11. COMMON RAIL DIRECT INJECTION (CRDI) DIESEL ENGINE
To meet emissions requirements, and to obtain further improvement in fuel economy in vehicles, the automobile research centers are booming towards a newer engine technology with the use of electronics. This paper deals with a solution for achieving the euro norms by use of “Electronic controlled Common Rail Direct Injection of diesel” in the CI engines. The CRDI system, which is controlled by Electronic Control Unit (ECU), is used to reduce pollution and to improve fuel economy. CRDI uses the Fuel Injection Equipment with the capability of operating at high pressures will be required to achieve the ultra low emissions challenges and low noise demands. This paper clearly deals with the importance of various sensors like Acceleration pedal sensor, crankshaft speed sensor, rail pressure sensor), electronic injector, high-pressure pump and accumulator, etc. The CRDI system enables the diesel vehicles to meet the stringent emission levels required by EURO-3 and beyond.
Fast increasing population of automotive vehicles has given rise to a serious problem of environmental pollution; many manufacturers’ tries to overcome this problem. One of the important techniques is the fuel injection technique in diesel engines. It is not easier one to achieve electronic controlled diesel injection due to need of higher injection pressure unlike gasoline port injection.
Conventional Diesel system:
In the conventional diesel using the inline jerk type pump pressurizes system the fuel. The maximum pressure achieved by this pump is about 210 bar. The mechanical injector used for the injection of the fuel is operating in a range of pressure i.e. NOP, NCP. (Nozzle Opening Pressure, Nozzle Closing Pressure). Mechanical metering of fuel quantity to be injected in the cylinder. Fuel injection timing mechanically controlled by fuel injection pump and variable with engine speed.
Drawbacks of Conventional Diesel system:
1. Noisy engine operation.
2. Fuel injection pressure varies during the injection process.
3. High fuel consumption at low engine speeds(due to imperfect A/F ratio).
4. High emissions at low engine speeds(due to imperfect A/F ratio)
5. Mechanical control of exhaust gas recirculation in to intake manifold.
6. Poor air fuel mixture causes particulate emissions.
Common Rail Direct Injection system:
A common rail engine is designed to supply constant fuel pressure to electronically controlled injectors through a shared fuel reservoir. This means that the fuel supply is not dependent on the engine speeds. A common rail system is built by :
1. A high pressure pump with pressure regulator and inlet metering valve.
2. A rail which contains a pressurized reserve of fuel.
3. Injectors which inject precise amounts of fuel into the cylinder as required.
4. A Diesel Control Unit – the ‘brain’ of the system, which precisely controls injector flow and timing as well as rail pressure while continuously monitoring the operating conditions of the engine.
Block Diagram of CRDI system:
A feed pump delivers the fuel through a filter unit to the high pressure pump. The high pressure pump delivers fuel to the high pressure accumulator (the Rail). The injectors, inject fuel into the combustion chamber when the solenoid valve is actuated. Because the injection pressure is independent of engine speed and load, the actual start of injection, the injection pressure, and the duration of injection can be freely chosen from a wide range.
Features of CRDI system:
• Injection pressure, independent of speed.
• High pressure injection.
• Reduced NOx emissions and engine noise.
• Modular system, easily adapted to different engine types.
• Compact design.
• Full electronic control and interface with other vehicle functions.
Electronic Control Unit (ECU):
The ECM uses digital electronic technology the input section provides an interface to the sensors generating sensors supply voltages and converting analogue sensors signals into digital signals. The 8-BIT MICROPROCESSORS then calculate output values as results of input variables together with programmed operational curves stored in memory, following a sequence of programmed instructions to obtain the required output values. Digital signal from Microprocessor is converted into analogue by D/A converter. Microprocessor output values converted into power signals by output stage and used to control the position of the actuators.
Block Diagram of ECU:
The Common Rail Direct Injection system has an Electronic Control Unit which gets the input from the following important sensors.
1. Acceleration pedal sensor
2. Crankshaft speed sensor
3. Rail pressure sensor
The Acceleration sensor sends the signal input to the ECU, according to acceleration pedal movement. The crank shaft sensor access the speed of the engine and it is supplied to the control unit. The Electronic control access the pressure in the rail or high pressure accumulator by the Rail pressure sensor.
Electronic Diesel Injector:
The advantage of the system derives from its injectors and advanced control of the injection process. The injectors have a balanced valve-independent of pressure allowing the use of an ultra–compact, fast–acting solenoid that enables precision in injection quantity and timing.
At the same time, a self – regulating accelerometer pilot control (APC) function accurately monitors each injector to ensure an optimized combustion throughout the engine life. The atomization by the fuel injectors is excellent, which promotes thorough combustion at all engine speeds.
High Pressure Accumulator (Rail):
The high pressure Accumulator (Common Rail) is used to store the fuel at high pressure. When the nozzle actuator opens it supplies the high pressure fuel to the injector. A pressure sensor is placed in the CR to monitor the pressure in the rail to the ECU.
1. Spherical type
2. Cylindrical type
The spherical shape of high pressure accumulator is well adopted to small and compact engines.
High Pressure Pump:
A common-rail injection system separates the two functions – generating pressure and injecting – by first storing the fuel under high pressure in a central container (rail), then delivering it to the individual injection valves (injectors) only on demand.
The pressure regulator monitors the high pressure in the rail by delivering the excess fuel to the tank after the signal received from the Electronic Control unit.
Diesel feed pump & Filters:
The Diesel Feed Pump is used to pump the fuel from the tank to the High Pressure Supply Pump through the fuel filters which separate the dirt particles from the fuel.
NOx & PM: From the Automotive research centers it is claimed that a reduction of 25% in NOx and 65% in particulates from present acceptable levels. It achieves EURO 3 and US 98 legislation norms and beyond.
Advantages of CRDI system:
• Compact Design:
The compact design of the injector outline enables the DCR system to be used on 2 or 4 valve per cylinder engines.
• Modular System:
With one electronically driven injector per engine cylinder, the system is modular and can be used on 3, 4, 5, and 6 cylinder engines.
• Low Drive Torque:
As the pumping of the pressure rail is not phased with the injection, the Common Rail system requires a low drive torque from the engine.
• Independent Injection Pressure:
The injection pressure is independent of the engine speed and load, so enabling high injection pressures at low speed if required.
• Lower NOx & CO2Emissions:
Injection sequences which include periods both pre and post the main injection can be utilized to reduce emissions particularly NOx, enabling the system to meet the stringent emissions levels required by EURO-3 and beyond.
• Noise Reduction & NOx Control:
The inclusion of pilot injection results in a significant reduction in noise.
• Full Electronic Control:
Common Rail offers all the benefits of full electronic-control fuel metering and timing, as well as the option to interface with other vehicle functions.
Reliability of the electronic components is determined only after implementation of the system in cars.
Common Rail marks a fundamental change in diesel technology and is happening now. More and more vehicles are being introduced with Common Rail systems. It is calculated that by 2004, 70% of new diesel vehicles will incorporate Common Rail injection systems and 5 million such systems will already be on the road.
1. SAE Automotive Engg. International November 2001, April 2001, Sept’ 2000.