Fuel systems on cars
2) Fuel: From tank to engine
The modern fuel tank is fitted with a sealed filler cap. Set within the fuel tank is the fuel level sender which sends a signal to the fuel gauge indicator on the dashboard. Fuel is drawn by a pre-pump (lifter pump),also within the tank, through a filter 'sock' or strainer to prevent bulky debris rom entering the fuel line; the pre-pump maybe located outside but near to thefuel tank. Also connecting to the tank is the fuel return pipe and often on top of the tank can also be found a pressure sensor. Associated with the tank there is often an expansion vessel to accommodate thermal expansion of the fuel. Finally attached to the top of the tank, or top of the expansion vessel, there is a fuel vapour recovery pipe that is part of the evaporative loss emission control system employed to prevent fuel vapour release to the environment (see later in this series).
Fuel is pumped from the tank by the pre-pump through the fuel line towards the engine: on some systems a further 'main' fuel pump may lie between the fuel tank and engine. The fuel, now under considerable pressure, passes through an in-line fuel filter that removes any fine particulates before they reach the engine. A clogged fuel filter puts a strain on the fuel pump so routinely changing the fuel filter can help prolong the pump's service life. On reaching the engine compartment the fuel line connects to the fuel rail mounted on the engine. The injectors feed directly off this pressurized fuel rail. Mounted onto the end of the fuel rail after the injectors is the fuel pressure relief (FPR) valve which governs the fuel pressure in the line all the way back to the fuel pump(s).
The critical component within the FPR is a spring loaded diaphragm whose role is to maintain a constant force on the relief valve. Fuel, pressurized by the action of the fuel pump, is forced against the valve and excess pressure allows fuel to bleed past to the return fuel line back to the fuel tank. Attached to the top of the fuel pressure regulator is a vacuum line connected to the inlet manifold. The vacuum partially offsets the action of the diaphragm spring. When the engine is at idle, the throttle plate is closed and the manifold vacuum is at its greatest (lowest pressure), the strong vacuum sucks back on the diaphragm against the spring pressure allowing a reduction in the fuel pressure needed to open the relief valve. This reduction in fuel pressure helps minimize fuel consumption per each injection cycle when at idle. Conversely, when the engine is running at high speed, the throttle is wide open creating little or no vacuum in the plenum. This reduces the pull back on the FPR diaphragm and consequently the fuel pressure in the fuel rail is at its highest.
In 'common rail' designs the injectors are controlled by the ECU. The ECU controls the injection cycle time (pulse width) and, in very modern systems, the actual injection pattern. When the car is at idle the injection times are short and when under load the injection times are longer. Similarly when the engine is cold (as indicated by the engine coolant sensor to the ECU) the injection times are longer to ensure a richer starting mixture. The combination of injection time, along with the fuel pressure in the fuel rail, dictates the engine speed and fuel consumption.
In recent years evaporative loss emission control systems (EVAP) have been introduced to prevent the escape of fuel vapour from cars. The fuel vapour that evaporates from the surface of the fuel in the tank is conveyed by the vapour relief pipe to a charcoal filled canister. The fuel vapour is stored in the charcoal for later use by the engine. Access to the stored vapour held in the canister is controlled by electrically operated valves. When the engine is cold the charcoal canister purge valve is kept closed. At normal operating temperature (signalled from the coolant temperature sensor to the ECU) the purge valve opens allowing the stored vapour to contribute to the air in the inlet manifold. The charcoal canister also has a vent valve operated either electrically or passively, in response to internal pressure drop, which when open introduces fresh air to help flush residual vapour out of the charcoal canister.
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on Jul 15, 2011 | Mercedes-Benz E-Class Cars & Trucks