AGK ENGINES - CHAPTER 4 - PISTON ENGINE FUEL SYSTEMS

What are the main requirements for piston engine fuel?

It must contain a significant amount of energy, form a vapour but not too readily, and burn quickly yet controllably to produce an even pressure rise.

What is the calorific value of a fuel?

It is the amount of heat energy released for a given mass of fuel when burned.

What type of petrol is used in aircraft piston engines?

Aviation gasoline (AVGAS).

Can car petrol be used in aero engines?

No, only specially formulated AVGAS must be used.

What is the density of AVGAS at 15°C?

Approximately 0.72 kg per litre.

What is vapour lock?

A condition where fuel vaporises in the lines or pumps, creating gas pockets that restrict or stop fuel flow.

Why is AVGAS more prone to vapour lock than kerosene?

Because AVGAS is more volatile.

What is detonation in a piston engine?

The extremely rapid, spontaneous combustion of part of the mixture after ignition has begun, causing a violent pressure rise.

What are the main symptoms of detonation?

Knocking sound, power loss, rough running, and increased cylinder temperature.

What physical damage can detonation cause?

Local hot spots and piston crown failure.

List four principal causes of detonation.

Poor fuel anti-knock quality; excessively weak mixture; high temperature or pressure in the mixture before burning; high engine temperature.

What is pre-ignition?

Ignition of the fuel/air mixture before the spark plug fires, caused by hot spots in the cylinder.

How does pre-ignition differ from detonation?

Pre-ignition occurs before normal ignition; detonation occurs after ignition.

What are common causes of pre-ignition?

Hot spots or carbon deposits inside the cylinder and excessively weak mixture.

What are the symptoms of pre-ignition?

Loss of power, rough running, overheating, and running-on after engine shutdown.

What is 'running-on' in a piston engine?

When the engine continues to run briefly after ignition is turned off, caused by pre-ignition.

How is detonation controlled in aviation fuels?

By using fuels with high octane (anti-knock) ratings and adding lead additives.

What does octane rating indicate?

A fuel's resistance to detonation or knocking.

How is octane number determined?

By comparing detonation characteristics to mixtures of iso-octane and heptane.

What does an 87-octane rating mean?

The fuel has the same detonation resistance as a mix of 87% iso-octane and 13% heptane.

What are common octane grades of AVGAS?

100/130 and 115/145.

What is the main additive used to control detonation?

Tetra-ethyl lead (TEL).

What is 100LL fuel?

A lower-lead version of AVGAS 100, dyed blue, with the same anti-knock rating but less lead content.

What colour is AVGAS 100?

Green.

What colour is AVGAS 100LL?

Blue.

List factors that can still cause detonation even with correct fuel.

High temperature, high pressure, carb heat at high power, low RPM with high manifold pressure, or high compression/supercharged engines.

If the correct grade of fuel is unavailable

can a substitute be used?,A higher-rated fuel may be used temporarily, but never a lower-rated one.

What is the chemically correct or stoichiometric air-fuel ratio?

15:1 by mass.

Why is the stoichiometric ratio not used in normal operation?

It produces excessively high temperatures that can cause detonation.

How is detonation avoided with mixture control?

By enriching the mixture about 10-20% to cool combustion.

What is the typical mixture strength used at normal cruise power?

About 15% rich of stoichiometric.

What are typical mixture ranges in aero engines?

From 8:1 (rich) to 20:1 (lean).

What is the best power mixture?

About 20% rich, giving maximum power and cooling.

What is the mixture ratio at take-off?

Up to 30% rich for additional cooling.

What are the signs of an excessively rich mixture?

Black exhaust smoke, carbon deposits, and spark-plug fouling.

How does a lean mixture affect combustion temperature?

It burns cooler gases but raises engine temperatures due to slower combustion heat transfer.

What happens with an extremely weak mixture?

Popping back through the inlet due to mixture still burning as the inlet valve opens.

What effect does a lean mixture have on CHT and EGT?

Cylinder Head Temperature increases; Exhaust Gas Temperature decreases.

What mixture gives best fuel economy?

About 20% lean of stoichiometric (≈17:1), giving best specific fuel consumption.

Why must the mixture be enriched at idle?

Because exhaust gas remains in the cylinder, diluting the new mixture and making it over-weak.

What RPM should be used at idle to prevent plug fouling?

About 1000-1200 RPM.

What does EGT stand for?

Exhaust Gas Temperature.

What is EGT used for?

To fine-tune the mixture ratio during cruise flight.

At what ratio is peak EGT achieved?

At the stoichiometric ratio of 15:1.

What is the ideal power cruise ratio?

About 12:1 — mixture enriched until EGT drops slightly from peak.

What is the ideal economy cruise ratio?

About 17:1 — mixture leaned slightly beyond peak EGT.

Why must mixture leaning be done only at cruise power?

Because excessive leaning at high power can cause detonation.

How do petrol and diesel engines differ in emissions?

Petrol engines emit more carbon monoxide but less soot than diesels.

What does the carburettor do?

Mixes fuel and air in the correct ratio and atomises the fuel for efficient combustion.

What controls air entering the engine?

The throttle valve.

What are the two main types of fuel/air mixing systems?

Carburettor and fuel injection.

What drives fuel to the carburettor?

An engine-driven pump providing positive pressure.

How does the float carburettor regulate fuel level?

A float and needle valve maintain constant fuel level in the chamber.

What causes fuel to flow into the venturi throat?

Pressure drop in the venturi draws fuel up through the jet.

What regulates fuel flow to maintain mixture?

A calibrated restrictor called the jet.

What happens to fuel flow as airspeed increases?

Air velocity increases, pressure drops, and more fuel is drawn in automatically.

What does the throttle butterfly control?

The amount of air (and mixture) entering the cylinders.

What happens when the float drops?

It opens the needle valve to admit more fuel.

Why is fuel pumped under pressure to the carburettor?

To prevent vapour lock and ensure steady supply.

Why does the basic carburettor need refinements?

Because it doesn't compensate for air density, speed, or altitude changes.

What is the function of the pressure balance duct?

It equalises pressure between the intake and float chamber to maintain the correct mixture at all airspeeds.

What is the function of the diffuser in a carburettor?

It adds air to the fuel flow to correct non-linear fuel delivery and maintain the correct mixture.

How does the diffuser aid combustion?

It improves atomisation by mixing air and fuel into a fine emulsion.

Why is mixture control necessary?

Because air density decreases with altitude, causing a richer mixture unless compensated.

How is mixture control achieved?

By a valve regulating pressure or air entering the float chamber or diffuser; may be manual or automatic.

What colour is the manual mixture control lever?

Red.

What are the two main uses of mixture control?

To compensate for altitude changes and to set lean mixtures for economy cruise.

What is the purpose of the slow running jet?

To provide a rich mixture at idle when venturi suction is minimal.

Where is the slow running jet located?

Just at the edge of the almost-closed throttle butterfly.

What is the idle cut-off valve?

A shut-off operated by the mixture control to stop fuel flow during engine shutdown.

What is the purpose of the power enrichment (economiser) system?

To provide extra fuel at high power for cooling.

How does the economiser system work?

A needle jet connected to the throttle linkage opens at high power to increase fuel flow.

What is weak cut or lean cut?

Engine hesitation caused by a sudden throttle opening before fuel flow increases.

What device prevents weak cut?

The accelerator pump.

How does the accelerator pump work?

It injects a small shot of fuel into the venturi when the throttle is rapidly opened.

What is carburettor icing?

Formation of ice in the carburettor venturi or throttle due to fuel vaporisation and pressure drop.

How much can temperature drop inside the carburettor?

Up to 35°C below ambient.

When can carb icing occur?

Any time humidity is high and temperature in the intake falls below 0°C, even on warm days.

At what conditions is carb icing most likely?

High humidity with OAT between -5°C and +35°C.

What is the first symptom of carb icing on a fixed-pitch propeller?

Unexplained drop in RPM and rough running.

What is the first symptom of carb icing on a variable-pitch propeller?

Reduction in manifold pressure.

List common symptoms of carburettor icing.

Loss of RPM; rough running; reduced performance; reduced manifold pressure; decreased EGT; jammed throttle.

What is the purpose of carburettor heat?

To provide hot air to prevent or remove carburettor ice and as an alternate air source.

Where does carb heat air come from?

A heat exchanger around the exhaust system.

Why is carb heat air unfiltered?

Because it bypasses the main air filter — use caution on dusty ground.

What happens when carb heat is selected?

RPM and power drop temporarily.

What does a temporary rough running after selecting carb heat indicate?

Ice is being melted and ingested by the engine.

What is the normal RPM drop when testing carb heat before take-off?

About 100-200 RPM.

List disadvantages of using carb heat.

Reduced power; possible mixture enrichment; potential for detonation if used at high power.

What does the induction system do?

Supplies the mixture or air to each cylinder.

What does the induction system include?

Air filter, intake manifold, and distribution pipes to cylinders.

Where is the inlet riser on many Lycoming engines?

Inside the oil sump to aid fuel vaporisation.

What is the advantage of having inlet and exhaust on opposite sides of the cylinder?

Improved gas flow and efficiency.

What is the alternate air source on a fuel-injected engine?

An unheated bypass around the intake filter.

Why must care be taken when using alternate air on the ground?

It is unfiltered and can draw in dust or debris.

Does alternate air cause a power drop on injected engines?

No, because it is unheated.

What causes induction system icing?

Super-cooled water droplets freezing on the air intake screen.

What are the symptoms of induction icing?

Same as carburettor icing — power loss, rough running, possible stoppage.

Can carb heat clear intake icing?

No, because it does not heat the intake screen.

Should light aircraft fly in icing conditions?

No, they are not certified for flight in icing.