Editing Wankel engine (section)

===Mechanical disadvantages===
Although many of the disadvantages are the subject of ongoing research, the current disadvantages of the Wankel engine in production are the following:<ref name="M9jv4">{{cite web|url= http://www.brighthubengineering.com/machine-design/4948-the-wankel-engine-part-iii-problems-and-disadvantages/ |title=Wankel Engine - Part III - problems and disadvantages |first1=John |last1=Sinitsky |publisher=BrighthubEngineering.com |date=11 September 2008 |access-date=2014-02-01}}</ref>
; Rotor sealing: The engine housing has vastly different temperatures in each separate chamber section. The different expansion coefficients of the materials lead to imperfect sealing. Additionally, both sides of the apex seals are exposed to fuel, and the design does not allow for controlling the lubrication of the rotors accurately and precisely. Rotary engines tend to be overlubricated at all engine speeds and loads, and have relatively high oil consumption and other problems resulting from excess oil in the combustion areas of the engine, such as carbon formation and excessive emissions from burning oil. By comparison, a piston engine has all functions of a cycle in the same chamber giving a more stable temperature for piston rings to act against. Additionally, only one side of the piston in a [[four-stroke engine|(four-stroke) piston engine]] is exposed to fuel, allowing oil to lubricate the cylinders from the other side. Piston engine components can also be designed to increase ring sealing and oil control as cylinder pressures and power levels increase. To overcome the problems in a Wankel engine of differences in temperatures between different regions of housing and side and intermediary plates, and the associated thermal dilatation inequities, a heat pipe has been used to transport heat from the hot to the cold parts of the engine. The "heat pipes" effectively direct hot exhaust gas to the cooler parts of the engine, resulting in decreases in efficiency and performance. In small-displacement, charge-cooled rotor, air-cooled housing Wankel engines, that has been shown to reduce the maximum engine temperature from {{convert|231|to|129|°C}}, and the maximum difference between hotter and colder regions of the engine from {{convert|159|to|18|C-change}}.<ref name="XFvJn">{{citation| title=SAE paper 2014-01-2160 |first1=Wei |last1=Wu |publisher=University of Florida|display-authors=etal}}</ref>
; Apex seal lifting: Centrifugal force pushes the apex seal onto the housing surface forming a firm seal. Gaps can develop between the apex seal and trochoid housing in light-load operation when imbalances in centrifugal force and gas pressure occur. At low engine-rpm ranges, or under low-load conditions, the gas pressure in the combustion chamber can cause the seal to lift off the surface, resulting in combustion gas leaking into the next chamber. Mazda developed a solution, changing the shape of the trochoid housing, which meant that the seals remained flush with the housing. Using the Wankel engine at sustained higher revolutions helps eliminate apex seal lift-off, making it viable in applications such as electricity generation. In motor vehicles, the engine is suited to series-hybrid applications.<ref name="VPlGg">{{cite web |url=http://wardsauto.com/vehicles-amp-technology/mazda-design-breakthrough-may-give-rotary-new-life |title=Mazda Design Breakthrough May Give Rotary New Life |date=29 February 2012 |first1=Roger |last1=Schreffler |website=wardsAuto.com |access-date=10 April 2015 |archive-url=https://web.archive.org/web/20150203022737/http://wardsauto.com/vehicles-amp-technology/mazda-design-breakthrough-may-give-rotary-new-life |archive-date=3 February 2015 |url-status=dead}}</ref> NSU circumvented this problem by adding slots on one side of the apex seals, thus directing the gas pressure into the base of the apex. This effectively prevented the apex seals from lifting off.<ref>Norbye, Jan P., ''The Wankel engine Design Development Applications'', p.134</ref>

Although in two dimensions the seal system of a Wankel looks to be even simpler than that of a corresponding multi-cylinder piston engine, in three dimensions the opposite is true. As well as the rotor apex seals evident in the conceptual diagram, the rotor must also seal against the chamber ends.

Piston rings in reciprocating engines are not perfect seals; each has a gap to allow for expansion. The sealing at the apexes of the Wankel rotor is less critical because leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the mainshaft case. Although sealing has improved over the years, the less-than-effective sealing of the Wankel, which is mostly due to lack of lubrication, remains a factor reducing its efficiency.<ref name="Eberle Klomp 1973 p. 454">{{cite conference |last1=Eberle |first1=Meinrad K. |last2=Klomp |first2=Edward D. |title=SAE Technical Paper Series |chapter=An Evaluation of the Potential Performance Gain from Leakage Reduction in Rotary Engines |date=1973-02-01 |volume=1 |doi=10.4271/730117 |page=454}}</ref>

The trailing side of the rotary engine's combustion chamber develops a squeeze stream that pushes back the flame front. With the conventional one or two-spark-plug system and homogenous mixture, this squeeze stream prevents the flame from propagating to the combustion chamber's trailing side in the mid and high-engine speed ranges.<ref name="SAE720357">{{citation|title= Combustion characteristics of Rotary Engines. SAE paper 720357|first1= K |last1= Yamamoto |publisher= Mazda|display-authors=etal}}</ref> Kawasaki dealt with that problem in its US patent {{patent|US|3848574}}; Toyota obtained a 7% economy improvement by placing a glow-plug in the leading side, and using Reed-Valves in intake ducts. In two-stroke engines, metal reeds last about {{convert|15000|km|abbr=in}} while carbon fiber, around {{convert|8000|km|abbr=in}}.<ref name="SAE790435"/> This poor combustion in the trailing side of the chamber is one of the reasons why there is more carbon monoxide and unburned hydrocarbons in a Wankel's exhaust stream. A side-port exhaust, as is used in the [[Renesis (Engine)|Mazda Renesis]], avoids port overlap, one of the causes of this, because the unburned mixture cannot escape. The [[R26B|Mazda 26B]] avoided this problem through the use of a three spark-plug ignition system and obtained a complete conversion of the aspirated mixture. In the 26B, the upper late trailing spark plug ignites before the onset of the squeeze flow.<ref name="SAE 930677">{{citation|title=Study of Direct Injection Stratiefied Rotary Engine SAE Paper 930677 |last1= Kagawa, Okazaki ... |publisher= Mazda |display-authors=etal}}</ref>