Scotch yoke
The 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:
- Fewer moving parts.
- Under ideal engineering conditions, force is applied directly in the line of travel of the assembly.
- Smoother operation: as the motion is sinusoidal, the velocity is cosinusoidal and the acceleration is also sinusoidal (assuming constant angular velocity).
- Higher percentage of the time spent at top dead center (dwell) improving theoretical engine efficiency of constant volume combustion cycles.[1]
- 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 mitigated. The longer the distance between the piston and the yoke, the less wear occurs, but inertia is increased, making such increases in the piston rod length realistically only suitable for lower RPM (but higher torque) applications.[2][3]
Disadvantages
The disadvantages and engineering challenges are:
- Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures. (Note: such wear can be mitigated through the use of a triple-sleeve - slipper - bearing, as used in the example of the Bourke Engine.[4][5])
- Increased heat loss during combustion due to extended dwell at top dead center offsets any constant volume combustion improvements in real engines.[1] Note: this statement is misleading, as demonstrated by working Bourke Engines which make use of precisely this extended dwell time to good effect, to effect "detonation". However, in an engine which is not designed around Bourke principles, such as Otto Cycle engines, this statement is very much correct.
- 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. Note: this statement is again misleading, again as demonstrated by working Bourke Engines, which use the pressurised air-fuel mixture in the chamber to force exhaust gases out. In the Bourke design, some exhaust gases (comprising water vapour and carbon dioxide) are deliberately left in the chamber in order to help catalyse the Hydrocarbon fuel Hydrogen-Oxygen "detonation" of the following cycle. It would however be completely inappropriate to attempt this in a standard two stroke based around Otto Cycle engine design because of the completely different chemical combustion involved.[6]
Applications
This setup is most commonly used in control valve actuators in high pressure oil and gas pipelines.
Although not a common metalworking machine nowadays, a Shaper uses a Scotch yoke which has been adjusted to provide a slow speed forward stroke and a faster return.
It has been used in various internal combustion engines, such as the Bourke engine, SyTech engine,[7] and many hot air engines and steam engines.
Experiments have shown that extended dwell time will not work well with constant volume combustion Otto Cycle Engines.[1] Gains might be more apparent in Otto Cycle Engines using a stratified direct injection (diesel or similar) cycle to reduce heat losses.[8].
In the case of the Bourke Engine, ignition takes place 90 degrees before top dead center to effect a carbon-based fuel burn: compression continues, increasing temperatures to in excess of 1800 F, where the extended dwell time helps to effect a hydrogen-oxygen "detonation" of the remaining mixture.[9]. The resultant hydrogen-oxygen burn, with flame speeds of 5,000 ft per second, is over extremely rapidly[10].
References
- ^ a b c "Science Links Japan | Effect of Piston Speed around Top Dead Center on Thermal Efficiency". Sciencelinks.jp. 2009-03-18. http://sciencelinks.jp/j-east/article/200609/000020060906A0236528.php. Retrieved 2011-12-06.
- ^ Bourke Engine Documentary, Published 1968, p50, "Appraising Engine Efficiency" para2
- ^ Bourke Engine Documentary, Published 1968, p51, "Important Factors in Engine Design"
- ^ Bourke Engine Documentary, Published 1968, p38 para4
- ^ Bourke Engine Documentary, Published 1968, p51, "The Bourke Slipper Bearing"
- ^ Bourke Engine Documentary, Published 1968, p38 "Bourke Cycle Chemistry Defined"
- ^ "The SyTech Scotch Yoke Engine". AutoSpeed. http://www.autospeed.com/cms/A_0948/article.html. Retrieved 2008-07-08.
- ^ "Effect of the Ratio Between Connecting-rod Length and Crank Radius on Thermal Efficiency". Science Links Japan. http://sciencelinks.jp/j-east/article/200623/000020062306A0851764.php. Retrieved 2008-07-08.
- ^ Bourke Engine Documentary, Published 1968, p33, p34, p38, p57-59
- ^ Bourke Engine Documentary, Published 1968, p36 para2
External links