Design of Engine Electronic Injection System Based on MC68HC9S12 Single Chip Microcomputer

Electronically controlled injection technology is an important technical measure for energy conservation and environmental protection of automobile engines. At present, the automobile industry has generally realized electronic injection, and the motorcycle industry has also produced a number of electronic injection vehicles. Research on electronic injection of gasoline engines for motorcycles has been carried out at home and abroad. The "XDZ50DQT electronic injection motorcycle project" of Xindazhou Honda Motorcycle Co., Ltd. was identified as a national technical innovation project by relevant parties in 2000. In 2003, Chunlan Group improved the original electronic injection system and successfully applied it to the 125 scooter produced in Europe. The ignition angle is intelligently controlled by the CDI contactless electronic ignition device, so that the engine can reach the best state under any working conditions. Tianjin Internal Combustion Engine Research Institute has conducted a lot of basic theoretical research and has achieved many research results. Tianjin Motorcycle Technology Center has successfully developed the FAI fuel electronic injection system suitable for four-stroke intake and two-stroke cylinder direct injection. In 2002, Yan Fuwu and others from Wuhan University of Technology developed the LH150 motorcycle electronic injection system and successfully put it into production.

This paper studies the electronic fuel injection technology for single-cylinder motorcycle engines and designs an engine electronic fuel injection control system to achieve precise control of the air-fuel ratio and improve combustion efficiency.

2 System Design

The structure of the electronic fuel injection control system is shown in Figure 1. This system is designed based on a 125cc single-cylinder four-stroke gasoline engine. The entire system includes sensors, ECU (Electronic Control Unit), injectors, and fuel injection pumps. Among them, the sensors include crankshaft position sensor, camshaft position sensor, air valve position sensor, engine temperature sensor, and air temperature sensor. The ECU includes Freescale's MC68HC9S12XS128, as well as signal conditioning circuits, fuel injection drive circuits, fuel pump drive circuits, and ignition drive circuits.

2.1 Sensor selection and installation

In order to obtain the position signals of the crankshaft and camshaft, the crankshaft position sensor and the camshaft position sensor use Hall switch sensors. A small magnet is fixed at the specific position of the starting clutch and the small chain plate respectively. The position of the crankshaft and camshaft is determined by the square wave signal generated when the magnet passes through the sensor. Since the sensor is small and difficult to install, a sensor circuit board is designed to make the sensor signal stable and not affected by adverse environments such as oil, dust, etc. The installation of the sensor on the engine is shown in Figure 2.

The throttle position sensor is a linear potentiometer, as shown in Figure 3. Its resistance is proportional to the throttle opening. The throttle opening is obtained by powering the potentiometer and using A/D acquisition.

The engine temperature sensor and air temperature sensor use thermistor sensors. Their resistance value changes linearly with the temperature, thus measuring the engine cylinder temperature and air temperature.

2.2 Selection of actuator

The actuator includes a high-pressure package for ignition, a fuel injection pump (injection pump) that provides oil pressure to the fuel system, and a fuel injector (injector) for injecting fuel.

The high-voltage package, also known as the ignition coil, consists of a primary coil, a secondary coil and an iron core. When in use, the primary coil is charged first, and a voltage of 200 to 300 V is self-induced in the primary coil; then, the mutual induction with the secondary coil generates a high voltage of 18 to 20 kV, and the voltage generated depends on the turns ratio of the two coils; finally, the high voltage is transmitted to the spark plug for ignition.

The fuel injection pump outputs an oil pressure of 300 kPa, with constant pressure output. It uses a pulse signal to drive the plunger to move and compress the fuel to obtain pressure. The injector has its own high-pressure fuel inlet nozzle, and the injection amount is accurate. The flow rate and injection pulse width are shown in Figure 4, and the atomization effect is good. The driving voltages corresponding to the three lines are 14.2 V, 13.2 V, and 12.2 V from top to bottom.

3 Control System Hardware Design

The Freescale MC68HC9S12XS128 microcontroller is used as the control chip, the IGBT v2040s chip is used to control the ignition, the Power MOSFET IRF3205 is used to control the fuel injection pump and the injector, and the switch function is realized by controlling the gate voltage to perform low-end control on the actuator. The actuator control circuit is shown in Figure 5. MC74HC125AD is a phase-matching device.

Since the electrical environment of the whole vehicle is relatively harsh, the anti-interference performance of the hardware circuit is very important. First of all, the PCB circuit board is designed and drawn through Protel DXP software, which not only reduces the volume of the circuit board, but also enhances the anti-interference ability. Secondly, the input signal must be processed by the corresponding signal conditioning circuit before entering the single-chip microcomputer, and the interference of the processed signal is greatly reduced. For the signals of the crankshaft and camshaft, the threshold comparison circuit is designed using the threshold comparator LM339, as shown in Figure 6. This circuit not only converts the original excitation signal of the engine into a square wave signal, but also can process the sensor signal after modification. In addition, the single-chip microcomputer has an internal A/D module, and a low-pass filter circuit needs to be used for filtering the analog quantity. M74HC04M1R is an inverter. 4 Control system software design

The control system program was written and debugged on Codewarrior IDE. The control program was designed based on the working conditions of the energy-saving car. The engine of the energy-saving car has three main processes during the race: starting, idling and accelerating. The control program flow is shown in Figure 7.

Determination of ignition timing: Calculate the speed through two crankshaft signals, and then find the set ignition advance angle, and then ignite at the corresponding time.

Determination of injection timing: The compression stroke is determined based on the camshaft signal, and the injection is synchronized with the crankshaft signal of the compression stroke.

Determination of fuel injection amount: first set the basic fuel injection amount, and then correct the fuel injection amount according to the throttle opening, cylinder temperature, and air temperature.

Calculation method of injection duration:

TI=TP·FC+FV

Wherein, TI is the duration of gasoline injection (ms); TP is the basic injection time (ms); FC is the correction coefficient of the basic injection time; TV is the injector invalid injection time (ms).

Among them, FC is calculated by the following formula:

FC=g(FAT，FTP，FCT)

Where, FAT is the air temperature correction coefficient; FCT is the cylinder temperature correction coefficient; FTP is the throttle opening correction coefficient.

5 Actual test results

The system first completed the verification test on the engine, and then used an energy-saving car to do a road test. The results show that the system can work stably and has obvious fuel-saving effects. Further comparative tests were conducted using carburetors, commercially available electronic fuel injection systems, and self-developed electronic fuel injection systems. The test site was a plastic track in the playground, and the test personnel were weighted to 50 kg. The test method was to drive 5 laps on the track, ignite and accelerate 10 times, with each lap time within 57 seconds, and finally record the fuel consumption. At least 3 data were used as the basis for analysis, and overtime results were invalidated. Try to choose clear and windless (or light wind) weather, and do not make any changes to the machinery on that day. The measured test data is shown in Figure 8.

Conclusion

This paper completes the selection of sensors and actuators, as well as the software and hardware design of the engine electronic fuel injection control system, and carries out installation and road tests. The designed control system works stably, and the test data show that the fuel saving effect is obvious, which lays a foundation for subsequent in-depth research.