{{Short description|Condition in which an engine turns beyond its design limit}} {{For|the aircraft flight condition|Overspeed (aeronautics)}} {{multiple issues|1= {{More citations needed|date=May 2020}} {{Expert needed|engineering|talk=|reason=layout and cohesion|date=May 2020}} }} '''Overspeed''', also referred to as '''over-rev''', is a condition in which an engine is allowed or forced to turn beyond its design limit. The consequences of running an engine too fast vary by engine type and model and depend upon several factors, the most important of which are the duration of the overspeed and the speed attained. With some engines, a momentary overspeed can result in greatly reduced engine life or catastrophic failure.<ref>[https://patents.google.com/patent/US9291107B2/en || Google Patents: Engine overspeed shutdown systems and methods]</ref> The speed of an engine is typically [[Tachometer|measured]] in [[revolutions per minute]] (rpm).<ref>[https://www.iso.org/obp/ui#iso:grs:7000:1389 || OBP: ISO 7000 — Graphical symbols for use on equipment]</ref> {{Citation needed|date=May 2020}}
==Examples of overspeed== * In a propeller aircraft, an overspeed will occur if the [[propeller (aircraft)|propeller]], usually connected directly to the engine, is forced to turn too fast by high-speed airflow while the aircraft is in a dive, moves to a flat [[blade pitch]] in cruising flight due to a governor failure or feathering failure, or becomes decoupled from the engine.{{Citation needed|date=May 2020}} * In a jet aircraft, an overspeed results when the [[axial compressor]] exceeds its maximal operating rotational speed. This often leads to the mechanical failure of turbine blades, [[flameout]] and destruction of the engine.{{Citation needed|date=May 2020}} * In a ground vehicle, an engine can be forced to turn too quickly by changing to an inappropriately low [[gearbox|gear]].{{Citation needed|date=May 2020}} * Most [[Regulator (automatic control)|unregulated]] engines will overspeed if power is applied with no or little [[mechanical work|load]].{{Citation needed|date=May 2020}} * In the event of [[diesel engine runaway]] (caused by excessive intake of combustibles), a [[diesel engine]] will overspeed if the condition is not quickly rectified.{{Citation needed|date=May 2020}} An example is a diesel engine powering equipment at an [[oil well]] head. Suppose the operators hit a pocket of natural gas. In that case, it will come to the surface and the engine will take in the flammable gas and rapidly increase speed until the engine is destroyed, unless the air intake is shut off, starving the engine of fuel and oxygen.
==Overspeed protection== Sometimes a regulator or [[Governor (device)|governor]] is fitted to make engine overspeed impossible or less likely. For example: * Many [[steam engine]]s use a [[centrifugal governor]], which closes a throttle at high rpm to restrict steam flow as engine speed increases.{{Citation needed|date=May 2020}} * In motor vehicles, [[automatic transmission]]s will change gear to prevent the engine from turning too quickly. Additionally, almost all modern vehicles are fitted with an electronic [[rev limiter]] device that will cut fuel supply or sparks to the engine to prevent overspeed.{{Citation needed|date=May 2020}} * Some aircraft have [[constant-speed propeller|constant-speed units]] that automatically change propeller pitch to keep the engine running at the optimal speed.{{Citation needed|date=May 2020}} * Large [[diesel engine]]s are sometimes fitted with a secondary protection device that actuates if the governor fails.<ref>[http://www.amot.com/products/air-intake-shutoff-valves AMOT Products]{{Dead link|date=October 2025 |bot=InternetArchiveBot }}.</ref> This consists of a flap valve in the air intake. If the engine overspeeds, the airflow through the intake will rise to an abnormal level. This causes the flap valve to snap shut, starving the engine of air and shutting it down.{{Citation needed|date=May 2020}}
== Different overspeed occurrences and prevention ==
=== Internal combustion engines === An excerpt presented by the [[San Francisco Maritime National Park Association]] illustrates the types of overspeed systems with [[Governor (device)|governor]] and engine control.<ref name=":22">{{Cite web|url=https://maritime.org/doc/fleetsub/diesel/chap10.htm|title=Submarine Main Propulsion Diesels - Chapter 10|website=maritime.org|access-date=2019-04-02}}</ref> Overspeed governors are either centrifugal or hydraulic.<ref name=":22" /> Centrifugal governors depend on the revolving force created by its own weight.<ref name=":22" /> [[Hydraulics|Hydraulic]] governors use the [[centrifugal force]] but drive a medium to accomplish the same task.<ref name=":22" /> The overspeed governor is implemented on most marine [[Diesel engine|diesel engines.]]<ref name=":22" /> The governor is a safety measure that acts when the engine is approaching overspeed and will trip the engine off if the regulator governor fails.<ref name=":22" /> It trips off the engine by cutting off fuel injection by having the centrifugal force act on levers linked to the governor collar.<ref name=":22" />
=== Turbines === Overspeeds for power plant [[turbine]]s can be catastrophic, resulting in failure due to the turbines' shafts and [[Turbine blade|blades]] being off balance and potentially throwing their blades and other metal parts at very high speeds.<ref>{{Cite book|title=Operators guide to general purpose steam turbines: An overview of operating principles, construction, best practices, and troubleshooting|last=Perez|first=R. X.|publisher=John Wiley & Sons|year=2016|location=Hoboken, NJ}}</ref> Different safeguards exist, which include a mechanical and electrical protection system.<ref name=":32">{{Cite web|url=http://pdfs.semanticscholar.org/bee4/9611336cfdb8887c6b1475b6a3979b95d772.pdf|archive-url=https://web.archive.org/web/20190304020351/http://pdfs.semanticscholar.org/bee4/9611336cfdb8887c6b1475b6a3979b95d772.pdf|url-status=dead|archive-date=2019-03-04|title=Turbine Overspeed Systems and Required Response|last=Taylor|first=Scott|date=June 2009|website=Semantic scholat|s2cid=15076138 |access-date=March 14, 2019}}</ref>
Mechanical overspeed protection is in the form of sensors.<ref name=":32" /> The system relies on the [[centripetal force]] of the shaft, a spring, and a weight.<ref name=":32" /> At the designed point of overspeed, the balance point of the weight is shifted, causing the lever to release a valve that makes the trip oil header to lose pressure due to draining.<ref name=":32" /> This loss of oil affects the pressure, and moves a trip mechanism to then trip the system off.<ref name=":32" />
An electrical overspeed detection system involves a [[gear]] with teeth and probes.<ref name=":32" /> These probes detect how fast the teeth are moving, and if they are moving beyond the designated [[Revolutions per minute|rpm]], it relays that to the logic solver (overspeed detection). The logic solver trips the system by sending the overspeed to the trip relay, which is connected to a [[solenoid]]-operated valve.<ref name=":32" />
== Mechanical vs. electrical governors on turbines <!-- Duplicate material with Turbine subheading above --> == In turbines and many other mechanical devices used for power generation, it is critical that the response times for overspeed prevention systems be as precise as possible.<ref name=":42">{{Cite journal|last1=Smith|first1=Sheldon S.|last2=Taylor|first2=Scott L.|date=2009|title=Turbine Overspeed Systems And Required Response Times|url=https://oaktrust.library.tamu.edu/handle/1969.1/163090 |journal=Turbomachinery and Pump Symposia |doi=10.21423/R19W7P}}</ref> If the response is off by even a fraction of a second, it can lead to turbines and its driven load (i.e. compressor, generator, pump, etc..) suffering catastrophic damage, and can put people at risk.'''<ref name=":42" />'''
=== Mechanical === Mechanical overspeed systems on turbines rely on an equilibrium between the centripetal force of the rotating shaft imparted on a weight attached to the end of a turbine blade.<ref name=":42" /> At the specified trip point, this weight makes physical contact with a lever that releases the trip oil header, which directly moves a trip bolt and/or a hydraulic circuit to activate stop valves to close.<ref name=":42" /> <!-- Previous sentence quite similar to the second paragraph of Turbines in the section above -->Because the contact with the lever occurs over a relatively limited angle, there is a maximum trip response time of 15 [[Millisecond|ms]] (i.e. 0.015 sec).<ref name=":42" /> The issue with these devices has less to do with response time as it does with response latency and variability in the trip point due to systems sticking.<ref name=":42" /> Some systems add two trip bolts for redundancy, which enables response latency to be reduced by half.<ref name=":42" />
=== Electrical === Electrical overspeed systems on turbines rely on a multitude of probes that sense speed through measuring the passages of the teeth of a spur gear.<ref name=":42" /> Using a digital logic solver, the overspeed system determines the propeller shaft rpm given the ratio of the gear to the shaft.<ref name=":42" /> If the shaft rpm is too high, it outputs a trip command which de-energizes a trip relay.<ref name=":42" /> Overspeed response varies from system to system, so it is key to check the original equipment manufacturer's specification to set the Overspeed trip time accordingly.<ref name=":42" /> Typically, unless specified otherwise, the response time to change the output relay will be 40 ms.<ref name=":42" /> This time includes the time required for the probes to detect speed, compare it to an overspeed set-point, calculate results, and finally output the trip command.<ref name=":42" />
== Overview of overspeed detection system <!--Vague section heading doesn't reflect section content-->== When configuring, testing, and running any overspeed systems on turbines or diesel engines, one factor considered is timing.<ref name=":22" /> This is because the response to overspeed is usually too fast for people to notice.{{blockquote|text=There is a strong argument to instrument the trip systems in such a way that the total system response can be measured. This way during a test a change in the response could indicate a degradation that might compromise system protection or point out a failing component.|source=Scott, 2009, p.161<ref name=":32" />|author=|title=}}
The responsibility of calibrating the correct overspeed response for a specific system falls on the manufacturer. However, variability is always present, and it is important for the owner/operator to understand the system in the event of maintenance, replacement, or retrofitting of outdated or worn out parts.<ref name=":32" /> After overspeed has occurred, it is essential to check all machinery parts for stress.<ref name=":52">{{Cite book|title=Modern marine engineering|last=National Marine Engineers' Beneficial Association (U.S.). District 1.|publisher=MEBA|oclc=28049257}}</ref> The first place to start for [[impulse turbine]]s is the rotor.<ref name=":52" /> At the rotor, there are balance holes<ref>{{cite conference|url=https://www.epj-conferences.org/articles/epjconf/pdf/2016/09/epjconf_efm2016_02080.pdf |title=Application of the balancing holes on the turbine stage discs with higher root reaction |first1=Lukáš |last1=Mrózek |first2=Ladislav |last2=Tajč |first3=Michal |last3=Hoznedl |first4=Martin |last4=Miczán |conference=EFM15 – Experimental Fluid Mechanics 2015 |date=28 March 2016 |doi=10.1051/epjconf/201611402080 |volume=114 |series=EPJ Web of Conferences}}</ref> that equalise the pressure difference between turbines, and if warped, would require the replacement of the entire rotor.<ref name=":52" />
==See also== *[[Airlines PNG Flight 1600]] *[[Overclocking]]
==References== {{reflist}}
[[Category:Engines]] [[Category:Engine problems]] [[Category:Mechanisms (engineering)]]