The formation of internal diesel injector deposits (IDIDs) in heavy-duty engines is a growing problem as engine technology becomes more advanced while fuel blends become more diverse, posing new challenges for mixing and compatibility. IDIDs have a variety of causes that can be challenging to pinpoint due to the number of factors involved, such as engine operation effects, fuel types, fuel additives, and fuel contamination. The aims of this study were to characterize IDIDs formed in an injector from an engine operating on a biofuel blend contaminated with coolant, gain a deeper understanding of the underlying formation mechanisms, and identify potential markers of coolant contamination in failed field injectors. In this study, a failed injector from the field was examined that was known to have fuel contamination from coolant. Laboratory experiments using the thermal deposit test (TDT) were carried out to generate deposits from a test fuel spiked with coolant. The laboratory and field deposits were characterized and compared using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), Fourier transform infrared attenuated reflectance spectroscopy (FTIR-ATR), and pyrolysis combined with gas chromatography (Py GC-MS). The results indicate that the deposits generated in the TDT were found to be primarily composed of sodium carboxylates originating from the organic acid technology additives in the coolant. The deposits were found to have structures with similarities to grease soaps, oleogels, or paraffin wax, suggesting that similar formation mechanisms may be involved. In contrast, the field injector deposits consisted of three distinct types: a cracked layer composed of sulfate salts and metal carboxylates, a globular cluster layer consisting of metal carboxylates, and particulate deposits that differ from the surroundings. The high proportion of sodium carboxylates in the globular cluster deposits was the key similarity to the laboratory deposits. In addition to the high sodium content, particulate deposits containing silicon and aluminum or aluminum and nitrogen were identified as potential markers of coolant contamination in IDIDs.

 To reduce carbon emissions in heavy-duty transportation, renewable fuels like biodiesel and hydrotreated vegetable oil are increasingly blended with fossil fuels as drop-in alternatives. However, these blends can lead to issues such as the formation of insoluble materials, or soft particles, within the fuel system. These precipitates, composed of inorganic salts and organic aggregates, cause filter clogging, nozzle fouling, and internal injector deposits, negatively impacting engine performance, increasing fuel consumption, and causing drivability issues. This study investigates internal injector deposits through an accelerated laboratory thermal test, replicating the deposits observed in injectors from heavy-duty vehicles. The goal is to understand the chemistry behind these deposits and explore the formation of inorganic salts, such as calcium crystals, and soft particle deposits. Temperature plays a critical role in deposit formation, influencing both morphology and composition. FTIR-ATR and SEM-EDX analyses reveal that metal carboxylates form between 100 ◦C and 170 ◦C, while calcium sulfate crystals form above 170 ◦C. The test successfully replicates the characteristics of real-world deposits, with findings suggesting that calcium sulfate deposits primarily form in the presence of engine oil contaminants. This points to engine oil leakage as a significant factor in the formation of internal diesel injector deposits (IDIDs). This research highlights the value of laboratory testing as a cost-effective alternative to engine tests for studying deposit formation in drop-in fuel systems. 

 In recent years, deposit formation in fuel systems for heavy-duty engines, using drop-in fuels, have become increasingly common. Drop-in fuels are particularly appealing because they are compatible with existing engines, allowing for higher proportions of alternative fuels to be blended with conventional fuels. However, the precipitation of insoluble substances from drop-in fuels can result in fuel filter clogging and the formation of internal injector deposits, leading to higher fuel consumption and issues with engine drivability. The precise reasons behind the formation of these deposits in the fuel system remain unclear, with factors such as operating conditions, fuel quality, and fuel contamination all suggested as potential contributors. In order to reproduce and study the formation of internal injector deposits, for heavy-duty engines under controlled conditions and to facilitate a more precise comparison to field trials, a novel injector test rig has been developed. This newly constructed, non-firing rig includes the main components of heavy-duty vehicle engines and uses an electric motor to simulate the revolutions per minute of an engine. A tailored run cycle has been developed to enable the continuous monitoring of injector performance during the deposit formation process, as well as to meticulously mimic the actual operations of a real engine. The deposits formed on injectors during the rig tests were analyzed using scanning electron microscopy with energy dispersive X-ray (SEM-EDX), Fourier-transform infrared spectroscopy (FTIR), and pyrolysis connected to gas chromatography-mass spectroscopy (Py GC-MS). This work presents the outcome of the analysis of injector deposits using the test rig, and compares these findings with deposits gathered from field operations. The deposits obtained from the injector test rig were found to be similar in terms of deposit location, composition, and microstructure, with both sets of deposits containing metal carboxylates and derivatives of engine oil additives. These similarities demonstrate that the test rig effectively reproduces the formation of injector deposits observed in real-world conditions.