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Gasoline (petrol) internal-combustion engines power most automobiles, but some engines use diesel fuel, electricity, natural gas, solar energy, or fuels derived from methanol (wood alcohol) and ethanol (grain alcohol). The purpose of a gasoline car engine is to convert gasoline into motion so that your car can move. Currently the easiest way to create motion from gasoline is to burn the gasoline inside an engine. Therefore, a car engine is an internal combustion engine -- combustion takes place internally.

Almost all cars today use a reciprocating internal combustion engine because this engine is:

Relatively efficient (compared to an external combustion engine)

Relatively inexpensive (compared to a gas turbine)

Relatively easy to refuel (compared to an electric car)

These advantages beat any other existing technology for moving a car around.

Most gasoline engines work in the following way: Turning the ignition key operates a switch that sends electricity from a battery to a starter motor. The starter motor turns a disk known as a flywheel, which in turn causes the engine’s crankshaft to revolve. The rotating crankshaft causes pistons, which are solid cylinders that fit snugly inside the engine’s hollow cylinders, to move up and down. Fuel-injection systems or, in older cars, a carburetor deliver fuel vapor from the gas tank to the engine cylinders.

The pistons compress the vapor inside the cylinders. An electric current flows through a spark plug to ignite the vapor. The fuel mixture explodes, or combusts, creating hot expanding gases that push the pistons down the cylinders and cause the crankshaft to rotate. The crankshaft is now rotating via the up-and-down motion of the pistons, permitting the starter motor to disengage from the flywheel.

If you put a high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel a thing into motion. This fact is behind a car engine. A cycle is created that allows one to set off explosions hundreds of times per minute, and that energy is harnessed to drive the car.

Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are:

Intake stroke

Compression stroke

Combustion stroke

Exhaust stroke

Here's what happens as the engine goes through its cycle:

The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. Then the piston moves back up to compress this fuel/air mixture making the compression stroke. Compression makes the explosion more powerful. When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline and combustion stroke begins. The gasoline charge in the cylinder explodes, driving the piston down. Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe. This is exhaust stroke. Now the engine is ready for the next cycle, so it intakes another charge of air and gas and the cycle continues.

A. Engine

The basic components of an internal-combustion engine are the engine block, cylinder head, cylinders, pistons, valves, crankshaft, and camshaft.

The lower part of the engine, called the engine block, houses the cylinders, pistons, and crankshaft. The components of other engine systems bolt or attach to the engine block. The block is manufactured with internal passageways for lubricants and coolant. Engine blocks are made of cast iron or aluminium alloy and formed with a set of round cylinders.

The core of the engine is the cylinder, with the piston moving up and down inside the cylinder. Most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine, the cylinders usually are arranged in one of three ways: inline, V or flat (also known as horizontally opposed or boxer). See below for more details.

The upper part of the engine is the cylinder head. Bolted to the top of the block, it seals the tops of the cylinders. Pistons compress air and fuel against the cylinder head prior to ignition. The top of the piston forms the floor of the combustion chamber. A rod connects the bottom of the piston to the crankshaft. Lubricated bearings enable both ends of the connecting rod to pivot, transferring the piston’s vertical motion into the crankshaft’s rotational force, or torque. The pistons’ motion rotates the crankshaft at speeds ranging from about 600 to thousands of revolutions per minute (rpm), depending on how much fuel is delivered to the cylinders.

Fuel vapor enters and exhaust gases leave the combustion chamber through openings in the cylinder head controlled by valves. The typical engine valve is a metal shaft with a disk at one end fitted to block the opening. The other end of the shaft is mechanically linked to a camshaft, a round rod with odd-shaped lobes located inside the engine block or in the cylinder head. Inlet valves open to allow fuel to enter the combustion chambers. Outlet valves open to let exhaust gases out.

A gear wheel, belt, or chain links the camshaft to the crankshaft. When the crankshaft forces the camshaft to turn, lobes on the camshaft cause valves to open and close at precise moments in the engine’s cycle. When fuel vapor ignites, the intake and outlet valves close tightly to direct the force of the explosion downward on the piston.

B. Engine Types

The blocks in most internal-combustion engines are in-line designs or V designs. In-line designs are arranged so that the cylinders stand upright in a single line over the crankshaft. In a V design, two rows of cylinders are set at an angle to form a V. At the bottom of the V is the crankshaft. In-line configurations of six or eight cylinders require long engine compartments found more often in trucks than in cars. The V design allows the same number of cylinders to fit into a shorter, although wider, space. Another engine design that fits into shorter, shallower spaces is a horizontally opposed, or flat, arrangement in which the crankshaft lies between two rows of cylinders.

Engines become more powerful, and use more fuel, as the size and number of cylinders increase. Most modern vehicles have 4-, 6-, or 8-cylinder engines, but car engines have been designed with 1, 2, 3, 5, 12, and more cylinders.

Diesel engines, common in large trucks or buses, are similar to gasoline internal-combustion engines, but they have a different ignition system. Diesels compress air inside the cylinders with greater force than a gasoline engine does, producing temperatures hot enough to ignite the diesel fuel on contact. Some cars have rotary engines, also known as Wankel engines, which have one or more elliptical chambers in which triangular-shaped rotors, instead of pistons, rotate.

Electric motors have been used to power automobiles since the late 1800s. Electric power supplied by batteries runs the motor, which rotates a driveshaft, the shaft that transmits engine power to the axles. Commercial electric car models for specialized purposes were available in the 1980s. General Motors Corporation introduced a mass-production all-electric car in the mid-1990s.

Automobiles that combine two or more types of engines are called hybrids. A typical hybrid is an electric motor with batteries that are recharged by a generator run by a small gas- or diesel-powered engine. These hybrids are known as hybrid electric vehicles (HEVs). By relying more on electricity and less on fuel combustion, HEVs have higher fuel efficiency and emit fewer pollutants. Several automakers have experimented with hybrids.

In 1997 Toyota Motor Corporation became the first to mass-produce a hybrid vehicle, the Prius. It became available in Japan in 1997 and in North America in 2000. The first hybrid available for sale in North America, the Honda Insight, was offered by Honda Motor Co., Ltd., in 1999. Honda later introduced a hybrid version of the Honda Civic. In August 2004 the Ford Motor Company became the first U.S. automaker to release a hybrid vehicle when it began production of the Ford Escape Hybrid, the first hybrid sport- utility vehicle (SUV). The Escape Hybrid was released for the 2005 model year.

C. Displacement (CC)

The combustion chamber is the area where compression and combustion take place. As the piston moves up and down, you can see that the size of the combustion chamber changes. It has some maximum volume as well as a minimum volume. The difference between the maximum and minimum is called the displacement and is measured in liters or CCs (Cubic Centimeters, where 1,000 cubic centimeters equals a liter). Here are some examples:

A chainsaw might have a 40 cc engine. A motorcycle might have a 500 cc or a 750 cc engine. A sports car might have a 5.0 liter (5,000 cc) engine. Most normal car engines fall somewhere between 1.5 liter (1,500 cc) and 4.0 liters (4,000 cc) If you have a 4-cylinder engine and each cylinder displaces half a liter, then the entire engine is a "2.0 liter engine." If each cylinder displaces half a liter and there are six cylinders arranged in a V configuration, you have a "3.0 liter V-6."

Generally, the displacement tells you something about how much power an engine can produce. A cylinder that displaces half a liter can hold twice as much fuel/air mixture as a cylinder that displaces a quarter of a liter, and therefore you would expect about twice as much power from the larger cylinder (if everything else is equal). So a 2.0 liter engine is roughly half as powerful as a 4.0 liter engine.

You can get more displacement in an engine either by increasing the number of cylinders or by making the combustion chambers of all the cylinders bigger (or both).

D. Parts of an Engine

The parts of an engine vary depending on the engine's type. For a four-stroke engine, key parts of the engine include the crankshaft, one or more camshafts and valves. For a two-stroke engine, there may simply be an exhaust outlet and fuel inlet instead of a valve system. In both types of engines, there are one or more cylinders and for each cylinder there is a spark plug, a piston and a crank A single sweep of the cylinder by the piston in an upward or downward motion is known as a stroke and the downward stroke that occurs directly after the air-fuel mix in the cylinder is ignited is known as a power stroke.

Piston: A piston is a cylindrical piece of metal that moves up and down inside the cylinder.

Piston rings: Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes:

They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.

They keep oil in the sump from leaking into the combustion area, where it would be burned and lost. Most cars that "burn oil" and have to have a quart added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly.

Connecting rod: The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates. It transmits motion or power from one moving part to another, especially the rod connecting the crankshaft of a motor vehicle to a piston. Also called pitman

Valves: The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

Crank shaft: The crank shaft turns the piston's up and down motion into circular motion just like a crank on a jack-in-the-box does.

Spark plug: The spark plug supplies the spark that ignites the air/fuel mixture so that combustion can occur. The spark must happen at just the right moment for things to work properly.

Sump: The sump surrounds the crankshaft. It contains some amount of oil, which collects in the bottom of the sump (the oil pan).

E. Combustion and Explosion

All internal combustion engines depend on the chemical process of combustion and explosion, that is the reaction of a fuel with oxygen. See also: stoichiometry.

The most common fuels in use today are made up of hydrocarbons and are derived from petroleum. These include the fuels known as diesel, gasoline and liquefied petroleum gas. Some have theorized that in the future hydrogen might replace such fuels. The advantage of hydrogen is that its combustion produces only water (the chief disadvantage of using hydrogen is that presently no method exists for efficiently producing it in quantities sufficient to power internal combustion engines on a large scale). This is unlike the combustion of hydrocarbons which also produces carbon dioxide - a major cause of global warming.

Whatever the choice of fuel, all internal combustion engines rely on the effects of a controlled explosion, where a gaseous fuel reacts with oxygen. The explosion products (hot gases) occupy a larger volume than the original compressed fuel/air mixture and the resulting high pressure in the cylinders drives the engine's pistons down. Relieving the pressure after this has occurred (either by opening a valve or exposing the exhaust outlet) allows the piston to return to its uppermost position (Top Dead Center - TDC). The piston can then proceed to the next phase of its cycle (which varies between engines). The resulting heat from the explosion is a waste product and is removed from the engine either by an air or liquid cooling system. (An ideal, 100% efficient engine would run and remain at ambient temperature and convert all the energy in the fuel to kinetic energy - not into heat).

All internal combustion engines must have a means of ignition to promote combustion. Most engines use either an electrical or a compression heating ignition system. Electrical ignition systems generally rely on a lead-acid battery and an induction coil to provide a high voltage electrical spark to ignite the air-fuel mix in the engine's cylinders. This battery can be recharged during operation using an alternator driven by the engine. Compression heating ignition systems rely on the heat already present in the compressed air in the engine's cylinders to ignite the fuel when it is injected.

F. How an Engine works

Each movement of the piston from top to bottom or from bottom to top is called a stroke. The piston takes two strokes (an upstroke and a downstroke) as the crankshaft makes one complete revolution. When the piston is at the top of a stroke, it is said to be at top dead center. When the piston is at the bottom of a stroke, it is said to be at bottom dead center. These positions are rock positions.

The enclosed end of a cylinder has two openings. One of the openings, or ports, permits the mixture of air and fuel to enter, and the other port permits the burned gases to escape from the cylinder. The two ports have valves assembled in them. These valves, actuated by the camshaft, close off either one or the other of the ports, or both of them, during various stages of engine operation. One of the valves, called the intake valve, opens to admit a mixture of fuel and air into the cylinder. The other valve, called the exhaust valve, opens to allow the escape of burned gases after the fuel-and-air mixture has burned. Later you will learn more about how these valves and their mechanisms operate.

The following paragraphs explain the sequence of actions that takes place within the engine cylinder: the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. Since these strokes are easy to identify in the operation of a four-cycle engine, that engine is used in the description. This type of engine is called a four-stroke-Otto-cycle engine, named after Dr. N. A. Otto who, in 1876, first applied the principle of this engine. Each stroke corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete.

Intake or Admission Stroke

The first stroke in the sequence is the intake stroke. During this stroke, the piston is moving downward and the intake valve is open. This downward movement of the piston produces a partial vacuum in the cylinder, and air and fuel rush into the cylinder past the open intake valve, the exhaust valve is meanwhile held shut by a spring. This action produces a result similar to that which occurs when you drink through a straw. You produce a partial vacuum in your mouth, and the liquid moves up through the straw to fill the vacuum.

Compression Stroke

When the piston reaches bottom dead center at the end of the intake stroke (and is therefore at the bottom of the cylinder) the intake valve closes and seals the upper end of the cylinder. Now both intake and exhaust valves are closed. As the crankshaft continues to rotate, it pushes the connecting rod up against the piston. The piston then moves upward and this upward movement compresses the combustible mixture in the cylinder. This action is known as the compression stroke .

In gasoline engines, the mixture is compressed to about one-eighth of its original volume. (In a diesel engine the mixture may be compressed to as little as one-sixteenth of its original volume.) This compression of the air-fuel mixture increases the pressure within the cylinder. Compressing the mixture in this way makes it more combustible; not only does the pressure in the cylinder go up, but the temperature of the mixture also increases.

Power or Expansion Stroke

As the piston reaches top dead center at the end of the compression stroke (and is therefore at the top of the cylinder), the ignition system produces an electric spark. The spark ignites the fuel-air mixture. The intake and exhaust valves are closed. Having been ignited, the fuel-air mixture burns. In burning, the mixture gets very hot and expands in all directions. The pressure rises to about 600 to 700 pounds per square inch. Since the piston is the only part that can move, the force produced by the expanding gases forces the piston down. This force, or thrust, is carried through the connecting rod to the crankpin on the crankshaft. The crankshaft is given a powerful twist. Since it is the only stroke and event that furnishes power to the crankshaft, it is usually called the power stroke, although it is sometimes called the expansion stroke for purposes of instruction. This turning effort, rapidly repeated in the engine and carried through gears and shafts, will turn the wheels of a vehicle and cause it to move along the highway.

Exhaust Stroke

After the fuel-air mixture has burned, it must be cleared from the cylinder. Therefore, the exhaust valve  is opened by the cam/lifter mechanism as the power stroke is finished and the piston starts back up on the exhaust stroke. The piston forces the exhausted fuel of the cylinder past the open exhaust valve.

The four strokes (intake, compression, power, and exhaust) are continuously repeated as the engine runs.

Valve Trains and Camshaft

The valve train consists of the valves and a mechanism that opens and closes them. The opening and closing system is called a camshaft. The camshaft has lobes on it that move the valves up and down, as shown in the attached Figure.

Most modern engines have what are called overhead cams. This means that the camshaft is located above the valves, as you see in Figure 5. The cams on the shaft activate the valves directly or through a very short linkage. Older engines used a camshaft located in the sump near the crankshaft. Rods linked the cam below to valve lifters above the valves. This approach has more moving parts and also causes more lag between the cam's activation of the valve and the valve's subsequent motion. A timing belt or timing chain links the crankshaft to the camshaft so that the valves are in sync with the pistons. The camshaft is geared to turn at one-half the rate of the crankshaft. Many high-performance engines have four valves per cylinder (two for intake, two for exhaust), and this arrangement requires two camshafts per bank of cylinders, hence the phrase "dual overhead cams."

Valves let the air/fuel mixture into the engine and the exhaust out of the engine. The camshaft uses lobes (called cams) that push against the valves to open them as the camshaft rotates; springs on the valves return them to their closed position. This is a critical job, and can have a great impact on an engine's performance at different speeds.

The key parts of any camshaft are the lobes. As the camshaft spins, the lobes open and close the intake and exhaust valves in time with the motion of the piston. It turns out that there is a direct relationship between the shape of the cam lobes and the way the engine performs in different speed ranges. To understand why this is the case, imagine that we are running an engine extremely slowly -- at just 10 or 20 revolutions per minute (RPM) -- so that it takes the piston a couple of seconds to complete a cycle. It would be impossible to actually run a normal engine this slowly, but let's imagine that we could. At this slow speed, we would want cam lobes shaped so that:

Just as the piston starts moving downward in the intake stroke (called top dead center, or TDC), the intake valve would open. The intake valve would close right as the piston bottoms out.

The exhaust valve would open right as the piston bottoms out (called bottom dead center, or BDC) at the end of the combustion stroke, and would close as the piston completes the exhaust stroke. This setup would work really well for the engine as long as it ran at this very slow speed.

But what happens if you increase the RPM? Let's find out.

When you increase the RPM, the 10 to 20 RPM configuration for the camshaft does not work well. If the engine is running at 4,000 RPM, the valves are opening and closing 2,000 times every minute, or 33 times every second. At these speeds, the piston is moving very quickly, so the air/fuel mixture rushing into the cylinder is moving very quickly as well. When the intake valve opens and the piston starts its intake stroke, the air/fuel mixture in the intake runner starts to accelerate into the cylinder. By the time the piston reaches the bottom of its intake stroke, the air/fuel is moving at a pretty high speed. If we were to slam the intake valve shut, all of that air/fuel would come to a stop and not enter the cylinder. By leaving the intake valve open a little longer, the momentum of the fast-moving air/fuel continues to force air/fuel into the cylinder as the piston starts its compression stroke. So the faster the engine goes, the faster the air/fuel moves, and the longer we want the intake valve to stay open. We also want the valve to open wider at higher speeds -- this parameter, called valve lift, is governed by the cam lobe profile.

Any given camshaft will be perfect only at one engine speed. At every other engine speed, the engine won't perform to its full potential. A fixed camshaft is, therefore, always a compromise. This is why carmakers have developed schemes to vary the cam profile as the engine speed changes.

Camshaft Arrangements

There are several different arrangements of camshafts on engines. We'll talk about some of the most common ones. You've probably heard the terminology: Single overhead cam (SOHC), Double overhead cam (DOHC) and Pushrod. Let's start by looking at single overhead cams.

Single Overhead Cams This arrangement denotes an engine with one cam per head. So if it is an inline 4-cylinder or inline 6-cylinder engine, it will have one cam; if it is a V-6 or V-8, it will have two cams (one for each head).

The cam actuates rocker arms that press down on the valves, opening them. Springs return the valves to their closed position. These springs have to be very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms.

A single overhead cam (SOHC)

On single and double overhead cam engines, the cams are driven by the crankshaft, via either a belt or chain called the timing belt or timing chain. These belts and chains need to be replaced or adjusted at regular intervals. If a timing belt breaks, the cam will stop spinning and the piston could hit the open valves.

Double Overhead Cam (DOHC)

A double overhead cam engine has two cams per head. So inline engines have two cams, and V engines have four. Usually, double overhead cams are used on engines with four or more valves per cylinder -- a single camshaft simply cannot fit enough cam lobes to actuate all of those valves.

The main reason to use double overhead cams is to allow for more intake and exhaust valves. More valves means that intake and exhaust gases can flow more freely because there are more openings for them to flow through. This increases the power of the engine.

Pushrod Engines

Like SOHC and DOHC engines, the valves in a pushrod engine are located in the head, above the cylinder. The key difference is that the camshaft on a pushrod engine is inside the engine block, rather than in the head.

The cam actuates long rods that go up through the block and into the head to move the rockers. These long rods add mass to the system, which increases the load on the valve springs. This can limit the speed of pushrod engines; the overhead camshaft, which eliminates the pushrod from the system, is one of the engine technologies that made higher engine speeds possible.

The camshaft in a pushrod engine is often driven by gears or a short chain. Gear-drives are generally less prone to breakage than belt drives, which are often found in overhead cam engines.

D. Fuel Supply

The internal-combustion engine is powered by the burning of a precise mixture of liquefied fuel and air in the cylinders’ combustion chambers. Fuel is stored in a tank until it is needed, then pumped to a carburetor or, in newer cars, to a fuel-injection system.

The carburetor controls the mixture of gas and air that travels to the engine. It mixes fuel with air at the head of a pipe, called the intake manifold, leading to the cylinders. A vacuum created by the downward strokes of pistons draws air through the carburetor and intake manifold. Inside the carburetor, the airflow transforms drops of fuel into a fine mist, or vapor. The intake manifold delivers the fuel vapor to the cylinders, where it is ignited.

All new cars produced today are equipped with fuel injection systems instead of carburetors. Fuel injectors spray carefully calibrated bursts of fuel mist into cylinders either at or near openings to the combustion chambers. Since the exact quantity of gas needed is injected into the cylinders, fuel injection is more precise, easier to adjust, and more consistent than a carburetor, delivering better efficiency, gas mileage, engine responsiveness, and pollution control. Fuel-injection systems vary widely, but most are operated or managed electronically.

High-performance automobiles are often fitted with air-compressing equipment that increases an engine’s output. By increasing the air and fuel flow to the engine, these features produce greater horsepower. Superchargers are compressors powered by the crankshaft. Turbochargers are turbine-powered compressors run by pressurized exhaust gas.

Lubrication System

The lubrication system makes sure that every moving part in the engine gets oil so that it can move easily. The two main parts needing oil are the pistons (so they can slide easily in their cylinders) and any bearings that allow things like the crankshaft and camshafts to rotate freely. In most cars, oil is sucked out of the oil pan by the oil

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