December 2021 - 72

lean-rich-lean repeatedly. The wide-band
sensor being used today is far superior and
allows calibration engineering teams to
squeeze more efficiency out of today's
internal combustion engines and provide
the optimal catalytic convertor feed gas to
achieve the lowest possible tailpipe emission
output. Additionally, these sensors
are typically ready for work shortly after
engine startup. Narrow-band sensors can
still be found downstream of the catalytic
converter and are primarily used for monitoring
catalyst efficiency. However, some
vehicles use this sensor to " fine tune " the
catalyst's feed gas by trimming fuel in a
way so that the catalyst can do everything
it can to further reduce tailpipe emissions.
AIR-FUEL RATIO
Today's pump gas typically contains up to
10 percent alcohol which means that the
proper air-fuel ratio is now less than the
customary 14.7:1 ratio we experienced in
the past. Some vehicles are designed to
operate on varying percentages of alcohol
therefore the ECM needs to have a clear
understanding of this content so that the
proper stoichiometric air-fuel ratio can be
targeted. Some vehicles use direct measurements
to detect alcohol content and
others use inferencing to estimate alcohol
content. Today, with most pump gas containing
approximately 10 percent alcohol,
the stoichiometric air-fuel ratio target is
in the lower 14's. In most cases the scan
tool offers air-fuel ratio PIDs in different
formats such as equivalence ratio, lambda,
phi, and air-fuel ratio.
SPEED DENSITY
This method is still in use today however
some of the behind-the-scenes routines
may have changes. Essentially, a speed
density system uses a 3-D table to express
the volumetric efficiency profile of the
engine across a wide range. The system
typically uses engine speed on one axis
and manifold pressure on the other. This
table is generated during engine development
and then written to the engine calibration
residing within the engine control
module. This " map " is used as a baseline
reference to understand what the percentage
of cylinder fill is when operating
within specific coordinates. For example,
say the engine was operating at 1600 rpm
and the manifold pressure was 50 kPa. The
reference table states that the cylinder fill
capabilities are 75.09 which means that
the engine can fill the cylinder 75.09 percent
of the actual volume of the cylinder.
So, on an engine with a 4,278cc engine,
the cylinder volume is 713cc. Essentially,
this means that air mass entering the cylinder
would only fill it to 75.08 percent of
the 100 percent rated size of the cylinder.
Since the engine controller knows the cylinder
size, the injector flow rate, the composition
of the fuel, density of the air, and
the desired air-fuel ratio, it will then know
how long to turn on the fuel injector. It will
then look to the result of the air-fuel ratio
indicated in the exhaust to see how that
math worked out. Any errors in hitting
that target will result in a correction issued
via fuel trim. For the driveability technician,
the manifold pressure sensor key on,
engine off (KOEO) reading should always
be referenced when performing analysis
on any driveability or emissions related
complaint. If this value isn't properly
reporting your station pressure, then the
powertrain control module (PCM) will
not be referencing the proper coordinates
within that table during engine operation.
MASS AIR FLOW SYSTEMS
Many engines use mass air flow sensors to
estimate/measure cylinder air flow. Most
engines utilize a single sensor while some
use multiple. The ECM uses the input
from the mass air flow sensor to calculate
the cylinder air flow. The output from the
mass air flow sensor can be used in conjunction
with other data parameters by
technicians to assess various conditions.
For example, if the technician suspected
that the engine wasn't flowing the amount
of air it should, either because they suspected
a restricted exhaust or an under
reporting sensor, they could use this as an
input. However, what I have found is that
where this value is sampled is not always
the best. Recently I was hosting a train72
PTEN DECEMBER 2021 www.VehicleServicePros.com
ing session and polled the class for where
one would typically want to sample the
mass air flow reading when performing
a volumetric efficiency test. The majority
of the attendees stated that while at wide
open throttle (WOT), the rpm just before
the shift point should be where the mass
air flow value should be sampled. Well,
this answer is correct only if that rpm is
where the engine under test is rated to
produce its maximum torque rating. But
in most cases, this is not where the engine
produces its maximum rated torque. So as
a best practice, one should try and sample
the mass air flow reading at the rpm where
the engine is rated to produce maximum
torque. From there, one can take that
value into a volumetric efficiency (VE)
calculator for calculation. Or one could
use some quick napkin math to roughly
figure the efficiency of the engine. To do
so, one needs to calculate using the following
equations.
To find cylinder air we need the following:
*
Engine speed in rpm
* Mass air flow (MAF) in
grams/sec
* Constant (based upon the number
of cylinders)
The equation is as follows: (MAF x
constant)/rpm
To find the constant we use the formula
120/cylinder count. Then one would need
to know what the theoretical 100 percentcylinder
air mass would be. Say that we
had a 2.7L 5-cylinder diesel, you would
take 2700cc and divide that by 5 to get
540cc or .540L. Next, we need to know
the mass of air that would fill that area.
To find this we'll reference a known value
and that is 1.184 grams of air at standard
temperature and pressure will fill 1L. So,
if we take 1.184 X 0.540 = 0.639. This
means that if we perform our measurement
from mass air flow and arrive at .639
grams/Cyl we would be at 100 percent
volumetric efficiency.
NEURAL NETWORKS
Mass air flow and speed density methods
have been the traditional pathway for pre
http://www.VehicleServicePros.com
December 2021

Table of Contents for the Digital Edition of December 2021
