Carburettors and Fuel Injection

A piston engine burns fuel only when it is mixed with the right amount of air. The carburettor and the fuel injection system are the two ways of doing that in aircraft engines. A carburettor lets the airflow itself draw fuel into the engine; a fuel injection system pumps the fuel and meters it under pressure. Both deliver a charge whose strength the pilot then adjusts with the mixture control.

The choice has operational consequences. A carburettor is simple and cheap but manufactures its own cold, so it can ice up on a warm, humid day. Fuel injection removes that problem and distributes fuel more evenly, but brings difficult hot starts and a greater risk of vapour lock. American material spells the word carburetor; the systems are the same.

How a float-type carburettor works

The float-type carburettor used on most light aeroplane engines has three main parts:

• The float chamber holds a small reservoir of fuel. A float operates a needle valve in the fuel inlet, admitting fuel as the level drops, so the level stays constant just below the outlet of the discharge nozzle.
• The venturi, or choke tube, is a narrowing of the air passage. Air accelerates through its throat and, as Bernoulli's principle predicts, its pressure and temperature fall. That depression acts on the discharge nozzle in the throat, and the higher pressure in the float chamber pushes fuel through a metering jet and out of the nozzle into the airstream.
• The throttle valve, downstream of the venturi, controls how much mixture reaches the cylinders.

The faster the airflow, the greater the depression and the more fuel is drawn in, so fuel is metered according to the volume of air passing through the venturi. Air is compressible and fuel is not, so a simple carburettor would enrich the mixture progressively as the airflow increased. A diffuser, which bleeds air into the fuel passage, corrects this. At idle the throttle is almost closed and the venturi depression is too small to draw fuel, so a separate slow-running (idle) circuit discharges fuel near the edge of the throttle valve, where the depression is high.

Throttle valve and accelerator pump

The throttle valve is a disc, or butterfly, on a spindle across the induction passage, linked to the pilot's throttle lever. Opening it lets more mixture into the cylinders and raises the manifold pressure and power; closing it does the opposite.

When the throttle is opened quickly, the air accelerates faster than the fuel flow can follow, and the mixture goes momentarily lean, so the engine can hesitate or stop. The accelerator pump, linked to the throttle, squirts extra fuel into the airstream whenever the throttle is opened, bridging that gap.

Because the accelerator pump discharges liquid fuel into the induction system, pumping the throttle before start is sometimes used as a substitute primer. It is dangerous: the fuel pools in the air intake, and a backfire through the carburettor can set it alight. It is done only on types whose flight manual permits it.

Priming the engine

A cold engine vaporises fuel poorly, because only its lightest fractions evaporate, so starting needs a deliberately rich mixture. On carburetted engines the primer, also called the priming pump or primer pump, is a small hand-operated pump that injects fuel directly into the induction manifold or the cylinder inlet ports. After use it must be pushed in and locked. A primer left unlocked keeps feeding fuel into the manifold and makes the mixture over-rich at idle.

Over-priming floods the engine, leaving liquid fuel in the induction system that can catch fire when the engine backfires on start. A fuel-injected engine is primed through the injection system itself, normally using the electric boost pump as the flight manual describes, so it needs no separate primer.

Fuel injection systems

In the indirect fuel injection system used on light aircraft engines, an engine-driven pump supplies fuel to a fuel control unit. There, a metering valve linked to the throttle meters fuel in proportion to the air admitted, and a mixture control valve connected to the pilot's mixture lever bleeds off part of the fuel pressure applied to the metering valve, so that the pilot can lean the mixture. The metered fuel is shared out to a nozzle at each cylinder's intake port, just upstream of the inlet valve. A fuel pressure or fuel flow gauge shows the metered fuel, which allows accurate leaning.

Compared with a carburettor, fuel injection:

• is free of carburettor icing, because no fuel evaporates upstream of the throttle and there is no carburettor venturi to chill the air;
• gives more uniform distribution of fuel between the cylinders, allowing more precise leaning;
• needs no slow-running circuit or separate primer;
• makes hot starts more difficult and is more prone to vapour lock;
• still suffers impact icing at the air intake, so it needs an alternate air source.

Vapour lock occurs when fuel boils in the lines and the vapour interrupts the flow. Fuel boils more readily when it is warm and when the pressure on it is low, so a hot climb to altitude is the classic case. The electric boost pump raises the pressure in the lines and keeps the fuel liquid.

Carburettor and induction icing

Induction icing is any ice that restricts the air path to the cylinders. It takes three forms:

Carburettor icing combines the first two. Fuel evaporating in the venturi absorbs latent heat from the air, and the pressure drop through the venturi and past the throttle cools it further. Together they can lower the temperature inside the carburettor by 20 °C or more, so moisture in the air freezes on the venturi walls and on the throttle valve even when the outside air is warm. EASA training material gives a range of outside air temperature from about −10 °C to +30 °C, with the most severe icing between about −2 °C and +15 °C. The FAA's Pilot's Handbook gives roughly 20 °F to 70 °F (about −7 °C to +21 °C) with relative humidity above about 80 per cent. Visible moisture is not needed.

Carburettor icing is most likely at low power. With the throttle nearly closed, the air accelerates through a very small gap and cools further, a small deposit of ice closes the gap quickly, and the engine produces little heat to warm the induction system. The symptoms depend on the propeller:

• Fixed-pitch propeller: a gradual loss of rpm, then rough running.
• Constant-speed propeller: the governor holds the rpm, so the first sign is a fall in manifold pressure, or a slow loss of airspeed in level flight.

Impact icing, or induction impact icing, is ordinary structural ice building up on the air intake and filter from the outside. It shows as a gradual, unexplained fall in manifold pressure and affects every piston engine, however it is fuelled. The airframe icing article describes the conditions.

Carburettor heat

Carburettor heat (carburetor heat in US material) replaces the normal filtered intake air with air heated in a shroud around the exhaust. The heated air is less dense, so selecting it always costs some power and makes the mixture richer.

When icing is suspected, the pilot selects full heat and leaves it on long enough to clear the ice, which can take a minute or more. With a fixed-pitch propeller the expected response is an initial further drop in rpm, often with roughness as the melted ice passes through the engine as water, followed by a rise in rpm as the ice clears. Switching the heat off at the first roughness leaves the remaining ice in place. If the rpm simply falls and stays down, there was no ice.

Partial heat is avoided unless a carburettor air temperature gauge is fitted and the flight manual allows it, because partly warmed air can be brought into the temperature range in which icing is worst.

Carburettor heat is normally selected cold for take-off: hot air costs power, enriches the mixture and makes detonation more likely at full throttle, and it bypasses the air filter. For the same reason its use on the ground is kept to the brief check in the run-up, since unfiltered air can carry dust into the engine. It is applied before power is reduced for a glide or long descent, and during a long glide the throttle is opened to a cruise setting from time to time to keep the engine, and so the heat supply, warm.

Alternate air sources

A fuel-injected engine has an alternate air source in place of carburettor heat. If impact ice blocks the air filter, an alternate air door admits warm, unfiltered air from inside the engine cowling. On many installations the door is spring-loaded and opens automatically under the suction of a blocked intake; on others it is selected with a cockpit control. Using it gives a small fall in manifold pressure and a slightly richer mixture. It should be selected at the first sign of an unexplained power loss in icing conditions.

In a carburetted engine the carburettor heat control admits unfiltered air from around the exhaust, so it also provides the air supply if the filter is blocked.

Carburettor fires and heat soak

A carburettor fire, or induction fire, usually happens during starting. The engine has been over-primed or the throttle pumped, liquid fuel lies in the induction system, and a backfire through the carburettor sets it alight. The correct action depends on whether the engine has started. If it has not, the pilot keeps cranking with the mixture at idle cut-off and the throttle open, so that the engine draws the flames into the cylinders where they burn out; stopping the starter leaves the fire burning under the cowling. If the engine has started, it is kept running so that it draws the burning fuel through. If the fire persists, the pilot follows the flight manual's engine fire procedure. Pumping the throttle feeds the fire.

Heat soak is the flow of heat from a hot engine into its surroundings after shutdown, when there is no cooling airflow. It vaporises fuel standing in the lines, pump and metering unit, which is why a fuel-injected engine is often difficult to restart twenty or thirty minutes after landing on a hot day. Flight manuals give a hot-start procedure, commonly running the electric boost pump for a short time with the mixture at idle cut-off and the throttle open to purge the vapour with cool fuel, then starting with a lean mixture and advancing it as the engine fires. Too much fuel in this process floods the engine, and the procedure differs between types, so the flight manual governs.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.