How is the injection timing of automobile engines achieved?

Engine ignition and other controls

Section 1 Engine ignition control system

1. Development of ignition control system

Ignition system is the most The basic principle is to control the primary current of the ignition coil and the power-off time through the power-off switch, thereby controlling the energy and time of ignition to ensure complete combustion of the mixture in the engine cylinder.

In traditional carburetor-type gasoline engines, the ignition control system has evolved from traditional (contact type) to non-contact type. During this process, the system's distributor still uses mechanical centrifugal and vacuum advance mechanisms to control the engine's ignition advance angle.

With the emergence and development of EFI systems, ignition control systems begin to use electronically controlled ignition devices (ESA). It can keep the engine in the optimal ignition advance state under any working conditions and realize three functions: power-on time control, ignition advance angle control and knock control.

2. Electronic ignition control system

Modern ignition control systems are all computer-controlled electronic control systems. It can be divided into two categories, one with a distributor and one without a distributor. But their main components and control principles are the same.

Composition:

(1) Ignition: including ignition control circuit, closing angle control circuit, igniter signal circuit, power transistor and its drive circuit, etc.

(2) Ignition coil and distributor The ignition coil adopts a high-energy ignition coil with a small primary coil resistance. In a system with a distributor, each cylinder uses an ignition coil; in a system without a distributor, the cylinders are grouped into groups and each group uses an ignition coil, or each cylinder uses an independent coil.

The composition of the electronic ignition control system is as shown in the figure

(1) ECU input signal

The ECU input signal, in addition to the throttle position sensor and input signal, In addition to signals from the throttle position sensor, air flow meter, water temperature sensor, etc., there are also the following signals from the crankshaft position sensor:

1) G signal

The so-called G signal, that is, the top dead center reference position signal. The crankshaft angle corresponding to its period is equal to the crankshaft angle corresponding to the working interval of each cylinder of the engine (180 degrees for four-cylinder engines and 120 degrees for six-cylinder engines). The crankshaft position corresponding to the phase of the G signal is equal to the upper stop of each group of pistons. The point position has a certain angle, usually 10 degrees before top dead center.

According to the G signal, the ECU can accurately calculate the time and engine speed required for each crankshaft rotation of 1 degree and one revolution. From the parameters input by the speed and other sensors, the ECU can look up the table to obtain the ignition advance angle and ignition coil energization time. According to the calculated time of the 1 degree signal, the power-on and power-off moments of the igniter after the G signal can be calculated, and finally the ignition control signal is output.

In the ignition control system without a distributor, some divide the top dead center position G signal into G1 and G2, and the two signals are 180 degrees apart (the crankshaft angle is 360 degrees). In the distributorless ignition control system of the Toyota Crown car, G1 is set near the top dead center of the sixth cylinder, and G2 is set near the top dead center of the first cylinder.

2) Ne signal.

The so-called Ne signal is the engine crankshaft speed signal.

Each pulse of the Ne signal indicates that the engine crankshaft has rotated through a fixed angle. In a general system, the Ne signal period is the time corresponding to the rotation of the rotating shaft by 30 degrees. In a more precise system, the Ne signal period is the time corresponding to the crankshaft rotation of 1 degree.

(2) ECU output signal

1) Ignition control signal IGt

IGt is actually the on-off control signal of the power transistor in the ignition. It is the ignition command signal output by the ECU to the ignition component, and is also the reference signal for the ignition component to calculate the closing angle.

After the IGt signal is output, when the piston position reaches the optimal ignition time memorized in the memory, the IGt signal disappears, that is, the ignition command is issued.

2) Identification of cylinder signals IGdA and IGdB

Each revolution of the crankshaft will generate multiple G signals, and the corresponding relationship between each G signal and the ignition cylinder should be constant. . In a system with a distributor, since the ignition cylinder is determined by the direction of the distributor head, there will be no problem. However, in a system without a distributor, the G signal alone cannot determine the specific ignition cylinder, so the cylinder identification signal IGd is added to the ECU output signal to determine the cylinder that needs ignition together with the G signal. In the simultaneous ignition mode without distributor, IGd is divided into IGdA and IGdB.

3. Distributorless ignition control system (DIL)

The distributorless ignition control system is a fully electronic ignition system.

Advantages: (1) Since there is no mechanical transmission, the energy loss and interference in the intermediate spark gap between the distributor head and the side electrode are reduced;

(2) Since there is no distributor , also makes the arrangement of various engine components easier and more reasonable.

Category: (1) Independent ignition method with one ignition coil per cylinder;

(2) Two piston position synchronized cylinders (the pistons of both cylinders reach the top dead center position at the same time , but one cylinder is the top dead center of the compression stroke, and the other cylinder is the top dead center of the exhaust stroke) *** Simultaneous ignition method using one ignition coil.

1) Simultaneous ignition mode without distributor

Cylinders 1 and 6, cylinders 2 and 5 and cylinders 3 and 4 are synchronized cylinders respectively. The two synchronized cylinders use one coil. , the method is that the spark plugs of the two synchronized cylinders are connected in series with the secondary coil of the special ignition coil. When the primary coil of the ignition coil is powered off, one cylinder is at the top dead center of the compression stroke, so it is effectively ignited; while the other cylinder is at the top dead center of the exhaust stroke, which is ineffective ignition. Since the pressure in the cylinder during the exhaust stroke is very low, and there are many conductive ions in the exhaust gas, the spark plug is easily broken down by high pressure, consumes very little energy, and will not affect the ignition of the cylinder during the compression stroke.

2) Distributor-less independent ignition mode control system

Since each cylinder has an independent ignition coil, even if the engine speed is as high as 9000r/min, the coil has a long energization time time (large closure angle), can provide sufficiently high ignition energy. Compared with the distributor system, at the same rotation speed and the same ignition energy, the current of the ignition coil per unit time is much smaller. Therefore, the coil is not suitable for heating and can be very compact. Generally, the ignition coil is press-fitted on the spark plug. This ignition control system is particularly suitable for multi-valve engines.

3. Optimal ignition advance angle and factors affecting the ignition advance angle

1. Optimum ignition advance angle

Definition: Can ensure the power of the engine The ignition advance angle that reaches the best value for economy, economy and emissions is called the optimal ignition advance angle.

Generally speaking, when the mixture is burned in the cylinder, its maximum combustion pressure (which can also be said to be the maximum output power of the engine) appears about 10 degrees after the top dead center of the crankshaft angle. As shown in Figure 3-4, curve A in the figure is the pressure waveform without combustion in the cylinder. It is a left-right symmetrical waveform centered on the top dead center (TDC). Curves B, C, and D respectively represent the combustion pressure waveforms when the ignition time is before 10 degrees of top dead center, around 10 degrees, and after 10 degrees. It can be seen from the figure that ignition at time II can achieve the best combustion pressure (the work is also the most, the amount of work can be seen in the shaded area) and no knocking occurs; while ignition at time I, although the combustion pressure is the highest, but Knocking occurs (sawtooth waveform on the upper part of crankshaft B). It can be seen that the optimal ignition advance angle is about 10 degrees before top dead center. But the optimal ignition advance angle is not static.

2. Factors affecting the ignition advance angle

1) The influence of engine speed on the ignition advance angle

As shown in Figure 3-5, the engine speed increases , the ignition advance angle should be increased.

In ordinary EFI systems, since a mechanical centrifugal regulator is used, the adjustment curve is quite different from the ideal ignition adjustment curve. When ESA is used, the actual ignition advance angle of the engine can be made close to the ideal ignition advance angle.

2) The influence of the absolute pressure of the intake manifold on the ignition advance angle

As shown in Figure 3-6, when the pipeline pressure is high (the vacuum is small and the load is large), it is required The ignition advance angle is small; conversely, when the pipeline pressure is low (high vacuum, small load), a large ignition advance angle is required. In ordinary EFI systems, due to the use of vacuum regulators, the adjustment curve is quite different from the ideal curve. When the ESA control system is used, the actual ignition advance angle of the engine can be made close to the ideal ignition advance angle.

3) The influence of octane number on ignition advance angle

Under certain conditions, engine knocking will occur. Knocking reduces engine power, increases fuel consumption, and overheats the engine, which is extremely harmful to the engine. The knocking of the engine is closely related to the quality of gasoline. The octane number is often used to express the anti-knock performance of gasoline. The higher the octane number of gasoline, the better the anti-knock performance, and the ignition advance angle can be increased; conversely, the lower the octane number of gasoline, the worse the anti-knock performance, and the ignition advance angle should be reduced. In ordinary ignition systems without electronic control, this is achieved by manually adjusting the initial position of the distributor. In EFI, in order to meet the needs of gasoline with different octane numbers, in actual application, different gasoline varieties can be selected. When leaving the factory, the switch is generally set to the unleaded gasoline position.

3. Control method of ignition advance angle

In the ESA control system, based on the signal sent by the relevant sensor, the ECU calculates the optimal ignition time and outputs the ignition timing signal ( IGt), controls igniter ignition. When the engine is started, the ignition time is directly controlled by the sensor signal to a fixed initial ignition advance angle without ECU calculation. When the engine speed exceeds a certain value, it is automatically converted to controlled by the ignition timing signal IGt of the ECU.

1. Initial ignition advance angle

In order to determine the ignition timing, the ECU determines the ignition time based on the top dead center position. In some engines, the ECU sets the first Ne signal zero-crossing point after the G1 or G2 signal as 10 degrees before the top dead center of the compression stroke. This point is used as a reference point when the ECU calculates the ignition timing. This angle is called the initial ignition advance angle, and its size varies with the engine.

2. Calculation of ignition advance angle

When the engine is working, the ECU finds the corresponding basic ignition advance angle from the data stored in the memory based on the intake manifold pressure (or intake air volume) and engine speed, and then based on the relevant The sensor signal value is corrected to obtain the actual ignition advance angle.

Actual ignition advance angle = initial ignition advance angle x basic ignition advance angle + corrected ignition advance angle (or retardation angle)

3. Control of ignition advance angle

The control of ignition advance angle includes two basic situations: ① Ignition time control during starting: when the engine starts, it ignites at a fixed crankshaft angle position, regardless of the engine's operating conditions . ② Ignition time control during normal engine operation after starting: The ignition time is determined by the basic ignition advance angle and correction amount determined by the intake manifold pressure signal (or intake air volume signal) and engine speed.

Correction items vary from engine to engine and are corrected according to the respective engine characteristic curves.

4. Knock control

Knocking is the most harmful fault phenomenon in the operation of gasoline engines. If the engine continues to produce knocking during operation, the spark plug electrode or piston may overheat, melt, etc., causing serious failure. Therefore, the occurrence of knocking must be prevented.

Knocking is closely related to the ignition time, and is also related to the octane number of gasoline.

In traditional ignition systems and ignition systems without knock control, in order to prevent the occurrence of knocking, the ignition time is often set far away from the knocking edge. This will inevitably reduce engine efficiency and increase fuel consumption.

The ignition system with knock control only leaves a small margin from the ignition moment to the edge of knock, or in other words, it works on the knock interface. In this way, the occurrence of knock is controlled and the occurrence of knock can be obtained more effectively. Engine power output.

1. Knock control system

Composed of: sensor and ECU.

From a hardware perspective, the knock control system is actually an ignition control system with a knock sensor added.

2. Knock control method

Working principle: The knock sensor is installed on the engine cylinder and uses the piezoelectric effect of the piezoelectric crystal to convert the vibration of the cylinder into The electrical signal is input to the ECU, and the ECU filters the signal output by the knock sensor, and simultaneously determines whether there is knock and the intensity of the knock, thereby delaying the ignition time. When the ECU has a knock signal input, the ignition control system adopts a closed-loop control method. If the knock is strong, the ignition delay angle is large; if the knock is weak, the ignition delay angle is small, and the ignition advance angle is delayed based on the original ignition advance angle until explosion. When the knock disappears, the current ignition time angle is maintained for a period of time. If no knocking occurs, the ignition advance angle is gradually increased until knocking occurs. When the engine knocks again, the ECU delays the ignition advance angle again, and the adjustment process is repeated.