Factors Affecting Aircraft Reciprocating Engine Operation


To prevent loss of power, all openings to the cylinder must close and seal completely on the compression and power strokes. In this respect, there are three items in the proper operation of the cylinder that must be operating correctly for maximum efficiency. First, the piston rings must be in good condition to provide maximum sealing during the stroke of the piston. There must be no leakage between the piston and the walls of the combustion chamber. Second, the intake and exhaust valves must close tightly so that there is no loss of compression at these points. Third, and very important, the timing of the valves (opening and closing) must be such that highest efficiency is obtained when the engine is operating at its normal rated rpm. A failure at any of these points results in greatly reduced engine efficiency.

Fuel Metering

The induction system is the distribution and fuel-metering part of the engine. Obviously, any defect in the induction system seriously affects engine operation. For best operation, each cylinder of the engine must be provided with the proper fuel/air mixture, usually metered by the carburetor. On some fuel-injection engines, fuel is metered by the fuel injector flow divider and fuel-injection nozzles.

The relation between fuel/air ratio and power is illustrated in Figure 1.

Factors Affecting Reciprocating Engine Operation
Figure 1. Power versus fuel/air mixture curve

Note that, as the fuel mixture is varied from lean to rich, the power output of the engine increases until it reaches a maximum. Beyond this point, the power output falls off as the mixture is further enriched. This is because the fuel mixture is now too rich to provide perfect combustion. Note that maximum engine power can be obtained by setting the carburetor for one point on the curve.

In establishing the carburetor settings for an aircraft engine, the design engineers run a series of curves similar to the one shown. A curve is run for each of several engine speeds. If, for example, the idle speed is 600 rpm, the first curve might be run at this speed. Another curve might be run at 700 rpm, another at 800 rpm, and so on, in 100-rpm increments, up to takeoff rpm. The points of maximum power on the curves are then joined to obtain the best power curve of the engine for all speeds. This best power curve establishes the rich setting of the carburetor.

In establishing the detailed engine requirements regarding carburetor setting, the fact that the cylinder head temperature varies with fuel/air ratio must be considered. This variation is illustrated in the curve shown in Figure 2. Note that the cylinder head temperature is lower with the auto-lean setting than it is with the auto-rich mixture. This is exactly opposite common belief, but it is true. Furthermore, knowledge of this fact can be used to advantage by flight crews. If, during cruise, it becomes difficult to keep the cylinder head temperature within limits, the fuel/air mixture may be leaned out to get cooler operation. The desired cooling can then be obtained without going to auto-rich with its costly waste of fuel. The curve shows only the variation in cylinder head temperature. For a given rpm, the power output of the engine is less with the best-economy setting (auto-lean) than with the best-power mixture.

Factors Affecting Reciprocating Engine Operation
Figure 2. Variation in head temperature with fuel/air mixture (cruise power)

The decrease in cylinder head temperature with a leaner mixture holds true only through the normal cruise range. At higher power settings, cylinder temperatures are higher with the leaner mixtures. The reason for this reversal hinges on the cooling ability of the engine. As higher powers are approached, a point is reached where the airflow around the cylinders do not provide sufficient cooling. At this point, a secondary cooling method must be used. This secondary cooling is done by enriching the fuel/air mixture beyond the best-power point. Although enriching the mixture to this extent results in a power loss, both power and economy must be sacrificed for engine cooling purposes.

Factors Affecting Reciprocating Engine Operation
Figure 3. Fuel/air curve for a water-injection engine

Many older, large, high-powered radial engines were influenced by the cooling requirements on fuel/air mixture, by effects of water injection. Figure 3 shows a fuel/air curve for a water-injection engine. The dotted portion of the curve shows how the fuel-air mixture is leaned out during water injection. This leaning is possible because water, rather than extra fuel, is used as a cylinder coolant. These types of systems are not used on modern aircraft.

This permits leaning out to approximately best-power mixture without danger of overheating or detonation. This leaning out gives an increase in power. The water does not alter the combustion characteristics of the mixture. Fuel added to the auto-rich mixture in the power range during dry operation is solely for cooling. A leaner mixture would give more power. Actually, water or, more accurately, the antidetonant (water/alcohol) mixture is a better coolant than extra fuel. Therefore, water injection permits higher manifold pressures and a still further increase in power.

In establishing the final curve for engine operation, the engine’s ability to cool itself at various power settings is, of course, taken into account. Sometimes the mixture must be altered for a given installation to compensate for the effect of cowl design, cooling airflow, or other factors on engine cooling. The final fuel/air mixture curves take into account economy, power, engine cooling, idling characteristics, and all other factors that affect combustion.

Figure 4 shows a typical final curve for a float-type carburetor. Note that the fuel/air mixture at idle is the same in rich and in manual lean. The mixture remains the same until the low cruise range is reached. At this point, the curves separate and then remain parallel through the cruise and power ranges.

Factors Affecting Reciprocating Engine Operation
Figure 4. Typical fuel-air mixture curve for a float-type carburetor

Note the spread between the rich and lean setting in the cruise range of both curves. Because of this spread, there is a decrease in power when the mixture control is moved from auto-rich to auto-lean with the engine operating in the cruise range. This is true because the auto-rich setting in the cruise range is very near the best power mixture ratio. Therefore, any leaning out gives a mixture that is leaner than best power.

Idle Mixture

The idle mixture curve shows how the mixture changes when the idle mixture adjustment is changed. [Figure 5] Note that the greatest effect is at idling speeds. However, there is some effect on the mixture at airflows above idling. The airflow at which the idle adjustment effect cancels out varies from minimum cruise to maximum cruise. The exact point depends on the type of carburetor and the carburetor setting. In general, the idle adjustment affects the fuel/air mixture up to low cruise on engines equipped with float-type carburetors. This means that incorrect idle mixture adjustments can easily give faulty cruise performance, as well as poor idling.

Factors Affecting Reciprocating Engine Operation
Figure 5. Idle mixture curve

There are variations in mixture requirements between one engine and another because of the fuel distribution within the engine and the ability of the engine to cool. Remember, a carburetor setting must be rich enough to supply a combustible mixture for the leanest cylinder. If fuel distribution is poor, the overall mixture must be richer than would be required for the same engine if distribution were good. The engine’s ability to cool depends on such factors as cylinder design (including the design of the cooling fins), compression ratio, accessories on the front of the engine that cause individual cylinders to run hot, and the design of the baffling used to deflect airflow around the cylinder. At takeoff power, the mixture must be rich enough to supply sufficient fuel to keep the hottest cylinder cool.

Induction Manifold

The induction manifold provides the means of distributing air, or the fuel/air mixture, to the cylinders. Whether the manifold handles a fuel/air mixture or air alone depends on the type of fuel metering system used. On an engine equipped with a carburetor, the induction manifold distributes a fuel/air mixture from the carburetor to the cylinders. On a fuelinjection engine, the fuel is delivered to injection nozzles, one in each cylinder, that provide the proper spray pattern for efficient burning. Thus, the mixing of fuel and air takes place at the inlet port to the cylinder. On a fuel-injection engine the induction manifold handles only air.

The induction manifold is an important item because of the effect it can have on the fuel/air mixture that finally reaches the cylinder. Fuel is introduced into the airstream by the carburetor in a liquid form. To become combustible, the fuel must be vaporized in the air. This vaporization takes place in the induction manifold, which includes the internal supercharger, if one is used. Any fuel that does not vaporize clings to the walls of the intake pipes. Obviously, this affects the effective fuel/air ratio of the mixture that finally reaches the cylinder in vapor form. This explains the reason for the apparently rich mixture required to start a cold engine. In a cold engine, some of the fuel in the airstream condenses out and clings to the walls of the manifold. This is in addition to that fuel that never vaporized in the first place. As the engine warms up, less fuel is required because less fuel is condensed out of the airstream and more of the fuel is vaporized, thus giving the cylinder the required fuel/air mixture for normal combustion.

Any leak in the induction system has an effect on the mixture reaching the cylinders. This is particularly true of a leak at the cylinder end of an intake pipe. At manifold pressures below atmospheric pressure, such a leak leans out the mixture. This occurs because additional air is drawn in from the atmosphere at the leaky point. The affected cylinder may overheat, fire intermittently, or even cut out altogether.

Operational Effect of Valve Clearance

While considering the operational effect of valve clearance, keep in mind that all aircraft reciprocating engines of current design use valve overlap. Valve overlap is when the intake and exhaust valves are open at the same time. This takes advantage of the momentum of the entering and exiting gases to improve the efficiency of getting fuel/air in and exhaust gases out. Figure 6 shows the pressures at the intake and exhaust ports under two different sets of operating conditions. In one case, the engine is operating at a manifold pressure of 35 "Hg. Barometric pressure (exhaust back pressure) is 29 "Hg. This gives a pressure acting in the direction indicated by the arrow of differential of 6 "Hg (3 psi).

Factors Affecting Reciprocating Engine Operation
Figure 6. Effect of valve overlap

During the valve overlap period, this pressure differential forces the fuel/air mixture across the combustion chamber toward the open exhaust. This flow of fuel/air mixture forces ahead of it the exhaust gases remaining in the cylinder, resulting in complete scavenging of the combustion chamber. This, in turn, permits complete filling of the cylinder with a fresh charge on the following intake event. This is the situation in which valve overlap gives increased power.

There is a pressure differential in the opposite direction of 9 "Hg (4.5 psi) when the manifold pressure is below atmospheric pressure, for example, 20 "Hg. These cause air or exhaust gases to be drawn into the cylinder through the exhaust port during valve overlap.

In engines with collector rings, this inflow through the exhaust port at low power settings consists of burned exhaust gases. These gases are pulled back into the cylinder and mix with the incoming fuel/air mixture. However, these exhaust gases are inert; they do not contain oxygen. Therefore, the fuel/air mixture ratio is not affected much. With open exhaust stacks, the situation is entirely different. Here, fresh air containing oxygen is pulled into the cylinders through the exhaust. This leans out the mixture. Therefore, the carburetor must be set to deliver an excessively rich idle mixture so that, when this mixture is combined with the fresh air drawn in through the exhaust port, the effective mixture in the cylinder will be at the desired ratio.

At first thought, it does not appear possible that the effect of valve overlap on fuel/air mixture is sufficient to cause concern. However, the effect of valve overlap becomes apparent when considering idle fuel-air mixtures. These mixtures must be enriched 20 to 30 percent when open stacks, instead of collector rings (radial engines) are used on the same engine. [Figure 7] Note the spread at idle between an open stack and an exhaust collector ring installation for engines that are otherwise identical. The mixture variation decreases as the engine speed or airflow is increased from idle into the cruise range.

Factors Affecting Reciprocating Engine Operation
Figure 7. Comparison of fuel/air mixture curves for open-stack and collector-ring installations

Engine, airplane, and equipment manufacturers provide a powerplant installation that gives satisfactory performance. Cams are designed to give best valve operation and correct overlap. But valve operation is correct only if valve clearances are set and remain at the value recommended by the engine manufacturer. If valve clearances are set wrong, the valve overlap period is longer or shorter than the manufacturer intended. The same is true if clearances get out of adjustment during operation.

Where there is too much valve clearance, the valves do not open as wide or remain open as long as they should. This reduces the overlap period. At idling speed, it affects the fuel/air mixture, since a less-than-normal amount of air or exhaust gases is drawn back into the cylinder during the shortened overlap period. As a result, the idle mixture tends to be too rich.

When valve clearance is less than it should be, the valve overlap period is lengthened. A greater than normal amount of air, or exhaust gases, is drawn back into the cylinder at idling speeds. As a result, the idle mixture is leaned out at the cylinder. The carburetor is adjusted with the expectation that a certain amount of air or exhaust gases is drawn back into the cylinder at idling. If more or less air, or exhaust gases, are drawn into the cylinder during the valve overlap period, the mixture is too lean or too rich.

When valve clearances are wrong, it is unlikely that they are all wrong in the same direction. Instead, there is too much clearance on some cylinders and too little on others. Naturally, this gives a variation in valve overlap between cylinders. This results in a variation in fuel/air ratio at idling and lowerpower settings, since the carburetor delivers the same mixture to all cylinders. The carburetor cannot tailor the mixture to each cylinder to compensate for variation in valve overlap. The effect of variation in valve clearance and valve overlap on the fuel/air mixture between cylinders is illustrated in Figure 8. Note how the cylinders with too little clearance run rich, and those with too much clearance run lean. Note also the extreme mixture variation between cylinders.

Factors Affecting Reciprocating Engine Operation
Figure 8. Effect of variation in valve overlap on fuel/air mixture between cylinders

Valve clearance also effects volumetric efficiency. Any variations in fuel/air into, and exhaust gases out of, the cylinder affects the volumetric efficiency of the cylinder. With the use of hydraulic valve lifters that set the valve clearance automatically engine operation has been greatly improved. Hydraulic lifters do have a limited range in which they can control the valve clearance, or they can become stuck in one position that can cause them to be a source of engine trouble. Normally engines equipped with hydraulic lifters require little to no maintenance.

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