The trailing side of the rotary engine's combustion chamber develops a squeeze stream which 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 , and Toyota obtained a 7% economy improvement by placing a glow-plug in the leading site, and using Reed-Valves in intake ducts. This poor combustion in the trailing side of chamber is one of the reasons why there is more carbon monoxide and unburnt hydrocarbons in a Wankel's exhaust stream. A side-port exhaust, as is used in the Mazda Renesis, avoids one of the causes of this because the unburned mixture cannot escape. The Mazda 26B avoided this problem through the use of a three spark-plug ignition system. (At the 24 Hours of Le Mans endurance race in 1991, the 26B had significantly lower fuel consumption than the competing reciprocating piston engines. All competitors had the same amount of fuel available due to the Le Mans limited fuel quantity rule.)