Dead centre
Encyclopedia
In a reciprocating engine
, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft
. The former is known as top dead centre (TDC) while the latter is known as bottom dead centre (BDC).
More generally, the dead centre is any position of a crank
where the applied force is straight along its axis, meaning no turning force can be applied. Many sorts of machines are crank driven, including unicycle
s, bicycle
s, tricycle
s, various types of machine press
es, gasoline
engines
, diesel engines, steam locomotive
s, and other steam engines. Crank-driven machines rely on the energy stored in a flywheel
to overcome the dead centre, or are designed, in the case of multi-cylinder engines
, so that dead centres can never exist on all cranks at the same time. A steam locomotive
is an example of the latter, the connecting rods being arranged such that the dead centre for each cylinder occurs out of phase with the other one (or two) cylinders.
cranks have dead centres at approximately 12 o'clock and 6 o'clock where simple pushing down of the pedal will not turn the chainwheel, but the rider's leg is able to apply tangential force at the pedal to overcome it. Fixed-gear bicycle
s (without a freehub
) use the momentum
of the bicycle and rider to keep the chainwheel turning even if the rider makes no attempt to pedal in a circular motion.
, top dead centre of piston #1 is the datum point from which ignition system
measurements are made and the firing order
is determined. For example, ignition timing
is normally specified as degrees before top dead centre (BTDC) although a very few small and fast-burning engines require a spark just after top dead centre (ATDC), such as the Nissan MA engine
with hemispherical combustion chamber
s, or hydrogen engines
.
Top dead centre for cylinder one is often marked on the crankshaft pulley, the flywheel
or dynamic balancer or both, with adjacent timing mark
s showing the recommended ignition timing settings as decided during engine development. These timing marks can be used to set the ignition timing either statically by hand or dynamically using a timing light
, by rotating the distributor
in its seat.
In a multi-cylinder engine, pistons may reach top dead centre simultaneously or at different times depending on the engine configuration
. For example:
The concept of top dead centre is also extended to pistonless rotary engine
s, and means the point in the cycle in which the volume of a combustion chamber is smallest. This typically occurs several times per rotor revolution; In the Wankel engine
for example it occurs three times for every one revolution of the rotor.
Finding the volume of the cylinder using TDC and BDC and multiplying it by the number of cylinders will give the engine displacement
.
s are commonly horizontal
, the relevant terms are front dead centre and back dead centre rather than "top" and "bottom".
If a single-cylinder steam engine stops in either of the dead centre positions it must be moved off the dead centre before it will re-start. In small engines this is done by turning the flywheel
by hand. In large engines the flywheel is moved with a lever or "turning bar". Both operations must be done with care to avoid the operator becoming entangled in the machinery. Even larger engines might require the use of a barring engine
.
Steam locomotive
s normally have two cylinders
with the cranks
set at right angle
s so one piston
will always be off the dead centre and no starting assistance is required.
Reciprocating engine
A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
. The former is known as top dead centre (TDC) while the latter is known as bottom dead centre (BDC).
More generally, the dead centre is any position of a crank
Crank (mechanism)
A crank is an arm attached at right angles to a rotating shaft by which reciprocating motion is imparted to or received from the shaft. It is used to change circular into reciprocating motion, or reciprocating into circular motion. The arm may be a bent portion of the shaft, or a separate arm...
where the applied force is straight along its axis, meaning no turning force can be applied. Many sorts of machines are crank driven, including unicycle
Unicycle
A unicycle is a human-powered, single-track vehicle with one wheel. Unicycles resemble bicycles, but are less complex.-History:One theory of the advent of the unicycle stems from the popularity of the penny-farthing during the late 19th century...
s, bicycle
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
s, tricycle
Tricycle
A tricycle is a three-wheeled vehicle. While tricycles are often associated with the small three-wheeled vehicles used by pre-school-age children, they are also used by adults for a variety of purposes. In the United States and Canada, adult-sized tricycles are used primarily by older persons for...
s, various types of machine press
Machine press
A machine press, commonly shortened to press, is a machine tool that changes the shape of a workpiece.-Servomechanism:A servomechanism press, also known as a servo press or a electro press, is a press driven by an AC servo motor. The torque produced is converted to a linear force via a ball screw....
es, gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
engines
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
, diesel engines, steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
s, and other steam engines. Crank-driven machines rely on the energy stored in a flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
to overcome the dead centre, or are designed, in the case of multi-cylinder engines
Engine configuration
Engine configuration is an engineering term for the layout of the major components of a reciprocating piston internal combustion engine. These components are the cylinders and crankshafts in particular but also, sometimes, the camshaft....
, so that dead centres can never exist on all cranks at the same time. A steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
is an example of the latter, the connecting rods being arranged such that the dead centre for each cylinder occurs out of phase with the other one (or two) cylinders.
Bicycles
BicycleBicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....
cranks have dead centres at approximately 12 o'clock and 6 o'clock where simple pushing down of the pedal will not turn the chainwheel, but the rider's leg is able to apply tangential force at the pedal to overcome it. Fixed-gear bicycle
Fixed-gear bicycle
A fixed-gear bicycle is a bicycle that has no freewheel, meaning it cannot coast, as the pedals are always in motion when the bicycle is moving....
s (without a freehub
Freehub
A freehub is a type of bicycle hub that incorporates a ratcheting mechanism, and the name freehub is a registered trademark of Shimano. A set of sprockets are mounted onto a splined shaft of the freehub to engage the chain...
) use the momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
of the bicycle and rider to keep the chainwheel turning even if the rider makes no attempt to pedal in a circular motion.
Reciprocating engine
In a reciprocating engineReciprocating engine
A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
, top dead centre of piston #1 is the datum point from which ignition system
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
measurements are made and the firing order
Firing order
The firing order is the sequence of power delivery of each cylinder in a multi-cylinder reciprocating engine.This is achieved by sparking of the spark plugs in a gasoline engine in the correct order, or by the sequence of fuel injection in a Diesel engine...
is determined. For example, ignition timing
Ignition timing
Ignition timing, in a spark ignition internal combustion engine , is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke...
is normally specified as degrees before top dead centre (BTDC) although a very few small and fast-burning engines require a spark just after top dead centre (ATDC), such as the Nissan MA engine
Nissan MA engine
The MA is an all aluminium straight-4 SOHC 0.9 L, 1.0 L, or 1.2 L engine made by Nissan. It had an 8-valve hemispherical cylinder head. Unusually, the specified ignition timing for the MA10 running on the specified 90 RON gasoline was 2 degrees after top dead centre, reflecting a very high flame...
with hemispherical combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
s, or hydrogen engines
Hydrogen vehicle
A hydrogen vehicle is a vehicle that uses hydrogen as its onboard fuel for motive power. Hydrogen vehicles include hydrogen fueled space rockets, as well as automobiles and other transportation vehicles...
.
Top dead centre for cylinder one is often marked on the crankshaft pulley, the flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
or dynamic balancer or both, with adjacent timing mark
Timing mark
A timing mark is an indicator used for setting the timing of the ignition system of an engine, typically found on the crankshaft pulley or the flywheel, being the largest radius rotating at crankshaft speed and therefore the place where marks at one degree intervals will be farthest apart.On older...
s showing the recommended ignition timing settings as decided during engine development. These timing marks can be used to set the ignition timing either statically by hand or dynamically using a timing light
Timing light
thumb|right|300px|Timing light, combination instrument with RPM, volt meter and dwell angle meter. The actual light is on the far end.The black clamp connects to the battery's -, the red clamp to +, the green one to the breaker side of the coil , the big black clamp in the foreground is an...
, by rotating the distributor
Distributor
A distributor is a device in the ignition system of an internal combustion engine that routes high voltage from the ignition coil to the spark plugs in the correct firing order. The first reliable battery operated ignition was developed by Dayton Engineering Laboratories Co. and introduced in the...
in its seat.
In a multi-cylinder engine, pistons may reach top dead centre simultaneously or at different times depending on the engine configuration
Engine configuration
Engine configuration is an engineering term for the layout of the major components of a reciprocating piston internal combustion engine. These components are the cylinders and crankshafts in particular but also, sometimes, the camshaft....
. For example:
- In the V-twinV-twinA V-twin engine is a two-cylinder internal combustion engine where the cylinders are arranged in a V configuration.- Crankshaft configuration :Most V-twin engines have a single crankpin, which is shared by both connecting rods...
configuration, the two pistons reach TDC at different times, equal to the angular displacementAngular displacementAngular displacement of a body is the angle in radians through which a point or line has been rotated in a specified sense about a specified axis....
between the cylinders. - In the flat twin configuration, two opposing pistons reach TDC simultaneously, which is also called 0° displacement - but one piston will be at TDC of the compression stroke, the other on TDC of the exhaust stroke.
- In the straight-4Straight-4The inline-four engine or straight-four engine is an internal combustion engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft....
configuration, the two end pistons (pistons 1 and 4) reach TDC simultaneously, as do the two centre pistons (pistons 2 and 3), but these two pairs reach TDC with an angular displacement of 180°. Similar patterns are found in almost all straight engineStraight engineUsually found in four- and six-cylinder configurations, the straight engine, or inline engine is an internal-combustion engine with all cylinders aligned in one row, with no offset...
s with even numbers of cylinders, with the two end pistons and two middle pistons moving together (not necessarily 180° out of phase however) and the intermediate pistons moving in pairs in mirror-image around the centre of the engine. - In the flatplane V8V8 engineA V8 engine is a V engine with eight cylinders mounted on the crankcase in two banks of four cylinders, in most cases set at a right angle to each other but sometimes at a narrower angle, with all eight pistons driving a common crankshaft....
and many larger V engineV engineA V engine, or Vee engine is a common configuration for an internal combustion engine. The cylinders and pistons are aligned, in two separate planes or 'banks', so that they appear to be in a "V" when viewed along the axis of the crankshaft...
s, the piston motion within each bank is similar to that of a straight engine, however in the crossplaneCrossplaneThe crossplane or cross-plane is a crankshaft design for V8 engines with a 90° angle between the cylinder banks.The crossplane crankshaft has four crankpins, each offset at 90° from the adjacent crankpins...
V8 and all V10V10 engineA V10 engine is a V engine with 10 cylinders in two banks of five with a distinct exhaust note.- Mechanics :The V10 is essentially the result of mating two even-firing straight-5 engines together. The straight-5 engine shows first and second order rocking motion...
engines the motion is far more complex.
The concept of top dead centre is also extended to pistonless rotary engine
Pistonless rotary engine
A pistonless rotary engine is an internal combustion engine that does not use pistons in the way a reciprocating engine does, but instead uses one or more rotors, sometimes called rotary pistons...
s, and means the point in the cycle in which the volume of a combustion chamber is smallest. This typically occurs several times per rotor revolution; In the Wankel engine
Wankel engine
The Wankel engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into a rotating motion instead of using reciprocating pistons. Its four-stroke cycle takes place in a space between the inside of an oval-like epitrochoid-shaped housing and a rotor that...
for example it occurs three times for every one revolution of the rotor.
Finding the volume of the cylinder using TDC and BDC and multiplying it by the number of cylinders will give the engine displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
.
Steam engine
As steam engineSteam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
s are commonly horizontal
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...
, the relevant terms are front dead centre and back dead centre rather than "top" and "bottom".
If a single-cylinder steam engine stops in either of the dead centre positions it must be moved off the dead centre before it will re-start. In small engines this is done by turning the flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
by hand. In large engines the flywheel is moved with a lever or "turning bar". Both operations must be done with care to avoid the operator becoming entangled in the machinery. Even larger engines might require the use of a barring engine
Barring engine
A barring engine is a small engine, usually a steam engine, that forms part of the installation of a large stationary steam engine. It is used to avoid the problem of the main engine stopping on its dead centre and so being unable to re-start....
.
Steam locomotive
Steam locomotive
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...
s normally have two cylinders
Cylinder (steam locomotive)
The cylinders of a steam locomotive are the components that convert the power stored in the steam into motion.Cylinders may be arranged in several different ways.-Early locomotives:...
with the cranks
Crank (mechanism)
A crank is an arm attached at right angles to a rotating shaft by which reciprocating motion is imparted to or received from the shaft. It is used to change circular into reciprocating motion, or reciprocating into circular motion. The arm may be a bent portion of the shaft, or a separate arm...
set at right angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...
s so one piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
will always be off the dead centre and no starting assistance is required.