Common rail: Components, working principle and functions

May 30, 2023

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Kunle Shonaike

Bosch launched the first common rail system in 1997. The system is named after the shared high-pressure reservoir (common rail) that supplies all the cylinders with fuel. With conventional diesel injection systems, the fuel pressure has to be generated individually for each injection. With the common rail system, however, pressure generation and injection are separate, meaning that the fuel is constantly available at the required pressure for injection.

Common rail systems have a modular design. Each system consists of a high-pressure pump, injectors, a rail, and an electronic control unit.

A common rail is one of the most important components in a diesel and gasoline direct injection system. The main difference between a direct and a standard injection is the delivery of fuel and the way how this one mixes with incoming air. In the direct injection system, the fuel is directly injected into the combustion chamber, skipping the waiting period in the air intake manifold. Controlled by the electronic unit, the fuel is squirted directly where the combustion chamber is hottest, which makes it burn more evenly and thoroughly.

The main advantages of the common rail direct fuel injection can be summarised in reduction of exhaust and noise emissions, better fuel efficiency and improved overall engine performance. The system consists of a high-pressure pump, injectors, a rail, and an electronic control unit.

The common rail is a long metal cylinder. It receives the fuel from the pump and distributes it to the injectors under extremely high pressure. The increase in the fuel pressure is a result of the latest engines’ design. Both diesel and gasoline engines tend to become smaller and lighter for better fuel efficiency and improved performance which increases the fuel pressure and sets entirely new standards for the manufacturing of a high-quality common rail.

First, the geometrical accuracy of the component is of critical importance. Precise design contributes to better common rail performance. Even a minimal size or shape fluctuation could lead to failures. Defining the right parameters in the design phase is essential, but what really matters is strictly following them during the manufacturing process.

The material selection is also a point that shouldn’t be underestimated. Good mechanical properties ensure strength and prevent corrosion. The used materials are usually steel and stainless steel. The common rail for diesel engine is made of steel, while the common rail for gasoline engine is made of stainless steel, because the fuel is too corrosive and stainless steel possesses better resistance to corrosion than steel.

Common rail direct injection

Most modern engine’s fuel systems use an advanced technology known as CRDi or common rail direct injection. Both petrol and diesel engines use a common ‘fuel-rail’ which supplies the fuel to injectors. However, in diesel engines, manufacturers refer to this technology as CRDi whereas petrol engines term it as gasoline direct injection or fuel stratified injection. Both these technologies have a similarity in design since they consist of “fuel-rail” which supplies fuel to injectors. However, they considerably differ from each other on parameters such as pressure & type of fuel used.

In common rail direct injection, the combustion takes place directly into the main combustion chamber located in a cavity above the piston crown. Today, manufacturers use CRDi technology to overcome some of the deficiencies of conventional diesel engines which were sluggish, noisy and poor in performance when implemented, especially in passenger vehicles.

The CRDi technology works in tandem with the engine ECU which gets inputs from various sensors. It then calculates the precise quantity of fuel and timing of injection. The fuel system features components which are more intelligent in nature and controls them electrically / electronically. Additionally, the conventional injectors are replaced with more advanced, electrically operated, solenoid injectors. They are opened by an ECU signal, depending upon the variables such as engine speed, load, engine temperature etc.

A common rail system uses a ‘common-for-all-cylinders’ fuel-rail or in simple words a ‘fuel distribution pipe.’ It maintains optimum residual fuel pressure and also acts as a shared fuel reservoir for all the injectors. In CRDi system, the fuel-rail constantly stores and supplies the fuel to the solenoid valve injectors at the required pressure. This is quite opposite to the fuel injection pump supplying diesel thru’ independent fuel lines to injectors in case of earlier generation (DI) design.

Mode of operation

With conventional diesel injection systems, the fuel pressure has to be generated individually for each injection. With the common rail system, however, pressure generation and injection are separate, meaning that the fuel is constantly available at the required pressure for injection. Pressure generation takes place in the high-pressure pump.

The pump compresses the fuel and feeds it via a high-pressure pipe to the inlet of the rail, which acts as a shared high-pressure reservoir for all injectors – hence the name ‘common rail.’

From there, the fuel is distributed to the individual injectors, which inject it into the cylinder’s combustion chamber.

High-pressure pumps

The high-pressure pump compresses the fuel and supplies it in the required quantity. It constantly feeds fuel to the high-pressure reservoir (rail), thereby maintaining the system pressure. The required pressure is available even at low engine speeds, as pressure generation is not linked to the engine speed. Most common rail systems are equipped with radial piston pumps. Compact cars also use systems with individual pumps which operate at a low system pressure.

Injectors

The injector in a common rail system consists of the nozzle, an actuator for Piezo injectors or a solenoid valve for solenoid valve injectors, as well as hydraulic and electrical connections for actuation of the nozzle needle.

It is installed in each engine cylinder and connected to the rail via a short high-pressure pipe. The injector is controlled by the electronic diesel control. This ensures that the nozzle needle is opened or closed by the actuator, be it solenoid valve or Piezo. Injectors with Piezo actuators are somewhat narrower and operate at a particularly low noise level. Both variants demonstrate similarly short switching times and enable pre-injection, main injection and secondary injection to ensure clean and efficient fuel combustion at every operating point.

Components of CRDi

Some types of fuel tanks also have a fuel sedimentor at the bottom of the filter to separate water content from the fuel.

Functions of the system

The diesel fuel injection system has four main functions:

Feeding fuel

Pump elements such as the cylinder and plunger are built into the injection pump body. The fuel is compressed to high pressure when the cam lifts the plunger, and is then sent to the injector.

Adjusting fuel quantity

In diesel engines the intake of air is almost constant, irrespective of the rotating speed and load. If the injection quantity is changed with the engine speed and the injection timing is constant, the output and fuel consumption change. Since the engine output is almost proportional to the injection quantity, this is adjusted by the accelerator pedal.

Adjusting injection timing

Ignition delay is the period of time between the point when the fuel is injected, ignited and combusted and when maximum combustion pressure is reached. As this period of time is almost constant, irrespective of engine speed, a timer is used to adjust and change injection timing – enabling optimum combustion to be achieved.

Atomising fuel

When fuel is pressurised by the injection pump and then atomised from the injection nozzle, it mixes thoroughly with air, thus improving ignition. The result is complete combustion.

Working principle of CRDi

A high-pressure pump supplies pressurised fuel. The pump compresses the fuel at the pressures of about 1,000 bar or about 15,000 psi. It, then, supplies the pressurised fuel via a high-pressure pipe to the inlet of the fuel-rail. From there, the fuel-rail distributes the fuel to individual injectors which then inject it into the combustion chamber.

Most modern CRDi engines use the unit-injector system with turbocharger which increases power output and meets stringent emission norms. Additionally, it improves engine power, throttle response, fuel efficiency and controls emissions. Barring some design changes, the basic principle and working of the CRDi technology remains primarily the same across the board. However, its performance depends mainly on the combustion chamber design, fuel pressures and the type of injectors used.

Advantages and disadvantages

Advantages

(1) Lower emissions: One of the reasons that common rail diesel engines were invented by vehicle manufacturers was because the government created stricter regulations on carbon emissions. Remember when big diesel trucks used to release a lot of black smoke into the air? You hardly see that anymore because the common rail diesel engine is designed to reduce those emissions. This is better for the environment and is one step closer to fighting global warming.

(2) More power: Studies have shown that vehicles with a common rail diesel engine will produce 25 per cent more power than a traditional diesel engine. This means the overall performance of the diesel engine will be improved.

(3) Less noise: Direct fuel injection systems were known for being noisy while driving. The common rail will reduce a lot of the noise that you might remember hearing. This makes it a more pleasant driving experience for you and those around you on the road.

(4) Fewer vibrations: There used to be a lot of vibrations felt in traditional direct fuel diesel engines. Now those vibrations have been reduced with the common rail direct injection system.

(5) Better mileage: Since the common rail diesel engine provides more power, that means you will get better mileage on your fuel. As a result, your fuel economy will be better as well. This means less money spent on fuel when you’re on the road.

Disadvantages

(1) Expensive vehicle: Vehicles with a common rail diesel engine are going to be more expensive than those with the traditional diesel engine. If you’re working for a company that supplies you with the vehicle, then it is no sweat. But if it is a personal vehicle, then you may not want to spend the extra money.

(2) Expensive parts: Since the common rail vehicles are more expensive, you can expect that replacement parts are going to be expensive as well.

(3) More maintenance: Common rail diesel engines will need more maintenance than a traditional diesel engine. Even if you do the maintenance yourself, it still takes more time, effort, and possibly expense.

Culled from the Internet

The oil specifically mentioned for servicing my Passat car is castrol oil. But the oil is scarce and if seen, quite expensive. Can I use any other type of oil? Thank you sir. Anonymous

I believe it’s just an agreement for marketing purposes. If you know the exact specification, you can buy any other brand that has the specifications.

I want to appreciate your selfless service in educating all of us. I bought a secondhand Toyota 2004 Hummer bus. I knew the engine was a suspect, but never knew it t would knock so early. The only option given by the mechanics is to buy a new engine costing N1.5m. Is that the only way out? Abrah

Sometime it is the only alternative you have. But depending on the damage to the old engine, you might still be able to salvage the damage. But only your mechanic can determine the damage.

Generic codes

P0697: Sensor reference voltage ‘C’ circuit open

Meaning

The control module has internal 5-volt reference buses called 5-volt reference. Each reference bus provided 5-volt reference circuit for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to the reference bus. The control module monitors the voltage on the 5-volt reference buses.

Possible causes

Meaning

The control module has internal 5-volt reference buses called 5-volt reference. Each reference bus provided 5-volt reference circuit for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to the reference bus. The control module monitors the voltage on the 5-volt reference buses.

Possible causes

P0699: Sensor reference voltage ‘C’ circuit high

Meaning

The control module has internal 5-volt reference buses called 5-volt reference. Each reference bus provided 5-volt reference circuit for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to the reference bus. The control module monitors the voltage on the 5-volt reference buses.

Possible causes

P0700: Transmission control system malfunction

Meaning

The transmission control module monitors the malfunction of sensors and actuators relating to transmission control. When the TCM detects a malfunction with the control, a signal is send to the engine control module to trigger the engine light or service engine soon light. The ECM stores the P0700 code and it means the TCM detected a malfunction with the transmission controls.

Tech notes

Since the P0700 is just and informative code, check the TCM for more codes to resolve the problem.

Possible symptoms

Possible causes

P0701: Transmission control system range/performance

Meaning

The transmission control module has detected other transaxle diagnostic trouble codes settings this DTC enables limp mode.

Possible causes

P0702: Transmission control module

Meaning

The code is triggered by the engine control module when there is a code stored in the transmission control module

Possible causes

P0703: Brake switch performance

Meaning

Engine control module detected accelerations and decelerations with no brake switch change

Tech notes

Check if the stop light works with the brake pedal. If the brake lights are inoperative, replace or adjust the brake switch.

Possible symptoms

Possible causes

P0704: Clutch switch input circuit malfunction

Meaning

When the clutch pedal is depressed, the voltage signal from the clutch switch to the Engine control module is low. If the ECM does not see this change from high to low when the vehicle is over 0 mph, the ECM sets code P0704.

When is the code detected?

ECM did not detect any movement in the clutch pedal position switch

Tech notes

Check clutch switch adjustment, the switch should open and close when the clutch pedal is depressed. If the switch is properly adjusted, replace the clutch switch to resolve the problem.

Possible causes

P0705: Transmission range sensor circuit malfunction

Meaning

The park/neutral position switch includes a transmission range switch. The transmission range switch detects the selector lever position when the shift lever is in the N or P shift position and sends a signal to the transmission control module.

When is the code detected?

The transmission range switch detects the selector lever position and sends a signal to the TCM.

Possible causes

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Contact: [email protected]

Kindly share this story:

All rights reserved. This material, and other digital content on this website, may not be reproduced, published, broadcast, rewritten or redistributed in whole or in part without prior express written permission from PUNCH.

Contact: [email protected]

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