How to carry out multi-stage intervention on turbocharger of diesel engine

Bottom line: Turbocharging usually only works when there is power demand. The word "intervention" of turbocharging can often be seen on the Internet. In fact, the word "intervention" is not accurate, because the turbine has always been driven by engine exhaust, and it has actually been working, but the effect and size vary with the turbine speed. There is a critical value according to speed. Turbocharger has two working states, one is "turbo positive pressure" and the other is "turbo negative pressure".

Basic principle and installation position of turbocharger;

Let's take a look at the installation position of the turbocharger. The following figure is a schematic diagram of a single turbocharger system. The turbocharger is usually installed behind the air filter and in front of the throttle.

In fact, the turbocharger can be regarded as adding a set of blowers to the ordinary naturally aspirated engine. The power source of the blower is engine exhaust gas. As long as the engine is running, the exhaust will drive the impeller of the turbine to rotate, and the impeller will drive the coaxial turbine. After the turbine rotates, it will continuously blow air into the intake manifold like a fan.

Many people have seen the following picture, which is also the source of many people being misled. The following figure shows the external characteristic curve of the engine. It should be noted that this external characteristic curve of the engine is measured when the throttle valve is fully open. In this state, the torque curve of the engine, that is, the green part below, can reach this state. In fact, if you drive very lightly in daily use, you can't reach this torque output at all.

Turbocharging usually has two working states:

Turbine negative pressure:

With the operation of the engine, the piston of the engine begins to reciprocate, constantly sucking air into the cylinder and discharging it through the exhaust pipe. At this time, the intake manifold generates negative pressure suction. Because the power of the turbine is driven by the exhaust gas of the engine, when the engine is running at a low speed, the exhaust gas is too small, the rotation speed of the turbine is relatively low, and the pressure generated by the turbine is far less than the air sucked by the intake manifold. At this time, although the turbine rotates, the speed is not enough, and the intake manifold is in a negative pressure state, which is only equivalent to a naturally aspirated engine, so the turbine does not work at all.

Turbine positive pressure:

When the engine speed increases, the engine exhaust pushes the turbine to rotate at high speed. Theoretically, when the turbine reaches a certain speed, a critical value is formed between the pressure generated by the turbine and the intake manifold pressure. Starting from this critical speed, the positive pressure generated by the turbine exceeds the pressure naturally inhaled by the piston, which is called "interference". Of course, the above is only a theoretical value. In fact, it is impossible to control the positive pressure of the turbine simply by relying on the engine speed, otherwise the power output of the engine will be difficult to control and the fuel consumption will surge.

Inlet pressure reducing valve, exhaust pressure reducing valve and intercooler

Air inlet and pressure relief valve:

In order to control the positive pressure speed of turbocharger, engineers designed the "inlet pressure reducing valve". The function of the intake pressure reducing valve is to discharge air from the intake manifold, thereby reducing the pressure of the intake manifold. That is to say, although the turbocharger is still rotating at high speed, the positive pressure generated by it will be discharged to the front of the turbocharger through the pipeline, that is, the internal pressure relief valve. Even if the turbine rotates at high speed, the pressure is positive. Of course, there are also some turbochargers with large displacement, which use pressure relief valves. Generally, when gas is discharged into the atmosphere, it will make a sniffing sound.

In addition, when decelerating and collecting oil, the intake and pressure relief valves of the engine should also be opened. This is because when decelerating, the throttle valve is closed, and the high-pressure air must be discharged in time, otherwise it will impact the turbine blades.

Exhaust pressure relief valve:

When the engine runs at high speed, the exhaust gas is sufficient, which will promote the turbine to rotate at high speed. If the exhaust pressure is not controlled, it will lead to excessive in-cylinder pressure and engine cylinder explosion. After the exhaust valve is opened, the engine exhaust gas will bypass me and pass directly. The power of the intermediate turbine. You may be interested in the pipe that Vin Diesel pulled out in Fast and Furious 8. In fact, he pulled the control valve of the exhaust pressure reducing valve. After the pipe was pulled out, it was equivalent to the complete closure of the exhaust and pressure relief valve, so his tattered old car accelerated like Viagra until it exploded. .

Intercooler:

Because the maximum speed of the turbine can reach 200,000 revolutions and the temperature can reach above 900℃, such a high temperature will lead to an increase in the intake temperature of the engine and a decrease in the air density, thus reducing the effective air volume and affecting the intake efficiency. So the intake air temperature must be cooled, that is, the intercooler is used to cool the air.

The turbocharger cannot always be in a positive pressure state.

Turbocharged engines are also EFI engines. Its basic combustion principle is to detect the oxygen content in the intake and exhaust gas through air flow sensor, air temperature sensor and oxygen sensor, so as to realize the closed-loop control of the engine. Therefore, in theory, as long as the engine intake increases, the fuel consumption will inevitably increase. Therefore, in order to reduce fuel consumption, the intake must be controlled, and the intake manifold should not be unnecessarily in a positive pressure state. That is to say, except when the engine is just started, it cannot be at positive pressure due to shortage. Even if the engine speed increases and the exhaust volume is sufficient, the intake volume should be controlled to reduce fuel consumption.

The control logic is also relatively simple. Generally speaking, air intake and pressure reducing valve are controlled by throttle opening and vehicle speed. When ECU judges that the current power demand does not need to be supercharged, the air inlet pressure reducing valve is normally opened, and the high-pressure gas at the rear end of the turbine will be discharged to the front end of the turbine. When ECU judges that pressurization is needed, the air inlet pressure reducing valve is closed. At this time, the pressure at the rear end of the turbine increases, and the air intake increases. That is to say, only when the ECU judges that there is power demand, the turbocharger is in the positive pressure state of the intake manifold.

For small displacement turbocharged engines, turbines with small inertia are often used, so that turbines are easier to be pushed. If it is opened gently every day, although the turbine speed is very high, the ECU detects that the power demand is not great, and the intake pressure reducing valve will be in a normally open state. When the accelerator accelerates, ECU judges that the acceleration demand is too large, and will control the pressure reducing valve to close, so that the intake pressure rises rapidly and a huge amount of air is ejected from the cylinder.

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