Efficiency Improvement of Boosted Low-Temperature Gasoline Combustion Engines (LTGC) using a Double Direct-Injection Strategy

Paper #:
  • 2017-01-0728

  • 2017-03-28
For lean/dilute and well-mixed gasoline compression-ignition engines, creating a partially stratified fuel mixture before auto-ignition can be beneficial to reduce the heat-release rate and the corresponding maximum rate of pressure rise. As a result, partial fuel stratification (PFS) can be used to increase load and/or efficiency without excessive ringing (i.e. without knock). In this work, a double direct-injection (D-DI) strategy is investigated for which the majority of the fuel is injected early in the intake stroke to create a relatively well-mixed background mixture, and the remaining fuel is injected in the latter part of the compression stroke to produce greater fuel stratification prior auto-ignition. Experiments were performed in a 1-liter single-cylinder engine modified for low-temperature gasoline combustion (LTGC) research. The main objective of this study is to quantify the efficiency gains possible by applying this D-DI fueling technique compared to a near-perfectly homogeneous mixture and to a slightly stratified mixture (all the fuel injected early in the intake stroke). For the D-DI fueling technique, the timing of the late injection as well as the fuel-fraction split between the early and late injections were independently varied. This study demonstrates that the D-DI fueling strategy is very effective for reducing the heat release rate, which allows CA50 to be significantly advanced without knock. Moreover, compared to fully premixed fueling, an efficiency gain of as much as 3%-units has been measured while maintaining very low NOx and soot emissions. Nonetheless, with the D-DI fueling, efficiency improvement is less than expected from the allowable CA50 advancement. Additional detailed analyses show that this is due to a combination of reduced combustion efficiency and increased heat transfer as stratification is increased beyond a certain level. Overall, this study provides key understanding about the practical use of multiple direct injections to exploit PFS for intake-boosted operation.
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