Substitution of Percent Load for Manifold Vacuum and
Analysis of Time in Mode in the Gasoline Test Procedure
Substitution of Percent Load for Manifold Vacuum:
The present nine-mode FTP defines power points in terms of manifold
vacuum. However, with the advent of supercharged and turbocharged engines,
EGR, and other advanced emission control systems, manifold vacuum can no
longer be used as the sole parameter in defining an engine's power points.
It would be desirable to redefine the nine-mode FTP power points in terms
of percent torque or power. This redefinition is expected to alleviate
problems encountered from engine to engine variation in manifold vacuum
versus horsepower (torque) relationships. The new nine-mode test procedure
would then be similar to the method used by the 13-mode diesel procedure.
As a basis for the conversion to percent of load from the manifold
vacuum test points currently in use, data from nineteen engines used in
past contractual work were used. Four of the nineteen engines were in-
volved in testing by Southwest Research Institute under Contract EHS
70-110 (Exhaust Emissions from Gasoline-Powered Vehicles Above 6,000-lbs
Gross Vehicle Weight, April 1972). Six engines were tested under Contract
EHS 70-110 (Baseline Characterization and Emissions Control Technology
Assessment of HD Gasoline Engines, November 1972), again conducted by
Southwest Research Institute. The remaining nine engines were tested
under Contract 68-01-0472 entitled, "Emission Control Technology Assess-
ment of Heavy Duty Vehicle Engines," December 1972.
Percentages of 10, 30, 60, and 90 percent of torque were earlier
derived by EPA personnel for the 19, 16, 10, and 3 in-Hg modes based on
four of the nineteen engines mentioned above. Primarily, these percent-
ages of torque were derived for SwRI use in Contract 68-01-0472 (Emis-
sions Control Technology Assessment of Heavy Duty Vehicle Engines,
December 1973). These percent power values, however, were at best,
approximations based on several manifold vacuum versus horsepower plots
and empirical judgement. Upon further detailed investigation different
percentages were arrived at.
Using the data from the nineteen engines, linear regressions were per-
formed on each set of data.for each engine. This resulted in the best fit
line for each set of data. That is, an equation (y = mx + b) was derived
with manifold vacuum as a linear function of percent power. All of the
data used yere taken at an engine speed of 2000 rpm (2300 rpm in the case
of the six engines tested under Contract EHS 70-110, "Baseline Characteri-
zation and Emissions Control Technology Assessment of HD Gasoline Engines,"
November 1972). This is the same engine rpm at which the present nine-mode
FTP is conducted. Subsequently, the percent power values corresponding to
3, 10, 16, and 19 in-Hg were determined for each individual engine. The
percent power values representing the different power points as presently
defined by manifold vacuum, were averaged together for each of the four power
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levels for all of the engines. The percent power points resulting were
10.8, 25,5, 55.0, and 89.5 percent. These percent power points are
analogous to the 19, 16, 10 and 3 in-Hg manifold vacuum respectively.
The 10 and 90 percent power points were verified. However, the above
analysis demonstrated that 25 percent and 55 percent should be used in
place of 30 percent and 60 percent power.
As can be seen from Figure 1, (Pg-4 ) there is considerable range in
manifold vacuum for a particular percent load point. In redefining the
3 in-Hg mode as 90 percent torque, a range of 1 1/4 to 4 3/4 inches of
mercury for manifold vacuum could be encountered. Also, at the 10 percent
torque level, formerly the 19 in-Hg mode, there is -1 to +1 1/2 in-Hg
variation from the 19 in-Hg reading depending on the particular engine.
The variability can be attributed to induction systems, valving, carburetion,
and timing.
Due to the variation, it might be expected that some large differences
in emission concentration will be encountered, particularly in the 10 and
90 percent torque mode. As can be seen from Figures 2, 3 and 4, (Pgs 5,6 and 7)
emission rates experience drastic changes. These changes in emissions
rates appear to be most serious in the proximity of the 10 and 90 percent
power level. When using the redefined power point of 10% (formerly 3 in-Hg),
a 1972 GM 427 CID, V-8 encountered decreases of 44%, 18%, and 35% in
the HC, CO, and NOx emission levels respectively. Also, at the 90% power
level (formerly 19 in-Hg), a 1972 GM 250 CID, 1-6 experienced increases of
41% and 63% in HC and CO, and a decrease of 14% in NOx. Although
the engines above are examples of extreme variation, it should be
expected that some engines will produce different emission levels when
tested using the percent load procedure. However, there is no simple solution,
and the 10, 25, 55, and 90 percent torque levels seem to be an adequate com-
promise.
^*-
The revised nine-mode gasoline test cycle would be as follows:
Observed \
Torque
Sequence (% of max.
No. Mode observed)
IIdle Idle
2 Cruise 25
3 PTA 55
4 Cruise 25 <
5 PTD 10
6 Cruise 25
7 FL 90
8 Cruise 25
9 CT CT
Using the above cycle and two EPA engines (1967 Ford 361 cu. in. V-8 and
1970 Chevrolet 350 cu. in. V-8) for testing, an operating procedure was
determined. The proposed nine-mode gasoline test cycle and procedural
changes to the regulations are attached (refer to the,appendix).
Manifold
Vacuum
in-Hg
Determined
At Time of
Test
1
1
Time in
Mode-sees.
70
23
44
23
17
23
34
23
43
Cumulative
Time-sees.
70
93
137
160
177
200
234
257
300
Weighting
Factors
.232
.077
.147
.077
.057
.077
.113
.077
.143
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-3-
Analysis of Time in Mode;
With regard to emission and fuel flow rates stabilization times, the
question arose whether the present times in mode for the nine-mode test
cycle were adequate in length for stabilization. Initially, a 1967 Ford
361 cu. in. V-8 and a 1970 Chevrolet 350 cu. in. V-8 were investigated
for their emission and fuel flow rates stabilization characteristics
during the nine-mode test cycle. The fuel flow rates stabilized in the
allotted times for all the modes and for both engines. However, the emis-
sion rates did not stabilize sufficiently for all the modes. Further,
there were no apparent trends when comparing the two engines.
It should be noted that inadvertently, the nine-mode FTP times in
mode, as they are currently structured, encourage the engine manufacturers
to have their engines stabilize quickly. That is, there is the possibility
that if the engine emission rates do not stabilize, the emission traces
could be integrated (integration is performed during the final three
seconds of modes one through eight) while they were still decreasing.
Higher values of emission rates would result, thus penalizing the manu-
facturer. It is therefore, to the engine manufacturers' advantage to
have their engines stabilize quickly after the transition from one mode
to another. This is to EPA's advantage and should not be done away with
arbitrarily.
In looking at several 1974 and 1975 gasoline engines, the emission
and fuel flow rates stabilization times were well within the present times
in mode. With this in mind, it would be very difficult to justify any
change in the times in mode as they now exist.
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-4-
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