Sylvie Willis
Direct injection diesel engines have been around since the first successful prototype in 1894. They have always utilized direct fuel injection, unlike petrol engines, which have only recently started using direct injection. The main purpose of the fuel injection system is to deliver fuel into the engine cylinders, while precisely controlling the injection timing, fuel atomization, and other parameters. Modern direct injection diesel engines use either a unit injector or a common rail system. The unit injector system combines the injector nozzle with the injection pump, and the common rail system uses a common fuel rail that stores fuel at extremely high pressures and feeds it to several fuel injectors. The fuel rack is able to rotate the plunger in the pump, and the rack is controlled by the governor.
| Characteristics | Values |
|---|---|
| How does direct injection work? | Fuel is sprayed into the combustion chamber or a pre-combustion chamber. |
| Fuel rack | The fuel rack rotates the plunger in the pump. |
| Fuel atomization | Fuel is atomized into very small particles to ensure complete combustion. |
| Fuel injection nozzles | Can be hole-type or throttling pintle type, and either closed or open. |
| Fuel metering | Carried out in the high-pressure pump or the fuel injector. |
| Fuel injection types | Pump-line-nozzle, unit injector, and common rail. |
| Fuel injection in diesel engines | Diesel engines have always utilized direct fuel injection. |
| Common rail systems | Found in most diesel passenger cars worldwide. |
| Port injection | Helps with cold starts and subsequent warm-up periods. |
| Direct injection drawbacks | More carbon buildup, decreased performance at lower combustion temperatures, added complexity, and more expensive replacement parts. |
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What You'll Learn
Fuel injection nozzles
The injection nozzle plays a key role in this process by determining the spray pattern and droplet size of the fuel as it enters the cylinder. The nozzle design can vary depending on the specific engine and application, but generally, it consists of a small orifice or set of orifices that shape the fuel spray.
One example of a direct injection system is the Bosch continuous injection system (CIS) used by Volvo and Volkswagen in the 1970s and 1980s. In this system, the fuel is sprayed continuously onto the back of the intake valve, and the spray pattern is controlled by a specially shaped tube. The amount of fuel admitted is regulated mechanically at the rotary admission valve.
Direct injection nozzles have some drawbacks. One issue is the aggressive carbon build-up that occurs in the intake ports and on the back of the valves due to the lack of detergents in the fuel. Additionally, atomized fuel does not perform well at lower combustion temperatures, leading to decreased performance and efficiency until the engine reaches its ideal operating temperature.
To address these issues, some maintenance procedures for direct injection engines include cleaning the injectors and using full synthetic motor oil. Synthetic oils resist breaking down and are less likely to evaporate, reducing the amount of oil that passes through the emissions system and ends up as carbon in the intake ports.
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Fuel atomization
Direct injection diesel engines have been in use since the first successful prototype in 1894. The purpose of the fuel injection system is to deliver fuel into the engine cylinders while controlling the injection timing, fuel atomization, and other parameters.
The application of direct injection (DI) technology can improve the atomization effect of heavy fuel, leading to reduced fuel loss and improved engine performance. Low-pressure air-assisted injectors, for example, are used in power systems like small-scale fuel-powered unmanned aerial vehicles, motorcycles, and off-board machines, demonstrating favourable atomization performance and significant fuel-saving effects.
However, direct injection engines also have some drawbacks. Finely atomized fuel does not perform well at lower combustion temperatures, leading to decreased performance and efficiency until the engine reaches its ideal operating temperature. Additionally, direct injection engines experience an aggressive amount of carbon buildup in the intake ports and on the back of the valves, which can hinder fuel efficiency and performance if not properly maintained.
Overall, while direct injection technology offers improved fuel atomization and engine performance, it also presents challenges related to carbon buildup and temperature sensitivity that must be addressed through proper maintenance and engine design.
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Injection timing
The injection timing is determined by the pump, which injects fuel into the cylinder as the piston reaches the BTDC (Before Top Dead Centre) point. Injecting diesel BTDC allows the air-fuel mixture to burn completely before the piston reaches the top, creating maximum pressure in the engine's cylinders. This pressure forces the piston down with significant force during the power stroke, moving the crankshaft.
The injection timing is set by manufacturers to balance emissions and power while adhering to legal emission limits. Advancing the timing can help engines make full use of the fuel injection process and increase power. However, if the advance is too far ahead, it can cause detonation, where the mixture pushes on the pistons as they move upward, potentially damaging the engine.
Direct injection engines, which inject fuel directly into the cylinder, offer more control over the fuel delivery process and enable advanced engine management techniques like Variable Valve Timing. They provide benefits such as lower emissions, improved fuel economy, and higher power compared to port fuel injection engines. However, direct injection engines are prone to carbon build-up due to the lack of detergents reaching the back of the intake valves.
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Injector design
Direct injection diesel engines have been around since the first successful prototype in 1894. However, the design has evolved over time, with modern engines using high-pressure common-rail fuel systems.
Direct injection diesel injector design is more complex than indirect injection. The injectors need to handle higher injection pressures, and the design and manufacture of the injectors are more intricate due to the integration between the injector and the engine. The optimisation of in-cylinder airflow is a challenging aspect of injector design, requiring powerful CFD simulation systems to ensure effective combustion.
The injectors in direct injection diesel engines are larger and heavier-duty than those in indirect injection systems. They also tend to be more expensive to replace due to their critical role in the engine's performance.
Some manufacturers have developed dual-injection systems, combining direct and indirect injectors to balance the benefits of both types. This approach helps manage carbon accumulation and improve fuel-air mixing, but it is typically found in higher-cost models due to added complexity and expense.
Overall, injector design in direct injection diesel engines is a complex and critical aspect of engine performance, requiring careful optimisation to achieve the desired combustion and engine efficiency.
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Direct vs indirect injection
Direct injection engines are currently the most common type of engine, but indirect injection engines still have their advantages and are used in many applications. The main difference between the two types of engines is where the fuel is injected. Direct injection engines inject fuel directly into the combustion chamber, while indirect injection engines inject fuel into a pre-chamber or swirl chamber that is connected to the main combustion chamber.
Indirect injection engines, also known as IDI engines, typically have high compression ratios, unique-looking cylinder heads that house the pre-chambers, and are often naturally aspirated rather than turbocharged. One advantage of IDI engines is that they are robust against low-quality fuel and are simple to maintain. Toyota, for example, continues to manufacture the 1HZ indirect non-turbo diesel engine for many markets. These engines are not suitable for high-speed highways in developed countries due to their low power output.
Direct injection engines, on the other hand, are more efficient because they do not have a pre-chamber, allowing for higher injection pressure and more precise injection events. The advent of electronic controls and the use of modern, fast-energizing, solenoid-equipped fuel injectors have further improved the efficiency of direct injection engines. Additionally, direct injection allows for more control during the fuel delivery process by atomizing the spray of fuel in the cylinder, providing better fuel distribution throughout the combustion chamber.
However, direct injection engines also have some drawbacks. One common problem is the build-up of carbon in the intake ports and on the back of the valves. This is because the detergents in gasoline, which help keep the ports and valves clean, are not as effective with direct injection. Another disadvantage is that atomized fuel does not perform well at lower combustion temperatures, leading to decreased performance and efficiency until the engine reaches its ideal operating temperature. Direct injection engines are also more complex and expensive due to the need for an additional high-pressure fuel pump and larger, heavier-duty injectors.
In conclusion, while direct injection engines offer improved efficiency and control over the fuel delivery process, indirect injection engines have the advantage of simplicity, robustness, and lower maintenance requirements. The choice between the two types of engines ultimately depends on the specific application and the user's needs.
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Frequently asked questions
Direct injection diesel engines are engines that inject fuel directly into the combustion chamber or a pre-combustion chamber. Modern direct injection diesel engines have two variants: the unit injector and the common rail.
In a common-rail system, fuel from the fuel tank is supplied to a common header (the accumulator), and then sent through tubing to the injectors, which inject it into the combustion chambers. The accumulator has a high-pressure relief valve to maintain pressure and return excess fuel to the fuel tank. The fuel is sprayed with the help of a nozzle that is opened and closed with a solenoid-operated needle valve.
Direct injection allows for more control during the fuel delivery process by atomizing the spray of fuel in the cylinder, providing better distribution of fuel throughout the combustion chamber.
