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A variant of the hybrid electric vehicle (HEV) is a split-axle, through-the-road parallel hybrid, which enables existing vehicles to be retrofitted into an HEV with minimal modification -achieved by placing electric motors in the hub of the otherwise non-driven wheels. The term 'through-the-road' (TTR) refers to the power coupling scheme between the internal combustion engine (ICE) and in-wheel motors (IWM), which is not through some mechanical device but through the vehicle itself, its wheels and the road on which it moves. The objective of the present work is to identify specific operating modes, power flow, torque-speed relationship, profiles and system architecture of the TTR-IWM hybrid -in particular a retrofit-conversion hybrid -in comparison to the ordinary parallel HEV. This paper also discusses three possible control architectures and the corresponding control parameters. The TTR-IWM hybrid's reduced operating modes and inability to charge its battery bank during vehicle stand-still present challenges which require a specific control strategy. Simulation results of acceleration show that the TTR-IWM hybrid has better fuel economy and acceleration performance, compared to the pre-transmission, parallel hybrid vehicle with on-board motor.

A split-parallel hybrid electric vehicle (HEV) is a particular sub-type of the HEV in which the internal combustion engine (ICE) and electric motor provide propulsion power to different axles. Due to the coupling of power through the vehicle itself, its wheels and the road on which it moves -rather than through some mechanical device -the vehicle is referred to as a 'through-the-road' (TTR) hybrid. A variant of the TTR hybrid has the motors in the hub of the otherwise non-driven wheels, called in-wheel motors (IWM). This configuration enables an existing vehicle to be converted into a hybrid vehicle with minimal modification. The present work identifies specific operating modes and power flow of the TTR-IWM hybrid, specifically a retrofit-conversion hybrid, in comparison to the ordinary parallel hybrid vehicle. It discusses control architecture, control objectives and system parameters, along with specific control issues of the retrofit TTR-IWM hybrid vehicle.Keywords-HEV, split-parallel configuration, through-the-road hybrid; operating mode of split-parallel hybrid; control of split-parallel hybrid; in-wheel motors.

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