Understanding Diesel Fuel's Combustion: Pressure Factors Explored

Diesel engines are the workhorses of industry, used in trucking, marine, rail shipping, and power plants. Unlike gasoline engines, diesel engines inject fuel directly into the cylinder during the power stroke, where it combusts due to high cylinder temperatures. This compression ignition occurs when the air in the cylinder is compressed by the piston to a much higher pressure than in a gasoline engine, up to 25:1 in some cases. This compression super-heats the air to 400° or more, causing the injected fuel to ignite instantly.

While the initiation of combustion differs between diesel and gasoline engines, one of the main differences is the type of combustion performed. Diesel fuel evaporates into vapour at higher temperatures than gasoline, and it is these extreme pressures and temperatures that cause the vapour to self-ignite.

Injector timing vs spark timing

Injector timing and spark timing are two different concepts in engine design, each with its own role in optimising engine performance.

Injector timing refers to the angle at which the injectors start to open or close, allowing fuel to be sprayed into the engine cylinders. This timing is crucial as it determines when the fuel is introduced into the cylinder and can affect the engine's performance and efficiency. The angle can vary from 0 to 720 degrees for 4-stroke engines, and the optimal timing depends on various factors such as engine speed, load, and fuel type.

On the other hand, spark timing, or ignition timing, refers to the timing of the spark in the combustion chamber relative to the current piston position and crankshaft angle. In spark ignition engines, such as gasoline engines, the spark plug ignites the fuel-air mixture, initiating combustion. Advancing or retarding the spark timing can affect the engine's performance, fuel economy, and power output. The spark timing is typically described as an angle before or after top dead center (BTDC or ATDC), indicating the position of the piston in the cylinder.

The main difference between injector timing and spark timing lies in their roles in the combustion process. Injector timing focuses on the introduction of fuel into the cylinder, while spark timing focuses on the ignition of the fuel-air mixture. In diesel engines, injector timing is crucial as it determines when the fuel is injected into the cylinder to take advantage of the point of maximum mixture compression. In gasoline engines, spark timing is essential as it controls the initiation of combustion by the spark plug.

Additionally, injector timing and spark timing are optimised differently. Injector timing is often tuned by monitoring the Lambda/AFR values and adjusting for the richest mixture at each RPM. Spark timing, on the other hand, can be set using a dynamometer to monitor engine power output or a knock sensor to find the correct timing by advancing it until knocking occurs and then slightly retarding it.

In summary, injector timing and spark timing are both critical aspects of engine performance, but they serve different functions. Injector timing focuses on fuel introduction, while spark timing focuses on ignition initiation. Each timing is optimised differently to achieve the desired engine performance, efficiency, and power output.

Compression ratios

The static compression ratio is the simpler way to measure the ratio, and it is calculated by dividing the volume of the cylinder when the piston is at the bottom of its stroke by the volume of the cylinder when the piston is at the top of its stroke. The dynamic compression ratio is a more advanced calculation that also accounts for gases entering and exiting the cylinder during the compression phase.

In general, a high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This is because internal combustion engines are heat engines, and higher compression ratios allow the same combustion temperature to be reached with less fuel while providing a longer expansion cycle. This results in increased mechanical power output and lower exhaust temperatures.

Diesel engines typically have higher compression ratios than petrol engines. This is because diesel engines lack spark plugs, so the compression ratio must increase the temperature of the air in the cylinder sufficiently to ignite the diesel fuel through compression ignition. Compression ratios for direct injection diesel engines are often between 14:1 and 23:1, while for indirect injection diesel engines, the ratio is typically between 18:1 and 23:1.

The compression ratio can be increased by making fundamental changes to the engine, such as increasing the stroke length or decreasing the size of the combustion chamber. However, it is important to note that higher compression ratios in diesel engines can lead to higher peak cylinder pressures and increased exhaust gas temperatures (EGTs). Lowering the compression ratio, on the other hand, can help reduce emissions of oxides of nitrogen, a toxic gas produced at high temperatures.

Fuel injection pressure

The higher injection pressure has a marked effect on fuel atomization, creating a finer mist. This, in turn, enhances combustion and contributes to the efficiency of modern diesel engines. The atomization, along with port swirl, is a key factor in today's diesel engines. The increased pressure also results in a denser fuel mixture, which leads to a stronger power output.

To achieve these high injection pressures, a unique pump called a high-pressure fuel pump is required. This pump is usually engine-mounted and driven by the engine gear train. A regulator, or fuel metering valve, controls the amount of pressure generated by the pump. After the pump pressurizes the fuel, it is stored in fuel rails, which act as accumulators and dampeners of vibration.

The fuel is then delivered to the injectors through lines branching off the fuel rails. The injector itself is a crucial component, with microscopic holes at its tip that deliver the fuel as a very fine mist. The droplet size is about seven times smaller than a human hair. This fine atomization is critical to ensuring complete fuel combustion and minimizing unburned droplets expelled from the engine.

The precise control of the injection system is managed by the Powertrain Control Module (PCM), which uses input from sensors to determine how much to open the fuel regulator and control the amount of pressure and fuel injected. This electronic control, coupled with the high-pressure injection, contributes to the cleaner and more efficient performance of modern diesel engines.

Fuel atomisation

Fuel Injection and Atomisation:

In a diesel engine, the fuel injection system plays a crucial role in atomisation. The injection pump pressurises the fuel to extremely high pressures, ranging from 17,000 to over 30,000 psi. This pressurised fuel is then injected into the combustion chamber, which is filled with superheated air. The high-pressure fuel spray breaks up into fine droplets as it enters the combustion chamber, creating a mist-like formation. This process is known as fuel atomisation.

Atomisation Quality and Engine Performance:

The quality of fuel atomisation directly impacts the performance and efficiency of a diesel engine. When diesel fuel is injected at high pressures, it creates the finest atomisation possible. This fine atomisation leads to an efficient and complete burn of the fuel. The droplet size, spray pattern, and pressure differences at the injector all influence the atomisation quality. Smaller droplets and uniform spray patterns promote better mixing with the air, resulting in more efficient combustion.

Atomisation and Emissions:

Combustion Chamber Design:

The design of the combustion chamber in a diesel engine is optimised for fuel atomisation. Most diesel engines feature a "Mexican Hat" (sombrero) design, where the combustion chamber is centred inside the piston crown. This design includes a contoured depression or bowl in the centre of the piston, with a cone-shaped protrusion directly under the fuel injector. This cone-in-crown shape promotes highly optimised fuel atomisation within the high-pressure combustion space. It ensures that the fuel spray is evenly distributed and promotes thorough mixing with the compressed air, resulting in efficient combustion.

Advancements in Atomisation Technology:

Modern diesel engines incorporate advanced fuel injection systems and engine management technologies to further enhance fuel atomisation. Common rail injection systems, for example, enable higher injection pressures and more precise fuel delivery, improving atomisation and combustion. Additionally, multiple injection events per power cycle (pilot injection) are now common, allowing for more controlled and optimised fuel atomisation. These advancements contribute to increased engine efficiency, reduced emissions, and improved overall performance of diesel engines.

Fuel ignition temperature

The flash point of a fuel is the temperature at which it emits enough flammable vapours to ignite when exposed to a source of ignition, such as an open flame. This is distinct from the fire point, which is the temperature at which the vapours of the fuel will continue burning after the ignition source is taken away, usually for a minimum of five seconds. The auto-ignition temperature is the temperature at which the fuel spontaneously ignites without the need for a spark, flame, or any other ignition source.

The flash point is an important characteristic used to distinguish between flammable fuels, such as petrol, and combustible fuels, such as diesel. It is also used to assess the fire hazards of fuels. If a fuel has a flash point below 37.8°C (100.0°F), it is considered flammable, while those with a higher flash point are deemed combustible.

Diesel fuel has a flash point that varies between 52°C and 96°C (126°F to 205°F). This means that diesel fuel typically needs to be in a relatively hot environment to reach its flash point. It's worth noting that the flash point is an empirical measurement that can vary depending on equipment and test protocol variations.

The auto-ignition temperature of diesel fuel is approximately 400°F or 210°C. This is significantly higher than the flash point, highlighting the importance of understanding these different temperature thresholds for safe fuel handling and storage.

In a diesel engine, the air is compressed in the cylinder to a much higher pressure than in a gasoline engine, up to a ratio of 25:1. This compression superheats the air to 400°C or higher. At this temperature, when diesel fuel is injected into the cylinder, it ignites instantly due to the high cylinder temperature, without the need for a spark plug. This is known as compression ignition.

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