The investigation of the impact of mild hybrid electric city bus application on performance, fuel economy and emissions

The city buses are the primary transportation method of nearly all cities for many years, thanks to passenger comfort, flexibility of daily routes, minimum infrastructure investment, different size and powerpack options. Since that vehicles interacts with many people every day, the need of enviromental actions are getting more important in time. As that is also a case for all automotive products, the global OEM's are under increasing pressure of the rulemakers and customers to develop enviromental friendly vehicles. New materials, production methodologies, design concepts have been always in scope of them, however the "electrification" and "software based intelligent vehicle technologies" have dominated the last 5-10 years of innovations and R&D investments. As a result of this trend, the number of electric vehicles are increasing exponentialy year by year in all segments including heavy duty vehicles like trucks and buses. The Total cost ownership is the primary assessment criteria in purchasing of city buses, who are mostly purchased by tenders and considering a mininum life time of 5 years. That creates a good opportunity to cover the delta cost of expensive high tech battery and power electronic components of the electric buses. As the electrified city bus has significantly less energy and maintenance cost, the total cost of ownership becomes more feasible against a conventional diesel automatic bus. The ongoing R&D investments and scale of economics are also motivating the electrification trend by reducing the delta costs. Therefore, we expect to see more electric bus models and sales in future. The studies mostly focused on the full hybrid electric and battery electric vehicles, which have more CO2 emission reduction potential with higher delta costs. However, this study proposes an alternatif way, which is a low cost 48V electrification solution with >20% CO2 – fuel consumption improvement potential in a city bus application. The electric components at 48V voltage level does not need any onboard high voltage safety equipments and that brings an opportunity to provide an electric power with minimum cost. The author prefers to combine the limited power and fuel economy advantage of a 48V mild hybrid electric powerpack with the semi-automatic transmission which has a mechanical clutch system with less friction losses over the common technology of full automatic transmission (with hydrodynamic clutch system). Normally, the semi-automatic transmission has a limited application in city bus segment, because of the driving comfort and clutch durability problems. The proposed powerpack is aiming to eliminate those natural disadvantages by utilizing the 48V electric powerpack with a proper sizing, control strategy and configuration. Firstly, the automatic and semi-automatic diesel variants will be tested to measure the performance and fuel consumption metrics. AVL Cruise 1D simulation tool is used for modelling all conventional vehicle variants and correlating with the measurements. Finally, the mild hybrid powerpack proposal will be built on the semi-automatic vehicle model for assessment of performance, clutch durability and performance benefits.