Piston Engine Fuel Systems

Question 1

Characteristics of AVGAS

Answer 1

Composition 85Carbon 15Hydrogen. Burn Rate 18-24 M/S
Be able to form a vapour but not to quickly.
Be able to burn quickly but controllably giving a manageable rise in pressure that can be efficiently translated into movement of pistons. Good Anti-knock Capability. Lead is added to control detonation.

Question 2

What is detonation

Answer 2

Extremely rapid spontaneous combustion of part of mixture after burning has begun. Rapid, violent and uneven rise in pressure. Can often be heard as knocking sound in engine. Power output reduces markedly and cylinder temperature rise rapidly. Engine runs rough.

Question 3

Cause of Detonation

Answer 3

High engine temperature
Fuel with poor anti-knock qualities,
Incorrect mixture ratio
Anything that unduly raises the temperature or pressure of mixture before it burns.

Question 4

What is pre-ignition

Answer 4

Uncontrolled Ignition. Ignition of mixture before spark plug fires.
Caused by local hotspots inside cylinder eg small carbon deposits Primary cause is weak mixture..

Question 5

Pre-ignition and detonation symptoms

Answer 5

Loss of power, overheating, rough running

However Pre-Ignition worsens with increased RPM and detonation reduces with RPM.

Question 6

What is Octane rating.

Answer 6

Fuel assigned number to indicate its resistance to detonation. Also known as anti-knock value. Higher number= higher resistance.
Additives are also added which are effective for controlling detonation.

Question 7

What is mixture ratio

Answer 7

Ratio of air to fuel. Ideal 15:1

Question 8

Rich Mixture and Lean Mixture ratios/characteristics

Answer 8

Rich 8:1 Lean 20:1 Lean gives elevated cylinder head temperature and reduced power but reduces fuel consumption.

Question 9

What is EGT

Answer 9

Exhaust Gas Temperature. Affected by ratio of mixture. Often used as a method for adjusting mixture ration in flight. Peak EGT is 15:1

Question 10

What are the cruise settings

Answer 10

Power Cruise. Ratio 12:1. Set by leaning mixture to achieve peak EGT. Then slowly enrich mixture until EGT drops slightly.
Economy Cruise.
17:1 Set by leaning mixture until peak EGT is achieved and then leaning it slightly more until EGT drops slightly.

Question 11

What are the functions of the Carburettor and fuel injection system

Answer 11

Amount of air delivered to engine is controlled by the throttle. Both systems infer the amount of air entering the engine induction system and add sufficient fuel to maintain correct mixture ratio.

Question 12

What is the basic principle of a carburettor

Answer 12

U-tube is formed between fuel chamber an fuel jet in carburettors throat which both have same pressure due to atmospheric air. Carburettor throat has Venturi tube and when engine is running air is drawn in from inlet and accelerates passed Venturi resulting in pressure drop. Fuel is then drawn up u-tube and into air flowing through the flow.

Question 13

Problems with the basic carburettor system

Answer 13

Does not take into account air density.
Ram effect of air would result in leaner mixture with increased airspeed.
Dosent take into account special mixture settings required at very low/high power settings.
Would get a progressively richer mixture as engine speed increases and altitude increases.

Question 14

What is the Pressure Balance Duct

Answer 14

Ensures correct mixture ratio is maintained across a range of air speeds and flight conditions.

Question 15

How does pressure balance duct work

Answer 15

High airspeeds result in intake pressure rising causing drop in fuel flow=weaker mixture.
PBD connects intake and float chamber ensuring pressure remains same.

Question 16

What is a diffusor

Answer 16

Helps maintain a correct fuel mixture ratio over a wide range of engine operations.

Question 17

What is mixture control

Answer 17

Air density decreases with altitude. Mass of air passing through throat will reduce with altitude. Result is richer mixture as aircraft climbs. Mixture control valve is fitted.

Question 18

What is slow running jet.

Answer 18

At very small throttle openings pressure drop in Venturi is negligible resulting in insufficient flow of fuel from main jet. SRJ is placed at edge of almost closed and designed to provide a rich mixture at idle.

Question 19

What is Power enrichment

Answer 19

At high power settings power enrichment systems needed to cool cylinders. At high throttle settings an additional needle jet lifts to allow increased fuel flow.

Question 20

What is the accelerator pump

Answer 20

When throttle opens rapidly air flow to engine increases instantaneously. However slight delay before reduced pressure in throat causes more fuel to be added to mixture. When throttle is rammed accelerator pump injects quantity of fuel directly into Venturi.

Question 21

Carburettor icing

Answer 21

While throat produces drop in pressure it also produces drop in temperature. Further temperature is caused by fuel evaporating as it emerges from nozzle. At moderate or high levels of humidity ice will form in intake duct or throat restricting amount of air that can enter the engine causing a marked reduction in engine power.
For fixed pitch propellor engine first sign of icing is unexplained drop in RPM.
For variable pitch manifold pressure reduces.

Question 22

Symptoms of Carburettor icing

Answer 22

Loss of RPM, reduced performance, Rough running, decreased EGT, Jammed throttle Reduced inlet manifold pressure.

Question 23

What is Carburettor Heating System

Answer 23

Hot air drawn from heat exchanger wrapped around exhaust system. 2 advantages as it prevents/removes ice from forming in the throat of carburettor body.
Provides an alternative source of air in event of primary air intake being blocked by ice.

Question 24

What can potentially happen when carburettor heat is switched on

Answer 24

Expect a drop in RPM. If carb heat is selected when ice is present, engine may misfire and round roughly as ice is dislodged and ingested by engine.

Question 25

What are disadvantages of heated air

Answer 25

Reduces engine power, Use of heated air at high power settings can cause detonation,
As hot air is less dense it will enrich the mixture.

Question 26

What are fuel injection systems.

Answer 26

Delivers fuel under pressure to engine. Provide a more reliable fuel delivery, better icing protection and quicker throttle response.

Question 27

How is fuel delivered in a inject system

Answer 27

An engine driven pump is required. Supplies fuel at pressure to the injector assembly.