Engine Operation Flashcards
What are the stages in a four-cycle engine?
I: Intake
C: Compression
P: Power
E: Exhaust
What stage is this?
Intake
What stage is this?
Compression
What stage is this?
Power
What stage is this?
Exhaust
Magneto Questions: • Define what a magneto is. • How many are there? • What does it do? • How does it function if the electrical system is lost?
A magneto is a self-contained source of electrical impulses, using the physical motion of a coil and a fixed magnetic field to develop ignition voltage.
There are two magnetos.
Each cylinder in your airplane engine has two spark plugs, each fired by a different magneto. This has two advantages: better combustion efficiency, and safety. The engine will run on either magneto if one should develop a problem.
Magnetos are totally independent of the aircraft electrical system.
During engine run-up, you turn off the left magneto and notice a drop in RPM. Then you turn off the right magneto and notice no drop. What is likely the issue?
When you turn the ignition off, with a key or with switches, you are connecting the electrical output of the magneto to the metal block of the engine where it is shorted to electrical ground and cannot fire the spark plugs. This “shorting out” is done through a wire called a P-lead, and if a P-lead is broken its associated magneto can fire the spark plugs even with the ignition in the OFF position. For this reason, you should treat all propellers with respect—moving the propeller might cause a magneto to start the engine unexpectedly if a P-lead has broken.
As you cut the ignition sources in half you will lose some power, reflected as a drop in revolutions per minute (rpm). If no drop occurs when one magneto is shut off, that magneto probably has a broken P-lead, and the flight should be delayed until a mechanic checks it. Some authorities recommend checking for a broken P-lead just before shutting the engine down after a flight, by turning the ignition switch to its “OFF” position momentarily while at idle power; if the engine continues to run, there is probably a broken P-lead.
You should check your engine’s magnetos each time you are in the runup area preparing for takeoff. Magnetos can develop faults that are not readily detectable in cruising flight but which might rob the engine of the power required for takeoff.
When is carburetor icing possible?
Power loss due to carburetor ice is a possibility any time the outside air temperature is between 20°F and 70°F, with high relative humidity or visible moisture in the air. Warning: Your engine can develop carburetor ice on a nice clear day in July, if the relative humidity is high enough!
Induction Systems: Carburetor
Almost all aircraft carburetors are “updraft” carbs; air is drawn in at the bottom and flows upward. Excess fuel caused by improper starting technique flows downward into the bottom of the cowling or onto the ground.
As air flows upward through a restriction called the venturi, its speed increases and pressure drops (remember Bernoulli’s Theorem?). The air in the float chamber is vented to the outside, so the difference between outside air and the reduced pressure in the venturi forces fuel out of the float chamber, through the main discharge nozzle, and into the airstream. The amount of airflow through the carburetor is governed by the position of the throttle valve—you control that directly from the cockpit. The amount of fuel metered into the mixture is governed by the mixture control, and you control that in the cockpit as well. When you “step on the gas” in your automobile, or add power in your airplane, you are really controlling airflow directly and fuel flow indirectly. You actually increase power by stepping on the air, not the gas.
Induction Systems: Fuel Injection
A manifold pressure gauge acts like a barometer, measuring the pressure in the intake manifold downstream (on the engine side) of the throttle butterfly. (This is identified in Figure 2-3 as the throttle valve, and serves the same function in a fuel injection system—control of the amount of air entering the induction system.) At power settings near idle, when the descending pistons are trying to pull air past the closed throttle butterfly, the manifold pressure is low. As you open the throttle to increase power, the pressure measured in the intake manifold increases as airflow increases. Without turbocharging, manifold pressure will approach but never equal the outside air pressure (unless you shut the engine down, something I don’t recommend unless all three wheels are on the ground).
