Ch. 2 - B - The Powerplant and Related Systems Flashcards
what are the two categories of engines?
Reciprocating and Turbine
Describe the basics of reciprocating engine operation
Converts chemical energy into mechanical energy within the cylinders. A fuel-air-mix is compressed within the cylinder by a piston. At moment of compression, the spark plug fires to ignite the mixture. The combustion creates energy which is converted to work and transferred via the crankshaft and connecting rods to drive the propeller.
Moveable pistons attach to the crankshaft by the connecting rods. Together, they change the back and forth motion generated by the pistons into rotary force which drives the prop.
The Four-Stroke Cycle
- Intake: Piston moves away from the cylinder head on intake stroke. Intake valve opens and draws fuel-air-mix into the combustion chamber.
- Compression: Piston moves back towards the cylinder head, intake valve closes and fuel-air-mix is compressed.
- Power: The compression nears completion and the spark plug fires to ignite the mixture. The power stroke is caused by the rapidly expanding gasses from the controlled burn and it drives the piston back providing power to rotate the crankshaft.
- Exhaust: The piston moves back away from the cylinder head and expels the burnt gases through opened exhaust valve.
This takes place several hundred times per minute. Each cylinder operates on a different stroke. Continuous rotation of crankshaft is provided by the precise timing of the power strokes in each cylinder. Depends on ancillary systems like induction, ignition, fuel, oil, cooling, and exhaust.
What is an Induction System?
Bring Outside air into the engine, mix it with fuel to the correct proportion, and deliver it to the cylinders where combustion occurs.
With regard to induction, what do the throttle and the mixture controls do?
The throttle controls the amount of the fuel-air-mix being delivered to cylinders.
The mixture control adjusts the ration of the fuel air mix itself.
What does the Tachometer display?
Engine speed controlled by the throttle in revolutions per minute.
What is the difference between a fixed-pitch propeller and a constant-speed propeller?
Fixed-Pitch: Engine speed controlled by the throttle and displayed on Tachometer.
Constant-Speed: Propeller speed is adjustable through propeller control within the cockpit. Moving the propeller control sets the engine rpm. Throttle controls the engine output as reflected on a manifold pressure gauge (NOT the Tachometer). Propeller control is displayed on tachometer. The gauge displays the pressure of the fuel-air-mix inside the intake manifold in inches of mercury.
Both have mixture controls .
Intake Port
A port near the engine compartment though which air flows through a filter. If filter is clogged, alternative air sources are available.
Generally describe the function of a carburetor
Air from the induction system arrives at the carb which mixes the air with fuel and delivers it to the combustion chamber.
What is the operating principle of float type carburetor ?
The float carb relies on the pressure differential between the air inlet and the venturi throat.
Describe the effect of the venturi throat
The shape of the venturi increases the inlet air’s velocity and decreases its pressure, creating an area of low pressure. Meanwhile, the float chamber is vented to the outside so it remains equal to atmospheric pressure. The higher atmospheric pressure in the float chamber forces fuel fuel through the discharge nozzle.
List and describe all components of a float style carburetor.
Air inters from outside the plane through the inlet. This air flows into a venturi chamber that increases velocity and decreases density of the air.
The nozzle is being fed fuel from the float chamber where a float gives us a way to gauge our fuel level. The float chamber is vented to the outside so it stays equal to the atmospheric pressure, which is always higher pressure than the low pressure area created by the venturi.
The passage though which fuel flows from the float chamber to the discharge nozzle has a mixture needle capable of restricting the rate of flow into the discharge nozzle. The full rich position leaves the entrance to the passage open, while leaning the mixture causes the needle to descend into the entry hole to limit flow.
The fuel enters the float chamber directly through the fuel inlet.
An air bleed connects the discharge nozzle to back to the float chamber by another route. This allows some air to mix with the fuel being drawn out f the discharge nozzle. This is to decrease fuel density and promotes more complete fuel vaporization.
Just beyond the venturi, the chamber widens to accommodate the throttle valve. The throttle controls the flow of the fuel-air-mix leaving the venturi. After passing by the throttle valve, the fuel-air-mix is routed to the combustion chambers.
Describe leaning and enriching the mixture, and why it is necessary.
Carbs are calibrated at sea level such that the proper fuel-air ratio is achieved when it is set to FULL RICH position. As altitude increases the inlet air density starts to decrease. The inlet air is less dense, but the fuel density remains roughly the same. This means that more fuel particles than there are air particles in the same volume of air. If the fuel-air-mix is too rich then it may lead to engine roughness, normally caused by carbon buildup on spark plugs. The excessively rich mix lowers the temp in the cylinders inhibiting complete combustion
Why lean the mixture?
To compensate for decreased inlet air density, we need to limit the rate of fuel flow. Decreasing the flow from the float chamber results in decreased fuel density.
For runup at high elevation airports, or for climbing or cruising altitudes.
Why enrich the mixture?
If descending from high altitudes, the mixture will have to be enriched. as the air density increases the fuel needs to match. An excessively lean mix will result in high engine temps.
If the engine temp gauge is creeping toward the warm side what should be done to the mixture?
Increasing engine temps mean you need to enrich the mix. If the air-fuel ratio is starved of fuel, it will burn hot.
If engine temp gauge is falling to the cold side, what should be done to the mix?
The mix needs to be leaned if the engine temp is getting too cold. If there is too high of a fuel to air ratio then the vapor will be too laden with fuel to burn properly. Too much fuel will lead to cold temps in cylinders and fouled spark plugs.
What is the main disadvantage of the float style carb?
Carburetor Icing. The process of fuel vaporization and the decreasing air pressure in the venturi causes a sharp temp drop within the carb. If water vapor in the air condenses when carb temp is at or below freezing ice forms on the inside surfaces including the throttle valve.
Carb Ice restricts the flow of the fuel-air-mix out of the carburetor which reduces power and causes engine failure.
During what weather conditions is carb icing most likely?
When intake air is below 21 degrees Celsius (70 degrees F), and relative humidity is above 80%
Due to venturi, carb ice may occur at 38 degrees C (100 degrees F) with humidity as low as 50%
What are the symptoms of Carb ice?
In fixed propeller planes, the first indication is a drop in RPM, followed by engine roughness and fuel starvation. For constant speed propeller planes these will be accompanied by shifts in manifold pressure.
During what circumstances is carb ice most dangerous?
During descents, because reduced power may allow for carb ice to build unnoticed. When one tries to add power back in, power will not be there, causing a hazardous emergency landing.
What is the purpose of a carburetor heat system?
It is designed to combat carburetor icing by rerouting the intake air over exhaust cylinders (or other heat source). When ON, a valve cuts off airflow from the normal intake port and draws it from an alternate air intake which passes around the exhaust pipes or other heat source from the engine.
How does using carb heat impact engine performance, and why does it have that effect?
With carb heat engaged there will be a slight decrease in engine power. This is because the heated air is less dense than the outside air. With lowered air density, the mixture enrichens because the fuel density remains the same making for a disproportionate amount of fuel in the mix.
For fixed-pitch propeller planes, how can one tell if carb ice is present via instruments?
When ice is present and you engage carb heat there should be a drop in RPM followed by a gradual increase in RPM as the ice melts. If there is no ice then there will be a slight decrease in RPM, and it will remain constant.
With regard to monitoring carb heat, what is different about Constant-Speed propellers vs. Fixed-Pitch ?
Constant-Speed propellers will display the drop in engine performance due to carb heat via the manifold pressure gauge. For fixed-pitch, the drop in RPM is observed via the tachometer.
When should you use full carb heat?
Whenever you reduce the engine RPM below the normal operating range, or when you suspect the presence of carb ice. Always check POH for specific guide.
What are the advantages of fuel injection over float carburetor designs?
Lower fuel consumption, increased horsepower, lower operating temperatures, and longer engine life.
PREVENTS CARB INDUCTION ICING
What are the four basic components of a fuel injection system?
- Fuel Pump
- Fuel control unit
- Fuel manifold valve
- Fuel discharge nozzles (plural)
Regarding Fuel Injection: What are the components of the fuel pump system; what is their function ?
The pump system moves fuel from the tank to the fuel control unit. There is a electric boost pump provides pressure to the fuel control unit for initial engine starting.
The engine-driven boost pump has its own route from which to draw fuel from the tank, potentially bypassing the electric boost pump. The engine-driven pump provides fuel under pressure to the fuel control unit after engine start. The output pressure changes as engine speed changes.
Regarding Fuel Injection: What is the fuel control unit, what does it do?
The fuel control unit meters fuel going to the engine based on the position of the throttle. If the throttle fully engaged, maximum fuel will be delivered to the fuel manifold valve.
Regarding Fuel Injection: what is the manifold valve and what is its function?
The fuel manifold valve receives fuel being sent from the fuel control unit. The manifold valve distributes fuel evenly to all cylinders and provides a fuel shutoff when the mixture is put in the idle-cut off position.
What is the fuel discharge nozzle and what is it’s function?
The fuel discharge nozzle mixes air with the fuel received from the from the manifold valve and injects the fuel-air mixture into the cylinder intake port. There is one nozzle per cylinder.
What are the disadvantages of fuel injection systems?
Increased Sensitivity to fuel contaminants, and more complex starting procedures especially when the engine is hot.
What problem do Supercharging and Turbocharging remedy?
Operating a reciprocating engine at high altitudes decreases engine efficiency as a result of lower air density. Any normally aspirated engine will suffer this problem, but if the intake air can be compressed more fuel could be added to mix, increasing engine power output.
Describe what a supercharger does for an engine
Sea-level performance may be achieved at higher altitudes if a supercharger is used. A supercharger compresses the intake air using a pump driven by the engine. If air can be compressed, more fuel can be added to the mix leading to increase in overall engine output.
However, since engine power is used to drive the pump it decreases the net power increase added by the supercharger.
Describe what a turbocharger does for an engine
A turbocharger is a more efficient version of a supercharger. Turbocharging is more efficient because it pressurizes the air using a mechanism driven by engine exhaust gases instead of using engine power and which would otherwise be wasted through vents. Both super and turbochargers are fuel injected.
What are the four main parts of an ignition system and what is the system’s function?
The function of the ignition system is to provide the spark that ignites the fuel-air mix in the cylinder.
The four main parts of an ignition system: Magnetos, Spark Plugs, Interconnecting wires, and the ignition switch.
There are upper and lower magneto wires. The upper wires are black in the textbook, and they connect to the opposite magneto. For instance, the left cylinders would have an upper and lower spark plug. The upper magneto wires connect to the right most magneto, while the lower magneto wires from the same cylinder would run to the left most magneto and vice versa. The lower magneto wires run to the magneto on the same side. Each magneto is connected to one of the two spark plugs in each cylinder.
What are magnetos, describe how they function.
They are self, contained, engine-driven units that supply electrical current to the spark plug. It uses a permanent magnet to generate electrical current completely independent of the of the aircrafts electrical system. The system begins when you engage the starter, and the crankshaft starts to turn. Anytime the crankshaft is operating, so are the magnetos.
What is a dual ignition system and what does it do?
It means there are two individual magnetos, separate wires, spark plugs and other components. It is to enhance reliability because if one goes out, the engine will still run, albeit with decreased performance.
Each magneto fires one of the two spark plugs in each cylinder. The firing of two spark plugs increases engine performance by improving combustion.
How are magnetos controlled from the cockpit?
The magnetos are controlled via the ignition switch. The ignition switch has several positions: Off, Right, Left, Both, and Start (moving from left to right in orientation).
How do you identify malfunctioning magnetos in the preflight check?
When moving the ignition switch from both to only the right one observes the corresponding drop in RPM. It should drop within a range specified by the manufacturer. Same for only the left magneto. If there is too sharp of a decrease in RPM it is evidence of possible fouled spark plugs, broken or shorted wires from magnetos to plugs, or improperly timed firing of plugs.
Why is it so important to have the ignition switch set to “off” position following engine shutdown?
If the ignition switch is not set to the off position, the engine can still fire and turn over, even if the battery and master switches off. The magnetos require no outside source to generate power which would turn the propeller. There is potential for serious injury.
What would be the impact of broken or loose wires in the ignition system?
If the ignition switch is properly set to the off position, but the ignition switch’s ground wire is disconnected or broken the magneto will continue to fire, not responding to the ignition switch input. If this occurs you MUST move the mixture to the idle cutoff position to stop the engine, then have it serviced.
What is normal combustion and what is detonation?
Normal combustion occurs in a very controlled, predictable manner. The burn starts at the ignition source of the spark plug and burns directionally away from the spark until it is all consumed. This causes a smooth build up of temp and pressure that ensures the expanding gases deliver maximum force to the piston at the exact right time in the power stroke.
Detonation is the opposite. It is an uncontrolled burn which puts stress on engine components by increasing the pressure and temperature. If not addressed, it will cause failure of the pistons, cylinder, or valves. Less severe cases include engine overheating, roughness, or loss of power.
Describe the parts and functions of a general aviation turbocharging system.
- Ram air intake
- Turbocharger uses a turbine (driven by exhaust gases) and a compressor that pressurizes incoming air.
- The throttle body regulates how much air reaches the intake manifold.
- Intake manifold supplies the pressurized air to the cylinders.
- Used gases exit the cylinders and is ducted through the exhaust manifold which runs the turbine which drives the compressor.
- The exhaust leaving the exhaust manifold reaches a fork in which one pipe leads to the turbine which drives the compressor, while, the other fork leads to the Waste Gate, after that, those two pipes reconnect and exits through the exhaust discharge.
The waste gate controls the amount of exhaust passing through the turbine and the waste gate position is controlled by engine pressure.
Turbocharging can also supply air for cabin pressurization, but also increases engine power output.
