Thermostat Heater Control

The primary objective for the thermostat heater control is for improvement in fuel economy and thermal efficiency. The system consists of a high temperature 208°F (98°C) instead of a 194°F (90°C) thermostat that has a resistive heater within a wax element. See Fig 1. The heater is controlled by PCM dependent on engine speed, throttle position, engine load, vehicle speed, air charge temperature, transmission oil temperature and engine coolant temperature. During low speed, low load and low air charge temperature conditions, the thermostat heater is OFF and engine is allowed to operate at an elevated coolant temperature. This should result in lower internal friction and higher thermal efficiency, both leading to improved fuel economy.

Fig 1: Identifying Thermostat Heater Control

Courtesy of FORD MOTOR CO.

During high speed, high load, high temperature conditions (air charge, transmission oil or engine coolant), PCM output is energized with a duty cycle to thermostat heater. This heats the wax and forces thermostat to rapidly open wider allowing extra coolant to flow from radiator. This will reduce coolant temperature and improve with performance demand. The heater is only capable of supplying a small amount of additional heat to wax element. It is not capable of opening thermostat alone. The thermostat is 100 percent duty cycle for short calibrated time and than duty cycle is reduced to a maximum of 70 percent ON and 30 percent OFF. Unheated, thermostat will begin to open at a coolant temperature of about 208°F (98°C), and will be fully open at 239°F (115°C). Energizing heater will reduce opening temperature to about 176°F (80°C), and the fully open temperature to 230°F (110°C).