Parametric Study to understand Ethanol Partially Premixed Combustion
Abstract: As global concern increases regarding pollution does the demand to develop more ecient and environmental friendlier propulsion system come in high focus. The main systems for transportation have been dominated by the Otto and Diesel engine for a long time, but both are trade-o in local and global emissions, i.e. NOX, soot, HC and CO versus CO2. This has driven the evolution to concepts as homogeneous charge compres- sion ignition, HCCI and later partially premixed combustion, PPC. Although HCCI has been shown to have ultra-low engine-out emissions in soot and NOX, has it been proven to be unreliable in terms of combustion control and lack in eciency at higher loads. PPC on the other hand is proven to be a good contester in both elds. This takes us to where we are now, where the PPC analyzing and testing phase is ongoing. PPC has been run with diesel as fuel, but with only a 25% of the conventional diesel combustion range, due to self-ignition 6. This problem has been solved by running high octane fuels with high resistance to ignite. These have shown to reduce emissions in soot and NOx as a result of longer air and fuel mixing period and high EGR values for a low combus- tion temperature. Ethanol has been tested with good results in eciency, combustion control and emissions. The growing market for renewable fuel in Sweden and globally 1 indicates that this fuel is worth our eort for further investigation. This thesis attempts to characterize PPC with ethanol as fuel. It has been shown, based, and conrmed, that earlier testing by Manente 3 with double injection, EGR=50% and =1.5 are suitable for PPC operation. New results shows that an injection strategy with a pilot injection around -35 CAD ATDC, -5 CAD ATDC for second injection and an injection ratio around 52% give good eciency, low emissions and low acoustic noise at 12 bar IMEPg. In this operation does the second injection provide with control over the combustion duration. Earlier it has been stated that start of combustion (SOC) was triggered by second injection, but results show no connection. Trends show that a combination of, inlet temperature, lambda, and start of rst injection are key factors, as they all coincide in creating a fuel and air mixture which is igniting at its certain temperature. One can divide the combustion sequence in two parts. The pilot injection which provides with a sucient fuel and air mixture which kinetically ignites due to the high temperature just before TDC, causing a high rate of heat release. Followed by the second injection, which is partially kinetically ignited and partially stratied that burns in a diusion ame. Concerning combustion sensitivity to parameter variation, inlet temperature has shown to be an important factor, as variation of 5-10 oC clearly aects combustion in all aspects. With too high temperature, SOC and phasing are advanced resulting in loud acoustic noise and high combustion temperature, with too low inlet temperature SOC and phasing are retarded which also leads to high noise, as well as higher emissions with decreasing combustion eciency. Small variations of rail pressure, EGR and lambda does not aect combustion at the same proportion, but even so they are as important to be combined properly to gain a sucient ignition delay and combustion temperature.
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