Downloaded from www.Manualslib.com manuals search engine 7 FUEL INJECTION
It is the function of any fuel injection system to ensure
that the correct mass ratio of air and fuel is delivered to
the engine under all operating conditions. We will concen-
trate on TBi (Throttle Body Injection or alternatively
called single point fuel injection systems) in this docu-
ment.
The availability of powerful low cost microprocessors has
made it possible to fit FI (Fuel Injection) systems to a
larger percentage of vehicles. The ECU (Engine Control Unit)
evaluates input sensor data and calculates the required
output signals to control the engine. The most important
function of a FI system is to measure the air mass entering
the engine and calculate the injector opening duration to
ensure the correct A/F ratio under specific engine operating
conditions. The A/F ratio has a direct effect on the power
output of the engine, fuel consumption and exhaust gas
emissions. It is therefore necessary to exercise precise
control over the opening duration of the injector.
A number of operating conditions exist where the A/F ratio
is deliberately modified and forced to deviate from the
calculated ratio to ensure better drivability and smoother
engine operation. These deviations are classified as A/F
corrections and will later be examined in detail in this
document.
Downloaded from www.Manualslib.com manuals search engine 7.1 AIR MASS TO FUEL MASS RATIO
The theoretical air mass to fuel mass ratio required by an
internal-combustion engine for complete combustion is
14.7:1. This ratio is also called the stoichiometric ratio.
The A/F ratio determines the fuel consumption, maximum
engine power output and exhaust gas emission levels. Unfor-
tunately there is no single A/F ratio that optimises these
three requirements.
The ratio of actual air mass supplied to the engine divided
by the theoretical requirement is defined as lambda.
ë = Air mass supplied/theoretical requirement
Where ë = Lambda
ë = 1
The air mass supplied matches the theoretical amount.
ë < 1
A lack of air resulting in a rich mixture. Increased engine
power outputs are obtained for 0.85 < ë < 1.
0.75 < ë < 0.85
A rich mixture suitable for transient conditions where a
sudden load change is experienced.
ë > 1
An excess of air resulting in a lean mixture together with a
reduction in engine power output. Optimum fuel consumption
takes place with 1 < ë < 1.2.
ë > 1.3
Lean mixture making it impossible to achieve reliable igni-
tion.
