Use of Early Exhaust Valve Opening to Improve Combustion Efficiency and Catalyst Effectiveness in a Multi-Cylinder RCCI Engine System: A Simulation Study

Low Temperature Combustion (LTC) strategies such as Reactivity Controlled Compression Ignition (RCCI) can result in significant improvements of fuel economy and emissions reduction. However, they can produce significant carbon monoxide and unburnt hydrocarbon emissions at low load operating conditions due to poor combustion efficiencies at these operating points, which is a consequence of the low combustion temperatures that cause the oxidation rates of these species to slow down. The exhaust gas temperature is also not high enough at low loads for effective performance of turbocharger systems and diesel oxidation catalysts (DOC). The DOC is extremely sensitive to exhaust gas temperature changes and lights off only when a certain temperature is reached, depending on the catalyst specifications. Uncooled EGR can increase combustion temperatures, thereby improving combustion efficiency, but high EGR concentrations of 50% or more are required, thereby increasing pumping work and reducing volumetric efficiency. However, with early exhaust valve opening, the exhaust gas temperature can be much higher, allowing lower EGR flow rates, and enabling activation of the DOC for more effective oxidization of unburnt hydrocarbons and CO in the exhaust. In this paper, a multi-cylinder engine system simulation of RCCI at low load operation with early exhaust valve opening is presented, along with consideration of the exhaust aftertreatment system. The combustion process is modeled using the 3D CFD code, KIVA, and the heat release rates obtained from this combustion are used in a GT-Power model of a turbocharged, multi-cylinder light-duty RCCI engine for a full system simulation. The post-turbine exhaust gas is fed into GT-Power’s aftertreatment model of the engine’s DOC to determine the catalyst response. It is confirmed that opening the exhaust valve earlier increases the exhaust gas temperature, and hence lower EGR flow rates are needed to improve combustion efficiency. It was also found that exhaust temperatures of around 457 K are required to light off the catalyst and oxidize the unburnt hydrocarbons and CO effectively. Performance of the DOC was drastically improved and higher amounts of unburnt hydrocarbons were oxidized by increasing the exhaust gas temperature.

A First Application of Thermographic Phosphors in a Marine Two-Stroke Diesel Engine for Surface Temperature Measurement

Phosphor thermometry is applied for the first time in a large-bore two-stroke diesel engine. The work proves the practicality of phosphor thermometry in large-bore engines. The experiments were conducted on the MAN 4T50ME-X marine research engine equipped with an optical cylinder head. By employing a thin surface coating of CdWO4 phosphor, cycle resolved temperature measurements of the cylinder wall were obtained. Motored and fired engine operations were tested at engine loads covering the low and medium engine load range. Phosphor thermometry proved to be successful in retrieving the temperature with standard deviations ranging around 1–8 K. Experimental considerations like detector linearity, coating thickness and an automated phosphor calibration routine will be addressed.

Numerical and Experimental Analysis of Ignition and Combustion Stability in EGR Dilute GDI Operation

This paper discusses the characteristics of EGR dilute GDI engines in terms of combustion stability. A combined approach consisting of RANS numerical simulations integrated with experimental engine testing is used to analyze the effect of the ignition source on flame propagation under dilute operating conditions. A programmable spark-based ignition system is compared to a production spark system in terms of cyclic variability and ultimately indicated efficiency. 3D-CFD simulations are carried out for multiple cycles with the goal of establishing correlations between the characteristics of the ignition system and flame propagation as well as cycle-to-cycle variations. Numerical results are compared to engine data in terms of in-cylinder pressure traces. The results show that an improved control over the energy released to the fluid surrounding the spark domain during the ignition process has beneficial effects on combustion stability. This allows extending the dilution tolerance for fuel/air mixtures. Although affected by cyclic variability, numerical results show good qualitative agreement with experimental data. The result is a simple but promising approach for relatively quick assessment of stability improvements from advanced and alternative ignition strategies.

Investigation of High Percentage Acetone-Butanol-Ethanol (ABE) Blended With Diesel in a Constant Volume Chamber

Bio-butanol, a promising alternative transportation fuel, has its industrial-scale production hindered significantly by high cost component purification process from acetone-butanol-ethanol (ABE) broth. The purpose of this study is to investigate the possibility of using ABE-Diesel blends with high ABE percentages as an alternative transportation fuel. An optical-accessible constant volume chamber capable of controlling ambient temperature, pressure and oxygen concentration was used to mimic the environmental conditions inside a real diesel engine cylinder. ABE fuel with typical volumetric ratios of 30% acetone, 60% butanol and 10% ethanol were blended with ultra-low sulfur diesel at 80% vol. and were tested in this study. The ambient temperature was set to be at 1100K and 900K, which represents normal combustion conditions and low temperature combustion conditions respectively. The ambient oxygen concentrations were set to be at 21%, 16% and 11%, representing different EGR ratios. The in-cylinder pressure was recorded by using a pressure transducer and the time-resolved Mie-scattering image and natural flame luminosity was captured using a high-speed camera coupled with a copper vapor laser. The results show that the liquid penetration is reduced by the high percentage of ABE in the blends. At the same time, the soot formation is reduced significantly by increasing oxygen content in the ABE fuel. Even more interesting, a soot-free combustion was achieved by combining the low temperature combustion with the higher percentage ABE case. In terms of soot emission, high ABE ratio blends are a very promising alternative fuel to be directly used in diesel engines especially under low-temperature combustion conditions.

Impact of Very High Injection Pressure on Soot Emissions of Medium Speed Large Diesel Engines

Measures exist to adjust tailpipe NOx emissions to assigned values, for example cooled exhaust gas recirculation (EGR) or a SCR catalyst in conjunction with urea. The situation is quite different with soot when use of a trap is not feasible for reasons of cost, space requirements and maintenance. Due to the highly complex soot formation and oxidation process, soot emissions can’t be targeted as easily as NOX. So how can soot be kept within the limits? In principle, soot can be controlled by allocating sufficient oxygen and establishing good mixing conditions with vaporized fuel. The most effective measures target the injection system, e.g. increasing injection pressure, applying multiple injections, optimizing nozzle geometry. To investigate the impact of very high injection pressure on soot, an advanced injection system with rail pressure capability up to 3000 bar and a Bosch injector was installed at the Large Engines Competence Center (LEC) in Graz. Full load and part load operating points at constant speed and in accordance with the propeller law were investigated at the test bed to quantify the impact of high injection pressure on soot emissions. Test runs were conducted with both SCR and EGR while varying injection timing and air-fuel ratios. Use of a statistical method, Design of Experiments (DOE), helped reduce the number of tests. Optical investigations of the spray and combustion were conducted. The goal was to obtain soot concentration history traces with the two color method in order to better understand how soot originates and to be able to calibrate 3D CFD FIRE spray models for use with injection pressures of up to 3000 bar. Very low soot emissions can be achieved using high pressure injection, even when EGR is applied. DOE results provide a clear picture of the relationships between the parameters and can be used to optimize set values for the whole speed and load range. A reliable spray break up model can be used in further 3D CFD simulation to investigate how to reduce soot emissions.

Controlled ASSCI With Moderate Auto-Ignition for Engine Knock Suppression in a GDI Engine With High Compression Ratio

This paper presents an experimental study on controlled ASSCI (Assisted Spark Stratified Compression Ignition) for engine knock suppression in a GDI engine with high compression ratio. The direct injection is used for forming desired stoichiometric stratified mixture at WOT condition without turbo-charging. The engine is filled with 20% cooled external EGR and the ignition timing is maintained at MBT point. The combustion characteristics of the desired stoichiometric stratified mixture show two-stage heat release, where the first stage is caused by spark ignition and the second stage is due to moderate auto-ignition. Compared with engine knock, the second stage heat release of controlled ASSCI shows smooth pressure curve without pressure oscillation. This is due to the low energy density mixture around the cylinder wall caused by cooled external EGR. The stratified mixture could suppress knock. Fuel economy and combustion characteristics of the baseline and the controlled ASSCI combustion were compared. The baseline GDI engine reaches a maximum of 8.9 bar BMEP with BSFC of 291 g/(kWh), the controlled ASSCI combustion achieves a maximum of 8.3 bar BMEP with BSFC of 256 g/(kWh), improving the fuel economy over 12% while maintaining approximately the same power. CA50 (the crank angle of 50% heat release) of the controlled ASSCI is detected at 8.4° CA ATDC, which is 17.4° CA advanced than that of the baseline while the combustion duration of the controlled ASSCI is 52.84dG CA, 16.6° CA longer than that of the baseline caused by diluted mixture and two-stage heat release. The COV of the controlled ASSCI is 1.4%, 2.1% lower than that of the baseline. The peak pressure (Pmax) and the maximum pressure rise rate (PRRmax) of the controlled ASSCI are 59.7 bar and 2.2 bar/° CA, 22.9 bar and 1.5 bar/° CA higher than that of the baseline respectively. The crank angle of Pmax and PRRmax of the controlled ASSCI are 11° CA ATDC and −1° CA ATDC, 15.4° CA and 17.2° CA earlier than that of the baseline. The results show that controlled ASSCI with two-stage heat releases is a potential combustion strategy to suppress engine knock while achieving high efficiency of the high compression ratio gasoline engine.

Diesel Spray Rate-of-Momentum Measurement Uncertainties and Diagnostic Considerations

Interpretation of combustion and emissions outcomes in diesel engines is often enhanced by accurate knowledge of the transient fuel delivery rate and flow characteristics of the injector nozzle. Important physical characteristics of these flows, including velocity profile and flow separation or cavitation effects, are difficult to measure directly, but can be characterized from a flow-averaged perspective through the measurement of nozzle flow coefficients, namely the discharge, velocity, and area contraction coefficients. Both the transient fuel mass flow rate and the flow-averaged nozzle coefficients can be found by measuring the mass and momentum flux of the fuel stream leaving the nozzle during injection through the application of an impingement technique, where fuel is sprayed onto the face of a transducer calibrated for force measurement in close proximity to the nozzle. While several published experiments have employed the spray impingement method to quantify rate-of-injection, the experimental setup and equipment selections vary widely and may contribute to disagreements in measured rate-of-injection. This paper identifies and provides estimates of measurement uncertainties that can arise when employing different experimental setups using the impingement method. It was observed that the impingement technique was sensitive to the design of the strike cap, specifically the contact area between the cap and transducer diaphragm, in addition to fuel temperature. Conversely, we observed that the impingement technique was relatively insensitive to angular and vertical misalignment, where the uncertainty can be estimated using control volume analysis. Transducer selection, specifically those with low acceleration sensitivity, high resonant frequency, and integrated electronics piezoelectric circuitry substantially reduce the noise in the measurement.

A Study of Combustion Inefficiency in Diesel LTC and Gasoline-Diesel RCCI via Detailed Emission Measurement

Reduction of engine-out NOx emissions to ultra-low levels is facilitated by enabling low temperature combustion (LTC) strategies. However, there is a significant energy penalty in terms of combustion efficiency as evidenced by the accompanying high levels of hydrocarbon (HC), carbon monoxide (CO), and hydrogen emissions. In this work, the net fuel energy lost as a result of incomplete combustion in two different LTC regimes is studied. The first LTC strategy, partially premixed compression ignition (PPCI), is investigated using a single, high pressure, in-cylinder injection of diesel fuel along with the application of exhaust gas recirculation (EGR). The second strategy includes dual-fuel application – reactivity controlled compression ignition (RCCI) of port injected gasoline and direct injected diesel. Moderate to high levels of EGR are necessary during engine operation in either of the two LTC pathways. A detailed analysis of the incomplete combustion products was conducted while the engine was operated in the aforementioned LTC modes. Speciation analysis of hydrocarbons was performed by sampling the exhaust gas in an FTIR. The total HC and the CO emissions were simultaneously measured using an FID and an NDIR, respectively. The production of hydrogen during the combustion process was also evaluated using a mass spectrometer. Engine tests were conducted at a baseline load level of 10 bar IMEP in the PPCI and RCCI modes. Load extension tests, up to 17 bar IMEP, were then conducted in the RCCI mode by increasing the gasoline-to-diesel fuel ratio. Test results indicated that CO, H2, and light HC made up for most of the combustion in-efficiency in the PPCI mode while heavier HC and aromatics were significantly higher in the RCCI mode.

Investigation on Spray and Soot Lift-Off Length of an ABE-Diesel Blend in a Constant Volume Chamber With Diesel Engine Conditions

Acetone-Butanol-Ethanol (ABE), an intermediate product in the ABE fermentation process for producing bio-butanol, is considered as a promising alternative fuel because it not only preserves the advantages of oxygenated fuel, which typically emit less pollutants compared to conventional diesel, but also lowers the cost of fuel recovery for each individual component during the fermentation. In this work, 20% ABE with component ratio of 3:6:1 and 80% ultra-sulfur diesel by volume, referred as ABE20, and pure diesel, referred as D100, were injected and combusted in a constant volume chamber with the ability to mimic high temperature and high pressure conditions of real diesel cylinder near the top dead center. By adjusting intake partial pressure and injection timing, the ambient oxygen concentration and temperature for fuel injection can be controlled. Ambient temperatures were set at 1100K, 900K and 700K to cover conventional temperature combustion and low temperature combustion, while the ambient oxygen concentrations were set at 21%, 16% and 11% to cover different EGR ratios separately. Spray and natural flame images were captured by a high speed camera coupled with a copper vapor laser as a light source. The results show that spray liquid penetration and soot lift-off length are shorter and much longer for ABE20 than those for D100 separately under all tested conditions, which form a much bigger gap from spray tip to the combustion area for ABE20. A big gap reduces the local equivalence ratio at the combustion area and then suppresses the soot formation due to the gap is the most effective area for air-fuel mixing processes. Indeed, the natural flame luminosity which represents the soot emission level of ABE20 is significantly lower than that of D100 at all tested conditions. At the same time, ABE20 performed a similar combustion phasing with D100 under high ambient temperature, but experienced an aggressive retardation under low ambient temperatures especially with low ambient oxygen concentrations. In addition, ABE20 did not show a stronger concentrated premixed combustion since its heat release rate peak is lower than that of D100, which was also confirmed by its longer combustion duration. Therefore, ABE20 expresses a high potential to reduce soot emissions but it also has to face combustion deterioration at low temperature combustion conditions.

Gas and Particle Emissions From a Diesel Engine Operating in a Dual-Fuel Mode Using High Water Content Hydrous Ethanol

Diesel engines running in a dual-fuel fumigation mode using injection of a high volatility fuel into the intake air and direct injection of diesel fuel can reduce NOX and particulate matter emissions. Fuels such as methanol, hydrogen, gasoline, and anhydrous ethanol have been studied as fumigants; however there has been less published regarding the use of high water content hydrous ethanol. Current production of ethanol yields anhydrous (200 proof) ethanol with no water content. The distillation and dehydration processes used to remove excess water from fermented starches during production require large amounts of input energy, reducing the renewability of the resulting fuel. This paper describes an experimental investigation of an aftermarket fumigation system provided by CleanFlex Power Systems, LLC. Experiments to measure gaseous and particulate emissions were conducted using 120 proof hydrous ethanol and non-oxygenated ultra-low sulfur diesel fuel. A John Deere 4045HF475 Tier 2 engine was modified to incorporate the fumigation system in the intake plumbing downstream of the charge-air cooler, just prior to the intake manifold. Data was collected for dual fuel fumigation combustion and compared to diesel only combustion. This study shows that the fumigation system achieved lower levels of NOX and soot proportional to the fumigant energy fraction (FEF), but increased CO and hydrocarbon levels as compared to diesel-only combustion modes. The results suggest that increasing the FEF by using lower water content or better mixing through port-injection may increase the emissions reduction potential of hydrous ethanol fumigation.