combustion engines can contain any number of combustion chambers
(cylinders), with numbers between one and twelve being common, though
as many as 36 (Lycoming R-7755) have been used. Having more cylinders
in an engine yields two potential benefits: first, the engine can have a
larger displacement with smaller individual reciprocating masses, that
is, the mass of each piston can be less thus making a smoother-running
engine since the engine tends to vibrate as a result of the pistons
moving up and down. Doubling the number of the same size cylinders will
double the torque and power. The downside to having more pistons is
that the engine will tend to weigh more and generate more internal
friction as the greater number of pistons rub against the inside of
their cylinders. This tends to decrease fuel efficiency and robs the
engine of some of its power. For high-performance gasoline engines
using current materials and technology, such as the engines found in
modern automobiles, there seems to be a point around 10 or 12 cylinders
after which the addition of cylinders becomes an overall detriment to
performance and efficiency. Although, exceptions such as the W16 engine
from Volkswagen exist.
- Most car engines have four to eight cylinders with some high
performance cars having ten, 12 — or even 16, and some very small cars
and trucks having two or three. In previous years, some quite large cars
such as the DKW and Saab 92, had two-cylinder or two-stroke engines.
- Radial aircraft engines had from three to 28 cylinders; examples
include the small Kinner B-5 and the large Pratt & Whitney R-4360.
Larger examples were built as multiple rows. As each row contains an
odd number of cylinders, to give an even firing sequence for a
four-stroke engine, an even number indicates a two- or four-row engine.
The largest of these was the Lycoming R-7755 with 36 cylinders (four
rows of nine cylinders), but it did not enter production.
- Motorcycles commonly have from one to four cylinders, with a few
high performance models having six; although, some ‘novelties’ exist
with 8, 10, or 12.
- Snowmobiles Usually have one to four cylinders and can be both 2
stroke or 4 stroke, normally in the in-line configuration however there
are again some novelties that exist with V-4 Engines
- Small portable appliances such as chainsaws, generators, and
domestic lawn mowers most commonly have one cylinder, but two-cylinder
- Large reversible two cycle marine diesels have a minimum of three to
over ten cylinders. Freight diesel locomotives usually have around 12
to 20 cylinders due to space limitations as larger cylinders take more
space (volume) per kwh, due to the limit on average piston speed of
less than 30 ft/sec on engines lasting more than 40000 hours under full
ignition system of an internal combustion engines depends on the type
of engine and the fuel used. Petrol
engines are typically ignited by a
precisely timed spark, and diesel engines by compression heating.
Historically, outside flame and hot-tube systems were used, see hot bulb
mixture is ignited by an electric spark from a spark plug — the timing
of which is very precisely controlled. Almost all gasoline engines are
of this type. Diesel engines timing is precisely controlled by the
pressure pump and injector.
occurs as the temperature of the fuel/air mixture is taken over its
autoignition temperature, due to heat generated by the compression of
the air during the compression stroke. The vast majority of compression
ignition engines are diesels in which the fuel is mixed with the air
after the air has reached ignition temperature. In this case, the
timing comes from the fuel injection system. Very small model engines
for which simplicity and light weight is more important than fuel costs
use easily ignited fuels (a mixture of kerosene, ether, and lubricant)
and adjustable compression to control ignition timing for starting and
reciprocating engines, the point in the cycle at which the
fuel-oxidizer mixture is ignited has a direct effect on the efficiency
and output of the ICE. The thermodynamics of the idealized Carnot heat
engine tells us that an ICE is most efficient if most of the burning
takes place at a high temperature, resulting from compression — near
top dead center. The speed of the flame front is directly affected by
the compression ratio, fuel mixture temperature, and octane rating or
cetane number of the fuel. Leaner mixtures and lower mixture pressures
burn more slowly requiring more advanced ignition timing. It is
important to have combustion spread by a thermal flame front
(deflagration), not by a shock wave. Combustion propagation by a shock
wave is called detonation and, in engines, is also known as pinging or
So at least in gasoline-burning engines, ignition timing is largely a
compromise between a later “retarded” spark — which gives greater
efficiency with high octane fuel — and an earlier “advanced” spark that
avoids detonation with the fuel used. For this reason, high-performance
diesel automobile proponents, such as Gale Banks, believe that
There’s only so far you can go with an air-throttled engine
on 91-octane gasoline. In other words, it is the fuel, gasoline, that
has become the limiting factor. … While turbocharging has been
applied to both gasoline and diesel engines, only limited boost can be
added to a gasoline engine before the fuel octane level again becomes a
problem. With a diesel, boost pressure is essentially unlimited. It is
literally possible to run as much boost as the engine will physically
stand before breaking apart. Consequently, engine designers have come
to realize that diesels are capable of substantially more power and
torque than any comparably sized gasoline engine.
burn faster and more efficiently when they present a large surface
area to the oxygen in air. Liquid fuels must be atomized to create a
fuel-air mixture, traditionally this was done with a carburetor in
petrol engines and with fuel injection in diesel engines. Most modern
petrol engines now use fuel injection too — though the technology is
quite different. While diesel must be injected at an exact point in
that engine cycle, no such precision is needed in a petrol engine.
However, the lack of lubricity in petrol means that the injectors
themselves must be more sophisticated.
reciprocating engines continue to use a carburetor to supply fuel into
the cylinder. Although carburetor technology in automobiles reached a
very high degree of sophistication and precision, from the mid-1980s it
lost out on cost and flexibility to fuel injection. Simple forms of
carburetor remain in widespread use in small engines such as lawn
mowers and more sophisticated forms are still used in small
gasoline engines used in automobiles have mostly moved to fuel
injection systems (see Gasoline Direct Injection). Diesel engines have
always used fuel injection system because the timing of the injection
initiates and controls the combustion.
Autogas engines use either fuel injection systems or open- or closed-loop carburetors.
internal combustion engines now require a fuel pump. Diesel engines
use an all-mechanical precision pump system that delivers a timed
injection direct into the combustion chamber, hence requiring a high
delivery pressure to overcome the pressure of the combustion chamber.
Petrol fuel injection delivers into the inlet tract at atmospheric
pressure (or below) and timing is not involved, these pumps are
normally driven electrically. Gas turbine and rocket engines use
internal combustion engines like jet engines and rocket engines employ
various methods of fuel delivery including impinging jets, gas/liquid
shear, preburners and others.
Oxidiser-Air inlet system
engines such as solid rockets have oxidisers already within the
combustion chamber but in most cases for combustion to occur, a
continuous supply of oxidiser must be supplied to the combustion
air is used with piston engines it can simply suck it in as the piston
increases the volume of the chamber. However, this gives a maximum of
1 atmosphere of pressure difference across the inlet valves, and at
high engine speeds the resulting airflow can limit potential output.
Superchargers and turbochargers
supercharger is a “forced induction” system which uses a compressor
powered by the shaft of the engine which forces air through the valves
of the engine to achieve higher flow. When these systems are employed
the maximum absolute pressure at the inlet valve is typically around 2
times atmospheric pressure or more.
are another type of forced induction system which has its compressor
powered by a gas turbine running off the exhaust gases from the engine.
Turbochargers and superchargers are particularly useful at high altitudes and they are frequently used in aircraft engines.
Duct jet engines use the same basic system, but eschew the piston engine, and replace it with a burner instead.
liquid rocket engines, the oxidiser comes in the form of a liquid and
needs to be delivered at high pressure (typically 10-230 bar or 1–23
MPa) to the combustion chamber. This is normally achieved by the use of
a centrifugal pump powered by a gas turbine — a configuration known as a
turbopump, but it can also be pressure fed.
a four-stroke engine, key parts of the engine include the crankshaft
(purple), connecting rod (orange), one or more camshafts (red and blue),
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 (grey and green), and for each
cylinder there is a spark plug (darker-grey, gasoline engines only), a
piston (yellow), and a crankpin (purple). A single sweep of the
cylinder by the piston in an upward or downward motion is known as a
stroke. The downward stroke that occurs directly after the air-fuel mix
passes from the carburetor or fuel injector to the cylinder (where it
is ignited) is also known as a power stroke.
A Wankel engine has a triangular rotor that orbits in an epitrochoidal
(figure 8 shape) chamber around an eccentric shaft. The four phases of
operation (intake, compression, power, and exhaust) take place in what
is effectively a moving, variable-volume chamber.
four-stroke internal combustion engines employ valves to control the
admittance of fuel and air into the combustion chamber. Two-stroke
engines use ports in the cylinder bore, covered and uncovered by the
piston, though there have been variations such as exhaust valves.
Piston engine valves
piston engines, the valves are grouped into ‘inlet valves’ which admit
the entrance of fuel and air and ‘outlet valves’ which allow the
exhaust gases to escape. Each valve opens once per cycle and the ones
that are subject to extreme accelerations are held closed by springs
that are typically opened by rods running on a camshaft rotating with
the engines’ crankshaft.
combustion engines—as well as piston engines—usually have valves that
open and close to admit the fuel and/or air at the startup and
shutdown. Some valves feather to adjust the flow to control power or
engine speed as well.
combustion engines have to effectively manage the exhaust of the
cooled combustion gas from the engine. The exhaust system frequently
contains devices to control pollution, both chemical and noise
pollution. In addition, for cyclic combustion engines the exhaust
system is frequently tuned to improve emptying of the combustion
chamber. The majority of exhausts also have systems to prevent heat from
reaching places which would encounter damage from it such as
heat-sensitive components, often referred to as Exhaust Heat Management.
For jet propulsion internal combustion engines, the ‘exhaust system’
takes the form of a high velocity nozzle, which generates thrust for the
engine and forms a colimated jet of gas that gives the engine its name.
generates a great deal of heat, and some of this transfers to the
walls of the engine. Failure will occur if the body of the engine is
allowed to reach too high a temperature; either the engine will
physically fail, or any lubricants used will degrade to the point that
they no longer protect the engine. The lubricants must be clean as
dirty lubricants may lead to over formation of sludge in the engines.
Cooling systems usually employ air (air cooled) or liquid (usually
water) cooling while some very hot engines using radiative cooling
(especially some Rocket engines). Some high altitude rocket engines use
ablative cooling where the walls gradually erode in a controlled
fashion. Rockets in particular can use regenerative cooling which uses
the fuel to cool the solid parts of the engine.
is a component of reciprocating engines. It is located in a cylinder
and is made gas-tight by piston rings. Its purpose is to transfer force
from expanding gas in the cylinder to the crankshaft via a piston rod
and/or connecting rod. In two-stroke engines the piston also acts as a
valve by covering and uncovering ports in the cylinder wall.
jet engine forms of internal combustion engines, a propelling nozzle
is present. This takes the high temperature, high pressure exhaust and
expands and cools it. The exhaust leaves the nozzle going at much
higher speed and provides thrust, as well as constricting the flow from
the engine and raising the pressure in the rest of the engine, giving
greater thrust for the exhaust mass that exits.
reciprocating internal combustion engines end up turning a shaft. This
means that the linear motion of a piston must be converted into
rotation. This is typically achieved by a crankshaft.
flywheel is a disk or wheel attached to the crank, forming an inertial
mass that stores rotational energy. In engines with only a single
cylinder the flywheel is essential to carry energy over from the power
stroke into a subsequent compression stroke. Flywheels are present in
most reciprocating engines to smooth out the power delivery over each
rotation of the crank and in most automotive engines also mount a gear
ring for a starter. The rotational inertia of the flywheel also allows a
much slower minimum unloaded speed and also improves the smoothness at
idle. The flywheel may also perform a part of the balancing of the
system and so by itself be out of balance, although most engines will
use a neutral balance for the flywheel, enabling it to be balanced in a
separate operation. The flywheel is also used as a mounting for the
clutch or a torque converter in most automotive applications.
internal combustion engines require some form of system to get them
into operation. Most piston engines use a starter motor powered by the
same battery as runs the rest of the electric systems. Large jet
engines and gas turbines are started with a compressed air motor that
is geared to one of the engine’s driveshafts. Compressed air can be
supplied from another engine, a unit on the ground or by the aircraft’s
APU. Small internal combustion engines are often started by pull
cords. Motorcycles of all sizes were traditionally kick-started, though
all but the smallest are now electric-start. Large stationary and
marine engines may be started by the timed injection of compressed air
into the cylinders — or occasionally with cartridges. Jump starting
refers to assistance from another battery (typically when the fitted
battery is discharged), while bump starting refers to an alternative
method of starting by the application of some external force, e.g.
rolling down a hill.
Heat shielding systems
systems often work in combination with engine cooling and exhaust
systems. Heat shielding is necessary to prevent engine heat from
damaging heat-sensitive components. The majority of older cars use
simple steel heat shielding to reduce thermal radiation and convection.
It is now most common for modern cars are to use aluminium heat
shielding which has a lower density, can be easily formed and does not
corrode in the same way as steel. Higher performance vehicles are
beginning to use ceramic heat shielding as this can withstand far
higher temperatures as well as further reductions in heat transfer.
combustions engines require lubrication in operation that moving parts
slide smoothly over each other. Insufficient lubrication subjects the
parts of the engine to metal-to-metal contact, friction, heat build-up,
rapid wear often culminating in parts becoming friction welded together
e.g. pistons in their cylinders. Big end bearings seizing up will
sometimes lead to a connecting rod breaking and poking out through the
Several different types of lubrication systems are used. Simple
two-stroke engines are lubricated by oil mixed into the fuel or injected
into the induction stream as a spray. Early slow-speed stationary and
marine engines were lubricated by gravity from small chambers similar
to those used on steam engines at the time — with an engine tender
refilling these as needed. As engines were adapted for automotive and
aircraft use, the need for a high power-to-weight ratio led to increased
speeds, higher temperatures, and greater pressure on bearings which in
turn required pressure-lubrication for crank bearings and
connecting-rod journals. This was provided either by a direct
lubrication from a pump, or indirectly by a jet of oil directed at
pickup cups on the connecting rod ends which had the advantage of
providing higher pressures as the engine speed increased.
engines require one or more systems to start and shutdown the engine
and to control parameters such as the power, speed, torque, pollution,
combustion temperature, efficiency and to stabilise the engine from
modes of operation that may induce self-damage such as pre-ignition.
Such systems may be referred to as engine control units.
Many control systems today are digital, and are frequently termed FADEC (Full Authority Digital Electronic Control) systems.
On Board Diagnostics (also known as OBD) is a computerized system that
allows for electronic diagnosis of a vehicles’ powerplant. The first
generation, known as OBD1, was introduced 10 years after the
U.S. Congress passed the Clean Air Act in 1970 as a way to monitor a
vehicles’ fuel injection system. OBD2, the second generation of
computerized on-board diagnostics, was codified and recommended by the
California Air Resource Board in 1994 and became mandatory equipment
aboard all vehicles sold in the United States as of 1996.