Air to Fuel Ratio

All combustion processes require two or more substances which will react with each other to convert the chemical energy - stored by the substances - to heat energy. This heat energy then enables such processes to be used for some particular application such as raising the temperature of a fluid (e.g. water, oil, air, etc) in a heat exchanger.

In the application to internal combustion engines, the combustion process causes a dramatic increase in the temperature and pressure of the gaseous mixture in the combustion chamber. The action of the pressure on the piston crown and the movement of the piston causes mechanical energy to be transferred to the connecting rod (and so on via the transmission system to the road wheels of a vehicle).

i.e. MECHANICAL ENERGY, called WORK = PRESSURE PISTON CROWN PROJECT

AREA x PISTON MOVEMENT = PAL per stroke

Usually, with the burning of hydrocarbon fuels air is the other substance because of it being the cheapest supply of oxygen. It is the oxygen that is needed for the combustion of the hydrocarbon. The nitrogen, which with the oxygen comprises the mixture called air, is carried into the combustion chamber but serves no useful purpose in producing hear energy. Minute amounts of it group with minute amounts of oxygen and such groups are called Nitrous Oxide (NO2) emissions in the exhaust gases.

The ratio of the Air to the FUEL, measured by mass in kg, or volume, in m3, lies within a relatively small band of values for good combustion conditions.

Note:
(i) Atmospheric air comprises approximately 23% O and 77% N2 by mass or 21% O and 79% N2 by volume.
(ii) For a typical hydrocarbon mixture, called lead-free petrol, the oxygen required per kilogram of the petrol is 3.36kg, for complete combustion.
Such a value is called the OPTIMUM, or STOICHIOMETRIC RATIO of OXYGEN TO FUEL RATIO, by mass.
Therefore, since only 23% of the air supply would be the oxygen, the OPTIMUM, or STOICHIMOETRIC,
Air to Fuel Ratio = 3.36
0.23= 14.6 to 1 by mass i.e. kg of air per kg of the petrol.

The combustion process can be sustained by a relatively small band of ratios, beyond which it is difficult and undesirable to maintain such processes. The band of air to fuel ratios approximately relates to plus or minus 40% of the optimum value for the particular fuel, by mass.

However, the useful range of air to fuel values, which produce advantages lies within an even narrower spectrum and for most engines, lies in the band approximately relating to plus or minus 20% of the optimum value for the particular fuel, by mass.

So, for the lead-free petrol, given above, the useful range of air to fuel ratios is approximately: - 11-7 to 1 up to 17.5 to 1 by mass.

To discover the advantages, from correct tuning (i.e. adjustment) of the air to fuel ratio- and hence the mixture strength - it is necessary to us an engine test bed, a vehicle rolling - road test bed and/or the skills of an engine technician.

Such differences of the mixture strength, produces the relationship between combustion efficiency (hence m.p.g.) and engine power. Such a graphical relationship, for a constant engine speed - maintained by the test bed dynamometer for a chosen throttle setting is shown: -




RICH MIXTURE are those with air to fuel ratios which are LOWER than that for the OPTIMUM ratio (i.e. minimum air for complete combustion under laboratory conditions).

Such mixtures improve the power output until a limiting value is reached after which the combustion process becomes stifled due to lack of oxygen.

LEAN, OR WEAK MIXTURES are those with air to fuel ratios which are GREATER than that for the OPTIMUM RATIO. Such mixtures improve the COMBUSTION EFFICIENCY until a limiting value is reached after which the combustion process becomes stifled due to a lack of fuel.