Multec 3.5 Fuel Injector Application Manual
Fundamentals
Delphi Energy and Chassis Systems
Revision:
11/05-1
2-15
Note
volatility. The volatility of gasoline is measured by the RVP (Reid Vapor
Pressure). Typically, RVP is stated as the pressure required to suppress
the formation of vapor at 100
°
F (38
°
C.)
Fuel in the vapor state
contains less heating energy by volume than fuel in the liquid state
causing a lean air fuel ratio.
Figure 2-4 shows the relationship between fuel vapor formation and fuel
type, RVP, and pressure. For equivalent RVP levels, E10 fuels form
vapor at lower temperatures than E0 (straight gasoline). Higher system
pressures suppress vapor formation.
The fuel vapor causes two problems:
•
Vapor "bubbles" above the metering orifice restrict the flow of liquid
fuel through the metering orifice.
•
Vapor "bubbles" do not deliver the same energy content as liquid fuel.
These two problems cause a lean shift, as the volume passed through the
metering orifice contains some percentage of vapor rather than 100%
liquid fuel. This lean shift can vary in severity, and in some cases may not
be noticeable by the driver of the vehicle. However, potential driveability
impacts are:
•
No engine start
•
Long engine crank time
•
Engine stalls after start; stall will typically occur repeatedly after
several starts
•
Engine idles roughly (misfires) after start. Rough idle may or may not
subside after some period of time
Because of this, it is critical that the fuel system be validated under severe
conditions to determine if acceptable hot re-start performance can be
achieved.
Hot re-start problems typically occur when the following conditions exist
in combination:
•
High ambient temperatures
•
Vehicle is operated under high load (e.g. trailer towing) before it is
shut down
•
Vehicle re-start is attempted approximately 15 – 45 minutes after the
engine is shut down.