Reconditioning Service for Motor Vehicle Engine Components




Common rail injection

Petrol Injection System Testing and Calibration

Fuel injection was in the beginning applied only to compression-ignition (aka Diesel) engines. In the 1970's, they were gradually adapted to gasoline (petrol) engines. In all modern petrol engines, carburettors have been supplanted by fuel injection systems. At the beginning, fuel injection units were purely mechanical assemblages but gradually they have evolved into electrically operated, electronically controlled devices. Irrespective of the principle of operation though, they need to be constructed of extremely high precision components that must be assembled with the greatest care and calibrated perfectly so that the system can perform optimally. The basic requirement that a petrol F.I. system should fulfill can be stated quite simply: it should deliver the right quantity of fuel in relation to the quantity of air aspirated by the engine to ensure complete combustion at all times. However, this does not imply that the fuel to air ratio should remain constant under all conditions. In practice, the quantity of mixture delivered to the engine varies greatly, and the fuel to air ratio can also vary somewhat, but within strict limits.

The air temperature, engine temperature, engine speed, engine load and throttle pedal position are some of the factors that determine the quantity of fuel that must be metered by the fuel injection system. The engine control unit constantly monitors the inputs from sensors that detect changes in these parameters and computes accurately the quantity of fuel that must be supplied at any given instant. As the demand keeps changing rapidly according to operating conditions, the computerized engine control unit has to work on the fly, exercising continuous control over the petrol injection system and the ignition timing too.

Fuel injection technology is now firmly entrenched in engine design, as this fuel metering system is very reliable and performs its role distinctly better than the traditional carburettor in every respect such as to render the latter obsolete. One reason for the efficiency of fuel injection is its ability to atomise the fuel so that it can mix perfectly with the air to provide a highly explosive mixture that burns completely. Other reasons are the very precise control it can exercise over the quantity of fuel injected and its timing. The electronic control circuitry is able to monitor and respond rapidly to changes in every relevant operating parameter with ease. The net result is that this technology is able to deliver more power, better driveability, greater fuel economy and improved emissions control.



The Evolution of Fuel Injection Technology

The earlier Diesel and gasoline injection systems used multi-lobe camshafts to operate separate plungers that supplied high pressure fuel to each injector at the precise moment it was needed. In contrast, most of the modern systems use a single high pressure electric pump to supply fuel to a common 'rail'. This is a long and narrow chamber containing fuel that is connected through metal tubing to all of the injectors. Thus, constant high pressure is maintained at the inlet of every injector.

The needle, or 'pintle' as it is called, blocks the opening of the nozzle. In the case of electromagnetic injectors, sometimes referred to as solenoid actuated injectors, the pintle is attached to an iron armature which is pulled up when a copper winding is energized by an electric current triggered by the control unit. As the pintle is drawn up it unblocks the openings in the nozzle, causing the pressurized fuel to be sprayed into the inlet manifold of petrol engines. There can be some variation in the positioning of the injector in the inlet manifold. Sometimes it is located just behind the intake valve, but more often it is placed higher up in the manifold. In the case of Diesel engines, the fuel is injected directly into the combustion chamber or sometimes into a 'pre-combustion chamber'. The fuel in a Diesel engine is injected from the nozzle with such force that it can easily penetrate the skin, and this could potentially cause fatal blood poisoning!

The latest innovation is the piezoelectric injector, which as its name implies, works on the piezoelectric principle. Many engines today come equipped with piezo injection units, but this technology is still evolving. Interestingly, certain types of inkjet printer were the first to exploit the piezoelectric crystal's curious property of changing dimensions when a voltage was applied across its faces. The crystal's rapid expansion was deployed to eject ink in controlled amounts onto paper.



The Need for Spot-on Calibration

Every type of injection unit contains mechanical components. Many of these components have to be machined to a degree of precision that surpasses the dimensional tolerance specifications drawn up for other mechanical parts of a motor vehicle. In fact, what is needed is nearly the type of precision involved in watchmaking.

The reason for this will become apparent if one considers the units of measurement applicable to the metering of fuel. The volume of fuel injected is measured in microliters and the duration of injection and intervals between injection measured in milliseconds. The pressure in the system is typically in the range from 20 to 150MPa.

It will thus be clear that unless perfectly adjusted, the fuel injection unit will not deliver the best performance and fuel economy of which it is capable. So it is important to check and service the fuel injection system of a vehicle at the recommended intervals. The filters should also be replaced at the same time. They perform an important role because even very fine dust particles can clog the injector nozzles.

We are fully equipped to test and calibrate both Diesel and petrol fuel injection pumps and injectors. We use the latest injector and nozzle testing equipment for this work. Special precautions have to be taken to service petrol and Diesel injection equipment. Different parts of the injector units from even the same engine cannot be interchanged as they are all selectively assembled at the factory. This is due to their dimensional tolerances being so fine. The same rule applies to injector units that have come in for service. Therefore, the parts from each injector unit are put into separate bins once they are disassembled.

The batches of parts are also cleaned separately in ultrasonic cleaning baths containing a special cleaning fluid. After careful re-assembly, the injector units are calibrated and tested by experienced technicians using highly specialised equipment. Once this operation has been performed satisfactorily the spray pattern and fineness of atomisation of the fuel can be checked visually as these can be clearly observed through the window of a special testing unit.

The following equipment are used to check the pumps and calibrate the injectors:

Hartridge Series 1150 machine for calibrating Diesel mechanical injector pumps of upto 12 cylinders. This unit has a 15 H.P. motor!

AG Precision (IMT) test bench for calibrating Diesel mechanical injector units upto 8 cylinders.

Hartridge AVM2-PC test bench for calibrating common rail pumps and injectors. This is an advanced machine, fully computerised and able to deal with upto 8 injectors simultaneously.

Hartridge IFT-70 common rail injector testing unit.

We also have equipment for servicing rotary fuel pumps like CAV and Delphi. All calibrations are done in accordance with the manufacturers' specifications.

Testing and calibration of petrol injectors is also undertaken. Many of the machines and test benches are kept in an air conditioned clean room to ensure the highest standards of precision, consistency and reliability.