Editing Four-stroke engine (section)

===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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