Page 207 - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

2
Engine systems 191

Defi nition

Lambda ( λ ) value
A λ value of 1 is a ratio of 14.7:1 by
mass of air to fuel.

Figure 2.224 Steel fuel tank in position

Petrol is ignited in the combustion chamber by a spark arcing across the
electrode gap of a spark plug. Diesel fuel ignites following injection into the
high-temperature air charge. The high temperature is obtained by compression
of the air charge. The air charge on petrol engines is matched to the amount of
fuel delivered. In diesel engines, a full air or gas charge is required to raise the
temperature by compression.
The fuel on the vehicle is held in a tank fi tted in a safe position ( Fig. 2.224 ).
Recent construction legislation requires that the tank is unlikely to be ruptured
in a vehicle collision. The positioning and protection of the tank are considered
at the design stage of the vehicle and tested during development. The tank is
fi tted with a fi ller neck and pipework from the fi ller cap to the tank. Also fi tted are
the outlets to the atmospheric vent or evaporative canister and the fuel feed and
return pipes to the engine. The fuel gauge is located in the fuel tank. Fuel supply

and return lines are made from steel pipes, plastic pipes and fl exible rubber
joining hoses, depending on application and the type of fuel used.
A pump to supply fuel to the engine is fi tted into or near the tank on petrol
injection vehicles. On carburettor vehicles, a mechanical lift pump ( Fig. 2.225 ) is
fi tted to the engine and is operated by a cam on the camshaft or crankshaft, or
an electric pump is fi tted in the engine compartment. Diesel-engined vehicles
using a rotary fuel injection pump may use the injection pump to lift fuel from the
tank. Alternatively, they may have a separate lift pump similar to the ones used
on carburettor engines. A separate priming pump fi tted in the fuel line may also
be used.
The carburettor was the traditional method of mixing petrol with air as it enters
the engine ( Fig. 2.226 ). However, a simple carburettor is only capable of
providing a correct air and fuel mixture ratio within a very small engine speed
range. For road vehicles a wide engine speed range and a wide engine load are
required. To respond to the speed and load variations, complex carburettors are
used ( Fig. 2.226 ).
There are two basic carburettor designs, the fi xed venturi and the variable venturi
types ( Fig. 2.227 ). The term choke is often used to describe the venturi and this
gives the alternative carburettor defi nitions of fi xed choke and variable choke
types. The usual meaning of the term choke is to describe the engine cold-start
device fi tted to the carburettor.