Determining PHEV fuel and electric energy consumption in WLTP
(note CD=Charge Depleting i.e. running on electric only; CS=Charge sustaining i.e battery at min level and running on ICE + regeneration energy only)
After the vehicle has been preconditioned by driving at least one WLTC cycle, the
battery is charged to the maximum SOC level while the vehicle is soaked in a
climate-controlled area at 23°C (Figure 4). The vehicle is tested over multiple WLTC
test cycles, until the test break-off criterion is reached, that is, the net energy
change of the battery from the start to the end of the test cycle is less than 4 % of the
cycle energy at the wheels. The cycle in which this criterion is fulfilled is referred
to as the confirmation cycle (n+1), whereas, in the previous cycle, the transition from
CD to CS operation is considered to have taken place; hence, it is called the
transition cycle (n). After a maximum of 120 minutes, the battery is recharged to the
maximum SOC level to determine the charge energy, EAC.
Figure 4: Schematic overview of CD mode test procedure in WLTP.
Calculation of the average weighted fuel consumption, C, in WLTP is done according
to the following formula:
C = UF .C1 + (1 – UF) .C2
Where:
C = weighted fuel consumption in liters per 100 kilometers;
C1 = fuel consumption in liters per 100 kilometers in CD mode;
C2 = fuel consumption in liters per 100 kilometer in CS mode; and
UF = utility factor as a function of the electric range RCDC, defined as the distance
driven up to and including the transition cycle
The energy consumption, EC (in Wh/km), follows from dividing the recharged energy,
EAC, by the equivalent all-electric range, EAER.
The EAER is determined by taking the total driven distance up to and including the
transition cycle, n, and subtracting the parts during which the combustion engine
was in operation. Calculation of this engine operation share is based on comparing
the CO2 mass emissions for the CS cycle with the average emissions during the cycles
driven in CD mode, as follows:
Where:
RCDC = the CD range up to and including the transition cycle n (in km),
MCO2,CS = the CS CO2 mass emission (in g/km), and
MCO2,CD,avg = the arithmetic average CO2 mass emission of the CD test (in g/km).
The actual CD range, RCDA (in km), is calculated by summing the distances driven up
until the cycle preceding the transition cycle, and performing a CO2-based linear
interpolation during the transition cycle to establish the transition point from CD to
CS mode, as follows:
Where:
MCO2, CS = the CS CO2 mass emission (in g/km);
MCO2,n,cycle = the CO2 mass emission during the transition cycle, n (in g/km);
MCO2,CD,avg, n-1 = the arithmetic average CO2 mass emission of the CD test from the
beginning up to and including test cycle, n–1 (in g/km);
dc = the distance driven during test cycle, c, of the CD test (in km); and
dn = the distance driven during transition cycle, n, of the CD test (in km).
Simples
basically the MPG is make believe, and bears no relation to any real driving cycle, and is not really comparable to pure ICE MPG. Really they should quote the electric range, and then the MPG once the battery is depleted as separate numbers which will allow people to have a better idea of what they will get over their own use cycle. Our friends had an Outlander PHEV that was 130 MPG or something daft on the old average cycle, but in reality returned little better than 30mpg average over a 'normal' useage pattern (and that included them having it plugged in!).
