Sylvie Willis
Pilot fuel is a small quantity of fuel used for ignition in internal combustion engines. The pilot ignition method is a form of indirect compression ignition, where the pilot fuel is introduced into the cylinder by an injector. Dual-fuel engines, which use a mixture of gaseous fuel and liquid pilot fuel, have been found to match the brake power and fuel economy of engines operating on pure diesel fuel. The quantity of pilot fuel is an important parameter for controlling the combustion process, with very few studies investigating its effects in detail.
| Characteristics | Values |
|---|---|
| Definition | Pilot fuel is a small quantity of fuel used for ignition in internal combustion engines. |
| Ignition method | Pilot fuel is introduced into the cylinder by another injector, a pilot injector, or by mixing with the gas in a gas injector. |
| Injector design | Two options are available: dual-fuel-two-injectors or dual-fuel-single-injector. |
| Fuel types | Diesel, methane, propane, natural gas, hydrogen, rapeseed methyl ester, rubber seed oil, Fischer-Tropsch fuel, and dimethyl ether. |
| Engine types | Dual-fuel engines, tri-fuel engines, turbocharged direct injection diesel engines, and heavy-duty engines. |
| Engine speed | Experiments conducted at constant engine speeds of 1000 rpm and 1500 rpm. |
| Pilot fuel quantity | Different quantities used in experiments: 0.125, 0.15, 0.19, 0.23, 0.22, and 0.312 kg/h or MPa. |
| Impact on emissions | Increasing pilot fuel quantity reduces CO and HC emissions. Dual-fuel mode operation may increase exhaust emissions compared to neat diesel/oil operation. |
| Engine efficiency | Dual-fuel combustion may negatively impact engine efficiency and increase carbon monoxide emissions. |
| Combustion stability | Pilot fuel properties, such as H2 concentration and charge-air temperature, affect combustion stability and ignition delay time. |
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What You'll Learn
Pilot fuel ignition method
The pilot ignition method involves injecting a small amount of fuel into the combustion chamber before the main fuel injection. This process is called pilot injection. Diesel fuel is usually used as the pilot fuel, which creates a highly active in-cylinder mixture at the moment of ignition. This mixture then ignites the main fuel, accelerating hydrogen combustion and improving combustion isotropy.
There are two methods of introducing the pilot fuel into the cylinder: separate-entrance and co-entrance. The separate-entrance method uses another injector, increasing the complexity and cost of the fuel injection system. However, it offers flexibility in timing and placement, allowing the pilot fuel to enter the cylinder independently of the main gas injection event. On the other hand, the co-entrance method is simpler and more cost-effective, but it lacks flexibility in timing and placement of the ignition zones, and the pilot fuel may not mix well with air.
The pilot fuel can be introduced into the cylinder by a separate injector (pilot injector) or by mixing it with the gas in a gas injector before entering the cylinder. The use of diesel as a pilot fuel is attractive due to its wide availability and lower cost compared to other candidates such as dimethyl ether, Fischer-Tropsch fuel, and rapeseed oil.
Pilot injection is an effective technique for reducing ignition delay and high rates of pressure rise at the start of injection, resulting in lower NO and noise levels, although it may lead to high hydrocarbon emissions. Adjusting the timing and pressure of pilot injection can improve spray jet penetration and overall combustion. Additionally, increasing the quantity of pilot fuel can enhance combustion and reduce specific emissions under certain operating conditions.
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Pilot fuel quantity
Pilot fuel is used for ignition in diesel engines. The pilot fuel can be introduced into the cylinder by a separate injector, a pilot injector, or by mixing with the gas in a gas injector.
The quantity of pilot fuel used is important as it affects the performance and emissions of the engine. For example, at very light loads with a small pilot fuel quantity, CO and HC emissions were found to be relatively high. This was attributed to an excessively lean mixture, where the resulting flame from the pilot ignition could not propagate throughout the combustion chamber, leading to only partial oxidation.
Increasing the pilot fuel quantity can help reduce these emissions. As the volume of the charge affected by the combustion of the pilot fuel envelope becomes larger, the burned fraction of the gaseous fuel increases, thereby reducing CO and HC emissions.
Imran et al. studied the effects of pilot fuel quantity on combustion and exhaust emissions characteristics of a DI diesel engine. They investigated three pilot fuel quantity settings at an engine speed of 1500 rpm and a load of 0.503 MPa brake mean effective pressure (BMEP): 0.125, 0.252, and 0.312 MPa BMEP.
Additionally, the effect of pilot fuel quantity is more significant at low speeds and loads. At high speeds and loads, the quantity of fuel is sufficient to produce a stable flame, and the impact of pilot fuel quantity becomes less noticeable.
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Pilot fuel pump
A pilot fuel pump is a stand-alone unit that comprises a radial piston pump and the necessary filters, valves, and control systems. Pilot fuel is used to ignite the gas fuel in dual-fuel injectors, which are used in heavy-duty engines for commercial applications. The injector is the same size as a diesel injector, allowing it to be fitted into the original hole without modification.
The pilot fuel can be introduced into the cylinder in several ways. One method is through a separate injector, known as the dual-fuel-two-injectors option, which may require substantial modifications to the cylinder head. Another method is to use a combined unit with the gas injector, called the dual-fuel-single-injector design. This design has been successfully applied to heavy-duty engines, allowing them to match the brake power and fuel economy of engines operating on pure diesel fuel.
The quantity of pilot fuel used can affect engine performance. For example, at very light loads with a small pilot fuel quantity, CO and HC emissions were found to be relatively high due to an excessively lean mixture. Increasing the pilot fuel quantity can reduce these emissions by enlarging the burned fraction of the gaseous fuel.
The influence of pilot fuel quantity is more visible at low speeds and loads, where the amount of natural gas (NG) is insufficient to sustain a stable combustion. However, at high speeds and loads, the quantity of NG is typically enough to produce a stable flame, making the effect of pilot fuel quantity less significant.
Overall, the use of pilot fuel pumps and the optimization of pilot fuel quantities play a crucial role in improving engine performance, reducing emissions, and enhancing combustion processes in dual-fuel engines.
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Pilot fuel injector design
Pilot fuel is a small quantity of fuel used for ignition, which can be introduced into the cylinder by a separate injector (dual-fuel-two-injectors) or by a combined injector (dual-fuel-single-injector). The dual-fuel-single-injector design offers the advantage of being able to fit into the original injector hole without modification, making it a convenient option for converting diesel engines to dual-fuel operation.
The pilot fuel injector design plays a crucial role in the performance and emissions of dual-fuel engines. The size of the pilot fuel injection is a critical factor, as it should be kept above a certain value to ensure continuous combustion of the gaseous fuel. Reducing the pilot fuel quantity below 5-10% of the ultimate design level can lead to poor atomization and deteriorated combustion. Therefore, injector design must take into account the minimum quantity of pilot fuel required to maintain stable combustion.
The timing and pressure of pilot injection also significantly impact engine performance and emissions. Advancing the timing and increasing the pressure of pilot injection improves spray jet penetration and overall combustion. Optimized timing can also help achieve two-stage auto-ignition, improving engine performance and reducing emissions at part load. Additionally, the delayed injection of gaseous fuel after pilot injection by approximately 1 ms is crucial for efficient combustion.
Another important consideration in pilot fuel injector design is the placement of ignition zones. With a co-entrance system, where the pilot fuel travels along with the gas jets, there is reduced flexibility in the timing and placement of ignition zones, and the pilot fuel may not have access to sufficient fresh air. On the other hand, a separate-entrance system offers more flexibility in ignition zone placement, allowing for efficient ignition of gas jets by establishing ignition zones ahead of the gas jets.
In summary, the design of pilot fuel injectors involves critical decisions about injector type (dual-fuel-two-injectors or dual-fuel-single-injector), injector timing and pressure, and the placement of ignition zones. These factors collectively influence the performance and emissions, and combustion stability of dual-fuel engines.
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Pilot fuel properties
Pilot fuel is used for ignition in engines that run on two types of fuel. It is usually introduced into the cylinder by an injector.
Biodiesel fuels have great potential as alternative fuels, either for conventional mono-fuel compression ignition engines or for NG-diesel dual-fuel engines as a pilot. This is due to their properties being fairly similar to diesel fuel. Biodiesel is defined as "fuel comprised of mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats". It has a higher cetane number than petrodiesel, and a relatively high LHV of around 40 MJ/kg. It is also easy to handle because its boiling point is higher than that of diesel. It is an environmentally friendly fuel with low smoke and PM emissions as it contains up to 10% oxygen.
Rapeseed methyl ester (RME), a product of methanol transesterification of rapeseed oil, is also a popular candidate for pilot fuel. It performs very closely to diesel fuel in terms of performance and thermal efficiency, while exhibiting very similar ignition delay and peak cylinder pressure levels. This is due to the similar physical and chemical properties of the two fuels.
The use of biodiesel as a pilot fuel has been shown to result in a smoother combustion process, with lower levels of noise and reduced NOx emissions. When used as a pilot fuel for NG engines, no smoke readings could be detected.
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Frequently asked questions
Pilot fuel is a small quantity of fuel used for the purpose of ignition.
Pilot fuel for diesel engines refers to a small quantity of diesel fuel used to ignite the main fuel, which may be natural gas, methane, or propane.
Using pilot fuel in diesel engines can help match the brake power and fuel economy of engines operating on pure diesel fuel. It also allows for the use of dual-fuel injectors without requiring substantial modifications to the engine. Additionally, pilot fuel can improve combustion stability and reduce harmful exhaust emissions.
