The Engine
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This page is very image intense and has flash animations of the inner workings of the engine. It may take a while to load - be patient, I think it's worth it and I'm sure that you will too. You may need to alter your browsers security settings to allow it to play the flash movies.
If the animations don't display properly, check that your ‘security' setting is at ‘medium'. Go to the tools menu on explorer, select ‘internet options', select the ‘security' tab. Now adjust the slider to ‘medium', click ‘apply' and then refresh the page. You'll now be able to see the animations whilst reading the text - have fun - Dave.
I'll try to explain in fairly simple terms
how the engine works. As we go along, we'll also have a look at the differences
between Petrol and Diesel engines, then we'll discuss some of the modifications
that engine manufacturers use to try to get more speed and power from
their cars.
This section is very involved and I can't possibly explain to you in
a few minutes what mechanics take months to learn! You will need to
read this page several times to get the best benefit from it. Look
at the images and read the text that goes with them, then study the
engine animation of a single cylinder to see what happens deep inside
your training car.
The most popular engine for cars is the petrol driven internal
combustion engine. We put the fuel (petrol) into the car
and it is converted into the energy we need to drive the car along
the road. The more petrol we put into the engine by pressing the
gas pedal, the faster we go. The term 'internal combustion' basically
means that the fuel is burnt inside a ‘combustion chamber' within the engine. A combustion chamber is an area inside
the engine where a fuel and air mixture is introduced and ignited
by a spark plug When this mixture of petrol
and air is ignited, it explodes with tremendous power and it's this
explosive power that pushes the car forwards along the road.
Most cars have four completely separate cylinders inside
the engine, and a mixture of petrol and air is ignited in each of
them. These cylinders are usually arranged in a straight line. Inside
each cylinder there's a piston and several
other component parts. The piston rises and falls inside the engine
as I'll describe below. It's the piston that compresses the petrol
air mixture before it's ignited, and it's the piston that's pushed
back down the cylinder by the explosion in the chamber.
The
diagram here shows one of the pistons inside one of the chambers.
The parts are:
A. Camshaft (opens
and closes the valves)
B. Tappet (pushed by the camshaft, moves the
valves)
C. Valve spring (keeps the valve normally shut)
D. Spark plug (or glow plug in a diesel)
E. Exhaust port (where waste fuel is pushed out)
F. Piston
G. Crankcase
H. Crankshaft (turned by the piston - turns
the engine and makes the car go)
I. Big end
J. connecting rod (nicknamed conrod or
just rod)
K. cylinder
L. Gudeon pin (attaches piston to conrod
M. Combustion chamber (where the fuel
explodes)
N. Inlet port
O. Valve
You certainly don't need to know about all the parts, but I'll explain
what the main ones do as we go along.
The Four Stroke Cycle
As I've said, most cars have four cylinders and their engines are usually
called ‘four stroke' systems. Some smaller engines, like those fitted
to mopeds, have only two strokes.
The ‘stroke' is simply when the piston moves either all the way up or all the way down inside the cylinder. As you might guess, in the four stroke engine each of the four strokes accomplishes something different. So, lets have a look at the four strokes and see what happens:
Induction
The first stroke is called Induction. This is when the fuel and air mixture
is drawn into the cylinder by the piston going down and producing
suction. As the piston travels down the cylinder it creates a vacuum
above it and the fuel mixture is drawn into the cylinder in the empty
space left by the piston The piston starts at the top, the camshaft turns
and pushes on the tappets which causes the intake valve to open (on the
left), and the piston moves down to let the engine take in a cylinder
full of air and fuel (shown in light blue). This is also sometimes called
the intake stroke. Only the tiniest drop of fuel needs to be mixed into
the air. The correct ratio of petrol is between 14:1 and 17:1 depending
on the engine manufacturer.
Compression
Compression is the second of the four strokes. This is the stage when
the fuel and air mixture is compressed and forced into the top of the
cylinder ready for ignition by the spark plug. The camshaft has turned,
pushed the tappets which have in turn allowed the inlet valve to return
to the closed position. The piston moves back up and compresses
this fuel/air mixture. Compressing the mixture makes the explosion more
powerful. As the valves are both closed, the cylinder is sealed and the
mixture can't escape.
Ignition
As the piston reaches the top of the compression stroke, the spark plug ‘fires'
and ignites the highly compressed fuel and air mixture. The piston is
then forced back down the cylinder by the resulting explosion,
turning the crankshaft and generating the propulsion for the engine which
makes the car go along the road. The diagram shows the piston on it's
way down the cylinder just after the mixture has ignited.
Exhaust
This is when the exhaust gases (after the fuel is burnt) are forced out
of the engine. Once the piston hits the bottom of its stroke, the exhaust
valve opens (on the right). The piston travels back up inside
the cylinder and this time it ‘pushes' the exhaust gasses out through
the now open exhaust vent. Now the engine is ready for the next cycle,
so it intakes another charge of air and gas. And we are back to the intake
stroke. This cycle of the four separate strokes keeps going on and on
until we turn the ignition off.
Now that you've looked at each stroke of the four stroke engine, let's
put it all together and see an animation of the piston as it rises and
falls. This animation is courtesy of How Stuff Works
|
Diesel Engines
A Diesel engine is almost the same as a petrol engine but it burns a
different type of fuel. Diesel is named after its inventor Rudolf Diesel,
a German who first developed this type of fuel in the early 1900's.
Diesel engines have mainly been used in lorries and buses because of
their increased power and their reliability. They tend to be noisy and
smelly compared to a petrol engine but are more economical to run.
The difference between a petrol and a diesel lies in the way the fuel is introduced into the engine and ignited. You may remember from above that in a petrol engine the fuel and air are mixed before being forced into the cylinders. In a diesel engine the compression stroke only compresses air and not fuel. On the ignition stroke the air is compressed to a very high pressure and this generates enormous heat which then ignites the fuel that is injected into the combustion chamber at that precise moment of maximum pressure. There is no need for a spark plug in a diesel engine. The compressed air is hot enough to cause the fuel to explode. This is because a diesel engine has a far higher "compression ratio" than does a petrol engine (Diesel engines usually 20 to 1, petrol engines typically 9 to one).
When
the engine is cold, the insides of the cylinders can't get hot enough
to ignite the fuel and start the car. A diesel engine therefore has extra
components called ‘glow plugs' in each cylinder. The glow plugs are pre
heated by power from the car battery before the engine is started. That's
why you should always turn on the ignition and wait for the glow plug
warning light to go out before trying to start the engine. When the light
goes out the plugs are hot enough to ignite the fuel in the cylinders.
A Diesel engine will normally last longer than a petrol engine. If you look after it you should get 250,000 miles without major problems.
Now let's have a closer look at the other parts of the engine shown on the first diagram. You can scroll back up to the diagram and use the text links to come to the relevant part of this section if you like, so that you can look at the parts and read the description.
The camshaft is a metal bar that sits along the top of all the cylinders. It rotates the ‘cams' which push on the tappets. The tappets then push on the valves and open or close the valve ports to let fuel into the cylinders and push the exhaust gasses out. diagram
The core of the engine is the cylinder. The piston moves up and down inside the cylinder. The engine described here has one cylinder. That is typical of most model engines, but some have more than one cylinder (four, six and eight cylinders are available). diagram
The plug supplies the heat that ignites the air/fuel mixture so that combustion can occur. diagram
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. Most modern engines don't just have one inlet valve and one exhaust valve per cylinder . . they have two or more of each. diagram
A
piston is a cylindrical piece of metal that moves up and down inside
the cylinder. The fit and finish between it and the cylinder is very
sensitive, the better the fit the better the engine. Two methods exist
of ensuring this fit is good
1) Piston rings are
a hard metal ring that sits around the top edge of the piston, they
are made of ring steel and keep themselves pushed outward into the
liner. They effectively provide a sliding seal between the outer edge
of the piston and the inner edge of the cylinder. Generally speaking,
when cold,
ringed engines have less compression and are therefore a bit easier
to start
2) ABC is the more
modern approach. The piston is ever so slightly barrel shaped and made
of Aluminium, and the liner is tapered with the narrow end at the top.
The liner is made from a Chromed Brass, hence ABC. The idea is that
when cold the piston and liner fit is very stiff, however at the optimum
running temperature, the coefficients of expansion of the various metals
are such that the fit between the two is perfect. Because of the lower
component count (i.e. no rings) they are much more reliable at higher
RPM's, in fact they are able to safely reach RPM's that ringed engines
could only dream of, 40,000 plus. They are also harder to seize because
the hotter the engine gets the further the liner gets away from the
piston. The only draw back is because the piston and liner are tight
when cold they can be hard to turnover and thus start. diagram
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 CCs (Cubic Centimetres, where 1,000 cubic centimetres
equals a litre) or Cubic Inches (where 1cc = .06 cubic inches). Generally,
the displacement tells you something about how much power an engine has.
A cylinder that displaces 2cc can hold twice as much fuel/air mixture
as a cylinder that displaces 1cc, and therefore you would expect about
twice as much power from the larger cylinder (if everything else is equal). diagram
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. The connection to the crankshaft is called the big end, and the connection to the piston is called the little end. The inner part of the little end is a steel pin that goes through the piston, this pin is called the gudeon pin. diagram
The crank shaft turns the piston's up and down motion into circular motion. The crank shaft rotates and it's this rotation that, through various mechanical connections, turns the wheels of the car and makes us go along the road. diagram
Modifications and engine ‘tuning'
Using all of this information, you can begin to see that there are lots of different ways to make an engine perform better. Manufacturers are constantly playing with all of the following variables to make an engine more powerful and/or more fuel efficient:
Increase the compression ratio -- Higher compression ratios produce more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burst into flame (before the spark/glow plug ignites it). Better quality fuel can prevent this sort of early combustion, but it is a fine balance
More
air/fuel into each cylinder -- If you can cram more air
(and therefore fuel) into a cylinder of a given size, you can get
more power from the cylinder (in the same way that you would by increasing
the size of the cylinder). Turbochargers and
super chargers pressurise the incoming air to effectively cram more
air into a cylinder.
Cool the incoming air -- Compressing air raises its temperature. However, you would like to have the coolest air possible in the cylinder because the hotter the air is, the less it will expand when combustion takes place. Therefore, many turbo charged and super charged cars have an ‘inter cooler'. An inter cooler is a special radiator through which the compressed air passes to cool it off before it enters the cylinder.
Let air come in more easily -- As a piston moves down in the intake stroke, air resistance can rob power from the engine. Air resistance can be lessened dramatically by putting two intake valves in each cylinder. Some newer engines are using polished intake manifolds to eliminate air resistance there
Let exhaust exit more easily -- If air resistance makes it hard for exhaust to exit a cylinder, it robs the engine of power. Ultimate power is developed with no exhaust, but this tends to upset the neighbours, so you must strike a compromise between power and noise.
Make everything lighter -- Lightweight parts help the engine perform better. Each time a piston changes direction, it uses up energy to stop the travel in one direction and start it in another. The lighter the piston, the less energy it takes.
Inject the fuel -- Fuel injection allows very precise metering of fuel to each cylinder. This improves performance and fuel economy. Most modern cars are fuel injected anyway, but performance cars can have high pressure injectors fitted to dramatically increase fuel flow into the engine.
I do hope that you've enjoyed reading about the engine in your car and how it works. When you have a little bit of knowledge about the engine you'll realise what a truly amazing invention it really is.