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 , and the maximum difference between hotter and colder regions of the engine from . 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 remain flush with the housing. Using the Wankel engine at sustained higher revolutions helps eliminate apex seal lift off, lending it viable in applications such as electricity generation. In motor vehicles, the engine is suited to series-hybrid applications. 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. 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. 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. Kawasaki dealt with that problem in its US patent ; Toyota obtained a 7% economy improvement by placing a glow-plug in the leading side, and