1. Correlation Improvements
Solving interpolation problems
2017

2. Interpolation in NEDC correlated Introduction
In WLTP the CO2 influence of vehicle body shapes and optional equipment can be interpolated, as long as it is resulting from aerodynamics, rolling resistance and mass.
Additionally also other road load effects can be interpolated (e.g. brake friction), with the new Amendment 3 of the UNECE GTR and with EU WLTP 2nd Act.
As every measurement has its inaccuracies it is important, that the two points defining the interpolation line are not too close and not too far away from each other. This is already defined for overall road load and CO2.
But there are definitions missing:  There is no minimum delta for each physical component (i.e. mass, aerodynamics, RR). Both WLTP and NEDC correlated.  There is no minimum delta for CO2 in NEDC correlated.
The missing definition can result in unrealistic CO2 values for interpolated vehicles.
The problem in WLTP is limited, as optional equipment produces a wider range and there is a definition of minimum CO2. Implementation should be fast, but no intermediate solution necessary.
The problem for NEDC correlated is more significant, therefore in addition an intermediate solution should be applied.

3. Interpolation in NEDC correlated Example
interpolation line, physically correct
road load overestimated
road load underestimated
High
Low
solution
interpolation line, measured
interpolation line, physically correct
High
interpolation line, measured
Low
Extrapolation is often necessary, as High and Low is strictly related to WLTP High und Low, but in NEDC a different vehicle could be the best or worst case.
There are already cases with errors > 15 g/km of CO2. There are both situations observed, vehicles, where CO2 is underestimated and vehicles, where CO2 is overestimated.
The error can go into both directions within one family, depending on the slope and whether extrapolation is done to lower or higher end.

4. Interpolation in NEDC correlated process of solution
Problem:
Solution:
today
existing WLTP approvals
new WLTP approvals,
but new regulation not in force
new regulation in force
Option 1: use wrong CO2
Option 2: post calculation
Option 1: use wrong CO2
Option 2: post calculation
Option 3: pre-usage of an agreed new regulation
use new regulation 
Usage of Options 2 und 3 would not follow the regulation. A confirmation by authorities and EU Commission would be required (e.g. by publishing on DG Clima website).
Options 2 and 3 could be the same technical solution.

5. Interpolation in NEDC correlated Road Load Family
The road load family is not strictly bound to an Interpolation Family. So within an IP Family different RL families could apply. That concept is not included in the NEDC correlation, but would be necessary to avoid total confusion and problems. This is less urgent. It has to come together with WLTP 2nd Act.
Proposal for NEDC correlated for decoupled road load families:
NEDC road load values of vehicle High and vehicle Low have to be calculated within their road load families as applied in "WLTP".
For every road load family in the interpolation family, a respective NEDC High and Low road load is calculated for the purpose of interpolation.
The road load interpolation is done within the road load family.
The CO2-inteprolation is done within the Interpolation Family on the basis of the cycle energy.

6. Solution for NEDC correlated List of proposals:
Proposal from ACEA: Require H to be higher than L, for cycle energy, CO2 and phases.  This shall prevent strange CO2 values in the case H and L are close together.
Copy from WLTP: Require a minimum 5g-range for CO2 in case of extrapolation.  This only solves new approvals, it is not so severe. But best possible interpolation should be ensured in future.
Proposal from EU Kom: Use road load correlation procedure not only for High and Low, but for every vehicle.  This can be also used as post-calculation for approved vehicles.  This solves the very critical road load interpolation problem, having too small deltas between Low and High.  This solves the WLTP 2nd Act new requirements, that have to be included also in the correlation process. ACEA supports the proposal from EU Kom.
Proposal from ACEA: Allow the OEM, to additionally increase interpolated CO2 values.  If problems occur in the future, that are not identified today, this could solve at least one side of the problem.
Note: The basic correlation process, formulas, etc. remain unchanged. It is only the application, that is improved, and stringency is increased in terms of the interpolation rules.

7. Interpolation in NEDC correlated Example for the proposed solution
Example for NEDC correlation, CO2-range:
High
High
WLTP Interpolation
Low
WLTP Interpolation
Low
Correlation
Correlation
Correlation
Correlation
tyre B
tyre A
bodyshape
tyre B
tyre A
bodyshape
Extrapolation not allowed, if delta between H and L too small  A higher "H" shall be defined.

8. Interpolation in NEDC correlated Example for the proposed solution
Example for NEDC correlation, road load solution:
WLTP Interpolation
WLTP Interpolation
and CO2MPAS
road load
and CO2MPAS
road load
road load
road load
and CO2MPAS
road load
and CO2MPAS
road load
road load and CO2
CO2
veh.2, tyre B
veh.2, tyre B
veh.1, tyre B
veh.1, tyre B
veh.2, tyre A
veh.2, tyre A
veh.1, tyre A
veh.1, tyre A
To ensure correct road load, it is correlated from WLTP, and only CO2 is interpolated on NEDC level.

9. Interpolation in NEDC correlated Conclusion
The proposal is providing a long term solution and a post-calculation solution at the same time.
Vehicles, that are already type approved, and where extrapolation is necessary, should already use that new system of the direct road load calculation. For that purpose, ACEA requests publishing that on DG Clima website.
A drafting proposal is available. Any input and feedback is highly welcomed.
The improved regulation should be implemented as soon as possible and should apply to all new approvals.
Solving it is very urgent and also necessary for approved vehicles, in order to fulfil the spirit of the regulation and to avoid wrong or even ridiculous CO2 values.
At the same time, the initially decided formulas, the CO2-stringency and the concept remains unchanged. It just closes identified loopholes.

11. Consolidated diagram by Pablo
WLTP H
CO2(&R/L)Transfer
R/L(only)Transfer
R/L only: by
WLTP L
R/L only: by
CO2: by COMPAS
R/L: by
CO2: by COMPAS
R/L: by
NEDC H
CO2: by
extrapolation
Min 5g, if extrapolation
CO2: by
NEDC L

12. Solution for WLTP

13. WLTP new issues task FORCE Minimum Range of Road Load parameters
Interpolation
criterion
Road Load Family
Interpolation Family
mass
different between H and L RR
air drag
min. range
4% cycle energy
5 g/km CO2
max. range
35% cycle energy
20%/30g CO2
extrapolation
unclear
3 g/km CO2
(within 30g/km)
If the difference e.g. of cd*A is too small, road load calculation could fail.
Interpolation
criterion
Road Load Family
Interpolation Family
mass
Deltamin: 30 kg RR
Deltamin: 1 kg/t
Deltamin: 1 RR class*
air drag cd*A
Deltamin: m²
min. range
4% cycle energy
5 g/km CO2**
max. range
35% cycle energy
20%/27g CO2**
extrapolation
allowed for the purpose of using the class-RR for IP-Family
3 g/km CO2**
(within 30g/km)  
()
Plus additional rules:
Parameter of "High" has to be higher than the parameter for "Low".
If difference is not achieved, High vehicle may be worsened artificially.
If no difference or difference smaller than minimum (RR, cd*A or mass), worst case of family is to be applied.
Information:
All proposed min-ranges are approx. 3 times of the measurement tolerance (10kg, 0.3 kg/t, 0.015m²).
* If the road load family concept is not used, the criteria from road load family is to be applied in addition.
** See separate proposal.

14. WLTP new issues task FORCE Minimum Range of Road Load parameters – way forward
Proposal was presented in IWG #20 and considered positive in 09/2017.
A text proposal was developed by OICA members.
Japan will discuss in JAMA meeting mid of October.
OICA will discuss and further develop until IWG #21.
Paper for IWG #21 and GRPE in Geneva 01/2018 should be available.
If confirmed by IWG, it will go into the GTR. Confirmation expected for IWG #21.
If confirmed by EU WLTP, it will go into WLTP 2nd Act. This will depend also on the decision in Geneva.