Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/660061
34 | April 2016 The Chassis opening temperature of the thermostat is maintained. These three modes are fxed to the actual design opening temperature of the thermo- stat. This fxed temperature is going to be a compromise that the engine designers/ engineers have to take into account when designing an engine as the performance, fuel economy and emissions output are very temperature dependent. For maximum performance the engine designer wants a lower opening temperature 185 degrees Fahrenheit (85 degrees Celsius) for maximum engine power. Cooler operat- ing temperatures reduce engine knock and allow optimized ignition timing, especially under full load, but if the temperature is maintained this low then emissions increase and fuel economy decreases. The fuel economy/emission engine designer wants a higher operating temperature of 230 degrees F (110 degrees C), as higher operating temperatures provide better fuel economy under part throttle, city driving, and partial load situ- ations and drastically reduce emissions and increase fuel economy. If these two characteristics could be combined you would have the best of both worlds, and that is exactly what a mapped thermostat achieves. The mapped thermo- stat still uses a wax pellet to open, and a spring to close, but it also incorporates a small heater inside the wax pellet that can be controlled by the PCM. Using a combina- tion of inputs such as engine load, engine rpm, vehicle speed, intake air and coolant temperatures, the PCM can now control the characteristics of the thermostat. If the opening temperature of the wax pellet is set at 230 degrees F (110 degrees C) without any electrical or PCM interven- tion the thermostat would open fully at 230 Art courtesy of Mahle Inc. Here is a temperature/engine load/vehicle speed graph that illustrates how the engine's cool- ant temperature is regulated due to operating conditions.