Carburation is the process of mixing air and fuel into the correct ratio for efficient combustion and then delivering this vaporised fuel to the inlet manifold before being sent to the cylinders where it is combusted.
Regardless of complexity of a carburettor, all different types of carburettor systems work by utilising a Venturi.
Principles of a Float type carburettor
Air enters the carburettor through the air intake and moves to a venturi (choke), the velocity of air increases; thus reducing pressure, as explained in the Bernoulli principle.
For an increase in velocity (kinetic/dynamic energy) of a fluid, a decrease in pressure (potential/static energy) occurs – to maintain equilibrium in the total Energy.
Ek + Ep = Et
This decreased pressure in the venturi forces out fuel from the fuel discharge nozzle, this nozzle in turn sucks fuel via jets from the carburettor bowl/float chamber. The fuel vaporizes and cools in the venturi because of the lower pressure, from here the fuel is transferred to the inlet manifold before being sent to the cylinders for combustion.
Construction of components and their function
The float chamber is a reservoir for fuel in the carburettor and provides a near constant level of fuel to the main discharge nozzle.
Fuel level in the float chamber is regulated by a float and needle valve system; closing the needle valve as the float rises with increased fuel level and opening the valve as the float lowers with reduced fuel level.
The chamber is filled either by a pump or a gravity fed system and is filtered before entering the chamber.
The pressure in the float chamber is slightly higher than that of the diffuser, and is at near atmospheric pressure using a pressure balance duct to prevent a vacuum from forming in the float chamber as fuel is discharged.
The fuel level in the float must always be lower than the height of the fuel discharge nozzle to prevent fuel overflow, the height of the float can be adjusted with a setscrew or adjustment of the float arm/pivot.
A float which fails to float sufficiently will cause too much fuel to be entered into the float chamber which will lead to flooding in a stationary engine as a result of fuel being pushed out through the discharge nozzle.
Main Metering System
This is the part of the carburettor which supplies fuel to the engine during all speeds other than idle. It comprises of the following components:
Decreases the pressure at the discharge nozzle by increasing velocity of air at the fuel discharge nozzle. It also restricts the amount of air during full throttle. This is where the butterfly valve regulated by the throttle is situated.
2. Main Metering Jet.
A threaded valve with a small hole running through it which regulates the fuel flow between the float chamber and the fuel discharge nozzle when the butterfly valve (throttle) is fully open. It is controlled from the cockpit by means of the mixture control. A richer mixture means more fuel is allowed to pass, a leaner mixture means less fuel is allowed to pass.
3. Main Fuel Discharge Nozzle and Diffuser.
A fuel nozzle is situated in the throat of the venturi in the lowest pressure area.
As air flows through the venturi, the nozzle is exposed to a pressure difference between the low pressure venturi and the float chamber which is vented to atmospheric pressure, this is referred to as a metering force. This metering force causes fuel to discharge from the discharge nozzle in a fine spray vaporising in the air.
As the throttle is increased, increasing airflow, the flow of fuel also increases; this needs to be done in the same ratio, resulting in the need for a diffuser. PTO
A diffuser is situated in the main fuel passage between the main metering jet and the fuel discharge nozzle. It is perforated to bleed/draw air to allow a constant ratio of fuel to air in the choke, regardless of throttle setting. The air is blead/drawn from an area which is out of airflow and close to atmospheric pressure.
These perforations are situated below the static fuel level, being covered at a low RPM, as the RPM increases, the fuel level in the diffuser drops opening up these perforations bleeding out air into the tube at intake pressure thus raising the pressure slightly above that in the venturi.
The Idling System
When the engine is at idle and the butterfly valve (throttle) is in the fully closed position, the air velocity in the venturi is so low that sufficient fuel cannot be drawn from the discharge nozzle; to prevent the engine from stopping, an idle fuel passageway is added to discharge fuel just past the butterfly valve.
The area on the engine side of the butterfly valve has a low pressure because of the suction from the pistons, this creates the metering force required to draw fuel from the carburettor bowl which is then discharged using an idling jet. The idling jet is adjustable to set the idle speed of the engine.
An adjustable idle air bleed is incorporated into the system to atomise fuel before it enters the manifold.
When the throttle is opened sufficiently to prevent the engine from stopping, only the main discharge nozzle will operate and no fuel will be discharged from the idle jet.
The Mixture Control System
With an increase in altitude, air density is decreased for a given volume.
The venturi system utilises an increase in velocity of air to create a low pressure to discharge a fixed volume of fuel, thus same amount of fuel is discharged regardless of altitude.
The mixture will become richer as altitude increases as there is now lower volume of air to mix with this fixed amount of fuel.
To maintain the prefect ratio of fuel to air, a manual or automatic fuel control system is implemented.
The manual systems are namely a Needle Type and a Back Suction Type:
• Needle Type Mixture Control System.
A needle valve situated between the float chamber and the main metering jet is control from the cockpit. Moving the mixture control in the cockpit to rich opens the valve increasing fuel flow and moving the control to the lean position closes the valve restricting the flow of fuel.
• The Back Suction Method.
An atmospheric line with an adjustable valve which is controlled from the cockpit is connected to the float chamber, this regulates the pressure in the float chamber to control the pressure gradient between the venturi and the float chamber.
By opening the valve and increasing the pressure in the float chamber, the mixture is made richer and alternately when the valve is closed, the pressure in the float chamber decreases (because of the vacuum created as fuel is discharged) thus leaning the mixture.
The Accelerating System
When the throttle is opened abruptly, a large volume of air moves through the carburettor. The main metering system is slow to introduce fuel to this influx of air, the mixture is momentarily leaned as there is a greater volume of air than fuel. This will cause the engine to accelerate slowly and is referred to as a flat spot, this may even lead the engine to stall.
An accelerator pump is installed to prevent this from occurring. A piston pump which is attached to the throttle draws fuel from the float chamber and sprays it to the main metering system or a discharge nozzle in the venturi if opened quickly, if opened slowly, fuel will flow back into the float chamber. Thus it is important not to open the throttle quickly when the engine isn’t running as it could lead to flooding.
Economizer (Fuel Enrichment System)
When an engine produces high amounts of power at high throttle levels, maximum heat is produced. More fuel is added into the main metering system to cool the combustion chambers, this cooling prevents detonation (engine knocking) – where combustion takes place out of phase. It utilizes a needle valve which remains closed until throttle reaches 60-70.
Idle Cut off
This is a system which is used to stop the discharge of fuel into the carburettor by means of the mixture control unit. Setting the mixture control in the cockpit to lean will cut the fuel supply completely.
Dangers of cutting the engine with the ignition key before cutting the fuel:
• When the ignition key is turned off, the spark plugs no longer create a spark however air and fuel may will still be supplied to the engine and if the engine is still very hot or hot spots are present, ignition of this mixture may continue causing the engine to keep running or kick backward.
• This combustible mixture may go through the cylinders unburned and ignite in the exhaust manifold.
• Unburned fuel may remain in the cylinders or induction system and could cause kickback during the next start.
By cutting the mixture before turning the ignition key to off, the spark plugs continue to operate and will burn away all the combustible fuel in the cylinders and once all is burnt it will bring the engine to a safe stop.
Disadvantages of a float type carburettor
• Inverted flight is not possible given the gravity based float system.
• Abrupt manoeuvres move the float system too abruptly.
• Low pressure system means fuel is not fully vaporized.
• Difficulty of discharging fuel into supercharged systems.
• Icing tendency.
The discharge nozzle utilised low pressure to discharge fuel; this drop in pressure leads to cooling of the air molecules, coupled with the release of latent heat during vaporization of fuel result ice forming in the venturi and the butterfly valve even in high ambient air temperatures.
a) Fuel Evaporation Icing (Refrigeration Icing) between +5°C and +27°C.
Fuel Evaporation icing occurs because of the process where fuel is evaporated from liquid to gas to create the correct mixture of fuel vapour to air for combustion, during this process, latent heat is released to change state.
This creates a drop in temperature just after the discharge nozzle where icing will form.
If there is moisture in the air such as days with high humidity (greater than 50%) as well as days with building clouds, this moisture will freeze in the venturi just past the discharge nozzle.
This accumulation of ice will lower the manifold pressure and affect the mixture ratio thus interfering with the fuel flow.
b) Throttle Icing (between 0°C and +5°C)
Throttle ice occurs because of the temperature drop owing to the pressure drop caused by the venturi.
Moisture in the air will freeze at or near the butterfly valve because of this temperature drop.
As with Fuel Evaporation Icing, accumulation of ice will lower the manifold pressure and affect the mixture ratio thus interfering with the fuel flow however in this case, the butterfly valve may also be obstructed by the accumulation of ice.
c) Impact Icing (between -10°C and 0°C.
Super cooled water droplets freeze on impact with the aircraft and accumulate which will cause excess parasite drag, hinder movement of control surfaces and thin the context of engine function, block air intakes limiting airflow and ultimately will cause engine failure.
This will usually form with visible moisture in the air such as rain, snow, sleet or clouds, particularly in thunderstorms.
Identification and Effects of Carburettor Icing
1. Initial symptoms will include rough running of the engine which will include excessive vibration.
2. Manifold pressure will drop in the case of helicopters and constant speed propellers
a. This may initially not be noticed as the governor will automatically increase throttle to maintain manifold air pressure and RPM
3. A continuation of performance loss.
a. In helicopters this will lead to overpitching and will result in low rotor RPM as the engine is unable to sustain rotor RPM.
4. Engine failure.
a. Carburettor Heat
An alternate air supply can be selected by the pilot with either a manual system, utilizing automatic systems or carb heat assist which will inlet air from the warmer parts of the engine, this application of carb heat will bypass the external air intake. With the air being warmer and thus less dense, this will result in the mixture of fuel to air to be richer resulting in degradation of performance.
The risk of carburettor icing is particularly high during low power settings such as during descent.
In the Robinson R22, the yellow arc of the carburettor in between -15°C and +5°C, carb heat should be applied to keep the carburettor air temperature (CAT) out of this arc. Refer to section 4 of the Pilots operating hand book for the correct procedure.
b. Vigilance of the surrounding environment is critical such as not flying into known icing conditions as prevention is better than cure as well as being weary of humidity, cloud, fog, snow, etc.
c. Check the spread between outside air temperature and dew point in the metrology report, as these two get closer the likelihood of icing increases.
Air induction systems (Carburettor)
Robinson R22 helicopters use a flexible duct to direct air to an air box which then runs through a filter and on to the carburettor. An alternate hot air intake which collects warm air next to the warm engine also run runs to the air box, the amount of which is controlled from the cockpit using the carb heat control.
Alternate induction systems (Turbo Charger ; Supercharger)
As engine performance drops with an increase in air density as altitude increases, superchargers are used to overcome this.
Super Chargers use the rotation of the engine to drive a compressor which boosts the amount of airflow to the engine. This artificially increases the density of air in the induction system which increases the pressure gradient between the inlet manifold and the induction stroke, increasing volumetric efficiency of the engine, allowing more fuel into the system and as such increasing horsepower. This is beneficial to maintain sea level power with a gain in altitude.
By adding the supercharger which is connected to the engine by a belt or gear system, weight is added, however the approximate 40% power increase will make up for this for the same fuel consumption. And because it is connected to the engine it does utilise some of the available horsepower.
This compression of air will increase temperature which may lead to detonation, to overcome this, fuel is added after the carburettor to utilise the beneficial cooling effect of vaporising fuel. An intercooler may also be used to assist cooling.
They are most efficient when the throttle is fully open and the engine speed as low as possible before detonation occurs. Engine management is critical.
With the use of superchargers, engines are exposed to higher temperatures and pressure which may affect longevity if poorly managed by the pilot.
Turbochargers are also dynamic compressors like the superchargers (turbocharger is a contraction of the word turbosupercharger), it however uses the engine exhaust gasses to drive a compressor to feed more air into the engine. Another major difference is that it operates before the carburettor.
As mentioned, horsepower decreases with altitude gain, to regain sea level pressure, air is compressed to normal levels in what is called Turbo Normalising, and any more pressure increase is called Turbo Charging.
A lot of heat is create (more so than a supercharger) because of the increased movement of air, this heat reduces the density of air – which opposes the desired effect of increasing the amount of air, so again, intercoolers are used to decrease temperature.
An intercooler sits between the Turbo and the engine, the warm air will pass through channels with cooling fins attached which are cooled by external airflow, reducing temperature.
Because of the fact that Turbos are driven by exhaust gas, it takes time for the compressor to spool up, this is called bootstrapping or turbo lag. This lag results in non-linear power output which makes power control difficult. Twin, Parallel, Sequential and Two Stage Turbo Charging are methods used to overcome this lag.
Higher engine RPM will create more exhaust gas which turns the compressor faster which in turn forces more air into the engine. Engines may not be able to handle this added pressure so a wastegate is added to the exhaust system which opens a flap to expel excess air. Wastegates are most often controlled with engine oil which closes the system which is opened by a spring system.
Both of these systems add intricacy to power management as well as maintenance.
Fuel Injection systems
A fuel injection system uses an engine driven fuel injection pump to supply fuel at a high pressure to a fuel/air control unit. This unit meters fuel and air to the engine according to the throttle and mixture input by the pilot. This metered fuel then flows to a fuel manifold valve (distributor) which distributes the fuel equally to each cylinder; each cylinder has its own fuel injector/discharge nozzles which vaporizes the fuel, and is situated in the inlet port ahead of the inlet valve in a Continuous/Constant Flow System, or directly into the combustion chamber of a Direct Injection System.
Fuel injected systems have many benefits as opposed to carburetted systems, namely:
1. Less susceptible to icing (only fuel temperature drop occurs at or near the cylinder – due to fuel vaporisation).
2. Uniform fuel distribution which leads to even heating of the cylinders and Improved fuel economy (in carburettor systems most cylinders must run richer to ensure optimal operation of the leanest cylinder).
3. Improved fuel/air ratio which leads to a higher degree of atomization which improves fuel economy.
4. Power output is higher (2 ; 3).
5. Easier engine management as well as diagnostics with electronic systems.
6. Smoother throttle response.
7. Starting is made easier – particularly in cold conditions.
8. Reduced possibility to backfire.
The disadvantages of this system are:
1. Warm fuel will vaporize in the fuel lines making it difficult to start a hot engine.
2. The system is less resilient to fuel quality as the parts are finer.
3. More expensive than carburettor systems in acquisition costs.
4. Still susceptible to impact icing.
Aero Engine Fuel
Types of aviation fuel
1. Jet Fuel often referred to as Aviation Turbine Fuel (ATF/Avtur).
It is a kerosene (paraffin) based fuel which has a clear to straw like colour, it is most commonly used in turbine engine aircraft and to a lesser extent compression ignition engines.
Jet fuel has a high flash point meaning a higher temperature is required to ignite it, this makes it safer for transporting and handling.
The most commonly used jet fuel in civil aviation is Jet-A1.
Jet A1 has a flash point from 38°C and will self-ignite at 210°C. Jet A1 freezes at -47°C.
There are a few alternate types of jet fuel that are used in military application, the only commonly encountered in civil aviation are Jet A (mainly in the USA), which varies only slightly to Jet A-1, and Jet B which has a lower freezing temperature of -60°C and is used only in cold climates – it also has a lower flash point making it more volatile.
Specific gravity ~ 0.79 at ISO. (Which means that it weighs 0.79kg per litre, where water weighs 1kg per litre).
2. AVGAS (Aviation Gasoline).
AVGAS is used in reciprocating aircraft engines and differs substantially from Jet fuel, it has a low flash point and is thus highly flammable and more volatile making proper handling and storage imperative.
The most common Octane of AVGAS and the only available in South Africa is 100LL (low lead) (100/130) and is dyed blue for identification purposes. Although new types are being tested to phase out the use of 100LL because of its expensive and toxic tetraethyl lead (TEL); which is added as it delays the onset of detonation; this allows the engine to produce more power.
100 high lead fuel is coloured green for identification purposes.
The grades of AVGAS include 80/87, 91/96, 100/130, 108/135 and 115/145.
The two numbers refer to the lean mixture/rich mixture i.e. 100 lean mixture/130 rich mixture.
-These numbers relate to the detonation value of the fuel mixture in the engine relative to the hydrocarbon iso-octane which is rated at 100 and heptane which rated at 0; higher numbers detonate later allowing for more compression and are used in high compression engines as such.
Specific gravity ~ 0.72 at ISO.
3. MOGAS (Motor Vehicle Gasoline)
Certain aircraft with low compression engines or with supplemental type certificates may allow the use of MOGAS. There are many associated problems with using automotive fuels in aircraft engines, namely, poor quality control of fuel, high volatility, and increased engine wear owing to lower lead content and increased chances of vaporisation.
Specific gravity ~ 0.75 at ISO.
Fuels have varying agents added for specific benefits without compromising the integrity of the fuel.
• Anti-static agents to prevent a spark.
• Anti-Corrosion agents to prevent rust.
• Anti-icing agents
• Biocides to prevent micro bacterial growth.
• Metal deactivators to prevent the formation of gummy residues created by ions of residual metals in the fuel during the oxidation process.
• Colour dyes for identification purposes.
Fuel quality is very strictly controlled during manufacture and distribution to ensure a product which is universally standard to ensure optimum output of the engine. During transfer and storage of fuel, various filters are used to remove sediment and other particles from the fuel.
Water most often compromises the quality of fuel. It will be entrained in the fuel but will settle in the bottom of the tanks given its higher density to fuel. Water will enter the fuel system either through leakages but of greater concern is water vapour present in the air that will condensate in the fuel tanks – it is therefore good practise to fill the fuel tanks of the aircraft after flight to limit the water vapour containing air in the fuel tanks.
Inspection – fuel strainers and drains
Prior to each flight the various components of the fuel system should be checked for leaks, corrosion and icing, from the fuel tanks, to the fuel lines, pumps and filters.
The fuel its self also needs to be inspected from where it is stored prior to refuelling as well as the fuel in the tanks and strained at varying points as indicated in the operating handbook.
Fuel needs to be checked using a clean and clear strainer for bacterial growths, metal shards as well as water, water is denser than fuel and will sink to the bottom of the strainer being clearly visible – continue straining until no more water is visible.
It is good practice to let fuel settle after refuelling before straining for inspection.
Furthermore it is important to check that the appropriate fuel is in the aircraft by means of its colour. AVGAS is light blue, Jet-A1 is clear to straw like in colour with an oily texture.
Aircraft are refuelled using either a low pressure gravity fed systems or high pressure systems (to increase speed of refuelling as well as prevent water from entering).
Volumetric top off valves which utilize float valves are installed in the refuelling system to cut the fuel supply before an overflow occurs, caution is still to be taken as there may be a delay of it may not be functioning.
• The aircraft should be to the earthed to the fuelling system so that any charge will flow along the earth wire, preventing a spark from igniting vapours and causing an explosion. After this connection, before the fuel caps are removed, the hose nozzle should be bonded to the aircraft to ensure no spark will occur near the fuel vapour in the tank of the aircraft.
• The aircraft as well as the refuelling truck or bowser should have parking brakes applied and be chocked in place to prevent movement. The refuelling truck or bowser must be positioned in such a manner that it does not have to reverse to leave the site.
• Other than the Halon fire extinguisher on board the aircraft, there should be an appropriate fire extinguisher on hand.
• Mobile devices may not be used in the vicinity of an aircraft being refuelled.
• It is forbidden to smoke within 15 meters of an aircraft being refuelled.
• Aircraft engines as well as APU’s should be switched off during refuelling.
• When refuelling from jerry cans it is to be ensured that the cans are certified for fuel so to prevent static build up, the funnel should also be bonded to the aircraft prior to refuelling.
• Do not refuel an aircraft within 30m of radar equipment.
• Flash bulbs and photographic equipment should not be used during the refuelling process.
• Refuelling should only be done by appropriately trained personnel.
• Check that fuel caps are appropriately fasted after the process.
• Ensure that the correct units are used, i.e. litres vs gallons vs kg vs pounds.
• It is the responsibility of the pilot to ensure that the correct fuel is added to the aircraft. – Stickers will be placed on both the bowser as well as most aircraft, colours of the fuel must also be referenced as mentioned in the Aero Fuels section.
Starting Procedures and Precautions
For engines to go from a stationary state to creating mechanical energy by burning the stored fuel, they must be started. This is done in a piston powered aircraft by using a starter motor which turns the engine at a relatively low speed until the magnetos spark the fuel causing the combustion process to ensue.
Starting an aircraft engine isn’t as simple as turning the keys in your car and hoping for the best, certain factors need to be considered and precautions should be taken. Refer to the pilots operating handbook for the appropriate procedures and checklist to start the engine appropriately (Section 4 of the RH22 Pilot’s operating handbook).
It is critical that the engine oil be checked prior to starting as it is fundamental to reliable operation and engine longevity as it cools the engine by reducing friction between moving parts as well as removing wear particles from the engine. It must be checked for quantity (4qts in the R22, 7qts in the R44), contamination as well as consistency (Less viscous for cold oil and more viscous for warm oil). Always ensure that the correct oil type is used – Thinner oils are often used for newer or overhauled engines – refer to the POH.
It is important to check that there is a positive oil pressure within 30 seconds of starting the engine otherwise it should be shutdown to prevent damage, the gauge might be faulty but it’s not worth the costly risk.
Hydraulic Lock (Hydraulicing)
If an engine has been stationary for a long period of time, a thin layer of fuel and oil build up on the cylinder walls in the combustion chambers or intake pipes. Starting in such a scenario may result in this incompressible liquid film taking up extra space during the compression stroke preventing the pistons from moving to top of dead centre and may lead to the cylinder blowing out or the push rod being bent. To prevent this, the engine must be turned by hand, or cranked with the fuel disengaged, this will remove the thin film layer – being cautious to ensure that the magnetos are off to prevent ignition. In cases of extreme build up, the spark plugs must be removed to allow the liquid to drain out – this is to be done by a licenced mechanic or engineer.
Engine Priming and Mixture Control
Engines start a lot easier with a richer mixture, particularly in cold weather. So the mixture is placed at the full rich position for start. Mixture adjustments are only made once the engine is running, where the mixture may be leaned as decreased air density (i.e. high altitude) will result in a richer mixture (at altitude there is less air particles for the same amount of fuel particles). For sea level flights the mixture always in the full rich position.
A richer mixture will cool the engine while a leaner mixture will heat up the engine.
If an exhaust gas gage is installed, it will indicate an increase in temperature while the mixture is leaned, once the needle stops while leaning, the ideal mixture is achieved. If there is no EGT gauge installed, ideal mixture is achieved just.
Certain systems make use of a primer pump which sprays fuel into the induction valve close to the inlet valve (RH44). Other systems require the throttle to be pumped (R22), however caution is to be taken as this may cause the pooling of fuel in the venturi because of the accelerator pump injecting fuel there – if a backfire were to take place, this build-up of fuel will cause an engine fire.
This is a situation that occurs when there is too much fuel in the cylinders because of priming for too long. To remove this excess fuel, the engine must be cranked by engaging aircraft starter motor with the fuel mixture in the cut off position, the throttle in the open and the magnetos on. Continue tis cranking for no more than 20 seconds at a time with 1 minute rest intervals to prevent the starter motor from burning out. Once the engine starts, slowly move the mixture to full rich and adjust the throttle accordingly.
The manifold pressure gauge will indicate the approximate atmospheric air pressure.
Unnecessary electronics must be switched off and the circuit breakers inspected to prevent a short circuit of the systems. The fuel shut-off valve must be open and the mixture
Recognition of Malfunctions and Remedial action
i. Engine Failing to Start
a. Fuel Starvation – check the fuel shutoff valve and fuel mixture control.
b. Incorrect priming.
c. Weak battery, the engine won’t crank or will crack slowly.
d. Water in carburettor.
e. Broken starter motor.
f. Broken wiring.
g. Clutch may be engaged (re run through start-up checklist).
ii. Rough Running
Most cases of rough running engines are also accompanied by lower power being available as well as other indications as mentioned below.
a. Blue Smoke
Worn piston rings will cause oil to seep into the combustion chamber releasing a blue smoke as the oil is burnt.
b. Black Smoke
When there is too much fuel being burnt in the combustion chamber, black smoke will be seen coming out of the exhaust, this may be as a result of too rich a mixture, over priming or an unlocked primer pump. A brief red exhaust flame may also be noticed.
When the fuel in the cylinders burns too slowly to complete combustion by the end of the exhaust stroke and this still burning fuel then ignites fuel in the intake manifold or induction system as soon as the intake valve opens for the new cycle. Having too lean a mixture is the most common cause, however, faulty ignition wires, spark plugs, valve settings, fuel injector nozzles, etc. may also prevent full combustion from taking place leading to backfiring. It is usually isolated to one or two cylinders.
d. Afterfiring (Afterburning)
In this case, the mixture is too rich but as with back firing, the fuel in the cylinders burns too slowly however instead of reacting with fuel in the intake or induction system, air from the exhaust system ignites causing an explosion in the exhaust system. Leaning the mixture may resolve this, however faulty spark plugs, valve settings, fuel injector nozzles, etc. may also be the cause.
This occurs when the temperature and pressure in a cylinder is so high that the compressed mixture spontaneously combusts out of phase resulting in the piston being forced back by the explosion. This is as a result of high manifold pressure (power setting), high intake temperature, using octane of too low a rating, or too lean a mixture. Indications of its imminent onset may include high cylinder head temperatures and pressures, if this noticed, the mixture should be richened (fuel has cooling properties), power setting lowered and airspeed increased (to allow cooling airflow).
When hot spots form in the cylinder because of carbon or other build ups, areas that have been roughened from detonation, or any other damages such as cracked valves and damaged spark plug insulators. The result is that the mixture is ignited prematurely. Other than rough running, backfiring and sudden cylinder head temperature increases with be noted. Engines which run at high temperature because of factors such as lower than specified fuel grade or running a mixture which is too lean will develop these hot spots. Incorrect ignition timing is also a factor. Isolated to only one cylinder.
Running the engine within its manifold limits and cylinder head temperatures, at the correct mixture setting with the proper grade of fuel is the best way to prevent Detonation which in Turn will prevent Pre-Ignition.
It can be noted that the correct grade of fuel is important and should no alternative be available, a higher grade need be used.
Warming up, power and systems check
An engine be must be properly warmed up before applying high settings such as during take-off to prevent damage to the various components of the engine. A rapid increase in power will increase the cylinder head temperature rapidly which will fatigue the metal components leading to cracks. This also give the oil time to become more viscous as it heats up which improves it’s lubricating benefits by allowing it to move quicker and into smaller spaces.
In the Robinson R22, The engine is initially idled at 55%, only once the clutch light goes off; confirming correct tension of the belts, is the engine rolled up to between 70 and 75% for warm up. This varies from manufacturer and the appropriate numbers must be checked in the POH, these are however usually indicated with green arcs which relate to vibration.
At this point the various engine gauges must be inspected: Oil Pressure Gauge, Oil Temperature Gauge, and Cylinder Head Temperature Gauge. These three readings are complimentary to one another, i.e. a low oil pressure reading will mean that there is not enough oil in the system, leading to an increase in both the oil temperature and cylinder head temperature. This can be used as a cross check for faulty gauges.
• The Oil Pressure Gauge reads at what pressure the oil is being passed through the engine pressure pump and relief valve.
• The Oil Temperature Gauge is usually placed just after the oil cooler before entering the engine as a test of the oil cooling system, one may be placed after the engine.
• The Cylinder head temperature gauge read the temperature of the hottest cylinder.
The oil pressure must be in the green within 30 seconds or the engine must immediately be shut down. It may take some time for the oil and cylinder temperature readings to go into their green arcs. During this time and 70-75% power, the following checks can be completed as the system heats up.
i. The Magneto Check
Move the key from both to left, back to both, then to right and once these checks are complete, back to both. A slight drop in RPM will be noted each time the switch is moved from both, this indicates that the opposite magneto is properly grounded.
Cutting out of the engine indicates that the selected magneto is not working.
An abrupt drop in RPM is an indication of faulty spark plugs.
A Slow but significant RPM drop is caused by a faulty valve or incorrect ignition timing.
These symptoms will usually be associated with rough running of the engine.
ii. The Carburettor heat check
The Carburettor heat should be applied taking note of the temperatures before and after the application to confirm an increase in temperature. A slight decrease in engine power may also be noted.
iii. Sprag-Clutch/Freewheeling unit Check.
A rapid closing of the throttle is done to see that the engine tachometer drop is much faster than that of the rotor RPM to ensure that the engine RPM won’t decay the Rotor RPM in an autorotation. Slowly increase the throttle after this check.
In cold temperature it make take much longer to reach the suitable temperatures, it may even be necessary to pre heat the engine with heated cover prior to start in extreme conditions.
Power Settings & Limitations
Engines as well as gearboxes have limitations to their capabilities, it is important to be cognisant of this and manage the power settings accordingly. Below is a table of the power plant limitations of the Lycoming 0-320 and 0-360 as found on the Robinson R22 helicopters as found in section 20 of the Robinson Pilots Operating Handbook.
Oil and Pressure Limitations
Operating Limitations of The Lycoming 0-320 and 0-360
Engine Maximum Speed 2652 RPM (104%)
Cylinder Head Max Temp 500°F (260°C)
Oil Maximum Temperature 245°F (118°C)
Minimum during idle 25 psi
Minimum during flight 55 psi
Maximum during flight 95 psi
Minimum during start and warm up 115 psi
Minimum Oil for take off 4 qt
Normal operating ranges are indicated with green arcs.
Precautionary or special operating procedure ranges are indicated with yellow arcs.
Red arcs are used to indicate the operating limits and should not be exceeded during normal operation.
Manifold Pressure Limitations
Maximum Continuous Power: This is the maximum amount of power that can be used for extended periods – longer than 5 mins.
Maximum Take-off Power: This is the amount of power that can be used up to a maximum specified time limit, in the R22 it can be used for 5mins.
These are not fixed values as Manifold Air Pressure is affected by both Pressure Altitude and Temperature.
With reference to the table below, lets assume we are at 4000ft and the temperature is 30°C, this means we have 21.8 inches Maximum Continuous Power. Next we add 0.9 (as stated on the bottom of the chart for The Maximum Take-off Power, which gives us 21.8 + 0.9 = 22.7.
*When the engine is off, the Manifold Pressure will indicate 29.92 Inches Mercury (1013hPa).
Most often we don’t have values which are so exact, so we round the values up or down for use in the calculation. Linear Interpolation as approximation is the most mathematical correct manner of calculation but is not necessary as there is negligible practical discrepancy in the result.
Example: OAT=24°C. Pressure Altitude = 3220ft.
We round the values to 25°C and 3000ft and use the average of the 4 values as indicated in the block below.
(22.2+22.5+21.8+22.0)/4=22.125 we use 22.1 as Maximum Continuous Power and add 0.9 to get 23 as the value for Maximum Take-Off Power.
Older Models of R22 Helicopters make use of the chart below, Note 1 inchHG is subtracted from Maximum Take-off Power to give us Maximum Continuous Power.
The orange line represents 3200ft
The Red Line represents 24°C
Where they intersect at 24.1 is MTP
We subtract 1 inch for MCP which gives us 23.1
Any time we use more than Maximum Take-Off Power we are over exerting the engine and quite possibly other systems such as the main rotor gear box, this is a situation called over boosting.
When we fly at power settings greater than Maximum Continuous Power for longer than 5min excessive strain is places on various components, because of the long period of added vibrations and increase in temperature, this may be indicated on the temperature and pressure gauges. The danger however lies in decrease in component lifespans and burden of increased unscheduled maintenance.
Avoidance of Rapid Power Changes
Power changes should also be gradual, avoiding rapid changes in power as this leads to sudden temperature changes which can lead to thermal shock resulting in cracked cylinder heads.
When descending it is advisable to not too lower the power setting too much but rather keep the engine doing some work as power settings which are too low reduces the amount of pressure which keeps the piston ring on the cylinder walls, this reduced pressure allows oil to leak past and make contact with hot surfaces – this is called glazing.
Abnormal Instrument Readings and Warning Lights
*Chart based on R22 and R44 refer to section 3 of the POH.
Light Colour Reason Remedial Action Note
Oil Red Loss of Engine Power or Oil Pressure. Check Oil Pressure gauge to confirm the light. Shutdown if on the ground or land immediately if airborne. Continued operation without oil pressure will cause serious engine damage and engine failure may occur.
Main Rotor Temp Amber Excessive Temperature in Main Rotor Gearbox If light is accompanied by any noise, vibration, or temperature rise, land immediately. I there are no other indications, land as soon as practical. Break in fuzz of new components may activate these warning light.
Main Rotor Chip Amber Metallic Particles in Main Rotor Gearbox
Tail Rotor Chip Amber Metallic Particles in Tail Rotor Gearbox
Low Fuel Amber One gallon of useable fuel in the tank. Land immediately This is 5 minutes of fuel at cruise power
Clutch Amber Indicates that the clutch actuator circuit is on. This means that it is either engaging or disengaging the clutch – re-tensioning the belts. If the light remains on for longer than 8 seconds, pull the clutch circuit breaker, reduce power and land immediately. The clutch light often comes on while on the ground or during flight to retention the belts.
Alternator Amber Indicates low voltage and possible alternator failure Turn off nonessential electrical equipment, switch the alternator off and then on again. If light stays on, land as soon as practical. Continued flight will result in loss of the tachometer.
Brake Amber Indicates rotor brake is engaged Release immediately in flight or before starting The engine will not start with the rotor brake engaged.
Starter Amber Starter motor is engaged If starter light does not go off when the ignition switch is released, immediately cut the engine and have motor serviced.
Governor off Amber Governor is off Land as soon as practical controlling RPM with manual throttle inputs. Avoid rapid power changes.
Low RPM Amber light and Horn The rotor RPM is critically low Lower collective, roll on the throttle and if in forward flight, apply aft cyclic to restore RPM. The horn and caution light are disabled when the collective is full down
Hydraulic Failure None Hydraulic failure due to possible leak or mechanical defect Land as soon as possible avoiding rapid control inputs. Controls will be very stiff.