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Emissions Testing

How full emissions tests are done.

Courtesy NREL

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This article was first published in 2002.

The US National Renewable Energy Laboratory runs emissions tests on cars using alternative fuels. This story, sourced from the NREL, shows the steps in that process, which are very similar to normal legal emissions tests undertaken by manufacturers in countries around the world.

The Soak Room

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As part of the US federal emissions testing procedure (FTP), vehicles must sit (soak) for a period of time at ambient temperature before they are put into a SHED for evaporative testing or on the dynamometer for emissions testing. This happens in the "soak room."

Before testing, we must "precondition" each test vehicle. Preconditioning includes procedures such as draining the fuel tank and filling it with test fuel, purging the evaporative canister, performing a heat build on the fuel tank, and running driving cycles on the dynamometer. Preconditioning ensures that the emissions measured are from the test fuel and that the fuel that was previously in the tank has no effect. Following the full prep procedure, we allow the vehicle to cool in the soak room for 12 to 36 hours before testing. The soak period allows the vehicle's engine to stabilize to ambient temperature.


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The vehicles are fuelled here during the testing process. The hoses you see dispense specially blended test fuels that are stored in temperature-controlled rooms. For some of the test procedures, the fuel must be cooled to below ambient temperature. In the fuel storage rooms you will find gasoline, methanol (M85), ethanol (E85), or other fuel blends.

Evaporative Emissions

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The SHED (sealed housing for evaporative determination) test is performed in an airtight room to determine the evaporative hydrocarbon emissions a vehicle emits when it isn't running. The diurnal (ie day/night) breathing loss and hot soak tests are evaporative tests. We conduct the diurnal test before the federal emissions test procedure, after the vehicle has been in soak for 12 to 36 hours. Then we fuel the vehicle with the 12.8 degree C (55 degrees F) test fuel and push it into the SHED. The diurnal test lasts for one hour, during which the fuel tank is heated from 15.6 to 28.9 degrees C (60 degrees F to 84 degrees F).

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To accomplish this, we install a temperature sensor in the fuel tank and place a heater under the tank. The sensor sends the current fuel tank temperature reading to a computer, which in turn controls the heater to raise the fuel temperature at the desired rate. We take samples of the air in the SHED for analysis at the beginning and end of the hour.

Immediately after the dynamometer emissions test, while the engine and other components are still warm, we perform the hot soak test. For this test, the vehicle is driven into the SHED and samples are taken at the beginning and end of the test. The hot soak test takes an hour; no "heat build" on the tanks is necessary.


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The chassis dynamometer is used for exhaust emissions tests. We roll the vehicle onto the chassis dynamometer, which allows the wheels to turn and uses inertia weights and horsepower settings to simulate real world driving conditions.

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We attach a heated transfer hose to the tail pipe to collect the exhaust gases and direct them into the constant volume sampler (see breakout box). This system dilutes the exhaust with ambient air and directs a portion of the mixture for analysis. A heated hose is especially important when we are testing methanol fuel vehicles, because exhaust from methanol vehicles usually has a higher vapour content than that emitted by gasoline vehicles. If the temperature of the transfer hose is lower than the dew point of the exhaust gas, water may condense on the walls of the hose. Because methanol and formaldehyde are soluble in water, you can lose a significant amount of these compounds in the condensation on the hose walls. By preventing the condensation, a heated line ensures an accurate sample.

Constant Volume Sampler (CVS)
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Motor vehicle exhaust is a complex mixture of unburned fuel, combustion products (including a high water vapour content), and intake air that exits the tail pipe at high temperatures. In simulated driving conditions on the dynamometer, the volume of the exhaust emitted from the tail pipe is continually changing depending on where in the driving cycle the vehicle is (for example, during acceleration, during deceleration, at constant speed, or at idle).

To quantitatively determine emission rates from vehicles, we use a constant volume sampling system, which maintains a constant total flow rate of vehicle exhaust plus dilution air. With a CVS system, as exhaust flow increases, such as during heavy acceleration, the dilution air is automatically decreased and the sampling source is representative of exhaust variations.

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Once the vehicle is properly mounted on the dynamometer with the sampling system in place and all the controls and instrumentation are set, we drive the vehicle following a speed-versus-time trace known as a driving cycle. To do this, the driver starts the engine and attempts to follow the driving cycle shown on a computer monitor by accelerating and braking the vehicle. Some typical driving cycles include the FTP-75, IM240, HWFET, US06 (aggressive driving cycle), and the Cold CO.

Control Room

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The control room contains the equipment used to control the dynamometer and analyse the exhaust gas for regulated emissions. We route a portion of the diluted exhaust from the constant volume sampler system to fill large TedlarŽ bags. TedlarŽ is a thin film made of polyvinyl fluoride. Because of its high tensile and tear strength, thermal stability, and chemical inertia, it is an ideal material for collecting exhaust samples. Its chemical inertia property minimizes the sample loss caused by compounds clinging to or reacting with the inner surface of the bag. We run samples from each bag through different analysers to measure the amounts of regulated components, such as total hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).

Impinger Cart

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The impinger cart is a mobile system used for extracting aldehydes and alcohols from the emission gas. The yellow liquid is a solution designed to collect the desired components from gas as it bubbles through the impinger. After the test is completed, the impingers are taken back to the laboratory, where the solution is prepared for analysis on the liquid chromatograph.

Aldehydes, which are primary decomposition products from combustion of alcohol fluids, are considered air toxins. Alcohol emissions come from unburned alcohol fuel. Detailed measurement of these constituents is important for understanding the complete emission picture.

Analytical Lab

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Here in the analytical lab, we conduct detailed analyses of the exhaust gases, including the quantification of detailed hydrocarbons, alcohols, and aldehydes. Exhaust gas may be composed of hundreds of different hydrocarbon compounds. Using a gas chromatograph (GC), we perform an analysis for each of these separate hydrocarbon components.

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Because some of these compounds, such as benzene and 1,3-butadiene, are considered toxic to humans, they are regulated by the government. The types of hydrocarbons emitted can be very different from fuel to fuel. The GC gives a profile of the hydrocarbon emissions known as a chromatogram. Details from this profile are used to compare how the hydrocarbon emissions from different fuels react to form ozone.

The liquid samples from impingers are placed in separate vials and analysed using a liquid chromatograph. These are analysed for aldehydes - such as formaldehyde - that may be present in the exhaust.

Test Results

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Test data sheets are generated at each step of the process. After testing is completed, we input all the data into a computer for final calculations. NREL staff members tabulate, analyse, and prepare reports summarizing the data.

Here is an example test data sheet:

Project Number: XXX

Test Date : 9/03/97

Make: Dodge Model: Ram 250

Tag #: G42-64706

VIN #: 2B4HB25T7NK135233

Year: 1992

Engine: 5.2 L V8 CNG

Odometer: 5481

Device(s): CNG factory conversion

# cylinders: 8

Inertia Weight: 4750

Indicated Load: 14.1

Fuel System: CNG

Trans. Type: Automatic



Mass Emissions Mass Emissions  
Exhaust Dilution Air Mass Emissions
Concentrations Concentrations Concentrations

Start Time: 3:30 PM

Stop Time: 4:11 PM

Cold Transient Phase

Barometric Pressure: 761.7

Relative Humidity: 21.87

Sample (CVS) Temp.: 91.3

Ambient Temp. (c): 24.96

Vmix: 2670

Miles: 3.60

THC 148.9753 PPM

THC 5.3938 PPM

THC 7.2056 GMS

CH4 124.1221 PPM

CH4 2.7490 PPM

CH4 6.1433 GMS

NOx 23.2068 PPM

NOx 9.4885 PPM

NOx 2.7186 GMS

CO 178.3837 PPM

CO 0.9255 PPM

CO 15.6344 GMS

CO2 1.4878 %

CO2 0.0521 %

CO2 1998.8563 GMS

Stabilized Test Phase

Barometric Pressure: 761.7

Relative Humidity: 21.63

Sample (CVS) Temp: 110.2

Ambient Temp. (c): 25.75

Vmix: 4494

Miles: 3.89

THC 25.5160 PPM

THC 6.2428 PPM

THC 1.6883 GMS
CH4 21.9231 PPM

CH4 3.0060 PPM

CH4 1.6330 GMS
NOx 3.8560 PPM

NOx 0.5137 PPM

NOx 0.6848 GMS
CO 35.2783 PPM

CO 1.3598 PPM

CO 5.0470 GMS
CO2 1.0245%

CO2 0.0521 %

CO2 2278.7788 GMS

Hot Transient Phase

Barometric Pressure: 761.4 Relative Humidity: 21.33  
Sample (CVS) Temp.: 105.9 Ambient Temp. (c): 26.7  
Vmix: 2627 Miles: 3.60  
THC 84.7822 PPM THC 5.3938 PPM THC 3.9689 GMS
CH4 80.2993 CH4 2.0470 PPM CH4 3.8549 GMS
NOx 14.3281 PPM NOx 0.5137 PPM NOx 1.6452 GMS
CO 47.8457 PPM CO 1.3598 PPM CO 4.0429 GMS
CO2 1.3561% CO2 0.0502% CO2 1788.4855 GMS

Weighted Result

THCwm = 0.9398
CH4wm = 0.8629
NMHCwm = 0.0769
NOxwm = 0.3722
COwm = 1.8766
MPG = 12.29


So as you can see, a full manufacturer-level emissions test is a complicated procedure that requires a laboratory of test and analysis equipment. It?s sure not something that your local modification workshop is equipped to do...

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