Scotch yoke

The Scotch yoke, also spelled Scotch Yoke and scotch yoke, is a mechanism for converting the linear motion of a slider into rotational motion or vice-versa. The piston or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a pin on the rotating part. The shape of the motion of the piston is a pure sine wave over time given a constant rotational speed.

Advantages

The advantages compared to a standard crankshaft and connecting rod setup are:
*High torque output with a small cylinder size.
*Fewer moving parts.
*Smoother operation.
*Higher percentage of the time spent at top dead center (dwell) improving theoretical engine efficiency of constant volume combustion cycles though actual gains have not been demonstrated. [http://sciencelinks.jp/j-east/article/200609/000020060906A0236528.php Science Links Japan | Effect of Piston Speed around Top Dead Center on Thermal Efficiency ] ]
*In an engine application, elimination of joint typically served by a wrist pin, and near elimination of piston skirt and cylinder scuffing, as side loading of piston due to sine of connecting rod angle is eliminated.

Disadvantages

The disadvantages are:
*Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.
*More heat loss during combustion due to extended dwell at top dead center off sets any constant volume combustion improvements in real engines. ibid]
*Lesser percentage of the time spent at bottom dead center reducing blowdown time for two stroke engines, when compared with a conventional piston and crankshaft mechanism.

Applications

This setup is most commonly used in control valve actuators in high pressure oil and gas pipelines.

It has been used in various internal combustion engines, such as the Bourke engine, SyTech engine [cite web |url=http://www.autospeed.com/cms/A_0948/article.html |title=The SyTech Scotch Yoke Engine |format=HTML |work=AutoSpeed |accessdate=2008-07-08] , and many hot air engines and steam engines.

Experiments have shown that extended dwell time will not work well with constant volume combustion (Otto, Bourke or similar) cycles. ibid] Gains might be more apparent using a stratified direct injection (diesel or similar) cycle to reduce heat losses. [cite web |url=http://sciencelinks.jp/j-east/article/200623/000020062306A0851764.php |work=Science Links Japan |title=Effect of the Ratio Between Connecting-rod Length and Crank Radius on Thermal Efficiency |format=HTML |accessdate=2008-07-08] ]

ee also

References

External links

* [http://www.brockeng.com/mechanism/ScotchYoke.htm Brock Institute for Advanced Studies: Scotch Yoke]
*" [http://demonstrations.wolfram.com/ComparingSimpleCrankSliderAndScotchYokeMechanisms/ Comparing Simple Crank/Slider and Scotch Yoke Mechanisms] " by Fred Klingener, The Wolfram Demonstrations Project; Active demo.
* Another demo [http://wiki.tcl.tk/20841] with Tcl scripting language