The positions of an engine's piston at the top or bottom of its stroke
This article is about engineering of crank-driven machinery. For other uses, see
Dead center
.
In a
reciprocating engine
, the
dead centre
is the position of a piston in which it is either 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
).
[1]
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
unicycles
,
bicycles
,
tricycles
, various types of
machine presses
,
gasoline
engines
,
diesel engines
,
steam locomotives
, 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 more) cylinders.
Bicycles
[
edit
]
Bicycle
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 bicycles
(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.
Reciprocating engine
[
edit
]
In a
reciprocating engine
, top dead centre of piston #1 is the point from which
ignition system
measurements are made and the
firing order
is determined. For example,
ignition timing
is normally specified as degrees of crankshaft rotation
before top dead centre
(
BTDC
).
[2]
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
combustions
, or
hydrogen engines
.
[
citation needed
]
Top dead centre for cylinder one is often marked on the crankshaft pulley, the
flywheel
or
harmonic balancer
or both, with adjacent
timing marks
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:
- In the
V-twin
configuration, the two pistons reach TDC at different times, equal to the
angular displacement
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-4
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 engines
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
V8
and many larger
V engines
, the piston motion within each bank is similar to that of a straight engine, however in the
crossplane
V8 and all
V10
engines the motion is far more complex.
The concept of top dead centre is also extended to
pistonless rotary engines
, 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 (although only once per revolution of the engine output shaft, since the output rotates at three times the speed of the rotor).
[
citation needed
]
Finding the volume of the cylinder using TDC and BDC and multiplying it by the number of cylinders will give the
engine displacement
.
Steam engines
[
edit
]
As
steam engines
are commonly
horizontal
, the relevant terms are front dead centre and back dead centre rather than "top" and "bottom".
[
citation needed
]
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 restart. 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 locomotives
normally have at least two
double acting cylinders
, which enables the
cranks
to be set so that at least one
piston
will always be off the dead centre and no starting assistance is required. In the common case of a two piston locomotive, the cranks are set at
right angles
, so that whenever one piston is at dead centre the other is in mid-stroke, and giving four equally spaced power strokes per revolution.
[
citation needed
]
Other machines
[
edit
]
This term is also used in the realm of production equipment. A mechanical
punch press
employs a crankshaft similar to that found in an engine. In the punch press the crankshaft drives a ram which when it is farthest away from the platen of the press is considered to be in the position of top dead centre.
[3]
See also
[
edit
]
The dictionary definition of
top dead center
at Wiktionary
References
[
edit
]