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Combustion phase disorder and pollutant surge
1. Injection timing misalignment and mixture ratio fluctuation
Pressure increase stage:
Instantaneous high pressure may increase the injection advance angle (such as from the standard 15°CA BTDC to 20°CA BTDC), the fuel is injected in the early stage of the compression stroke, the temperature in the cylinder is low, forming an "over-rich mixture area", and a large amount of carbon soot (PM) is generated after combustion.
Pressure drop stage:
Instantaneous low pressure causes injection delay (such as delay to 5°CA BTDC), the fuel is injected after the ignition moment, the local oxygen-rich environment causes intense combustion, the temperature in the cylinder rises sharply (over 2300K), and the generation of nitrogen oxides (NOx) surges.
Data comparison:
When the pressure fluctuates by ±10%, PM emissions can increase by 40%~60% compared with stable conditions, and NOx increases by 30%~50% (Source: SAE paper "Common Rail Pressure Fluctuation Impact on Diesel Emissions").
2. Increasing unevenness of each cylinder's operation
In a multi-cylinder engine, pressure fluctuations may cause the difference in injection volume between cylinders to exceed 15% (normal should be <5%):
Due to excessive instantaneous high-pressure injection volume in a cylinder, "incomplete combustion → soot + CO" emissions occur;
Due to insufficient instantaneous low-pressure injection volume in the adjacent cylinder, the air-fuel ratio is too high, causing "high-temperature oxygen-rich → NOx" emissions.
Typical phenomenon:
Intermittent black smoke (at low pressure) and colorless transparent exhaust gas (at peak pressure, NOx surges but no soot) can be seen alternating in the exhaust pipe.

Deterioration of pollutant characteristics and post-treatment load
1. Particle size distribution and capture difficulty of soot (PM)
Stable pressure condition:
Soot particles are mainly fine particles (<0.1μm), which are easily captured by DPF (diesel particulate filter).
Fluctuating pressure conditions:
More nanoparticles (0.01~0.1μm) are formed in the high pressure stage, and the number concentration can increase by 2~3 orders of magnitude (such as from 10⁷/cm³ to 10⁹/cm³), and the risk of penetrating the DPF increases significantly;
Coarse particles (＞1μm) are generated in the low pressure stage, resulting in faster DPF clogging (such as the clogging cycle is shortened from 5000 km to 2000 km).
2. Synergistic pollution of NOx and unburned hydrocarbons (HC)
High pressure-high temperature period:
The amount of NOx generated can reach 1.5 times that of stable conditions (such as from 800ppm to 1200ppm), and some fuel is cracked by high temperature to generate toxic HC compounds such as formaldehyde (HCHO) (concentration increased by 50%~80%).
Low pressure - low temperature period:
Unburned HC (such as benzene) emissions increase, reacting with NOx in the atmosphere to generate photochemical smog precursors, exacerbating secondary pollution.

Chain damage to post-treatment system
1. DPF overload and shortened life
Frequent regeneration leads to carrier damage:
Pressure fluctuations cause soot emissions to increase in a pulsed manner, and the efficiency of DPF passive regeneration (relying on exhaust heat to burn soot) is insufficient, and active regeneration (fuel injection to assist combustion) needs to be started frequently. The active regeneration frequency increases from once every 10 hours to once every 3 hours, causing cracks in the DPF carrier due to high-temperature oxidation (>650℃), and the life span is reduced from 800,000 kilometers to less than 300,000 kilometers.
Accelerated ash accumulation:
Fluctuating combustion may cause oil dilution (fuel seeping into the crankcase), and oil ash (such as calcium and magnesium compounds) enters the DPF with exhaust gas, and the ash capacity is saturated in advance (normal ash capacity is about 15g/L, and abnormal 6~8g/L is blocked).
2. Collapse of SCR system efficiency
Risk of catalyst poisoning:
Pressure fluctuations may cause intermittent "fuel-rich combustion", and unburned HC reacts with SCR catalysts (such as vanadium-based) to form vanadium sulfate (V₂(SO₄)₃) to cover the active sites, causing the NOx conversion rate to drop sharply from 95% to below 40%.
Intensified ammonia slip:
In order to cope with fluctuating NOx emissions, the ECU may over-inject urea, and unreacted ammonia (NH₃) is discharged with the exhaust gas, forming "secondary pollution" (ammonia emission limit is 0.01ppm, and can reach more than 0.1ppm in abnormal cases).

Regulatory compliance and fault warning
1. Real road emissions (RDE) test failure
In the European RDE or Chinese real road emissions test, pressure fluctuations may cause:
PM-RDE value exceeds the limit (0.023g/km) by more than 2 times;
NOx-RDE value exceeds the limit (0.08g/km) by 1.5 times, triggering a vehicle emissions recall.
2. OBD diagnostic system false alarm
Pressure fluctuations may cause multiple fault codes (such as P0236 - common rail pressure fluctuation is too large, P242F-DPF is inefficient), causing the on-board diagnostic system to misjudge it as a post-treatment system failure and light up the emission warning light.
V. Typical cases and countermeasures
Case: A National VI light truck exceeded the standard for annual emission inspection
Phenomenon: free acceleration smoke value 1.2m⁻¹ (limit 0.5m⁻¹), exhaust black smoke erupts periodically.
Cause: The common rail pressure sensor harness has poor contact, the pressure fluctuates between 120 and 160 MPa (standard value 140 ± 5 MPa), and each cylinder alternately has excessive and insufficient fuel.
Solution: Replace the sensor and repair the harness, and the smoke value drops to 0.3m⁻¹.
Preventive measures
Accurately control the pressure source: Install a high-precision common rail pressure sensor (accuracy ± 0.5% FS), and use an oscilloscope to detect the pressure waveform every 20,000 kilometers (the fluctuation amplitude should be < ± 2MPa).
Maintain the consistency of the injector: Use a test bench to match the opening pressure of each cylinder injector (difference < 1MPa), and replace the worn needle valve pair in time (must be replaced when the wear amount is > 0.003mm).
Post-processing system maintenance: Clean the DPF every 50,000 kilometers and detect the activity of the SCR catalyst (regeneration or replacement is required when the conversion rate is less than 85%).