1. Institute for Internal Combustion Engines and Thermodynamics
Analysis of ACEA proposals for introducing a transfer function for emission correction between moderate and extended driving conditions S. Hausberger, S. Lipp
Stefan Hausberger
RDE test procedure 1

2. The presentations aims at:
Analysing the background and rationale of a Transfer Function
Considering the possible options for setting up such a function
Analysing open issues
Suggesting work plan and time table

3. Background: effects not covered by the actual normalisation tools
The normalisation tools adjust emissions measured in PEMS trip to normal conditions
+ Normal power distribution (CLEAR)
+ Normal CO2/km levels (EMROAD)
NOx emissions from diesel cars depend beside engine power also on following parameters which are not considered in the actual normalisation tools:
Driving dynamics: high power gradients limit proper controlling of EGR, of NH3 fill levels of the SCR and of NSC regeneration.
High power and rpm levels: occurring at high altitude differences (m/100km), high loading, high speeds, high driving dynamics high power limit EGR rates, high rpm increases space velocity in catalysts (conversion efficiency is typically reduced at higher space velocities, low temperatures and low NH3 fill level in case of SCR)
The “ideal” normalisation tool would also consider these effects to allow low CF limit values for RDE testing in combination with cost efficient technology selection (no need to adjust hardware to worst case test condition combination which hardly occurs in real world driving).

4. Background: Idea of a “Transfer Function”
Working hypothesis: a “Transfer Function” in addition to the existing normalisation tools could reduce variabilities in emission results due to effects not covered yet.
Assumption: additional correction of emissions or adjustment of CF allows an agreement on lower base CF value since margins for influence of driving conditions not considered yet are then lapsed.
Several options for the method exist, below one of them is shown as schematic example.
“Severity” = function of e.g.: *95 percentile of v x apos, *Altitude to be overcome (m/km), *Weight of vehicle + loading, *Ambient temperature, * Maximum velocity

5. Examples of NOx emissions EU6 #01, JRC test at EURO 6 diesel car without NOx aftertreatment
Measured NOx (“Mean”) correlated with R² = 0.73 to 95 percentile of v*a<0.1 After CLEAR evaluation (“weighted”) the correlation as well as the slope is lower but not eliminated

6. Examples of NOx emissions EU6 #02, JRC test at EURO 6 diesel car with SCR
Measured NOx (“Mean”) correlated with R² = 0.48 to 95 percentile of v*a<0.1 After CLEAR evaluation (“weighted”) the correlation as well as the slope is lower but not eliminated

7. Examples of NOx emissions EU6 #18, TUG test at EURO 6 diesel car with NOx storage catalyst
Measured NOx (“Mean”) correlated with R² = 0.69 to 95 percentile of v*a<0.1 After CLEAR evaluation (“Weighted”) the correlation as well as the slope is lower but not eliminated
The normalization works perfect for CO2 for all tested vehicles as shown in figure left:
CO2 depends mainly on engine load and not on dynamics (< 5% influence)
NOx depends on dynamics of driving conditions to a much larger extent

8. Thresholds for Driving Dynamics
The upper threshold discussed for valid PEMS trips and the v*a+ 95 percentiles from the trips collected at TUG are shown in the figure below.
Between v x apos values from WLTP and the upper thresholds reasonable effects on NOx emission levels are expected. Rough check of test data available suggests ~ +50% to +80% in Nox for measured data and + 20% to + 50% for NOx normalised by CLEAR.
~ +70 % NOx

9. Options for Transfer Function
Transfer function may be applied between:
end of moderate and end of extended conditions or
using the WLTC value as centre (i.e. no correction).
We suggest to start with the option b) since it is already well defined while option a) needs a definition of “moderate boundaries” first. Functions established for b) can be converted into a) later on with low effort if necessary.
Upper and lower thresholds for abnormal driving shall remain since extrapolation of emissions from too soft or too aggressive driving is most likely meaningless.

10. Options for Transfer Functions (II)
The function can describe:
Change of emissions
Change of the CF
as function of the severity of driving conditions.
Both functions are mathematically linked and can be derived from each other:
D Emissions = Ei/EWLTP = F (Severity of driving conditions) = D CF
Emission correction = 1 / D CF
(similar if difference instead of ratio is used, then Diff-CF can be converted to Diff-Emission by multiplication with limit value (e.g. 80 mg NOx)

11. Parameters to be considered in a transfer function
The transfer function shall be a combination of linear equations to be robust (adjustment to non linear functions possible in later step)
With Ai....Constants to be elaborated
Pi....Parameters with effects on NOx considered Ctf...Correction value (either for the CF or for the emissions)
Parameters relevant for NOx:
Dynamics of the driving style (95 percentile of v x apos)
Altitudes to be overcome (m/km)
Weight of vehicle + loading
Ambient temperature
Maximum velocities
Engine speed (option tbd)
Such a function can be elaborated by multiple regression analysis:
Deviation of NOx emissions against e.g. WLTP test value of the vehicle as function of the parameters listed above.
Analysis can use measured emissions, CLEAR results, EMROAD results (we suggest to analyse all 3 options)
Analysis shall result in generic correction function to be applied for all technologies.
Figures use a = (v(i+1) – v(i-1) )/(2x3.6) in all cases

12. Set up of the transfer function
To run multiple regression analysis sufficient data is needed to analyse effects of parameters on NOx emissions
Data collection necessary
For each vehicle test data shall cover range between moderate and extended conditions for all relevant parameters. Unlikely to cover entire range with existing measurements.
Example test data set (values for total trip, analysis to be done separately for U,R,MW) Shortcomings of this set: PEMS only at low T, chassis dyno at normal T high mass only in PEMS, lower mass on chassis dyno, gradient range limited, v_max limited.
v_max
To be completed
Parameters for statistical analysis to be calculated from test data (see slide 13)

13. Data sources for set up of the transfer function
Data sources suggested to elaborate function by multiple regression analysis:
Measurements on vehicles with technologies and calibration close to EURO 6c Step 1 and 2
Simulation of vehicles (also useful for requested analysis on effects on air quality since PHEM can then provide updated EU6c HBEFA emission factors)
Input technology:
Vehicle data: 4 vehicles: small car, medium class, upper class, N1-III
6-gear MT
Engine raw exhaust gas maps: a) EU6c from HBEFA3.2 b) EU6 SCR measured c) EU6 NSK measured d), e) as b),d) but adjusted to 2020 technology assessment Transient effects raw exhaust: I) none II) generic
Aftertreatment: SCR, NSK, NSK&SCR Conversion = F(T,SV) Open: transient effects
Input boundaries:
Test cycles: WLTC, NEDC, approx. 8 PEMS trips PEMS trips to be selected from data base to cover low, medium, high v*a+ areas in urban, road motorway with different mh/km. Different vehicle loading, different ambient temperatures
Figures use a = (v(i+1) – v(i-1) )/(2x3.6) in all cases

14. Details to be elaborated and tested
Target: set up equation, which calculates the change in (NOx) emissions between actual test conditions and WLTP condition.
Important: statistical analysis shall include all parameters Pi with high influence on NOx to obtain correct relations (Ai values). Single parameters can be eliminated later on easily.
Options for Ctf
Absolute emissions : (g/km)i - (g/km)moderate
Relative emissions: (g/km) i / (g/km) moderate
Difference in CF factor (Cfi – CFmoderate)
Ratio in CF factors (Cfi / CFmoderate)
Instead of „moderate“ also „WLTC“ could be used as reference in the analysis
Statistical analysis may show favourite (if one option gives highest significance and correlation)
Options for v*a+
Absolute v*a+: (m²/s³)i - (m²/s³)moderate
Relative v*a+: (m²/s³) i / (m²/s³) moderate
Instead of „moderate“ also „WLTC“ could be used as reference in the analysis
Statistical analysis may show favourite (if one option gives highest significance)
Options for other parameters as for v*a+

15. Details to be elaborated and tested
Options for Ctf
Basis for calculation of delta or diff emissions can be
Measured emissions (if successful may replace existing normalisation tools)
Results from CLEAR
Result from EMROAD
If functions gained from b) and c) are different, each tool would have specific set of constants for the transfer function.

16. Details to be elaborated and tested
Time resolution of the transfer function:
Minimum = separation into urban, road, motorway averages. Corrected total trip can be calculated easily from the three corrected parts.
Reason: v*a+ limits and thus severity of driving at given v*a+ depend on road category.
This option is suggested as base case.
Alternatives:
Correction of MAWs: seems to be also possible but would need different limits for v*a+ Reason: MAW typically are shorter than the 90 minute trips composed by Heinz Steven from the WLTP db for elaboration of the 95 percentile limits.  shorter trips have more variability in v*a+ than longer trips and thus most likely would result in higher 95 percentiles for the v*a+ 95percentiles.  different reference value for v*a+ severity.
Correction of 1 Hz data: seems to be possible if “Diff-Emission” is used but would give identical result as base case (linear equations).
If “D Emission” are used 1Hz application is meaningless since cause and impact on emission changes are not necessarily closely linked in time (e.g. NSC regeneration events, NH3 storage capacity of SCR, Oxygen storage of 3-way catalyst).

17. Details for single parameters
ToDo: 1) check by Heinz
2) elaborate min share accel.
Dynamics of the driving style (95 percentile of v x apos)
+ Good correlation with NOx for unbiased driving
- Critical, for trips with short full load accelerations and long cruise control phases.
Such trips would result in high 95 v*a+ percentiles and thus strong emission reductions by the transfer function.  misuse would be very attractive!
Suggested solution:
Define minimum “normal acceleration share” for urban, road, motorway.
Correct 95 percentile of v*a+ with this share if acceleration time in the trip was lower.
Example (acceleration time % values to be validated by Heinz?)
Example Motorway: trip has only 60 seconds v*a with a above 0.1 m/s² and gives v*a+ = 30m²/s³
Correction with (157 – 60) = 97 seconds with v*a+ = 0:
V*a+Corr = (30 * * 0)/157 = 30 * 60/157 = (alternative: calc. 95 percentile from 157s sample)
Such a correction would be applied only if No. of seconds with a>0.1m/s² is below minimum value (e.g. 157 or 22% for MW). Thus the correction would not appear for “normal” driving.

18. Test cycles for simulation work
WLTP
PEMS trips with:
3 different altitude gains (m/100km)
With each
1 trip with low v*a+
1 trip s with average v*a+
1 trip trips with high v*a+
Additional some with v_max up to >145 km/h
Simulated with different loading and (optional gear shift behaviour)
To cover entire range of parameters of the transfer function.
Already available cycles at TUG cover already large area.
Missing:
V_max above 130 km/h
altitude gains >(1000m/100km)

19. Suggested rough timeline
Topic
Who
until
Collection of measured Emission data (PEMS and WLTP) > 3 vehicles “Step 2 like” > 3 vehicles “Step 1 like”
ACEA
Analysis of these data to
validate feasibility of transfer function
elaborate best design of transfer function (relevant parameters, diff or delta, etc. by checking significances and correlation coefficients)
TUG
Set up PHEM models as shown before
Analyse simulation results and elaborate preliminary transfer function
Further improvements, discussions etc.
All ?