although air and gasoline vapor will burn when mixed in proportions ranging between eight to one which is considered to be rich and thirty to one which is used in some lean-burn automobile engines complete combustion only occurs with an air gasoline ratio of fourteen point seven to one by weight this ratio is termed the stoichiometric or chemically correct ratio at this ratio all of the oxygen in the charge combines with all of the hydrogen and carbon during combustion but the purposes of this lesson will restrict our numerical definition of the chemically correct ratio to two significant
figures thus will specify fifteen to one as a chemically correct ratio for air and gasoline it's important to remember that the chemical correct mixture does not give the best results in a piston engine because the temperature of combustion is so high at this ratio that power can be lost through detonation in practice particularly in a carburetor induction system mixing of the air and fuel particles and their distribution is less than perfect this results in some parts of the induction system having a richer mixture and others having a weaker mixture than the optimum strength in mixture
strength may even exist between one cylinder and another adjacent to it as is evident in this example of a simple automotive twin carburetor induction system where the difference in the distance that the air fuel stream has to travel between the carburetor and say the number one and number two cylinders will have a quantifiable effect on performance having a slightly rich mixture does not have much effect on power since all of the oxygen is still consumed and the excess of fuel merely serves to slightly reduce the effective volumetric efficiency of the engine in fact the cooling
effect of the evaporating fuel can to some extent be beneficial in cooling the charge temperature below the level where it might otherwise detonate running the engine on a weak mixture however rapidly reduces power since some of the inspired oxygen is not being utilized the power reduction thus suffered is much greater than that which results from a slight richness of mixture it is therefore quite common to run aircraft engines which are fitted with carburetors when maximum power rather than best fuel economy is the objective but somewhat richer than they chemically correct mixture for example about 12.5
to 1 to ensure that no cylinder is left running at severely reduced power from being unduly weak a mixture which is weaker than the chemically correct ratio besides burning at a lower temperature than that which would occur with the chemical correct ratio also burns at a slower rate than the chemically correct ratio because of the greater proportion of nitrogen in the cylinder power output thus decreases as the mixture is weakened but because of the increase in efficiency which results from cooler burning and the associated decreased occurrence of detonation the falling power is proportionally less than
the decrease in fuel consumption thus the specific fuel consumption decreases as the mixture strength is weakened below 15 to 1 or economical cruising at moderate power air/fuel ratios of eighteen to one may be used and advanced in the ignition timing being necessary to allow for the slower rate of combustion the current number of modern aircraft with the facility to vary the ignition timing in flight is limited however following the recent introduction of electronic engine control arise in this number is likely with extremely weak mixtures the gases may still be burning when the exhaust valve opens
exposing the valve to high temperatures which may cause the vow to crack or distort as the inlet valve opens the heat of the exhaust gas is remaining in the cylinder may still be so high that it could ignite the mixture in the induction system and popping back occurs through the induction manifold this slow-burning also causes overheating because a certain amount of the heat generated in combustion is not converted into work by expanding the gases in the cylinder as it should be it therefore remains in the cylinder and has to be dissipated by the cooling system
the mixture requirement is ultimately dependent upon engine speed and power output this diagram shows typical air fuel mixture curves note that a rich mixture is required for starting and slow running there are two reasons for this the first reason concerns the fact that fuel will only burn when it's vaporized and is mixed with air when first starting the engine will be cold and there is little heat to assist the vaporizing process therefore only the lightest fractions of the fuel will vaporize to make sure that there is sufficient fuel vapor in the cylinders to support combustion
a rich mixture is therefore required turns the valve timing as we've already learnt the exhaust valve timing is given a certain amount of lag so that full advantage can be taken of the considerable inertia of the gases at normal engine speeds to obtain efficient scavenging of the burnt gases and to give impetus to the incoming charge however as engine speed reduces the gas velocity Falls and more of the burnt gases remain in the cylinder whilst at still lower speeds such as idling speed there is the tendency for exhaust gases to be sucked back into the
cylinder by the descending piston before the exhaust valve closes the consequent dilution of the induction gases is such that to maintain smooth running a rich mixture is required the crews portion of the diagram shows that this is really the only time during which the pilot has any choice in what mixture to set time is no object then during cruising flight only moderate power is required from the engine thus an economy cruise setting of approximately seventeen point four to one can be used fuel consumption will decrease accordingly other hand if speed is of the essence then
a mixture setting which gives performance cruise power can be selected say somewhere in the region of fourteen point four to one the fuel consumption will of course rise in some cases by as much as 20% when higher power is required for instance when climbing the mixture is enriched to about eleven to one the extra fuel in vaporizing cools the mixture and reduces the tendency to detonate when full power is selected for takeoff the mixture must be enriched to about ten to one apart from the cooling effect the excess fuel is wasted or there is insufficient
oxygen available for it to burn completely the higher power results from a greater weight of charging used in a given time and not because of mixture enrichment in practice excess fuel vapor is not scavengers vapor because the oxygen is shared out to some extent so that carbon monoxide is produced during combustion as well as carbon dioxide with very rich mixtures some of the carbon fails to combine with oxygen at all and is exhausted as black smoke because the density of the air decreases with increase of altitude as the aircraft altitude increases the weight of air
drawn into the cylinder decreases for a given intake velocity the pressure drop in the carburetor venturi will decrease as ambient density decreases however the fuel flow due to this pressure drop will not decrease by the same proportion and so the mixture will become richer if we are to operate the engine efficiently when flying at altitude then it becomes imperative that we have some method of defining and regulating the mixture strength we'll look at two methods which can be used to maintain the correct mixture control the first method that will examine presumes some knowledge of the
effects that mixture change will have on the engine bear in mind that as the mixture control is moved from fully rich to a weaker setting the air fuel ratio approaches the chemically correct value of approximately 15 to 1 at this ratio all of the air and fuel are consumed and the heat released by combustion is at its maximum more heat means more power with a fixed pitch propeller the RPM will rise an air speed would increase as more power is produced if the mixture is weakened even more the RPM will drop pushing the mixture control
back to where the chemically correct mixture was found will bring the engine speed back up but this cannot be maintained without risking the generation of detonation to prevent this happening the mixture control should be moved to a slightly richer position when the RPM will fall slightly the method utilizes the exhaust gas temperature gauge the exhaust gas temperature system is powered by a thermocouple which is fitted into the exhaust pipe of the hottest cylinder on the engine the thermocouple produces a voltage which is directly proportional to its temperature the voltage is indicated on a gauge which
is calibrated to show exhaust gas temperature if the mixture control is moved towards lean the temperature will peak at the ratio of 15 to 1 should be remembered that this ratio should not be used as detonation can occur the mixture control should then be moved from the peak exhaust gas temperature position towards rich and the exhaust gas temperature will drop the aircraft's flight manual will specify a temperature drop which will give the rich cruise setting this concludes the lesson on mixture you
