Entry ID:

A633232
Edited by:

Xuenyl
Date: 17 September 2001

The four-stroke internal combustion engine running on petroleum spirit
(a.k.a. petrol and gasoline) forms the basis for the power plants of the
majority of modern automobiles and has changed little since its invention
in the late nineteenth century.

A Little History

The original four-stroke engine was patented in 1862 by a French scientist
called Alphonse Beau de Rochas. Unfortunately, having perfected the theory,
the Frenchman failed to build an operational engine and it was the
German engineer

Nikolaus A. Otto
who built the first working prototype some five years later.

Engines based on Otto's principles were designed and built by another,
more famous German engineer

Gottlieb Daimler
, who was the first to use the now widespread vertical cylinder in 1885 and then a
two cylinder engine in the original 'V' arrangement in 1889.

Unlike Otto, the name Daimler is widely known today due to

work in a link to Jaguar-Daimler here
- see Daimler - Motoring Pioneers in A10114732(not edited yet)

the eponymous brand of luxury cars (currently produced by Chrysler) and the
involvement of Daimler-Chrysler with McLaren Cars, Ltd, of Formula 1 racing fame.

The Otto Cycle

Otto's eventual reward for his effort was the naming of the four-stroke cycle
which gives the engine its name. The Otto cycle forms the basic principle of
operation of the engine but to understand how it functions it is necessary to
know a little bit of engine anatomy.

Animated Four Stroke Engine

Basic Anatomy of the Four-Stroke Engine

The basic operating unit of a four stroke engine is the cylinder, so named
because it normally takes the form of a cylindrical space in the engine block
within which sits a flush fitting piston, also cylindrical and attached via a
piston rod (which has the potential to move back and forth at its joint with the piston)
through the open end of the cylinder to the crankshaft. The latter is a long metal rod
which usually runs the length of the engine. There are U-shaped bends offset
from the crankshaft sometimes called crankpins (similar to an old fashioned hand drill)
which are normally at 90 but can also be at 180 degrees to each other.

Looking along the axis of the shaft, you would see one at each of the
four points of the compass in a 4 cylinder engine. Each piston rod is attached
to one of these crankpins and as the piston moves up and down in the cylinder,
its 'reciprocating' motion is translated into rotation as the crankpins move
around the crankshaft. The rotational motion of the crankshaft is later used to
move the wheels of the vehicle via the clutch, the gearbox and the final drive.

Four Strokes, One Cycle

Each up or down movement of a piston is termed a stroke, hence the
piston moves up and down twice and the crankshaft rotates twice in once cycle.
Each stroke is associated with a different phase of the cycle:

1. 
   Induction (Suck*)
   
   
   In this first phase, the piston moves down, away from the top of the cylinder,
   increasing the space available between the two. This creates a negative pressure
   which, when an inlet valve in the top of the cylinder opens, draws a fuel and
   air mixture into the cylinder.
   
   
   
2. 
   Compression (Squeeze)
   
   
   The inlet valve closes and the piston moves back toward the top of the cylinder,
   compressing the fuel and air mixture. The difference between the volume of
   fuel and air mixture when the piston is maximally down and maximally up is
   termed the compression ratio. It is normally between 8:1 and 10:1 but
   can be 12:1 or more if high octane fuels are used. Compression increases
   the potential of the fuel/air mixture to burn.
   
   
   
3. 
   Combustion (Bang)
   
   
   An electrical spark generated by the spark plug in the roof of the cylinder
   ignites the fuel/air mixture, generating large amounts of heat and causing
   the mixture to expand rapidly, driving the piston downward and actively turning the crankshaft.
   
   
   
4. 
   Exhaust (Blow)
   
   
   The last stroke involves the opening of the second valve (the exhaust valve)
   in the cylinder and the return of the piston to the upper position. The returning
   piston forces the remaining gases in the cylinder out through the exhaust valve
   and into the engine's exhaust system.
   

Other Important Engine Parts

The four-stroke cycle applies to a single cylinder operating a crankshaft
but many practical engines have several cylinders. There is also the problem
of controlling the opening and closing of the valves. These are explained below.

Multiple Cylinders

The vast majority of internal combustion engines use more than one cylinder.
This is entirely a question of efficiency. The limitation of the Otto Cycle is
that it only provides power to turn the crankshaft a quarter of the time.
The logical solution is to have four cylinders with pistons turning the
crankshaft so at any time there is always one cylinder in the power stroke
and the crankshaft is turned at a fairly even rate.


Rotary - firing is the direction its going to go


An even more powerful method is to use extra cylinders at intermediate
points in the cycle so that one power stroke starts before the previous
one has finished.

Engines with 6, 8, 10, 12 and more cylinders have been created, either in
straight rows or with the cylinders split into two banks in a V shape
converging on the crankshaft. This allows for efficient transfer of
power to the crankshaft without making the engine too large.
Horizontally opposed engines with the banks of cylinders directly
opposite each other have also been designed but these are generally
limited to aircraft, which typically have multiple spark plugs per
cylinder as a fail safe device.

Camshafts and Valves

The inlet and exhaust valves of each cylinder need to open and close
at precise intervals during the cycle. The most appropriate method of
achieving this was found to be the mechanical linking of the valve
timing to the crankshaft. Thus, when the engine turns more quickly,
the valve openings sped up in proportion.

Originally, this was achieved by a timing chain driven via a sprocket
on the crankshaft to a sprocket on the camshaft, with a tensioner
fitted approximately halfway between the two sprockets to take out
the inevitable any slack in the chain. The method prevailed until the
advent of overhead camshaft engines, when chains were phased out
in favour of a flexible rubber/canvas composite belt.

The camshaft is so named because distributed along its length are
teardrop shaped cam lobes. As the crankshaft rotates once every
two strokes, the teardrop cams rotate at half the speed of the crankshaft.
Therefore, each pushrods (which rest on the cams) are pushed upward by
the tail of the teardrop once every four strokes.

The pushrods used on overhead valve engines raise one end of the
see-saw like rockers, the other end of which presses down on the valve.
The valves are shaped like mushrooms with their flattened heads inside
the cylinder, held in the upward position by springs. The downward
motion of the rockers pushes the head of the valve further into the
cylinder and creates a space around the 'stem' of the valve,
allowing the flow of gas into or out of the cylinder.

< my_interpretation_of_what_Straightalker_suggested>


With an internally fitted camshaft, involving many moving parts
(including cam buckets, rods and tappetts), the overhead valve
system was prone to regular and expensive repair bills.
Furthermore, the gap between the tappet and the valve required
routine adjustment by an experienced mechanic simply to ensure
the efficient running of an engine

Overhead camshafts were fitted at the top of the engine directly
over the valves thus doing away with the need for push rods
and tappets. Although early overhead cams still needed adjustment
of the cam followers (the equivalent of tappets), by 19XX
these had been superceded THE INVENTOR'S NAME's
hydraulic tappets, which require no routine maintenence whatsoever.

</ my_interpretation_of_what_Straightalker_suggested>

Fuel Systems

The fuel and air mixture which is supplied through the inlet valve
is traditionally supplied by a device called a carburettor. The most
important bit of the carburettor is a narrow tube known as a venturi,
through which air flows on its way to the inlet valve. Because a
vacuum is created in the far end of the venturi by the intake
stroke of the Otto Cycle, fuel is drawn from a jet into the venturi,
vaporising as it passes through the tiny nozzle of the jet. The flow
through the venturi is controlled by the engine's throttle, the higher
the setting, the more air, and therefore the more fuel is drawn
through the carburettor and the faster the engine can work.

Fuel Injection

Most modern petrol driven cars have fuel injection systems like
diesles where vapourised fuel is injected directly into the cylinder,
removing the necessity for a carburettor. The injection units are
normally electronically controlled, allowing for much more
accurate regulation of the amount of fuel which enters the
cylinder, producing more power, more economy and
fewer emissions.

Electrical Systems

As has already been alluded to, combustion in a four-stroke
petrol engine occurs when the piston is at the top of the cylinder.
Combustion is initiated by a precisely timed electrical spark,
which typically occurs just before the end of the compression
stroke, in order to allow the burn time to take hold. The spark
is provided by the spark plug, which is the end point of an
engine's electrical system. The important bits of a spark plug
are the two closely placed contacts inside the cylinder between
which an electrical pulse arcs (or 'jumps') creating a spark.

The electrical system on a vehicle is generally referred to as a
12 volt system, which reflects both the static supply from
a standard battery and the demand from a distributor.

However, when an engine is running, the supply is modified by
an alternator to produce a systems voltage of between
13.5 and 14.5 volts. The alternator gives out an average of
15 to 25 amps current under normal running conditions,
which is sufficient to keep the battery charged. However as
more electrical components are switched on, for example,
headlights, wipers etc., the alternator output increases accordingly.
Typically an alternator can give out as much as 70 amps at maximum output.

In order for combustion to occur, a spark is deliverd to the
cylinder via the spark plug which ignites the fuel/air mixture
within the cylinder. The spark is provided by the distributor
via High Tension Leads, known as HT leads or simply as
spark plug leads.



The number of cylinders per engine dictates how many plugs
and plug leads there are. The spark is distributed to each plug
lead by the rotor arm within the distributor which passes a
contact in the distributor cap where the plug leads are fitted.



The original source of the spark is generated by the ignition
coil which feeds the distributor via a single high tension lead
known as the king lead. The coil can generate up to 70kV in
standard points ignition systems. This is a high tension feed and
does not feed the points in any way. There are 2 circuits to an
ignition system; primary and seconday: Primary is the standard 12V
feeds and the secondary is the high tension/voltage feed.

In modern systems, the old points and condensor distributors have
been superceded by electronic ignition modules and are more reliable
as there are no moving parts, so no wear and tear and no regular
maintenance or adjustments are needed as on points systems.



The modules do the same as the points in making and breaking a
circuit only modules do it electronically rather than mechanically.
Yet further advancement was realised by the distributorless ignition
system, where the ignition is controlled solely by an

electronic control unit
(ECU) with various electronic sensors placed on the engine.

<needsWork>

Distributor

The distributor is possibly the most technical part of the electrical
system. Its task is to connect the electrical energy from the coil to
the spark plug supplying each cylinder in turn. Electricity is supplied
via a low tension lead to the distributor that has, at its centre, a rotor
* arm that passes four equally spaced contacts,
each of which are connected (via high tension leads) to the
spark plugs. For the engine to produce more power, the cylinders
must fire more rapidly, which means the rotor arm must spin faster.

The rotor arm speed is governed by the speed of the camshaft as the
distributor is driven by the camshaft either via a gear or offset keyway.
The vacuum pipe seen on early distributors actually moved the base plate
in the distributor, which in turn had the effect of advancing the timing
marginally, the unit shaped like a bellows on the side of the distributor
was called the advance retard unit.

Starter Motor

The electrical starter motor is essentially quite a simple gadget
which turns the cranshaft and allows fuel and air to be sucked
into the cylinders to start the Otto cycle, which is self perpetuating
once the engine is running. It is powered from the battery and
can be activated once the ignition has been switched on, the latter
providing power to the coil and thereby the sparkplugs.

Summary

As can hopefully be seen from this article, the petrol powered
four-stroke internal combustion engine is something approaching
a technological miracle. A complicated piece of equipment in which
each component is intimately related to several others and dependant
on them for its proper functioning. It is a testament to the original
design that the essential principles have changed little in 130 years
despite the improvements in materials science and electronics
which have lead to significant improvements in the power,
efficiency and reliability of these engines. There are few other
inventions which can claim such lasting success.

The following might be incorporated... somehow

A few comments on OHC versus OHV

• 
  Put simply overhead cam engines are more fuel efficient, use
  less horse power to be driven hence increased power to the
  engine and ultimately final drive, hence a faster car in mph terms.
  
• 
  There are less moving parts so weight of engine and wear
  and tear also vastly reduced. There is no regular adjustments to
  make on OHC engines, cambelts need to be changed but the intervals
  for change are quite long in terms of time or mileage, typically
  100,00 miles or 3 years, some cars today it's even higher.
  
• 
  Valve timing is more accurate with OHC as the tolerances can
  be a lot closer in terms of the gap between cam lobe and valve
  head.hydraulic tappets means any wear is automatically taken up
  without need for manual adjustment hence running costs are lower
  as less servicing required.
  

OHV

usually run via a timing chain that wears and rattles over a period
of time, too many moving parts used to make what is a simple operation
work. Apart from chain and tensioner, there are cam buckets that sit
on the cam lobes, push rods then sit in these up to the top end where
they sit under the tappet. the tappet has a nut and screw in one end
that is used to set the gap between the tappet and the valve, this requires
regular adjustment to prevent rattles and ensure the best possible operating
efficiency in opening and closing the valves the right length of time
for each cycle.




<aside> The term 'petrolheads' is one which qualified motor
engineers/mechanics and technicians refer to as enthusiatic amateurs
who 'play' around with engines and motors without any formal qualifications
or mechanical training. The

Top Gear
presenters are a classic example in my view of petrolheads.</>


Radio 4 technology: The combustion engine

library.thinkquest.org Car Engine History