Hideyuki Ogawa

APPROACHES TO EXTREMELY LOW EMISSIONS AND EFFICIENT DIESEL COMBUSTION WITH OXYGENATED FUELS

Abstract Significant improvements in smoke, particulate matter, NOx, THC, engine output and thermal efficiency were simultaneously achieved with highly oxygenated liquid fuels. Engine noise was also remarkably reduced for oxygenates with higher ignitability. The improvements in exhaust emissions and thermal efficiency depended almost entirely on the oxygen content in the fuels regardless of the oxygenate-diesel fuel blending ratio or type of oxygenate. Smoke emission decreased sharply and linearly with an increase in oxygen content and disappeared entirely at an oxygen content above 38 wt %, even at stoichiometric conditions. Smoke-free, low NOx diesel combustion with oxygenated fuels was achieved at stoichiometric conditions with the adoption of very high exhaust gas recirculation (EGR). NOx, THC and CO emissions were almost completely removed with a combination of high EGR and a three-way catalyst over a wide range of brake mean effective pressure (b.m.e.p.). The maximum b.m.e.p. with the highly oxygenated fuels was significantly higher than that with the conventional diesel fuel because b.m.e.p. was released from the smoke limits.

Characteristics of low temperature and low oxygen diesel combustion with ultra-high exhaust gas recirculation

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

Catalytic effects of metallic fuel additives on oxidation characteristics of trapped diesel soot

The oxidations of trapped diesel soots containing catalytic metals such as Ca, Ba, Fe, or Ni were characterized through thermogravimetric analysis with a thermobalance. Soot particles were generated by a single cylinder IDI diesel engine with metallic fuel additives. A two-stage oxidation process was observed with the metal-containing soots. It was found that the first stage of oxidation is catalytically promoted by metal additives resulting in an enhanced reaction rate and a reduced activation energy. Soot reduction in the rapid first stage increases with increases in metal content. Soots containing Ba and Ca are oxidized most rapidly due to the larger reduction during the first stage. The second stage of oxidation is also slightly promoted by metal addition. The ignition temperature of the collected soot is substantially reduced by the metal additives.

Characteristics of Diesel Soot Suppression with Soluble Fuel Additives

Experiments on a large number of soluble fuel additives were systematically conducted for diesel soot reduction. It was found that Ca and Ba were the most effective soot suppressors. The main determinants of soot reduction were: the metal mol-content of the fuel, the excess air factor, and the gas turbulence in the combustion chamber. The soot reduction ratio was expressed by an exponential function of the metal mol-content in the fuel, depending on the metal but independent of the metal compound. A rise in excess air factor or gas turbulence increased the value of a coefficient in the function, resulting in larger reductions in soot with the fuel additives. High-speed soot sampling from the cylinder showed that with the metal additive, the soot concentration in the combustion chamber was substantially reduced during the whole period of combustion. It is thought that the additive acts as a catalyst not only to improve soot oxidation but also to suppress soot formation. Furthermore, the additives resulted in decreased ignition temperatures and enhanced oxidation of the additive-containing soot.

Improvements in premixed charge compression ignition combustion and emissions with lower distillation temperature fuels

Abstract The charge mixture in a premixed charge compression ignition (PCCI) engine with direct in-cylinder injection early in the compression stroke is still heterogeneous even at the compression end. Direct injection of a low-volatility fuel, such as diesel fuel, early in the compression stroke results in adhesion of unevaporated fuel on the cylinder liner wall. It may be possible to improve both mixture formation and homogeneity, and decrease wall wetting by using higher-volatility fuels with distillation temperatures lower than the in-cylinder gas temperature early in the compression stroke. This research addressed the potential for improvements in early direct injection type PCCI combustion with a higher-volatility fuel, experimentally and computationally. A normal heptane + isooctane blended fuel with ignitability similar to diesel fuel in PCCI operation was used as the higher-volatility fuel. The experimental results showed that the deterioration in thermal efficiency that occurs with advanced injection timings with ordinary diesel fuel could be eliminated with the higher-volatility fuel without significantly altering the total hydrocarbons (THC) and CO emissions. With early injection timings, the rate of heat release with diesel fuel is smaller than with higher-volatility fuels. This result suggests that with diesel fuel there is significant fuel adhesion to the cylinder liner wall and also absorption into the lubricating oil.

Combustion control and operating range expansion in an homogeneous charge compression ignition engine with direct in-cylinder injection of reaction inhibitors

Abstract Light naphtha, which exhibits two-stage ignition, was induced from the intake manifold and water or a low-ignitability fuel, which does not exhibit low temperature oxidation, was directly injected early in the compression stroke for ignition suppression in an homogeneous charge compression ignition (HCCI) engine. Their quantitative balance was flexibly controlled to optimize ignition timing according to operating conditions. Ultra-low NOx and smokeless combustion without knocking or misfiring was realized over a wide operating range with water or alcohol injection. The water injection significantly reduced the low-temperature oxidation, which suppressed the increase in charge temperature and the rapid combustion caused by the high-temperature oxidation. 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. Alcohols inhibit low-temperature oxidation more strongly than other oxygenated or unoxygenated hydrocarbons, water, and hydrogen. Chemical kinetic modelling with methanol showed a reduction of OH radical before the onset of low-temperature oxidation, and this may be the main mechanism by which alcohols inhibit low-temperature oxidation.

Dependence of premixed low-temperature diesel combustion on fuel ignitability and volatility:

A comprehensive study of fuel property effects in internal combustion engines is required to enable fuel diversification as well as the development of applications to advanced engines for operation with a variety of combustion modes. The objective of this paper is to investigate the effects of fuel ignitability and volatility over a wide range of premixed low-temperature combustion (LTC) modes in diesel engines. A total of 23 fuels were prepared from commercial gasoline, kerosene, and diesel as baseline fuels and with the addition of additives, to generate a cetane number (CN) range from 11 to 75. Experiments with a single-cylinder diesel engine operated in moderately advanced-injection LTC modes were conducted to evaluate these fuels. The combustion phasing is demonstrated to be a good indicator to estimate the in-cylinder peak pressure, exhaust gas emissions, and thermal efficiency in the LTC mode. Fuel ignitability affects the combustion phasing by changing the ignition delay. The predicted cetane number (PCN) based on fuel molecular structure analysis can be fitted to the ignition delays with a higher coefficient of determination than CN, suggesting good potential as a fuel ignitability measure over a wide range. The stable operating load range in the smokeless LTC mode depends more on the actual ignition delay or PCN rather than CN. With fixed injection timing and intake oxygen concentration, O2in, only when PCN < 40, the load range can be expanded significantly to higher loads. By holding the combustion phasing at top dead centre and varying intake oxygen concentration, the nitrogen oxides and smoke emissions become limitations of the load expansion for some fuels. The effects of fuel volatility on the characteristics of LTC are small compared to ignitability. Finally, the operational injection timing range and robustness of the LTC to fuel ignitability are examined, showing that the advantageous ignitability range becomes narrower, with fuel ignitability decreasing.

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.

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.

Effect of two-stage injection on unburned hydrocarbon and carbon monoxide emissions in smokeless low-temperature diesel combustion with ultra-high exhaust gas recirculation

The unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions from smokeless low-temperature diesel combustion (LTC) with ultra-high exhaust gas recirculation (EGR) can be attributed to lowered combustion temperatures as well as to under-mixing of fuel-rich mixture along the combustion chamber walls, overly mixed fuel-lean mixture at the spray tails, and fuel missing the piston bowl and entering the squish zones. Two-stage injection has the potential to reduce UHC and CO emissions through decreasing the ratios of these mixtures. This study investigates the effects of two-stage fuel injection by varying the dwell between the two injections as well as the fuel quantity in each injection on the UHC and CO emissions, experimentally with a single-cylinder diesel engine. With the optimized dwell and injection ratio, two-stage injection can reduce the UHC and CO emissions, but these emissions are still at high levels in the ultra-high EGR smokeless LTC regime. Computational fluid dynamics simulations of the in-cylinder spray and mixture formation processes showed that with the two-stage injection, over-rich mixture in the squish zones can be significantly avoided but the over-lean mixtures at centre of the combustion chamber are little reduced, and these would likely be a significant source of UHC and CO emissions.