Repsol YPF

Piston engines

In the period from the beginning of powered flight, in 1903, until the end of World War II, there were great advances in aircraft internal combustion engines.

Engine power and reliability increased dramatically, while the weight-to-power ratio fell steadily.

Developments in engine design made since the end of World War II have resulted in continuous improvement in thermal efficiencies and fuel consumption in modern engines.

This has allowed for the development of high-performance aircraft to meet today's market demands.

Internal combustion engines

An internal combustion engine burns fuel in a chamber, thus transforming the energy contained in the fuel into mechanical energy.   

The majority of aviation engines are of the reciprocating piston type in which a piston moves back and forth in a cylinder. The engine’s power is generated by the force exerted on the piston by the rapid expansion of gases resulting from the combustion of a compressed fuel-air mixture. The power from the motion of the piston is transmitted through a connecting rod to a crankshaft, which is coupled to the propeller.

Air intake system

Air is drawn into the engine through the intake system. It passes though an air filter to keep dust and contaminants out of the engine. A throttle (a disk attached to a rotatable shaft) mounted in the air intake controls the airflow. When the pilot opens the throttle, the disk tilts, allowing more air to enter the engine. With more air, the engine can use more fuel and produce more power to take off or climb.

An engine that uses intake air at atmospheric pressure is said to be naturally aspirated. The amount of air a naturally aspirated engine can use is limited by the local air density (barometric pressure) and pressure losses in the intake system. To get more air into an engine (boost the air pressure), small compressors are usually used to pressurize the intake air.

There are two ways to drive the compressor, off the engine crankshaft (supercharging) or on a common shaft with a turbine driven by the engine's exhaust gas (turbocharging). To maintain boost pressure at a relatively constant amount over a wide range of engine speeds, a certain type of pressure regulation is needed. Turbocharging is preferred since it extracts energy from exhaust gases that would otherwise be wasted, so it is more efficient than supercharging, which takes energy from the crankshaft.

Since air density decreases along with atmospheric pressure as altitude of the aircraft increases, less and less air is drawn into a naturally aspirated engine as an aircraft climbs. This limits the maximum speed and altitude that can be achieved. This limitation was recognized even before World War I, but turbochargers were not developed until the mid-1920s. They were so successful that naturally aspirated engines were virtually obsolete in high-performance aircraft by the early 1930s.

Carburetion

In the intake system of an engine, air mixes with a small amount of vaporized fuel to produce a homogeneous fuel-air mixture. The carburetor is the most successful of the many devices developed to discharge the correct amount of fuel into the intake air stream.

The heart of a carburetor is the venturi – a converging-diverging nozzle. The diameter of the nozzle decreases to a minimum at the throat and then increases to the discharge end. As air passes through the venturi, its velocity increases up to the narrowest portion (throat) because the cross-sectional flow area decreases. As the air velocity increases, its pressure decreases, creating a vacuum that draws fuel out of the carburetor's fuel bowl through a tiny jet. Additional jets are used to enrich the mixture during acceleration and to supply sufficient fuel at idle. A hand-operated primer is used on many engines to enrich the mixture for cold starts.

Carburetors do not control fuel flow precisely enough for critical or high-performance applications. In part this is because they are volume-flow based and difficult to calibrate for all operating conditions.

Fuel injection

The second most important type of fuel system design is the fuel injection system.

Fuel injectors are mounted at the intake port of each cylinder, where they spray fuel onto the intake valves. To enrich the mixture during cold starts, an additional cold-start injector may be used. This injector adds additional fuel to the intake air for a short period of time while the engine warms up. 

The first advantage of fuel injection is more uniform fuel distribution to each cylinder compared to carburetion. Fuel-injected engines also respond more rapidly than carbureted engines when the pilot changes control settings. An additional advantage is the elimination of carburetor icing. Disadvantages of fuel injection compared to carburetion are increased complexity, more moving parts, very narrow passages in an injector that may become plugged, and higher vapour lock tendencies.

Fuel injection systems used in general aviation are generally not as sophisticated as those used in modern automobile engines.

They operate at a lower pressure and provide a continuous flow of fuel to the intake ports, rather than the solenoid-actuated, timed injection typical of automobile systems.

The aircraft system uses an engine-driven fuel pump and generally includes an auxiliary electric fuel pump, which stops vapour from forming and acts as a backup for the engine driven pump. Fuel filters are installed both upstream and downstream of the main pump to remove particulate matter from the fuel that could cause injector plugging.

In some systems, a diaphragm pressure regulator maintains pressure and routes excess fuel back to the fuel tank.

Engine configurations

Aircraft piston engines have been built in several different configurations. The in-line and "V" engines are very similar to those used in automobiles. Some early designs had separate cylinders to minimize weight, but later designs used the familiar engine block. These were water cooled because the integral cylinder blocks are ideally suited to liquid cooling.

The radial engine configuration is unique to aviation. Here the crankshaft is in the circular centerpiece of the engine and the cylinders radiate out from it in a plane perpendicular to the crankshaft. In this design, each cylinder gets equal airflow, therefore, most radials are air cooled. An early yet interesting design is the rotary engine, in which the engine block rotates about a fixed crankshaft.
Horizontally opposed (boxer) engines are the third major configuration. These can be considered an extreme example of a "V" engine, in which the angle between the pistons is 180 degrees. The cylinders lie in a plane parallel to the wings. Most of these engines are air cooled. Horizontally opposed engines have been used in almost all small aircraft built since World War II.