Investigation into
Hydrogen Sulfide Odor Problem
on a Privately-Owned, Catalytic Converter-Equipped
1975 Automobile
June 1975
Technology Assessment and Evaluation Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Environmental Protection Agency
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Background
The Michigan-Ohio District Office of the Environmental Protection
Agency was contacted in April 1975 by a person living in the Cleveland,
Ohio area about the unpleasant odor emanating from his brand-new 1975
catalytic converter-equipped Plymouth Gran Fury. The "very pungent
and acrid odor" had been noticeable from the time the owner took
delivery of the car, and was at its worst when the car was idling or
moving in dense traffic, according to the owner. Along with the
unpleasant odor, irritations of the throat and nose were experienced
by the owner and his family. Taking the car to the dealer did not
result in any alleviation of the odor, although a driveability problem
(hesitation on tip-in) was solved.
District Office personnel contacted the Emission Control Technology
Division for technical advice on the problem. Descriptions of the
odor and the driving modes when it occurred led EPA to speculate that
hydrogen sulfide (H~S) was being formed over the oxidation catalyst due
to the carburetor being set too rich. On advice of EPA the owner
changed brands of gasoline, seeking one with lower sulfur content,
but no reduction in the odor occurred.
The situation was seen by EPA personnel as an opportunity to
look into the cause of recently reported odor problems on in-use
catalytic converter-equipped vehicles, and to respond in a helpful
manner to a concerned citizen, so the owner was invited to bring his
car to the Motor Vehicle Emission Laboratory (MVEL) for a thorough
investigation. This repbrt covers that investigation, which took
place in early June 1975.
Vehicle Description
The car was a 1975 Plymouth Gran Fury sedan, with a 360 cubic
inch engine and oxidizing catalytic converter. Optional equipment
included automatic transmission, power brakes, power steering and air
conditioning. The car is described in detail in the table on the
following page.
Test Procedure
Tests included the following: driveability, visual inspection,
state of tune in as-received condition, and a hot start 72 FTP exhaust
emissions test. After the car was tuned to manufacturer's specifications
another hot start 72 FTP was run.
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TEST VEHICLE DESCRIPTION
Chassis model year/make - 1975 Plymouth Gran Fury
Emission control system - Oxidation Catalyst
Engine
type 4 stroke Otto cycle, OHV, V-8
bore x stroke 4.00 x 3.58 in./101.6 x 90.9 mm
displacement 360 CID/5900 cc
compression ratio . '. 8.40:1
maximum power @ rpm 170 hp/127 kW @ 4000 rpm
fuel metering Holley 2 bbl carburetor
fuel requirement .91 RON unleaded
Drive Train
transmission type 3-speed automatic
final drive ratio not known
Chassis
type body/frame, front engine, rear wheel drive
tire size' !!'.!!'..'!!.'!! GR 78 x 15
curb weight . not weighed
inertia weight 500° lb-
passenger capacity 6
Emission Control System
basic type oxidation catalyst, engine modifications,
exhaust gas recirculation, positive
crankcase ventilation, evaporative
hydrocarbon emission control
Manufacturer's Tuning Specifications
basic timing 6° BTDC
engine idle speed 750 rpm in Neutral
idle CO concentration 0.3% (measured at upstream end of converter)
As-Received Tuning Conditions
odometer reading 2460 miles
basic timing 10° BTDC
engine idle speed 1200 rpm in Neutral
idle CO concentration 9.0%
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Results
In the on-the-road driveability test the car was rated as good,
with no hesitation, stumble, surge, or run-on. The visual inspection
was performed with the vehicle at floor level and on a lift. No holes
were seen in either body or trunk sheet metal. The exhaust system
was in good condition, although the threaded plug in the catalytic
converter (removable for idle CO analysis) was only finger-tight. The
inside of the tailpipe was covered with soot, as was the inside of
the converter plug. On the carburetor, the limiter caps were missing
from the idle adjustment screws.
The car's as-received state of tune was checked by EPA personnel
using a Sun Model C-65 Infra-Red Engine Performance Tester. It was
found that idle speed was 1200 rpm and basic timing was 10° BTDC,
both of which were outside of the manufacturer's specifications. Idle
CO concentration was 9% at the sample point in the upstream end of
the converter and at the tailpipe.
When the car was driven from the tune-up area to the chassis dyna-
mometer test cell the rotten-egg odor characteristic of H_S was
detected by the driver and other personnel.
A hot start 72 FTP was run on the car in the as-received
condition. Results are listed below.
As Received
72 FTP Hot Start
Mass Emissions
grams per mile
( (grams per kilometre)
Fuel Economy
Test No. HC_ CO NOx (Fuel Consumption)
16-9340 1.92 93.63 2.54 11.78 miles/gallon
(1.19) (58.18) (1.58) (19.97 litres/100 kilometres)
The engine was then tuned to manufacturer's specifications (see
Test Vehicle Description) and another hot start 72 FTP was run, with
the following results.
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Tuned to Manufacturer's Specifications
72 FTP Hot Start
Mass Emissions
grams per mile
(grams per kilometre)
Fuel Economy
Test No. HC (X) NOx (Fuel Consumption)
16-9341 .13 6.24 3.38 11.77 miles/gallon
(.08) (3.88) (2.10) (19.98 litres/100 kilometres)
After the car had been tuned to manufacturer's specifications, no
H_S odor was detected. However, driveability problems (hesitation and
run-on) were noted.
Conclusions
The cause of the H2S odor problem was the extremely rich carburetor
setting. The idle CO concentration of 9% indicates that the air-fuel
(A/F) ratio was about 11:1, whereas for this particular car it should
have been about 15:1. From the tailpipe idle CO concentration of 9%,
the soot in the tailpipe, and the hot start 72 FTP mass emissions of
almost 94 grams per mile (gpm) CO, it is obvious that the catalyst
could not promote oxidation, because insufficient oxygen was present
in the engine exhaust. It was however causing H«S to be formed, in
line with reported experimental work.Q'
When the air-fuel mixture was leaned out to the proper setting
the catalyst was able to light off and do the job for which it was
intended, thus the low HC and CO emissions in the second test.
Retarding the timing from 10° to 6° BTDC would ordinarily tend to
reduce NOx formation. However in this case NOx formation was suppressed
by the rich mixture, so that the effect of leaning out the A/F ratio
was greater than retarding the timing and NOx increased from 2.54 to
3.38 gpm.
The effect of the retuning on the car's fuel economy was negligible.
The absence of idle adjustment screw limiter caps suggests that they
were removed in order to solve lean operation driveability problems.
(1) Barnes and Summers, "Hydrogen Sulfide Formation Over Automotive
Oxidation Catalysts," Society of Automotive Engineers Paper No.
750093, February 1975.
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5
This investigation supports previous analyses and data (SAE 750093),
and suggests that H2S problems encountered with in-use cars are due to
mixture enrichment (and/or air injection malfunction) rather than
catalytic converter malfunction. The cause of H2S formation is believed
to be either poor quality control at the factory or maladjustment
of air-fuel ratio arising from attempts to correct driveability problems.
If this condition - the tradeoff between driveability and H2S formation! -
is widespread among in-use cars then the solution will require proper
training and equipping of mechanics.
U.S. GOVERNMENT PRINTING OFFICE: 1975-650-518/ 1
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