Emission Effect of Engine Faults and Service

Hannu Jääskeläinen

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Abstract: Engine emissions may increase as the engine deteriorates. Normal engine wear typically causes an increase of PM emissions and a decrease of NOx in mechanical engines. In engines with EGR or NOx aftertreatment, engine wear may also lead to increased NOx emissions. A number of studies investigated the effect of engine service on emissions.

Engine Faults


Engine emissions can be affected—sometimes significantly—as the engine deteriorates due to normal wear and/or lack of proper service. A number of studies carried out before the wide spread application of technologies such as EGR, DPFs and NOx aftertreatment have shown that diesel engines usually deteriorate to have higher PM, CO and HC emissions and lower NOx emissions. The effect of deterioration on emissions for engines with modern aftertreatment systems is considerably more complex.

Modern engines rely very heavily of the aftertreatment system to limit emissions, especially of NOx and PM. Conversion efficiencies for NOx can exceed 95% while those for PM can exceed 99% and even a small deterioration in aftertreatment system performance can have very significant impacts on emissions even if the engine itself continues to perform as intended.

In cases where the aftertreatment system performs properly, engines with DPFs and high performance NOx aftertreatment systems such as urea SCR can tolerate some deterioration of engine-out emissions before tailpipe emissions become excessive. With the very high filtration efficiency for DPFs, small changes in engine-out PM emissions rarely translate into increased tailpipe PM emissions. However, the DPF may need to be actively regenerated more frequently which could increase the fuel consumption penalty. Urea SCR systems that have the capability to adapt to changes in engine-out NOx emissions can adjust urea dosing to maintain tailpipe NOx emissions but with the downside of potential increases in urea consumption.

Engines with high performance aftertreatment systems are also typically equipped with diagnostic systems (OBD systems in on-road vehicles) whose role is to detect malfunctions in the engine and aftertreatment system that can increase emissions and inform the operator that service is required. In some cases, these fault notifications can also be communicated to fleet managers or even regulatory authorities (though regular vehicle inspections or telemetrically) to ensure faults are corrected in a timely fashion.

Fuel injection system faults are the most common reasons for increased engine-out PM emissions. For older mechanical fuel injection systems, this includes problems with the fuel injection pump such as transient air/fuel ratio control and maximum fuel stop settings. Normal engine wear can result in decreased injection pressures and delayed fuel injection timing [1104]. A number of different engine malfunctions can cause retarded or delayed injection timing, which would increase PM emissions while decreasing NOx [1327]. Increased intake air restriction, intake air leaks, improperly set injector lash, disabling of throttle delay, and injection timing retard can have the same result [1328][1329]. Problems with fuel injectors are also common.

In vehicles without on-board diagnostic systems capable of reliably detecting emission related faults, detection of faults that increase emissions needs to start with emission measurements using portable instrumentation during vehicle inspections. For engines without DPFs, faults resulting in increased PM/CO/HC emissions are often detected using smoke opacity meters. For engines with DPFs, more sensitive detection methods are required. Engine failures resulting in increased NOx are more difficult to detect and correct through mechanical repairs [4166][4356]. While portable NOx instrumentation is available, NOx emissions can vary widely depending on engine temperature, load and differing applications of legitimate Auxiliary Emission Control Devices (AECDs) by different engine manufactures that makes interpretation and a reliable determination of compliance very difficult during vehicle inspection.

The effect of engine faults and maintenance procedures on emissions usually follows the NOx/PM trade-off. Engine faults that increase PM usually reduce NOx emissions and vice versa. This also means that repairs to correct high PM emissions can increase NOx emissions and vise versa. Restoring engine settings and performance to factory settings is the only way to balance the two and to ensure that PM or NOx emissions do not become excessive.

Tampering with engine fuel and emission systems is another common cause of increased emissions from in-use engines and vehicles. Tampering has been common in both mechanical and electronically controlled engines. This topic is further discussed under Emission Tampering.

Common Faults

Common diesel engine faults and a qualitative estimate of their frequency of occurrence are listed in Table 1. The table includes information from work done in 2001 [1330] as well as additional information to reflect advances in diesel engine technology since that time. Many of the faults shown in the table are also applicable to engines fueled by natural gas or gasoline.

Dirty air filters and leaky turbocharger oil seals are relatively common. More serious turbocharger damage or problems with intercoolers are not. Intake and exhaust valve timing and leaks are also relatively rare. If a valve leak is significant, engine performance will deteriorate and/or noise and vibration will increase significantly.

Fouled or leaking injectors will result in poor fuel penetration and atomization within the combustion chamber. Even though the overall fuel-to-air ratio may be sufficient for complete combustion, local fuel-rich zones resulting from poor fuel vaporization and mixing will also increase HC, CO, and PM. Spray hole erosion may be common in older high mileage engines. Installation of incorrect sized injectors could occur during replacement or rebuild. This may simply raise the maximum fuel delivered to another certified rating level with no increase in emissions. In some cases however, the mismatch between the existing turbocharger/intake system and the larger capacity injector may lead to high PM emissions.

The use of incorrect parts during repair or rebuild can similarly result in higher smoke in some cases, but is believed to be relatively rare. Very worn engines with leaky valve guides, or worn piston rings, are likely to be found near the end of an engine’s useful life.

Table 1
Effect and frequency of faults in heavy-duty diesel engines
ComponentsEffect on EmissionsFrequency
Air filter clogging (dirty)Increased PM and CO; can increase full throttle PM considerablyExtent of blockage varies, but is relatively common
Turbocharger seals wornCan leak oil and cause increased PM and hydrocarbonsMinor oil leaks are common in older engines
Turbocharger damageSignificant damage is catastrophic, but minor damage has little effect on emissionsMinor nicks on turbo are common
Intercooler internal leaksCoolant induction can cause white smokeRare
Intercooler - restricted coolant flowHigh charge temperature will increase PM and NOxUnknown
Valve timingIncorrect valve timing can have minor emissions effectRare
Valve leaksLoss of compression and high PM; engine is hard to startRelatively rare, self correcting due to poor startability
Governor RPM settingIncreased RPM setting can increase hydrocarbons, CO and PM in some trucksCommon among independent trucks (tampering)
Maximum fuel stop settingIncreased hydrocarbons, CO and PM at full throttleRelatively rare but can occur for certain engine models (tampering)
Injection timing settingAdvance causes increased NOx, retard increased hydrocarbons, CO and PMUnknown
Air-fuel ratio controlCauses excessive PM during accelerationCommon among independent trucks (tampering)
Worn injector spray holes Increase hydrocarbons, CO and PMOccurs in older trucks
Injector pluggingAsymmetric spray can cause increase hydrocarbons, CO and PMOccurs in older trucks
Injector tip crackingExcessive PM, but is catastrophic to engineUnknown
Incorrect injector sizeEffect can vary, but hydrocarbons, CO and PM increase with increasing injector sizeCould be common in replacement of injectors
Worn piston ringsHigh PM from low compression/oil leakRelatively rare, as vehicle is hard to start
Leaking valve sealsBlue smoke from oil consumption, hydrocarbons increasedUnknown
Wrong part numbersMinor effects if mismatch is not severeUnknown, but could be a problem with aftermarket parts
EGR valve - low EGR flowIncreased NOx emissionsUnknown
EGR valve - excessive EGR flowIncreased particulate matter and CO emissionsUnknown
Diesel particulate filter damagedIncreased CO and PM emissionsUnknown
Diesel particulate filter blockageMay have little noticeable effect on emissionsSevere blockage would be self correcting as the engine may loose power
NOx aftertreatment damage or malfunctionIncreased NOx emissionsUnknown