Sylvie Willis
Diesel engines are internal combustion engines that use the heat generated by compressing air to ignite the atomised diesel fuel injected into the cylinder. This is in contrast to spark-ignition engines, such as petrol or gasoline engines, where the airflow is reduced to regulate torque output. Diesel engines maximise airflow and rely on the compression of air to regulate torque output. The fuel is injected at the top of the compression stroke, timed similarly to the firing of a spark plug. Modern diesel engines inject fuel gradually to ensure a slower burn of the fuel, which lowers peak pressure and temperature in the cylinder.
| Characteristics | Values |
|---|---|
| How diesel fuel enters the cylinder | Fuel is injected at the top of the compression stroke, timed similarly to the firing of a spark plug. |
| When the fuel enters the cylinder | The fuel is introduced as the piston approaches the top dead center of its stroke. |
| How the fuel is introduced | The fuel is introduced under high pressure either into a precombustion chamber or directly into the piston-cylinder combustion chamber. |
| Injection pressure | Injection pressures are typically in the range of 7 to 70 megapascals (1,000 to 10,000 pounds per square inch). |
| Type of injection | Diesel engines use direct fuel injection, i.e., diesel fuel is injected directly into the cylinder. |
| Purpose of gradual injection | To ensure a slower burn of the fuel, lowering peak pressures and temperatures, reducing engine noise and vibration, and allowing a higher average combustion pressure. |
| Fuel distribution | Highly atomized, pressurized fuel is distributed evenly throughout the cylinder, resulting in increased power and fuel economy, and smoother operation. |
| Fuel ignition | Ignition occurs due to the elevated temperature of the air in the cylinder caused by mechanical compression. |
| Fuel type | The fuel used in modern high-speed diesel engines is derived from the heavier residues of crude oil left over after refining more volatile fuels. |
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What You'll Learn
Injector's role: atomised fuel delivered to cylinders
Injectors are a critical component of a diesel engine's fuel system. The primary role of injectors is to deliver a precise amount of atomised and pressurised fuel into each cylinder. Injectors are located in the combustion chamber, inlet manifold, or throttle body. The atomised fuel is distributed evenly throughout the cylinder, resulting in increased power and fuel economy, decreased engine noise, and smoother operation.
The injector is essentially a fuel pump that sends fuel under pressure into a common rail, where the injectors are attached. The injectors then spray the fuel into the combustion chamber. This spraying of the fuel can be managed electronically, allowing for more precise control over the quantity of fuel injected and the timing of injection. This electronic control leads to reduced fuel consumption, lower pollutant emissions, improved efficiency, and enhanced engine performance.
There are four primary systems for injecting fuel:
- Individual pump and injector for each cylinder.
- Combined pump and injector for each cylinder (unit injector type).
- One pump serving injectors for several cylinders (distributor type).
- Pumps in a common housing with injectors for each cylinder (common rail system).
The common rail system is becoming increasingly popular for on-road applications, while the in-line and distributor types are used in off-road vehicles and industrial machines. Modern diesel engines inject fuel gradually to ensure a slower burn of the fuel, lowering peak pressures and temperatures in the cylinder. This gradual injection also reduces engine noise and vibration, allowing for higher average combustion pressure and more power.
The performance of diesel engines is heavily influenced by their injection system design. The injection system controls the injection timing, fuel atomisation, and other parameters. Modern injection systems utilise extremely high injection pressures and sophisticated electronic control methods to optimise engine performance, emissions, and noise characteristics.
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Fuel injection timing: at the top of the compression stroke
The injection timing of fuel in a diesel engine is critical to its performance and efficiency. Modern diesel engines inject fuel gradually to ensure a slower burn of the fuel, reducing peak pressures and temperatures in the cylinder. This gradual injection also allows for a higher average combustion pressure, increasing power.
The timing of the injection pump determines when fuel is injected into the cylinder as the piston reaches the BTDC (bottom dead centre) point. The fuel injects right before the piston reaches the top of the compression stroke, timed similarly to the firing of a spark plug. This is when the air-fuel mixture reaches its maximum pressure, with the high-pressure air forming intense temperatures, causing the diesel to combust spontaneously.
The precise injection timing can be checked by measuring the injector pump's stroke at TDC (top dead centre) using a dial indicator. The timing can be adjusted to alter when the engine injects the fuel and, therefore, when combustion occurs. Adjusting the timing can impact the engine's performance, sound, and emissions. For example, early injection provides lower soot and higher NOx emissions than late injection.
The fuel injectors deliver a precise amount of atomised and pressurised fuel into each cylinder. The atomised, pressurised fuel is distributed evenly throughout the cylinder, resulting in increased power and fuel economy, decreased engine noise, and smoother operation.
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Fuel ignition: caused by high temperatures in the cylinder
Diesel engines are internal combustion engines that function by compressing air, which increases the air temperature inside the cylinder. Finely atomised diesel fuel is then injected into the cylinder, where it ignites due to the high temperature of the compressed air. This is known as a compression-ignition engine (CI engine).
The diesel engine was invented by Rudolf Diesel, and the first ignition took place on 10 August 1893. However, it was not until the 1990s that the detailed mechanisms of diesel combustion were fully understood.
In a diesel engine, the combustion process begins at the end of the injection phase, before a homogeneous mixture of air and fuel can be formed. The atomised fuel absorbs heat from the surrounding heated compressed air, vaporising and mixing with the air. As the piston moves closer to the top dead centre (TDC), the temperature of the mixture reaches the fuel's ignition temperature, causing ignition.
The high air-fuel ratio in diesel engines means they run at significantly leaner global air-fuel ratios than the stoichiometric ratio. This lean burn enables heat dissipation by excess air, contributing to the high thermal efficiency of diesel engines.
Modern diesel engines inject fuel gradually to ensure a slower burn of the fuel, lowering peak pressures and temperatures in the cylinder. This reduces engine noise and vibration and allows for higher average combustion pressure, resulting in increased power and fuel economy.
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Fuel injection methods: four primary systems
In a diesel engine, ignition occurs due to the elevated temperature of the air in the cylinder, caused by mechanical compression. This is in contrast to engines that use spark plug ignition, such as petrol or gas engines.
Diesel engines compress air, or a combination of air and residual combustion gases (known as exhaust gas recirculation, or EGR), during the compression stroke. This increases the air temperature inside the cylinder, so that when the atomised diesel fuel is injected into the combustion chamber, it ignites.
Diesel fuel injectors are a critical component of the fuel system. They deliver a precise amount of atomised and pressurised fuel into each cylinder, which results in increased power and fuel economy, decreased engine noise, and smoother operation.
There are four primary systems for injecting fuel:
- Individual pump and injector for each cylinder: This system provides a separate pump and injector for each cylinder.
- Combined pump and injector for each cylinder (unit injector type): This system combines the pump and injector into a single unit for each cylinder.
- One pump serving injectors for several cylinders (distributor type): In this system, a single pump serves multiple cylinders, supplying fuel to their respective injectors.
- Pumps in a common housing with injectors for each cylinder (common rail system): This system is gaining popularity for on-road applications. It features pumps and injectors housed together, with each cylinder having its own injector.
Modern injection pumps often use the plunger and cam method of fuel injection, and common rail systems use piezoelectricity for extremely precise fuel injection at very high pressures.
The fuel injection system can be categorised into low-pressure and high-pressure sides. The low-pressure side includes the fuel tank, fuel supply pump, and fuel filter, while the high-pressure side includes the high-pressure pump, accumulator, fuel injector, and fuel injector nozzle.
The fuel injection system is a critical component of any diesel engine, and its optimal operation is essential for peak performance.
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Fuel additives: preventing premature pump failure
In a diesel engine, ignition occurs due to the elevated temperature of the air in the cylinder, which is caused by mechanical compression. This is in contrast to engines that use spark plug ignition, such as petrol or gas engines.
Diesel fuel injectors are responsible for delivering a precise amount of atomized and pressurized fuel into each cylinder. Modern diesel fuel injectors use piezoelectricity, which allows for extremely precise handling of the high pressures found in common-rail applications.
One of the leading causes of diesel fuel pump failure is contaminated fuel. Diesel fuel can become contaminated with dirt, water, and other impurities, causing damage to the fuel pump and injectors. Water, in particular, can rapidly oxidize ferrous metal (steel) components, leading to injection component seizure and governor/metering component failure.
To prevent premature pump failure, it is essential to use high-quality, clean diesel fuel and regularly replace fuel filters. Installing a fuel water separator can also help remove water and impurities from the fuel before it reaches the engine. Additionally, fuel pumps in diesel vehicles require lubrication to operate smoothly. Running them without adequate lubrication can lead to premature wear and failure.
Fuel additives can play a crucial role in preventing premature pump failure by addressing issues related to contaminated fuel and lubrication. Here are some ways fuel additives can help:
- Lubricity additives: These additives increase the fuel's lubricity, reducing friction and wear on pump components. Examples include LX4 Lubricity Extreme, which is designed to protect CP4 fuel injection pumps prone to failure under certain conditions.
- Cetane number boosters: Raising the cetane number of diesel fuel improves ignition speed and acts as a lubricant, protecting fuel system components like pumps and injectors from premature failure. Diesel Extreme, for instance, adds seven points to the diesel's cetane score and also cleans and lubricates injectors.
- Stabilizers: Stabilizers help prevent fuel degradation, ensuring that it remains usable over time.
- Water-removing additives: As water is a common enemy of diesel fuel systems, additives that can remove excess water can be beneficial. For example, Diesel Extreme helps remove water and other contaminants from the fuel.
- Anti-gel additives: In extreme cold conditions, diesel fuel can transform into a waxy, gel-like substance, restricting fuel flow and causing damage. Anti-gel additives prevent this from happening and can also help clear built-up gel in lines and filters.
By using appropriate fuel additives, maintaining clean fuel, and ensuring proper lubrication, diesel engine owners can help prevent premature pump failure and keep their engines running smoothly.
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Frequently asked questions
Diesel fuel enters the cylinder via the fuel injector, which delivers a precise amount of atomised and pressurised fuel.
Atomised fuel is distributed evenly throughout the cylinder, resulting in increased power and fuel economy, decreased engine noise, and smoother operation.
The fuel is injected at the top of the compression stroke, timed in the same way as the firing of a spark plug. The fuel starts to burn as soon as it atomises in the combustion chamber due to the temperature of the compressed air.
