designs such as , G. J. Watt, 1974, for a "Wankel Engine Cylinder Generating Machine", , "Apparatus for machining and/or treatment of trochoidal surfaces" and , "Device for machining trochoidal inner walls", and others, solved the problem. Wankel engines have a problem not found in reciprocating piston four-stroke engines in that the block housing has intake, compression, combustion, and exhaust occurring at fixed locations around the housing. This causes a very uneven thermal load on the rotor housing. In contrast, reciprocating engines perform these four strokes in one chamber, so that extremes of "freezing" intake and "flaming" exhaust are averaged and shielded by a boundary layer from overheating working parts. The use of heat pipes in an air-cooled Wankel was proposed by the University of Florida to overcome this uneven heating of the block housing. Pre-heating of certain housing sections with exhaust gas improved performance and fuel economy, also reducing wear and emissions. The boundary layer shields and the oil film act as thermal insulation, leading to a low temperature of the lubricating film (approximate maximum on a water-cooled Wankel engine). This gives a more constant surface temperature. The temperature around the spark plug is about the same as the temperature in the combustion chamber of a reciprocating engine. With circumferential or axial flow cooling, the temperature difference remains tolerable. Problems arose during research in the 1950s and 1960s. For a while, engineers were faced with what they called "chatter marks" and "devil's scratch" in the inner epitrochoid surface. They discovered that the cause was the apex seals reaching a resonating vibration, and the problem was solved by reducing the thickness and weight of the apex seals. Scratches disappeared after the introduction of more compatible materials for seals and housing coatings. Another early problem was the build-up of cracks in the stator surface near the plug hole, which was eliminated by installing the spark plugs in a separate metal insert/ copper sleeve in the housing, instead of a plug being screwed directly into the block housing. Toyota found that substituting a glow-plug for the leading site spark plug improved low rpm, part load, specific fuel consumption by 7%, and also emissions and idle. A later alternative solution to spark plug boss cooling was provided with a variable coolant velocity scheme for water-cooled rotaries, which has had widespread use, being patented by Curtiss-Wright, with the last-listed for better air-cooled engine spark