Carburetion is the process of supplying the correct amount of fuel for mixing with the air induced into the engine to inform a suitable combustible mixture.
Chemically correct Mixture is 1:15 (1 part of fuel to 15 parts of air).
Air drawn into the engine cylinder, governed by the degree of throttle opening, enters the carburetor through the air intake.
At the restriction caused by the choke tube the velocity of air increases.
The increasing velocity of air, with in the choke tube & in the region of the jet is accompanied by a decrease in pressure below that of the atmosphere.
At the fuel in the float chamber is subjected to the atmosphere pressure a higher pressure, which is higher than around the jet, fuel issues from the jet & mix with the air stream flowing to the engine cylinder.
The fall in the level of fuel in float chambers lowers the float, which lifts the needle valve off it’s seating permitting more fuel to flow into the float chamber to replace that being issued from the jet.
Limitation to this is that any increase in airflow will increase pressure differences acting across the jet & consequently increases fuel flow. The fuel flow however, does not vary proportionality with greater airflow, & the mixture obtained from the simple carburetor becomes progressively richer as the engine speed increases.
Throttle – is also known as Butterfly.
Needle Valve – is also known as Flat Valve
Air Vent – is also known as Breather
Due to maneuvering of A/c it is not possible to provide equal atmospheric pressure in the air intake.
To prevent upsetting the rate of discharge of fuel from the jet, the atmospheric pressure in the air intake & in the float chamber must be equal.
Equalized pressure conditions are obtained by admitting atmospheric pressure to the float chamber through a tube or duct opening into the air intake.
This pressure balance duct also supplies air to the diffuser, & is used in some carburetors to provide altitude mix control.
A Diffuser is fitted to prevent main jet supplying excessive fuel as the engine speed increases, it ensures that the fuel flow is kept directly proportional to the volume of air flowing through the choke.
Fuel enters the diffuser through the jet & when the engine is not running, the fuel level in the diffuser & the annular space is the same as that in the float chamber.
During operation, as engine speed increases, the fuel level in the annular space falls thereby uncovering some of the radial holes.
This allows air to flow through the holes into the diffuser, thus reducing pressure difference acting across the jet & counteracting the tendency of jet to supply excessive fuel.
The diffuser in addition to fulfilling it’s primary function, also breaks down or emulsifies the fuel before the fuel is pushed into the air stream flowing to the induction manifold.
Slow Running jet is used for idle rpm operations, it allows the flow of fuel for slow running rpm.
Power jet – Mix strength supplied by the diffuser for cruising conditions is too weak for engine to develop its full power output. To supply the extra fuel necessary to give the desired mixture strength for full power, the size or number of jets must be increased.
Enrichment jets are used for short periods such as Take off, it provides carburetor with a very rich mixture fro a short period of time.
Accelerator pump – is used to assist in sudden opening of throttle, It inject fuel into air-stream to make up for this deficiency.
Sometimes a spring operated delayed pump (action pump) maintains the flow for a few seconds even after the throttle opening has ceased.
Mixture Control – density of air decreases as altitude increases & consequently less amount of air is induced for the same velocity of air through the choke.
Therefore fuel flow is decreased, as altitude increases to match fuel/air ratio.
Cut out valve – A device incorporated in slow running jets to prevent fuel from being drawn into induction system.
ANTI – Gravity devices are incorporated in carburetor design to assist in maintaining the power output of engine while negative ‘G’ conditions prevail during Aerobatics.
Float needle Anti – G stops – An adjustable stop fitted above the float needle, in conjunction with a collar on the lower end of the needle restrict the needle movement & fuel flow during aerobatics to allow only the entry of sufficient fuel to keep the engine running maximum power conditions.
ANTI G Ball Valve – A ball valve fitted to the pressure balance vent in the float chamber, that prevents fuel spillage into the choke during negative “G” flying conditions.
ANTI – G Stand Tube – prevents Engine cutting due to fuel starvation, it is precluded by a stand tube, which feeds the metering jet from the centre of the float chamber.
Two types of icing
- Carburetor icing – In carburetor as throttle valve cause velocity to increase, press decreases it also causes temperature to decrease, furthermore as heat is required to convert a liquid into a gas, fuel which vaporizes in Induction system absorbs heat from induced air and the surrounding metal, resulting in additional decrease in temperature.
When temperature of air is less than O degrees Celsius the moisture content of air begins to precipitate & form as ice.
The added constriction caused by the ice formation, results in an increased air velocity & progressively further decrease of the temperature.
With these conditions ice builds up more rapidly in the system causing loss of engine power, rough running & jamming of the throttle valve.
- Impact Icing – This form of icing occurs most readily at temperature of Oo Celsius to -7o Celsius & is caused by rain droplets turning into ice on striking the exposed surfaces of the carburetor.
The ice adheres & builds up around the air intake, disturbing the airflow, upsetting the mixture strength & resulting in rough running & loss of power & may even stop the engine.
In fuel injection carburetor – there are no float chambers.
The choke is known as the large venturi & inside it a small boost venturi is fitted : this has the effect of lowering the pressure still further & this depression in the boost venturi acts upon a diaphragm which in turn regulates the flow of fuel into the carburetor by means of a Poppet valve.
In this type of carburetor the choke is retained purely as a means of measuring the airflow.
Fuel Injectors are actually not carburetors but engine driven pump, which supplies fuel direct to the cylinder via supercharger or Turbo charger in accordance with engine requirements.
As the flow of fuel is governed by boost & rpm, no altitude control of the mixture strength as is fitted in the conventional float type carburetor is required.
As we know as altitude increases air pressure decreases, results in less exhaust gases remaining in the cylinders at the end of each exhaust stroke & consequentially, the Volumetric efficiency increases i.e. More mixture goes into each cylinder at a given induction pressure.
In the float type carburetor the extra fuel is automatically drawn off by the increased depression in the choke, but in a fuel injector, compensation is necessary to avoid weakening of mixture.
Here compensation of mixture is provided by the back pressure capsule, which although integrated with the boost pressure capsule is sealed from it & is subjected to atmospheric pressure.
Thus – as altitude increases pressure capsule is compressed & the main metering needle responds accurately to capsule movement. It is essential for it not to be a tight fit in its supporting bushes.
Volumetric Efficiency increases as altitude increases because of air density decreasing as altitude increases. Therefore after the exhaust stroke ends the fuel/air mixture after burning is totally scavenged & more fuel/air mixture is allowed to enter fully, therefore more fuel/air mixture enters as altitude increases, at low altitudes the fuel/air mixture entering the carburetor is less, because at low altitude scavenging of exhaust gases is less.
The carburetor measures the fuel/air mixture to deliver the mix to cylinders by weight and not by volume.
The throttle valve measures the fuel/air mixture in volume and delivers to the intake manifold.
Carburetor measures fuel/ air mixture by weight.
Throttle valve measures fuel/air mixture by volume.
It is important of warm up the eng before T/o since some portion of the gasoline in fuel/air mix does not vaporize and is still in liquid form when it enters the engine. It is vaporized by the engine. It is important to warm up the engine before T/o. This is done so that there is sufficient engine heat to vaporize all the fuel and develop full power during T/o.
On applying Carburetor heat mixture becomes rich because of the low density of hot air, therefore you must decrease the fuel/air mix to lean the mixture.
Min 50% Relative humidity is required for icing to occur (In Carburetor) and the icing hazards increasing as the Rel humidity increases.
Danger of Carburetor icing is – 50c to 300c. No Carburetor icing occurs below –50c because of no water vapors present in air, any small amount of moisture if present is frozen and is therefore harmless. Extremely low temperature causes the engine to run lean because density of air is low in hot air and the fuel flow is constant therefore normally ISA conditions Air density: 10, Fuel Flow: 5, cold temp Air density: 12, Fuel Flow: 5, because of high density of air. Mixture because lean. (More air less fuel)
In high temperature conditions (summer etc.) Air density decreases Fuel Flow remains constant and normal Air density: 10, Fuel Flow: 5, In Summer Air density: 8, Fuel Flow: 5, because of low density of air, mixture because rich (Less air more fuel)
An Important point to remember while using Carburetor Heat – Hot Air supplied to Carburetor for Carburetor de-icing is not filtered therefore avoid ground usage.
When a hot engine (with fuel injection system) is shutdown the heat inside the cowling may cause the fuel in the lines to vaporize. Therefore at the next start, the vapor present may cause the fuel in the lines to vaporize, therefore at the next engine start the vapor in the lines can prevent the engine from getting enough fuel to start the engine.