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178 Vasile Gabriel Nenerica et al. i) To raise fuel pressure to a specified value and to spray into the combustion chamber so that air and fuel mixture is as good as possible and combustion is more complete. ii) Fuel injection-start at some point and finish within a well established time. iii) Fuel-injection to be made in accordance with the engine combustion process in terms of position and shape of the jet. iv) To inject an appropriate amount of fuel to the engine load at any time. Since fuel injection should be done with great precision, both in volume and time, and at a very high pressure, it is required that the clearances among moving parts that are pumping circuit be very small (1-3 microns). This leads to the need of a better execution of the pumping element , the relief valve and the spray especially with the highest precision encountered in engineering. As a result, size, shape and mutual position deviations of these parts work surfaces are prescribed and produced in very tight limits, and the same surface roughness is very small that the injection equipment is manufactured at high precision, which requires special attention from the operation and repair units. Fuel injection equipment is made up, in the order passed through by the fuel, of the following: fuel tank, fuel pump, low pressure pipes, filter battery (pre-cleaning filter and final filter), the injection pump with regulator, high pressure pipes, injection and spraying. The main functions of the high pressure system are provided by the high pressure injection pump. Thus, the pressure of injection, the dosage amount of fuel per cycle and cylinder, the injection advance, as well as the optimum injection characteristics during injection are achieved by the injection pump. Injection pumps are classified according to several criteria: І. Depending on the service cylinder engine there can be distinguished: a) individual pumps; b) injector pump; c) rotary distributor pumps; d) pump line, characteristic of this class is that each cylinder of the engine is served by one outlet item. II. After dose adjustment method of diesel: a) invariable aspiration and partial discharge (eg, piston pumps, gate); b) variable intake and total discharge (rotary distributor pumps); III. By mode of operation of the pumping element: a) mechanical (cam); b) electromagnetic actuator;
Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 179 The main problem with injection pumps is to reach high injection pressures. Values up to 140 MPa maximum injection pressure can be ensured only by piston pumps. Size injection pressure implies high requirements to accuracy of execution of the piston and cylinder pumping element as well as to sealing the couple parts from the external environment. These requirements have led to reduction of functional clearances between piston and cylinder to values of 1.5 ... 3.0 μm and production of certain construction with increased plunger length compared to its diameter. This implementation involves fine grinding operations, with extremely limited deviations in form, surface quality and their mutual position, and running operations of the piston and cylinder mating. Piston-cylinder torque thus obtained has the component parts interchangeable. 2. Common Rail System Common rail is a direct injection type, used in internal combustion engines, particularly with compression ignition engines. The most important aspect of a common rail engine is that, distribution of fuel to the injectors is a main pipeline (hence the common rail) with high pressure to each injector separately. The basic idea of the system is creating pressure for injection to occur independently of the engine speed, and fuel pressure at low speed is still maximum. The so called pressure, common rail, has meantime reached that the German Bosch company and Japanese DENSO to build power pumps that can sustain pressures up to 2000 bar, presented in 17/09/2007 at IAA (Internationale Automobil-Ausstellung) Frankfurt. This system, developed after more than 15 years, but was brought to commercial use only a few years ago. The system was manufactured by Fiat in the '80s, but those who have implemented for the first time in a vehicle for sale, was Mercedes. After them, others have followed, one of them being Peugeot that in 2000 had spectacular sales increases on vans equipped with such system. The visible difference from the old system is that fuel injection does not go directly to the injectors to pump but it builds a common platform and from there goes directly to the injectors. Development of common rail injection system was done trying to follow the principles of gas injection. Many ideas are taken from this system. The advantages that this system brings from previous versions of diesel injection systems are: 1. Harm reduction due to better fuel spray into cylinder and due to better control over the amount of fuel entering the cylinder during injection, CO2 emissions be reduced by up to 20% and carbon monoxide level is reduced by up to 40%. Unburnt hydrocarbons are decreased by 50%.
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