Cap 5 Engine Fuel Systems Flashcards
Avgas
Avgas burns around 18-24m/s
Anti knock prooperties allowing to resist detonation (octan number says if is resistance or not)
Contains 85% carbon and 15% hydrogen
Avgas colouring
Avgas 100 dyed green, Octane rating 100
Avgas 100LL dyed blue ,octane rating of 100 (more3 greener)
Avgas 80 dyed red Considered a leaded fuel
MOGAS motor gasolene authorised certain a/c only
Detonation
Shockwave frown ignition triggers detonation
Esxtremely rapid combustion from detonation source
Excessevely high pressure at TDC cause mechanical shock
Cause of detonation
1) Fuel with poor antiknock properties
2) incorrect mixture
3) anything that might unduly raise temperature or pressure(low rpm with high manifold absolute pressure)
4) high engine temp
5) carb heat
6) turbo/supercharge
Principle causes of pre ingition
1) hot spot within cycliner (carbon deposits on valve)
And is more destruction that autodetonation
2)ignition malfunction damage spark plug
3)cylinder head temperature (CHT)
Mixture ratio
Correct mixture»_space;> 15:1 (Air :fuel) correct air fuel ration is the one cause the exact right amount of oxygen to the correct amount of fuel by mass .also where the exhaust gas temp is max
Rich mixture»_space;8:1
Lean mixture»20:1
When is lean keep an eye on cht( cylinder head temp) it could get hot
Additional fuel metereing
- it takes no account of air density
- it would result in poregressively richer mixture as engine speed and /or altitude increase
- ram air effect in the inlet would weaken the mixture with increased airspeed
- no provision for special mixture setting required at very low and very high power setings
Ram effect
“Pressure balance duct “ counteracts the ram air error by balancing the same ram pressure with the inside of the carburator
Speed squared error
Diffuser reduce the pressure inside carburettor to a level that cancel out the difference of 1/4pressure static pressure dropped.reastabilizing the pressure differential inside the carbuarator and the inlet when the speed is doubled
Mixture control
Mixture control equalise the pressure between the chamber and the Venturi so the mixture gets leaner when pulled back
Slow running jet
When the butter fly is partially closed ( when is in idle) the slow running jet is made to keep the engine in idle without shutting it down.BUT WHEN THE MIXTURE IS FULLY PULLED TO BACK THEN THEN PLUNGER IN THE SLOW RUNNING JET IS CLOSED SHUTTING DOWN THE ENGINE
POWER ENRICHMENT
WHEN THE THROTTLE IS AT HIGHER END, A POWER ENRICHMENT CAM MAKES MORE FUEL TO GET INTO VENTURI SO MORE POWER IS AVAILABLE WHEN YOU PUT FULL THROTTLE FORWARD
ACCELERATOR PUMP CAM
ACCELERATOR PUMP WHEN YOU NEED TO GIVE FULL POWER
Inlet carburettor temperature
The temperature of the inlet carburettor can drop upto 25 degrees due to drop in press and fuel evaporation.
At moderate or high humidity ice will from in the throat
Causing stuck the throttle and /or reduction power
Symthoms of carburator icing
Loss of rpm Reduced performance Reduced inlet manifold absolute pressure(MAP) Decreased exhaust gas temperature Rough running Jammed throttle Engine stops
SIGNS OF CARBURATOR INLET ICING
FIXED PITCH PROPELLER : UNEXPLAINED DROP RPM
Variable pitch propeller : DROP IN MANIFOLD ABSOLUTE PRESSURE
FUEL INJECTION
INDIRECT FUEL INJECTION
FUEL IS CONTROLLED BY THE THROTTLE LEVER.
PRESSURE GAUGE TELLS THE PRESSURE OF THE ELECTRIC PUMP OF THE FUEL .
ENGINE SHUT DOWN IS MANAGED THROUGH THE ENGINE MANIFOLD VALVE
Pressure balancing in the carb? (Ram effect)
At high airspeeds the intake pressure rises because of the ram effect of
the air entering it. This increases the pressure in the throat which causes
a drop in the fuel flow from the U-tube resulting in a weaker mixture. To
avoid this problem a pressure balance duct connects the intake and the
float chamber. The duct ensures that the pressure in the intake and float
chamber remain the same
Diffuser (due to increase in speed the pressure doesn’t reduce lineary but exponentially)?
Air from the pressure balance duct is fed through a perforated tube called
the diffuser. Fuel enters the diffuser from the main jet. At idle RPM the
levels in the diffuser and float chamber are almost equal. When the engine
is running, the fuel level in the diffuser falls because of the slight
restriction caused by the jet. This uncovers some of the perforations in the
diffuser allowing air from the pressure balance duct to flow in. This
additional air displaces some of the fuel and thus reduces the amount of
fuel reaching the nozzle.
Misture control (for increase in altitude)
To compensate for the
effect of changing air density, a mixture control valve is fitted. The valve is
either manually controlled or operated automatically by an aneroid
capsule.
Slow running mixture
The slow running jet is placed just at the edge of the
almost closed butterfly to take advantage of the strong pressure drop in
the constriction caused by the valve. This jet is designed to provide a rich
mixture at idle
Power enrichment
At high power settings a power enrichment system is required to provide
the rich mixture needed to cool the cylinders.
The needle is connected to the throttle linkage so that it operates when
the throttle position is set towards high power.
The needle closes during all but high power settings and for this reason it
is sometimes confusingly referred to as an economiser system
Accelerated pump ?
When the throttle is opened rapidly, the air flow to the engine increases
almost instantaneously. But there is a slight delay before reduced
pressure in the throat causes more fuel to be added to the mixture. This
can cause the engine to hesitate or even stop - a phenomenon known as
weak cut (or lean cut). The solution is an accelerator pump.
The throttle lever is connected by a suitable system of links to a small
piston type pump in the carburettor. When the throttle lever is moved
rapidly forward the accelerator pump injects a quantity of fuel directly into
the venturi
