Aircraft Engine Types
Straight or In-Line:
As the name indicates, straight or in-line aircraft piston engines have cylinders in a line, much like automobile engines, which is why they were popular in early aircraft. The main advantage to an in-line piston engine in an aircraft is that the engine is narrow, allowing the plane to have a more narrow front fuselage that reduces drag. However, because the airflow around the engine is poor, this type of piston engine must be water-cooled, which increases the weight-to-power ratio of the aircraft. These engines also require additional maintenance and have a greater risk of failure than other types of aircraft piston engines.
Developed during World War I for military aircraft, the rotary piston engine came about when military personnel determined that inline aircraft engines were too heavy for military operations. In this type of aircraft piston engine, the entire engine rotated with the prop, which created additional airflow for cooling. These types of engines were bulky and awkward, and found not to be practical for commercial use.
The V-type piston engine is basically two in-line engines welded together to form a V shape. It is composed of two banks of cylinders sharing a common crankshaft. The V-type piston engine has also been used in the automotive industry, as well as in trains and ships due to the tremendous torque they provide. Most of these aircraft piston engines are water-cooled, but shear power balanced the extra weight of the cooling system. However, during World War II, the planes were more vulnerable due to bullet strikes that damaged the cooling systems.
Example: Curtis OX-5
The W-type piston engine is basically three in-line engines welded togeter to form a W shape. It is composed of three banks of cylinders sharing a common crankshaft.
Example: Napier Lion
Far more complex than the V-type piston engine, the radial piston engine produced smooth and efficient running. The engine consists of one or more rows of odd-numbered cylinders arranged in a circle around a central crankshaft. With a relatively small crankcase and one crank per row, this type of aircraft piston engine had a much better power to weight ratio than v-type engines. The engines cool evenly and run smoothly due to the cylinder arrangement's exposure to air.
Also known as flat or boxer engines, these aircraft piston engines have two banks of cylinders on opposite sides of a central crankcase. These engines can be air or liquid cooled, but are most often air cooled. Reliability, simplicity and easy maintenance have made this type of piston engine the most popular aircraft engine for more than half a decade. In addition to their efficiency, the engine can run on many different types of fuels as well.
Turbine engines are classified according to whether the compressor is centrifugal flow, axial flow or a combination of both centrifugal and axial. The type of engine is further classified by the path the air takes through the engine and how power is produced. There are four different types of turbine engines - turbojet, turboprop, turbofan and turboshaft.
A turbojet engine was first developed in Germany and England prior to World War II and is the simplest of all jet engines. The four sections of a turbojet engine are the compressor, combustion chamber, turbine section and exhaust. The compressor passes air at a high rate of speed to the combustion chamber which contains the fuel inlet and igniter. Expanding air drives the turbine and accelerated exhaust gases provide thrust. These engines are limited on range and endurance and today are mostly used in military aviation. They are known for being slow to respond to throttle applications at slow compressor speeds.
Between 1939 and 1942, a Hungarian designer, Gyorgy Jendrassik designed the first turboprop engine. However, the design was not implemented into an actual aircraft until Rolls Royce converted a Derwint II into the RB50 Trent which flew on September 20, 1945 as the first turboprop jet engine. A turboprop engine drives a propeller through a reduction gear, allowing optimum propeller performance to be achieved at much slower speeds than the operating RPM. With their ability to perform well at slow airspeeds and fuel efficiency, turboprop engines are often used in small, commuter aircraft and agricultural applications due to their greater reliability offsetting their higher initial cost.
Example: Pratt & Whitney PT6
Turbofan jet engines were designed to merge the best features of the turbojet and turboprop. By diverting a secondary airflow around the combustion chamber, additional thrust was created. Two separate streams of air pass through a turbofan engine. One passes through the engine core while the second bypasses the core. The Gloster E28/39 which flew for the first time on May 15, 1941 was one of the first times a turbofan engine was used for military or commercial aircraft.
The fourth type of jet engine is known as the turboshaft. Most of the energy produced by the expanding gases drives a shaft connected to a turbine through a single stage of reduction gearing rather than producing jet thrust. Turboshaft engines are predominantly used by helicopters. The first turboshaft engine was built by the French firm, Turbomeca in 1949.