Common Rail Direct Injection

Diesel engine technology has advanced by seeming light-years over the last decade or so. Gone

are the days of sulfur laden black, sooty diesel smoke spewing out of the stacks of trucks and

buses thus clogging our airways after the oil shock of the late seventies and early eighties are

now just a memory. Though diesels have always been very fuel efficient, stringent emissions

laws and expectations of performance by the car buying public have forced developments that

have taken the lowly diesel from an embarrassment to be endured all the way to clean air and

economical powerhouse champions.

Old Mechanical Indirect-Injection: Diesels of yore relied on a simple and effective--yet not

altogether efficient and accurate method of distributing fuel to the engine's combustion

chambers. The fuel pump and injectors on early diesels were completely mechanical, and

though precision machined and ruggedly built, the working pressure of the fuel system was

not sufficiently high enough (injection pressure of 100-125 bar) to render a sustained and

well-defined spray pattern of fuel. And in these old mechanical indirect systems, the pump had

to do double duty--not only supplying fuel system pressure, but also acting as the timing and

delivery device (pump pressure forced the mechanical injectors to open). Additionally, these

elementary systems relied on simple mechanical inputs (there were no electronics yet) such as

fuel pump RPMs and throttle position to meter their fuel delivery.

Subsequently, they often delivered a shot of fuel with a poor and ill-defined spray pattern

that was either too rich (most often) or too lean, that resulted in either a rich belch of sooty black

smoke or insufficient power and a struggling vehicle. To make matters worse, the low pressure

fuel had to be injected into a pre-chamber to insure proper atomization of the charge before it

could mosey into the main combustion chamber to do its work. Hence the term indirect-

injection. And if the engine was cold and the outside air was cold, things really got lethargic.
Though the engines had glow plugs to help them start, it would take several minutes of running

time before they were sufficiently heat soaked to allow smooth running Why such a bulky, multi-

stage process? And why so much trouble with cold temperatures? The main reason is the nature

of the diesel process and the limitations of early diesel technology. Unlike gasoline engines,

diesels have no spark plugs to ignite their fuel mixture--they depend on heat generated by the

intense compression of air in the cylinders to ignite the fuel when it's sprayed into the

combustion chamber. And when cold, they need the assistance of glow plugs bolster the heating

process. And furthermore, since there is no spark to initiate combustion, the fuel must be

introduced to the heat as an extremely fine mist in order to properly ignite.

Another requirement for shifting over to electronic diesel injection was the nature of fuel

combustion in diesel engines. The actual combustion process is normally considered to take

place in the three distinct phases namely (1) Delay period, (2) Uncontrolled combustion and (3)

Controlled combustion. Delay period divided into two parts -the physical delay or the mixing

period and the chemical delay or the interaction period. During physical delay, the fuel particles

are atomized and vaporized. The fuel thoroughly mixes with air and is raised to self ignition

temperature. During the chemical delay, certain chemical reactions start between the fuel and air.
 The fuel which enters the cylinder during delay period gets properly atomized and

vaporized. The finally divided particles of fuel get enough time to have intimate contact with the

oxygen of air. Consequently when the actual combustion starts the fuel gets burnt in an

uncontrolled manner and the pressure rise in the cylinder takes place at an enormous rate.

Due to rapid increase in pressure, a sudden load is applied to the engine structure and this may

result in fatigue failure of the engine components. The engine has a rough running and

produces a violent noise called diesel knock. The knock results in a drop in the engine power

and efficiency. The growth of pressure rise dP/dα (kgf/cm2 per degree crank travel) is influenced

by the delay angle (angle turned during the delay period), the amount of fuel evaporated during

delay period, degree of turbulence, the penetration of fuel throughout the cylinder and its proper

mixing with air. If the delay period is long, there will be enough time for evaporation of fuel and

its mixing with air. So for smooth running of the engine, the delay period must be minimum

or the accumulation of fuel during this period should be minimized. The fuel is supplied

during delay in number of stages, avoiding its accumulation thus converting uncontrolled

combustion during second phase into controlled combustion similar to third phase, achieving

gradual pressure rise, noise free operation, better pick up/quick response and highest fuel

economy along with reducing exhaust emissions.

The New Way: Electronic Common Rail Direct Injection (CRD)

Modern diesels owe their resurgence in popularity to advances in fuel delivery and engine

management systems that allow the engines to return power, performance and emissions

equivalent to their gasoline counterparts, while simultaneously producing superior fuel economy.

It's the high pressure fuel rail and the computer controlled electronic injectors that make all

the difference. In the common rail system, the fuel pump charges the fuel rail at a pressure of
up to 25,000 psi--but unlike indirect injection pumps--it is not involved in fuel discharge.

Under the control of the onboard computer, this fuel quantity and pressure accumulates in the

rail independently of engine speed and load.

Each fuel injector is mounted directly above the piston within the cylinder head (there is

no pre-chamber) and is connected to the fuel rail by rigid steel lines that can withstand the high

pressure. This high pressure allows for a very fine injector orifice that completely atomizes

the fuel and precludes the need for a pre-chamber.

The actuation of the injectors comes via a stack of piezo electric crystal wafers that

move the jet needle in tiny increments allowing for the spray of fuel. Piezo crystals function

by expanding rapidly when an electric charge is applied to them. Like the fuel pump, the

injectors are also controlled by the engine computer and can be fired in rapid succession

several times during the injection cycle. With this precise control over injector firings, smaller,

staggered quantities of fuel delivery (5 or more) can be timed over the course of the power stroke

to promote complete and accurate combustion. In addition to timing control, the short duration,

high pressure injections allow a finer and more accurate spray pattern that also supports better

and more complete atomization and combustion.

Through these developments and improvements, the modern common rail direct injection

diesel engine is quieter, more fuel efficient, cleaner, and more powerful than the indirect

mechanical injection units they have replaced.

 Common Rail Direct injection
System (CRDI)
BOSH EDC16 DENSO EDC‐U2P High Pr. Pump

Advantages of common rail:

Fuel pressure available on demand.....

• Higher injection pressures and finer atomization of fuel.....

• Injection pressure created independent of engine speed.....

• Multiple injections per cylinder combustion are possible.

Benefits of common rail:

• Reduction of overall exhaust emissions.....

• Reduction of particulate emissions.....

• Reduction of noise emissions.....

• Improved fuel efficiency.....

• Higher performance.
Common Rail Diesel Fuel Systems

Examples of typical common rail system maximum fuel pressures:

• Bosch:

Generation 1: up to 1350 bar (19845 psi). Unijet

Generation 2: up to 1600 bar (23520 psi) EDC 16

Generation 3: up to 2000 bar + (29400 psi)

• Denso:

1st generation: up to 1450 bar (21315 psi) ECD-U2P

2nd generation: 1800 bar + (26460 psi) HP3/HP4

• Delphi

Multec: up to 2000 bar, Direct acting diesel common rail system: up to 2000 bar

Various systems differ in design, components layout and specific functions. However, all operate
in a similar way.

The fuel system can be divided into three basic circuits

High Pr. Delivery
Fuel rail pr. control valve
• Low pressure supply circuit
High Pr. Pump
Fuel metering • High pressure delivery circuit
Low Pr. Supply
control valve
Common Rail
• Fuel leak back and return
Fuel rail Pr. Sensor
Fuel Filter

Electric Fuel Pump

ECM Fuel Injectors

Fuel Leak back and return

Example: Bosch EDC16

High pressure fuel pump

The High pressure pump is the interface between the low pressure and the high pressure side of
the fuel system.

Bosch CP3 Bosch CP1 Denso HP4 Denso HP3

Basic function:

To ensure that enough fuel is delivered at sufficient pressure across the engine’s entire operating
range. This includes delivery of sufficient fuel for a rapid engine start and pressure increase in
the rail.

Pump has several pumping chambers

When solenoid de energized, valve is open

= LOW fuel volume intake to pump.

When solenoid energized, valve is closed

= HIGH fuel volume intake to pump.

 1. Pump piston
2. Pressure valve
3. to common rail
4. Suction valve
5. Return
6. Safety valve
7. Feed (from the
tank)
8. Gear pump
9. Fuel metering
control valve
10. Throttle bore
11. Control piston
12. Lubricating-oil
bore
13-- High Pressure
13 Hi h
Pump

1. Pump piston
2. Pressure valve
3. to common rail
4. Suction valve
5. Return
6. Safety valve
7. Feed (from the
tank)
8. Gear pump
9. Fuel metering
control valve
10. Throttle bore
11. Control piston
12. Lubricating-oil
13
bore
High Pressure Pump
13 Hi h
Advantages of fuel intake volume regulation:

• Only the required volume of fuel is supplied to the common rail from the high pressure
pump.....

• Reduced fuel flow around system results in lower fuel return flow temperature.....

• Reduced parasitic load on engine from high pressure pump contributes towards further
reductions in exhaust emissions.

High pressure regulator valve

High pressure fuel
delivery to common tail
• Fitted to back of HP pump, Controls high pressure
• fuel delivery to common rail.

• Excess fuel returns to tank.

High pressure
regulator valve
• Fuel cooler is required to cool the Fuel return
(to fuel tank)

• return fuel flow. Low pressure fuel inlet

(from fuel tank)

High pressure accumulator (Common Rail)

• Fuel is supplied to the common rail at high pressure

from the high pressure pump.
• The rail stores the fuel and distributes it to the
individual injectors.
• It also damps pressure vibrations caused by the
high pressure pump and injection processes.
• Typical volume of fuel held in common rail: 16 – 20cm³.
• Typical fuel rail pressure with engine idling and at running
temperature:
• approximately between 300 – 400 bar (4410 – 5880 psi)
• Typical maximum possible fuel rail pressure:
• approximately between 1600 – 2000 bar (23520 – 28400 psi)

Fuel rail pressure sensor

Monitors the fuel pressure in the common rail, Typically a piezo resistive type sensor.

Connected by three wires:

• 5 Volt supply from engine ECM.

• Sensor ground via engine ECM.

• Linear signal voltage output to ECM.

Signal utilization:

To enable the engine ECM to determine the fuel rail pressure.....

Used by the ECM as part of the calculation for the % duty cycle applied to the rail pressure
control solenoid and fuel metering solenoid.

Rail pressure limiter valve

• A mechanical pressure limiter valve is fitted to some systems.

• It is located at the end of the fuel rail. Fuel Return

• Its function is to relieve rail pressure if abnormally

high system pressure is generated.

• If excessive fuel pressure is generated, the valve opens Rail Pr. Limiter valve (Mechanical)

a fuel return port and excess fuel is returned back to the fuel tank

Fuel return to fuel tank

Valve opens at 230 MPa (2300 bar)

Valve closes at 50 MPa (500 bar)

Fuel rail pressure control valve solenoid

• A rail pressure control valve solenoid is fitted to Fuel return

the common rail on some systems.

• The valve controls fuel pressure by opening and closing Rail pressure control
valve (solenoid)

a return port in the rail.

• Excess fuel returns to the fuel tank via the fuel return.

• Receives battery voltage supply from engine ECM.

• Energized by engine ECM via a negatively triggered PWM.

• Operating frequency: approximately 1000Hz

• Used in conjunction with fuel metering solenoid, the rail pressure solenoid

• provides more accurate and faster control of pressure, particularly when

• reducing rail pressure during overrun.

• Fuel pressure is supplied to the injector

• needle seat area and also to a small chamber

• above the injector piston via a calibrated inlet port.

• When the solenoid is energized, the injector valve

Fuel spray
• opens. Into cylinder

• Fuel pressure is relieved above the injector piston and returns to the fuel tank via the injector

• leak back (return) ports. This creates a pressure difference above and below the injector piston.

• Fuel pressure below the injector needle lifts the needle thereby injecting the fuel into the
cylinder.

• Maximum stroke of solenoid valve: approximately 50 micrometers (0.05 mm).