Dead centre (engineering)

<h2 id="bicycles">Bicycles</h2>
<p><a href="/facts/Bicycle/nxBVrlBF">Bicycle</a> cranks have dead centres at approximately 12 o'clock and 6 o'clock where simple pushing down of the pedal will not turn the <a href="/facts/Chainwheel/wXFhx3Rz">chainwheel</a>, but the rider's leg can apply tangential force at the pedal to overcome it. <a href="/facts/Fixed-gear_bicycle/r0LISGBU">Fixed-gear bicycles</a> (without a <a href="/facts/Freehub/BtBGzm6U">freehub</a>) use the <a href="/facts/Momentum/TTkGv5eY">momentum</a> of the bicycle and rider to keep the chainwheel turning even if the rider makes no attempt to pedal in a circular motion.
</p>
<h2 id="reciprocating-engine">Reciprocating engine</h2>
<p>In a <a href="/facts/Reciprocating_engine/ItLAxmW3">reciprocating engine</a>, top dead centre of piston #1 is the point from which <a href="/facts/Ignition_system/7Rqqltpr">ignition system</a> measurements are made and the <a href="/facts/Firing_order/ssapth5e">firing order</a> is determined. For example, <a href="/facts/Ignition_timing/hqpb5yaG">ignition timing</a> is normally specified as degrees of crankshaft rotation before top dead centre (BTDC).<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> A very few small and fast-burning engines require a spark just after top dead centre (ATDC), such as the <a href="/facts/Nissan_MA_engine/QDdk5nBn">Nissan MA engine</a> with hemispherical <a href="/facts/Combustion/ohQoWj8z">combustions</a>, or <a href="/facts/Hydrogen_vehicle/dMKIgm1F">hydrogen engines</a>.
</p><p>Top dead centre for cylinder one is often marked on the crankshaft pulley, the <a href="/facts/Flywheel/x6yw1efR">flywheel</a> or <a href="/facts/Harmonic_balancer/atFoQTY2">harmonic balancer</a> or both, with adjacent <a href="/facts/Timing_mark/yB5WaqNn">timing marks</a> showing the recommended ignition timing settings as decided during engine development. The timing marks can be used to set the ignition timing either statically by hand or dynamically using a <a href="/facts/Timing_light/lW4jDUSF">timing light</a>, by rotating the <a href="/facts/Distributor/VySYIImn">distributor</a> in its seat.
</p><p>In a multi-cylinder engine, pistons may reach top dead centre simultaneously or at different times depending on the <a href="/facts/Engine_configuration/0zC4dXgG">engine configuration</a>. For example:
</p>
<ul><li>In the <a href="/facts/V-twin/utETANz0">V-twin</a> configuration, the two pistons reach TDC at different times, equal to the <a href="/facts/Angular_displacement/qoqvM9Np">angular displacement</a> between the cylinders.</li>
<li>In the <a href="/facts/Flat_twin/A2h1JIdG">flat twin</a> 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.</li>
<li>In the <a href="/facts/Straight-4/NVgxSDFb">straight-4</a> 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 <a href="/facts/Straight_engine/T1537bJF">straight engines</a> 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.</li>
<li>In the flatplane <a href="/facts/V8_engine/W7Q5Uw29">V8</a> and many larger <a href="/facts/V_engine/nNIeYP6c">V engines</a>, the piston motion within each bank is similar to that of a straight engine, however in the <a href="/facts/Crossplane/C6zKSq4H">crossplane</a> V8 and all <a href="/facts/V10_engine/vBhr3Vqt">V10</a> engines the motion is far more complex.</li></ul>
<p>The concept of top dead centre is also extended to <a href="/facts/Pistonless_rotary_engine/PhMUuxVY">pistonless rotary engines</a>, 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 <a href="/facts/Wankel_engine/IstzAyor">Wankel engine</a> 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).
</p><p>Finding the volume of the cylinder using TDC and BDC and multiplying it by the number of cylinders will give the <a href="/facts/Engine_displacement/qkHXacPv">engine displacement</a>.
</p>
<h2 id="steam-engines">Steam engines</h2>
<p>As <a href="/facts/Steam_engine/f7hN8vzm">steam engines</a> are commonly <a href="/facts/Horizontal_plane/Hhsu9C8x">horizontal</a>, the relevant terms are front dead centre and back dead centre rather than "top" and "bottom".
</p><p>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 <a href="/facts/Flywheel/x6yw1efR">flywheel</a> 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 <a href="/facts/Barring_engine/6R5SeBjP">barring engine</a>.
</p><p><a href="/facts/Steam_locomotive/cXEQwhFk">Steam locomotives</a> normally have at least two <a href="/facts/Double_acting_cylinder/o0TxbdjM">double acting cylinders</a>, which enables the <a href="/facts/Crank_(mechanism)/1a3bCxyE">cranks</a> to be set so that at least one <a href="/facts/Piston/z7pN46u7">piston</a> 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 <a href="/facts/Right_angle/psaslRq7">right angles</a>, so that whenever one piston is at dead centre the other is in mid-stroke, and giving four equally spaced power strokes per revolution.
</p>
<h2 id="other-machines">Other machines</h2>
<p>This term is also used in the realm of production equipment. A mechanical <a href="/facts/Punch_press/hR6BuPL1">punch press</a> employs a crankshaft similar to that found in an engine. In the punch press, the crankshaft drives a ram that when it is furthest away from the platen of the press is deemed to be in the position of top dead centre.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a>
</p>
<h2 id="see-also">See also</h2>
<p> The dictionary definition of top dead center at Wiktionary
</p>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Sen, Mohan (2006). Basic Mechanical Engineering. Firewall Media. p. 297. ISBN 9788170089612. <a href="9788170089612" target="_blank">9788170089612</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Principles of Automotive Vehicles. Technical Manual. United States Department of the Army. October 1985. pp. 2−33, 2−34. <a href="https://books.google.com/books?id=OP0XAAAAYAAJ&pg=PA33" target="_blank">https://books.google.com/books?id=OP0XAAAAYAAJ&pg=PA33</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Miller, Rex; Miller, Mark Richard (2004). Audel Machine Shop Tools and Operations. Audel Technical Trades Series. Vol. 9 (5th ed.). Wiley Publishing, Inc. p. 345. ISBN 9780764568619. <a href="9780764568619" target="_blank">9780764568619</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
</ol>

Dead centre (engineering) open-in-new

Dead centre (engineering)