Is diesel burning? It burns, and quite strong. Its residue, which did not participate in pre-mixed combustion, is consumed in the combustion phase at an adjustable rate.
Combustion in diesel engines is very difficult. Until the 90s of the last century, its detailed mechanisms were not well understood. The combustion temperature of diesel fuel in the combustion chamber also varied from case to case. For decades, the complexity of this process did not seem to succumb to researchers' attempts to reveal its many secrets, despite the availability of modern tools, such as high-speed photography used in transparent engines, the computing power of modern computers, and many mathematical models designed to simulate combustion in diesel engine. Application of laser visualization of the sheet to the traditional process of burning diesel fuel in the 90s of the last century has become the key to a significant deepening of understanding of this process.
This article will examine the most established process model for a classic diesel engine. This conventional combustion of diesel fuel is primarily controlled by mixing, which can occur due to diffusion of fuel and air before ignition.
Combustion temperature
At what temperature does diesel burn? If earlier this question seemed complicated, now it can be answered quite clearly. The combustion temperature of diesel fuel is about 500-600 degrees Celsius. The temperature must be high enough to ignite the mixture of fuel and air. In cold countries where low ambient temperatures prevail, engines had a glow plug that warms the inlet to help start the engine. That's why you must always wait until the heater icon on the dashboard goes out before starting the engine. It also affects the combustion temperature of diesel fuel. Consider what else there are nuances in his work.
Features
The main prerequisite for burning diesel fuel in a burner, the temperature of which is regulated from the outside, is its unique way of releasing chemical energy stored in it. To perform this process, oxygen must be available to it to facilitate ignition. One of the most important aspects of this process is the mixing of fuel and air, which is often called the preparation of the mixture.
Diesel Combustion Catalyst
In diesel engines, fuel is often injected into the engine cylinder at the end of a compression stroke, just a few degrees from the crankshaft angle to top dead center. Liquid fuel is usually injected at high speed in the form of one or more jets through small holes or nozzles in the tip of the injector, sprayed into small droplets and penetrates the combustion chamber. The atomized fuel absorbs heat from the surrounding heated compressed air, evaporates and mixes with the surrounding high-temperature high-pressure air. As the piston continues to move closer to top dead center (TDC), the temperature of the mixture (mainly air) reaches the ignition temperature. The combustion temperature of Webasto diesel fuel is no different from the similar temperature of other diesel grades, reaching about 500-600 degrees.
The rapid ignition of some pre-mixed fuel and air occurs after a period of ignition delay. Such rapid ignition is considered the beginning of combustion and is characterized by a sharp increase in pressure in the cylinder as the fuel-air mixture is consumed. The increased pressure resulting from pre-mixed combustion compresses and heats the unburned part of the charge and shortens the delay before ignition. It also increases the rate of evaporation of the remaining fuel. Its atomization, evaporation, mixing with air continue until all of it is burned. The combustion temperature of kerosene and diesel fuel in this regard may be similar.
Characteristic
First, let's deal with the notation: then A is air (oxygen), F is fuel. Diesel combustion is characterized by a low overall A / F ratio. The lowest average A / F is often observed at peak times. To avoid excessive smoke generation, A / F at peak times is usually maintained above 25: 1, well above the stoichiometric (chemically correct) equivalence ratio of about 14.4: 1. This also applies to all diesel fuel activators.
In turbocharged diesel engines, the idle A / F ratio can exceed 160: 1. Therefore, the excess air present in the cylinder after fuel combustion continues to mix with burning and already exhausted gases. When the exhaust valve is opened, the excess air together with the combustion products is depleted, which explains the oxidative nature of the diesel exhaust.
When is diesel burning? This process occurs after the evaporated fuel mixes with the air to form a locally rich mixture. Also at this stage, the proper combustion temperature of diesel fuel is achieved. However, the overall A / F ratio is small. In other words, we can say that most of the air introduced into the cylinder of a diesel engine is compressed and heated, but never participates in the combustion process. Oxygen in excess air helps to oxidize gaseous hydrocarbons and carbon monoxide, reducing them to extremely low concentrations in exhaust gases. This process is much more important than the combustion temperature of diesel fuel.
Factors
In the process of diesel combustion, the following factors play the main role:
- The induced charge of air, its temperature and its kinetic energy in several dimensions.
- Sprayability of injected fuel, spray penetration, temperature and chemical characteristics.
Although these two factors are the most important, there are other parameters that can significantly affect engine performance. They play a secondary but important role in the combustion process. For instance:
- Inlet design It has a strong influence on the movement of charge air (especially at the moment when it enters the cylinder) and on the mixing speed in the combustion chamber. From this, the combustion temperature of diesel fuel in the boiler may change.
- The inlet design may also affect the charge air temperature. This can be achieved by transferring heat from the water jacket through the surface area of the inlet.
- Inlet valve size. It controls the total mass of air admitted into the cylinder for a finite time.
- Compression ratio. It affects evaporation, mixing speed and combustion quality, regardless of the combustion temperature of diesel fuel in the boiler.
- Injection pressure. It controls the duration of the injection for a given nozzle hole parameter.
- Spray geometry, which directly affects the quality and temperature of combustion of diesel fuel and gasoline through the use of air. For example, a larger angle of the spray cone can place fuel on top of the piston and outside the combustion tank in DI diesel engines with an open chamber. This condition can lead to excessive "smoking", as fuel is denied access to air. Wide cone angles can also lead to fuel spatter on the cylinder walls, rather than inside the combustion chamber where required. Sprayed onto the cylinder wall, it will eventually be moved down into the oil pan, which will shorten the life of the lubricating oil. Since the spray angle is one of the variables that affect the speed of mixing of air in the fuel stream near the outlet of the injector, it can have a significant impact on the overall combustion process.
- Valve configuration that controls injector position. Two-valve systems create an inclined position of the injector, which implies uneven spraying. This leads to disruption in the mixing of fuel and air. On the other hand, designs with four valves allow vertical installation of the injector, a symmetrical arrangement of the atomization of fuel and equal access to available air for each of the nozzles.
- Position of the upper piston ring. It controls the dead space between the upper area of the piston and the cylinder liner. This dead space traps air, which is compressed and expanded, without even participating in the combustion process. Therefore, it is important to understand that the operation system of a diesel engine is not limited to the combustion chamber, atomizer nozzles and their immediate surroundings. Combustion includes any part or component that may affect the end result of a process. Therefore, no one should have doubts about whether diesel fuel is burning.
Other nuances
It is known that diesel combustion is very depleted with A / F ratio:
- 25: 1 at peak torque.
- 30: 1 at rated speed and maximum power.
- Over 150: 1 idling for turbocharged engines.
However, this additional air is not included in the combustion process. It heats up quite a bit and is depleted, as a result of which the diesel exhaust becomes poor. Even considering that the average air-fuel ratio is poor, if appropriate measures are not taken during the design process, the areas of the combustion chamber can be rich in fuel and lead to excessive smoke emissions.
The combustion chamber
A key objective in its design is to ensure that fuel and air are sufficiently mixed to mitigate the effects of fuel-rich areas and allow the engine to achieve its performance and emissions. It has been found that turbulence in the movement of air inside the combustion chamber is useful for the mixing process and can be used to achieve this goal. The vortex created by the inlet can be amplified, and the piston can create compression as it approaches the cylinder head to provide more turbulence during the act of compression due to the correct design of the cup in the piston head.
The design of the combustion chamber has the most significant effect on particulate emissions. It can also affect unburned hydrocarbons and CO. Although NOx emissions depend on the design of the bowl [De Risi 1999], bulk gas properties play a very important role in their exhaust levels. However, due to a compromise with NOx / PM, the design of the combustion chambers had to evolve as the NOx emission limits decreased. This is mainly required in order to avoid an increase in PM emissions that might otherwise occur.
Optimization
An important parameter for optimizing the diesel combustion system in the engine is the fraction of available air involved in this process. The coefficient K (the ratio of the volume of the piston cup to the gap) is an approximate measure of the fraction of air available for combustion. A decrease in engine displacement leads to a decrease in the relative coefficient K and to a tendency to deteriorate combustion characteristics. For a given displacement and at a constant compression ratio, the K coefficient can be improved by choosing a longer stroke. Factor K and a number of other factors, such as engine packaging, openings and valves, and so on, can influence the selection of the ratio of cylinder diameter to engine.
Possible difficulties
A particularly significant problem when setting the maximum cylinder to stroke ratio is the very complex packaging of the cylinder head. This is necessary to accommodate a four-valve structure and a Common-Rail fuel injection system with a centrally located injector. The cylinder heads have a complex structure due to the many channels, including water cooling, cylinder head retaining bolts, inlet and outlet openings, injectors, glow plugs, valves, their rods, recesses and seats, as well as other channels used for exhaust gas recirculation some designs.
Combustion chambers in modern direct injection diesel engines can be called open or repeated.
Open cameras
If the upper hole of the bowl in the piston has a smaller diameter than the maximum of the same parameter of the bowl, then it is called return. Such bowls have a "lip." If not, then this is an open combustion chamber. In diesel engines, these designs with a bowl type "Mexican hat" have been known since the 20s of the last century. They were used until 1990 in heavy-duty engines until the moment when the return bowl became more important than it was before. This form of the combustion chamber is intended for relatively advanced injection times, where the cup contains most of the burning gases. It is not very suitable for slow injection strategies.
Diesel engine
It is named after the inventor of Rudolph Diesel. It is an internal combustion engine in which ignition of the injected fuel is caused by the increased temperature of the air in the cylinder due to mechanical compression. A diesel engine works by compressing air only. This increases the air temperature inside the cylinder to such an extent that the atomized fuel injected into the combustion chamber spontaneously ignites.
This differs from spark ignition engines such as gasoline or gas (using gaseous fuel rather than gasoline). They use a spark plug to ignite the air-fuel mixture. In diesel engines, glow plugs (combustion chamber heaters) can be used to facilitate starting in cold weather, as well as with a low compression ratio. The original diesel engine runs on a constant pressure cycle of gradual combustion and does not produce an acoustic shock.
general characteristics
The diesel engine has the highest thermal efficiency among all practical internal and external combustion engines due to a very high degree of expansion and inherent lean combustion, which allows heat to dissipate excess air. A small loss in efficiency is also prevented without direct injection, since unburned fuel is not present when the valve is shut off, and fuel does not directly enter the exhaust pipe from the inlet (injection) device. Low-speed diesel engines that are used, for example, on ships, can have a thermal efficiency exceeding 50 percent.
Diesels can be designed as push-pull, and four-stroke. They were originally used as a more effective replacement for stationary steam engines. Since 1910, they have been used in submarines and ships. Use in locomotives, trucks, heavy equipment and power plants followed later. In the thirties of the last century, they found a place in the design of several cars.
Advantages and disadvantages
Since the 70s of the last century, the use of diesel engines in larger road and off-road vehicles in the USA has increased. According to the British Society of Automobile Manufacturers and Manufacturers, the EU average for diesel cars is 50% of total sales (among them 70% in France and 38% in the UK).
In cold weather, starting high-speed diesel engines can be difficult because the mass of the block and cylinder head absorbs compression heat, preventing ignition due to a higher surface to volume ratio. Previously, such units use small electric heaters inside the chambers, called glow plugs.
Kinds
Many engines use resistive heaters in the intake manifold to heat the incoming air and to start or until the operating temperature is reached. Electrical resistive heaters of the engine block connected to the mains are used in cold climates. In such cases, it must be turned on for a long time (more than an hour) in order to reduce startup time and wear.
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The conditions in a diesel engine are different from a spark ignition engine due to a different thermodynamic cycle. In addition, the power and frequency of its rotation are directly controlled by the supply of fuel rather than air, as in an engine with a cyclic cycle. The combustion temperature of diesel fuel and gasoline may also vary.
An average diesel engine has a lower power to weight ratio than a gasoline one. This is due to the fact that the diesel engine must operate at lower speeds due to the structural need for heavier and stronger parts to withstand the working pressure. It is always caused by a high degree of compression of the engine, which increases the forces on the parts due to inertia forces. Some diesels are for commercial use. This is repeatedly confirmed in practice.
Diesel engines usually have a long stroke. This is mainly necessary to facilitate the achievement of the necessary compression ratios. As a result, the piston becomes heavier. The same can be said of the connecting rods. Through them and the crankshaft, it is necessary to transfer more force to change the momentum of the piston. This is another reason why a diesel engine should be stronger at the same power output as a gasoline one.