011 Part2 Emissions and Fuel Economy Calculations

Emissions and Fuel Economy Calculations

Part II Instrumentation, Emissions Testing And Emission Standards

2103555 Engine and Emissions Control

Emissions Calculation : Bag Emissions Test• In each phase of a vehicle emissions test, the sampling

system fills one pair of bags; one bag is filled with dilute exhaust gas and the other with ambient air.

• Samples are analyzed as soon as possible after the phase ends to avoid the effects of gas reactions or permeation.

• After selecting and setting the analyzer ranges, the analytical system records the gas concentrations from the sniff sample and begins the zero/span calibration at the bench.

• After the zero/span calibration is successfully completed, the contents of the sample and ambient bags are pumped to the analyzers.

• The analyzers calculate concentration values by the use of bag mass equations

Emissions Calculation : Bag Emissions Test

CVS Bag Mass• The CVS concept for calculating the mass of pollutants

emitted by a vehicle during a "Bags Only" test (no modal sampling) involves subtracting the ambient mass from the diluted sample mass. Equation (1) shows this calculation:

where,• is the density of constituent x at standard conditions [kg/m3],• is the total CVS volume of dilute exhaust at standard conditions [m3],• is the sample bag (diluted exhaust) concentration of constituent x[ppm or %],• is the ambient bag concentration of constituent x [ppm or %], and• DF is the bag dilution factor

⎥⎦⎤

⎢⎣⎡

⎟⎠⎞⎜

⎝⎛ −⋅−⋅⋅=

DF

11CCVMass Ambient

xSamplex

CVSTotalxρ …..[grams] (1)

xρCVSTotalV

SamplexC

AmbientxC


CVS Bag MassDF is the bag dilution factor defined by Equation (2):

where,13.4 is the percent of CO2 in the exhaust under stoichiometric conditions,

is the sample bag concentration of CO2,is the sample bag concentration of HC, andis the sample bag concentration of CO.

( ) 4SampleCO

SampleHC

SampleCO 10CCC

4.13DF

2

−⋅++= …… (2)

SampleCO2C

SampleHCC

SampleCOC


FTP Composite Bag MassThe bag mass values obtained from Equation (1) are used to calculate an FTP-75 composite bag mass. This composite value is a weighted average of bags 1-3 as shown in Equation (3):

where,

are the bag mass values of constituent x for bags 1-3 [grams],

are the actual distances traveled by the vehicle during bags 1-3 [miles].

[grams/mile] …(3)


SFTP Composite Bag MassSupplemental Federal Test Procedure (SFTP) emissions are calculated by weighting NMHC, CO and NOx mass emissions from • the FTP (35%), • the A/C (SC03) test (37%), and • the high-speed/high-accel (US06) test (28%).

The weighted NMHC and NOx values are added to form a composite standard. When vehicles without air conditioning are tested, a 72% weighting factor is applied to the FTP composite value along with the 28% weighting factor for the US06 test.


SFTP Composite Bag Mass• For vehicles with air conditioning:

[4][5][6]

• For vehicles without air conditioning:[7][8][9]

Emissions Calculation : Bag Emissions TestNon-Methane Organic Gas (NMOG)• Emissions research showed that aldehydes and ketones possessed

a high ozone forming potential while methane was relatively nonreactive in the atmosphere.

• With this knowledge, the California Air Resources Board required all TLEVs, LEVs and ULEVs to be certified using non-methane organic gas (NMOG) bag mass standards.

• NMOG is defined as all nonoxygenated hydrocarbon constituents, except for methane, that contain 25 or fewer carbon atoms and all ketones, aldehydes, alcohols and ethers containing five or fewer carbon atoms.

• As shown in Equation (10), NMOG emissions for each bag are determined by subtracting methane hydrocarbons from the total hydrocarbon measurement and then adding aldehydes, ketones and alcohols.

NMOG = THC - CH4 + Aldehydes + Ketones + Alcohols (10)• The composite NMOG value is then calculated using Equation (3).

Emissions Calculation : Bag Emissions TestNon-Methane Organic Gas (NMOG)

Emissions Calculation : Bag Emissions TestReactivity Adjustment Factors (RAF)

• Reactivity adjustment factors are adjustments to certification NMOG tailpipe standards and emissions based on the ozone forming potential of the NMOG constituents.

• For TLEVs, LEVs and ULEVs operating on fuels other than gasoline, automotive manufacturers are required to multiply tailpipe NMOG emissions by RAF values.

Emissions Calculation : Bag Emissions TestReactivity Adjustment Factors (RAF)

• The California Air Resources Board (CARB) has established the RAF values shown in Table 1. However, CARB allows Manufacturers to use these RAF values or to generate their own reactivity adjustment factors.

Table 1 CARB Reactivity Adjustment Values

0.500.50 1.0 LPG

0.430.43 1.0 CNG

not applicablenot applicable not applicable Ethanol

0.410.41 0.41 M85

0.940.94 0.98 Phase 2 Gasoline

Light-Duty ULEVLight Duty LEV Light-Duty TLEV Fuel


Reactivity Adjustment Factors (RAF)The procedure for calculating and applying a RAF to tailpipe emissions is as follows:1. The gram/mile values of the individual hydrocarbon NMOG

constituents are multiplied by their respective Carter Maximum Incremental Reactivities (MIR) to determine their Ozone Forming Potential (OFP) in grams of ozone/mile. The MIRs are determined from mathematical models using laboratory reactivity data. The OFP for each constituent are then summed to achieve a total OFP.

….. (g O3/mi) (11)


Reactivity Adjustment Factors (RAF)

2. The total Ozone Forming Potential (g O3/mi) is divided by the total NMOG (g/mi) to yield a Specific Reactivity (SR).

(12)

3. The Specific Reactivity is divided by a Baseline Reactivity (BR) to obtain the Reactivity Adjustment Factor.

(13)

Modal Mass• The primary reasons for making modal mass

measurements are to understand how engine operating parameters affect exhaust emissions under dynamic conditions and to determine the effectiveness of exhaust gas after-treatment devices.

• To understand modal calculations, it is necessary to understand what happens to the volume of exhaust gas between the vehicle's engine and the CVS during a modal test.

Emissions Calculation : Modal Emissions Test

Modal Mass• The pathways for combustion gas as it is transported

from the vehicle's engine to the CVS during a modal test of dual catalytic converter system are shown belowed.


Modal Mass• During a dual catalyst test, four modal samples are

continuously extracted from the sample stream: – feedgas (VFG), - intermediate (VINT), – tailpipe (VTP) and - tracer (VTCR).

• The volume of gas emitted by the vehicle's engine and at various points along the exhaust stream can be determined by applying the principle of conservation of mass as shown in Equation 14.

VCVS = VENG - VFG - VINT - VTP + VDIL - VTCR (14)


Modal Mass• The undiluted modal extractions (VFG, VINT and VTP)

are transported from the sample point through heated lines and into a sample conditioning unit (SCU).

• In the SCU, water vapor condenses and is removed from the sample. The "dry" sample then proceeds to an analytical bench for measurement.

• Modal concentrations for each constituent, x, are calculated as "flow weighted" averages as shown in Equation 15:

(15)


Modal Mass• The concentrations are averaged over a 1 second period

using 10 Hz measurements. • Modal mass is determined by multiplying this

concentration by the density of the constituent being measured, the volume of the sample removed, and a water correction factor. This calculation is shown in Equation 16:

(16)

where (17)where,– H/C is the hydrogen to carbon ratio of the fuel, and– CCO2

is the concentration of CO2 in %.


O2Hxxx KVCModalMass ⋅⋅⋅= ρ

Modal Mass• The water correction factor is used to account for the

volume of water removed in the SCU. Without using this correction factor, the measured concentrations would be higher.

• The bag emissions must be corrected for the modal extractions. The modal correction for each constituent is equal to the mass of gas lost due to sampling at each of the analytical benches.

• For modal feedgas and intermediate sampling, it is assumed that the portion of contaminant gas lost through feedgas extraction would have been converted by the upstream catalytic converter(s) in the same ratio as gas that had actually gone through the converter.


Modal Mass• Under this assumption, the feedgas modal correction is:

(18)• For modal tailpipe sampling, the correction factor is

given by: (19)

• For modal CO2 tracer sampling, the correction must account for the dilution air:

(20)where R is the modal dilution factor defined below:

(21)• Modal calculations become more involved if the test

vehicle uses secondary air to assist in converting HC and CO in the catalyst.


Catalyst Efficiency• The basic calculation for conversion efficiency across a

catalyst is as follows:

• Pre-catalyst and post-catalyst refer to the modal sampling points in the exhaust stream. Some engineers prefer to analyze catalyst performance in terms of catalyst inefficiency, which is simply 100 minus percent efficiency.


Air-to-Fuel Ratio• The air-to-fuel ratio can be measured using an air-to-fuel sensor.

The Spindt formula also allows the engineer to calculate the air-to-fuel ratio based on engine emissions constituents:

where

where– HC = Carbon equivalent HC emissions percentage– CO = CO emissions percentage– CO2 = CO2 emissions percentage– K = Oxygen-hydrogen equilibrium constant = 3.5– c = Carbon weight percentage of fuel– h = Hydrogen weight percentage of fuel


Evaporative Emissions• The hydrocarbon mass calculation for enhanced evaporative

VV/VT SHED hot soak and diurnal emissions for gasoline fueled vehicles is given by

where– K = 297 is the ratio of the average HC molecule (CH2.3) for

evaporative emissions to the universal gas constant (R),– Ci is the initial hydrocarbon concentration [ppmC3],– Cf is the final hydrocarbon concentration in [ppmC3],– Pi is the initial barometric pressure [in Hg],– Ti is the initial temperature [oR], and– Vi is the initial SHED volume [ft3] given by Equation (28):

Vi = Vn - Vehicle Volume - Bag Precharge …..(28)– where

• Vn is the nominal SHED volume [ft3], and • Vehicle Volume = 50 ft3.


ECE (R48) and old EU Measurement Method (80/1268/EEC)

• European Union: TA up to 31 DEC 1995. FR up to 31 DEC 1996.• Average fuel consumption is determined in a test made up of 3 parts:

– 1. Urban cycle simulating urban driving– 2. Constant speed at 90 km/h– 3. Constant speed at 120 km/h

• Test results are averaged and expressed in l/100 km.

• For volumetric measurement fuel consumption (FC) is calculated:

FC = V (1 + a (T0 - Tf)) × 100 l/100 kmD

where– V = Volume in litres of fuel consumed– a = Coefficient of volumetric expansion of fuel– T0 = Reference temperature (°C)– Tf = Fuel temperature (°C) measured at volume measuring point– D = Distance covered during the test (km)

FUEL CONSUMPTION FUEL ECONOMY

Carbon Balance Fuel EconomyEU – New Measurement Method (93/116/EC)• Implementation: TA mandatory:1/96, FR mandatory: 1/97• Fuel consumption is determined according to the carbon

balance method from the CO2, CO and HC emissions measured in g/km during the urban/extra-urban cycle.

FC (gasoline) = 0.1154 1) [(0.866×HC) + (0.429×CO) + (0.273×CO2)]D

1) Diesel:0.1155, FC = Fuel consumption in l/100 km, D = Fuel density


Carbon Balance Fuel EconomyU.S. – Fuel Economy Measurement Methods• The carbon balance method is used for determining EPA fuel

economy. The calculation is based on a carbon mass balance; morethan 99.9% of the carbon in the fuel appears in the exhaust gas as CO2, CO, HC and oxygenated hydrocarbons. The equation used to calculate EPA fuel economy for gasoline fueled vehicles is:

FE (gasoline) = 5174×104×CWF×SG .[(CWFxMHC)+(0.429×MCO)+(0.273×MCO2)]×[(0.6×SG×NHV)+5471]

where,– CWF is the carbon weight fraction of the test fuel– SG is the specific gravity of the test fuel– NHV is the net heating value of the test fuel [Btu/lbm]– MHC is the total hydrocarbons [g/mi] in the exhaust gas– MCO is the carbon monoxide [g/mi] in the exhaust gas– MCO2 is the carbon dioxide [g/mi] in the exhaust gas– 0.6 is defined as R, the percentage change in fuel economy

relative to the percentage change in volumetric heat content (Btu/lbm).


Carbon Balance Fuel EconomyU.S. – Fuel Economy Measurement Methods• The equation used to calculate EPA fuel economy for diesel fueled

vehicles is:

FE (diesel) = 2778 .(0.866×MHC) + (0.429×MCO) + (0.273×MCO2)

• Combined fuel economy value is calculated in miles/gallon by applying the results from US EPA III and Highway (HWFET) Cycle:

HWFET [mpg] = 1 .0.55 / EPA III + 0.45 / HFE

• Conversion: Fuel consumption in l/100 km to fuel economy in mi/gal

FC (l/100 km) = 235.215 .FE (mi/gal)


What is CAFE?• Corporate Average Fuel Economy (CAFE) is the average

fuel economy of all the passenger automobiles or light trucks in a manufacturer’s fleet for a given model year.

CAFÉ Testing and CAFE Numbers• In U.S.,each vehicle’s fuel economy values are based on the results

of tests required by the U.S. EPA. • During the test program, prototypes are driven by a professional

driver under controlled laboratory conditions, on a dynamometer.Two tests are conducted: an urban or “city” cycle which includes a cold start, stop-and-go driving, and periods of idling, and a “highway”cycle that consists primarily of steady-speed driving.

• These tests are used to certify that vehicles meet the U.S. Federal emissions standards, but they also yield fuel economy figures.

• Manufacturers test pre-production prototypes of the new car models and submit the test results to U.S. EPA. U.S. EPA then confirms the accuracy of the figures provided by the manufacturers.

CAFE Testing And Adjustment

CAFE Calculation• The city and highway fuel economy test results are “harmonically”

averaged in a ratio of 55 percent city and 45 percent highway to get the combined fuel economy for each model. Then, the production-weighted harmonic average of the combined fuel economy values for all the models in a manufacturer’s fleet is calculated.

• The harmonic average is equivalent to averaging the reciprocals of the fuel economy values of the vehicles (ie, the fuel consumption in gallons per mile). This calculation weights the lower fuel economy vehicles more heavily.

• For example, a manufacturer of 2,000 vehicles, half of which achieve 20 mpg and half of which achieve 30 mph, would have a CAFE of:


Criteria for Establishing or Amending CAFE Standards• The standard must be set through rulemaking (public

notice and comment) at the “maximum feasible average fuel economy level” taking into account these criteria:

• Technological feasibility;• Economic practicality;• The effect of other Federal motor vehicle standards

on fuel economy; and,• The need for the nation to conserve energy.


Credits/Penalties, Carryforward/Carrybackward• Compliance is determined first by considering each manufacturer’s

actual average fuel economy achievement for the model year (as measured by U.S. EPA), and comparing that to the applicable standard.

• If the achievement is greater than the standard, the manufacturer is in compliance, and the law permits the surplus to be used as credits to be carried forward or backward up to three years to help offset shortfalls in other years. These credits can only be used to offset shortfalls; they cannot be cashed or traded.

• If the achievement is less than the standard, the manufacturer may still comply with the law by the use of carryforward credits earned during the prior three years, or carrybackward credits projected to be earned in the next three years. If those credits are not sufficient for fully offset the shortfall, then the manufacturer is subject to civil penalties.


Credits/Penalties, Carryforward/Carrybackward• For U.S. case, the amount of the civil penalty is set by

statute, and is subject to alteration by NHTSA only under very limited circumstances. The formula is:Penalty = $5.50 x (Standard - CAFE) x 10 x Number of

Vehicles Sold - Available Credits

• Example: If a manufacturer’s CAFE is 26.5 mpg, with 3.5 million cars in its domestic passenger car fleet, the CAFE standard is 27.5 mph, and the manufacturer has no credits, then the penalty to the manufacturer is:

Penalty = $5.50 x (27.5 - 26.5) x 10 x 3.5 Million

= $ 192.5 Million


011 Part2 Emissions and Fuel Economy Calculations

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