Leanen
Under Leanen (from English lean = lean, slender) refers to the leaning of the air-fuel mixture in petrol engines , as aircraft engine in aircraft can be used. Leanen reduces the proportion of fuel in the fuel-air mixture that is fed to the engine in the partial load range, which lowers fuel consumption and thus increases the range. In most models, leaning is done manually by the pilot using the mixture control (condition lever) and less often automatically. In contrast, when the power requirement is high, a richer mixture is set, which ensures higher power and lower temperatures on highly loaded engine parts.
Basics
Air-fuel mixture
In an internal combustion engine , the fuel-air mixture only burns completely at a mass ratio (in contrast to the volume ratio ) of 1: 14.7 (1.0 kg of fuel to 14.7 kg of air ). With this ratio, the carbon (C) and the hydrogen (H) of the fuel (different hydrocarbon compounds - C x H y ) react with the oxygen (O) of the air, a distinction is made between complete and incomplete combustion as follows:
Complete combustion of long-chain hydrocarbons
- Octane and oxygen react under optimal conditions to form carbon dioxide and water .
Ordinary, incomplete combustion of long chain hydrocarbons
- If the combustion is incomplete, octane and oxygen react to form carbon dioxide, carbon monoxide , soot , short-chain hydrocarbons ( propene ) and water, and incomplete combustion in the engine also produces nitrogen oxides and possibly sulfur oxides . These types of burns occur when the combustion temperature is too low or not enough oxygen is available.
This creates carbon dioxide (CO 2 ) and water (H 2 O) in the form of water vapor . However, this is an idealized representation of the running processes, which, as further studies have shown, are much more complex in reality.
In the case of a stoichiometric mixture, the air-fuel ratio is precisely dimensioned such that the air mass is present that theoretically oxidizes all fuel to H 2 O and CO 2 . From this ideal stoichiometric fuel ratio (1: 14.7), a higher proportion of fuel (“rich”, “rich”, English: rich, e.g. 1:13) or a higher proportion of air (“lean”, “poor”) , English: lean, e.g. 1:16) can be deviated from within certain limits. However, the mixture should only be made leaner to such an extent that it remains ignitable (up to approx. 1:30).
In contrast to cars, airplanes are operated at very different altitudes. At high altitudes, there is no longer enough oxygen available for combustion because of the “thin air”. The optimal technical solution would be to charge the engine (e.g. to force more air into the cylinder with the help of a turbocharger ) - this would preserve the engine's full power. The next best technical solution is discussed here leaning ( Leanen ). Less fuel is added to the cylinder when required to compensate for the lower amount of oxygen in the cylinder. The engine runs on "low flame" (less oxygen, less fuel) and delivers less power .
Failure to lean at higher altitudes would result in increased fuel consumption, reduced range and an increase in operating costs.
The increased combustion temperatures are manageable for the engine in the partial load range.
Motor vehicle
In its main features, the mixture formation of a regular gasoline engine , z. B. in a motor vehicle analogous to that in an aircraft engine. One differentiates:
Classic mixture preparation
Carburettors or manifold injection were used to regulate the fuel-air mixture in motor vehicle engines. These ensure a rich, performance-optimized mixture in the full load range and a relatively lean, fuel-saving mixture in the partial load range, with an accelerator pump briefly enriching the mixture when accelerating.
It is assumed that the air pressure is greater than 900 mbar. The significant mixture enrichment z. B. when driving on passes in high mountains leads to a drop in performance, which is usually accepted with the carburetor engine. If you are on the road for longer trips in the high mountains, however, there is the possibility of saving fuel or achieving better emissions by setting the carburettor leaner. Individual engines with electronically controlled intake manifold injection with air flow meter also have an additional switch controlled by a height measuring cell, which, from a certain height, gives the control unit a signal to lean the mixture.
Current mixture preparation
In the days of carburetor technology, height correctors were installed as additional equipment on the carburettor, which automatically leaned the mixture by means of a barometer box and solenoid valves when the air pressure fell. Newer motor vehicle engines have air mass meters that provide the engine control unit with a reference value for the air mass that is sucked in. In the partial load range, precise metering of the amount of fuel in relation to the available combustion air is achieved after measurement by the lambda probe , which measures the residual oxygen content in the exhaust gas. The control unit corrects the amount of fuel injected to the stoichiometric value of lambda 1 based on the feedback from the lambda probe. For example, when accelerating or driving very fast, the mixture is slightly enriched (lambda 0.8 ... 0.9), since more power and lower temperature load are achieved). The background to this is the increasingly strict exhaust gas standards that would not be able to be met without modern mixture preparation.
plane
In the case of aircraft piston engines , the different operating environments result in some differences which require the engine to be leaned. At higher altitudes, an aircraft is exposed to significantly lower air pressures ( barometric altitude formula ), which causes the mixture to enrich. Therefore manual or automatic intervention in the mixture formation is necessary.
In addition, aircraft engines usually fly at lower ambient temperatures . Therefore the cooling works more efficiently. A powerful and therefore intensive cooling is actually only required when starting under full load and when rolling (almost no airflow). A richer mixture then supports the cooling of the engine and especially the exhaust valves thanks to its higher fuel content:
An aircraft engine under full load on the ground would also be damaged by pre-ignition ( detonation ) and overheated exhaust valves (top lands / cylinder head gaskets) at a lambda of 1 . Therefore, when rolling, starting and climbing - that is, whenever a lot of power is required - a slightly over-rich mixture (lambda 0.8 ... 0.9) or even richer as engine protection (with a reduction in performance) is used. Characterized decreases the EGT (exhaust gas temperature -. Dt exhaust gas temperature ), and consequently the CHT ( cylinder head temperature -. Dt cylinder head temperature ) at überfettem mixture. This side effect also lowers the engine temperature somewhat. The fuel consumption increases at take-off due to the over-rich mixture, but is reduced due to the emaciation during cruise. The best engine efficiency is set there with approx. Lambda 1.1, which also results in the highest exhaust gas temperature, which, however, is tolerated without damage in the partial load range.
Leanen therefore serves to protect the engine from damage due to overheating or hypothermia under all flight conditions and to find the best combination of high performance, low fuel consumption and the best range for the current performance requirements and the current environmental conditions (air pressure, air and engine temperature) . Control whether geleant right, by the EGT indicator ( exhaust gas temperature - dt. Exhaust gas temperature gauge done in cockpit).
In addition to the manual mixture setting, some aircraft engines also have an automatic altitude correction and automatic mixture enrichment, for example with injection engines . But carburetors can also provide an altitude-corrected mixture with the help of a vacuum unit , which relieves the pilot of this task. The mixture lever ( condition lever ) is only required in such aircraft to switch off the engine. A carburetor with automatic altitude correction can be found in the Piaggio P.149 or the Dornier Do 27 , for example .
The engines from Thielert and SMA based on the diesel principle have a turbocharger and fully electronic engine control or "single lever control". This eliminates the need for leanen and the turbocharger provides the necessary oxygen supply even at great heights.
Lean process
For piston engines with low power and without control by an EGT display , the mixture is depleted (slowly pull out the red mixture control button) until the engine runs rough, then enrichment takes place (turn the red mixture control button two or three turns) until the engine runs smoothly.
Modern aircraft engines with higher power require a different process. The exhaust gas temperature measuring device ( EGT ) is used here to regulate the mixture . About five minutes after reaching cruising altitude, when the engine temperatures have stabilized, the mixture is depleted with the mixture lever until the exhaust gas temperature reaches its peak EGT .
You have to remember this peak value. Most EGTs have an additional, adjustable reference pointer in addition to the actual temperature indicator . This is set to the displayed peak value. The mixture is then enriched again (turn the red mixture control knob) until the exhaust gas temperature drops by 50 ° F ( Fahrenheit ). This corresponds to two graduation marks on the EGT scale . This ensures a combustion and exhaust gas temperature that does not damage the engine even in continuous operation. The fuel consumption is very cheap, the range of the aircraft and the power level of the engine are optimal, but not maximal.
1st alternative:
Occasionally, an associated, correct fuel flow is also set for the respective selected power setting using a table. This method also works without control by an exhaust gas temperature meter. The input parameters for this table are speed and intake manifold (under) pressure.
2nd alternative:
To achieve better consumption values , the mixture in engines with manifold injection can be leaned to the peak EGT value (recommendation by Lycoming and Continental ) or even to 50 ° C. For this purpose, it is emaciated from the peak value until a drop of 50 ° C EGT is displayed. This means that fuel consumption is kept to a minimum. However, this procedure should only be used if the engine is equipped with an engine monitoring instrument that displays the EGT (exhaust gas temperature) and the CHT (cylinder head temperature) for each individual cylinder (so that it can be seen if a cylinder is running too lean). It is also advisable to retrofit the engine with coordinated injection nozzles for using this method so that the leaning process takes place as simultaneously as possible in all cylinders and so that the thermal and mechanical load remains homogeneously distributed over all cylinders.
Operating areas
A distinction can be made between three operating ranges in which different mixture settings are required:
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Operation from idle to approx. 50% engine power
This is the range that is also set for an approach without engine power. The engine is running at too low an operating temperature, possibly with too low a speed and with a mixture that is too rich. The consequences are cold sludge formation and dangerous lead-containing deposits over longer periods of time. Therefore this operating area should be avoided. If that is not possible, the highest acceptable output must be set and then the mixture must be made lean. Exception: It is not allowed to land on the approach. A full mixture must be set here in order to be prepared if a go - around is necessary.
Operation with engine powers from 50% to 75%
This is essentially the cruising flight . In this area you must always lean, in aircraft with fixed propellers up to maximum speed and in constant-speed propellers up to maximum speed. The engine runs economically and in a favorable temperature range. There is only a slight tendency towards deposits, no thermal overload, maximum self-cleaning effect of the spark plugs , minimum pollutant emissions. This area should be chosen whenever it is reasonable.
Operation with an engine power over 75%
This power is needed for take-off and climb flights . It should only be called up by the engine when the mixture is full. Impoverishment creates the risk of overheating and knocking burns . However, the full mixture also leads to higher deposition rates and unnecessarily high fuel consumption. For this reason, this area may only be used for take-offs and climbing at traffic lap height.
Risks
- When taking off and climbing (with> 75% power) from high-altitude airfields, a fully rich mixture can be too rich due to the lower air density, which is why the engine runs rough and with reduced power. Then the pilot should lean just enough until the engine runs smoothly again. A lean mixture setting can cause the engine to overheat.
- Set a fully rich mixture before every increase in output.
- Carefully readjust the mixture setting when descending from great heights.
- Set the mixture to full on the final approach.
Single / double / multiple
In single-engine aircraft with a controllable pitch propeller (Fig. 1), the engine setting console has three levers (here to slide):
- black = thrust lever ("throttle")
- blue = propeller pitch
- red = mixture lever
On single-engine aircraft without a controllable pitch propeller (Fig. 2), the engine setting console has two levers:
- black = thrust lever ("throttle")
- red = mixture lever
To enrich ( full rich ) the mixture lever (red) is pushed in. To lean the mixture lever (red) is pulled out very slowly. To switch off the engine, the mixture lever (red) is pulled out completely. The mixture then becomes completely lean, the engine only sucks in air without fuel and thus goes out dry . In contrast to the carburettor engine, where the engine is stopped by switching off the ignition, in an aircraft engine this is done by controlling the fuel supply using extreme leanen.
In two-engine airplanes (Fig. 3) the engine setting console ( throttle quadrant ) has 6 levers - in pairs for the right and left engine. With the two red levers (on the right in Figure 3), both engines can be leaned separately. On the left (black) are the two thrust levers (power lever or "throttle lever"). In the middle (not visible in the picture because very far forward) are the levers for the propeller pitch (angle of attack).
In aircraft with more than two engines, the number of controls increases accordingly.
literature
- Bachmann, Faber, Senftleben: Danger manual for pilots , Motorbuch Verlag, Stuttgart 1981, ISBN 3-87943-656-8
- Ernst Götsch: Luftfahrzeug-Technik , 3rd edition, Motorbuch Verlag, Stuttgart 2003, ISBN 3-613-02006-8
- Jeppesen Sanderson: Privat Pilot Manual 2001, ISBN 0-88487-238-6
- Lührs, Henrik and Wahn, Mirko (eds.): Advanced PPL-Guide , Volume 1, Allgemeine Luftfahrzeugkunde, airCademy Verlag, London, Meerbusch 2011, ISBN 978-3-943188-02-8
- Wolfgang Kühr: The private aircraft pilot , technology I, volume 1, Friedrich Schiffmann Verlag, Bergisch Gladbach 1981, ISBN 3-921270-05-7