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Pressure Sensitivity of HCCI Auto-Ignition Temperature for Primary Reference Fuels
Abstract:
Some fuels with the same research octane number (RON) have
different HCCI engine performance. Therefore RON alone cannot be
used for determining auto-ignition in HCCI combustion. The current
research focuses on creating an HCCI fuel index suitable for
comparing different fuels for HCCI operation. More thorough studies
are needed to map the fuel effects. One way to characterize a fuel
is by using the Auto-Ignition Temperature (AIT). The AIT and the
amount of Low Temperature Heat Release (LTHR) together describe the
auto-ignition properties of the fuel. Both can be extracted from
the pressure trace. The assumption is that the pressure and
temperature are known at inlet valve closing (IVC) and that the
mass in the cylinder does not change after IVC.
The purpose of this study was to map the AIT of different
Primary Reference Fuels (PRF) for HCCI combustion at different
cylinder pressures. Different pressure levels were achieved by
changing inlet air temperatures in 5 steps from 50°C to 150°C. A
Cooperative Fuel Research (CFR) engine with variable compression
ratio was used. The compression ratio was varied from 5.5 to 15.5
to keep combustion phasing, defined as 50% of total heat released,
constant at 3±1° after TDC. The experiments were carried out in
lean operation with a constant equivalence ratio of 0.33 and with a
constant engine speed of 600 rpm.
The results showed that the AITs of the PRFs ranged from around
580 K for PRF 0 to up to 800 K for PRF 100. Auto-ignition was
defined as the point where the rate of heat released had reached
0.2 J/CAD. At the lowest inlet air temperatures all fuels except
PRF 100 showed Low Temperature Heat Release (LTHR) from about 3%
LTHR/Total Heat Release for PRF 95 with up to 19% for PRF 0. Amount
of LTHR was found to decrease linearly with increasing octane
rating.
The low octane PRFs ignited at almost the same temperature
independent of the cylinder pressure. The high octane number PRFs
(PRF 95 - 100) displayed a wide range of auto-ignition temperatures
resulting from the different inlet air temperatures. PRF 80, PRF 85
and PRF 90 showed an intermediate behavior. A constant
auto-ignition temperature was seen at the low inlet air temperature
but at higher inlet air temperatures the IAT quickly raised when
the LTHR disappeared.