Kraków 2016-04-22
Aero-engines – Introduction – Part 1
Classification of internal combustion-piston engines
This article is the first in a series discussing aircraft engines from a historical perspective, with particular emphasis on engines used in Poland.
When we look at an airplane and we see a propeller, we say a propeller plane, and when we don’t see a propeller we say a jet airplane. Simple and logical. However, the issue of propulsion of airplanes and other aircraft is more complicated. Before we present the general division of drives, we will start from the ancient past, i.e. from the 19th century. It has been dubbed the "Age of Steel and Steam" because it was then that steam engines developed. These engines contributed to the creation of the railroad. Mounted on ships, they significantly accelerated sea transport. They significantly increased the production capacity of factories whose machines were powered by locomobiles. However, the steam engine did not find its way into aviation. Although attempts were made to mount a steam engine on balloons to force the balloon to move in the right direction. All attempts ended in failure, already at the design stage. The main problem was too much weight at low power. An additional difficulty was the complicated operation and the need to have a sufficient amount of fuel (coal, wood) on board, which was not a problem in the case of steam engines and steam boats. The situation changed when the first efficient internal combustion engines were constructed, which used gasoline to generate energy.
We owe the creation of gasoline to scientists who tried to solve the problem of petroleum residues, from which only kerosene was extracted. Kerosene was the primary source of light for powering kerosene lamps. Rapeseed oil and olive oil were also in common use, which were also tried to be used as fuel for internal combustion engines. It turned out that gasoline has a lot of energy. First, gasoline was created, and then, looking for applications for it, an internal combustion engine was built, which we now call piston. The piston engine led to the development of motoring. A car (a wagon without a horse) and a motorcycle (a bicycle with a motor) were created.
The piston engine was smaller and lighter than the steam engine, although more complicated in construction. It had an additional mixture ignition system. It was harder to start. However, during operation, its operation was simple. The amount of fuel by volume was much smaller. The features of the piston (internal combustion) engine meant that it began to be successfully installed on the first aircraft. The piston engine also found its way to airships.
From the first flight of the Wright brothers in 1903, until the outbreak of the Great World War, several dozen more or less successful piston-engine airplanes were built and flown. The Great World War consolidates the position of a piston engine equipped with a propeller as the basic propulsion of an aircraft. Initially, the piston engine used in an airplane did not differ from the piston engine used in cars. Over time, the differences grew larger and larger.
Division of piston engines.
Piston aircraft engines in the first years of their existence were more often called internal combustion engines. Already during the Great World War, they were divided into radial and in-line engines.
And here the problem immediately arises. At the beginning of aviation, the Anzani engine was very popular, which was used by Louis Blériot on his plane during the English Channel crossing. This engine was one of those units that could work without failure for a long time, although they could overheat. This engine is a 3-cylinder, commonly called star engine. But its cylinders aren’t spaced 120 degrees apart as it should be on a radial engine, only 72 degrees. So it’s not your typical star. As we show below, it can be called a "W" government engine, although there is only one cylinder per row. This example only shows that the split is a contractual matter.
The Anzani company was a factory founded by the Italian Alessandro Anzani. He developed and produced engines for cars, motorcycles, boats and airplanes. Plants in the UK and France were also launched. In 1905, Alessandro Anzani developed the first 3-cylinder engine, which was used by Louis Blériot in 1909. Then, water-cooled 4-cylinder engines and many others (6-cylinder, 7-cylinder) in-line and radial, and even a 20-cylinder engine were developed.
Classification of star engines.
Star motors are divided into rotary and moving shaft motors, which we would call traditional. Rotary engines developed in the first years of aviation were especially interesting. These will be discussed further in Part 2. Traditional radial motors are single, double, or multi-star motors. Also about them will be a comprehensive description.
Description to the photo: WN-3 engine for single star, 7-cylinder engine. This engine was used in several aircraft designs.
Division of inline engines.
The breakdown of in-line engines is even more complicated. They are generally divided into single-row and multi-row. Multiple rows may be V-shaped (two-rows) or in a "W" -shaped (three-row) pattern. Contrary to appearances, the latter were very popular. To darken the image even more, they can be inverted even more. The multi-row can also be stacked or the so-called boxer. And closing the matter of in-line engines, they can be naturally aspirated or supercharged, and cooled with air or liquid. This division does not exhaust the whole topic yet, but it will all become clear over time.
Description for the photo: One of the best engines of the Second World War Roll-Royce Merlin XX. "V" engine, 12-cylinder, 1,390hp, 1940.
Reciprocating engines – Work system.
As early as the beginning of the 20th century, basic working arrangements were already in place; two-stroke and four-stroke. Four-stroke systems are in the lead. Two-stroke systems due to; higher fuel consumption, lower compression ratio, generated noise, difficult lubrication – they have not been used in aviation.
Reciprocating engines – Number of cylinders.
Two-cylinder engines are the most popular for ultra-light aircraft. In the 70-years of adventures, the GDR engine from the Trabant 601 was widely used. Due to its good rotational speed, it was well suited to power hang gliders and powered sailplanes. Series-produced aircraft usually have 4-cylinders. Military and commercial aircraft in the interwar period usually had 12 cylinders. During World War II, 12 or 24 cylinders.
It is a bit different with radial engines. The most popular are 5-, 7- and 9-cylinder systems. However, in multi-star systems, the 9-cylinder system in one star definitely leads. The most powerful one used was the quad-star engine with 36 cylinders.
What is important for a reciprocating engine?
At this point, it is worth mentioning that aircraft engines are only overhead valves, due to the need to obtain maximum compression of the mixture in the cylinder. A low valve system was not used.
Carburetors were responsible for delivering fuel to the cylinders. Their design was often very complex, so that the engine was powered also in inverted flight. It is also a rule to use two carburettors and two spark plugs per cylinder. During World War II, fuel injection was introduced, and over time, its electronic control system was introduced. However, fuel injection did not completely eliminate aviation carburetors.
A reciprocating engine requires cooling, although it operates at lower temperatures than automotive engines and is also more complicated to cool. This is due to the necessity to maintain narrower operating regimes so that the engine is fully operational at all times. Aircraft engines are air-cooled or liquid-cooled. Liquid cooling is based on similar principles as in car engines. Air cooling, on the other hand, seems to be easier, but difficult to implement at the design stage. Polish constructors have found this out many times. The engines were either overcooled or overheated.
The matter of engine cooling does not end with these general statements. Aircraft engines contain large amounts of oil which; lubricates, maintains, collects the resulting filings and cools the engine. Therefore, oil coolers are used.
It is now widely believed that aero piston engines have stalled, and car engines have developed significantly. There are many reasons for this. First of all – much less aircraft engines are produced. Secondly – the market is dominated by only a few producers. Thirdly – Every customer wants to have a reliable, proven engine, so they will not buy an engine in which one cylinder has been cut off to save weight and works jerkily. Fourth – Aircraft engines run on a different fuel. Attempts to create an engine that will work well on aviation and car fuel have failed so far, due to the difference in price of these fuels. The price of fuels is a political price and not a result of the actual production costs and producer margin. On the other hand, many models of engines running on car gasoline have already been created. These engines are used in ultralight and light airplanes and small helicopters.
Drive unit.
The engine is the element that makes the propeller rotate, which in the early days of aviation was called the drive wheel. The principle is this – the reciprocating motion sets the engine crankshaft in motion. The end of this shaft extends beyond the motor housing. Usually it has a spline on which the hub is mounted to which the propeller is directly mounted. And such a system is enough for the plane to fly. But there may be two more components in the path between the propeller and engine crankshaft.
The first will be the reducer. The reducer reduces the speed transmitted from the shaft to the propeller so that a larger diameter or more blade propeller can be used. The advantage is that such an airplane has more thrust at lower air speeds. Can take a larger load. It has a shorter inrun. The downside is the lower top speed. This solution was already used in the 1920s.
The second element is the blade setting angle adjustment system. The adjustable propeller, i.e. the so-called variable pitch, gives the possibility of changing the return and the value of the generated thrust. The main benefit is the possibility of using the propellers for braking and smooth adjustment of the power train thrust. In the so-called flag position, the idle engine propeller resistance during flight is reduced.
Aircraft power units have to work at different altitudes, and we know that as altitude increases, air density decreases, and therefore the amount of oxygen. As a result, engine performance drops. Therefore, already before World War II, supercharging systems were used, i.e. pre-compressing the air in front of the cylinders. For this purpose, centrifugal compressors or turbochargers are used. Their design solutions are very different. For example, two-stage. The second top-up is launched at an even higher level.
Supercharged aircraft power units are sometimes equipped with an intercooler, i.e. a radiator. It is located between the supercharger and the cylinders. When the air is compressed in the compressor, its temperature increases. At higher temperatures, air has a lower density which reduces the boost effect. By lowering the temperature in the intercooler, its density increases. Some variants of the 2,000-2,100hp Pratt & Whitney R-2800 Double Wasp engine, developed in 1937, and installed on many aircraft, were fitted with the intercooler.
However, it must be remembered that the installation of one or another element in the drive unit results only from the cost-effect function. Therefore, the same motor can be combined with different propellers, reducers, fuel and cooling systems etc. The motor itself can be overloaded or with reduced power in order to achieve its longer service life between repairs.
Aero-piston combustion engine has several significant advantages. In small planes, it is definitely more economical. Usually, it can be easily installed in the fuselage or nacelles of the aircraft by connecting it through the engine mount. It has easy-to-remove covers for easy handling. Such an engine generates much less noise than turbine drives. It is possible to work on different fuels. The cost of purchasing a 1,000 HP reciprocating engine is significantly lower than that of a turbine engine. The new PZL Kalisz ASz-62 IR engine in 2005 cost about PLN 600,000.
However, the internal combustion piston engine also has disadvantages. It is made of a large number of elements, which increases the risk of failure. In the cylinder, the piston reciprocates, producing rapid and large load changes. Drive units have many accessories, which extends the time of their maintenance and repairs. The propeller of the power unit is a potentially very dangerous component and requires special attention. The commercial aviation piston engine has been completely replaced by turbine engines; propellers and turbo-jets, which turned out to be more viable and more efficient.
Aircraft in a historical context.
From a historical perspective, internal combustion engines were instrumental in the development of aviation. The countries with a well-developed automotive industry participated in the construction of aircraft engines: France, UK, USA, Italy, Germany. Designers of aviation engines focused on reducing the weight of the engine and increasing the rotational speed of the shaft, and thus the propeller. A big problem at that time was to ensure failure-free operation of the engine. Efforts have been made to improve cooling and lubrication.
The first successful in-line (4-cylinder and 6-cylinder), star (5-cylinder) and rotary (9-cylinder) aircraft engines used the air flowing around the plane and the motor for cooling. It was only after the Great War that the development of liquid-cooled engines reached their peak during the Second World War. They competed with star bikes that could achieve higher powers. But the disadvantage of star engines was their large cross-sectional area, which did not make them predestined for fast fighter planes.
Despite the same general structure of car and airplane motors as early as in the 1920s, there was a clear separation of one engine from another. 12-cylinder and larger engines were increasingly used in aero engines. In automobilism, it remained with 6-cylinder and smaller motors.
Internal combustion engines in aviation reached their peak in the 1950s. The best designs were developed in the USA. They were used to propel the air giants; Convair B-36 Peacemaker bombers, Convair XC-99 transport bombers, Lockheed Constitution transport bombers.
The advent of turbojet engines has caused the measurement of internal combustion engines in commercial aviation.
Piston engines are still in use today. They are used in motor gliders, power hang gliders, ultralight and light airplanes, gyroplanes, a few airships and sometimes in helicopters (an extremely successful, cheap Robinson R.22, R.44 helicopter). Piston-combustion engines are used wherever high power and high flight altitudes are not needed.
Written by Karol Placha Hetman