Place the missing words to complete the statement about engine performance and combustion. Missing Words: air, combustion, energy, fuel, heat, spark How well an engine runs depends on the combustion process. To ensure [ Select... ] takes place efficiently, various engine performance systems are used. Today's systems are designed to achieve as close to complete combustion as possible. Basically, if all of the [ Select... ] that enters an engine's cylinder is burned, combustion is complete. The requirements for complete combustion are simple. However, achieving these is not. Complete combustion will occur when the correct amount of [ Select... ] is mixed, in a sealed container, with the correct amount of fuel. The mixture is compressed and shocked by the correct amount of [ Select... ] at the correct time. Air entering the cylinder via the intake valve is mixed with fuel. This mixture is compressed, which greatly increases the amount of [ Select... ] released during combustion. Once compressed, the mixture is ignited and burns. The spent exhaust gases then flow out from the cylinder through the exhaust valve. The amount of air and fuel must be precisely controlled, as should the [ Select... ] for ignition. Because the engine runs at different speeds, loads, and temperatures, these requirements are very difficult to meet