Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.2
 
Fuel Metering Systems 
Vehicle fuel delivery systems comprise of a fuel storage facility, fuel pump(s) 
and metering devices. Fuel delivery systems in automotive engines are responsible for 
controlling the amount, location and method of fuel introduced to the combustion 
chamber. The method through which fuel is introduced to the combustion chamber 

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2.2 Fuel Metering Systems 
43 
ultimately determines several important characteristics of engines such as the fuel 
consumption, power output and emissions. 
Due to hydrocarbon fuels not having high enough volatility to rapidly produce 
fuel vapour, successful air-fuel mixture is created through a fuel atomisation process. 
This means breaking liquid up into small droplets that can mix with air and evaporate 
easier than the fuel in liquid form. Atomisation is achieved through high relative 
velocities between the fuel and the ambient air. This can be realised through dispersion 
of fuel at high speed into a relatively slow-moving air, such is the case with pressure 
and rotary atomisers or alternatively expose a slow moving liquid fuel to high moving 
stream of air [103]. The following sections will briefly discuss the three prominent 
fuel metering systems used in gasoline internal combustion engines. 
2.2.1
 
Carburettor Systems 
Carburettors are the earliest fuel delivery systems used in internal combustion 
engines. They rely upon a Venturi effect created in the air inlet duct. Fuel delivery 
pump into carburettor is often connected to camshaft or the distributer, supplying 
volumes of fuel proportional to engine speed. Narrower section in the inlet duct 
increases the air velocity through it as the lowering piston generates suction, while the 
connection to the float chamber of the carburettor induces a vacuum effect and sucks 
in fuel. Often the idling characteristics mean not enough suction is created and 
additional ducts and orifices are used for compensation.
Inherently, carburettors lack in fuel metering accuracy and additionally 
provide for low efficiency air-fuel mixing. Defrance and Versille [104] addressed 
mixing issue with a heated charge-mixing device, fitted in line between the engine and 
the carburettor to increase homogeneity of air-fuel mixtures and enable reductions in 
fuel consumption. Although reliable and simple in construction, lack of accurate fuel 
metering control means that carburettors in modern automobiles are nearly completely 
superseded by more fuel efficient mechanical or electronic fuel injection systems.