Does diesel fuel burn? It burns, and quite strongly. Its residue that did not participate in pre-mixed combustion is consumed in the variable rate combustion phase.
Combustion in diesel engines is very difficult. Until the 1990s, 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 seemed to defy researchers' attempts to unravel its many secrets, despite the availability of modern tools such as high-speed photography used in "transparent" engines, the processing power of modern computers, and many mathematical models designed to simulate combustion in diesel The application of sheet laser imaging to the traditional diesel combustion process in the 1990s was the key to greatly improving the understanding of this process.
This article will coverthe 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 the diffusion of fuel and air before ignition.
Combustion temperature
At what temperature does diesel fuel burn? If earlier this question seemed difficult, now it can be given a completely unambiguous answer. 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 predominate, engines had a glow plug that warms the intake port to help start the engine. This is why you should always wait until the heater icon on the dashboard goes out before starting the engine. It also affects the combustion temperature of diesel fuel. Let's consider what other nuances there are in his work.
Features
The main prerequisite for burning diesel fuel in an externally controlled burner is its unique way of releasing the chemical energy stored in it. In order to carry out this process, oxygen must be available to it in order to facilitate combustion. One of the most important aspects of this process is the mixing of fuel and air, often referred to as pre-mixing.
Diesel combustion catalyst
In diesel engines, fuel is often injected into the engine cylinder at the end of the compression stroke, just a few degrees of crankshaft angle before top dead center. The liquid fuel is usually injected at high speed in one or more jets through small holes or nozzles in the injector tip, atomized into fine droplets, and enters 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 (mostly air) reaches its ignition temperature. The combustion temperature of Webasto diesel fuel is no different from that of other diesel grades, reaching about 500-600 degrees.
Fast ignition of some pre-mixed fuel and air occurs after a period of ignition delay. This rapid ignition is considered the start of combustion and is characterized by a sharp increase in cylinder pressure as the air-fuel mixture is consumed. The increased pressure resulting from pre-mixed combustion compresses and heats the unburned portion of the charge and shortens the delay before it ignites. It also increases the rate of evaporation of the remaining fuel. Its spraying, evaporation, mixing with air continues until all of it is burned. The combustion temperature of kerosene and diesel fuel in this respect 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 under peak torque conditions. To avoid excessive smoke generation, peak torque A/F is typically maintained above 25:1, well above the stoichiometric (chemically correct) equivalence ratio of about 14.4:1. This also applies to all diesel combustion activators.
In turbocharged diesel engines, the A/F ratio at idle can exceed 160:1. Consequently, the excess air present in the cylinder after the combustion of the fuel continues to mix with the burning and already exhausted gases. When the exhaust valve is opened, excess air is exhausted along with the products of combustion, which explains the oxidative nature of diesel exhaust.
When does diesel fuel burn? This process occurs after the vaporized fuel mixes with air to form a locally rich mixture. Also at this stage, the proper combustion temperature of diesel fuel is reached. However, the overall A/F ratio is small. In other words, it can be said that most of the air entering the cylinder of a diesel engine is compressed and heated, but never participates in the combustion process. The oxygen in the excess air helps to oxidize gaseous hydrocarbons and carbon monoxide, reducing them to extremely low concentrations in the exhaust gases. This process is much more important than the combustion temperature of diesel fuel.
Factors
The following factors play a major role in the diesel combustion process:
- The induced charge of air, its temperature and its kinetic energy in several dimensions.
- Atomization of injected fuel, splash 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 example:
- Design of the inlet. It has a strong influence on the movement of charge air (especially at the moment when it enters the cylinder) and on the mixing rate in the combustion chamber. This can change the combustion temperature of diesel fuel in the boiler.
- The design of the intake port can also affect the charge air temperature. This can be achieved by transferring heat from the water jacket through the surface area of the inlet.
- Intake valve size. Controls the total mass of air admitted to the cylinder over a finite time.
- Compression ratio. It affects evaporation, mixing speed and combustion quality, regardless of the combustion temperature of the diesel fuel in the boiler.
- Injection pressure. It controls the injection duration for a given nozzle opening parameter.
- Atomization geometry, which directly affects the quality and combustion temperature of diesel fuel and gasoline forair use account. For example, a larger spray cone angle can place fuel on top of the piston and outside of the combustion tank in open chamber DI diesel engines. This condition can lead to excessive "smoking" as the fuel is denied access to air. Wide cone angles can also cause fuel to splatter on the cylinder walls rather than inside the combustion chamber where it is required. Sprayed onto the cylinder wall, it will eventually move down into the oil pan, shortening the life of the lubricating oil. Because the spray angle is one of the variables that affects the rate of air mixing in the fuel jet near the injector outlet, it can have a significant effect on the overall combustion process.
- Valve configuration that controls the position of the injector. Two-valve systems create a tilted injector position, which means uneven spraying. This leads to a violation of the mixing of fuel and air. On the other hand, four-valve designs allow vertical injector mounting, symmetrical fuel atomization, and equal access to available air for each atomizer.
- Position of the upper piston ring. It controls the dead space between the top of the piston and the cylinder liner. This dead space traps air that compresses and expands without even participating in the combustion process. Therefore, it is important to understand that the diesel engine system is not limited to the combustion chamber, injector nozzles andtheir immediate environment. Combustion includes any part or component that can affect the end result of the process. Therefore, no one should have any doubts about whether diesel fuel burns.
Other details
Diesel combustion is known to be very lean with A/F ratio:
- 25:1 at peak torque.
- 30:1 at rated speed and maximum power.
- More than 150:1 at idle for turbocharged engines.
However, this additional air is not included in the combustion process. It heats up quite a lot and is depleted, as a result of which the diesel exhaust becomes poor. Even though the average air-fuel ratio is poor, if proper measures are not taken during the design process, combustion chamber areas can be rich in fuel and result in excessive smoke emissions.
Combustion chamber
A key design goal is to ensure sufficient mixing of fuel and air to mitigate the effects of fuel rich areas and allow the engine to reach its performance and emissions targets. It has been found that turbulence in the movement of air within the combustion chamber is beneficial to the mixing process and can be used to achieve this. The vortex created by the inlet can be amplified and the piston can createsqueezing as it approaches the cylinder head to allow for more turbulence during the act of compression due to the correct cup design in the piston head.
Combustion chamber design has the most significant impact on particulate emissions. It can also affect unburned hydrocarbons and CO. Although NOx emissions depend on the design of the bowl [De Risi 1999], the properties of the bulk gas play a very important role in their exhaust gas levels. However, due to the NOx/PM trade-off, combustor designs had to evolve as NOx emission limits decreased. This is mainly required to avoid the increase in PM emissions that would otherwise occur.
Optimization
An important parameter for optimizing the combustion system of diesel fuel in the engine is the proportion of available air involved in this process. The K factor (ratio of piston cup volume to clearance) is an approximate measure of the proportion of air available for combustion. Reducing the displacement of the engine leads to a decrease in the relative coefficient K and to a tendency to worsen the combustion characteristics. For a given displacement and at a constant compression ratio, the K factor can be improved by choosing a longer stroke. The selection of the cylinder bore to engine ratio can be affected by the K factor and a number of other factors such as engine packaging, bores and valves, and so on.
Possible difficulties
A particularly significant problem when setting upThe maximum ratio of cylinder to stroke lies in the very complex packaging of the cylinder head. This is necessary to accommodate the four-valve design and the common-rail fuel injection system with the injector located in the center. Cylinder heads are complex due to the many passages including water cooling, cylinder head retaining bolts, intake and exhaust ports, injectors, glow plugs, valves, valve stems, recesses and seats, and other passages used for exhaust gas recirculation in some designs.
Combustion chambers in modern direct injection diesel engines may be referred to as open or secondary combustion chambers.
Open cameras
If the upper hole of the bowl in the piston has a smaller diameter than the maximum of the same bowl parameter, then it is called returnable. Such bowls have a "lip". If not, then this is an open combustion chamber. In diesel engines, these Mexican hat bowl designs have been known since the 1920s. They were used until 1990 in heavy duty engines to the point where the return bowl became more important than it used to be. This form of combustion chamber is designed for relatively advanced injection times, where the bowl contains most of the burning gases. It is not well suited for delayed injection strategies.
Diesel engine
It is named after inventor Rudolf Diesel. It is an internal combustion engine in which ignition of the injected fuel is caused by increasedair temperature in the cylinder due to mechanical compression. Diesel works by compressing only air. This raises the temperature of the air inside the cylinder to such an extent that the atomized fuel injected into the combustion chamber ignites spontaneously.
This is different from spark ignition engines such as gasoline or LPG (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 aid starting in cold weather and also at low compression ratios. The original diesel operates on a constant pressure cycle of gradual combustion and does not produce a sonic boom.
General characteristics
Diesel has the highest thermal efficiency of any practical internal and external combustion engine due to its very high expansion ratio and inherent lean combustion, allowing excess air to dissipate heat. A small loss of efficiency is also prevented without direct injection, since unburned fuel is not present when the valve closes, and fuel does not flow directly from the intake (injector) device to the exhaust pipe. Low speed diesel engines, such as those used in ships, can have thermal efficiencies in excess of 50 percent.
Diesels can be designed as two-stroke or four-stroke. They were originally used aseffective replacement for stationary steam engines. Since 1910 they have been used on 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 1970s, the use of diesel engines in larger on- and off-road vehicles in the US has increased. According to the British Society of Motor Manufacturers and Manufacturers, the EU average for diesel vehicles 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 as the mass of the block and cylinder head absorbs the heat of compression, preventing ignition due to the higher surface to volume ratio. Previously, these units use small electrical heaters inside chambers called glow plugs.
Views
Many engines use resistance heaters in the intake manifold to heat the intake air and to start or until operating temperature is reached. Electric resistive engine block heaters connected to the mains are used in cold climates. In such cases, it needs to be turned on for a long time (more than an hour) to reduce start-up time and wear.
Block heaters are also used for emergency power supplies with diesel generators, which need to quickly offload power in the event of a power outage. In the past, a wider variety of cold start methods have been used. Some engines, such as the Detroit Diesel, used a system to introduce small amounts of ether into the intake manifold to start combustion. Others have used a mixed system with a methanol-burning resistance heater. An impromptu method, especially on non-running engines, is to manually spray an aerosol can of essential liquid into the intake air stream (usually through the intake air filter assembly).
Differences from other engines
Diesel conditions are different from spark ignition engine due to different thermodynamic cycle. In addition, the power and speed of its rotation is directly controlled by the supply of fuel, and not air, as in a cyclic engine. The combustion temperature of diesel fuel and gasoline may also differ.
The average diesel engine has a lower power-to-weight ratio than a gasoline engine. This is because the diesel has to run at a lower RPM due to the structural need for heavier and stronger parts to withstand the operating pressure. It is always caused by a high compression ratio of the engine, which increases the forces on the part due to inertia forces. Some diesels are for commercial use. This has been repeatedly confirmed in practice.
Diesel engines usuallyhave a long stroke. Basically, this is necessary to facilitate the achievement of the required compression ratios. As a result, the piston becomes heavier. The same can be said about the rods. More force must be transmitted through them and the crankshaft to change the momentum of the piston. This is another reason why a diesel engine needs to be stronger for the same power output as a gasoline engine.
