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Fundamentals Multec 3.5 Fuel Injector Application Manual
2-14 Delphi Energy and Chassis Systems
Revision: 11/05-1
2.2.7 Engine-Vehicle Environment
2.2.7.1 Impact of High Engine Temperatures on Combustion
As the engine and engine compartment temperatures increase, several
factors must be considered to obtain optimum combustion. Hot air
entering the induction system is lower in density and results in a reduced
mass air flow rate. To maintain the optimum air/fuel ratio, the engine
controller must reduce the amount of metered fuel. Speed density
systems, which do not have the ability to directly measure intake airflow,
utilize an inlet air temperature sensor to estimate the reduction in mass
airflow at elevated temperatures. Mass airflow systems are capable of
reading reduced airflow rates directly from the calibrated air flow meter
Refer to Section 5 Note : Low pulse-widths, such as at idle or during overrun conditions,
could fall below the injector minimum working flow range under elevated
temperature conditions. This could cause pulse-to-pulse variations that
directly affect idle quality. The impact on idle quality depends on the
injector firing scheme. Typically the minimum commanded pulse width is
limited in the engine control software.
It is important to consider these operating conditions when determining
the proper flow size for the injector.
2.2.7.2 High Ambient Temperature Startability
Reference section 8.4.1
for Hot Fuel Handling
Tests. While high ambient temperature conditions must be evaluated for most
engine components, several conditions in combination can cause specific
problems for the fuel system.
In general, fuel system components reach their peak temperatures after the
vehicle has been shut down. This period is usually referred to as the soak
period. It is during this soak period that problems may occur if the vehicle
is re-started.
During normal operation, the fuel injector does not typically see extreme
temperatures because the fuel flowing through the tip helps dissipate heat
energy. When the vehicle is shut down, fuel is no longer flowing through
the injector. Injector tip temperatures rise and can eventually reach an
equilibrium temperature with their surrounding environment in the intake
manifold or cylinder head.
The problems typically encountered are due to the premature vaporization
of fuel, either upstream of the metering orifice in the injector or as liquid
fuel passes through the metering orifice and "flashes" to vapor. Although
the fuel system is under pressure, the temperature can rise to the point that
this pressure is no longer able to suppress formation of vapor.
The likelihood that a particular fuel will vaporize is characterized by its