Vehicle Thermal Modelling for Improved Drive Cycle Analysis of a Generic City Bus

In the UK local authorities are being incentivized to purchase low emission vehicles through schemes such as the Ultra-Low Emissions Bus (ULEB) scheme. To qualify vehicles must prove they meet certain emissions limits over a pre-described drive cycle. One of the key factors that will impact vehicle performance is the operation of the Heating Ventilation and Air Conditioning (HVAC) system, a large consumer of energy in the system, thus it is vital to have an understanding of the thermal behaviour of the cabin and HVAC loads required to maintain comfortable conditions. This paper outlines a lumped parameter model of a generic city bus for use in determining such loads and assisting in the creation of control strategies to minimize energy consumption. Using steady-state analysis it is possible to determine the average power consumption required to maintain an internal cabin temperature over a single MLTB drive cycle. The dynamic analysis shows that by changing the HVAC strategy it is possible to reduce the energy consumption by removing the need to operate the system at full power for the entire duration of the cycle. By implementing a hysteresis based control strategy that overheats the environment then switches to a low power mode to allow the temperature to recover the model was able to demonstrate potential energy savings of up to 26.6% when compared to the baseline case operating at a constant steady power output.