cylinder (where it is ignited) is also known as a power stroke. A Wankel engine has a triangular rotor that orbits in an epitrochoidal (figure 8 shape) chamber around an eccentric shaft. The four phases of operation (intake, compression, power, and exhaust) take place in what is effectively a moving, variable-volume chamber. Valves All four-stroke internal combustion engines employ valves to control the admittance of fuel and air into the combustion chamber. Two-stroke engines use ports in the cylinder bore, covered and uncovered by the piston, though there have been variations such as exhaust valves. Piston engine valves In piston engines, the valves are grouped into 'inlet valves' which admit the entrance of fuel and air and 'outlet valves' which allow the exhaust gases to escape. Each valve opens once per cycle and the ones that are subject to extreme accelerations are held closed by springs that are typically opened by rods running on a camshaft rotating with the engines' crankshaft. Control valves Continuous combustion engines—as well as piston engines—usually have valves that open and close to admit the fuel and/or air at the startup and shutdown. Some valves feather to adjust the flow to control power or engine speed as well. Exhaust systems Internal combustion engines have to effectively manage the exhaust of the cooled combustion gas from the engine. The exhaust system frequently contains devices to control both chemical and noise pollution. In addition, for cyclic combustion engines the exhaust system is frequently tuned to improve emptying of the combustion chamber. The majority of exhausts also have systems to prevent heat from reaching places which would encounter damage from it such as heat-sensitive components, often referred to as Exhaust Heat Management. For jet propulsion internal combustion engines, the 'exhaust system' takes the form of a high velocity nozzle, which generates thrust for the engine and forms a collimated jet of gas that gives the engine its name. Cooling systems Combustion generates a great deal of heat, and some of this transfers to the walls of the engine. Failure will occur if the body of the engine is allowed to reach too high a temperature; either the engine will physically fail, or any lubricants used will degrade to the point that they no longer protect the engine. The lubricants must be clean as dirty lubricants may lead to over formation of sludge in the engines. Cooling systems usually employ