Usman Asad

Tightened intake oxygen control for improving diesel low-temperature combustion

The low-temperature combustion (LTC) operation in diesel engines is challenged by the higher cycle-to-cycle variation with heavy exhaust gas recirculation (EGR) and high unburnt hydrocarbon and carbon monoxide emissions. Small variations in the intake charge dilution can further decrease the combustion efficiency, which in turn escalates the successive fluctuations of the cylinder charge, adversely affecting the stability and efficiency of the LTC operation. In this work, improvements in the promptness and accuracy of combustion control as well as tightened control on the intake oxygen concentration have been combined to enhance the robustness and efficiency of the LTC operation in diesel engines. The empirical set-up consisted of a set of field programmable gate array (FPGA) modules that were coded and interlaced to execute on-the-fly combustion event modulations on either a cycle-by-cycle or within-the-same-cycle basis. The cylinder pressure traces were analysed to provide the necessary feedback for the combustion control algorithms. A methodology for estimating the indicated mean effective pressure for the current engine cycle helped to stabilize the LTC cycles. Engine dynamometer tests demonstrated that such systematic and prompt control algorithms were effective to optimize the LTC cycles for improved fuel efficiency and exhaust emissions. Moreover, a strategy for enabling load transients within narrow LTC operating corridors was implemented and shown to improve the load management of the LTC cycles. The reported techniques were in part to establish a model-based control strategy for robust diesel LTC operations.

Implementation Challenges and Solutions for Homogeneous Charge Compression Ignition Combustion in Diesel Engines

Homogeneous charge compression ignition (HCCI) combustion in diesel engines can provide cleaner operation with ultralow NOx and soot emissions. While HCCI combustion has generated significant attention in the last decade, however, till date, it has seen very limited application in production diesel engines. HCCI combustion is typically characterized by earlier than top-dead-center (pre-TDC) phasing, very high-pressure rise rates, short combustion durations, and minimal control over the timing of the combustion event. To offset the high reactivity of the diesel fuel, large amounts of exhaust gas recirculation (EGR) (30–60%) are usually applied to postpone the initiation of combustion, shift the combustion toward TDC, and alleviate to some extent, the high-pressure rise rates and the reduced energy efficiency. In this work, a detailed analysis of HCCI combustion has been carried out on a high-compression ratio (CR), single-cylinder diesel engine. The effects of intake boost, EGR quantity/temperature, engine speed, injection scheduling, and injection pressure on the operability limits have been empirically determined and correlated with the combustion stability, emissions, and performance metrics. The empirical investigation is extended to assess the suitability of common alternate fuels (n-butanol, gasoline, and ethanol) for HCCI combustion. On the basis of the analysis, the significant challenges affecting the real-world application of HCCI are identified, their effects on the engine performance quantified, and possible solutions to overcome these challenges explored through both theoretical and empirical investigations. This paper intends to provide a comprehensive summary of the implementation issues affecting HCCI combustion in diesel engines.

A Thermal Response Analysis on the Transient Performance of Active Diesel Aftertreatment

Diesel fueling and exhaust flow strategies are investigated to control the substrate temperatures of diesel aftertreatment systems. The fueling control includes the common-rail post injection and the external supplemental fuel injection. The post injection pulses are further specified at the early, mid, or late stages of the engine expansion stroke. In comparison, the external fueling rates are moderated under various engine loads to evaluate the thermal impact. Additionally, the active-flow control schemes are implemented to improve the overall energy efficiency of the system. In parallel with the empirical work, the dynamic temperature characteristics of the exhaust system are simulated one-dimensionally with in-house and external codes. The dynamic thermal control, measurement, and modeling of this research intend to improve the performance of diesel particulate filters and diesel NOx absorbers.

EGR Oxidation and Catalytic Fuel Reforming for Diesel Engines

Exhaust gas recirculation (EGR) treatment techniques that include combustible substance oxidation, catalytic fuel reforming, and partial bypass-flow control have been experimentally investigated on a single cylinder diesel engine. Application tests are conducted to investigate the effects of the reformed gases on the diesel combustion characteristics and exhaust emissions. This research is aimed at stabilizing and expanding the limits of heavy EGR during steady and transient operations by enhancing the premixed combustion that may significantly alleviate problems with soot formation and cyclic variations. Additionally, the heavy treated EGR is applied to enable in-cylinder low temperature combustion. A preliminary investigation on the effects of water addition to the high temperature catalyst bed is also conducted. The potential of EGR reforming is also examined for possible generation of synthetic EGR (CO2) at low engine loads. The effectiveness of the treated EGR on engine emission and operating characteristics are therefore reported.Copyright

Study of Cylinder Charge Control for Enabling Low Temperature Combustion in Diesel Engines

Suitable cylinder charge preparation is deemed critical for the attainment of a highly homogeneous, diluted, and lean cylinder charge which is shown to lower the flame temperature. The resultant low temperature combustion (LTC) can simultaneously reduce the NOx and soot emissions from diesel engines. This requires sophisticated coordination of multiple control systems for controlling the intake boost, exhaust gas recirculation (EGR), and fueling events. Additionally, the cylinder charge modulation becomes more complicated in the novel combustion concepts that apply port injection of low reactivity alcohol fuels to replace the diesel fuel partially or entirely. In this work, experiments have been conducted on a single cylinder research engine with diesel and ethanol fuels. The test platform is capable of independently controlling the intake boost, EGR rates, and fuelling events. Effects of these control variables are evaluated with diesel direct injection and a combination of diesel direct injection and ethanol port injection. Data analyses are performed to establish the control requirements for stable operation at different engine load levels with the use of one or two fuels. The sensitivity of the combustion modes is thereby analyzed with regard to the boost, EGR, fuel types and fueling strategies. Zero-dimensional cycle simulations have been conducted in parallel with the experiments to evaluate the operating requirements and operation zones of the LTC combustion modes. Correlations are generated between air-fuel ratio (λ), EGR rate, boost level, in-cylinder oxygen concentration and load level using the experimental data and simulation results. Development of a real-time boost-EGR set-point determination to sustain the LTC mode at the varying engine load levels and fueling strategies is proposed.Copyright

Diesel pressure departure ratio algorithm for combustion feedback and control

The pursuit for higher efficiency and ultra-low exhaust emissions from diesel engines requires the combustion process to be precisely controlled so as to minimize departures from the intended engine operation. The combustion control system must be able to perform corrective actions on a cycle-by-cycle basis, with a robust feedback on the combustion process. The combustion phasing, commonly represented by the crank angle of 50% heat release and derived from the measured cylinder pressure data, shows a strong correlation to the efficiency and the engine-out nitrogen oxide emissions. To accurately estimate the combustion phasing from the derived heat-release rate, the authors previously introduced and experimentally validated a computationally efficient diesel pressure departure ratio algorithm, against selected cases of boost, engine load and exhaust gas recirculation. In this work, the formulation of the pressure departure ratio algorithm is presented in detail along with its implementation to enable combustion control during both transient and steady-state engine operations. Engine tests demonstrate that the algorithm was effective in stabilizing the combustion process on a cycle-by-cycle basis for a range of engine speeds, load and exhaust gas recirculation, which included conventional and low-temperature diesel combustion modes.

An engine cycle analysis of diesel-ignited ethanol low-temperature combustion:

Modification of the fuel–air charge properties has the potential to improve the load range of low-temperature combustion with ultra-low nitrogen oxide emissions (less than 0.2 g/kW h) and ultra-low smoke emissions (less than 0.01 g/kW h). The ignition characteristics of the cylinder charge are altered by injecting the highly reactive diesel fuel into a homogeneous lean air–fuel mixture of low-reactivity fuel. The ethanol–diesel combination has been of particular recent interest since ethanol is a renewable biofuel. The additional advantages of ethanol include excellent anti-knock properties, high volatility and reduction in the compression work through charge cooling. In this work, a detailed investigation using diesel-ignited ethanol experiments was conducted on a high-compression-ratio (18.2:1) diesel engine. The emissions, the combustion performance and the thermal efficiency characteristics are analysed at different values of the exhaust gas recirculation, the intake boost pressure, the ethanol fraction and the diesel injection timing. The empirical investigations supported by detailed zero-dimensional engine cycle simulations indicate that a diesel injection timing close to top dead centre provides direct control over the ignition timing across the engine load range. The nitrogen oxide–soot trade-off of conventional diesel combustion, which is affected by exhaust gas recirculation, is minimized to achieve clean combustion over a wide load range (indicated mean effective pressure, 4–17 bar) with increased ethanol fraction and moderate intake dilution through a combination of modulation of the exhaust gas recirculation level and an increase in the intake boost pressure. The operation at low loads is constrained by the minimum diesel amount necessary for stable and efficient combustion while progressively retarded combustion phasing is necessary at higher loads to satisfy the physical engine constraints (peak cylinder pressure, less than 170 bar; peak pressure rise rate, less than 15 bar/deg crank angle). The improved understanding of this combustion strategy through experimental and theoretical research provides the necessary guidance for obtaining clean efficient full-load operation (demonstrated at an indicated mean effective pressure of 19.2 bar).

Speciation Analysis of Light Hydrocarbons and Hydrogen Production During Diesel Low Temperature Combustion

The simultaneous reduction in engine-out NOx and soot emissions with diesel low temperature combustion (LTC) is generally accompanied by high levels of hydrocarbon (THC) and carbon monoxide (CO) emissions in the exhaust. To achieve clean diesel combustion in terms of low regulated emissions (NOx, soot, THC, and CO), the exhaust combustibles must be dealt with the exhaust aftertreatment (typically a diesel oxidation catalyst). In this work, engine tests were performed to realize LTC on a single-cylinder common-rail diesel engine up to 12 bar IMEP. A single-shot fuel injection strategy was employed to push the diesel cycles into LTC with exhaust gas recirculation (EGR). The combustibles in the exhaust were generally found to increase with the LTC load and were observed to be a function of the overall equivalence ratio. A Fourier transform infrared (FTIR) spectroscopy analysis of light hydrocarbon emissions found methane to constitute a significant component of the hydrocarbon emissions under the tested LTC conditions. The relative fraction of individual species in the hydrocarbons also changed, indicating a richer combustion zone and a reduction in engine-out THC reactivity. The hydrogen production was found to correlate consistently with the CO emissions, largely independent of the boost pressure or engine load under the tested LTC conditions. This research intends to identify the major constituents of the THC emissions and highlight the possible impact on exhaust aftertreatment.© 2011 ASME

An Analysis of the Production of Hydrogen and Hydrocarbon Species by Diesel Post Injection Combustion

The effects of post injection on the combustion efficiency, exhaust emissions, and in-cylinder hydrogen generation were experimentally investigated in a modern heavy duty diesel engine. As the post injection was moved away from top dead center (TDC), the test results generally showed increasing carbon monoxide (CO) and total hydrocarbons (THC), fairly constant nitrogen oxide (NOx) emissions while the smoke emissions were more sensitive to the post injection timing. Hydrogen production was observed to be higher at later post injection timings. In a majority of instances, hydrogen production and carbon monoxide formation were very well correlated. Additional tests explored the effects of the overall air-to-fuel ratio on the in-cylinder hydrogen production and the experimental results indicated that a lower air-to-fuel ratio seemed to promote the in-cylinder generation of hydrogen. However, the increased hydrogen production was offset by less efficient power production from the post injection combustion. A Fourier transform infrared (FTIR) spectroscopy analysis of hydrocarbon emissions was carried out in an attempt to determine the effects of diesel post injection timing on individual light hydrocarbon species.Copyright

Effects of Intake Air Humidity on the NOX Emissions and Performance of a Light-Duty Diesel Engine

The effects of intake air humidity on the performance of a turbo-charged 4-cylinder diesel engine have been investigated. The relative humidity of the intake charge was varied from 31 to 80% at a fixed ambient air temperature of 26°C. The intake humidity was controlled to within ±1% of the desired value by using a steam generator-equipped intake-air conditioning system. The tests were conducted at 3 load points (4.1, 9.1 and 15 bar BMEP) at engine speeds of 1500, 2500 and 3500 RPM without exhaust gas recirculation. The results indicate that increasing the intake air moisture leads to a reduction of 3∼14% in the NOX emissions for the tested conditions. The smoke was found to increase with speed but no significant increase in the smoke values was observed with the increased humidity. The CO and HC emissions were found to be largely insensitive to the humidity levels and were otherwise extremely low. The emissions have been analyzed on both the volumetric (ppm) and brake-specific basis to provide an insight into the effect of humidity on the quantitative results.Copyright