WEIGHT OF THE EXHAUST GAS IN 4 ST PETROL ENGINE

Assuming complete combustion, the exhaust gas will be all carbon dioxide, water as steam and nitrogen associated with the air.
Again assuming that petrol is all Octane (which is not true) this applies:
Quote: Molecular weights of the above reagents are C8H18 114, O2 32, CO2 44, H2O 18; therefore 1 kg of fuel reacts with 3.51 kg of oxygen to produce 3.09 kg of carbon dioxide and 1.42 kg of water. Endquote
From: <a href="https://en.wikipedia.org/wiki/Gasoline" target="_blank" rel="nofollow noopener noreferrer">Gasoline</a>
In addition to the above is (approximately) 12 kg of Nitrogen and other inert gases.
So,
Per kg of fuel burnt the weight of exhaust gas will be about 17.32 kg.