Liquid Petrol is relatively heavy. Its S.G. (Specific gravity) is around 0.72. (Specific gravity is measured against pure Water, where 1 litre weighs exactly 1 Kilo (part of the way the metric weight system was devised)). This tells us that Petrol weighs 0.72 X Water (1 Kg), thus 1 litre of Petrol will weigh 0.72 Kgs.
As a vapour, Petrol is said to be 'heavy' as it relates to the surrounding air. It will tend to sink to the ground or into the lowest level.
Note that Petrol as a liquid does NOT burn!
It must be vapourised and mixed with air, adding the Oxygen necessary for combustion. ('Burning' or 'combustion' are other names for 'oxidation'.) In the environmental conditions we live in (standard or average pressure of 1013 millibars and an ambient temperature of +15C) Petrol will only vapourise very slowly. That process must be accelerated to make it burn efficiently. Liquid Petrol must be induced to vapourise and to do so requires all manner of mixing and atomisation processes performed by the Carburettor or Injectors.
Petrol in its raw state has a relatively low 'Octane' rating. The numbering of a fuel's Octane rating (the higher the number the higher the rating) indicates how much compression a fuel will stand before it combusts. Spontaneous combustion (Dieselling) is to be avoided in a spark ignition engine as it results in 'Detonation', causing knocking or pinking, overheating, loss of power and possibly resulting in engine damage.
To sum up, we require a fuel to be 'squeezed' or compressed in the engine's compression chamber by the rising piston as much as possible in order to get the maximum reaction from it. We must also have precise control of ignition and 'time' it to happen at just the right point in time. Such ignition should only be initiated by the spark plug. If detonation were allowed to occur it would probably happen too fast and too early, pushing the piston back down the bore (retard its progress) whilst it is still travelling upward on the compression stroke. That would severely reduce power output.
To make Petrol an efficient ('knock free') fuel we must increase its Octane rating by adding other chemicals such as Lead and Benzine. If Lead is released into the atmosphere is dangerous for the environment and has recently been removed from Petrol. Benzine is a powerful carcinogen and is just as dangerous (if not more so).
When a petrol / air mixture is burnt sizeable amounts of harmful or poisonous elements and compounds are expelled from the exhaust.
Although not poisonous in low concentrations, the harmful one on the world agenda right now is CO2 or Carbon dioxide. Much has been said and written on the effects of increasing levels of CO2 in the atmosphere and you'll be aware of that.
The poisonous gases include CO (Carbon monoxide) which is a colourless, odourless killer of anything that breathes if present in high enough concentrations. SO2 (Sulphur Dioxide) manifests as acid rain amongst other things. The Lead and/or Benzine added to Petrol combine with other elements to make up many other poisons when burned. Even the smoke or smog that hangs over our cities as a result of heavy traffic on congested roads is unpleasant, without considering just what it contains, what harm it can do if breathed.
Note also that Diesel has been a very popular alternative to Petrol up to date, mainly because of two factors;
1. That Diesel may be compressed far more than Petrol before it combusts (the desired effect in this case) which results in better fuel economy (the more the charge can be squeezed, the more useful reaction can be derived from less fuel).
2. That Diesel was initially thought to produce less harmful pollutants than Petrol. However, later research has shown that assumption to be incorrect as Diesel merely produces different pollutants. 'Particulates' are now well known to be dangerous to all living creatures and the environment itself. Ultra - fine particles are also harmful and they are being poured into the atmosphere by every stroke of every Diesel running.