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..
Each cylinder
contains a piston that travels up and down inside the cylinder bore. All the pistons
in the engine are connected through individual connecting rods to a common crankshaft.
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.
How an Engine Works
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. For
more information on the cooling system, click here.
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.
Notice: The information on this site is
not intended as a substitute for the advice of a professional who is
qualified to examine, diagnose and repair your vehicle.
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
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.