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Smoothing HCCI Heat Release with Vaporization-Cooling-Induced Thermal Stratification using Ethanol
Abstract:
Ethanol and ethanol/gasoline blends are being widely considered
as alternative fuels for light-duty automotive applications. At the
same time, HCCI combustion has the potential to provide high
efficiency and ultra-low exhaust emissions. However, the
application of HCCI is typically limited to low and moderate loads
because of unacceptably high heat-release rates (HRR) at higher
fueling rates.
This work investigates the potential of lowering the HCCI HRR at
high loads by using partial fuel stratification to increase the
in-cylinder thermal stratification. This strategy is based on
ethanol's high heat of vaporization combined with its true
single-stage ignition characteristics. Using partial fuel
stratification, the strong fuel-vaporization cooling produces
thermal stratification due to variations in the amount of fuel
vaporization in different parts of the combustion chamber. The low
sensitivity of the autoignition reactions to variations of the
local fuel concentration allows the temperature variations to
govern the combustion event. This results in a sequential
autoignition event from leaner and hotter zones to richer and
colder zones, lowering the overall combustion rate compared to
operation with a uniform fuel/air mixture.
The amount of partial fuel stratification was varied by
adjusting the fraction of fuel injected late to produce
stratification, and also by changing the timing of the late
injection. The experiments show that a combination of 60 - 70%
premixed charge and injection of 30 - 40% of the fuel at 80°CA
before TDC is effective for smoothing the HRR. With CA50 held
fixed, this increases the burn duration by 55% and reduces the
maximum pressure-rise rate by 40%. Combustion stability remains
high but engine-out NOx has to be monitored carefully.
For operation with strong reduction of the peak HRR,
ISNOx rises to around 0.20 g/kWh for an IMEPg
of 440 kPa. The single-cylinder HCCI research engine was operated
naturally aspirated without EGR at 1200 rpm, and had low residual
level using a CR = 14 piston.