Understanding the effect of operating conditions on thermal stratification and heat release in a homogeneous charge compression ignition engine

Abstract

Thermal stratification of the unburned charge prior to ignition plays a significant role in governing the heat release rates in a homogeneous charge compression ignition (HCCI) engine. A deep understanding of the conditions affecting thermal stratification is necessary for actively managing HCCI burn rates and expanding its operating range. To that end, a single-cylinder gasoline-fueled HCCI engine was used to characterize the relationship between key operating conditions, such as intake temperature, residual gas fraction, air-to-fuel ratio, and swirl, and thermal stratification. The recently developed Thermal Stratification Analysis was applied to calculate the unburned temperature distribution prior to ignition from heat release.A comparison between re-induction of exhaust gas with an air-to-fuel ratio of 24:1 and air dilution with an air-to-fuel ratio of 43:1 shows that the presence of internal residuals increases the burn duration by 34% and broadens the temperature distribution by as much as 15%. The results from an intake temperature and combustion phasing sweep at an air-to-fuel ratio of 20:1 show that heat release rates increase with advancing CA50 phasing; however, the temperature distributions broaden by 48% when comparing the most advanced to most retarded cases. To add further insight by removing the effect of combustion phasing, an equivalence ratio sweep is compared to an intake temperature sweep. It is shown that a significant part of the broadening of the distributions can be attributed exclusively to the increased intake temperature which elevates the maximum TDC temperature while leaving the wall region unaffected. However, combustion phasing plays a role as well, with earlier combustion phasing being responsible for an additional broadening of the temperature distribution.The addition of swirl elongates the burn duration by broadening the temperature distribution, with this effect being slightly larger at earlier combustion phasings. However, swirl significantly increases heat transfer losses and reduces efficiencies by as much as 1.8 percentage points. Finally, a load sweep with compensation to ensure constant combustion phasing indicates that higher loads result in increased heat release rates and narrower temperature distributions by as much as 20%.