An Assessment of Thermal Barrier Coatings for The Low-Temperature Combustion Family: From HCCI To GCI

Thermal barrier coatings (TBCs) reduce in-cylinder heat transfer losses and increase thermal efficiency. Beyond the efficiency improvement, many challenges associated with low-temperature combustion (LTC) could be potentially improved with TBC. Therefore, the investigation of the effects of TBC in LTC serves as the motivation of this thesis. The thesis includes both experimental and computational investigations, which are mainly divided four-fold. First, this dissertation experimentally demonstrated the feasibility and comprehensively investigated the effects of thick thermal barrier coatings on pure Homogeneous Charge Compression Ignition (HCCI) combustion (i.e., low residual and high compression ratio) with two different fuels (conventional gasoline and wet ethanol 80). A deterioration of the high load limit was not observed, which implies that the charge heating penalty does not occur in pure-HCCI. Both combustion and thermal efficiency increased for the thicker TBC with a reduced intake temperature requirement. It is also observed that a dense top sealing layer results in a significant improvement to unburned hydrocarbon (UHC) emissions. Then, a parametric computational investigation into the effects of various coating properties on pure-HCCI combustion was performed. A zero-dimensional (0D) thermodynamic model of the engine cycle was established and coupled to a 1D transient heat transfer model of the coating and piston. Three parameters were thoroughly investigated independently: thermal conductivity (k), coating thickness, and volumetric heat capacity (s). The results revealed that the volumetric efficiency actually increases with a thicker coating due to a reduction in heat transfer during the compression stroke, which lowers the required intake temperature to reach autoignition. The results also indicate that the optimal coating configuration for pure-HCCIis a combination of the lowest k, the lowest s, and the thickest coating before reaching the charge heating limit. Since LTC contains a big family tree, ranging from HCCI to stratified LTC such as Gasoline Compression Ignition (GCI), it was desired to explore the effects of TBCs in GCI combustion, firstly by understanding GCI combustion through the Partial Fuel Stratification (PFS) combustion strategy. PFS was successfully enabled at a 1.6 bar boost level. The peak pressure rise rate (PPRR) was successfully reduced by up to 30% with the latest injection event and the lowest split fraction. However, a new double late injection strategy was also proposed that enables another 27% reduction in PPRR, which indicates that the φ distribution has been broadened dramatically, thereby unlocking further potential for higher loads. Last, this dissertation established a preliminary guideline for TBCs with GCI via thermodynamic modeling. The coating performance was evaluated with two candidates. The results show that increasing coating thickness increases the thermal efficiency of GCI combustion with a trend of diminishing returns. Charge heating was much less than expected due to the high level of intake boost. A study of the intake and exhaust valves revealed an exhaust valve peak surface temperature of ~1000K, which could be a concern for coating temperature durability. It was shown that coating the piston and firedeck was very rewarding in terms of efficiency improvement with low charge heating. However, it is not worth coating the liner clearance due to a minimal efficiency gain