Chapter 55: Air Cooling and Liquid Cooling
"The engine of an aircraft is not a single machine, but a system abbreviated as the engine, consisting of the engine, propellant or fuel system, and accessories necessary for normal and effective operation, such as pipes, instruments, and devices fixed on the aircraft. Because its purpose is to provide continuous power to the aircraft, we can refer to it simply as the engine, which is also the core of the aircraft, just like the heart in humans."
"In 1883, when the first gasoline-powered internal combustion engine, or piston engine, was invented, it laid a solid foundation for the successful flight of the first aircraft, making human flight a reality. As technology continued to evolve and advance, aerospace technology has developed to the point where airplanes have become an indispensable part of warfare, including reconnaissance, air combat, bombing, transportation, and more. Airplanes have opened up new fields for human life and brought about a new technological high ground in the industrial era. Whoever can firmly occupy this emerging high ground will be able to stand out from the crowd and dominate the world for a long time to come..."
Zhang Yu has never been good at public speaking, and even on the grand opening day of China Aviation Industry Group's Aviation Power Company, he couldn't come up with a decent speech no matter how hard he tried to gather all his literary talent. He could only manage something mediocre, but judging from the workers' enthusiastic expressions, it seemed that such plain language was more effective in moving them.
"The Chinese people's dream of flying into the blue sky has been around since ancient times, starting with the heroic Feng Ru. Since then, we Chinese people have not stopped moving forward. When we have better conditions than Mr. Feng, we should achieve even more. China Aviation Power Company is committed to providing the best, strongest, most effective and most sustainable power for the Chinese people's blue sky dream, allowing our planes to fly higher, farther and better."
On April 9, 1919, China Aviation Industry Group Aviation Power Company became the first company to be established under the group, located in Yulin City, southeastern Guangxi Province. This is another large power company established since the city's industrial development strategy was established, and previously, the power company of China Heavy Industry Group was established here. It is precisely because a good environment for manufacturing power equipment has been formed here, coupled with convenient transportation, that AVIC Power Company settled here. Of course, after the company was formally established, many factories under it should also be put into production. However, before this, there is another significant story about AVIC Power Company worth telling.
Shortly after the aviation project was established, the autonomous region's industrial department has considered the core component of the spacecraft - the engine. At present, with the current level of industry, Zhonghang Power can only revolve around piston engines, and due to technical strength and industrial level constraints, it is still some distance away from jet engines. However, regarding whether the engine should be researched as air-cooled or liquid-cooled, there was a lot of controversy in the original power research group, just like people entering the information age holding their own views and debating endlessly.
Piston engines are fueled by gasoline and use the four-stroke principle. Of course, this is also why piston engines run at extremely high speeds, with dozens of ignitions per second in the cylinders, and the high-temperature and high-pressure working environment causes the cylinder temperature to be very high, so piston engines must be equipped with efficient cooling systems.
There are only two ways to cool down, one is to use the circulation of coolant to take away heat through evaporation, which is liquid cooling, and the other is to use the cooling effect of high-speed airflow to take away heat, which is air cooling.
For this reason, the liquid-cooled engine cylinder is arranged in a straight line or V-shape, and there is a heat dissipation sleeve inside the engine casing. The coolant with a certain pressure circulates in the sleeve to take away the heat. In this way, the cooling system of the liquid-cooled engine becomes more complicated, including the water tank, water pump, radiator, piping system, etc., which brings no small weight to the engine.
However, the air-cooled engine is directly centered on the crankshaft, arranged in a star shape, with many heat dissipation fins on the outside of the cylinder. The high-speed airflow generated during flight can dissipate the heat from the cylinder wall, thereby achieving the purpose of cooling, so it is also called an air-cooled radial engine.
The autonomous region's focus on developing the aviation industry is primarily for military purposes, with civilian use being mainly for transportation needs. The stimulation of the latter is secondary. Therefore, the ultimate goal of the autonomous region is to develop excellent aircraft for military use, and once that is achieved, the economic benefits of civilian use can be easily resolved.
As a result, the debate over the final research direction entered a white-hot stage. The higher aviation requirements required that the piston engine should have large horsepower, light weight, good reliability, small windward area and so on. To achieve these requirements, there are only a few means, such as increasing cylinder diameter, increasing number of cylinders, increasing stroke, and adding mechanical or turbocharger devices. The aviation power research group quickly split into two opposing organizations for their respective opinions, one insisting on air cooling as the mainstay, and the other advocating liquid cooling.
After some research, the air-cooled group firmly believed that for air-cooled engines, they must go down the path of radial engines. Since the entire engine's front face will be exposed to the oncoming airflow, installing a large number of heat sinks on the cylinder body becomes an ideal cooling arrangement, so radial engines are basically all air-cooled, and the radial layout also became the only choice for air-cooled engines.
To get enough horsepower, we can work on increasing the number of cylinders and cylinder bore. The number of cylinders is often limited by the front face of the engine and cannot be arranged too much, and increasing the number of cylinders will definitely be very difficult. Increasing the cylinder diameter also has the same problem, but they found through technical data that increasing the number of layers in a star shape is not a bad idea.
If a single-layer star has 7 cylinders, a double layer is 14 cylinders. Continuing this calculation can make the engine have three or four layers of stars, and then it can have dozens or even forty cylinders. In this way, different cylinder numbers and star layers can be selected according to different power requirements, and manufacturing an engine with 3,000-4,000 horsepower is no longer a dream.
However, this idea also has problems. After multi-layer star arrangement, the layers behind will have serious cooling problems. The air that has passed through has already been heated in front, and by the time it reaches the back, it is high-temperature gas, which greatly reduces the cooling effect. If no special compensation is made for the rear layers, the lifespan and horsepower of the rear cylinders will be affected.
If increasing the stroke is considered as another method to increase effective horsepower, increasing the stroke will quickly increase the windward area. After all, the windward area increases with the square of the radius, and the resistance is proportional to the windward area. Since the piston engine works by burning heat, the higher the combustion temperature, the greater the output per unit volume. It is unrealistic to resist such high temperatures with the heat resistance of engine materials. More practical cooling technologies are needed to reduce cylinder body temperature and extend lifespan. Otherwise, air-cooled groups cannot withstand the provocation of liquid-cooled groups.
For liquid-cooled units, they are well aware of the advantageous conditions for their counterparts. Air-cooled engines directly utilize the natural environment air flowing through the engine to cool the cylinder body. To a certain extent, air-cooled engines can install many heat sinks on the cylinder body, effectively increasing the heat dissipation area. In other words, the heat sink area of air-cooled engines does not reach the so-called limit, and the heat dissipation efficiency is very good. In other words, if the number of star-shaped layers is not large, and the number of cylinders is certain, using an air-cooled method can achieve a very good power output while also ensuring an excellent cooling effect.
This is not a big challenge for the liquid-cooled unit, but the air-cooled one has low cooling efficiency. Even if the air-cooled unit makes multi-cylinder, multi-row high-power air-cooled engines through various means to gain certain actual use capabilities, the liquid-cooled unit will at most lose its application in transport-type aircraft engines. They cannot make liquid-cooled engines useless on combat aircraft or fighters.
The main advantage of liquid cooling is to use a coolant, such as water or specialized coolant oil, to flow through the cylinder wall and take away heat from the engine. The heat is then released into the air through a dedicated radiator. The thermal conductivity of the coolant is much higher than that of air, which facilitates heat transfer and allows for efficient removal of large amounts of heat from the engine. This enables the engine to reach higher working temperatures, which is beneficial for increasing power per unit volume, and also makes it easier to achieve uniform cylinder temperature, improving engine operating conditions and extending engine life.
The design of the heat exchanger is not a problem for the liquid cooling team, they have enough confidence to manufacture one that can be free from the constraints of engine cylinder shape and layout, requiring large numbers of long and narrow heat transfer fins. If necessary, it could even allow coolant pipes to wind in and out between fin packs, forming multi-pass heat exchangers which would greatly increase heat transfer efficiency, the latter's heat transfer efficiency will far exceed that of directly connected heat transfer fins with cylinder block.
The heat transfer efficiency of water cooling is higher than that of air cooling, and the cooling performance is better, which is an undisputed fact and also the advantage of liquid cooling. However, water cooling requires additional pipes and radiators, making production and manufacturing more complicated and maintenance more cumbersome, which is a major drawback compared to air cooling. Moreover, from the perspective of European liquid-cooled engine applications, coolant leakage is still a problem that cannot be ignored.
As a result, when the power requirement of the engine is not high, the various problems brought about by the water-cooled radiator, pipes and other accessories are very prominent, making it heavier and more complex than air-cooled engines, with no advantages. For an engine with a certain power output, its volume and weight are not a problem, and the simplicity of air-cooled engines is very obvious, with liquid cooling having no advantage to speak of. However, when the power requirement of the engine is very high, the limitations of air-cooled engines become apparent, with a large number of heat sinks not only unable to effectively dissipate heat but also greatly increasing weight, and the efficiency advantages of liquid cooling systems become evident, with the total system weight actually becoming lighter.
Both sides have their own advantages and disadvantages, making it difficult for each other to give up their ideas, wanting the other party to accept their correct ideas, which leads to debate. However, the result of the debate is often that both parties become more convinced that their own ideas are correct, and it can be said that the debate is ineffective from start to finish. Therefore, the phrase "practice is the sole criterion for testing truth" allows them to find a good way to persuade each other, which is to make achievements and compare with each other.
The Ministry of Industry and the military clearly pointed out that the aviation industry should first serve the military, so the developed aircraft models are nothing but fast and highly maneuverable fighter jets, transport planes with large range and good maneuverability and economic benefits, and various bombers, reconnaissance and passenger planes for long, medium and short distances can be gradually derived. The competition between the two groups started from the first type, that is, fighter jets.
For fighter planes, it is known from the blood and fire practice of European warfare that pursuing speed and altitude is an eternal theme, and this theme will continue. The pros and cons of air-cooled radial engines and liquid-cooled inline (including V-shaped) engines on fighter planes cannot be separated from the comparison of speed and altitude.
For speed, increasing horsepower and reducing resistance is the only way.
Liquid-cooled engines have a natural advantage in increasing horsepower. One of the huge advantages of inline or V-shaped liquid-cooled engines is their slender body shape, which can bring benefits such as small windward area and small resistance. By placing the inline engine longitudinally inside the front fuselage, the streamlined cowl can maintain a streamlined aerodynamic shape, which helps to further reduce drag. With this arrangement, fighter planes using inline engines look like sharp heads, which is very beneficial for pilots' driving.
The radial engine of the air-cooled group must expose the entire front of the engine to the head-on airflow in order to maintain good air cooling, making it difficult to use a streamlined cowl to reduce drag. Therefore, fighter planes with radial engines are all blunt-nosed. The blunt nose also makes the view in front of the cockpit poor, and in order to improve the view, only the pilot's cockpit can be driven, which further increases the wind resistance and loses speed.
When flying in the air, this shortcoming is not obvious after wind tunnel test, but it is more difficult for the aircraft to take off and land, so it fails a little bit in speed.
The speed issue is not a big deal for both of them, and the gap is not obvious. However, their respective shortcomings in height are immediately highlighted.
In the high-altitude environment, the air is thin, and the intake of piston engines will be greatly affected. In order to ensure the effective operation of the engine, certain measures must be taken, and mechanical supercharging or turbocharging has become a necessity.
A mechanical supercharger is driven directly by the engine's crankshaft via a belt or gear. This provides a mechanical advantage because the turbine in a turbocharger can spin much faster than its RPM equivalent of the engine.
A turbocharger does not directly draw power from the engine, but instead uses the exhaust gases emitted by the engine to drive a turbine, which in turn drives a compressor to compress air into the engine's intake. Turbocharging also takes some of the engine's power, because the increased back pressure in the exhaust system reduces the engine's output. However, turbocharging takes much less power than mechanical supercharging, has a lighter system weight and is more reliable. The disadvantage is that the throttle response is slow, requiring the engine speed to rise to a certain level before it can work normally.
The latter is not a problem for fighter jets, as the engine has already been running at high speed when climbing upwards, and starting the exhaust gas turbine is not a problem. However, throttle response is an issue, so on fighter jet engines, turbocharging and mechanical supercharging must be used together to complement each other.
Of course, the technology of mechanical supercharging and turbo-supercharging is not a problem for both groups, as it has been widely used in the automotive industry. Perhaps this is also due to the maturity of automotive technology, which gives liquid-cooled engines more confidence to continue with liquid cooling, as cars are also liquid-cooled, including internal combustion locomotives that will be used on railways. However, what defeated air-cooled engines was not these, but facts.
As a fighter engine, the superiority of turbocharging is obvious and inevitable. However, it is very difficult to apply turbocharging to air-cooled radial engines because the cylinder heads face outwards, so each cylinder intakes and exhausts separately. Mechanical supercharging would require installing a compressor for each cylinder's intake tract, which would be a huge challenge; applying turbocharging technology would also require installing an exhaust turbine for each cylinder's exhaust tract, greatly increasing system complexity and cost.
In comparison, the cylinder heads of an inline or V engine are lined up in a row and can use a single intake and exhaust manifold, requiring only one centralized supercharger or turbocharger, greatly simplifying the system and increasing efficiency. In terms of high-altitude performance, inline or V engines also have an advantage over radial engines. If a radial engine is forced to adopt these two technologies, it will thoroughly lose its proud advantages of structural simplicity and light weight, becoming more complex and cumbersome than liquid-cooled engines.
It is more beneficial for liquid-cooled groups that, whether it is mechanical supercharging or turbo supercharging, ideally, the charged air should be intercooled to reduce temperature and increase density, so that under the same intake pressure, more air can be injected into a unit volume, mixed with more fuel, and burned to produce better power output. The cylinder head of the star-shaped engine is separately inhaled, making it difficult to adopt intermediate cooling. In-line or V-shaped engines use a unified flow device and then divide the flow into each cylinder, making it easier to adopt intermediate cooling. From the perspective of intermediate cooling, in-line or V-shaped engines are more beneficial again.
So overall, the liquid-cooled engine won a comprehensive victory on fighter planes, but the air-cooled group did not give up. Fighter planes equipped with liquid-cooled engines must have a huge demand for coolants such as water, which greatly limits their range of use. The combat survivability of air-cooled engines is also noteworthy.
Imagine if the enemy hits the water-cooled pipe and dedicated radiator of a liquid-cooled engine, just damaging one part can make the cooling capacity lost. However, the air-cooled team did not think about why the enemy could hit their own radiators and other locations, why not directly attack the cockpit to strike the pilot? The latter is also an effective means, so there is no so-called combat loss rate competition between liquid-cooled and air-cooled fighters, no need for a large amount of cooling water and diverse startup environments, this is where the application advantages of the air-cooled team lie.
In any case, the application of aircraft engines, Zhang Yu's suggestion includes test results from another time and space, and liquid-cooled fighter planes still occupy an advantageous position, but air-cooled ones are not a complete failure, because they can also be used for the second type of aircraft. Transport planes that require powerful engines need radial air-cooled engines, and there are many more unknown fields that require air-cooled engines. Without extensive practical testing, it is impossible to arrive at the truth, and the prospects for air-cooled engines are equally broad.
No matter what, when the air-cooled faction and the liquid-cooled faction insist on their own opinions and strive for them, it is already a manifestation of self-confidence in their abilities. Whether they win or lose, it is progress in the aviation industry, because this shows that the technical talents in the autonomous region have grown up and will be able to drive the progress of the entire industry after developing into more fields, regardless of which industry.
Of course, such an ending is not the final result, nor is it a reason for a large aviation power company to be established. The aviation power research group originally started a long and fierce competition over whether to use liquid cooling or air cooling, and now that the power company has been established, I believe that the road of competition in the future will be even longer, but for now they all have their own tasks, which is to steadily produce engines, whether it's the current piston type or the future jet type.
