Tie Li

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).

Effect of split injection on stratified charge formation of direct injection spark ignition engines

Abstract The effects of split injection with various dwells and mass ratios on the spray and mixture characteristics in an ambient environment similar to the late stage of compression stroke in direct injection spark ignition (DISI) engines were investigated by using the laser absorption scattering (LAS) technique. Through splitting the fuel injection process with appropriate dwells and mass ratios, some benefits for the stratified charge formation of DISI engines can be achieved. First, the phenomenon of high-density liquid phase fuel piling up at the leading edge of the spray can be circumvented and the subsequent reduction in the spray penetration length for both liquid and vapour phases is seen. Second, the radial width of the ‘combustible mixture’ (equivalence ratio of vapour φv in a range of 0.7 ≤ φv ≤ 1.3) is significantly extended. Finally, the quantity of ‘over lean’ (φv < 0.7) mixture in the spray is significantly reduced. These results are believed to contribute to the stratified lean operation and the reduction in smoke and unburned hydrocarbons (UBHC) emissions of DISI engines. Further, the mechanism behind these effects of the split injection was clarified by analysing the interactions between the two pulsed sprays and the spray-induced ambient air motion using the LIF-PIV technique.

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 PCNu2009<u200940, 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.

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.

Volatile Organic Compounds in Exhaust Gas from Diesel Engines under Various Operating Conditions

The volatile organic compounds (VOC) from diesel engines, including formaldehyde and benzene, are of concern and remain as unregulated harmful substances. These substances are positively correlated with total hydrocarbon (THC) emissions, but the VOC and aldehyde compounds at light load or idling conditions are more significant than THC. When coolant temperatures are low at light loads, there are notable increases in formaldehyde and acetaldehyde, and with lower coolant temperatures the increase in aldehydes is more significant than the increase in THC. With ultra-high exhaust-gas recirculation (EGR) suppressing in-cylinder soot and NO x formation, VOC increase drastically with intake oxygen content below 14 per cent. These trends correlate well with the drastic increase in THC emissions. Oxidation catalysts are effective for reducing some VOC emissions, including aldehydes and some unsaturated hydrocarbons. However, aromatics and methane generated from ultra-high EGR, low-temperature smokeless combustion, are hardly reduced with the catalysts, particularly under overall rich conditions.

Characterization of mixture formation processes in DI gasoline engine sprays with split Injection strategy via laser absorption and scattering (LAS) technique

In order to investigate the effect of split injections on mixture formation processes in Direct Injection (DI) gasoline engine sprays, an experimental study was conducted applying the laser absorption and scattering (LAS) technique to the sprays using double pulse injections with various dwells and mass ratios. The effects of various dwells and mass ratios between the pulsed injections on the spatial concentration distributions in the spray, the penetration of vapor and liquid phases, and the mean equivalence ratios of the vapor phase and overall spray, were clarified. It was found that the phenomenon of high concentration liquid spray piling up at the leading edge of the spray is avoided by the double injections with enough dwell or appropriate mass ratio. The maximum penetration length of the spray significantly decreases, especially for the liquid phase with high concentration. Moreover, the mean equivalence ratios including vapor phase and overall spray are significantly affected by the double pulse injections with various dwells and mass ratios.

Characterization of Mixture Formation Processes in D.I. Gasoline Sprays by the Laser Absorption Scattering (LAS) Technique - Effect of Injection Conditions

Mixture formation processes play a vital role on the performance of a D.I. Gasoline engine. Quantitative measurement of liquid and vapor phase concentration distribution in a D.I. gasoline spray is very important in understanding the mixture formation processes. In this paper, an unique laser absorption scattering (LAS) technique was employed to investigate the mixture formation processes of a fuel spray injected by a D.I. gasoline injector into a high pressure and temperature constant volume vessel. P-xylene, which is quite suitable for the application of the LAS technique, was selected as the test fuel. The temporal variations of the concentration distribution of both the liquid and vapor phases in the spray were quantitatively clarified. Then the effects of injection pressure and quantity on the concentration distributions of both the liquid and vapor phases in the spray were analyzed.

Influence of fuel properties on operational range and combustion characteristics of premixed diesel combustion with high volatility fuel

The effects of fuel properties including ignitability, volatility, and compositions on operational range and combustion characteristics of premixed diesel combustion with various high volatility model fuels and an ordinary diesel fuel were examined in a direct injection diesel engine. The indicated mean effective pressure was limited by knocking with high-intake oxygen concentrations and by unstable combustion or significant increases in CO and total hydrocarbon emissions with low-intake oxygen concentrations regardless of fuels. The fuel volatility has little effect on the combustion characteristics and the stable operational range in premixed diesel combustion. With increasing octane number, the combustion phasing is retarded, and higher intake oxygen concentrations can be employed within the tolerance limits of rapid combustion, expanding the stable premixed diesel combustion indicated mean effective pressure range. The operational range of premixed diesel combustion with normal heptane and toluene blend fuels shifts to higher intake oxygen concentrations when compared with primary reference fuels with the same research octane numbers, showing lower ignition characteristics than primary reference fuel. The silent, low-NOx, and smokeless operation with high thermal efficiency was possible with both primary reference fuel and normal heptane and toluene blend fuel when the intake oxygen concentration is optimized corresponding to indicated mean effective pressure.

Impacts of multiple pilot diesel injections on the premixed combustion of ethanol fuel

Engine experiments were carried out to study the impact of multiple pilot injections of a diesel fuel on dual-fuel combustion with a premixed ethanol fuel, using compression ignition. Because of the contrasting volatility and the reactivity characteristics of the two fuels, the appropropriate scheduling of pilot diesel injections in a high-pressure direct-injection process is found to be effective for improving the clean and efficient combustion of ethanol which is premixed with air using a low-pressure port injection. The timing and duration of each of the multiple pilot injections were investigated, in conjunction with the use of exhaust gas recirculation and intake air boosting to accommodate the variations in the engine load. For correct fuel and air management, an early pilot injection of fuel acted effectively as the reactivity improver to the background ethanol, whereas a late pilot injection acted deterministically to initiate combustion. The experimental results further revealed a set of pilot injection strategies which resulted in an increased ethanol ratio, thereby reducing the emission reductions while retaining a moderate pressure rise rate during combustion.