This content is not included in
your SAE MOBILUS subscription, or you are not logged in.
Boosted HCCI Combustion Using Low-Octane Gasoline with Fully Premixed and Partially Stratified Charges
Abstract:
High-load HCCI combustion has recently been demonstrated with
conventional gasoline using intake pressure boosting. The key is to
control the high combustion heat release rates (HRR) by using
combustion timing retard and mixture stratification. However, at
naturally aspirated and moderately boosted conditions, these
techniques did not work well due to the low autoignition reactivity
of conventional gasoline at these conditions. This work studies a
low-octane distillate fuel with similar volatility to gasoline,
termed Hydrobate, for its potential in HCCI engine combustion at
naturally aspirated and low-range boosted conditions. The HCCI
combustion with fully premixed and partially stratified charges was
examined at intake pressures (Pin) from 100 to 180 kPa
and constant intake temperature (60°C) and engine speed (1200
rpm).
First, the key parameters for high-load HCCI operation were
investigated, including 1) intermediate temperature heat release
(ITHR), which determines the potential of using combustion retard
to control the HRR, and 2) φ-sensitivity, which determines the
effectiveness of mixture stratification for controlling the HRR. It
is found that consistent with the low octane number, Hydrobate
shows strong ITHR and φ-sensitivity even at Pin = 100
kPa. However, the ITHR of Hydrobate is not enhanced by increasing
intake pressure, which is different from gasoline at Pin
= 100 - 180 kPa, but similar to gasoline at Pin ≥ 180
kPa. For high-load operation, it is found that with a fully
premixed charge, Hydrobate allowed higher loads than conventional
gasoline for Pin ≺ 160 kPa, mainly due to the lower
intake temperature allowing a higher fueling rate. However, for
Pin ≥ 160 kPa, the maximum load reached by Hydrobate was
lower than gasoline due to the higher level of EGR required, which
limited the oxygen available for combustion. On the other hand,
higher thermal efficiency was consistently observed with Hydrobate
at all Pin tested, mainly due to the lower intake
temperature and more advanced combustion phasing. Applying partial
fuel stratification to Hydrobate combustion further improved
thermal efficiency compared to the fully premixed case, since it
allowed even more advanced combustion phasing; however, it only
slightly extended the high-load limit, again due to the limited
oxygen availability. This work indicates that low-octane gasoline
is a superior HCCI fuel for naturally aspirated and modestly
boosted operations.