Working Principle Of Diesel Engine High-Pressure Common Rail Fuel Injection System

Working Principle of Diesel Engine High-Pressure Common Rail Fuel Injection System

Earlier we talked about the working principle of the diesel engine mechanical fuel pump and the working principle of the electronic control fuel pump.

https://www.zwcummins.com/news/working-principle-of-mechanical-fuel-injection-85619231.html
 

Working principle of diesel engine mechanical injection pump (plunger injection pump)

https://www.zwcummins.com/news/working-principle-of-diesel-engine-electronic-85619240.html

Working principle of diesel engine electronically controlled pump (electronic control injection pump/electronic governor pump)
 

Now, let's talk about the working principle of the diesel engine high-pressure common rail fuel system (both layman and professional versions).

 

Ⅰ. First, understand the core essence:

High-pressure common rail: diesel is continuously pressurized and stored in a common high-pressure rail (common rail pipe) for pressure stabilization and storage. All injectors share this high-pressure fuel source; the ECU directly controls the injector solenoid valves to independently control injection for each cylinder.

 

Three main modules: High-pressure fuel supply unit, Common rail pressure stabilization unit, Electronically controlled injector execution unit, ECU with vehicle-wide sensors.

 

Ⅱ. Complete fuel system process (step-by-step breakdown):

1. Low-pressure fuel supply stage (fuel tank → high-pressure pump inlet):

1. An electric fuel pump built into the fuel tank draws out the diesel;
2. Coarse filter → fine filter, filtering out impurities and water;
3. Diesel enters the low-pressure inlet chamber of the high-pressure fuel pump; excess diesel returns to the fuel tank via the return pipe, achieving preliminary pressure stabilization.

 

2. High-pressure fuel pump: continuously generates high pressure (core pressurizing component)

 

High-pressure pumps are mostly three-plunger radial pumps, driven by the engine crankshaft gears. They do not distinguish between cylinders and are only responsible for continuously pressurizing diesel:
 

1. Plunger downward stroke: The inlet valve opens, and low-pressure diesel is sucked into the plunger chamber;

2. Plunger upward stroke: The inlet valve closes, and the diesel is forcibly compressed, with pressure soaring to 1300~2200 bar (far exceeding mechanical pumps);

3. High-pressure diesel is one-way forced into the common rail accumulator pipe;

4. The pump has a built-in fuel pressure control valve (PCV valve): The ECU expels excess high-pressure oil in real time according to the rail pressure demand, dynamically maintaining the set pressure within the common rail.

 

Key point: The high-pressure pump only maintains rail pressure and does not control injection timing or injection volume, which is the essential difference from a plunger pump.

 

3. Common rail pipe: Public high-pressure accumulator "high-pressure oil reservoir" A thick, sealed high-pressure steel pipe stores high-pressure diesel, serving three purposes:

1. Pressure accumulation and stabilization: Buffers plunger pulsation supply from the high-pressure pump, keeping rail pressure stable throughout, without fluctuating with engine speed;

 

2. Uniform distribution of high-pressure oil: All cylinder injectors share the same pressurized oil source;

 

3. Equipped with a rail pressure sensor and pressure relief valve:

- Rail pressure sensor: Sends real-time actual rail pressure back to the ECU for closed-loop control;

- Pressure relief valve: Releases pressure protection when rail pressure exceeds the limit to prevent pipe rupture.

 

4. Electronically controlled injector: Independently controls injection (actuating terminal)

 

High-pressure oil from the common rail is delivered to each cylinder injector through high-pressure branch pipes. The injector has a built-in high-speed solenoid valve (piezoelectric/electromagnetic), directly controlled by ECU power to open and close. 

 

Complete Four-Step Fuel Injection
1. Standby state: The solenoid valve is powered off and closed. The high-pressure oil in the injector's control chamber keeps the needle valve tightly closed, sealing the nozzle and preventing fuel injection.

2. Injection start: The ECU powers the solenoid valve, opening the pressure relief hole, which quickly releases the high-pressure oil in the control chamber. The pressure above the needle valve drops, and high-pressure diesel from the common rail lifts the needle valve, atomizing fuel into the cylinder at ultra-high pressure.

3. Continuous injection: The solenoid valve stays powered, keeping the needle valve lifted and fuel injecting continuously. The longer it's powered, the more fuel is injected.

4. Stop injection: The solenoid valve is powered off, closing the relief hole. The control chamber rebuilds high pressure, pushing the needle valve down to seal the nozzle, stopping fuel injection immediately.

Advanced capabilities: Pre-injection, Main injection, Post-injection (something mechanical pumps simply can't do)

The ECU can drive the solenoid valve multiple times in one power stroke:

1. Pre-injection: Injects a small amount of diesel early into the cylinder, preheating the combustion chamber, reducing knocking noise, and preventing rough combustion.

2. Main injection: Injects a large amount of diesel to provide power.

3. Post-injection: Injects a small amount at the end of the power stroke to raise exhaust temperature, which helps regenerate the DPF particle trap and optimize emissions.

3. Electronic Closed-Loop Control System (ECU Brain)

1. Various sensors collect operating conditions:
- Throttle pedal: Driver load demand;
- Crankshaft speed/camshaft position sensors: Determine cylinder firing order and injection timing reference;
- Common rail pressure sensor: Provides real-time rail pressure feedback;
- Water temperature, intake pressure/temperature, boost pressure, exhaust sensors: Correct injection parameters.

2. ECU calculations and outputs:
The ECU has massive calibration MAPs, calculating in real-time based on RPM, load, water temperature, and altitude:

1. The target rail pressure needed, instructing the high-pressure pump regulator to adjust;
2. Injection advance timing per cylinder (when to start injection);
3. Injection duration (how much fuel to inject);
4. Timing to start pre-injection, main injection, and post-injection.
3. Closed-loop correction.

Rail pressure and engine speed are transmitted in real time, constantly fine-tuning parameters, dynamically and adaptively adjusting to all operating conditions throughout the process.

 

4. Extremely concise summary in one sentence:

 

The electric low-pressure pump supplies fuel → the high-pressure pump continuously increases pressure, storing high-pressure diesel in the common 'high-pressure fuel tank (common rail)' to stabilize pressure;

the ECU reads the vehicle's overall operating conditions, individually controls each cylinder injector solenoid valve, flexibly controlling injection timing, duration, and multiple injections;

rail pressure remains independently stable throughout, decoupled from engine speed, achieving high-precision combustion control.