Learn how piston engines operate
|Basic Piston Engine Principles|
|The Four-Stroke Cycle|
|Carburetors and Fuel Injectors|
|Piston Engine Controls|
|Engines with Carburetors|
Knowledge of a few general principles of engine operation helps pilots operate engines efficiently, extends the operating life of the power plant, and helps avoid engine failures.
Basic Piston Engine Principles
Reciprocating piston engines are the most common power plants on general aviation aircraft. These engines are virtually identical to automobile engines, with three important exceptions:
The Four-Stroke Cycle
A typical piston engine operates according to a four-stroke cycle.
Intake: The piston moves down in the cylinder, drawing in air and fuel through the open intake valve.
Compression: The intake and exhaust valves in the cylinder close and the piston moves up in the cylinder, compressing the fuel-air mixture.
Power: As the piston nears the top of the cylinder during the compression stroke, a burst of electricity from the ignition system generates a spark in the spark plugs. The sparks ignite the air/fuel mixture, which expands rapidly as it burns. The force of this expansion drives the piston back down in the cylinder. As the piston moves down, it turns the crankshaft, which turns the propeller.
Exhaust: When the piston reaches the bottom of the cylinder, the exhaust valve opens. The piston then moves back up in the cylinder, pushing the burned air/fuel mixture out of the cylinder.
Each cylinder cycles through these four strokes in turn, ensuring that at least one piston is always producing power.
Carburetors and Fuel Injectors
Most piston engines used in aircraft have either a carburetor or fuel injection system to deliver fuel and air to the cylinders. The carburetor mixes fuel and air before it enters the cylinders. Carburetors are common on smaller engines because they're relatively inexpensive. Larger engines usually have fuel-injection systems, which squirt fuel directly into the cylinders, where it mixes with air during the intake stroke.
The ignition system provides a spark to ignite the air/fuel mixture in the cylinders of a piston engine. Most modern aircraft engines use magnetos to generate the spark. Although not as sophisticated as the electronic ignition systems used in the latest cars, magnetos are useful in aircraft because:
Magnetos generate electricity when they rotate. So, to start the engine, the pilot must engage a battery-powered starter that rotates the crankshaft. After the magnetos begin rotating, they supply the spark to each cylinder to ignite the air/fuel mixture and the starter system is disengaged. The battery no longer has any part in the operation of the engine. If the battery (or master) switch is turned off, the engine continues to run.
Most aircraft engines are equipped with a dual ignition system—two magnetos that supply electrical current to two spark plugs for each cylinder. One magneto system supplies the current to one set of plugs; the second system supplies the current to the other set of plugs. This is why the ignition switch on the Cessna Skyhawk SP Model 172 (marked as MAGNETO on some planes) has five positions: OFF, L (left), R (right), BOTH, and START. With the switch in the L or R position, only one magneto supplies current and only one set of spark plugs fires. With the switch in the BOTH position, both magnetos supply current and both sets of spark plugs fire.
Advantages of Dual Ignition
Aircraft have dual ignition systems for safety and efficiency.
Operating the Ignition System
You should turn the ignition switch to BOTH after starting the engine and leave it on BOTH during flight. Turn it OFF after shutting down the engine. If you leave the ignition switch on BOTH (or L or R), the engine could fire if the propeller is moved from outside the airplane—even if the electrical master switch is off.
Before Takeoff Check
To make sure both ignition systems are operating properly, check each system during the engine run-up before takeoff. The normal procedure is to set the power at about 1700 rpm. Turn the ignition switch from BOTH to R, then back to BOTH, then to L, and then back to BOTH. You should see a slight drop in rpm each time you switch from BOTH to either R or L. If both magnetos are functioning normally, the drop should be no more than about 75 rpm.
Shutting Down the Engine
You should not shut down a piston engine by turning the ignition switch to OFF. Instead, move the mixture control to the idle cutoff position to turn off the fuel supply to the cylinders. After the engine stops, turn the ignition switch to OFF. This procedure ensures that no fuel remains in the cylinders and that the engine won't start accidentally if someone turns the prop or if carbon deposits inside the cylinders create hot spots that ignite residual fuel.
Piston Engine Controls
Most modern piston engines have two or three basic controls.
Carbureted engines also have carburetor heat to prevent the formation of or to melt carburetor ice. Engines of about 200 horsepower or more usually have cowl flaps to allow the pilot to adjust the amount of cooling air that flows over the engine. Opening the cowl flaps is especially important during high-power operations such as takeoff and prolonged climbs.
Piston engines are typically connected to a fixed-pitch or a constant-speed propeller.
Fixed-pitch propellers are bolted directly to the crankshaft of the engine and therefore always turn at the same speed as the engine. A fixed-pitch prop is somewhat like a transmission with only one gear. This configuration makes up for its lack of efficiency by being very simple to operate. The only gauge that you need to monitor is the tachometer.
A constant-speed propeller has a governor that adjusts the angle of the blades to maintain the rpm you select. This type of propeller makes much more efficient use of the engine's power. At low speed when maximum power is required (as during takeoff), you select maximum rpm or "full increase" with the propeller control, and the propeller blades meet the air at a small angle. During cruise, you adjust the rpm to a lower setting, and the blades take a bigger bite of the air while turning a lower speed.
Managing the Power
With a fixed-pitch propeller, managing power is simple. Push the throttle in, and rpm (and power) increases. Pull the throttle out, and rpm decreases. Be aware, however, that as airspeed increases, rpm tends to creep up, too. Monitor the tachometer carefully during descents at high speed to make sure that the rpm stays within limits.
A constant-speed propeller makes power management a bit more complicated. You must monitor the manifold pressure gauge, controlled by the throttle, and the tachometer, which shows the propeller rpm. You adjust rpm with the prop control.
When setting power with a constant-speed propeller, remember these basic rules to avoid overstressing the engine:
To increase power
To decrease power
Engines with Carburetors
Many aircraft piston engines use carburetors to combine air and fuel to create a combustible mixture that burns in the cylinders.
How a Carburetor Works
Outside air flows through an air filter, then into the carburetor. The air flows through a venturi, a narrow throat in the carburetor. The air accelerates in the venturi and its pressure drops according to Bernoulli's principle. The partial vacuum forces fuel to flow through a jet into the airstream where it mixes with the flowing air. The air/fuel mixture then flows into the intake manifold, which routes it to each cylinder.
The Right Ratio
The carburetor mixes air and fuel by weight. Piston engines generally produce maximum power when the air/fuel mixture is about 15:1.Carburetors are calibrated at sea-level pressure to meter the correct amount of fuel with the mixture control in the full rich position. As altitude increases, air density decreases. To compensate for this difference, the pilot uses the mixture control to adjust the air/fuel mixture entering the combustion chamber.
To control the amount of fuel that mixes with the air, most carburetors use a float in a fuel chamber. A needle attached to the float opens and closes an opening in the fuel line, metering the correct amount of fuel into the carburetor. The position of the float, controlled by the fuel level in the float chamber, determines when the valve opens and closes.
An air/fuel mixture that is too rich—that is, it contains too much fuel—causes excessive fuel consumption, rough engine operation, and loss of power. Running an engine too rich also cools the engine, causing below-normal temperatures in the combustion chambers, which leads to spark plug fouling, among other problems.
Operating with the mixture too lean—too little fuel for the current weight of air—results in rough engine operation, detonation, overheating, and a loss of power.
Vaporization of fuel and expansion of the air in the carburetor causes sudden cooling of the air/fuel mixture. The temperature may drop as much as 60 F (15 C) within a fraction of a second. This cooling causes water vapor in the air to condense, and if the temperature in the carburetor reaches 32 degrees F (0 C) the water freezes inside the carburetor passages. Even a slight accumulation of this deposit can restrict the flow of air into the carburetor, reducing power. Carburetor ice may also lead to complete engine failure, particularly when the throttle is partly or fully closed.
On dry days, or when the temperature is well below freezing, the moisture in the air generally doesn't cause carburetor ice. But if the temperature is between 20 F (-7 C) and 70 F (21 C), with visible moisture or high humidity, the pilot should be constantly on the alert for carburetor ice.
Indications of Carburetor Icing
For airplanes with fixed-pitch propellers, the first indication of carburetor icing is a drop in rpm on the tachometer. For airplanes with controllable pitch (constant-speed) propellers, the first indication is usually a drop in manifold pressure. In both cases, the engine may start to run rough. In airplanes with constant-speed propellers, rpm remains constant.
To prevent carburetor ice from forming and to eliminate ice that forms, carburetors are equipped with heaters. The carburetor heater preheats the air before it reaches the carburetor. This preheating melts ice or snow entering the intake, melts ice that forms in the carburetor passages (provided the accumulation is not too great), and keeps the air/fuel mixture above freezing to prevent formation of carburetor ice.
Using Carburetor Heat
When flying in conditions conducive to carburetor icing, monitor the engine instruments to watch for signs that ice is forming. If you suspect that carburetor ice is present, apply full carburetor heat immediately. Leave it on full until you're certain that all the ice has been removed. Applying partial heat or leaving heat on for too short a time may aggravate the situation.
When you first apply carburetor heat, expect a drop in rpm in airplanes equipped with fixed-pitch propellers; in airplanes equipped with constant-speed propellers, expect a drop in manifold pressure. If no carburetor ice is present, rpm or manifold pressure will remain lower than normal until the carburetor heat is turned off. If carburetor ice is present, expect a rise in rpm or manifold pressure after the initial drop (often accompanied by intermittent engine roughness). When you turn carburetor heat off, the rpm or manifold pressure rises above the value before heat was applied. The engine should also run more smoothly after the ice has melted.
In extreme cases of carburetor icing, after the ice has been removed you may need to apply just enough carburetor heat to prevent further ice formation.
Carburetor Heat as a Precaution
Whenever the throttle is closed during flight, especially as you prepare to land, the engine cools rapidly and vaporization of the fuel is less complete than if the engine is warm. If you suspect carburetor icing conditions, apply full carburetor heat before closing the throttle and leave the heat on.
Use of carburetor heat tends to reduce the output of the engine and increase the engine's operating temperature. Therefore, don't use carburetor heat when you need full power (as during takeoff) or during normal engine operation except to check for the presence or removal of carburetor ice.
Piston engines rated at more than 200 horsepower often use a fuel-injection system rather than a carburetor.
A fuel-injection system squirts fuel directly into the cylinders or just ahead of the intake valve. The fuel then mixes with air in the cylinders. Because this type of system requires high-pressure pumps, an air/fuel control unit, a fuel distributor, and discharge nozzles for each cylinder, it's generally more expensive than a carburetor.
As with an engine equipped with a carburetor, the pilot controls the flow of fuel by adjusting the mixture control.
Advantages of Fuel Injection
Fuel injection has several advantages over a carbureted fuel system, which compensate for its greater cost and complexity.
Disadvantages of Fuel Injection
Fuel injection does have some disadvantages, the most important being: