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Technical Paper

A Feasibility Study of Using DI Butanol as an Ignition Source for Dual-Fuel Combustion

The combustion of dual-fuel engines usually uses a pilot flame to burn out a background fuel inside a cylinder under high compression. The background fuel can be either a gaseous fuel or a volatile liquid fuel, commonly with low reactivity to prevent premature combustion and engine knocking; whereas the pilot flame is normally set off with the direct injection of a liquid fuel with adequate reactivity that is suitable for deterministic auto-ignition with a high compression ratio. In this work, directly injected butanol is used to generate the pilot flame, while intake port injected ethanol or butanol is employed as the background fuel. Compared with the conventional diesel-only combustion, dual-fuel operations not only broaden the fuel applicability, but also enhance the potential for clean combustion, in high efficiency engines. The amount of background fuel and the scheduling of pilot flame are investigated through extensive laboratory experiments.
Technical Paper

A Numerical Study on the Burning Velocity of a Spherical, Premixed Methane-Air Flame

As a first step toward better understanding of the effects of flame stretch on combustion rate in SI engines, the burning velocity of a premixed, spherical, laminar methane-air flame propagating freely at standard temperature and pressure was investigated. The underlying un-stretched burning velocity was computed using CHEMKIN 3.7 with GRI mechanism, while the Lewis number and subsequently the Markstein length were deduced theoretically. The burning velocity of the freely growing flame ball was calculated from the un-stretched burning velocity with curvature and stretch effects accounted via the theoretically deduced Markstein length. For the positive Markstein length methane-air flame, flame stretching reduces the burning velocity. Therefore, the burning velocity of a spark-ignited flame starts with a value lower than, and increases asymptotically to, the underlying un-stretched burning velocity as the flame grows.
Technical Paper

A Semi-Empirical Model of Spark-Ignited Turbulent Flame Growth

A semi-empirical turbulent flame growth model has been developed based on thermodynamic equilibrium calculations and experiments in a 125-mm cubical combustion chamber. It covers the main flame growth period from spark kernel formation until flame wall contact, including the effects of laminar flame speed, root mean square turbulence intensity, turbulent eddy size, and flame size. As expected, the combustion rate increases with increasing laminar flame speed and/or turbulence intensity. The effect of turbulent eddy scale is less obvious. For a given turbulence intensity, smaller scales produce higher instantaneous flame speed. However, turbulence of a smaller scale also decays more rapidly. Thus, for a given laminar flame speed and turbulence intensity at the time of ignition, there is an optimum turbulent eddy size which leads to the fastest combustion rate over the period considered.
Technical Paper

Adaptive Fuel Injection Tests to Extend EGR Limits on Diesel Engines

Exhaust gas recirculation (EGR) is effective to reduce nitrogen oxides (NOx) from diesel engines. However, when excessive EGR is applied, the engine operation reaches zones with higher combustion instability, carbonaceous emissions, and power losses. In order to improve the engine combustion process with the use of heavy EGR, the influences of boost pressure, intake temperature, and fuel injection timing are evaluated. An adaptive fuel injection strategy is applied as the EGR level is progressively elevated towards the limiting conditions. Additionally, characterization tests are performed to improve the control of the homogeneous charge compression ignition (HCCI) type of engine cycles, especially when heavy EGR levels are applied to increase the load level of HCCI operations. This paper constitutes the preparation work for a variety of algorithms currently being investigated at the authors' laboratory as a part of the model-based NOx control research.
Journal Article

An Empirical Study to Extend Engine Load in Diesel Low Temperature Combustion

In this work, engine tests were performed to realize EGR-enabled LTC on a single-cylinder common-rail diesel engine with three different compression ratios (17.5, 15 and 13:1). The engine performance was first investigated at 17.5:1 compression ratio to provide baseline results, against which all further testing was referenced. The intake boost and injection pressure were progressively increased to ascertain the limiting load conditions for the compression ratio. To extend the engine load range, the compression ratio was then lowered and EGR sweep tests were again carried out. The strength and homogeneity of the cylinder charge were enhanced by using intake boost up to 3 bar absolute and injection pressure up to 180 MPa. The combustion phasing was locked in a narrow crank angle window (5~10° ATDC), during all the tests.
Journal Article

An Enabling Study of Diesel Low Temperature Combustion via Adaptive Control

Low temperature combustion (LTC), though effective to reduce soot and oxides of nitrogen (NOx) simultaneously from diesel engines, operates in narrowly close to unstable regions. Adaptive control strategies are developed to expand the stable operations and to improve the fuel efficiency that was commonly compromised by LTC. Engine cycle simulations were performed to better design the combustion control models. The research platform consists of an advanced common-rail diesel engine modified for the intensified single cylinder research and a set of embedded real-time (RT) controllers, field programmable gate array (FPGA) devices, and a synchronized personal computer (PC) control and measurement system.
Technical Paper

An Enabling Study of Neat n-Butanol HCCI Combustion on a High Compression-ratio Diesel Engine

This work investigates the benefits and challenges of enabling neat n-butanol HCCI combustion on a high compression ratio (18.2:1) diesel engine. Minor engine modifications are made to implement n-butanol port injection while other engine components are kept intact. The impacts of the fuel change, from diesel to n-butanol, are examined through steady-state engine tests with independent control of the intake boost and exhaust gas recirculation. As demonstrated by the test results, the HCCI combustion of a thoroughly premixed n-butanol/air lean mixture offers near-zero smoke and ultralow NOx emissions even without the use of exhaust gas recirculation and produces comparable engine efficiencies to those of conventional diesel high temperature combustion. The test results also manifest the control challenges of running a neat alcohol fuel in the HCCI combustion mode.
Technical Paper

An HCCI Engine Fuelled with Iso-octane and Ethanol

This paper investigates Homogeneous Charge Compression Ignition (HCCI) combustion on an engine that is fuelled with ethanol, iso-octane, and ethanol/iso-octane. The engine is a four-stroke three cylinder indirect injection type diesel engine converted to a single cylinder HCCI operation. In order to clarify the effects of fuel chemistry on HCCI combustion, the trials were done at a constant engine speed, a fixed initial charge temperature and engine coolant temperature. The HCCI engine was fuelled with a lean mixture of air and fuel (ethanol, iso-octane or mixture of ethanol/iso-octane). The engine performance parameters studied here include indicated mean effective pressure (IMEP) and thermal efficiency. Heat-release rate (HRR) analysis was done to determine the effect of fuels on combustion on-set. The experimental results demonstrate that the addition of iso-octane to ethanol retards the on-set of combustion and subsequently leads to a reduction of the IMEP and thermal efficiency.
Journal Article

An Improvement on Low Temperature Combustion in Neat Biodiesel Engine Cycles

Extensive empirical work indicates that the exhaust emission and fuel efficiency of modern common-rail diesel engines characterise strong resilience to biodiesel fuels when the engines are operating in conventional high temperature combustion cycles. However, as the engine cycles approach the low temperature combustion (LTC) mode, which could be implemented by the heavy use of exhaust gas recirculation (EGR) or the homogeneous charge compression ignition (HCCI) type of combustion, the engine performance start to differ between the use of conventional and biodiesel fuels. Therefore, a set of fuel injection strategies were compared empirically under independently controlled EGR, intake boost, and exhaust backpressure in order to improve the neat biodiesel engine cycles.
Technical Paper

An Investigation of Near-Spark-Plug Flow Field and Its Effect on Spark Behavior

In the recent decades, the emission and fuel efficiency regulations put forth by the emission regulation agencies have become increasingly stringent and this trend is expected to continue in future. The advanced spark ignition (SI) engines can operate under lean conditions to improve efficiency and reduce emissions. Under such lean conditions, the ignition and complete combustion of the charge mixture is a challenge because of the reduced charge reactivity. Enhancement of the in-cylinder charge motion and turbulence to increase the flame velocity, and consequently reduce the combustion duration is one possible way to improve lean combustion. The role of air motion in better air-fuel mixing and increasing the flame velocity, by enhancing turbulence has been researched extensively. However, during the ignition process, the charge motion can influence the initial spark discharge, resulting flame kernel formation, and flame propagation.
Technical Paper

An Investigation on the Regeneration of Lean NOx Trap Using Ethanol and n-Butanol

Reduction of nitrogen oxides (NOx) in lean burn and diesel fueled Compression Ignition (CI) engines is one of the major challenges faced by automotive manufacturers. Lean NOx Trap (LNT) and urea-based Selective Catalytic Reduction (SCR) exhaust after-treatment systems are well established technologies to reduce NOx emissions. However, each of these technologies has associated advantages and disadvantages for use over a wide range of engine operating conditions. In order to meet future ultra-low NOx emission norms, the use of both alternative fuels and advanced after-treatment technology may be required. The use of an alcohol fuel such as n-butanol or ethanol in a CI engine can reduce the engine-out NOx and soot emissions. In CI engines using LNTs for NOx reduction, the fuel such as diesel is utilized as a reductant for LNT regeneration.
Technical Paper

An Open Cycle Simulation of DI Diesel Engine Flow Field Effect on Spray Processes

Clean diesel engines are one of the fuel efficient and low emission engines of interest in the automotive industry. The combustion chamber flow field and its effect on fuel spray characteristics plays an important role in improving the efficiency and reducing the pollutant emission in a direct injection diesel engine, in terms of influencing processes of breakup, evaporation mixture formation, ignition, combustion and pollutant formation. Ultra-high injection pressure fuel sprays have benefits in jet atomization, penetration and air entrainment, which promote better fuel-air mixture and combustion. CFD modeling is a valuable tool to acquire detailed information about these important processes. In this research, the characteristics of ultra-high injection pressure diesel fuel sprays are simulated and validated in a quiescent constant volume chamber. A profile function is utilized in order to apply variable velocity and mass flow rate at the nozzle exit.
Technical Paper

Boosted Current Spark Strategy for Lean Burn Spark Ignition Engines

Spark ignition systems with the capability of providing spark event with either higher current level or longer discharge duration has been developed in recent years to help IC engines towards clean combustion with higher efficiency under lean/diluted intake charge. In this research, a boosted current spark strategy was proposed to investigate the effect of spark discharge current level and discharge duration on the combustion process. Firstly, the discharge characteristics of a boosted current spark system were tested with a traditional spark plug under crossflow conditions, and results showed that the spark channel was more stable, and was stretched much longer when the discharge current was boosted. Then the boosted current strategy was used in a spark ignition engine operating under lean conditions. Boosted current was added to the spark channel with different timing, duration, and current levels.
Technical Paper

Clean Combustion in a Diesel Engine Using Direct Injection of Neat n-Butanol

The study investigated the characteristics of the combustion, the emissions and the thermal efficiency of a direct injection diesel engine fuelled with neat n-butanol. Engine tests were conducted on a single cylinder four-stroke direct injection diesel engine. The engine ran at 6.5 bar IMEP and 1500 rpm engine speed. The intake pressure was boosted to 1.0 bar (gauge), and the injection pressure was controlled at 60 or 90 MPa. The injection timing and the exhaust gas recirculation (EGR) rate were adjusted to investigate the engine performance. The effect of the engine load on the engine performance was also investigated. The test results showed that the n-butanol fuel had significantly longer ignition delay than that of diesel fuel. n-Butanol generally led to a rapid heat release pattern in a short period, which resulted in an excessively high pressure rise rate. The pressure rise rate could be moderated by retarding the injection timing and lowering the injection pressure.
Technical Paper

Computational Study of Reactivity Controlled Compression Ignition (RCCI) Combustion in a Heavy-Duty Diesel Engine Using Natural Gas

Reactivity controlled compression ignition (RCCI) combustion employs two fuels with a large difference in auto-ignition properties that are injected at different times to generate a spatial gradient of fuel-air mixtures and reactivity. Researchers have shown that RCCI offers improved fuel efficiency and lower NOx and Soot exhaust emissions when compared to conventional diesel diffusion combustion. The majority of previous research work has been focused on premixed gasoline or ethanol for the low reactivity fuel and diesel for the high reactivity fuel. The increased availability of natural gas (NG) in the U.S. has renewed interest in the application of compressed natural gas (CNG) to heavy-duty (HD) diesel engines in order to realize fuel cost savings and reduce pollutant emissions, while increasing fuel economy. Thus, RCCI using CNG and diesel fuel warrants consideration.
Technical Paper

Development of a Fuel Injection Strategy for Diesel LTC

A production V-8 engine was redesigned to run on low temperature combustion (LTC) with conventional Diesel fuel. Two fuel injection strategies were used to attain reduction in soot and NOx; a) early premixed injection strategy: fuel injected early during the compression stroke and b) late premixed injection strategy: fuel injected close to TDC with heavy EGR. The early premixed injection strategy yielded low NOx and soot but struggled to vaporize the fuel as noted in unburned hydrocarbons readings. The late premixed injection strategy introduced the fuel at higher in-cylinder temperatures and densities, improving the fuel's vaporization and limited the unburned hydrocarbon and carbon monoxide. The use of high EGR and high injection pressure for late premixed injection strategy provided sufficiently long ignition delay that resulted in partially premixed cylinder charge before combustion, and thereby prevented high soot, even in presence of high EGR.
Technical Paper

Diesel EGR Fuel Reformer Improvement with Flow Reversal and Central Fueling

Empirical work has been conducted with an EGR fuel reformer configured in a flow reversal and central fueling embedment to improve the fuel dispersion quality and the reforming energy efficiency. Comprehensive comparison analyses are made between the unidirectional flow and the periodic reversal flow embodiments of similar substrate size and properties; and between the inlet and central heating schemes. With a unidirectional EGR reformer, a large amount of supplemental heating is commonly required prior to reforming. The central-fueling and flow-reversal embedment in this study is shown to significantly reduce the supplemental heating energy. The EGR cooler loading for the two strategies is also analyzed. One-dimensional modeling analyses are conducted to evaluate the fuel delivery strategies and temperature profiles of the reformer at various reforming gas flow rates and engine-out exhaust temperatures and compositions.
Technical Paper

Effects of Fuel Composition Variations (H2:CO) for Biomass Gas HCCI Combustion

Research regarding higher efficiency engines and renewable energy has lead to HCCI engine technology as a viable option with the ability to utilize a variety of fuels. With a larger focus on environmental effects the ability of HCCI engines to produce low levels of NOx and potentially other combustion products is another attractive feature of the technology. Biomass gas as a renewable primary fuel is becoming more predominant regarding internal combustion engine research. The simulated fuel in this study replicates compositions derived from real-world gasification processes; the focus in this work corresponds to fuel composition variations and their effects regarding combustion phasing and performance. There are three biomass gas fuel compositions investigated in this study. All compositions consisted of combustibles of CH₄, CO, and H₂ accompanied by CO₂ then balanced with N₂. The CH₄ and CO₂ constituents of each fuel mixture are held constant at 2% and 5% respectively.
Technical Paper

Effects of Spark Discharge Energy Scheduling on Flame Kernel Formation under Quiescent and Flow Conditions

The breakdown phase is considered to have the highest electric-thermal energy transfer efficiency among all the discharge modes in a conventional spark ignition process. In this study, an external capacitor is connected in parallel with the spark plug in order to enhance the discharge energy and power during the breakdown phase. A constant volume combustion chamber is used to investigate the high power spark discharge under different background pressures and with varied flow velocities. Results show that the added parallel capacitance is effective in redistributing the spark energy. With the increase in parallel capacitance, the breakdown power and energy increase, though at the cost of reduced glow phase energy. The breakdown energy also increases with the increased background pressure. Then combustion tests are carried out to study the effects of the breakdown power enhanced spark on flame propagation under both quiescent and flow conditions via optical diagnosis.
Technical Paper

Emission Analysis of HCCI Combustion in a Diesel Engine Fueled by Butanol

Advances in engine technology in recent years have led to significant reductions in the emission of pollutants and gains in efficiency. As a facet of investigations into clean, efficient combustion, the homogenous charge compression ignition (HCCI) mode of combustion can improve upon the thermal efficiency and nitrogen oxides emission of conventional spark ignition engines. With respect to conventional diesel engines, the low nitrogen oxides and particulate matter emissions reduce the requirements on the aftertreatment system to meet emission regulations. In this paper, n-butanol, an alcohol fuel with the potential to be derived from renewable sources, was used in a light-duty diesel research engine in the HCCI mode of combustion. Control of the combustion was implemented using the intake pressure and external exhaust gas recirculation. The moderate reactivity of butanol required the assistance of increased intake pressure for ignition at the lower engine load range.