Piston Engine Fuel Systems Flashcards
Pressure Balance Duct
Venturi Effect
Ensure correct mixture ratio at high speed/low speed flights
EGT
Exhaust Gases Temp
Used for adjusting mixture ratio
Cooler when excess lean
Cool when excessive mixture due to excess fuel
Grades of AVGAS
AVGAS 100ll (Blue) - Low lead
AVGAS 100 (Green) - High Lead
AVGAS density
15 degrees about 0.72kg
Vapour Lock
Vaporise in fuel lines restricting fuel flow
Detonation definition and symptoms
Rapid combustion after burning has begun
- knocking sound
- reduce power output
- CT temp rises
- Rough running
- Damage cylinder
Causes of detonation
Fuel with poor anti knock qualities
Incorrect mixture ratio
Raising temp or pressure of mixture before burning (low rpm)
High engine temp
Definition of pre-ignition symptoms/causes
Ignition of mixture before spark plug has fired
- tendency for engine to run after shut down
- worsens with increased rpm
- weak mixture
- hot cylinder
- contaminated with carbon deposits
Octane Rating
Resistance to detonation
Power ratio/economic ratio/ideal ratio
Ideal - 15:1 (Very high combustion temp so not used)
8:1 (rich) to 20:1 (lean)
Ideal Cruise - 12:1
Economy - 17:1
EGT vs Mixture
Combustion cooler at richer mixture due to cooling effect
Cooler when mixture is very lean (Excess air in mixture)
Power Cruise - leaning mixture to achieve peak EGT then enriching more until EGT DROPS
Economy cruise - leaning mixture until peak EGT then leaning again until slight drop
Petrol vs Diesel (Carbon monoxide)
Petrol produced more carbon monoxide but less soot than diesel
Engine driven pump role
Ensure a positive supply of fuel under pressure to the carb under all engine conditions.
Float valve role
Control level of fuel in chamber
Throttle butterfly valve
Controls the amount of air passing through the throat
Pressure balance duct
Ensure that correct mixture ratio is maintained across a range of airspeeds and flight conditions
Increase in pressure in throat causes drop in fuel flow
Weak mixture
Diffuser
Ensure correct mixture ratio across whole speed range of the engine
Large addition of fuel leading excess rich mixture at high airflows
Slow Running Jet
Additional jet to cater for low power conditions
- places near butterfly to take advantage of strong pressure drop
Provide rich mixture at idle
Power Enrichment
Provide rich mixture needed to cool cylinder at high power settings
Accelerator Pump
Injects fuel directly into Venturi
Carb Icing most likely + symptoms
Dependent on humidity/ambient temp and throttle position
18/25 degrees + 30% humidity likely icing conditions
Cruise/descent power
Loss of RPM
Reduce Performance
Reduced inlet manifold pressure
Decreased exhaust gas temp
Rough running
Jammed throttle
Carb Heat advantages
Provides alternative source of air in event primary air intake becomes blocked
Prevents and removes ice in throat of carburettor
Carb heat disadvantages
Reduces engine power
Further reduce power when breaking down ice
Cause detonation at high power setting
Enrich mixture (hot air less dense)
Purpose of induction systems
Introduce the mixture into each cylinder
Induction Icing
Occur in airframe icing conditions
Fuel injection systems
Provide fuel delivery better icing protection and quicker throttle response than carburettor
Engine Driven Pump - Fuel injection
Supplies fuel at pressure to injector assembly
Manifold valve - injections
Distributes fuel from injector to fuel nozzle
Injector Assembly - Injection
Adjusts fuel flow according to throttle position and mixture setting
Specific fuel consumption is…
Mass of fuel required to produce unit power for unit time
Engine Driven Pump - Carburettor
Pumps filtered fuel into carburettor
Ensure positive supply of fuel under pressure in all engine conditions
Fuel Injection 5 components
Engine drive pump - supplies fuel at pressure to injector assembly
Electrically driven pump - also supplies fuel to injector assembly
Injector assembly - adjust fuel flow according to throttle/mixture
Manifold valve - distributes fuel from injector to fuel nozzle
Fuel nozzle - spray fuel into inlets
Mixture control valve
Only means of adjusting fuel to air ratio on fuel injector
Manifold valve - injection
Divides metered fuel between all cylinders
Provide positive means of stopping all fuel flow when mixture control set to idle cut off
CHT
Cylinder head temperature
Sensors fitted on all or the hottest cylinder
Measured by thermocouple no electrical supply
Affected by:
- power
- speed
- temp of cooling air
- ratio of mixture
Weak Mixture leads to
Lower RPM/Lower EGT/Higher CHT
Rich mixture leads to
Higher RPM/Higher EGT/Lower CHT
Fuel tanks vent purpose and location large a.c
Air intakes on the underside of the wing provide ram air for pressurisation
Tanks are vented at the wing tips
Allow air into the fuel tank to replace volume of fuel consumed by engine - equalisation of pressure
Helps reduce evaporation and fuel transfer
Fuel tanks vents purpose and location on light a.c
Single pipe on top of fuel tanks
Fuel booster pump failure will limit…
Max operating altitude
Vapor lock is fuel boiling. Ambient pressure drops at higher alt thus fuel changes to vapour more easily.
Spark Park Fouling causes
Prolonged idle power
Execessive priming
Use of high octane fuels
Continuous climb with a mixture setting fully rich
Safest method of priming a piston engine for start is
Injection of fuel in the cylinder intake port
Mixture Ratio
The ratio between the mass of fuel and mass of air
