Fuel Systems: Gasoline Fuel Systems

Gasoline fuel system principles	Gasoline fuel	The properties of gasoline must be balanced to give satisfactory engine performance over a wide range of operating conditions including heat, altitude, and driving patterns. The more effectively liquid gasoline is changed into vapor, the more efficiently it burns in the engine.
	Gasoline fuel characteristics	There are several important characteristics of hydrocarbon based fuels: volatility, octane rating & energy content. The most important characteristic of gasoline is its Research Octane Number (RON) or octane rating, which is a measure of how resistant gasoline is to premature detonation (knocking). A range of fuel additives are used to change a fuel's performance characteristics.
	Controlling fuel burn	Detonation is a violent collision of flame fronts in the cylinder, caused by uncontrolled combustion. The sudden rise in pressure can cause a knocking sound.
	Stoichiometric ratio	Stoichiometric ratio is the air-fuel ratio necessary for complete combustion.
	Air density	The density of air is its mass per unit volume.
	Fuel supply system	EFI is a circulation system. A pump draws fuel from the tank and sends it to solenoid-operated injection valves, where pressure is maintained by a fuel pressure regulator. Excess fuel flows back to the tank through a return line.
	Pressure & vacuum	As air pressure is reduced, a vehicle has to reduce the amount of fuel delivered to the engine to maintain the correct air-fuel ratio.

Carburetor operation	Carburation	A light vehicle under normal conditions needs an air-fuel ratio, by mass, of about 15 to 1. By volume, that's 11000 to 1.
	Carburetor system components	The main components in most carbureted systems are a float chamber, a venturi, the throttle, idle circuit, main circuit, a choke and an accelerator pump.
	Carburetor systems	Low speed and idling ports allow the engine to operate with a low throttle opening before the main system is operating fully.
	Metering jets	The main jet size is selected to provide the best mixture for fuel economy. An extra jet supplies additional fuel for maximum power.
	Accelerating	For acceleration, suddenly depressing the accelerator delivers extra fuel into the airstream.
	Carburetor barrels	A two-stage carburetor has a primary throttle open only from idle to medium speeds. At higher speeds, the secondary throttle opens to admit more air-fuel mixture.

Carbureted system components	The carburetor	The carburetor atomises the fuel and mixes it with air, and controls the delivery of the correct mixture to the engine.
	Mechanical fuel pumps	The mechanical fuel pump has a diaphragm separating two chambers. Moving the diaphragm down draws fuel into the pumping chamber. A spring then moves the diaphragm up, forcing fuel from the pump, into the carburetor.
	Electric fuel pumps	An electric fuel pump operates with the ignition switched on. It can be controlled so that it operates only if the engine is running.
	Tanks & lines	Most fuel tanks are in two parts joined by a weld around the flanges where the parts fit together. Baffles make the tank more rigid, prevent surging of fuel, and ensure fuel is available at the pickup tube.
	Fuel lines	The fuel tank is connected to the engine by fuel lines. A return line may carry excess fuel back to the tank, to keep fuel system components cool.
	Charcoal canister	Charcoal canisters are used in some emission systems as a means of preventing pollution to the atmosphere.
	Carburetor filters	Carburetor filters are used to prevent particles from entering the fuel carburetion/injection components.

EFI fuel supply system - principles	EFI principles	EFI systems employ injectors to spray the fuel into the intake system. An electronic control unit processes data received from various sensors to optimize the air fuel mixture at any given moment by adjusting the amount of fuel injected.
	Basic EFI principles	EFI is a pressurised, indirect-injection system with solenoid-operated injectors. In multi-point injection, one injector is in each intake manifold runner. Single-point injection uses one or two injectors in a carburetor-like throttle-body.
	Air supply	The design of the intake system determines how much air can be drawn into a cylinder at any given engine RPM. EFI can achieve uniform distribution of the air delivered to the cylinders.
	Air volume	The amount of air entering the engine must be measured, so that the amount of fuel injected into it forms a mixture to suit the engine operating conditions at that time.
	Multi-point injection systems	For any injection duration, if fuel is held at constant pressure, then, as manifold pressure varies, so does the amount of fuel delivered. That means fuel pressure must be held constant above manifold pressure.
	Simultaneous injection	In multi-point injection, the injectors can all be triggered simultaneously, twice per cycle. In a throttle-body system, the central injector is normally triggered on each ignition pulse. With two injectors, alternate triggering may be used.
	Efficient combustion	Adaptive learning is a form of feedback that lets fuel settings change as components age. The ECU memorizes its fuel settings for different operating conditions, and stores them for future use.

EFI fuel supply system - components	Fuel pumps	EFI fuel pumps operate electrically to provide fuel under pressure to the fuel rail and the injectors.
	Fuel filters	EFI fuel filters remove contaminants from the fuel, so that clean fuel can be supplied to the injectors.
	Tanks & lines	Most fuel tanks are in two parts joined by a weld around the flanges where the parts fit together. Baffles make the tank more rigid, prevent surging of fuel, and ensure fuel is available at the pickup tube.
	Fuel lines	The fuel tank is connected to the engine by fuel lines. A return line may carry excess fuel back to the tank, to keep fuel system components cool.
	Fuel rail	The fuel rail supplies fuel to the injectors under constant pressure.
	Fuel pressure regulator	The fuel pressure regulator controls the return of fuel to the fuel tank, to maintain the pressure in the fuel rail at a constant value above intake manifold pressure.
	Injectors	Injectors are solenoid-operated valves which deliver fuel in the form of an atomised spray, into the intake manifold, or the intake ports.
	Tachometric relay	The tachometer indicates engine RPM.
	Thermotime switch	The thermotime switch senses engine coolant temperature, to control the operation of the cold start injector, during cranking conditions.
	EFI sensors	EFI sensors include: wide band oxygen sensors, twin oxygen sensors, knock sensors, oil deterioration sensor, exhaust gas recirculation sensors and switches.
	Potentiometer	A potentiometer is a mechanically variable resistor.
	Auxiliary air valves	Auxiliary air valves allow additional air to bypass the throttle plate during cold start, and warm-up conditions.
	Idle speed control devices	Idle speed control devices allow the preset idling speed to be maintained automatically when additional loads are placed on the engine, during idling conditions.
	Inertia sensors	Inertia sensors shut off the fuel pump in the event of an accident, to minimize the danger of fuel spillage from a leak in the system.

Fuel system procedure	Replacing a fuel filter	There are two types of fuel filter: carbureted system filter and EFI system filter. It is important to follow the correct procedure for the type of vehicle you are servicing. The objective of this procedure is to show you how to remove and replace a fuel filter.

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