Engine Diesel Common Rail Principle

A diesel engine operates without an ignition spark, therefore the fuel is combusted due to auto-ignition. Combustion takes place in a cylinder. The air drawn into the cylinder is compressed by a piston so that its temperature rises sharply. The combustion causes the piston to move downward so that the crankshaft is turned and the engine runs.

Type of Combustion

A wide variety of different engine types were produced in the course of diesel engine development. One decisive difference between them is due to the design of the combustion chamber. Generally speaking, in automotive applications, there is difference between indirect-injection (IDI) engine and direct-injection (DI) engines.

IDI engine

IDI engines have a divided combustion chamber. Fuel is injected into a prechamber in which combustion is initiated. Less noise is generated in the IDI engine because of the prechamber principle.

DI engine

In the DI engine, the fuel is injected directly into the cylinder’s combustion chamber. DI engines save fuel up to 20% compared with IDI engines.

A ~ B  :  Ignition lag                        

B ~ C  :  Premixed flame                       

C ~ D  :  Diffusion flame

D ~ E  :  Post flame 

A ↔ D : Injection              

B ↔ E : Combustion                                       

Ignition lag (A~B)

The start of injection and the start of ignition are separated by a certain period of time.

Premixed flame (B~C)

The fuel that is injected prior to the start of ignition and mixed with the air combusts.

Diffusion flame (C~D)

That portion of the combusted fuel which burns as a very rapid premixed flame is primarily responsible for the pressure rise, and thus is the primary cause of both combustion noise and NOx

Post flame (D~E)

The soot formed primarily during the diffusion flame is oxidized in the post flame phase.

Diesel Knock

Fuel which did not burn during Ignition lag period combusts rapidly. It causes pulsating metallic sound.

Diesel Knock control

Ø  The ignitability of the fuel

    - high-cetane number : decrease Ignition lag time

Ø   The compression pressure (compression ratio, boost pressure)

     - increase compression pressure and temperature

Ø   The engine temperature (intake air temperature, cylinder temperature,   Glow system)

Mechanical Injection pump

Conventional vs CRDI

Components

·         Fuel tank

·         Fuel filter

·        Priming Pump

·         Injection Pump

·         Nozzles and hold assembly

Components

·         Fuel tank

·         Fuel filter

·         Low-pressure Pump

·         High-pressure Pump, Common Rail, Injector

·         ECU, sensors

Conventional vs Common Rail Direct Injection Engine (CRDI)

CRDI COMMON RAIL DIRECT INJECTION

CONCLUSION 

Modern Diesel Common Rail Systems typically achieve extremely high pressures of around 2,800 BAR (40,000 psi). Unlike traditional diesel injection systems, which used mechanical governors and hydraulic pressures to control injection quantity and timing, common rail systems are precisely controlled electronically via an Electronic Control Module (ECM). The ECM receives input from a variety of sensors located throughout the engine and engine subsystems. Following receipt of these inputs, the ECM calculates the optimal fuel injection quantity and timing to optimise performance, fuel economy, emission, and noise levels.