The Impact of Irregular Combustion for Diesel Generators

Irregular combustion in diesel generators refers to fluctuations in the combustion state within each cylinder across different cycles during stable operation, as well as combustion discrepancies between cylinders. The former is called "cycle variation", while the latter is referred to as "uneven cylinder operation". These irregularities not only affect the generator's fuel efficiency and power output but also increase emissions and reduce overall performance. This analysis will delve into the causes and impacts of irregular combustion for diesel generators, focusing on cycle variation, uneven cylinder operation, and the quenching effect of combustion chamber walls.

Cycle Variation

Cycle variation describes fluctuations in combustion within each cylinder across cycles. This fluctuation is primarily due to subtle changes in fuel-air mixture concentration near the spark plug and variations in gas turbulence. Differences in flame kernel formation timing across cycles cause variations in ignition timing, resulting in pressure fluctuations that negatively impact engine performance.

1. Causes

Cycle variation is closely linked to the concentration of the fuel-air mixture near the spark plug and the turbulence of the gas. The airflow inside the cylinder differs from cycle to cycle, creating inconsistencies in flame kernel formation and resulting in unstable combustion.

2. Impact

Cycle variation leads to higher fuel consumption and reduced engine power. The air-fuel ratio and ignition timing may not reach their optimal state in each cycle, increasing the risk of abnormal combustion.

3. Fuel-Air Mixture

Research shows that cycle variation is minimized when the excess air ratio (α) is between 0.8 and 0.9. When the mixture is too rich or too lean, cycle variation increases. Therefore, using a lean mixture to reduce CO emissions can exacerbate cycle variation, especially under high loads, limiting the applicability of lean mixtures.

4. Ignition Quality

Enhancing ignition energy or using multiple ignition points can reduce the impact of cycle variation. Adjusting the timing and location of ignition can effectively improve cycle stability, enhancing combustion efficiency and engine stability.

Uneven Cylinder Operation

In multi-cylinder diesel generators, combustion inconsistencies between cylinders are known as uneven cylinder operation. This phenomenon significantly affects overall generator performance. Since diesel generators commonly use premixed combustion, the composition of the mixture outside the cylinders plays a decisive role in the combustion process.

1. Major Causes

In the intake manifold, the distribution of air, fuel vapor, atomized droplets, and wall oil films is complex, making it challenging for each cylinder to receive an even mixture. Additionally, intake manifold design, intake overlap, and other factors contribute to differences in intake volume, airflow speed, and turbulence between cylinders, resulting in an uneven fuel-air mix.

2. Impact

Variation in fuel-air mix between cylinders prevents each cylinder from operating in an optimal state. Discrepancies in mixture concentration affect overall engine power, increase fuel consumption, and worsen emission performance. Intake manifold design is especially crucial since any asymmetry, improper assembly, or differences in flow resistance can impact mixture distribution.

Quenching Effect of Combustion Chamber Walls

The walls of the combustion chamber have a quenching effect on flames, preventing flames near the walls from fully propagating and resulting in unburned hydrocarbons (HC), a significant contributor to HC emissions in diesel exhaust. Quenching is closely related to combustion chamber temperature, pressure, and turbulence intensity in the cylinder.

1. Main Causes

Wall quenching occurs because of the cooling effect of the cylinder walls, which interrupts the flame chain reaction during combustion. The cooler wall reduces the temperature of the gas layer near it, halting combustion. Tests indicate that when the excess air ratio (α) is close to 1, the quenching layer thickness is minimized, while rich or lean mixtures and reduced engine load increase it. Higher combustion chamber temperature and pressure or increased turbulence within the cylinder can reduce the quenching layer thickness.

2. Impact

The thickness of the quenching layer can be used to estimate the volume of unburned areas, which helps predict the concentration of hydrocarbons in exhaust gases. Reducing quenching layer thickness and minimizing the ratio of combustion chamber surface area to volume can effectively lower hydrocarbon emissions.

Summary and Improvement Measures

Irregular combustion significantly impacts the fuel efficiency, power, and emission performance of diesel generators. To mitigate this phenomenon, the following approaches can be applied.

1. Optimize Intake System Design

Carefully design the shape and layout of intake manifolds to reduce flow resistance and asymmetry, promoting even fuel-air mixture distribution.

2. Enhance Ignition System Performance

Increase ignition energy, use multi-point ignition, or optimize ignition timing to reduce cycle variation and improve combustion efficiency.

3. Control the Quenching Effect

Adjust combustion chamber temperature and pressure and increase turbulence intensity in the cylinder to reduce quenching layer thickness and decrease hydrocarbon emissions.

Applying these measures in combination can effectively reduce irregular combustion in diesel generators, improving power performance, fuel efficiency, and reducing emissions, leading to more efficient and environmentally-friendly operation.