EPA Testing Methods Posted by Mark Shaw Around 1994 But Presumed To Still Be Valid |
Gas mileage
(This was not posted by Mark.) A Chrysler engineer told us that EPA gas mileage standards were originally developed based on a mid-1960s project where surveyors were sent to all major United States cities to discover the conditions of a typical commute. It should be noted that traffic was much lighter at that time, and that the highway test does not reach up to current average speeds (that is, did not exceed 70 mph). Thus, the test tends to be very optimistic, and does not give ample credit to cars that do very well on stop-and-go driving with long idle times; not does it measure adequately cars that have good gas mileage at 55, but poor mileage at more typical highway speeds. The result is that cars like the Spirit R/T, which use a surprising amount of fuel in typical stop and go traffic, benefit, while hybrids, which do surprisingly well in stop and go traffic, do not (though they benefit greatly on the highway tests!). The test also does not adequately measure the effect of variable displacement systems.
EPA Federal Test Procedure (FTP75)
Used to certify new vehicle designs and to check compliance of design over a period of time. These are done on only a few vehicles of each model. The test:
Run on a simulated driving cycle lasting 2477 seconds and 11.1 miles. It starts with a cold start after an overnight cool down (12 hours) at an ambient temperature of 20-30C. Test includes a shutdown for 10 minutes with a repeat of the first 505 seconds after a hot restart.
The overall average speed is about 21.1 mph with maximum of 56.7 mph and about 17.9% of the time the engine is running is spent at idle conditions.
The measurement method uses diluted air with a makeup pump supplying a known quantity. Samples of known percentage are collected in bags during various phases of the test. Total mass of each emission is calculated and averaged over the total test distance for data in grams/mile.
There is some empirical adjustments made for the vehicle weight (for dynamometer loading) and total dilluted air to be included.
For more detail see the SAE Handbook, Section 13. Particularily the following standards/practices: J254, general testing; J1094 - CVS testing; and J1506 - the FTP75 driving cycle.
Test limits are expressed in grams/mile and depend on vehicle class, weight, and year of manufacture. Generally speaking, light trucks are allowed 1.5-5X what passengers cars are allowed. Heavier versions of light duty trucks are allowed even more. For the 1981-present model years which are subject to local IM240 testing the limits look like:
Year For Cars HC CO NOx Total 1981-95 0.41 3.4 1.0 4.81 1994-95 0.25 3.4 0.6 4.25 (1994 starts phase in of Tier I standards to be complete by 1996) For "Light" Trucks Year Weight HC CO NOx Total 1981-83 any 1.7 18 2.3 22.0 1984-87 any 0.8 10 2.3 13.1 1988-90 any 0.8 10 1.2 12.0 1991-93 Under 3750 lbs GVW 0.8 10 1.2 12.0 1991-93 Over 3750 lbs GVW 0.8 10 1.7 12.5 1994-95 3751-5750 lbs GVW 0.32 4.4 0.7 5.42 1994-93 6000-8500 lbs GVW
and 3751-5750 Test0.32 4.4 0.7 5.42 1994-93 6000-8500 lbs GVW
and >5750 Test0.39 5.0 1.1 6.49
Starting in 1994 the HC are reduced by excluding the methane gases and using NMHC grams/mile instead of total grams HC.
Also in 1994 onwards there are slightly higher limits if the manufacturer will guarantee the system for 10 years/100,000 miles.
Passing the EPA FTP75 test and standards is NOT a trivial test that can be easily fooled.
Local IM240 Testing
Local test procedure used to more fully test the complete operation of 1981 and later cars and light trucks. Loading done on a dynamometer for a total test time of 240 seconds. The overall average speed is around 30 mph (should follow profile within 3 mph) so the total distance is about 2 miles. Speeds to 56.7 mph and with only about 3.8% of the time spent at idle.
A CVS (Constant Volume Sampling) method like the EPA FTP75 is not used, but rather a flow meter is employed to get average total gas flow and to calculate the total grams emitted.
Test limits are expressed in grams/mile and depend on vehicle class, weight, and year of manufacture. Generally speaking, light trucks are allowed 2X what passengers cars are allowed. Heavier versions of light duty trucks are allowed even more. For the 1981-present model years which are subject to local IM240 testing the limits look like:
The limits are considerably HIGHER than the EPA FTP75 cycle certification limits. The limits are to get tougher in 1997 with about a drop of 50%.
For Passenger Cars:
Year HC CO NOx Total
1981-82 2.0 60.0 3.0 64.2
1983-90 2.0 30.0 3.0 35.0
1991-96 1.2 20.0 2.5 23.7
(1982-84 high altitude passenger cars allowed higher limits) For "Light" Trucks
Year Weight HC CO NOx Total
1981-83 any 7.5 100.0 7.0 114.5
1984-87 any 4.0 90.0 7.0 101.0
1988-90 <6000 GVW 3.2 80.0 3.5 86.7
1988-90 >6000 GVW 3.2 80.0 5.0 88.2
1991-96 <6000 GVW 2.4 60.0 3.0 65.4
1991-96 >6000 GVW 2.4 60.0 4.5 66.9
(high-altitude light duty trucks are allowed higher limits)
Above limits are as used in Arizona, other states may vary somewhat.
The IM240 is always done on cars which are warmed up. Since the EPA FTP75 cycle has a cold start that can account for up to 60-80% of the total HC and CO from the test, then warm running emissions are usually less than 1/2 of the limits for the whole test. This makes the local IM240 limits even higher in relation to the EPA FTP75 cycle.
Passing the IM240 test and standards is not easy if your vehicle is a high fuel consumption model. Fuel efficient models which passed older tailpipe concentration tests will probably pass the IM240 unless the NOx is high due to a bad EGR function.
Local Tailpipe Concentration Testing
This has been the usual methods of testing cars locally and may only consist of an idle test or may be combined with a "light cruise" test on a dynamometer at about 20-30 mph steady state.
Tailpipe concentration testing is merely a "state of tune" test to find grossly out-of-adjustment engines. It is of little value in comparing emissions between two vehicles, unless both are non-catalyst vehicles with identical readings -- in this case the relative emissions are going to be almost directly related to the fuel consumption of the two vehicles. Trying to compare emissions between two technologies (even if they have the same tailpipe concentrations) is not a sure thing since you cannot determine the specific emissions rates (in grams/gallon). Some general observations are that most post 1980 cars will have about 1/10 the tailpipe concentrations of pre-1975 cars. The 1975-1980 models vary with the technology, since they are mostly open-loop.
Typical test limits for tailpipe concentrations (in Arizona) are as follows (note the included post-1980 limits before IM240):
Idle Cruise HC CO HC CO Year ppm % ppm % 1967-1971 450 5.00 --- --- 1972-1974 400 5.00 --- --- 1975-1978 250 2.00 --- --- 1979 220 2.00 --- --- 1980 220 1.20 --- --- 1981-1994 220 1.20 220 1.20
Pre-1981 were not normally cruise tested. NOx is also not normally tested. Also, it is possible that test stations using the IM240 test equipment may infer the tailpipe concentrations from total emissions during the sample time. This is not an exact correlation.
Comparing Two Vehicles
I have long used a simple gallons in, emissions out model for some time, but am trying to get a more accurate model. Bosch has published some data showing typical emissions and fuel consumption which can be used to define rates in grams/gallon. These rates are dependent on the air-fuel ratio which can be determined from the CO concentration before a catalyst. Then
a catalyst efficiency can be applied.
It is not a simple relationship such that vehicle A's mpg and emissions concentrations to those of vehicle B can be easily compared. If you limit the operation of both to the ideal air-fuel ratio (about 14.7, but varies with the fuel blend) then you could use the following general relations:
Non-Catalyst Emissions at A/F = 14.7
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HC output is approximately 17 grams/gallon
CO output is approximately 383 grams/gallon
NOx output is approximately 66 grams/gallon
Total emissions about 466 grams/gallon
Post-Catalyst (3-way) Emissions for A/F = 14.7
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HC output is approximately 1 grams/gallon
CO output is approximately 62 grams/gallon
NOx output is approximately 5 grams/gallon
Total emissions about 68 grams/gallon
With these numbers you can theorize all sorts of who's the worst conditions. If you then allow one of more to become detuned then the emissions rates go up for both cases. To further complicate the comparison, you need to compare exact technologies -- i.e. a two-stage catalyst with air-injection can have lower CO and HC for rich mixtures than a typcial 3-way catalyst which relies on mixture cycling. There or lots of other cases, as well. About the only vehicles than can be conveniently compared are those with engines less than 3-liters, with or without 3-way catalysts; and any engine without any aftertreatment.
Probably more than anyone wanting to read, but it points out the complexity of the subject; and the enormous possibility for misinterpretation.
When I hear someone say their 19xx YYYY passed the test for 1995 cars with flying colors... I wonder what really happened...... WAY too much to guess at in such a statement....
Mark
