The picture of a central positively charged nucleus of an atom surrounded by negatively charged swarming electrons explains so much detail in the nature of our world.  This is known as the atomic model.  The behavior of gas, liquids and solids are almost completely explained with just that model (after including quantum effects).  Basic things like weather, water behavior, rock mechanics and fire to name but a small few. 

The behavior of gas follows another simple rule using the atomic model when kept in a constant volume chamber.  The gas pressure is approximately equal to the number of gas molecules multiplied by the temperature (although this requires the correct units and proportionality constants to be used).  This means that if you double the temperature of the gas, you double the pressure in the vessel.  Similarly, if you double the number of gas molecules present, you again double the pressure in the vessel.

When burning gasoline in your car or burning other fossil fuels at the local power plant, the reaction chamber design has to incorporate the changes in pressure and temperature associated with the combustion reaction.  At the moment of combustion, if the reaction chamber is a fixed volume, the pressure is directly proportional to both the number of gas molecules and the temperature in the chamber.

The temperature of the chamber is largely driven by the energy released in the combustion reaction.  When the organic molecules being burned are mixed with sufficient oxygen and heat, they combine into soot, smoke, water (H₂O) and carbon dioxide (CO₂).  A high temperature burn with optimal oxygen will almost completely oxidize the fuel converting it more fully over to pure H₂O and CO₂.  The chemical energy released in the burning fuel in this oxidation reaction is what we typically refer to as fire.

The chemical potential energy in the fuel is actually in the form of electrons being further away on average from the nucleus of their atoms than is available in CO₂ and H₂O.  By converting the fuel completely over to CO₂ and H₂O, all the electrons on average are closer to the nucleus of the atoms to which they were bound than they were when they were part of the fuel.  Like dropping a weight from an altitude can turn a wheel and do work, allowing the negatively charged electrons to get closer to the positively charge protons in the nucleus can do work by releasing heat.  That heat is what causes energy transfer to the molecules sufficient to raise their orbital electrons to higher energy levels such that when they spontaneously drop to lower energy levels, they give off the light we associate with fire.

The number of gas molecules is also proportional to the pressure.  The pressure is an average of how many gas molecules are bouncing off the walls of the vessel in any given time interval.  The gas molecules are like little bullets bouncing off the wall continually resulting in an average force per area we call pressure.  If you double the number of molecules in a fixed volume, you basically double the number of these atomic bullets being fired at the walls and so you double the pressure.

This is the case for a typical combustion reaction where the fuel is initially liquid or solid and the combustion products being mostly CO₂ and heated H₂O are gaseous.  The initial volume taken up by the fuel is largely negligible because the phase change from a liquid or a solid over to a gas generally creates gas taking up around a thousand times more volume than the initial volume of the fuel.  In other words, a cubic inch of fuel will result in something in the range of 1000 cubic inches of gas.  It is this large expansion of volume that causes the pressure increase sufficient to move a piston or turn a turbine although we will have to wait until next week to consider changing volumes in the gaseous equations of state.