Search Results

This research investigates the influences of the injection timing, injection pressure, and compression ratio on the combustion and exhaust emissions in a single cylinder 1.0 L DI diesel engine operating with ultra-high EGR. Longer ignition delays due to either advancing or retarding the injection timing reduced the smoke emissions, but advancing the injection timing has the advantages of maintaining the thermal efficiency and preventing misfiring. Smokeless combustion is realized with an intake oxygen content of only 9-10% regardless of the injection pressure. Reduction in the compression ratio is effective to reduce the in-cylinder temperature and increase the ignition delay as well as to expand the smokeless combustion range in terms of EGR and IMEP. However, the thermal efficiency deteriorates with excessively low compression ratios.

Ultra-low NOx and smokeless operation at higher loads up to half of the rated torque is attempted with large ratios of cold EGR. NOx decreases below 6 ppm (0.05 g/(kW·h)) and soot significantly increases when first decreasing the oxygen concentration to 16% with cold EGR, but after peaking at 12-14% oxygen, soot then deceases sharply to essentially zero at 9-10% oxygen while maintaining ultra low NOx and regardless of fuel injection quantity. However, at higher loads, with the oxygen concentration below 9-10%, the air/fuel ratio has to be over-rich to exceed half of rated torque, and thermal efficiency, CO, and THC deteriorate significantly. As EGR rate increases, exhaust gas emissions and thermal efficiency vary with the intake oxygen content rather than with the excess air ratio.

The combustion characteristics in a partially premixed charge compression ignition (PCCI) engine with n-hexane were compared with ordinary diesel fuel to evaluate combustion improvements with lower distillation-temperature fuels. In the PCCI engine, a lean mixture was formed reasonably with early stage injection and the additional fuel was supplied with a second stage fuel injection after ignition. With n-hexane, thermal efficiency improved while simultaneously maintaining low NOx and smokeless combustion. A CFD analysis simulated the mixture formation processes and showed that the uniformity of the mixture with the first stage injection improves with lower distillation-temperature fuels.

The thermal cracking and polyaromatic hydrocarbon (PAH) formation processes of dimethyl ether (DME), ethanol, and ethane were investigated with chemical kinetics to determine the soot formation mechanism of oxygenated fuels. The modeling analyzed three processes, an isothermal constant pressure condition, a temperature rising condition under a constant pressure, and an unsteady condition approximating diesel combustion. With the same mole number of oxygen atoms, the DME rich mixtures form much carbon monoxide and methane and very little non-methane HC and PAH, in comparison with ethanol or ethane mixtures. This suggests that the existence of the C-C bond promotes the formation of PAH and soot.

Light naphtha, which exhibits two-stage ignition, was induced from the intake manifold for ignition enhancement and a low ignitability fuel or water, which does not exhibit low temperature oxidation, was directly injected early in the compression stroke for ignition suppression in an HCCI engine. Their quantitative balance was flexibly controlled to optimize ignition timing according to operating condition. Ultra-low NOx and smokeless combustion without knocking or misfiring was realized over a wide operating range. Alcohols inhibit low temperature oxidation more strongly than other oxygenated or unoxygenated hydrocarbons, water, and hydrogen. Chemical kinetic modeling for methanol showed a reduction of OH radical concentration before the onset of low temperature oxidation, and this may be the main mechanism by which alcohols inhibit low temperature oxidation.

Direct injection of various ignition suppressors, including water, methanol, ethanol, 1-propanol, hydrogen, and methane, was implemented to control ignition timing and expand the operating range in an HCCI engine with induced DME as the main fuel. Ultra-low NOx and smoke-less combustion was realized over a wide operating range. The reaction suppressors reduced the rate of low-temperature oxidation and consequently delayed the onset of high-temperature oxidation. Analysis of the chemical kinetics showed a reduction of OH radical in the premixed charge with the suppressors. Among the ignition suppressors, alcohols had a greater impact on OH radical reduction resulting in stronger ignition suppression. Although water injection caused a greater lowering of the temperature, which also suppressed ignition, the strong chemical effect of radical reduction with methanol injection resulted in the larger impact on suppression of oxidation reaction rates.

The fundamental parameters related to engine combustion and performances, such as, heating value, theoretical air-fuel ratio, adiabatic flame temperature, carbon dioxide (CO2), and nitric oxide (NO) emissions, specific heat and engine thermal efficiency were investigated with computations for a wide range of oxygenated fuels. The computed results showed that almost all of the above combustion-related parameters are closely related to oxygen content in the fuels regardless of the kinds or chemical structures of oxygenated fuels. An interesting finding was that with the increase in oxygen content in the fuels NO emission decreased linearly, and the engine thermal efficiency was almost unchanged below oxygen content of 30 wt-% but gradually decreased above 30 wt-%.

This study investigated the odorous components in the exhaust of DI diesel engines. The complete products of combustion are H2O and CO2, which have no odor. Therefore, other products of incomplete combustion like unburned fuel components, partially burned components, cracked products from thermal cracking and others are thought to be responsible for exhaust odor. The THC in the exhaust is the result of incomplete combustion. This study measured THC in the exhaust, and a good correlation was found between THC and exhaust odor at different engine conditions. The low boiling point hydrocarbon components, especially CH4 in diesel exhaust were found to show a good correlation with exhaust odor. Aldehydes in exhaust gases correlate with exhaust odor very well and among the aldehydes, formaldehyde is found to be the most important component in causing irritating odor. The other part of this study is devoted to the improvement in the odor assessment used for DI diesel engines.

This study investigated the effect of ignition delay and exhaust gas speed on exhaust odor in DI diesel engines. From the investigation of many engine parameters like injection timing and injection pressure, it has been found that the optimum ignition start position is more important than the shorter ignition delay, but the optimum ignition start position along with the shorter ignition delay is the best scenario for minimum odor. Further, it has been found that good mixture formation is more important than shorter ignition delay in reducing odor, but the optimum mixture formation together with shorter ignition delay results in the lowest odorous emissions. From the investigation of various fuels in the diesel engine, it seems that the combustion pattern and the raw odor of fuel are more important than ignition delay. A fuel with low raw odor and high cetane number with optimum boiling point significantly improves the exhaust odor.

The control of fuel ignition timing and suppression of rapid combustion in a premixed charge compression ignition (PCCI) engine was attempted with direct in-cylinder injection of water as a reaction suppressor. The water injection significantly reduced the heat release at low temperature oxidation, which suppressed the increase in charge temperature after the low temperature oxidation and the rapid combustion caused by the high temperature oxidation. The possible engine operating range with ultra low NOx and smokeless combustion was extended to a higher load range with the water injection. Rapid combustion was suppressed by reductions in the maximum in-cylinder gas temperature due to water injection while the combustion efficiency suffered. Therefore, the maximum charge temperature needs to be controlled within an extremely limited range to maintain a satisfactory compromise between mild combustion and high combustion efficiency.

Improvements in the fuel economy of passenger cars and trucks contribute directly to CO2 reductions. Recently low fuel consumption cars are being developed, however most cars are still old types. This study estimates the effect of new engine types, light weight cars, kinds of fuel, and running conditions on fuel economy and CO2 emissions with a new calculation method of fuel consumption. In this calculation method the fuel consumption and CO2 emissions are easily calculated from the indicated thermal efficiency and friction mean effective pressure for different engine types and sizes. By this method the overall fuel consumption and CO2 emissions from small passenger cars to heavy duty trucks were estimated for roads through the center of a small city. As a result, the influence of new type engines, light weight cars, fuel properties and driving patterns on fuel economy and CO2 was made clear.

This study proposes a new combustion chamber concept for small DI diesel engines. Reduction of fuel adhering to the cavity wall, improvements in mixture formation, and an optimum distribution of mixture inside and outside the cavity are the main characteristics of the combustion chamber. The spray formation and it's distribution inside and outside the combustion chamber was investigated photographically in a small DI diesel engine with transparent cylinder and piston. Optimization of the fuel spray distribution inside and outside the cavity was attempted by changing the shape of the cavity entrance and the location where spray impinges on the lip. In addition improvements in the mixture formation of the impinging spray and reductions in the fuel adhering to the cavity wall were attempted by introducing a small step on the cavity side wall. The results were confirmed by analyzing the combustion and emission in an actual DI diesel engine.

Smokeless and ultra low NOx combustion without knocking in a dual-fuel diesel engine with induced LPG as the main fuel was established with a uniquely developed piston cavity divided by a lip in the sidewall. A small quantity of diesel fuel was directly injected at early compression stroke into the lower part of the cavity as an ignition source for this confined area, and this suppressed explosively rapid combustion just after ignition and spark-knock like combustion at later stage. A combination of the divided cavity, EGR, and intake air throttling was effective to simultaneously eliminate knocking, and reduce THC and NOx significantly.

Since there is a trade-off relationship between NOx and particulates in exhaust gas emitted from a diesel engine, simultaneous reduction of the amounts of NOx and particulates in a combustion chamber is difficult. However, the amount of particulates produced in the combustion process could be reduced in a state of almost complete combustion, and the amount of NOx produced during the combustion process could be reduced by the use of a catalyst and reducing agent in the exhaust process. It has been demonstrated that the use of ethanol as a reducing agent on a silver-base catalyst in the presence of oxygen is an effective means for reducing NOx, although the mechanism of the reduction has not been elucidated. Therefore, in the present study, an NOx-reduction apparatus was conducted, and model experiments on NOx reduction were carried out in an atmosphere simulating exhaust gas emitted from a diesel engine and at the same catalyst temperature as that in a combustion chamber.

Small two stroke SI engines supplied with natural gas in the intake port are advantageous for low maintenance and low cost when used in co-generation systems for residential use. However in the engines with port injection systems, the unburned HC emissions are higher and thermal efficiency is lower than with 4 stroke engines. To overcome these disadvantages, an in-cylinder injection with a special low pressure injection nozzle system was attempted. The results showed that improvements in unburned HC emissions and thermal efficiency are possible due to the remarkable reduction in scavenging loss and the lean combustion.

Modern small DI diesel engines are operated at high loads and high speeds. In these engines the spray spreading on the cavity walls during the main combustion is kept approximately constant at all engine speeds to optimize the air utilization. However, spray spreading on the wall during the early and late part of combustion changes with engine speed due to the changes in air motion. At the end of impingement much of the spray moves outside the cavity due to a strong reverse squish when the injection timing is set near TDC. This causes incomplete combustion of fuel and increase emissions of HC and soot. Therefore, the study of the behavior of spray affected by the reverse squish is very important. In this study the fuel spray development under high injection pressure and high gas charging pressure was investigated photographically in a small direct injection diesel engine with a common rail injection system.

This study investigated the influence of aldehyde and hydrocarbon components (HC components) on exhaust odor in DI diesel engines. Aldehyde is an important odorous group in exhaust, and it correlates well with exhaust odor at any engine condition. Formaldehyde (HCHO) in the exhaust has been identified as an important component causing irritating odor. Water-washing of exhaust gases does not trap HC components, while most of the odorous components are trapped with remarkable odor reductions. This indicates that the HC components in the exhaust have no direct effect on exhaust odor. However, the exhaust odor increases with increases in the concentration of the low boiling point HC components. This maybe due to the increase in intermediate odorous compounds like aldehydes, organic acids, or other oxygenated compounds in the combustion condition where the low boiling point HC components increase.

Ultra low emissions and high performance combustion was achieved with a combination of high EGR, a three-way catalyst, and a highly oxygenated liquid fuel, neat dimethoxy methane (DMM), in an ordinary DI diesel engine. The smokeless nature of neat DMM effectively allowed stoichiometric diesel combustion by controlling BMEP with EGR. NOx, THC, and CO emissions were reduced with a three-way catalyst. At lower BMEP with excess air, the EGR effectively reduced NOx. High-speed video in a bottom view type engine revealed that luminous flame decreased with increased fuel oxygen content and almost disappeared with DMM.

Direct injection diesel engines emit a far more disagreeable exhaust odor at idling than gasoline engines, and with increasing numbers of DI diesel engines in passenger cars, it is important to promote the odor reduction research. High pressure injection in DI diesel engines promotes combustion and decreases particulate matter (PM) emissions, but injection pressures at idling and warm up are limited to 30∼40 MPa considering engine noise and vibration. In this pressure range, a part of the fuel adheres on the relatively cool combustion chamber walls and causes incomplete combustion, producing higher concentration of unburned HC and intermediate combustion components (aldehydes, other oxygenated compounds, etc.) with objectionable exhaust odors. To reduce the exhaust odor, oxidation catalysts are effective, but catalyst activity is poor at idling, when the exhaust gas temperature is low (about 100°C).

This study investigated the effect of high pressure injection and an oxidation catalyst on the exhaust odor of DI diesel engines. At idling an injection pressure of 60∼80 MPa resulted in the minimum exhaust odor, with the least aldehyde and minimum total hydrocarbon (THC). This is because of decreases in fuel adhering to the combustion chamber walls due to the shortest ignition delay at this pressure range. However, above 60 MPa there is no further shortening of the ignition delay and overleaning of the local mixture dominates at injection pressures above 100 MPa, where the exhaust odor increases again. The odor reduction at the optimum injection pressure and injection timing is not significant, and further experiments with an oxidation catalyst were performed. The oxidation catalyst was found less effective to reduce exhaust odor at long idling where the maximum catalyst temperature is only about 120°C.