Prerequisite Knowledge:
The last item an engine needs to run is air (specifically, oxygen). This article will examine how carbureted and fuel injected engines handle air.
Fuel Injection Engines
Fuel injection engines are relatively straight forward in regards to how they manage air. Air enters the engine through an induction port, passes through an air filter, and then is distributed to the cylinders.
Yeah, that's about it.
Carbureted Engines
Carbureted engines are where the air management gets more interesting. Carbureted engines rely on a device known as a venturi passage. A venturi is a constricted passage, where the air speeds up and the pressure drops as a result of the venturi effect. This helps the fuel vapourize into the air, before it is sent off into the cylinders. However, the vapourization of the fuel (not the venturi itself) causes the air temperature inside the carburetor to drop by more than 30 degrees Celsius (86 degrees Fahrenheit for the plebs out there). In certain conditions, this puts the aircraft at risk for carburetor icing, where ice forms inside the carburetor due to the air drop.
The last item an engine needs to run is air (specifically, oxygen). This article will examine how carbureted and fuel injected engines handle air.
Fuel Injection Engines
Fuel injection engines are relatively straight forward in regards to how they manage air. Air enters the engine through an induction port, passes through an air filter, and then is distributed to the cylinders.
Yeah, that's about it.
Carbureted Engines
Carbureted engines are where the air management gets more interesting. Carbureted engines rely on a device known as a venturi passage. A venturi is a constricted passage, where the air speeds up and the pressure drops as a result of the venturi effect. This helps the fuel vapourize into the air, before it is sent off into the cylinders. However, the vapourization of the fuel (not the venturi itself) causes the air temperature inside the carburetor to drop by more than 30 degrees Celsius (86 degrees Fahrenheit for the plebs out there). In certain conditions, this puts the aircraft at risk for carburetor icing, where ice forms inside the carburetor due to the air drop.
Carburetor ice constricts the carburetor venturi, severely limiting the flow of air into the engine. This can cause an extreme drop in power, and moderate engine roughness. To prevent this, a carbureted engine gives the pilot to switch from a normal, unheated air source, to a heated air source. This is known as carburetor heat.
Generally, carburetor heat is not used unless carburetor icing has been suspect, carburetor icing conditions are present, or under other conditions specified by the aircraft manufacturer. For example, in carbureted Cessna 172s, the tachometer has a green band on it. When operating below this green arc, carburetor heat must be applied.
Generally, carburetor heat is not used unless carburetor icing has been suspect, carburetor icing conditions are present, or under other conditions specified by the aircraft manufacturer. For example, in carbureted Cessna 172s, the tachometer has a green band on it. When operating below this green arc, carburetor heat must be applied.
Normally, with the carburetor heat set to "cold", air is drawn from the front of the aircraft, filtered, and sent to the carburetor. When the carburetor heat is set to "hot", a valve switches positions, which switches the air source to another part of the engine. This heated source is located elsewhere in the engine, and sent around the exhaust manifold in order to warm it up before sending it through the carburetor. However, this heated source is generally unfiltered, which is why it is generally not advised to use carburetor heat on the ground, during takeoff, or on landing, as it can cause debris to enter the engine.
When carburetor heat is applied, the engine's power will generally drop by a small amount. This is normal, as the hotter air being sent into the engine is less dense (higher temperature = lower density). This is another reason why carburetor heat should not be used during takeoff or landing, as it reduces the engine's power output.
If carburetor ice is present, the engine power will decrease dramatically, and it will begin running much rougher than before carburetor heat was applied. Again this is normal; as the heated air enters the carburetor, the ice begins to melt and break away, flowing through the rest of the engine and into the cylinders. The throttle should also be opened at this point, as higher power settings will better help the engine cope with the ice/water intake.
Carburetor advice should only be used in the full hot or full cold position, never in between. This is because partial carburetor heat can raise the temperature of the air in the carburetor into the icing danger zone (around 0 degrees Celsius, 32 degrees Fahrenheit).
When carburetor heat is applied, the engine's power will generally drop by a small amount. This is normal, as the hotter air being sent into the engine is less dense (higher temperature = lower density). This is another reason why carburetor heat should not be used during takeoff or landing, as it reduces the engine's power output.
If carburetor ice is present, the engine power will decrease dramatically, and it will begin running much rougher than before carburetor heat was applied. Again this is normal; as the heated air enters the carburetor, the ice begins to melt and break away, flowing through the rest of the engine and into the cylinders. The throttle should also be opened at this point, as higher power settings will better help the engine cope with the ice/water intake.
Carburetor advice should only be used in the full hot or full cold position, never in between. This is because partial carburetor heat can raise the temperature of the air in the carburetor into the icing danger zone (around 0 degrees Celsius, 32 degrees Fahrenheit).
The risk of carburetor icing is the greatest at high relative humidities, and temperatures around 10 degrees Celsius (50 degrees Fahrenheit). The dew point is the temperature at which any moisture present in the air will condense. When the difference between the temperature and dewpoint are small, the relative humidity is high, which means there is lots of water vapour in the air. This increases the potential for carburetor icing greatly.
Engine Aspiration Types
The density of the air directly affects the power it can generate, as denser air gives the engine more oxygen to use for combustion. However, air density drops with altitude, which means the engine produces significantly less power at high altitudes. This is a problem for normally aspirated engines, which simply draw in air from the outside and use that for the engine. Turbochargers and superchargers give the engine the ability to generate more power by compressing the air before it is used, increasing its density.
Turbocharger
Normally, the exhaust gases generated by the engine are dumped overboard, as they are simply waste gases from the combustion process. However, a turbocharger uses some of the exhaust gases to spin a turbine, which compresses the air that is entering the engine. This is essentially a "free" boost to the engine, as the exhaust gas is usually wasted anyway. However, turbochargers are not instant. Turbocharges take time to spool up and down; this delay is known as turbo lag.
Engine Aspiration Types
The density of the air directly affects the power it can generate, as denser air gives the engine more oxygen to use for combustion. However, air density drops with altitude, which means the engine produces significantly less power at high altitudes. This is a problem for normally aspirated engines, which simply draw in air from the outside and use that for the engine. Turbochargers and superchargers give the engine the ability to generate more power by compressing the air before it is used, increasing its density.
Turbocharger
Normally, the exhaust gases generated by the engine are dumped overboard, as they are simply waste gases from the combustion process. However, a turbocharger uses some of the exhaust gases to spin a turbine, which compresses the air that is entering the engine. This is essentially a "free" boost to the engine, as the exhaust gas is usually wasted anyway. However, turbochargers are not instant. Turbocharges take time to spool up and down; this delay is known as turbo lag.
Supercharger
Unlike a turbocharger, which uses exhaust gases to spin a turbine, a supercharger uses a direct connection to the engine, usually via a belt of some sort. A supercharger provides a much more immediate power boost with no lag due to this direct connection, but it also increases the load on the engine, which slightly increases fuel consumption as well. However, the performance benefit is much more significant.
That's it for air management! The next article will cover piston engine oil systems.
Unlike a turbocharger, which uses exhaust gases to spin a turbine, a supercharger uses a direct connection to the engine, usually via a belt of some sort. A supercharger provides a much more immediate power boost with no lag due to this direct connection, but it also increases the load on the engine, which slightly increases fuel consumption as well. However, the performance benefit is much more significant.
That's it for air management! The next article will cover piston engine oil systems.