Optimization of the Charge Motion in Internal Combustion Engines Driven by Sewage Gas for CHP-Units

In this study, the influence of the charge motion on the internal combustion in a spark ignition sewage gas-driven engine (150 kW) for combined heat and power units was investigated. For this purpose, the geometry of the combustion chamber in the immediate vicinity to the inlet valve seats was modified. The geometrical modification measures were conducted iteratively by integrative determination of the swirl motion on a flow bench, by laser-optical methods and consecutively by combustion analysis on a test engine. Two different versions of cylinder heads were characterized by dimensionless flow and swirl numbers prior to testing their on-engine performance. Combustion analysis was conducted with a cylinder pressure indication system for partial and full load, meeting the mandatory NOx limit of 500 mg m−3. Subsuming the flow bench results, the new valve seat design has a significant enhancing impact on the swirl motion but it also leads to disadvantages concerning the volumetric efficiency. A comparative consideration of the combustion rate delivers that the increased swirl motion results in a faster combustion, hence in a higher efficiency. In summary, the geometrical modifications close to the valve seat result in increased turbulence intensity. It was proven that this intensification raises the ratio of efficiency by 1.6%.

Model Based Design and Optimization for Large Bore Engines: Some Industrial Case Studies

Large bore reciprocating internal combustion engines are used in a wide variety of applications such as power generation, transportation, gas compression, mechanical drives, and mining. Each application has its own unique requirements that influence the engine design & control strategy. The system architecture & control strategy play a key role in meeting the requirements. Traditionally, control design has come in at a later stage of the development process, when the system design is almost frozen. Furthermore, transient performance requirements have not always been considered adequately at early design stages for large engines, thus limiting achievable controller performance. With rapid advances in engine modeling capability, it has now become possible to accurately simulate engine behavior in steady-states and transients. In this paper, we propose an integrated model-based approach to system design & control of reciprocating engines and outline ideas, processes and real-world case studies for the same. Key benefits of this approach include optimized engine performance in terms of efficiency, transient response, emissions, system and cost optimization, tools to evaluate various concepts before engine build thus leading to significant reduction in development time & cost.

Modeling the Formation of Pollutant Emissions in Large-Bore, Lean-Burn Gas Engines

This paper discusses chemical kinetic modeling used to analyze the formation of pollutant emissions in large-bore, lean-burn gas reciprocating engines. Pollutants considered are NOx, CO, HCHO, and UHC. A quasi-dimensional model, built as a chemical reactor network (CRN), is described. In this model, the flame front is treated as a perfectly stirred reactor (PSR) followed by a plug flow reactor (PFR), and reaction in the burnt gas is modeled assuming a batch reactor of constant-pressure and fixed-mass for each crank angle increment. The model treats full chemical kinetics. Engine heat loss is treated by incorporating the Woschni model into the CRN. The mass burn rate is selected so that the modeled cylinder pressure matches the experiment pressure trace. Originally, the model was developed for large, low speed, two-stoke, lean-burn engines. However, recently, the model has been formatted for the four-stroke, open-chamber, lean-burn engine. The focus of this paper is the application of the model to a four-stroke engine. This is a single-cylinder non-production variant of a heavy duty lean-burn engine of about 5 liters cylinder displacement Engine speed is 1500 RPM. Key findings of this work are the following. 1) Modeled NOx and CO are found to agree closely with emission measurements for this engine over a range of relative air-fuel ratios tested. 2) This modeling shows the importance of including N2O chemistry in the NOx calculation. For λ = 1.7, the model indicates that about 30% of the NOx emitted is formed by the N2O mechanism, with the balance from the Zeldovich mechanism. 3) The modeling shows that the CO and HCHO emissions arise from partial oxidation late in the expansion stroke as unburned charge remaining mixes into the burnt gas. 4) Model generated plots of HCHO versus CH4 emission for the four-stroke engine are in agreement with field data for large-bore, lean-burn, gas reciprocating engines. Also, recent engine tests show the correlation of UHC and CO emissions to crevice volume. These tests suggest that HCHO emissions also are affected by crevice flows through partial oxidation of UHC late in the expansion stroke.

Combustion and Emissions of Jet-A and N-Butanol in RCCI Operation

Jet-A was investigated in RCCI (Reactivity Controlled Compression Ignition) given that the fuel is readily available and has a similar cetane number compared to ultra-low sulfur diesel (ULSD#2). To promote emissions’ control, RCCI was conducted with direct injection (DI) of Jet-A and PFI (port fuel injection) of n-butanol. Combustion and emission characteristics of Jet-A RCCI were investigated for a medium duty DI experimental engine operated at constant boost and 30% EGR rate and compared to ULSD#2 RCCI and single-fuel ULSD#2 operation. DI fuel was injected at 5 CAD ATDC and constant rail pressure of 1500 bar. A 20% pilot by mass was added and investigated at timings from 15 to 5 CAD BTDC for combustion stability. The results showed that the effect of the pilot injection on Jet-A combustion was not as prominent as compared to that of ULSD#2, suggesting a slightly different spray and mixture formation. Ignition delay for Jet-A was 15–20% shorter compared to ULSD#2 in RCCI. When the pilot was set to 5 CAD BTDC, CA50 phased for ULSD#2 RCCI by 3 CAD later when compared to Jet-A RCCI. After TDC, the local pressure maximum for ULSD#2 RCCI decreased by 3 bar, resulting from a 15% difference in peak heat release rate between ULSD#2 and Jet-A in RCCI at the same pilot timing. NOx and soot levels were reduced by a respective maximum of 35% and 80% simultaneously in Jet-A RCCI mode compared to single-fuel ULSD#2, yet, were higher compared to ULSD#2 RCCI. Ringing intensity was maintained at similar levels and energy specific fuel consumption (ESFC) improved by at least 15% for Jet-A compared to ULSD#2 in RCCI. Mechanical efficiencies additionally improved at earlier pilot timing by 2%. In summary, Jet-A RCCI allowed for emissions control and increased fuel efficiencies compared to single fuel ULSD#2, however, injection should be further tweaked in order to reach lower soot levels.

Effects of Fuel Quality on Gaseous Emissions From an 8.9L Spark-Ignited Natural Gas Engine Operated Over Prime-Mover Cycles for Unconventional Well Development

Technology developments in directional drilling and hydraulic fracturing have led to increased natural gas reserves. Development of these unconventional resources is an energy intensive process. Prime-movers of unconventional well development were previously identified to be over-the-road trucks, drilling engines, and hydraulic stimulation engines. Diesel engines dominate these markets but industry is attempting to cut costs by using dual fuel and dedicated natural gas engines. On-road engines are subject to the transient FTP cycle for certification and off-road engines are subject to the 5-mode ISO 8178 D-2 cycle. It is well known that in-use activity can differ from certification activity. Significant in-use activity data for each prime-mover were collected and a Markov-Chain Monte-Carlo Simulation with a genetic algorithm was used to develop test cycles for each. The developed test cycles allowed for operation of a smaller yet similar engine within a controlled laboratory environment. Laboratory tests utilized a Cummins 8.9L ISL-G to analyze the emissions of new cycles compared to certification cycles and to examine the effects of fuel quality on emissions. The ISL-G is a spark-ignited engine used for heavy-duty trucks and could see market penetration in fleets serving the well development industry. It is similar in technology to the Waukesha LI7044, which is used in drilling operations — both employ air fuel ratio control and three-way catalysts. For the case of “pump” quality fuel, compressed natural gas was used. The developed OTR truck cycle produced higher brake-specific emissions of CO2, CO, NOx, and lower HC emissions compared to the FTP. The drilling and fracturing cycles tended to have lower CO2 and HC emissions but higher CO emissions when compared to the D-2 cycle. Two additional fuel blends were used on the new cycles and represented blends with higher ethane and propane fractions — which are common to shale gases that could fuel prime-movers in the future. The minimum recommended methane number for this engine was 75 and additional fuel blends had methane numbers of 75.5 (propane blend) and 75.3 (ethane blend). As expected, CO2 emissions increased with increased alkane concentration, while opposite trends were shown for THC and CH4. NOx emissions also tended to decrease with higher ethane and propane blends, across all cycles. For all cycles and fuels, HC emissions were predominately CH4 - 94–97%. Variations in activity and the effects of different fuels should be addressed when estimating emissions since using standard certification or emissions factors may not be representative of in-use emissions.

Influence of Charge Motion and Compression Ratio on the Performance of a Combustion Concept Employing In-Cylinder Gasoline and Natural Gas Blending

The present paper represents a small piece of an extensive experimental effort investigating the dual-fuel operation of a light-duty spark ignited engine. Natural gas (NG) was directly injected into the cylinder and gasoline was injected into the intake-port. Direct injection of NG was used in order to overcome the power density loss usually experienced with NG port-fuel injection as it allows an injection after intake valve closing. Having two separate fuel systems allows for a continuum of in-cylinder blend levels from pure gasoline to pure NG operation. The huge benefit of gasoline is its availability and energy density, whereas NG allows efficient operation at high load due to improved combustion phasing enabled by its higher knock resistance. Furthermore, using NG allowed a reduction of carbon dioxide emissions across the entire engine map due to the higher hydrogen-to-carbon ratio. Exhaust gas recirculation (EGR) was used to (a) increase efficiency at low and part-load operation and (b) reduce the propensity of knock at higher compression ratios (CR) thereby enabling blend levels with greater amount of gasoline across a wider operating range. Two integral engine parameters, CR and in-cylinder turbulence levels, were varied in order to study their influence on efficiency, emissions and performance over a specific speed and load range. Increasing the CR from 10.5 to 14.5 allowed an absolute increase in indicated thermal efficiency of more than 3% for 75% NG (25% gasoline) operation at 8 bar net indicated mean effective pressure and 2500 RPM. However, as anticipated, the achievable peak load at CR 14.5 with 100% gasoline was greatly reduced due to its lower knock resistance. The in-cylinder turbulence level was varied by means of tumble plates as well as an insert for the NG injector that guides the injection “spray” to augment the tumble motion. The usage of tumble plates showed a significant increase in EGR dilution tolerance for pure gasoline operation, however, no such impact was found for blended operation of gasoline and NG.

Influence of Injection Parameters and Operating Conditions on Ignition and Combustion in Dual-Fuel Engines

Dual-Fuel (DF) engines offer great fuel flexibility since they can either run on gaseous or liquid fuels. In the case of Diesel pilot ignited DF-engines the main source of energy is provided by gaseous fuel, whereas the Diesel fuel acts only as an ignition source. Therefore, a proper autoignition of the pilot fuel is of utmost importance for combustion in DF-engines. However, autoignition of the pilot fuel suffers from lower compression temperatures of Miller or Atkinson valve timings. These valve timings are applied to increase efficiency and lower nitrogen oxide engine emissions. In order to improve the ignition, it is necessary to understand which parameters influence the ignition in DF-engines. For this purpose, experiments were conducted and the influence of parameters such as injection pressure, pilot fuel quantity, compression temperature and air-fuel equivalence ratio of the homogenous natural gas-air mixture were investigated. The experiments were performed on a periodically chargeable combustion cell using optical high-speed recordings and thermodynamic measurement techniques for pressure and temperature. The study reveals that the quality of the Diesel pilot ignition in terms of short ignition delay and a high number of ignited sprays significantly depends on the injection parameters and operating conditions. In most cases, the pilot fuel suffers from too high dilution due to its small quantity and long ignition delays. This results in a small number of ignited sprays and consequently leads to longer combustion durations. Furthermore, the experiments confirm that the natural gas of the background mixture influences the autoignition of the Diesel pilot oil.

Comparative Analysis of EGR and Air Dilution in Spark-Ignited Natural Gas Engines

Industrial natural gas engines are used in a wide range of applications, each with unique requirements in terms of power density, initial cost, thermal efficiency, and other factors. As a result of these requirements, distinct engine designs have evolved to serve various applications. Heavy-duty spark-ignited engines can generally be divided into two broad categories based on their charge characteristics and method of emissions control. Stoichiometric engines are widely used in applications where first cost, absolute emissions and relative engine simplicity are more important than fuel consumption. In most of the developed world, stoichiometric engines are equipped with a three-way catalyst to control emissions of nitrogen oxides (NOx) as well as products of incomplete combustion and raw unburned fuel. Dilution of the charge mixture with excess air reduces the peak combustion gas temperature and associated heat rejection. As a result, lean burn engines are generally able to achieve higher efficiency and power density without inducing excessive component temperatures or end gas knock. NOx formation is mitigated by the reduced gas temperatures, such that most regulatory standards can currently be met in-cylinder. Significant obstacles exist to meeting more stringent future emissions regulations in this manner, however. Another possible strategy is to dilute the charge mixture with recirculated exhaust gas. This offers similar benefits as air dilution while maintaining the ability to use a three-way catalyst for emissions after-treatment. While similar principles apply in either case, the choice of diluent can have a significant impact on knock resistance, emissions formation, thermal efficiency, and other parameters of importance to engine developers and operators. This work aimed to examine the unique characteristics of EGR and air dilution from a thermodynamic and combustion perspective. A combination of cycle simulation tools and experimental data from a single-cylinder test engine was applied to demonstrate the impact of diluent properties on a fundamental level, and to illustrate departures from idealized behavior and practical considerations specific to the development of combustion systems for spark-ignited natural gas engines.

Homogenous Charge Compression Ignition (HCCI) Operation With Jet Fuel and Water Injection in a Single Cylinder Diesel CFR Engine

Conventional Navy jet fuel ‘5’ (JP-5) was operated in a Waukesha single cylinder diesel Cooperative Fuels Research (CFR) engine with intake port jet fuel injection under a range of compression ratios (CR). At the lowest CR of 12, a small range of engine loads in the lower torque or gross indicated mean effective pressure (gIMEP) range could be attained. As the engine CR was increased only a very limited range of loads were attained since heavy engine ‘knocking’ occurred. Navy jet fuel is a reactive fuel (cetane number 46), and thus could not tolerate higher CRs without premature combustion. Intake port water injection was then applied in order to cool the intake air charge and delay the jet fuel HCCI start of combustion. As a result, a range of HCCI operation (gIMEP from 1 to 5 bar) could be attained across a range of CRs. In general, with this approach, combustion phasing with port fuel and water injection advanced with increasing CR resulting in lower efficiencies at the higher CRs. Exhaust carbon monoxide (CO) was minimum in mid-HCCI operating range suggesting a trade-off of poor light load lean combustion, and the steadily diluted higher load operation with increasing water content. Companion analysis suggests that the thermal energy for water evaporation was principally provided by the engine walls. Further, the dilution effect of the water resulted in lower overall charge temperatures which further lowered the overall reactivity of jet fuel in the engine allowing reasonable HCCI operation.

Influence of the Spatial and Temporal Interaction Between Diesel Pilot and Directly Injected Natural Gas Jet on Ignition and Combustion Characteristics

In this paper, pilot-ignited high pressure dual-fuel (HPDF) combustion of a natural gas jet is investigated on a fundamental basis by applying two separate single-hole injectors to a rapid compression expansion machine (RCEM). A Shadowgraphy system is used for optical observations, and the combustion progress is assessed in terms of heat release rates. The experiments focus on the combined influence of injection timing and geometrical jet arrangement on the jet interaction and the impact on the combustion process. In a first step, the operational range for successful pilot self-ignition and transition to natural gas jet combustion is determined, and the restricting phenomena are identified by analyzing the shadowgraph images. Within this range, the combustion process is assessed by evaluation of ignition delays and heat release rates. Strong interaction is found to delay or even prohibit pilot ignition, while it facilitates a fast and stable onset of the gas jet combustion. Furthermore, it is shown that the heat release rate is governed by the time of ignition with respect to the start of natural gas injection — as this parameter defines the level of premixing. Evaluation of the time of gas jet ignition within the operability map can therefore directly link a certain spatial and temporal interaction to the resulting heat release characteristics. It is finally shown that controlling the heat release rate through injection timing variation is limited for a certain angle between the two jets.