Characterization of Combustion Chamber Deposits Formed During Homogeneous Charge Compression Ignition and the Impact of a Thermal Barrier Coating on Deposit Accumulation and HCCI Operability.

Combustion Chamber Deposits;Combustion;Homogeneous Charge Compression Ignition;Thermal Barrier Coating;Heat Transfer;Porosity;Mechanical Engineering;Engineering;Mechanical Engineering

Hoffman, Mark A.Sick, Volker ;

University of Michigan

关键词: Combustion Chamber Deposits;    Combustion;    Homogeneous Charge Compression Ignition;    Thermal Barrier Coating;    Heat Transfer;    Porosity;    Mechanical Engineering;    Engineering;    Mechanical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/96098/markhoff_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Combustion chamber deposits, CCD, have been shown to influence the operational range of homogeneous charge compression ignition, HCCI. CCD insulate the combustion chamber during the intake and compression strokes, preserving enough charge temperature to shift the HCCI operational range to lower loads where the HCCI fuel economy benefit over traditional spark-ignited combustion is at a maximum. However, the drive cycle dependent CCD accumulation and burn-off creates uncontrolled shifting of the HCCI operability range, which must be mitigated in a practical multi-mode engine. Ideally, the beneficial shift of HCCI operation to lower loads provided by CCD could be obtained while avoiding uncontrolled shifting of the operational range. To provide fundamental insight into CCD properties, CCD thermal diffusivity was non-destructively measured during HCCI combustion (in-situ firing), during engine motoring (in-situ motoring) and in a specially designed radiation chamber (ex-situ). The diffusivity measurement methodology utilized the phase lag in sub-CCD temperature signals and the one dimensional heat diffusion equation. Comparisons of the CCD diffusivity values determined from the different measurement environments allowed the separation of several thermal and morphological CCD characteristics. The interaction of fuel with the CCD morphology was shown to have no significant impact on the diffusivity of CCD accumulated on the cylinder head. CCD less than thirty micrometers were spatially sparse and contained line-of-sight pathways though which the ex-situ radiation could pass and the in-situ convection could not. Thicker CCD exhibited differential sensitivity to radiation and convection heat transfer modes, which was utilized to quantify the effective porosity of the CCD through a novel radiation penetration factor.The impact of thermal barrier coatings on CCD accumulation and HCCI operability was assessed by testing a piston coated with magnesium zirconate, MgZr. The radiation penetration factor determined the MgZr coating to have 2.5 times the effective porosity of CCD. Reductions in CCD accumulation resulted from the elevated surface temperature of the MgZr. In addition, reduced cylinder head CCD accumulation was speculatively attributed to interaction between the surface roughness of the MgZr piston and the fuel spray. Overall, the HCCI operability shift due to CCD accumulation was reduced by the MgZr piston.