To learn how the mod system works, I created this experimental new configuration for the default Citybus that adds a hybrid drivetrain similar to the hybrid buses found in larger cities like Chicago and NYC. This configuration includes a custom eCVT transmission, which has a built in electric motor for boosted torque and "regenerative braking" which slows down the vehicle using the resistance of the motor (which captures kinetic energy into the battery and, on a real hybrid vehicle, reduces wear-and-tear on the friction brake system). The engine on the bus even shuts off when it's not needed (such as when approaching a bus stop or a traffic signal) to conserve fuel!
This configuration is especially great on the "West Coast" map, as the increased torque helps when climbing the steep city inclines, and the regenerative braking likewise helps when descending these inclines.
There is a pre-made "citybus_hybrid" configuration, or you can change an existing configuration to hybrid by changing the transmission selection:
Beta Hybrid Citybus 5.1
A custom configuration for the Wentward DT40L bus that adds a real hybrid-electric drivetrain