﻿ Air-fuel ratio

# Air-fuel ratio

Under Leanen means the leaning of the fuel -air mixture in gasoline engines, which are used as aircraft engine in aircraft. By Leanen, the fuel portion of the fuel-air mixture which is supplied to the engine is reduced, which reduces fuel consumption and increases range. The loss of weight is done manually in most models by the pilot by means of the mixture lever (mixture control; condition lever ) and less automatically.

## Basics

### Air -fuel mixture

In an internal combustion engine, the fuel-air mixture burns only at a weight ratio (as opposed to volume ratio) of 1:14,7 completely (1.0 kg fuel to 14.7 kg of air ). At this ratio of carbon ( C) and hydrogen react (H ) of the fuel (various hydrocarbon compounds - C x H y ) with the oxygen (O ) of the air, one distinguishes between complete and incomplete combustion as follows:

Complete combustion of long-chain hydrocarbons

Ordinary, incomplete combustion of long-chain hydrocarbons

Thus will produce carbon dioxide (CO2 ) and water (H2O ) in the form of water vapor. It is, however, an idealized representation of the processes involved, which, as shown further investigations, in reality are much more complex.

At a stoichiometric mixture, the air -fuel ratio is just such that there is exactly the mass of air that theoretically oxidizes all fuel to H2O and CO2. From this ideal stoichiometric fuel ratio ( 1:14,7 ), by a higher proportion of fuel ( "bold", "rich", English: rich, for example, 1:13 ), or a higher proportion of air ( " lean", "arm", English: lean, for example, 1:16 ) to deviate within certain limits. The mixture should, however, be only so far emaciated that it remains capable of ignition ( until about 1:30 ).

Unlike cars airplanes are operated in very different heights. At high altitudes is due to the " thin air " is no longer enough oxygen available for combustion. The optimum technical solution would be to press with the help of a turbocharger more air (under pressure) into the cylinder - so the full power of the engine would remain. The second best technical solution is discussed here emaciation ( Leanen ). In this case, the cylinder is supplied with less fuel needed to compensate for the reduced amount of oxygen in the cylinder. The engine runs on " the back burner " (less oxygen, less fuel ) and dissipates less power.

Not to leanen At higher altitudes would have increased fuel consumption, range shortening and an increase in operating costs.

### Motor vehicle

In its substance, the mixture formation of a regular gasoline engine works, such as in a motor vehicle analogous to an airplane engine. We distinguish:

#### Classic mixture preparation

For controlling the air-fuel mixture carburetor or injection systems are used in motor vehicle engines. In this case, it is assumed that the air pressure is greater than 900 mbar. The significant enrichment of the mixture, for example in negotiating a high mountain pass leads to a drop in performance, which is accepted in cars. If you are on long journeys in the high mountains is however possible to save fuel and to achieve better emissions to where you set the carburetor leaner. Unique electronically controlled fuel injection engines with air flow meter also have an additional, controlled over a height measurement box, switch to the control unit sends a signal to the leaning of the mixture from a certain height.

#### Currently mixture preparation

At the time of gasification technology height correctors were in demand as options on the carburetor installed, which automatically means abmagerten barometer can and solenoid valves with low air pressure werdendem the mixture. Recent automotive engines have air mass meter that provide the engine control unit has a reference value for the intake air mass. An accurate metering of the fuel quantity in relation to the available combustion air is achieved by means of the lambda probe, which measures the residual oxygen content in the exhaust gas. The control unit according to the feedback correction of the lambda probe, the fuel injection amount to the optimum value. An enrichment of the mixture is excluded (except under hard acceleration - here a slight enrichment of the mixture is desired, as more power is achieved ). The background is the ever more stringent emission standards taken which would not be observed without modern mixture preparation.

### Aircraft

In aviation piston engines result from the different operating environment, some differences that require a Leanen the engine. An airplane is exposed to greater heights significantly lower air pressure ( barometric height formula ), which enriches the mixture. Therefore, a manual or automatic intervention in the mixture formation is required.

In addition, aircraft engines usually fly at lower ambient temperatures. Therefore, the cooling works efficiently. A powerful and so intense cooling is really only needed when starting under full load and the rollers (almost no wind ). A richer mixture then supported by its higher proportion of fuel to cool the engine and especially the intake valves:

An aircraft engine would be damaged in a lambda of 1 under full load to the floor by pre-ignition ( detonation ) and superheated inlet valves. Therefore, one uses a slightly überfettetes mixture ( while rolling, at the start and at low altitude ). Accepted the associated performance degradation to keep the ignition timing in a non-critical, late area. ( - Dt CHT cylinder head temperature) at überfettem mixture through the poorer combustion, the CHT decreases. Due to this side effect, the motor temperature is also lowered slightly. Consumption is higher at the start by the superfatted mixture contains, but is reduced by the loss of weight in cruise flight.

The Leanen serves to keep the engine under all flight conditions from damage due to overheating or hypothermia and to find the best combination of high level of performance, low fuel consumption and excellent range. The control whether geleant properly, by the EGT indicator ( exhaust gas temperature - dt exhaust gas temperature gauge ) take place in the cockpit.

In addition to manual mixture adjustment there is in some aircraft engines, an automatic height correction and automatic enrichment of the mixture, for example in Einspritzermotoren. But carburetor can provide you with the help of a vacuum unit, a height- corrected mixture, which relieves the pilot of this task. The mixture control lever (English mixture; condition lever ) is only needed in such aircraft to stop the engine. A carburetor with automatic height correction can be found for example in the Piaggio P.149 or the Dornier Do 27

The engines of the Thielert and SMA companies based on the diesel principle are turbocharged and a fully electronic engine control or "single lever control ." This eliminates the Leanen and the turbocharger ensures even at great heights for the required supply of oxygen.

## The end of the Leanens

For piston engines with low power or control by an EGT indication that the mixture is as long as impoverished ( the red mixture knob slowly draw out), until the engine runs rough, then a re-enrichment occurs ( red mixture knob two or three turns screw ) until the engine runs smoothly.

Modern aircraft engines with higher performance require a different method. Here, the exhaust gas temperature gauge (EGT ) is used for mixture regulation. About five minutes after reaching the cruise altitude when the engine temperatures have stabilized, the mixture with the mixture lever long as depleted until the exhaust gas temperature ( EGT peak german) reaches its peak.

This peak can be forgotten. In most EGT addition to the actual temperature indicator, an additional, adjustable reference indicator ( engl. reference pointer) is available. This is set to the indicated peak. Thereafter, the mixture is again ( red mixture screw knob ) if enriched to an exhaust gas temperature drop of 50 ° F ( Fahrenheit ) occurs. This corresponds to two ticks of the scale of the EGT. Thus, a combustion and exhaust gas temperature is given that the engine does not harm even in continuous operation. The fuel consumption is very low, range of the aircraft and power efficiency of the engine are optimal, but not a maximum.

Alternative 1: Occasionally, an associated, correct fuel is also used for the respective selected power setting flow ( fuel flow ) is set based on a table. This method also works without control by an exhaust gas temperature gauge. The input parameters for this table are speed and intake manifold pressure (below ).

Alternative 2: To achieve better fuel economy, the mixture may be for Injection Engines on the peak EGT (recommendation of Lycoming and Continental), or even at 50 ° C emaciated ( geleant ). To this end, emaciated from the peak of the extent, until a drop of 50 ° C EGT is displayed. Thus, a minimum fuel consumption is achieved.

## Operating ranges

One can distinguish three operating ranges in which different mixture settings are required:

Operating from idle to about 50 % engine power This is the area which is also set at a touch, without engine power. The motor runs at too low operating temperature, possibly too slowly and with too rich mixture. The result is cold sludge and hazardous lead-containing deposits with longer duration. Therefore, this operating range should be avoided. If this is not possible, the highest acceptable performance must be set and then the mixture be leaned. Exception: During the landing approach must not be geleant. It must here be full rich mixture setting, to be prepared in case a go-around is necessary.

Operation with engine outputs of between 50 % to 75 % That's essentially the cruise. In this area must be geleant always, for aircraft with fixed pitch propeller to the maximum speed and at constant speed propeller to maximum speed. The engine runs sparingly and in a low temperature range. There is only a slight tendency to form deposits, no thermal overload, maximum self-cleaning effect of the spark plugs, lowest emissions. This area should be used whenever feasible.

Operating with an engine power above 75% This power is used during takeoff and climb flights. You should be requested from the engine only at full rich mixture. By depleting the risk of overheating and of knocking combustion. However, the full rich mixture also leads to greater deposition rates and to unnecessarily high fuel consumption. Therefore, this area may only be used for the start and the climb to circuit altitude.

## Single engine / Zweimots / Mehrmots

For single-engined aircraft with variable pitch (Fig. 1), the engine settings console has three levers ( here for slide ):

• Black = push lever ( " throttle " )
• Blue = propeller pitch
• Red = mixture lever

For single-engined airplanes without pitch propeller (Fig. 2), the engine settings console has two levers:

• Black = push lever ( " throttle " )
• Red = mixture lever

For enrichment (English full rich) the mixture lever ( red) is pushed. For leaning the mixture lever ( red) is pulled very slowly. To stop the engine the mixture lever (red) is completely pulled out. The mixture then leans entirely on, the motor draws only air without fuel and is thus based dry. In contrast to the car petrol engine, where the engine is stopped by turning the ignition off, this is done on a flight motor by controlling the fuel supply due to extreme Leanen.

For two-engined airplanes (Figure 3), the engine settings console (English throttle quadrant) 6 lever - in pairs for the right and left engine. With the two red levers (on the right in Figure 2), both engines are geleant separately. Links (black ) are the two thrust levers (power lever or " throttle "). In the middle (not visible in the picture, because right at the front ) are the levers for the propeller pitch ( angle of attack).

For airplanes with more than two engines, the number of controls increases accordingly.

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