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A
Short Course on
Automobile Engines
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
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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.
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 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.
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