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Understanding Automotive Electronics 5 Part 9

Tham khảo tài liệu 'understanding automotive electronics 5 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | DIGITAL ENGINE CONTROL SYSTEM 7 The control system selects an operating mode based on the instantaneous operating condition as determined from the sensor measurements. Within any given operating mode the desired air fuel ratio A F d is selected. The controller then determines the quantity of fuel to be injected into each cylinder during each engine cycle. This quantity of fuel depends on the particular engine operating condition as well as the controller mode of operation as will presently be explained. Engine Crank During engine crank the controller compares the value from the coolant temperature sensor with values stored in a lookup table to determine the correct air fuel ratio at that temperature. While the engine is being cranked the fuel control system must provide an intake air fuel ratio of anywhere from 2 1 to 12 1 depending on engine temperature. The correct air fuel ratio i.e. A F d is selected from a ROM lookup table as a function of coolant temperature. Low temperatures affect the ability of the fuel metering system to atomize or mix the incoming air and fuel. At low temperatures the fuel tends to form into large droplets in the air which do not burn as efficiently as tiny droplets. The larger fuel droplets tend to increase the apparent air fuel ratio because the amount of usable fuel on the surface of the droplets in the air is reduced therefore the fuel metering system must provide a decreased air fuel ratio to provide the engine with a more combustible air fuel mixture. During engine crank the primary issue is to achieve engine start as rapidly as possible. Once the engine is started the controller switches to an engine warm-up mode. Engine Warm-Up The controller selects a warm-up time from a lookup table based on the temperature of the coolant. During engine warm-up the air fuel ratio is still rich but it is changed by the controller as the coolant temperature increases. While the engine is warming up an enriched air fuel ratio is still needed to .