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Technical Paper

Characteristics of Unburned Hydrocarbon Emissions in a Low Compression Ratio DI Diesel Engine

In a DI diesel engine, THC emissions increase significantly with lower compression ratios, a low coolant temperature, or during the transient state. During the transient after a load increase, THC emissions are increased significantly to very high concentrations from just after the start of the load increase until around the 10th cycle, then rapidly decreased until the 20th cycle, before gradually decreasing to a steady state value after 1000 cycles. In the fully-warmed steady state operation with a compression ratio of 16 and diesel fuel, THC is reasonably low, but THC increases with lower coolant temperatures or during the transient period just after increasing the load. This THC increase is due to the formation of over-lean mixture with the longer ignition delay and also due to the fuel adhering to the combustion chamber walls. A low distillation temperature fuel such as normal heptane can eliminate the THC increase.
Technical Paper

Dependence of Ultra-High EGR and Low Temperature Diesel Combustion on Fuel Injection Conditions and Compression Ratio

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.
Technical Paper

Characteristics of Diesel Combustion in Low Oxygen Mixtures with Ultra-High EGR

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.
Technical Paper

Combustion in a Two-stage Injection PCCI Engine With Lower Distillation-temperature Fuels

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.
Technical Paper

Chemical-Kinetic Analysis on PAH Formation Mechanisms of Oxygenated 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.
Technical Paper

Combustion Control and Operating Range Expansion in an HCCI Engine with Selective Use of Fuels with Different Low-Temperature Oxidation Characteristics

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.
Technical Paper

Combustion Control and Operating Range Expansion With Direct Injection of Reaction Suppressors in a Premixed DME HCCI Engine

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.
Technical Paper

Nature of Fundamental Parameters Related to Engine Combustion for a Wide Range of Oxygenated Fuels

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-%.
Technical Paper

Expansion of the Operating Range with In-Cylinder Water Injection in a Premixed Charge Compression Ignition Engine

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.
Technical Paper

Analysis of Contribution to SPM by Organic Matters Using High-Performance Liquid Chromatography (HPLC)

Most countries consider it is harmful for humans to inhale SPM of fine organic particles and elemental carbon less than 2.5 μ in diameter1,2). It is generally believed that organic matters in SPM are mainly composed of diesel exhaust particulate and soot from residential chimneys or industrial smokestacks3,4). To determine the contribution ratios of several organic substances to SPM, we characterized SPM, diesel exhaust particulate (DEP), powdered summer radial tire, and bitumen, using high performance liquid chromatography, field desorption mass spectrometry and linear theory.
Technical Paper

Low Emission and Knock-Free Combustion with Rich and Lean Biform Mixture in a Dual-Fuel CI Engine with Induced LPG as the Main Fuel

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.
Technical Paper

Time-Resolved Behavior of Unburned Hydrocarbon Components in Diesel Exhaust Under Transient Operations

Time resolved changes in unburned hydrocarbon emissions and their components were investigated in a DI diesel engine with a specially developed gas sampling system and gas chromatography. The tested transient operations include starting and increasing loads. At start-up with high equivalence ratios the total hydrocarbon (THC) at first increased, and after a maximum gradually decreased to reach a steady state value. Reducing the equivalence ratio of the high fueling at start-up and shortening the high fueling duration are effective to reduce THC emissions as long as sufficient startability is maintained. Lower hydrocarbons, mainly C1-C8, were the dominant components of the THC and mainly determined the THC behavior in the transient operations while the proportion of hydrocarbon (HC) components did not significantly change. The unregulated toxic substances, 1,3 butadiene and benzene were detected in small quantities.
Technical Paper

Ultra Low Emissions and High Performance Diesel Combustion with a Combination of High EGR, Three-Way Catalyst, and a Highly Oxygenated Fuel, Dimethoxy Methane (DMM)

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.
Technical Paper

Improvements of Diesel Combustion and Emissions with Two-stage Fuel Injection at Different Piston Positions

The fuel spray distribution in a DI diesel engine with pilot injection was actively controlled by pilot and main fuel injections at different piston positions to prevent the main fuel injection from hitting the pilot flame. A CFD analysis demonstrated that the movement of the piston with a cavity divided by a central lip along the center of the sidewall effectively separates the cores of the pilot and main fuel sprays. Experiments showed that an ordinary cavity without the central lip emitted more smoke, while smokeless, low NOx operation was realized with a cavity divided by a central lip even at heavy loads where ordinary operation without pilot injection emits smoke.
Technical Paper

Ultra Low Emission and High Performance Diesel Combustion with Highly Oxygenated Fuel

Significant improvements in exhaust emissions and engine performance in an ordinary DI diesel engine were realized with highly oxygenated fuels. The smoke emissions decreased sharply and linearly with increases in oxygen content and entirely disappeared at an oxygen content of 38 wt-% even at stoichiometric conditions. The NOx, THC, and CO were almost all removed with a three-way catalyst under stoichiometric diesel combustion at both the higher and lower BMEP with the combination of EGR and a three-way catalyst. The engine output for the highly oxygenated fuels was significantly higher than that with the conventional diesel fuel due to the higher air utilization.
Technical Paper

Cycle-to-cycle Transient Characteristics of Diesel Emissions during Starting

Changes in exhaust gas emissions during starting in a DI diesel engine were investigated. The THC after starting increased until around the 50th cycle when the fuel deposited on the combustion chamber showed the maximum, and THC then decreased to reach a steady value after about 1000 cycles when the piston wall temperature became constant. The NOx showed an initial higher peak just after starting, and increased to a steady value after about 1000 cycles. Exhaust odor had a strong correlation with THC, and at the early stage odor was stronger than would be expected from the THC concentration. The THC increased with increased fuel injection amounts, decreased cranking speeds, and fuels with higher viscosity, higher 90% distillation temperature, and lower ignitability.
Technical Paper

Smokeless, Low NOx, High Thermal Efficiency, and Low Noise Diesel Combustion with Oxygenated Agents as Main Fuel

Diesel combustion and emissions with four kinds of oxygenated agents as main fuels were investigated. Significant improvements in smoke, particulate matter, NOx, THC, and thermal efficiency were simultaneously realized with the oxygenates, and engine noise was also remarkably reduced for the oxygenates with higher ignitability. The improvements in the exhaust emissions and the thermal efficiency depended almost entirely on the oxygen content in the fuels regardless of the oxygenate to diesel fuel blend ratios and type of oxygenate. The unburned THC emission and odor intensity under starting condition with an oxygenate were also much lower than with conventional diesel fuel.
Technical Paper

Cycle-to-Cycle Transient Characteristics of Exhaust Gas Emissions from a Diesel Engine with Different Increasing and Decreasing Load Patterns

Cycle-to-cycle changes in diesel exhaust gas emissions were investigated under two transient operation patterns: One, “an interval step decreasing and increasing load”, where the fuel amount is rapidly decreased from high to low loads, and after an interval, Δtint the fuel amount is abruptly returned to the initial level. The other is “a ramp increasing load”, where the fuel amount is increased gradually. Except just after the step increase in fuel amounts, the THC emissions were almost completely determined by the piston wall temperature and fuel amount. However, the THC concentrations immediately after the step increase in fuel amounts were much higher than the value of the corresponding steady state operation with the same piston wall temperature. This overshoot concentration, ΔTHC, was almost constant at different intervals, Δtint and it can be suppressed by ramp increased loading.
Technical Paper

Improvement of Diesel Combustion and Emissions with Addition of Various Oxygenated Agents to Diesel Fuels

The effect of eight kinds of oxygenated agents added to diesel fuels on the combustion and emissions was investigated in a DI diesel engine. The results showed significant smoke and particulate suppression without increases in NOx with every oxygenated agent. The emissions decreased linearly with increasing oxygen content in the fuels, almost regardless of the kind of oxygenated agent. The improvement in smoke and particulate emissions with the oxygenated agent addition was more significant for lower volatility fuels. Combustion analysis with the two-dimensional two color method showed that soot concentration in the flame during the combustion process decreased with the addition of the oxygenated agent while the flame temperature distribution was almost unchanged.
Technical Paper

Simultaneous Reductions in Diesel NOx and Smoke Emissions with Aqueous Metal-Salt Solutions Directly Injected into the Combustion Chamber

The effect of several aqueous metal-salt solutions on NOx and smoke lowering in an IDI diesel engine were examined. The solutions were directly injected into a divided chamber independent of the fuel injection. The results showed that significant lowering in NOx and smoke over a wide operation range could be achieved simultaneously with alkali metal solutions which were injected just prior to the fuel injection. With sodium-salt solutions, for instance, NOx decreased by more than 60 % and smoke decreased 50 % below conventional operation. The sodium-salt solution reduced dry soot significantly, while total particulate matter increased with increases in the water soluble fractions.