Air/Fuel Ratio Effects On Combustion - Delphi Multec 3.5 Applications Manual

Multec 3.5 Fuel Injector Application Manual
2.2.1

Air/Fuel Ratio Effects on Combustion

Ref. Sec 2.2.1.1
Ref Sec. 2.2.7.2 & 8.4.1
Note
2.2.1.1 Stoichiometric Mixtures, Definitions
Delphi Energy and Chassis Systems
The goal of the Multec 3.5 Fuel Injector is to supply the correct fuel mass
to achieve the correct air/fuel ratio (A/F). Complete combustion will
depend, in general, on the following:
• The air and fuel must be in the proper portions (referred to as the
stoichiometric mixture or ratio); this proportion will depend upon the
chemistry of the fuel.
Note: Stoichiometric A/F refers to the quantitatively derived ratio of air
to fuel that will allow the chemical process of combustion to be delivered
to ideal equilibrium. In this manual, A/F is stated in terms of their
molecular weights – that is, molecular weight of air over molecular
weight of fuel.
• The mixture must be in vapor state, as liquid fuel is not combustible.
Note: In order to eliminate any confusion, it is important to note that for
the fuel injection process fuel must be in a liquid state in order to be
properly metered by the fuel injector. Vapor formation before the
injection process is highly undesirable and can cause a host of
driveability problems (See Sec. 2.2.7.2 and 8.4.1). However, it is
important for the actual combustion process that fuel is in the vapor
state. This is typically achieved through the fuel spray and particle size
characteristics of the liquid fuel after it is injected. Other factors, such as
injection time, fuel spray targeting, residence time and the air induction
characteristics all play a role in this process.
Throughout this manual, it should be assumed that when A/F ratios are
stated as being stoichiometric, it is in reference to standard non-
oxygenated fuels unless specifically stated otherwise. It should be noted
that oxygenates (MTBE, ethanol) have a higher (lower numerically)
stoichiometric air/fuel ratio than standard gasoline. This effectively
means that more fuel is needed for the same intake airflow to obtain
complete combustion.
As noted in the last section, stoichiometric A/F refers to the quantitatively
derived ratio of air to fuel that will allow the chemical process of
combustion to be delivered to ideal equilibrium. Typical values for this
are 14.7:1 for standard, non-oxygenated gasoline. Stoichiometry values
for alternate fuel blends are shown in Table 2-1.
Fundamentals
Revision: 11/05-1 2-3