Unit Injector

The pump -nozzle system (English Unit Injector System) is an injection system for internal combustion engines.

History

The pump -nozzle system is a development of the GM Diesel Division, which today (2010 ) and Detroit Diesel Corporation ( DDC) for Daimler AG is one. The Unit Injector System ( UIS) said unit was developed in the late 1930s and first used in relatively slow-moving ship, rail and truck diesel engines. The essential feature is the separate injection pump for each cylinder with very short pressure lines to the injection nozzle. The pumps are mechanically driven by the camshaft via tappet and bumpers.

The first engine with electronically controlled unit injectors put in his truck Volvo FH 12 before in 1993. Its 12 -liter engine D12A 4 -valve technology was equipped with an overhead camshaft and so-called unit nozzle holders (unit injectors). The unit injectors are mounted directly above the piston and driven by the overhead camshaft and electrically controlled by a control unit.

With respect to the legally required reduction of exhaust emissions in passenger cars to normal in the 1990s, diesel fuel injection systems ( distribution and in-line injection pump) were mainly due to the relatively long high-pressure lines and the associated limitation of the pressure gradient no longer viable.

Therefore, the pump -nozzle system was taken up by Bosch for Volkswagen, developed and used since 1998 in passenger car diesel engines of the Volkswagen Group. First vehicle with PD technique was the VW Passat B5 with a 1.9 -liter engine, which delivers 85 kW.

At the same Magneti Marelli developed for Fiat, the common-rail injection, which has since proven to be technically superior. Meanwhile, the achievable injection pressures of CR systems where the pump-nozzle system are equal. Since 2008, Volkswagen equips mainly due to cost and also for reasons of comfort its diesel engines gradually with common-rail technology.

Principle

Unlike the common-rail system in the pump-nozzle system of the injection pressure is generated separately for each cylinder. This takes place in a plunger pump having a plunger which is actuated by a respective cam on the camshaft and own a rocker arm.

To obtain a low-cost process for the injection pressure curve over time, a steeper pressure rise is required. To the kinematics of the actuation path of the working cam is designed such that the piston moves at a high speed of high acceleration. This is achieved purely mechanically by an oval cam shape.

The pressure build-up in the space below the piston, the Plungerraum can be controlled by opening and closing of a solenoid valve or a piezo actuated by an actuator valve. The valve is closed, the piston builds up pressure, and the fuel is injected by the injection valve. By opening the control valve, the injection is stopped, wherein an optimum combustion is achieved by an abrupt possible cancel of the injection process, with a fast drop in pressure. Piezo actuators work are three times faster than magnetic plates up to. For example, had the VW Passat 2.0 TDI Year 2005 (125 kW/170 hp), a pump - nozzle injection with piezo - actuated valves, which reached a peak pressure of 220 MPa.

Benefits

• Since the pressure in the pump -nozzle unit (PDE ) - and thus the injection pressure - is produced by the cams of the camshaft, the drive energy required to be applied only to the relevant to the injection area. Pump-nozzle systems are fault tolerant than common-rail injection (no high pressure pump, no rail), so that the failure of a pump -nozzle unit does not necessarily lead to a stop of the engine.
• The high pressure promotes a fine atomization of the injected by the fuel injection valves. Smaller droplets mean a smaller ratio of volume to surface area, which, depending on other conditions may result in a lower soot formation by itself.
• PD diesel have good efficiency and provide a very high torque up to medium speeds.
• The non-sealing plunger principle of the PDE allows the use of almost all fuels (gasoline, alcohol, ethanol, LPG, biofuels, etc.).
• Upon a failure of the injection valve ( jamming or fouling of the nozzle) can flow continuously into the combustion chamber, no fuel.

Disadvantages

• Due to the above-mentioned cam dependency injection can (time, that is, considered over the angle of rotation) only be triggered if the cam drives the pump. This means that the range of possible injection timings is restricted to a specific range around the top dead center, which, although the efficiency of, but not the smooth operation is performed ( see below).
• The system requires a relatively high overhead because of principle for each cylinder its own pump is necessary.
• As the injection timing and quantity can not be changed in small increments and pre-and post-injection are only to a limited extent, the motor run is cultivated as little. Furthermore, the exhaust gas temperature can not be varied fast enough. This is necessary in order to achieve 4 emission standards EURO.
• Because of the pressure buildup in the PDE ( pump -nozzle unit ) should take place as suddenly as possible, the drive energy required is to be applied only where relevant to the injection area. The associated high dynamic loading by changing the pressure build-up in the individual PDE requires a corresponding dimensioning of the camshaft and its drive design. For camshaft drive therefore a broad toothed belt drive or a spur gear is necessary. Chain drives can not transfer the high peak loads and thus tear due to the excessive necessary tensile stiffness and low damping capacity.

Future Development

Meanwhile, the common-rail systems with respect to the maximum pressure (more than 2000 bar) open. The cost of a pump-jet diesel engine of the latest generation are about 100 euros higher than in a comparable diesel engine with common - rail injection.