Fuel, Lubricant and Additive Effects on Combustion Chamber Deposits

There is continuing interest in understanding how fuel, fuel additives, and lubricants contribute to combustion chamber deposit (CCD) weights and compositions in order to better anticipate the impact of CCD on exhaust emissions and engine performance. For this reason, we have characterized a range of CCDs from bench engines and vehicles using solid state ¹³C Nuclear Magnetic Resonance Spectroscopy (NMR) and X-ray Photoelectron Spectroscopy (XPS). Differences in CCD composition and structure were related to the fuel, fuel additives, and engine oil used in the test. CCDs derived from most fuels run in modern engines are predominantly organic. The fraction of aromatic carbon ranges between 24 and 74% depending on fuels and test conditions over a test length of 1,000 to 20,000 miles. These aromatic carbons exist in predominantly 1 and 2 ring structures that are independent of the amount of aromatic carbon in the CCD. These 1 and 2 ring aromatic carbon units are even present in CCD produced from non-aromatic fuels and non-aromatic lubes. The major structural difference among CCD appears in the average aliphatic carbon chain length, which decreases with increasing amount of aromatic carbon in CCD. In general, intake valve detergent packages decreased the aromatic carbon content and increased the average aliphatic chain length in CCD. In some cases, the change in CCD composition occurs even though there is little change in the total amount of CCD produced. The amount of aromatic carbon in CCD increases with increasing mileage. This implies that some oxidative aromatization reactions and reactions, which eliminate aliphatic groups in CCD take place over time on the combustion chamber walls.