To answer your question is just a matter of taking care of our environment reducing CO2 emmision and greenhouse effect. .
Three things. First, any - ANY - form of energy conversion creates more pollution. All conversion methods are less than 100% efficient, which means that your plan will have exactly the opposite effect of what you want.
Second, an automobile engine is reasonably efficient at moving two tons of steel 50 mph. It is terrible at everything else, especially creating electrical energy from chemical energy. If petroleum is your energy source, a multi-megawatt power plant is at least twice as efficient as any other way to convert it to electricity. Of all of the ways there are to generate your own electricity, and automobile engine is the worst.
And third, you can't just "discharge" a capacitor into a commercial electric power grid and have any positive effect. The power grid is comprised of thousands of AC generators running in nearly-perfect synchronism. Connecting a 12 V capacitor to the grid will cause the capacitor to burst, and the grid will not even notice because your total energy is the equivalent of running a 60 W light bulb for 3 hours. It is possible to connect a local power source to the grid, but it takes an specialized inverter to convert your DC source into an AC sinewave source of the correct voltage, frequency and phase.
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