http://berc.berkeley.edu/storage-wars-batteries-vs-supercapacitors/
They’re working on it...
“The main shortcoming of supercapacitors is their low energy density, meaning that the amount of energy supercapacitors can store per unit weight is very small, particularly when compared to batteries. Additionally, the cost of supercapacitor materials often exceeds the cost of battery materials...”
The only function of the caps would be to act as a "spring" between the engine and the motors. Much like the function of a starting capacitor on an AC electric motor.
I doubt the size and weight of the capacitors would need to be anywhere near as much as a bank of lead/acid or lithium batteries...nor the cost.
Plus if properly matched, I'm betting they would last a long time.