induction system

The engine receives ram air through an intake in the lower front portion of the engine cowling. An air filter is placed at the intake end of the duct. This filter removes dirt, dust and foreign matter from entering the carburettor. The air passes through an air-box, then to the carburettor intake. In the event that the airflow to the carburettor becomes blocked by carburettor ice or intake ice, an alternate heated air source can be selected by the pilot by pulling out a carburettor heat control in the cockpit. Use of the heated air will result in approximately 75 to 100 RPM drop.

A throttle is located on the instrument panel in the cockpit. When the throttle is closed, it is pulled rearward toward the pilot until it is stopped by mechanical means. At this setting, the engine continues to run, but at “idle” speed (a few hundred RPM). As the throttle is moved forward, the throttle valve in the carburettor opens allowing more air into the carburettor, thus increasing the RPM. When the throttle is full forward maximum RPM results. The throttle can be locked into a set position with a friction lock so that in cruise flight the power setting will remain set. This relieves the pilot from constant attention to the throttle.


The carburettor provides 2 principal functions.

It mixes the fuel with the air in the proper proportion
It regulates the amount of air (and thus fuel) that enters the engine.

The the air is routed from the intake through ducts into the carburettor. The carburettor on most engines are of the updraft type; i.e. the carburettor is mounted on the bottom of the engine, and the fuel/air mixture is sucked upward to the engine.

When the carburettor heat control in the cockpit is pulled on, heated air enters the carburettor. The air source comes from inside the cowling, and passes through a “heat” box to warm the intake air. The heated air can be selected when atmospheric conditions are conducive to carburettor icing or the normal intake duct become blocked by ice at the induction port and air filter.

The carburettor is equipped with a small chamber containing fuel and a float valve. The valve maintains a constant amount of fuel in the chamber. This provides a constant and sufficient source of fuel to satisfy the fuel demands of the engine.

The main air duct of the carburettor is a tubular structure which decreases in diameter near the middle of the duct, then increases in diameter near the intake manifold end of the carburettor. This is called the “venturi”. This decreased diameter creates a vacuum in accordance to the Bernoulli principle. The fuel intake port is located in this section. A metered amount of fuel is sucked into the carburettor. The fuel vaporizes into fine particles in the intake air flow. This atomised fuel and air mixture is of proper proportion to cause correct burning of the fuel/air mixture in the engine.

The fuel / air mixture is set by design to be correct for operation at sea level. As the engine is operated over a range of altitudes and air densities, the pilot can adjust the mixture via manual means in the cockpit. It is called the “mixture control”. The correct mixture adjustment procedure is covered in the Pilot Operating Handbook (POH) for the given aircraft. Some aircraft are equipped with an Exhaust Gas Temperature Gauge in the cockpit. A proper fuel/air mixture will produce a given exhaust gas temperature. The pilot can adjust the fuel / air mixture to a fairly accurate measurement by observing that the exhaust gas temperature is within the proper range.

The throttle regulates the amount of fuel/air that enters the engine, thereby controlling the power that the engine develops. On aircraft with a “fixed pitch” propeller, the throttle directly controls the engine RPM. On aircraft with a variable pitch propeller, a Manifold Pressure Gauge directly measures the engine power being developed. A propeller pitch control controls the propeller blade angle. The power setting of the engine requires adjustment of both the throttle and propeller pitch control.

The carburettor has an accelerator pump which will provide a “burst” of additional fuel for quick development of maximum horsepower, such as performing a go around from landing approach. An economizer valve allows the engine to idle when the throttle closed.


The predominate forms of icing affecting engine operation are carburettor ice, throttle ice and induction ice.

Carburettor Icing

Carburettor icing is a constant concern to the pilot when operating in high humidity and visible moisture conditions. Whenever the outside temperature is 20° to 70° F, ice creation in the throat of the carburettor is a possibility. Due to the Bernoulli effect and the vaporization of the fuel in the venturi, the temperature of the fuel / air mixture can be as much as 50 degrees lower than the outside air.

Induction Icing

The air induction port at the front of the cowling can become partially or totally clogged with ice when air temperatures are 32° F or below while flying in visible moisture. This is known as “impact” ice, and is most prevalent when the Outside Air Temperature (OAT) in around 25° F and super-cooled moisture exists.

Throttle Ice

Throttle ice in the carburettor occurs most often when the throttle is partially closed. This can occur at low cruise speeds or near idle situations such as approach to landing. Some manufacturers recommend that the alternate air source (carburettor heat on) be used anytime the power setting of the aircraft is below a certain point even though high atmospheric moisture content is not present. For fixed pitch propeller configurations, most aircraft should use carburettor heat below 2000 - 2100 RPM. The location of ice in the carburettor is shown in the diagram at right.

The vaporization of the fuel in the venturi additionally cools the throttle area. Even a small amount of ice in the carburettor or the induction system will reduce power. Usually this condition is detected by a gradual drop in RPM (or Manifold Pressure). Application of carburettor heat will usually cause an additional temporary decrease in power, but as the ice melts, the power should be restored. If icing is persistent, it may be necessary to operate with some carburettor heat on continuously.

Fuel Injection

In these systems, the air intake system is similar to carburetted systems. However, the fuel is not vaporized in a venturi, but rather is injected directly into the engine cylinder just prior to the spark plugs firing. A specific amount of fuel is injected and an appropriate amount of air is vented though the air induction system to provide for proper combustion.

There are several advantages to fuel injected systems.

Less susceptibility to icing
Fuel flow is better controlled
Faster throttle response since the fuel is directly injected into the cylinder.
Better distribution of fuel to each cylinder.
Easier starting in cold weather.

Some disadvantages are:

Starting a hot engine can sometimes be difficult.
Vapour lock during ground operations on hot days.
Difficult engine re-start if engine quit due to fuel starvation.