Rajan Wilcox
The performance of diesel engines is heavily influenced by their injection system design. The main purpose of a fuel injection system is to deliver fuel into the engine cylinders while precisely controlling the injection timing, fuel atomization, and other parameters. The pressure at which diesel fuel is injected has a direct impact on the engine's efficiency, with higher pressures creating more efficiency. Today's diesel engines have considerably more fuel injection pressure than diesels of the past, with common rail injection systems delivering fuel at pressures as high as 28,000 psi. This raises the question of how many psi a diesel fuel injector can handle.
| Characteristics | Values |
|---|---|
| Purpose of fuel injection system | To deliver fuel into the engine cylinders, while precisely controlling the injection timing, fuel atomization, and other parameters |
| Main types of injection systems | Pump-line-nozzle, unit injector, and common rail |
| Fuel injection pressure range | 10,000-30,000 psi |
| Early Stanadyne DB2 injection pumps | 6,700 psi |
| Newer common-rail systems | 34,000-35,000 psi |
| Green Diesel Corp system | 160,000 psi |
| Fuel injection start | ≈15 BTDC on the compression stroke |
| Fuel atomization | Enabled by high-pressure injection; the biggest contributing factor to efficiency |
| Injection duration | The period of time during which fuel enters the combustion chamber from the injector |
| Injector lag | The difference between the actual SOI and indicated SOI |
| Injection delay | The time it takes for a pressure wave to travel between the pump and injector, influenced by the length of the line and the speed of sound in the fuel |
| End of injection (EOI) | The time in the cycle when fuel injection stops |
| Injected fuel quantity | The amount of fuel delivered to an engine cylinder per power stroke, expressed in mm3/stroke or mg/stroke |
| Injection pattern | The rate of injection of fuel, which often varies during the injection duration period |
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What You'll Learn
Injector nozzle sprays directly into the combustion chamber
The performance of diesel engines is heavily influenced by their injection system design. The fuel injection system delivers fuel into the engine cylinders while precisely controlling the injection timing, fuel atomization, and other parameters. The injector nozzle sprays directly into the combustion chamber.
The nozzle is the part of the nozzle body/needle assembly that interfaces with the combustion chamber of the engine. The nozzle holder or injector body is the part that the nozzle is mounted on. In conventional injection systems, this part mainly serves the function of nozzle mounting and nozzle needle spring preloading. In common rail systems, it contains the main functional parts: the servo-hydraulic circuit and the hydraulic actuator (electromagnetic or piezoelectric). Injector commonly refers to the nozzle holder and nozzle assembly.
The start of injection (SOI) or injection timing is the time at which injection of fuel into the combustion chamber begins. The SOI is often indicated by the time that an electronic trigger is sent to the injector or a signal from a needle lift sensor that indicates when the injector needle valve starts to open. Due to the mechanical response of the injector, there can be a delay between the indicated SOI and the actual SOI when fuel exits the injector nozzle into the combustion chamber. This delay is known as injector lag.
The end of injection (EOI) is the time in the cycle when fuel injection stops. The injected fuel quantity is the amount of fuel delivered to an engine cylinder per power stroke. It is often expressed in mm3/stroke or mg/stroke. Injection duration is the period of time during which fuel enters the combustion chamber from the injector. It is calculated by finding the difference between EOI and SOI and is related to injection quantity. The rate of injection of fuel often varies during the injection duration period.
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Injector lag and injection delay
The performance of diesel engines is heavily influenced by their injection system design. The 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 diesel fuel injection systems can reach very high injection pressures, and they utilize sophisticated electronic control methods. The higher the pressure at which diesel fuel is injected, the more efficient the engine becomes.
Injection delay, on the other hand, refers to the time difference between the start of delivery and the SOI. The start of delivery is when the high-pressure pump starts delivering fuel to the injector. The injection delay is influenced by the length of the line between the high-pressure pump and the injector, as well as the speed of sound in the fuel.
In some cases, there can also be a delay caused by the injector's response to the pressure wave travelling between the pump and the injector. This delay can affect the injection duration, which is the period during which fuel enters the combustion chamber from the injector. It is important to note that the injection rate can vary during this duration.
To achieve higher injection pressures, diesel fuel injection systems have evolved from mechanical systems operating at 2,000 to 3,000 psi to modern common rail injection systems that can reach pressures as high as 28,000 psi. This increase in pressure improves fuel atomization, contributing to the efficiency of today's diesel engines.
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Fuel atomization and injection timing
The performance of diesel engines is heavily influenced by their injection system design. The main purpose of the fuel injection system is to deliver fuel into the cylinders of an engine. However, it is how the fuel is delivered that makes the difference in engine performance, emissions, and noise characteristics. The injection timing must be controlled, and the correct amount of fuel must be delivered to meet the power requirements.
Fuel atomization, along with port swirl, is the biggest contributing factor to the efficiency of today's diesel engines. Fuel atomization refers to ensuring that the fuel atomizes into very small fuel particles. The higher the pressure at which the fuel is injected, the more efficient the atomization. Modern injection systems reach very high injection pressures, and common rail injection systems can inject fuel at pressures as high as 28,000 psi, far exceeding the 2,000 to 3,000 psi of mechanical systems of the past. The increased atomization of fuel from the common rail injection also created design changes to the piston and combustion chambers, as well as the design of the intake ports and valve train.
The start of injection (SOI) or injection timing is the time at which injection of fuel into the combustion chamber begins. It is usually expressed in crank angle degrees (CAD) relative to TDC of the compression stroke. There can be a delay between the indicated SOI and the actual SOI when fuel exits the injector nozzle, known as injector lag. The difference between the start of delivery and SOI is affected by the length of time it takes for a pressure wave to travel between the pump and injector and is influenced by the length of the line between the high-pressure pump and the injector, as well as the speed of sound in the fuel.
Advancing the injection timing can improve the mixture concentration distribution and increase the mass fraction of fuel evaporation. Injection timing is critical for several reasons. For example, too early and the result can be bent rods and piston damage, while late timing will result in an incomplete fuel burn, loss of power, and poor fuel economy.
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High-pressure fuel pumps
The performance of diesel engines is heavily influenced by their injection system design. Modern diesel fuel injection systems reach very high injection pressures, and utilise sophisticated electronic control methods. The main types of injection systems include pump-line-nozzle, unit injector, and common rail.
The purpose of common rail injection is to deliver high-pressure fuel to the injector. Fuel in a common rail system will be injected into the combustion chamber through the injector nozzle at pressures as high as 28,000 psi. This is far beyond the mechanical systems of the past that would inject fuel into the combustion chamber at 2,000 to 3,000 psi.
To inject diesel fuel at high pressures, a unique pump is required, known as a high-pressure fuel pump. This pump is usually engine-mounted and driven by the engine gear train. A regulator, or fuel-metering valve, controls the amount of pressure the pump makes, and the amount of fuel it intakes. The high-pressure fuel pump will produce the necessary high pressures regardless of the engine speed. After the pump has been pressurised, the fuel is stored in the fuel rails, which act as accumulators for the high-pressure fuel to be delivered to the injectors. The fuel rails also dampen vibrations from the high-pressure fuel pump and injection cycles from the injectors.
The CP3 is a radial-piston pump for high-pressure, common-rail injection systems. The common-rail system uses an accumulator rail(s) to maintain fuel at high pressure; this rail(s) feeds the fuel to the injectors. The CP3 pump is similar to the VP44, but the main difference is that the CP3 doesn't have a solenoid for fuel delivery to the injectors. The common-rail system uses either solenoid-valve or piezo-electric injectors to control fuel quantity and timing.
The P7100, or P-pump, is an inline-injection pump that uses a cam to actuate plungers to pressurise the fuel. According to some diesel enthusiasts, this is the best injection pump because of its extraordinary capabilities. Although it was replaced by the electronic VP44 pump on the 24-valve Cummins, some heavily modified trucks have taken a step back and switched the VP44 for a P-pump because of its ability to flow large amounts of fuel.
The HEUI was developed by Caterpillar and is used in the 7.3L Power Stroke V-8. This injector is significantly different from the Bosch injectors because it uses an engine-driven oil pump to feed high-pressure oil into the injector to pressurise the fuel. Because oil pressure is used to pressurise the fuel inside the injector, a high-pressure fuel pump is not needed. Fuel is fed to the injector at a relatively low pressure (50-70 psi) and a solenoid controls the high-pressure oil going into the plunging mechanism to ramp injection pressure up to 21,000 psi.
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Injector nozzle holes and injection pulses
The injector nozzle hole size and geometry are crucial design parameters that influence the combustion characteristics of the engine. The shape and dimensions of the injector seat and the injector sac are also important factors. These design elements not only impact combustion but also affect the stability of emissions and performance over the engine's lifetime.
The injection pulse refers to the electrical pulses that control the opening and closing of the injection valves. These pulses are calculated by the control unit based on sensor data regarding the engine's operating state. The pulse duration, or opening time, can be measured using an oscilloscope by connecting the measurement line to the signal line and grounding the other line. The pulse duration should increase during the acceleration phase and decrease to idle value when a constant engine speed is reached.
The injection pulse also plays a role in fuel delivery. With the use of a common rail, multiple injections per combustion cycle can occur, which is beneficial for cold-weather startups and enhances pressure reliability regardless of engine speed. The start of injection (SOI) refers to the time when fuel injection into the combustion chamber begins, and there may be a delay between the indicated SOI and the actual SOI due to injector lag.
Overall, the injector nozzle holes and injection pulses are key factors in optimizing fuel atomization, combustion efficiency, and engine performance while also minimizing emissions.
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Frequently asked questions
A diesel fuel injector is part of a fuel injection system, which delivers fuel into the engine cylinders. The injector, or nozzle, sprays directly into the combustion chamber.
The injector is controlled by the PCM (Powertrain Control Module). When the PCM commands the injector to open, fuel enters the injector and is sent through intricate passages to the injector's tip.
The psi of a diesel fuel injector varies depending on the engine. Older diesel engines injected fuel at 2,000 to 3,000 psi. Modern diesel engines can inject at 10,000 to 30,000 psi. Some injectors, like the Green Diesel Corp injector, can produce up to 160,000 psi.
The psi of a diesel fuel injector is influenced by the injection system design, the type of engine, and the specific demands placed on the engine. The injection system design includes the type of injection pump, the presence of an engine control unit (ECU), and the use of common rail technology.
Higher psi in diesel fuel injectors leads to increased fuel atomization, which improves the efficiency of the engine. Common rail injection systems, which operate at higher psi, also offer more reliable pressure regardless of engine speed and allow for multiple injections per combustion cycle.
