vehicle running on battery and engine.

I am guessing when you state you do not want electronics in the car you only mean the switching over of power, as any hybrid car using batteries will have to have electronics. =) You will need a transmission to couple over to electric motor power through a gearbox into the main drive shaft or use hub mounted electric motors then all you would need is to put the engine's transmission into neutral. The tricky part would be abapting a speed control system to match rates of speed between the two systems. This would require electronics. Otherwise you are stopping to switch over.

very good idea there..!

With out using electronics means we have to depend on the forces that will change accordingly with speed, remember the concept of mechanical automatic transmission which works only on the speed variation of the shaft. Similarly we can use the speed variation to have our design, but the problem is during the time of shifting from battery power to mechanical, the shaft of the driving wheels need to be exactly matched with the speed of the IC engine.

To achieve this even though we are running on battery for instantaneous change our IC engine also should run at same speed to produce 35 KMPH.

If we use electronics, we can have real time solution for this. Since you don't wanna use electronics in your design, I don't want to overstep the your concept..!