Fuel Mixture Theory Explained
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Internal components of the PCU are encapsulated in silicone to seal out
moisture and protect against vibration. |
The ‘chemically
ideal’ air-fuel mixture, by weight, in which air and gasoline are used,
occurs at approximately 14.6 parts air and 1 part fuel. This chemically
ideal air/fuel ratio is known as a stoichiometric mixture. Mixtures
with more air are identified as lean mixtures, and tend to occur with
an air/fuel ratio of greater that 14.7 to 1. A lean mixture results in
increased combustion temperatures, which allows for a more complete burn of
the fuel. The result is increased power output, however, the risk of severe
engine damage looms overhead for those seeking to gain engine performance by
venturing into this risky domain. Mixtures with less than 14.6 to 1 ratios
are identified as rich burning mixtures. Generally, the rich burn
limit for an engine at a normal operating temperature is about 6:1, whereas
the lean burn limit is about 22:1 for electronic injection engines, and 18:1
for carbureted engines. The 'ideal mixture' is a model for fuel efficiency -
for performance applications, the best mixture will not be the
'chemically ideal' mixture.
The ideal air/fuel ratio for a particular RPM range in a performance engine
is determined by many factors. Some of these factors are compression,
ignition timing, octane rating of the fuel, RPM, engine load, and air
temperature. Remember, the cooler the incoming air charge, the denser it
becomes. The result is that more air molecules can enter the engine at a
given time, which has an effect on the fuel mixture. A small alteration to
the air/fuel mixture will have a significant impact on engine performance.
This is where the PCU’s adjustability plays a central role in precise engine
tuning.
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Optional PCU Accessories, left
to right: PCU Steemwheel for remote adjustments,
Extension Harness for remote mounting of the PCU, AC Adapter
and Serial Interface cable for connectivity to a PC, and
Serial Interface Extension cable |
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Fine Adjustments Using The PCU
By allowing
precise adjustments to the fuel mixture at all RPM levels, as well as
individual pairs of injectors, the engine can be tuned according to many
factors, including weather conditions. For example, the advanced user might
want to increase the fuel volume (richen the mixture) when operating the
boat on an especially cool, dry day to compensate for the increased air
entering the engine and potentially avoid a lean condition. The result would
be increased performance, although this type of fine adjustment should only
be attempted by the experienced user, with the aid of exhaust gas
temperature monitoring devices installed.
Setting up and tuning the Pro Control Unit is a straightforward process,
which we will detail in Part Two of this series. The PCU arrives with a
baseline fuel curve for your engine already preloaded. It is highly
recommended that baseline exhaust temperature readings are recorded before
installing the PCU. The tuning process then begins with a +20% rich setting
on the Steamwheel or the internal Main encoder, then gradually and carefully
leaning the mixture down at the required RPM ranges. The idea is to cover
broad RPM ranges at first, and then work your way down to smaller RPM ranges
for an accurate tune.
The Brucato Pro
Control unit is a godsend to those that seek to fine-tune their
fuel-injected Mercury outboards with uncanny precision. This type of
custom programmability has been available to car enthusiast for years, and
now it is available to the outboard enthusiast. It is the first such
system to offer plug-and-play installation and a level of digital
sophistication previously unheard of for the engines it was designed to work
with.
The Pro Control
Unit itself costs $1000 US, which is significantly less expensive than
replacing a new factory ECU. When you consider the PCU's level of
adjustability, quality, and options available, its a real bargain, and an
extremely useful tool in the performance boater's arsenal. For pricing
on accessories and special pricing on PCU packages with popular accessories,
visit the
Brucato Machine
and Tool website.
Part Two of this series will focus on installation, setup, complete
tuning, and testing the PCU. These tests will be performed on two
engines: a modified Mercury 2.5, 260 horsepower (stock rating) EFI with a
modified digital ECU, and a mildly modified 2.4, 245 horsepower (stock
rating) Bridgeport EFI with a stock analog ECU.
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