Aircraft technical Basics: Introduction to Airplanes - Navy Training Courses Edition of 1944: Chapter 5 Engines

CHAPTER 5 ENGINES

WHAT THEY ARE

Engines are the "heart" of the POWER PLANT.

Briefly, gasoline mixed with air is converted by the engine into power that rotates the propeller. Rotation of the propeller drives an airplane through the air. All Naval airplane engines in use today are of the INTERNAL COMBUSTION type, so called because the fuel burns inside the engine rather than outside (as, for example, in a steam engine).

When a fuel such as gasoline is mixed with air and burned, it forms a hot, rapidly expanding gas. If the mixture is made to burn inside a closed chamber, the hot gas exerts great pressure on the chamber walls. That's fundamentally what goes on in the CYLINDERS of an internal-combustion engine. Each cylinder, however, has one movable end, called a PISTON, which tends to be pushed away from the far end of the cylinder every time a charge of fuel is burned inside.

Simple, isn't it? All you have to do now is harness the movement of the piston and make it do the kind of work you want. Of course, there are many more details to be worked out before you end up by having an aircraft engine, but the movement of the pistons in the cylinders is still the basis of the operation.

The force pushing the piston is passed to a CRANKSHAFT by means of a CONNECTING ROD, so that the crankshaft TURNS. Since the propeller is connected to the crankshaft, they spin around together. Once they are in motion, other pistons operate in a similar manner and the first piston is carried back toward the top of the cylinder, ready for another kicker of fuel.

VALVES in the top, or head, of the cylinder open and close to let out burned gases and take in fresh fuel. There are usually two valves in each cylinder. One controls the admission of fuel and is called the INLET, or INTAKE, valve. The other controls the leaving time of burned gases and is called the EXHAUST valve. These valves are operated by a system of gears and cams so they will open and close at exactly the right time.

FOUR-STROKE CYCLE

Each movement of the piston in the cylinder - whether up or down - is called a STROKE. The whole sequence of events - from the time a fuel charge enters the cylinder until it leaves as burned exhaust gas - is known as a CYCLE. Aircraft engines use a FOUR-STROKE CYCLE, and are called FOUR-CYCLE engines. In figure 16 you see the basic operations involved in the four-stroke cycle.

Figure 16.-The four-stroke cycle.

The first stroke, called the INTAKE STROKE, finds the piston moving down and the intake valve opening to admit a charge of fuel. The exhaust valve, of course, stays closed. The downward movement of the piston makes room for the introduction of the fuel into the cylinder through the intake opening.

The second stroke, called the COMPRESSION STROKE, finds both valves are closed and the piston moves upward to compress the charge of fuel. This compression makes the fuel mixture burn more efficiently and produce more power than it would if not compressed.

Stroke number three is the POWER STROKE. The piston is up toward the top of the cylinder as far as it will go, and the fuel mixture is fully compressed into the small remaining space. Both valves are closed. An electric spark from the SPARK PLUGS ignites the fuel, which burns and expands rapidly, forcing the piston downward.

The fourth stroke, called the EXHAUST STROKE, finds the piston moving upward again. The exhaust valve opens and the piston forces the burned gases out of the cylinder. That's the end of the cycle, and the cylinder is ready to start a new one by drawing in a new charge of fuel.

As you have noticed, the piston makes two trips upward and two trips downward during a single cycle. That means the crankshaft makes TWO COMPLETE REVOLUTIONS during each four-stroke cycle. To turn the statement around, the cylinder FIRES ONLY ONCE during every TWO revolutions of the crankshaft.

Today, aircraft engines have a considerable number of cylinders. The power strokes of the separate cylinders are timed so that the crankshaft will receive a series of evenly spaced pushes in the course of each revolution it makes. The greater the number of cylinders, the more smoothness you have in engine operation. But the powerful monsters that drive modern airplanes through the air at fabulous speeds operate on exactly the same basic principles as the chugging little engine that first boosted Orville Wright into the air. The big difference lies in their size and relative efficiency.

One way of measuring the efficiency of an air-plane engine is to compare the horsepower it produces with its weight. As a matter of fact, with engines both LIGHT ENOUGH and POWERFUL ENOUGH, man would doubtless have been able to fly long before the Wright. brothers did the trick. BUT they were not available. Even the engine used by the Wrights at Kitty Hawk was dangerously close to being too heavy for the job. It produced ONLY 16 horsepower.

Engineers and other scientists have constantly been slugging away at the problems of cutting down engine weight and jacking up horsepower. Their refinements have made possible the great airplane engines of the present day which produce approximately ONE HORSEPOWER for EACH POUND of engine weight. Development of stronger and lighter metals has been responsible for much of the weight decrease. Better grades of fuel, produced by modern chemistry, have resulted in much higher power output.

Fighting airplanes of World War I were considered wonderful if they had 400-horsepower engines and could travel 120 miles per hour. Some modern fighters carry 2,000-horsepower engines, can fly at least 400 miles per hour, and reach ceilings undreamed of a few years ago.

TYPES OF ENGINES

All aircraft engines can be classified as either IN-LINE or RADIAL engines. In-line engines have their cylinders arranged in straight lines. There are a number of variations in the way the straight rows of cylinders may be placed with respect to the crankshaft. Sometimes you find the cylinders in a single vertical line. Sometimes they're in an X, a W, or a V. They may have two, three, or four banks of in-line cylinders. Certain in-line engines are cooled by air, but the majority are liquid-cooled. Figure 17 diagrams some of the cylinder arrangements used for in-line and radial engines.

Figure I7.-Aircraft engine types.

Radial engines have their cylinders arranged in a circle around the crankshaft. Some have a single row of cylinders, while others are built with two or more rows. Radial engines are usually air-cooled, and have a natural advantage in weight because they don't have to carry a radiator full of liquid around with them.

Air-cooled radial engines predominate in the Navy's airplanes. The Army, however, uses liquid-cooled in-line engines to power a number of its craft. Both engines have advantages and disadvantages. Lightness is a factor favoring the air-cooled radial. In-line types are easier to streamline, and offer less resistance to the air.