Piston Engines

Question 1

What is a Horizontally Opposed Engine?

Answer 1

The most common type of reciprocating, air-cooled four-stroke, piston engine used in general aviation.

Uses even numbers of cylinders (four, six, or eight), is quite flat and so has a small frontal area, giving low parasite drag.

Question 2

What is a radial engine?

Answer 2

Typically found on older ACs. Have an odd number oc cyllinders to keep it balanced.

Advantages: less problem with cooling as so much surface area is getting air.

Disadvatages: tons of drag

Question 3

What type of engines do most ultralights use?

Answer 3

2 stroke. Very noisy and smelly.

Question 4

What does the term ‘four-stroke’ mean?

Answer 4

A ‘cycle’ of a piston engine is defined by the number of strokes that the piston must undergo to complete the combustion sequence.

In a four stroke engine, each one of the strokes in this cycle serves a specific purpose.

1) Intake (of fuel and air mixture)
2) Compression (of fuel and air mixture)
3) Power (ignition and combustion of fule and air mixture)
4) Exhaust (venting of fumes left over from combustion)

Question 5

What are the ‘fins’ on cylinders for?

Answer 5

Cooling

Question 6

Most GA AC engines are air cooled. What is a major disadvantage of liquid cooled?

Answer 6

Heavier. More complicated. More things to break/malfunction.

Question 7

What is a magneto?

Answer 7

It generates the electricity for the spark in the spark plugs and also controls which spark plugs fire when.

There is a magneto on the left and right sides and both of them are independantly connected to all 8 spark plugs on all four cylinders

Question 8

Why do you lose a little bit of power when shutting down either the left or right mags on runup checks?

Answer 8

Because both mags are wired to all four cylinders, if you lose one, you will lose a bit of power, but the other mag will still fire all cylinders.

You are verifying that the magnetos are operating correctly.

Question 9

What does it mean if you shut down one or the other magneto during a checklist and you do not get a drop in RMP? Or when you shut down and the AC is at idle and you turn the mags to off just for a second before putting it back to both?

Answer 9

It means the magneto did not actually shut down and is still ‘live’. If someone turns the propeller, it could start the engine.

Could be an idication of a broken P-Lead wire (the wire that grounds and shuts down the mags).

Another cause for little or no RPM drop could be due to improper magneto timing.

Question 10

What does selecting the right or left magneto do?

Answer 10

By selecting the right magneto, the left is grounded, and vice versa.

Question 11

What does the run-up checklist prove when you cycle the magnetos?

Answer 11

That each magneto can operate the ignition system without help from the other magneto

Question 12

The exhaust system is used to reduce noise and take pollution away from the cabin area. If you smell exhaust fumes, what should you do?

Answer 12

Turn off cabin heat as this could lead to carbon monoxide poisoning.

Question 13

What is the most common reason for carbon monoxide gas getting into the cabin?

Answer 13

The muffler gets a nick or small hole, the exhaust seeps into the air inside the shroud, which then gets pumped into the cabin when the cabin heat is turned on.

This could be fatal and is why we shut off the cabin heat immediately.

Question 14

What does the mixture control do?
What does the throttle do?

Answer 14

The Mixture control alters only the amount of fuel going in to the fuel/air mixture going into the cylinders.

The throttle manages the rate at which that mixture is pushed/sucked into the cyllinder.

Question 15

What is a rich and what is a lean mixture?

Answer 15

Rich: lots of fuel in the fuel/air mixture
Lean: reduced fuel in the fuel/air mixture

Question 16

What is the chemically correct mixture?
What is the best power mixture?

Answer 16

Chemically correct: 1:15 (1 lbs of fuel to 15 pounds of air)
Best Power: 1:14

Question 17

As you climb higher and higher, in terms of mixture, what happens?

Answer 17

When on takeoff you have full rich mixture, as you climb the air becomes thinner and thinner, which will cause the mixture to get richer and richer.

Question 18

If 1:15 is the ideal (chemically) mixture what would a 1:8 and 1:20 mixture be?

Answer 18

1:8 very rich
1:20 lean

Question 19

For normally aspirated engines, ss you climb and lean the mixture to balance the less dense air, what happens to engine power?

Answer 19

The higher you climb, the less dense the air so you have to reduce the fuel mixture. This leads to less power. Eventually you will not be able to climb any higher.

Question 20

What does leaning the mixture properly do?

Answer 20

-Saves fuel
-Increases efficiency of the engine
-Decreases spark plug fouling (as it does not burn as well and so leaves residue on spark plugs because not everything gets ‘burned up’
-Keeps combustion chamber clean, which decreases the possibility of pre-ignition

Question 21

What are the problems associated with too lean a mixture?

Answer 21

Rough engine running
Backfiring
Overheating
Sudden engine stoppage

Question 22

What problems are associated with too rich a mixture?

Answer 22

Rough engine running
Fuel wastage
Spark plug fouling
Combustion chamber depostis
Loss of power or engine failure

Question 23

What does applying carb heat do to the mixture?

Answer 23

Makes it richer (warm air is less dense)

Question 24

What do turbo or superchargers do and how do they work?

Answer 24

It will compress the thin air at high altitudes and compress them so that the fuel air mixture resembles that of lower altitudes.

They are powered by the exhaust, which turns the power turbine of the turbocharger

Question 25

What is a wastegate in a turbocharger?

Answer 25

It regulates the amount of exhaust directed to the turbine. In doing this it regulates the speed of the turbocharger.

When the wastegate is open, the engine is not turbocharged because the exhaust is being vented out.

When it is closed, the engine is turbocharged. The exhaust is directed trough the turbine before being ported overboard.

Question 26

What are the advantages of a turbocharger over a supercharger?

Answer 26

Lightweight and does not take power directly from the engine

Very hot operating, expensive, complicated maintenance

Question 27

What is a supercharger and how does it work?

Answer 27

An internally driven compressor that is powered directly from the engine, typically this will come off the crank shaft via a gear reduction system.

Unlike a turbo charger, which compresses the air prior to it being sent to the carburetor, a supercharger compreses the fuel/air mixture downstream of the carburetor.

Question 28

What are the advantages and disadvantages of a supercharger vs a turbocharger?

Answer 28

Advantages: Reliable, less expensive, not as hot
Disadvantages: Takes up to 16% power away from engine

Question 29

How does density altitude affect the performance of non-turbocharged and non-supercharged engines?

Answer 29

Hot,High, and/or Humid = High Density Altitude (low density air)
Low, Cool, and/or Dry = LowDensity Altitude (high density air)

Density altitude (and humidity) is the altitude the aircraft ‘thinks’ it is operating at. It changes the power output of the engine, the performance of the prop, and the lift created by the wings.

Question 30

What effects does a high density altitude have on prformance?

Answer 30

Take off roll will be longer, climb will take longer, landings will run longer, mixture needs to be adjusted correctly before takeoff but will still result in less power (because your AC thinks it is operating higher in the air than it is)