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EPA Testing Methods

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. Traffic was much lighter at that time, and the highway test did not exceed 70 mph. Thus, the test was, before 2008, optimistic for most cars, and applying a simple correction factor would result in more bias. The 2008 and newer test is more realistic for most drivers, including air conditioning, faster speeds, and more time spent stopped at simulated traffic lights.

EPA Federal Test Procedure (FTP75) for emissions compliance

These are done on only a few vehicles of each model. The test is run on a simulated driving cycle lasting 2,477 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 around 17.9% of the time, the engine is at idle.

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 are some adjustments made for the weight (for dynamometer loading) and total diluted 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 class, weight, and year. Generally, light trucks are allowed 1.5-5X what passengers cars are allowed (this was phased out). Heavier versions of light duty trucks are allowed even more. For the 1981-1995 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, completed 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 Test
0.32 4.4 0.7 5.42
1994-93 6000-8500 lbs GVW
and >5750 Test
0.39 5.0 1.1 6.49

Starting in 1994, the HC was reduced by excluding the methane gases and using NMHC grams/mile instead of total grams HC; and there were slightly higher limits if the manufacturer guaranteed the system for 10 years/100,000 miles [this might be mandatory now for all cars]. Passing the EPA FTP75 test and standards is not a trivial test that can be easily fooled.

Local IM240 Testing

This is a local test used to more fully test the complete operation of 1981 and later cars and light trucks. Loading is done on a dynamometer for a total test time of 240 seconds. The overall average speed is around 30 mph (following a profile within 3 mph) so the total distance is about 2 miles. Speeds go to just 56.7 mph and only about 3.8% of the time is spent at idle.

A CVS (Constant Volume Sampling) method like the EPA FTP75 is replaced by a flow meter to get average total gas flow and to calculate the total grams emitted. Test limits are expressed in grams/mile. Generally speaking, light trucks are allowed twice what passengers cars are allowed [this has changed since that time]. 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, warm running emissions are usually less than half 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):

Year Idle HC ppm Idle CO % Cruise HC ppm Cruise CO %
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 cars were not normally cruise tested, and NOx is not normally tested. 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

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:

Grams/Gallon (Air:Fuel = 14.7)HCCONOxTotal
Non Catalyst Emissions1738366466
3-Way Catalyst Emissions162568

With these numbers you can theorize all sorts of who’s-the-worst conditions. If you then allow one or 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.

What it’s like to do the testing

Marc Rozman said:

You drive the EPA cycle on the chassis rolls, they give you a TV screen, and it’s like a big video game. You have to pull a car in cold, after a 24 hour soak [sitting without running], you pull a vehicle in, strap it down on the chassis rolls, hook up a tail pipe, and sample. We have a big monitor on a set of casters, and we pull it by the car.

You start by doing a cold start on the vehicle, and there’s a trace on the screen, with two dots, and as you accelerate, the two dots move along with the trace. The trace comes up on the screen like a video game, you have to keep those two dots on the trace as you accelerate and brake according to the trace on the screen, that’s all part of the EPA cycle. It runs for 30-35 minutes, and then you have to stop the car, ten to twelve minutes soak [stop], restart the car, another ten minutes of driving, and that’s it. They sample exhaust gas and they judge our driving by keeping those dots on the trace of the screen.

You’re allowed so many errors, and as long as the driver did a good job, and as long as emissions were collected and came all correct, the car passed audit. If it failed, it went to a separate building where they would break it in more, run the engine more, and any big problems, correct per production specs, and then test it again.

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