Editing Four-stroke engine (section)

==Design and engineering principles==

=== Power output limitations ===
[[Image:Ciclo del motore 4T.svg|right|250px|thumb|The four-stroke cycle
<br>1=TDC
<br>2=BDC
<br><span style="margin:1px; background-color: #10ff00;">'''&nbsp;A: Intake&nbsp;'''</span>
<br><span style="margin:1px; background-color: #ffae21;">'''&nbsp;B: Compression&nbsp;'''</span>
<br><span style="margin:1px; background-color: #ff0000;">'''&nbsp;C: Power&nbsp;'''</span>
<br><span style="margin:1px; background-color: #639eff;">'''&nbsp;D: Exhaust&nbsp;'''</span>
]]

The maximum amount of power generated by an engine is determined by the maximum amount of air ingested. The amount of power generated by a piston engine is related to its size (cylinder volume), whether it is a [[two-stroke engine]] or four-stroke design, [[volumetric efficiency]], losses, air-to-fuel ratio, the [[calorific value]] of the fuel, oxygen content of the air and speed ([[revolutions per minute|RPM]]). The speed is ultimately limited by material strength and [[lubrication]]. Valves, pistons and [[connecting rod]]s suffer severe acceleration forces. At high engine speed, physical breakage and [[piston ring]] flutter can occur, resulting in power loss or even engine destruction. [[Piston ring]] flutter occurs when the rings oscillate vertically within the piston grooves they reside in. Ring flutter compromises the seal between the ring and the cylinder wall, which causes a loss of cylinder pressure and power. If an engine spins too quickly, valve springs cannot act quickly enough to close the valves. This is commonly referred to as '[[valve float]]', and it can result in piston to valve contact, severely damaging the engine. At high speeds the lubrication of piston cylinder wall interface tends to break down. This limits the piston speed for industrial engines to about 10&nbsp;m/s.

==== Intake/exhaust port flow ====
The output power of an engine is dependent on the ability of intake (air–fuel mixture) and exhaust matter to move quickly through valve ports, typically located in the [[cylinder head]]. To increase an engine's output power, irregularities in the intake and exhaust paths, such as casting flaws, can be removed, and, with the aid of an [[air flow bench]], the radii of valve port turns and [[valve seat]] configuration can be modified to reduce resistance. This process is called [[cylinder head porting|porting]], and it can be done by hand or with a [[CNC]] machine.

=== Waste heat recovery of an internal combustion engine ===
An internal combustion engine is on average capable of converting only 40-45% of supplied energy into mechanical work. A large part of the waste energy is in the form of heat that is released to the environment through coolant, fins etc. If somehow waste heat could be captured and turned to mechanical energy, the engine's performance and/or fuel efficiency could be improved by improving the overall efficiency of the cycle. It has been found that even if 6% of the entirely wasted heat is recovered it can increase the engine efficiency greatly.<ref>{{Cite journal|last=Sprouse III|first=Charles|last2=Depcik|first2=Christopher|date=2013-03-01|title=Review of organic Rankine cycles for internal combustion engine exhaust waste heat recovery|journal=Applied Thermal Engineering|volume=51|issue=1–2|pages=711–722|doi=10.1016/j.applthermaleng.2012.10.017}}</ref>

Many methods have been devised in order to extract waste heat out of an engine exhaust and use it further to extract some useful work, decreasing the exhaust pollutants at the same time. Use of the [[Rankine cycle|Rankine Cycle]], [[turbocharging]] and [[Thermoelectric generator|thermoelectric generation]] can be very useful as a [[waste heat recovery unit|waste heat recovery]] system.

==== Supercharging ====
One way to increase engine power is to force more air into the cylinder so that more power can be produced from each power stroke. This can be done using some type of air compression device known as a [[supercharger]], which can be powered by the engine crankshaft.

Supercharging increases the power output limits of an internal combustion engine relative to its displacement. Most commonly, the supercharger is always running, but there have been designs that allow it to be cut out or run at varying speeds (relative to engine speed). Mechanically driven supercharging has the disadvantage that some of the output power is used to drive the supercharger, while power is wasted in the high pressure exhaust, as the air has been compressed twice and then gains more potential volume in the combustion but it is only expanded in one  stage.

==== Turbocharging ====
A [[turbocharger]] is a supercharger that is driven by the engine's exhaust gases, by means of a [[turbine]]. A turbocharger is incorporated into the exhaust system of a vehicle to make use of the expelled exhaust. It consists of a two piece, high-speed turbine assembly with one side that compresses the intake air, and the other side that is powered by the exhaust gas outflow.

When idling, and at low-to-moderate speeds, the turbine produces little power from the small exhaust volume, the turbocharger has little effect and the engine operates nearly in a naturally aspirated manner. When much more power output is required, the engine speed and throttle opening are increased until the exhaust gases are sufficient to 'spool up' the turbocharger's turbine to start compressing much more air than normal into the intake manifold. Thus, additional power (and speed) is expelled through the function of this turbine.

Turbocharging allows for more efficient engine operation because it is driven by exhaust pressure that would otherwise be (mostly) wasted, but there is a design limitation known as [[turbo lag]]. The increased engine power is not immediately available due to the need to sharply increase engine RPM, to build up pressure and to spin up the turbo, before the turbo starts to do any useful air compression. The increased intake volume causes increased exhaust and spins the turbo faster, and so forth until steady high power operation is reached. Another difficulty is that the higher exhaust pressure causes the exhaust gas to transfer more of its heat to the mechanical parts of the engine.

=== Rod and piston-to-stroke ratio ===
The rod-to-stroke ratio is the ratio of the length of the [[connecting rod]] to the length of the piston stroke. A longer rod reduces sidewise pressure of the piston on the cylinder wall and the stress forces, increasing engine life. It also increases the cost and engine height and weight.

A "square engine" is an engine with a bore diameter equal to its stroke length. An engine where the bore diameter is larger than its stroke length is an [[oversquare]] engine, conversely, an engine with a bore diameter that is smaller than its stroke length is an undersquare engine.

=== Valve train ===
The valves are typically operated by a [[camshaft]] rotating at half the speed of the [[crankshaft]]. It has a series of [[Cam (mechanism)|cam]]s along its length, each designed to open a valve during the appropriate part of an intake or exhaust stroke. A [[tappet]] between valve and cam is a contact surface on which the cam slides to open the valve. Many engines use one or more camshafts "above" a row (or each row) of cylinders, as in the illustration, in which each cam directly actuates a valve through a flat tappet. In other engine designs the camshaft is in the [[crankcase]], in which case each cam usually contacts a [[push rod]], which contacts a [[rocker arm]] that opens a valve, or in case of a [[flathead engine]] a push rod is not necessary. The [[overhead cam]] design typically allows higher engine speeds because it provides the most direct path between cam and valve.

==== Valve clearance ====
Valve clearance refers to the small gap between a valve lifter and a valve stem that ensures that the valve completely closes. On engines with mechanical valve adjustment, excessive clearance causes noise from the valve train. A too-small valve clearance can result in the valves not closing properly. This results in a loss of performance and possibly overheating of exhaust valves. Typically, the clearance must be readjusted each {{convert|20000|mi|km}} with a feeler gauge.

Most modern production engines use  [[hydraulic lifters]] to automatically compensate for valve train component wear. Dirty engine oil may cause lifter failure.

===Energy balance===
Otto engines are about 30% efficient; in other words, 30% of the energy generated by combustion is converted into useful rotational energy at the output shaft of the engine, while the remainder being lost due to waste heat, friction and engine accessories.<ref name="OtoE">{{cite web |url=http://www.ecen.com/content/eee7/motoref.htm |title=Efficiencies of Internal Combustion Engines |first=Omar Campos |last=Ferreira |work=Economia & Energia |location=Brasil |language=pt |date=March 1998 |access-date=2016-04-11}}</ref> There are a number of ways to recover some of the energy lost to waste heat. The use of a turbocharger in diesel engines is very effective by boosting incoming air pressure and in effect, provides the same increase in performance as having more displacement. The Mack Truck company, decades ago, developed a turbine system that converted waste heat into kinetic energy that it fed  back into the engine's transmission. In 2005, BMW announced the development of the [[turbosteamer]], a two-stage heat-recovery system similar to the Mack system that recovers 80% of the energy in the exhaust gas and raises the efficiency of an Otto engine by 15%.<ref name="BMWTS">{{cite news |url=http://www.autoblog.com/2005/12/09/bmw-turbosteamer-gets-hot-and-goes/ |title=BMW Turbo Steamer Gets Hot and Goes |first=John |last=Neff |work=Autoblog |date=2005-12-09 |access-date=2016-04-11}}</ref> By contrast, a [[six-stroke engine]] may reduce fuel consumption by as much as 40%.

Modern engines are often intentionally built to be slightly less efficient than they could otherwise be. This is necessary for [[Vehicle emissions control|emission controls]] such as [[exhaust gas recirculation]] and [[catalytic converter]]s that reduce [[smog]] and other atmospheric pollutants. Reductions in efficiency may be counteracted with an [[engine control unit]] using [[Lean-burn|lean burn techniques]].<ref>{{cite book |title=Air pollution from motor vehicles: Standards and Technologies for Controlling Emissions |first1=Asif |last1=Faiz |first2=Christopher S. |last2=Weaver |first3=Michael P. |last3=Walsh |publisher=World Bank Publications |year=1996 |isbn=9780821334447}}</ref>

In the United States, the [[Corporate Average Fuel Economy]] mandates that vehicles must achieve an average of {{convert|34.9|mpgus|abbr=on|1}} compared to the current standard of {{convert|25|mpgus|abbr=on|1}}.<ref>{{cite web|url=http://www.nhtsa.gov/fuel-economy|title=Fuel Economy |publisher=National Highway Traffic Safety Administration (NHTSA)|location=US|access-date=2016-04-11}}</ref> As automakers look to meet these standards by 2016, new ways of engineering the traditional [[internal combustion engine]] (ICE) have to be considered. Some potential solutions to increase [[fuel efficiency]] to meet new mandates include firing after the piston is farthest from the crankshaft, known as top [[Dead centre (engineering)|dead centre]], and applying the [[Miller cycle]]. Together, this redesign could significantly reduce fuel consumption and {{NOx|link=yes}} emissions.

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<div style="font-style: italic; text-align: center;">
[[Image:Four stroke cycle start.png|200px|Top dead center, before cycle begins]]
[[Image:Four stroke cycle intake.png|200px|1 – Intake stroke]]
[[Image:Four stroke cycle compression.png|200px|2 – Compression stroke]]
<br>Starting position, intake stroke, and compression stroke.<br />
[[Image:Four stroke cycle spark.png|200px|Fuel ignites]]
[[Image:Four stroke cycle power.png|200px|3 – Power stroke]]
[[Image:Four stroke cycle exhaust.png|200px|4 – Exhaust stroke]]
<br>Ignition of fuel, power stroke, and exhaust stroke.
</div>

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