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An innovative high-pressure injection system
The Scania HPI (High Pressure Injection) system minimises harmful emissions without imposing unnecessary restrictions on fast build-up of torque. It is a system that is also highly fuel efficient as a result of infinitely variable minute adjustments in timing that ensure the delivery of fuel is always very precise.
Back to basic mechanics
The Scania HPI unit injector is in many respects similar to traditional mechanical injectors that have been used for many years. Fuel is pressed into the combustion chamber at high pressure by a mechanical plunger, driven by the engine’s camshaft. The innovation in the injector is Time Pressure Metering. The technique uses a separate feed of fuel to the injector to adjust timing.
By making infinitely small variations in the volume of injection and timing fuel introduced into the injector, it is possible to have precise control of the injection and thus how it affects consumption and emissions.
More than an economic argument
In addition to the significant benefits of high torque, good economy and low emissions, Scania HPI has other worthwhile benefits: easy, low cost maintenance, lower noise levels and warm fuel.
Low maintenance: the unit injector requires a new filter every 60,000 kilometres. Adjustments to the injector are needed during the running-in period and thereafter every 480,000 kilometres.
Lower noise levels: the hydraulic actuation within the barrel of the unit injector means far less noise is generated. This is the main reason for the distinctly different and quiet sound of the 12-litre engines.
Warm fuel: vehicles operating in very low temperatures have been known to experience problems with waxing of the fuel. This is greatly reduced with the 470 engine, because constantly recirculating hot fuel, warmed by the engine in the timing feed, is continuously being returned to the tank.
How does it work?
- Upper plunger
This plunger is pressed down at 12,500 N (the injector is made for forces up to 18,000 N), a force generated by the camshaft.
- Induction port for timing fuel
A strictly controlled volume of 'timing' fuel is pumped into the injector at this point. It provides a hydraulic link between the upper and middle plungers. The volume of 'timing' fuel permitted to create this link determines the precise timing of combustion fuel pressed into the combustion chamber through the open nozzle at the base of the injector. More fuel advances the timing, less fuel retards it. The volume of fuel is controlled by the Scania engine management system. By varying the volume, timing is adjusted to be exactly right for that particular combustion cycle.
- Return duct for timing fuel
Timing fuel is evacuated and returned to the tank. The passage of fuel through the injector causes a temperature rise, which means that fuel in the tank is warmed and therefore unlikely to suffer waxing, caused by low winter temperatures.
- Induction port for combustion fuel
Fuel for combustion is pressed into the lower body of the injector, ready for introduction to the open nozzle. The volume of fuel is controlled by the Scania engine management system.
- Lower plunger
Fuel is pressed at high pressure by the lower plunger into the combustion chamber. The pressure has to overcome the combustion pressure in the engine created by the rising piston.
- Open nozzle
The open nozzle allows fuel at high pressure to be introduced to the combustion chamber. The high-pressure causes fuel to combine with air to form a mist of a density required for efficient, clean combustion |
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