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Compared with petrol, diesel is the lower quality product of petroleum family. Diesel particles are larger and heavier than petrol, thus more difficult to pulverize. Imperfect pulverization leads to more unburnt particles, hence more pollutant, lower fuel efficiency
and less power.
Common-rail technology is intended to improve the pulverization process. Conventional direct injection diesel engines must repeatedly generate fuel pressure for each injection. But in the CRDI engines the pressure is built up independently of the injection sequence and remains permanently available in the fuel line. CRDI system that uses an ion sensor to provide real-time combustion data for each cylinder. The common rail upstream of the cylinders acts as an accumulator, distributing the fuel to the injectors at a constant pressure of up to 1600 bar. Here high-speed solenoid valves, regulated by the electronic engine management, separately control the injection timing and the amount of fuel injected for each cylinder as a function of the cylinder's actual need.
CRDi stands for Common Rail Direct Injection meaning, direct injection of the fuel into the cylinders of a diesel engine via a single, common line, called the common rail which is connected to all the fuel injectors.
Whereas ordinary diesel direct fuel-injection systems have to build up pressure anew for each and every injection cycle, the new common rail (line) engines maintain constant pressure regardless of the injection sequence. This pressure then remains permanently available throughout the fuel line. The engine's electronic timing regulates injection pressure according to engine speed and load. The electronic control unit (ECU) modifies injection pressure precisely and as needed, based on data obtained from sensors on the cam and crankshafts. In other words, compression and injection occur independently of each other. This technique allows fuel to be injected as needed, saving fuel and lowering emissions.
PRINCIPLE OF CRDi IN GASOLINE ENGINES.
Gasoline or petrol engines were using carburetors for supply of air-fuel mixture before the introduction of MPFI system .but even now carburetors are in use for its simplicity and low cost. Now a days the new technology named Gasoline Direct Injection (GDI) is in use for petrol engines. The GDI is using the principle of CRDi system. Now let us examine the various factors that lead to introduction of GDI technology.
The fall of carburettor.
For most of the existence of the internal combustion engine, the carburetor has been the device that supplied fuel to the engine. On many other machines, such as lawnmowers and chainsaws, it still is. But as the automobile evolved, the carburetor got more and more complicated trying to handle all of the operating requirements. For instance, to handle some of these tasks, carburetors had five different circuits.
DIRECT INJECTION SYSTEMS.
Direct injection means injecting the fuel directly into the cylinder instead of premixing it with air in separate intake ports. That allows for controlling combustion and emissions more precisely, but demands advanced engine
A fuel injector is nothing but an electronically controlled valve. It is supplied with pressurized fuel by the fuel pump, and it is capable of opening and closing many times per second. When the injector is energized, an electromagnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle.
The nozzle is designed to atomize the fuel -- to make as fine a mist as possible so that it can burn easily. The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU. The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe called the fuel rail supplies pressurized fuel to all of the injectors. Each injector is complete and self-contained with nozzle, hydraulic intensifier, and electronic digital valve. At the end of each injector, a rapid-acting solenoid valve adjusts both the injection timing and the amount of fuel injected. A microcomputer controls each valve's opening and closing sequence.
Spiral-Shaped Intake Port For Optimum Swirl:
The aluminum cylinder head for the CRDI engines is a new development. Among its distinguishing features are two spiral-shaped intake ports. One serves as a swirl port while the other serves as a charge port. Both ports are paired with the symmetrical combustion chamber, rapidly swirling the intake air before it enters the cylinders. The result is an optimum mixture, especially under partial throttle. The newly-designed injector nozzles (injectors) located in the middle of the cylinders provide for even distribution of fuel inside the combustion chambers
Integrated Port For Exhaust Gas Recycling:
Another novelty is the integrated port for exhaust gas recycling (EGR) in the cylinder head. Whereas older diesel engines lead exhaust gases outside around the engine the new CRDi engines are incorporated with a cast port for the direct injection motor which conducts the gases within the cylinder head itself. The exhaust gases recirculate directly from the exhaust side to the intake side. There are three advantages to this system. For one, it eliminates external pipes which are subject to vibration. Then, integrating EGR into the cylinder head means that part of the exhaust heat is transferred to the coolant, resulting in quicker engine warm-up. Finally, this new technique allows cooler exhaust gases and that means better combustion.
Precise Timing Courtesy Air Flow Metering:
The hot-film mass air-flow meter is located in front of the turbocharger's compressor permitting an exact analysis of the air-mass that is being taken in. This mass will alter depending on temperature or atmospheric pressure. Due to this metering system, the microcomputer that controls engine timing receives precise data. It is thus able to regulate exhaust-gas recycling according to engine load and speed in the interest of lowering nitrous oxide and particle emissions.
The compressed air from the turbocharger then flows through the intercooler which cools it down to 70 degrees centigrade. Since cool air has less volume than warm air, more air is taken inside the combustion chamber, thus amplifying the effect of the turbocharger.