Research methodology

The research methodology can be divided into 7 major stages. These stages are as follows and outlined in Figure 4.1.

1) Literature review

2) Formulation of research question

3) Proposal of a new methodology to solve the research question 4) Simulation setup

5) Investigation over standard conditions 6) Validation in real-world conditions 7) Interpretation and generalisation

Literature review Chapters 2 & 3

Formulation of research question Chapter 3

Proposal of a new methodology to solve the research question Chapter 5

Validation in real-world conditions Chapter 11

Learning Overview of HEVs

Chapter 2

Investigation over standard conditions Chapters 7, 8, 9 & 10

Interpretation and generalisation Chapters 12 & 13

Simulation setup Chapter 6

State-of-the-art of FE improvement in HEVs Chapter 3

Evaluation of traditional methodology over standard conditions – first study

Chapter 7

Evaluation of new methodology over standard conditions – first study

Chapter 9

State-of-the-art of FE variability in HEVs Chapter 3

Discussion & future work Chapter 12

Conclusions Chapter 13

Evaluation of the new and traditional methodologies in real-world conditions

Chapter 11

Learning Evaluation of traditional methodology

over standard conditions – second study Chapter 8

Evaluation of new methodology over standard conditions – second study

Chapter 10 Stage 3 Stage 1 Stage 2 Stage 4 Stage 5 Stage 6 Stage 7

4.1.1Literature review

The first and one of the most important stages of research is literature review. Literature review helps to understand the state-of-the-art and clarify the research question.

The review of related literature was discussed in two chapters, chapters 2 and 3. Chapter 2 provided an overview of HEVs and associated terminologies used throughout the thesis. This chapter also discussed the reasons for higher FE in HEVs compared to conventional vehicles.

Chapter 3 provided a review of current state-of-the-art of FE in HEVs. The importance of powertrain components for FE in HEVs and the approaches followed for the optimum selection of powertrain components of HEVs were discussed. This chapter generalised the approach followed in the reviewed literature for the optimum selection of powertrain components of HEVs and termed as traditional methodology. The reviewed literature indicates that variation in FE under different conditions i.e., FE variability is a problem in HEVs. The variation in atmospheric temperature, operation of air-conditioning, and variation in driving patterns are responsible for the problem. Among the three factors, driving patterns are affected by external factors like traffic which could not be controlled and therefore, the variations in driving patterns are less predictable. Even for a certain atmospheric temperature with the air conditioning off, the variation in driving patterns cannot be avoided. The FE variability due to the variation in driving patterns was chosen for study due to the relative importance of the factor compared to the other factors. The reviewed literature indicates that no study has yet addressed the problem and the traditional

methodology for the selection of powertrain components is insufficient to address the problem of FE variability.

4.1.2Formulation of research question

Due to the importance of FE variability in HEVs due to variation in driving patterns and the inability of the traditional methodology to address the FE variability, how to reduce the FE variability due to variation in driving patterns through the optimum selection of powertrain components was considered as the research question. The research question was discussed in chapter 3.

4.1.3Proposal of a new methodology

Due to the limitations of the traditional methodology to address the research question, a new methodology was required. The new methodology is conceptually improves upon the traditional methodology. In the traditional methodology, powertrain components of HEVs are generally optimised over a single driving pattern. The reason for considering a single driving pattern probably because the research was more focused on the development of better optimisation methods to improve FE and for that purpose the optimisation over a single driving pattern was sufficient. This indicates a conceptual flaw in the traditional methodology as real-world consists of different driving patterns. On the other hand, in the new methodology, powertrain components are optimised over a range of driving patterns representing different traffic conditions and driving styles simultaneously. Consideration of different driving patterns makes the concept of the new methodology potentially more applicable in the real-world. The proposal of the new methodology is discussed in chapter 5.

4.1.4Simulation setup

The research question was related to the optimisation of powertrain component sizes. Development and testing of each combination of powertrain component sizes is time consuming as well as expensive. The reviewed literature suggests that computer simulation is a preferred approach over the experimental study for the optimisation of powertrain component sizes. Based on the suggestion of the reviewed literature, computer simulation was considered to address the research question. A computer simulation model of a Toyota Prius HEV, pre-built in a vehicle-simulation-software was considered for the investigation. The simulation model of the Toyota Prius was considered as the benchmark vehicle for comparison. The design parameters, design constraints, optimisation method, and driving patterns were selected based on their usage in the reviewed literature. The performance of the simulation model of the Toyota Prius was considered as the design constraints for an optimum design to ensure that the performance of the optimum design should not be inferior compared to the Toyota Prius. The simulation set up of all the investigations is discussed in chapter 6.

4.1.5Investigation over standard conditions

Investigation over standard conditions is useful to establish any new concept, before investigation in more complex real-world conditions. The standard driving patterns, namely, NEDC, FTP, LA92, HWFET, and US06, generally used in the reviewed literature for studies related to FE, were considered as the standard conditions.

The traditional methodology was evaluated over standard conditions for FE variability, which has not been investigated in the reviewed literature. The study also

patterns. As the study has not been investigated before, first a preliminary study was conducted. As the preliminary study failed to provide desired results, another study was conducted by incorporating the learning from the analysis of the first study. The preliminary investigations of the traditional methodology over the standard conditions are discussed in chapter 7. Chapter 8 discusses the second study of the traditional methodology over the same standard conditions.

The new methodology was also investigated over the same standard conditions. As the new methodology considers a range of different driving patterns, two different approaches are possible to maintain charge sustaining after the end of the last driving pattern. As the first study which investigated the first approach failed to provide the desired results, learning from the first study was incorporated in the second approach which was investigated in the second study to improve the new methodology. The new methodology was compared with the traditional methodology as well as with the simulation model of the benchmark vehicle (Toyota Prius). The first study of the new methodology over the standard conditions is discussed in chapter 9. Chapter 10 discusses the second study of the new methodology over the standard conditions.

4.1.6Validation in real-world conditions

After the establishment over standard conditions, the optimum design produced by the new methodology over standard driving patterns was needed to be validated over driving patterns that were not used for the optimisation. For complete generality these driving patterns were selected to be real-world driving patterns. The new methodology was investigated over real-world driving patterns to validate the applicability of the new methodology in real-world conditions.

Speed-time data logged for a conventional vehicle driven by 10 drivers over a predefined route consisting of urban and highway driving patterns were considered as real-world driving conditions.

The optimum designs produced by the traditional methodology over standard conditions and the simulation model of the benchmark vehicle (Toyota Prius) were also evaluated over the same real-world driving patterns to understand the improvement of the new methodology in real-world conditions. The investigation over the real-world conditions are discussed in chapter 11.

4.1.7Interpretation and generalisation

This discussion is divided into two chapters, chapters 12 and 13. Chapter 12 interprets the results individually and comparatively in the context of the research question and discusses the potential of the new methodology to address the research question. The learning, limitations, and applications of the traditional and new methodologies are also discussed in chapter 12. The future direction of work related to this research is suggested in chapter 12. The major conclusions of the thesis are summarised in chapter 13.

4.2 Summary

 The approach followed for the research in this thesis has been discussed. The

research methodology consisted of 7 major stages, namely, literature review; formulation of research question; proposal of a new methodology; simulation setup; investigation over standard conditions; validation in real-world conditions; and interpretation and generalisation.