but also low friction, passes the intake port, it creates a vacuum which draws in the air and fuel. As it rotates, the space compresses against the side of the block, in turn compressing the A/F mixture. This part also contains the spark plug which fires, igniting the mixture and pushes the rotor on to the next stage. the exhaust port is again in a smaller section which allows the gasses to be expelled when the rotor compresses again against the side of the block.
A common problem of the rotary however is the tips of the rotors. As the wear down, there is a significant compression loss, and the engine burns a nasty amount of oil.
Here's a neat little animation found by Jorge that goes through construction and operation: YouTube
__________________________________________________ ___________
So there you have it. A rundown of the operational theory behind the engine, and how it is constructed. If there is anything you think is missing, please PM me and let me know!
So i am putting together some posts that will hopefully help out and will eventually be accompanied with FAQs and Common Problem sections, along with a glossary of terms and components. I will also be including images and hopefully animations at some point along the line. As the title states this is going to be a work in progress, so please be patient.
I plan on putting together 3 more of these; Transmission, Chassis and Driveablility. These will be worked on after i complete this one.
Again please be patient as i am going to be updating this little by little.
__________________________________________________ __________
DISCLAIMER: I AM NOT GOING TO BE HELD RESPONSIBLE FOR ANYTHING YOU SCREW UP BY WORKING ON YOUR CAR YOURSELF. IF YOU DON'T THINK YOU CAN HANDLE A JOB, DON'T DO IT! THIS IS SUPPOSED TO BE AN INFORMATIVE GUIDE, NOT A LICENSE TO CALL YOURSELF A MECHANIC!!
Basic Theory
An engine consists of mutiple moving parts working together to create the torque, and horsepower needed to make a car move. This is achieved by igniting a mixture of air and hydrocarbons, wether it be petrol (gasoline) or diesel. the method of igniting the two is different, however the theories behind making them combustible is the same. This section will focus on gasoline engines.
First we're going to look at the basics as to how the engine operates:
An internal combustion engine is essentially an air pump. It draws air in, compresses it, and pushes it back out. Only when you add the air fuel mixture and detonate it, do you get the power that makes the engine the marvel it is. There are 4 steps in the combustion process. Intake, compression, power and exhaust. Otherwise known as Suck, Squeeze, Bang, Blow.
Intake: As the piston travels downward in the cylinder, the intake valve is open, allowing the air/fuel mixture to be drawn in. The intake valve will hang open for a few degrees as the piston starts it's upward travel in the next step. This allows the speed of the inrushing air to pack a little extra in.
Compression: When the piston begins it's upward travel, the intake valve will close, sealing the cylinder and allowing the piston to compress the mixture until reaching the point where spark plug fires, igniting the mixture. The compression of the air causes it to heat up. This makes the ignition process easier.
Power: The air/fuel mixture has ignited and the thrust produced pushes the piston back down the cylinder. As the piston reaches the bottom of it's travel, the exhaust valve will begin to open to cool the cylinder and begin venting the spent gasses.
Exhaust: The piston once again is on it's way up, this time the exhaust valve is open, which allows the piston to push out the byproducts of the combustion process. Before reaching the top of the cylinder to begin the next intake stroke, the intake valve will open. This is to take advantage of the vacuum created by the escaping gasses and pull in a little extra air. This is valve overlap and also for what is referred to as the scavenging effect.
It is essential that the opening and closing of the valves be correctly timed in order for the engine to operate at it's peak efficiency. Opening the valves too early/late will result dramatically reduced power, if the engine starts at all and possibly engine damage depending on wether or not the engine is an interference fit. The camshaft, which controls the opening and closing of the valves, is timed to the crankshaft by means of either a belt, gear or a chain, though gears are not used any longer as far as i am aware, and will be discussed in the FAQ.
Engine Construction
There are two halves to every engine. The Bottom End which consists of the Engine Block and Crankshaft assembly; and the Top End which is the Cylinder Head and valve train. We'll start with the bottom end.
The engine block is an intricately cast piece of metal, these days it's aluminum but there are still a few manufacturers using iron blocks. Machined into this is the cylinder bores, oil gallies, and water jackets. Below this, machined into the supports, is a row of main bearing journals. These are where the crankshaft is held to the block. There are main bearing caps that are bolted over these journals. These must be torqued properly in order to withstand the immense pressures imparted on the crank during combustion. In order to protect the crank journals, plain bearings are fitted between the crank and the engine block. These bearings come in halves and are replaceable.
The crankshaft is responsible for changing the reciprocating (up and down) motion of the pistons into the rotational force that is transferred through the drivetrain. There are 2 types of journals on the crankshaft. Main and Rod. The main journals are the ones that are held by the main bearing journals in the block. The Rod Journals are what the Connecting Rods are bolted to. There are bearings on the con rods as well. They are just a smaller version of the main bearings and they do the same thing; protect the crank journals. The main journals are machined inline with each other down the centerline of the crank. The rod throws, where the connecting rods fit, are made off the centerline. A counterweight is made into the crank opposite of the rod throw. A crankshaft must be perfectly balanced in order to run smoothly at high rpms. Material is drilled out of the counterweights to balance the crank. Oil passages are drilled through the crank in order to lubricate the bearings.
The connecting rods are the key to transferring the combustion force from the piston to the crankshaft. The bottom end of the rod, like the main bearing journals, come in 2 halves. The smaller half is the rod cap. This bolts to the connecting rod to secure it to the crank. Like the Mains, these also require proper torquing, otherwise catastrophic engine failure will result. (Think big plumes of thick white smoke behind the F1 car of your choice :mrgreen: )
At the top of the connecting rod, snuggled tightly in the cylinder bore, is the piston. Securing the piston to the connecting rod is a wrist pin. This makes a pivot point to allow the con. rod to change the direction it leans when it is pushing the piston up, or pulling it down the bore. The piston is not perfectly fit into the cylinder. There is a small gap between the piston and the cylinder wall. To seal the cylinder, there are a series of rings around the piston. Fitted in grooves between the piston crown and piston skirt, there are the Top Compression Ring, Second Compression Ring, and the Oil Control Rings; respectively. There are holes drilled in the oil control ring grooves to allow oil to drain back to the oil pan.
The piston rings are made larger than the cylinder bore so when installed, the spring tension pushes them out against the cylinder walls. The action of the compressed gasses tightens them out and coupled with the sealing action of the small amounts of oil that get into the pores of the cylinder walls, provide a nearly air tight seal that enables the engine to reach compression pressures up to 200 psi.
Top End Construction
The engine's cylinder head, like the block, is intricately cast. Typically these days, the cylinder head is made of aluminum, however in the past it was also made out of cast iron.
In the head, is machined the combustion chambers, valve ports, oil drains, spark plug bores and, if the engine is an overhead cam engine, the camshaft journals are also machined in the top of the head. If it is a cam in block set up, then passages for the push-rods are drilled and studs for the rocker arms are bolted in.
Valve Train
The valve train consists of the camshaft, lifters, push-rods, rocker arms, valve springs, and valves. Again, if the engine is an overhead cam design, the push-rods and lifters are omitted. You may still have the rocker arms though.
The camshaft consists of cam journals which perform the same task as the crankshaft's main journals, and cam lobes which directly, or indirectly actuate the valves. The lobe is an egg shaped finger on the camshaft. The lobe's base circle is what the lobe would look like without the lobe. Each lobe has an opening and closing ramp and flank. The nose of the cam is what holds the valve open. There are 2 factors to a camlobe. Lift and duration. Lift is the height which the lobe raises the lifter, or pushes the valve. Duration is the amount, in degrees, the crankshaft rotates while the valve is open.
The valves fit through holes drilled in the head, through the intake and exhaust runners. This is called a valve guide. Valve guides can be integral or inserts. In an aluminum head, they must be inserts. The insert style guides are replacable. It keeps the valve from moving laterally which would lead to head and or engine damage. At the top of the guide, where the valve stem comes through, is a valve seal. This keeps the oil in the top of the cylinder head from being sucked down into the combustion chamber while a vacuum is being pulled in the intake manifold under idle, and deceleration conditions.
The top of the valve stem is grooved so that the valve keepers have something to fit into. The valve keepers are held on the outside by the valve spring retainer which fits over the top of the valve spring. The valve spring surrounds the valve stem and returns the valve to the closed position as the cam lobe rotates off the valve.
Valve Seats are machined into the valve ports in the combustion chamber. These provide the seal between the valve and valve port when the valve is closed.
Lifters ride on the cam lobes in a cam-in-block type engine. The lifters are made of the same material of the cam so that neither component is more prone to wear than the other. There is a tremendous load imparted on the lifters of up to 100,000 psi. To disperse this load, the lifter is designed with a convex (protruded face) and rides off center of the cam lobe. There is a taper of .0007" to .002" across the face and cause the lifter to spin. If the lifter fails to spin, the full load will be forced on the lifter causing it to wear. This will result in edge loading due to the convex shape of the face being worn concave (inverted face) and will rapidly wear the cam lobe. Lifters come in either Hydraulic or Solid types.
The construction of the hydraulic lifter is made up of a plunger, plunger return spring and check valve assemble. The hydraulic lifter maintains a zero lash condition in which there is no clearance between the valve and the valve operating mechanisms. This eliminates the need for periodic and sometimes labor intensive valve adjustments. The way it operates is simple. When ever there is clearance in the valve train, the plunger return spring expands, moving the pushrod upward against the rocker arm to take up the slack between the rocker arm face and the valve. The expanding lifter allows engine oil in to maintain the position. Because oil is a liquid, it cannot be compressed like air. This allows the oil to act as a solid spacer. The lifter is allowed to "leak down" as engine parts expand as they heat up. This allows the lifter to expel any excess oil. There is only a .0002" clearance between the lifter body and plunger, so leak down is very small. The reason it is so tight, is because foreign particles in the oil are generally larger than .0002" so they cannot wedge themselves between the plunger and lifter body.
Roller Lifters. Unlike regular lifters, roller lifters are not allowed to spin in their bores because there is a roller that spins on top of the cam lobe. They are held by either brackets or pins. If it spins, the roller will not work and quickly wear the cam lobe. This reduces the friction on the lobe and lifter drastically. Usually by 50%. This drop in friction results in an increase in horsepower, and fuel economy. The roller camshaft is made of steel and it's lobes have a different shape than standard cams to promote faster acceleration (valve opens faster).
The push rod is a simple metal rod that fits between the lifter and rocker arm. It is generally hollowed out from tip to tip to allow pressurized oil to flow through it from a hole in the top of lifter, to the rocker arm for lubrication of the top end of the valve train.
The rocker arm is a simple lever that transfers the motion of the cam and lifter to the valve. Picture the way a seesaw works on a playground. The pushrod fits under it on one end, it is bolted to the head by means of a threaded stud. The top of the valve fits under the other side. A rocker ball pivot is fitted on the topside and acts as the fulcrum. An adjustment nut is torqued on top of the rocker pivot. This nut will stop on a shoulder made into the stud and is called a positive stop arrangement. This arrangement is used on hydraulic lifter engines and requires no adjustment. Solid lifters periodically require adjustment. Because of this there are 2 types of rocker arms. Adjustable and Non-Adjustable. Only non-adjustable rockers can be used on positive stop arrangements.
Overhead Cam Hydraulic Lash Adjusters These work the same was as the hydraulic lifter. They sit in the head and keep the lash adjusted out of the valve train by means of a rocker arm. The camshaft rides on top of the rocker and pushes it down on top of the valve.
The cylinder head is bolted to the block using bolts designed specifically to handle the pressures of the engine. Between the head and the block is a gasket. The Cylinder head gasket seals the combustion chambers, cooling jackets and oil galleys. It must withstand the heat of combustion, and the expansion/contraction of the engine as it heats up and cools down.
In order to ensure that the engine gets the air it needs to run correctly, the opening of the valves must be precise. At the end of the cam and crank is a sprocket. The cam sprocket is twice as big as the crank sprocket so that it turns half as fast. The reason for this is that because the combustion process has 4 strokes, it takes 2 revolutions of the crank to complete the process. If the cam rotated at the same speed as the crank, the combustion would never occur because the piston would be too low in the cylinder for the force to do any good, providing there was any compression in the first place. Marks on the sprockets should line up with marks on the engine block and cylinder head(s) to ensure correct timing.
Induction
In order for the fuel to ignite, air must be fed into the cylinders. So far we've covered What happens to the air once it has mixed with the fuel and compressed in the cylinder, but there is a small part that plays a vital part in the performance, and idle quality of an engine's running.
Some of the things discussed here are going to be rehashed in Fuel Systems and probably referenced in Forced Induction, when I eventually get there.
Intake Manifold and Vacuum
The intake manifold is the airflow's link between the throttle body and the intake ports in the cylinder heads. The passages that carry this air to it's destination are called runners.
Because there are 2 different types of fueling systems, carburetted/throttle body injection and Fuel Injection, intakes are set up differently.
Carburetted and Throttle Body Injection manifolds carry the air/fuel mixture to the intake valve, atomizing the fuel on the way; whereas the intake manifold of a fuel injection system carries air only since the fuel is sprayed directly on the intake valve.
There is a distinct difference in the shapes of the two types of manifolds. Fuel injection manifold have larger runners and can feature sharper bends since they only carry air. Because the runners of a carburetted system must maintain the suspension of the fuel in the air, the runners are smaller and need to be straighter to maintain the velocity of the mixture. Should the velocity drop off, the suspended fuel mist could condense, form droplets and fall out of the airflow. Though large diameter runners flow well at high engine speeds, they will not support the fuel's suspension within the air, so a smaller diameter must be used, making the carburettor's intake manifold a compromise. Smaller runners provide the flow rate needed for the average rpm range of passenger cars. There is also a relationship between diameter and length of the runners. Larger diameter runners are usually shorter, and small runners are generally longer.
Dual Plane
Carburetted V8 engines typically used Dual Plane Manifolds, meaning that one barrel of the carb would feed 4 cylinders; The outside 2 runners on it's own side, and the inner two on the opposite side. Dual Planes have smaller runners and are much better suited for low rpm use.
Single Plane
These manifolds use both barrels to feed all 8 cylinders. These runners are larger and much more suited for high rpm use. These manifolds are the form that were adapted to Fuel Injection use and are found on pretty much all V8s nowadays, with few exceptions.
Siamese Runners
Siamese runners are used on older inline engines and use one runner to feed 2 cylinders.
Multiple Runner Manifolds
Multiple valve heads sometimes use multiple intake runners to improve volumetric efficiency of the cylinder. There is a butterfly valve that is used to maintain velocity and swirl at low rpm and flow rate in the higher ranges.
Variable runner length manifolds
There are some manufacturers that are using systems to change the length of the intake runners depending on the engine's rpm range. Short runners for low rpm, and long runners for high rpm. This ensures that power is maximised through out the rpm range.
manifold vacuum
There is a difference in pressure between the 14.7 psi of atmospheric pressure, and the pressure in the intake manifold. This is called Manifold Vacuum. The reason for this is because as the piston travels down during the intake stroke, the valve is open and the air rushes in to replace the area the cylinder was taking up. With the throttle closed, this translates into vacuum. As the throttle is opened, more air is allowed in which requires more fuel. This increases engine speed and at about 2750-3000 rpm, typically the air speed now rushing into the engine is maintained by sheer inertia and vacuum drops off.
Manifold vacuum is crucial to the operation of both carbureted and injected engines. With a carb, you need the vaccum to draw the air through the barrels to mix with the fuel.
Engine Configurations
There are a few different types of engine set ups. V, Inline, W, Opposed and Rotary. These configurations, with exception of the Rotary, refer to the way the cylinders are arranged. There are also sub categories which were discussed earlier, that refer to valve and camshaft location.
Inline This is a common set up with the cylinders arranged in a straight line: o o o o Inline 4 and 6 cylinder engines are the most common examples. There used to be Inline 8 cylinder engines, but their length and weight made them incredibly inefficient. A few companies even used an inline 5 cylinder engine.
V These engines are composed of 2 banks of cylinders that when looking at it from the front, the construction resembles a V. Today, the common angle is a 90 degree angle, but this can vary greatly depending on the application. Cylinders range from 4-12, some aircraft use 16. V4 applications are generally industrial such as forklifts.
W This is almost exclusively Volkswagen's set up as far as i know, with 8 and 12 cylinder arrangements. As with the V, looking at the arrangement from the front, makes it look like a W.
Opposed This is also referred to as a Boxer engine. The engine is flat and the cylinders are set up opposite from each other. Porsche and Subaru favor these engines heavily.
Rotary The rotary engine is pretty much related to Mazda, and does not use pistons, or valves. Instead, there is a triangle shaped rotor set inside an oblong engine block. There is a port for the intake, a spark plug and an exhaust port. The crankshaft is actually an eccentric shaft which moves the rotor. As it spins in the oblong block, the space between the rotor and block shrinks and expands. As the rotor tip, which has a special seal to ensure compression, but also low friction, passes the intake port, it creates a vacuum which draws in the air and fuel. As it rotates, the space compresses against the side of the block, in turn compressing the A/F mixture. This part also contains the spark plug which fires, igniting the mixture and pushes the rotor on to the next stage. the exhaust port is again in a smaller section which allows the gasses to be expelled when the rotor compresses again against the side of the block.
A common problem of the rotary however is the tips of the rotors. As the wear down, there is a significant compression loss, and the engine burns a nasty amount of oil.
Here's a neat little animation found by Jorge that goes through construction and operation: YouTube
__________________________________________________ ___________
So there you have it. A rundown of the operational theory behind the engine, and how it is constructed. If there is anything you think is missing, please PM me and let me know!