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Performance criteria provide a systematic and objective approach to comparing different hybrid powertrain topologies. It enables powertrain topologies to be compared based on criteria that are defined by vehicle class and usage profile. As a result, this potentially enables powertrain topologies to optimised for a given vehicle application.

The following criteria have been used in literature for powertrain optimisation:

 Fuel economy and emissions [47]

 Powertrain mass [48; 49]

 Estimated powertrain costs [50; 51]

 Longitudinal acceleration performance [15; 50]

Fuel ICE M INV B a t t Wheel C AC DC DC T T T Torque BUS DC BUS CVT T Clutch Cap DC

Fuel economy and emissions is a major driver for pushing alternative powertrain technologies. This is compounded by the rising costs in fuel and increased concern of global warming caused by CO2 emissions. In tandem with alternative powertrains, lowering the overall vehicle mass also aids in reducing emissions. Amongst the approaches that are being used to reduce vehicle mass includes using lighter materials, such as aluminium and composites. However, as the focus of this research is specific to powertrains, the investigation will be on optimising the powertrain to minimise its mass. There would also appear to be a trade-off between lowering emissions and powertrain costs; for example, EVs, which have lower tailpipe emissions when compared to CVs, are more expensive to manufacture. Therefore, this area will be investigated to identify such trade-offs.

These performance criteria will form the basis of the cost functions and the constraints used in the powertrain optimisation routine, as demonstrated by way of case studies in the upcoming chapters.

2.3.1 Tank-to-wheel Emissions

Tank-to-wheel emissions is a measure of comparing only the tail-pipe emission of the vehicle. This criterion was largely driven by the goal set by the European Automobile Manufacturers Association (ACEA) with the European Commission (EC) in 1998. This goal called for manufacturers to produce more fuel-efficient and lower emission vehicles. They voluntarily agreed to limit the fleet specific CO2 emission produced by new passenger vehicles to 140g CO2/km by 2008 [52].

Additionally, EU CO2 targets are predicted to drive a dramatic shift in the types of powertrain produced over the next decade [53]. In the short term, a new European (EU) fleet average target for less than 130g/km of CO2 emission has been set for all new vehicles produced after 2015, as per the ACEA agreement [52]. This is a further 7% reduction from the 2008 levels. Hence, it can be assumed that tank-to-wheel emission performance will be a growing concern and therefore of high importance to compare various powertrain topologies. Additionally, if the fleet average CO2 emissions of a manufacturer exceeds this limit, a penalty is imposed on the excess emissions for each car registered. This penalty amounts to a premium of €5 for the first g/km that is exceeded, €15 for the second g/km, €25 for the third g/km, and €95 for each subsequent g/km thereafter. From 2019, the cost will increase to a flat rate of €95 for every g/km exceeded.

There are also additional incentives given to manufacturers that to produce vehicles with extremely low emissions (below 50g/km). Each low-emitting car will be counted as 3.5 vehicles in 2012 and 2013, 2.5 in 2014, 1.5 vehicles in 2015 and then 1 vehicle from 2016 to 2019. This approach will help manufacturers further reduce the average emissions of their new car fleet [54].

2.3.2 Well-to-wheel Emissions

The tank-to-wheel analysis is a subset of the well-to-wheel analysis, which is used to determine the energy consumption and greenhouse gas (GHG) emission of a system [28]. The “system” is defined as every stage involved from fuel production (‘‘well’’) to its end use in a vehicle (‘‘wheel’’). Well-to-wheel studies in general form the basis for assessing the impacts of future fuel and powertrain options, particularly in terms of energy use and greenhouse gas emissions [28]. In order to assess the well-to-wheel CO2 emissions of various powertrain topologies, it is necessary to consider CO2 emissions associated with production of the fuel/source of energy (well-to-tank).

One example of well-to-wheel CO2 emission for various energy sources is summarised in Table 3. In this table, each energy source is paired with its respective powertrain type, such as CV, EV, and Fuel Cell EV (FCEV). Offer et al. estimated hydrogen CO2 emissions to be 76.9g CO2 MJ-1, based upon a value of 11 kgCO2 kgH2-1 for steam reforming natural gas and a calorific value of 143MJ kgH2-1 [10]. Electricity CO2 emissions are assumed to be 150 gCO2 MJ-1 based upon the 2008 UK average electricity emissions of 540 gCO2 kWh-1, which included 5.5% of electricity generation from renewable sources [10] (in 2011, this figure was increased to 594 gCO2 kWh-1, according the Department for Environment, Food and Rural Affairs (DEFRA), a public UK body [55]). Well-to-tank conversion factor for petrol is 14.10 gCO2/MJ [55].

The vehicle type used in the example shown in Table 3 is assumed to be a “medium vehicle” as defined by the National Travel Survey (NTS) [56]. After completing a drivecycle, the amount of electrical energy consumed by an EV or PHEV is determined by replenishing the charge in the battery back to its initial state from the electric grid. Subsequently, the amount of well-to-wheel CO2 emitted is then calculated by converting this consumed electrical energy into gram-CO2 using the data published by DEFRA. For a PHEV, its well-to-wheel CO2 output combines emissions from both its electrical and fossil fuel energy sources.

The purpose of this example is to clarify distinctions and significance of the well-to- tank, tank-to-wheel, and well-to-wheel CO2 emissions. The vehicle type is used to serve as an illustration, and won’t be used any further in this thesis.

It ought to be mentioned that this estimate does not include the following:

 emissions from construction and decommissioning of the infrastructure that is used to create and process the fuel

 emissions that result from commissioning and decommissioning of the electrical power plant, transmission lines, and charging station [57]

 manufacture and end-of-life disposal of the powertrain components within the vehicle.

Table 3: Example of well-to-wheel CO2 emissions for each fuel type (adapted from [10])

Powertrain Type CV FCEV EV

Energy Source Petrol Hydrogen Electricity

Well-to-tank emissions / gCO2 MJ-1 14.1 76.9 150 Tank-to-wheel emissions / gCO2 MJ-1 77.6 - - Well-to-wheel emissions / gCO2 MJ-1 91.7 76.9 150 Given fuel consumption / MJ mile-1 2.93 1.46 0.73 Well-to-wheel emissions / gCO2 mile-1 267 112 110 Well-to-wheel emissions / gCO2 km-1 167 70 68