Computational Study to Improve Thermal Efficiency of Spark Ignition Engine

The Co-Optimization of Fuels and Engines (Co-Optima) initiative from the US Department of Energy aims to co-develop fuels and engines in an effort to maximize energy efficiency and the utilization of renewable fuels. Many of these renewable fuel options have fuel chemistries that are different from those of petroleum-derived fuels. Because practical market fuels need to meet specific fuel-property requirements, a chemistry-agnostic approach to assessing the potential benefits of candidate fuels was developed using the Central Fuel Property Hypothesis (CFPH). The CFPH states that fuel properties are predictive of the performance of the fuel, regardless of the fuel's chemical composition. In order to use this hypothesis to assess the potential of fuel candidates to increase efficiency in spark-ignition (SI) engines, the individual contributions towards efficiency potential in an optimized engine must be quantified in a way that allows the individual fuel properties to be traded off for one another. This review article begins by providing an overview of the historical linkages between fuel properties and engine efficiency, including the two dominant pathways currently being used by vehicle manufacturers to reduce fuel consumption. Then, a thermodynamic-based assessment to quantify how six individual fuel properties can affect efficiency in SI engines is performed: research octane number, octane sensitivity, latent heat of vaporization, laminar flame speed, particulate matter index, and catalyst light-off temperature. The relative effects of each of these fuel properties is combined into a unified merit function that is capable of assessing the fuel property-based efficiency potential of fuels with conventional and unconventional compositions.

<div class="htmlview paragraph">To examine the effects of exhaust gas recirculation (EGR) on the exhaust emissions and on the cycle-by-cycle variation of combustion process in a methanol engine, the authors have tried to measure the emissions of total hydrocarbon, carbon monoxides, nitric oxides and formaldehyde and to record the cylinder pressure development histories of 1000 cycles duration. Moreover, the standard deviation of peak pressure Pmax, the heat release rate and the averaged pressure history are analyzed.</div> <div class="htmlview paragraph">The results are compared with those of gasoline fueled operation.</div> <div class="htmlview paragraph">The results are summarized as follows:</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"> <span class="li-label">(1)</span><div class="htmlview paragraph">In the case of a methanol-fueled engine operated under exhaust gas recirculation (EGR), similar to that already carried out in a gasoline-fueled engine, the concentration of NOx, in the exhaust gas can be decreased but those of formaldehyde (HCHO) and unburned methanol are increased.</div> </li> <li class="list-item"> <span class="li-label">(2)</span> <div class="htmlview paragraph">The effect of EGR, such as 20% of it, on Nox reduction is the same as those of hydrous methanol of 50% water content.</div> </li> <li class="list-item"> <span class="li-label">(3)</span> <div class="htmlview paragraph">In the case of EGR applied to an EFI engine, its output and thermal efficiency are decreased within the lean mixture region (λ>1.0) but these recover and the concentration of hydrocarbon is decreased by raising the fuel temperature.</div> </li> <li class="list-item"> <span class="li-label">(4)</span> <div class="htmlview paragraph">When the optimum conditions of the EGR and fuel temperature are selected, it is possible to improve the exhaust gas characteristics concerning NOx, THC, etc., while the decreased in engine output and thermal efficiency is reduced to the minimum.</div> </li> </ol> </div>

Numerical evaluation of heat transfer effects on the improvement of efficiency of a spark ignition engine characterized by cylinder variability

Gasoline compression ignition (GCI) is a practicable way to obtain low emissions and high thermal efficiency of gasoline-like fuels in internal combustion engines. In this paper, the research octane number (RON) and injection strategy were coordinated to optimize the GCI engine performance and emissions under high loads. The direct injection and port injection were used to achieve two injection strategies: direct injection (DI) and port injection plus direct injection (PIDI), and the primary reference fuels (PRF) with the RON of 60, 70, 80 and 90 were used. The results show that using lower RON fuels under high loads, DI mode can achieve higher efficiency, while PIDI mode can achieve lower combustion noise at an expense of slightly lower fuel economy. When the DI mode is converted to PIDI mode with a pre-injection ratio of 30%, using PRF70 under 12 bar and the exhaust gas recirculation (EGR) rate of 40%, the gross indicated thermal efficiency and the maximum pressure rise rate are reduced by 1% and by 2 bar/°CA, respectively, while the particle emissions also decrease significantly, thus achieving low emissions and high efficiency. However, under the same load and EGR rate, DI mode produces less regulated and unregulated emissions than PIDI mode. In addition, the effect of fuel RON was obvious, the lower RON fuels exhibit obvious three-stage heat release in PIDI mode, however, PRF90 with higher RON only exhibits two-stage heat release, and the peak value of the first-stage heat release rate is also lower than those of other fuels.

<div class="htmlview paragraph">Since ethanol is a renewable source of energy and it contributes to lower CO2 emissions, ethanol produced from biomass is expected to increase in use as an alternative fuel. It is recognized that for spark ignition (SI) engines ethanol has advantages of high octane number and high combustion speed and has a disadvantage of difficult startability at low temperature.</div> <div class="htmlview paragraph">This paper investigates the influence of ethanol fuel on SI engine performance, thermal efficiency, and emissions. The combustion characteristics under cold engine conditions are also examined.</div> <div class="htmlview paragraph">Ethanol has high anti-knock quality due to its high octane number, and high latent heat of evaporation, which decreases the compressed gas temperature during the compression stroke. In addition to the effect of latent heat of evaporation, the difference of combustion products compared with gasoline further decreases combustion temperature, thereby reducing cooling heat loss. As a result, torque and thermal efficiency increase compared to gasoline. Also, reductions in CO2, NOx, and THC are described.</div> <div class="htmlview paragraph">To improve combustion under cold conditions, methods of increasing fuel temperature or the compressed gas temperature during the compression stroke are considered. In the latter case, simulation showed valve timing to be effective. Based on simulation results, the effect of valve timing optimization on improving combustion at low temperature is investigated with an engine.</div>

<div class="section abstract"><div class="htmlview paragraph">In this paper, 20%H₂ (20% hydrogen in natural gas-hydrogen blends, by volume) is selected as the test fuel, and the ignition timing and EGR ratio are adjusted to optimize the performance, combustion, and emissions of a natural gas-hydrogen engine. An experimental investigation on the effect of ignition timings, EGR ratios on combustion behaviors and emissions of a spark-ignition engine fuelled with natural-gas and hydrogen blends was conducted.</div><div class="htmlview paragraph">When increasing the ignition timing at specified EGR ratio, engine power output will give its peak value at MBT timing. Large EGR introduction decreases engine power output and increases combustion duration. Effective thermal efficiency shows an increasing trend at the small EGR ratio and a decreasing trend with further increasing EGR ratio.</div><div class="htmlview paragraph">With advancing the ignition timing, the flame development duration is increased, and the rapid combustion duration and total combustion duration are decreased. All of these three kinds of combustion duration are increased with the increase of EGR ratio.</div><div class="htmlview paragraph">HC concentration is decreased and NOx concentration is increased with advancing the ignition timing while CO emission varies little with the ignition timing. NOx concentration is decreased with the increase of EGR ratio. HC emission is increased with the increase of EGR ratio. CO and CO₂ emissions show little variations with the EGR ratio and ignition timing.</div><div class="htmlview paragraph">Based on the above analysis, a MAP containing ignition timings and EGR ratios for 20%H₂ fuel blend is presented, and this will provide the reference and guidance to engine designers and engineers. In addition, through optimizing ignition timing and EGR ratio, high efficiency and low emission spark-ignition engine can be realized fueled with natural gas-hydrogen blends.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Nitric Oxide (NO) can significantly influence the autoignition reactivity and this can affect knock limits in conventional stoichiometric SI engines. Previous studies also revealed that the role of NO changes with fuel type. Fuels with high RON (Research Octane Number) and high Octane Sensitivity (S = RON - MON (Motor Octane Number)) exhibited monotonically retarding knock-limited combustion phasing (KL-CA50) with increasing NO. In contrast, for a high-RON, low-S fuel, the addition of NO initially resulted in a strongly retarded KL-CA50 but beyond the certain amount of NO, KL-CA50 advanced again. The current study focuses on same high-RON, low-S Alkylate fuel to better understand the mechanisms responsible for the reversal in the effect of NO on KL-CA50 beyond a certain amount of NO. Experiments were conducted to measure the responses of KL-CA50 and trace-autoignition CA50, the latter being indicative of CA50 at which end-gas autoignition starts to become measurable from the apparent heat-release rate. Chemical-kinetics simulations were conducted to reveal the role of NO for end-gas autoignition, with a specific focus on sequential autoignition in a thermally stratified end-gas.</div><div class="htmlview paragraph">The simulation results reveal that the magnitude of low-temperature heat release (LTHR) generally increases with NO. However, the relative importance of NO for enhancing LTHR diminishes when the LTHR inherent to a fuel’s chemistry is strong, such as at lower temperatures in a thermal boundary layer. This rendered more uniform LTHR within a hypothetical thermal boundary and led to a more sequential (<i>i.e.</i> slower) autoignition event. It was also revealed that a change in compression ratio influences the importance of intermediate-temperature heat release (ITHR) due to changes of the temperature-pressure history of the end-gas. Together with the condition where end-gas autoignition occurs more sequentially, the shorter time spent in LTHR and ITHR regime can counter the increase in autoignition reactivity at high NO levels and allow KL-CA50 to advance.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Some SI (spark-ignition) engines fueled with gasoline for industrial machineries are designed based on the conventional diesel engine in consideration of the compatibility with installation. Such diesel engine-based SI engines secure a combustion chamber by a piston bowl instead of a pent-roof combustion chamber widely applied for SI engines for automobiles.</div><div class="htmlview paragraph">In the development of SI engines, because knocking deteriorates the power output and the thermal efficiency, it is essential to clarify causes of knocking and predict knocking events. However, there has been little research on knocking in diesel engine-based SI engines. The purpose of this study is to elucidate knocking phenomena in a gasoline engine with a re-entrant piston bowl and swirl flow numerically and experimentally. In-cylinder visualization and pressure analysis of knock onset cycles have been experimentally performed. Locations of autoignition have been predicted by 3D-CFD analysis with detailed chemical reactions. The prediction accuracy of the location of autoignition has been examined by comparing with experimental results. Initial and boundary conditions for 3D-CFD are obtained by TPA (Three Pressure Analysis), which is a 1D engine cycle simulation using measured intake/exhaust and in-cylinder pressures as the input data. Locations and timing of autoignition have been observed through an endoscope attached to the cylinder head. The visualization area through the endscope is determined based on the location of autoignition predicted by 3D-CFD analysis. The in-cylinder pressure is also measured simultaneously, and the maximum amplitude of the pressure oscillations after applying a high-pass filter to the in-cylinder pressure is used as knock intensity (KI) to determine knock onset cycles. From results of the in-cylinder pressure measurements and analysis by TPA, the relationships between in-cylinder conditions and KI are analyzed. The comparison between visualized and calculated location of autoignition have been shown good agreement in the case that both the maximum and the low engine speed conditions.</div></div>

<div class="htmlview paragraph">Mass burning rate was varied by using two single-cylinder engines, each fitted with different combustion-chamber geometries. The slower-burning chamber incorporated a side-mounted spark plug, the faster-burning chamber combined a more compact combustion space and a centrally located spark plug.</div> <div class="htmlview paragraph">The faster-burning engine exhibited an improved efficiency-NOx emissions trade-off, primarily due to the engine's higher maximum thermal efficiency and greater tolerance to charge dilution by recirculated exhaust gas.</div> <div class="htmlview paragraph">Faster burning also reduced cyclic variability in combustion, increased the knock-limited thermal efficiency, but offered no advantage in regard to hydrocarbon emissions.</div>

<div class="section abstract"><div class="htmlview paragraph">Butanol, which is a renewable biofuel, has been regarded as a promising alternative fuel for internal combustion engines. When blended with diesel and applied to pilot ignited natural gas engines, butanol has the capability to achieve lower emissions without sacrifice on thermal efficiency. However, high blend ratio of butanol is limited by its longer ignition delay caused by the higher latent heat and higher octane number, which restricts the improvement of emission characteristics. In this paper, the potential of increasing butanol blend ratio by adding hot exhaust gas recirculation (EGR) is investigated. 3D CFD model based on a detailed kinetic mechanism was built and validated by experimental results of natural gas engine ignited by diesel/butanol blends. The effects of hot EGR is then revealed by the simulation results of the combustion process, heat release traces and also the emissions under different diesel/butanol blend ratios. To get a better fundamental understanding of the EGR effects, the explanations for the effects induced by hot EGR is aided by 0D simulations. The results indicate that the ignitable range of n-butanol blend ratio can be extended from 40% to 60% by the adoption of hot EGR. Besides, the thermal efficiency could be improved by adding appropriate fraction of hot EGR at n-butanol ratios higher than 30%. Generally, optimized thermal efficiency and emission characteristics of NOx and CO can be achieved with adding hot EGR by 5% and 10%. It is also find that improvements in PAH emissions can be obtained by adding hot EGR at n-butanol blend ratios smaller than 30% and PAHs with more than two rings could be reduced by adding n-butanol.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Partially premixed combustion (PPC) is one of several advanced combustion concepts for the conventional diesel engine. PPC uses a separation between end of fuel injection and start of combustion, also called ignition dwell, to increase the mixing of fuel and oxidizer. This has been shown to be beneficial for simultaneously reducing harmful emissions and fuel consumption. The ignition dwell can be increased by means of exhaust gas recirculation or lower intake temperature. However, the most effective means is to use a fuel with high research octane number (RON). Methanol has a RON of 109 and a recent study found that methanol can be used effectively in PPC mode, with multiple injections, to yield high brake efficiency. However, the early start of injection (SOI) timings in this study were noted as a potential issue due to increased combustion sensitivity. Therefore, the present study attempts to quantify the changes in engine performance for different injection strategies. Simulations were performed on a heavy-duty multi-cylinder compression ignition engine fueled with methanol. Two operating conditions with different engine load were chosen from the European stationary cycle. Three different injection strategies were applied: 1) SOI > −160°ca aTDC 2) SOI > −40°ca aTDC 3) SOI > −25°ca aTDC. The engine settings were selected to maximize the brake efficiency for each case and the sensitivity of combustion to inlet conditions was analyzed. For the high load operating point, the brake efficiency was 2.2 %pt. higher for case 1 compared to case 3, while this difference was only 0.5 %pt. for the low load operating point. However, the combustion phasing for case 1 and 2 at the high load point proved to be very sensitive to inlet temperature, inlet pressure and oxygen concentration.</div></div>

<div class="section abstract"><div class="htmlview paragraph">At part load and wide open throttle operation with stratified charge and lean mixture conditions the Direct Injection Spark Ignition (DISI) engine offers similar efficiency levels compared to compression ignition engines</div><div class="htmlview paragraph">The present paper reports on results of recent studies on the impact of the in-cylinder processes in DISI engines e. g. the injection, the in-cylinder flow, the mixture preparation and the ignition on the combustion, the energy conversion and the exhaust emission behavior. The analyses of the spray behavior, of the in-cylinder flow during compression as well as of the flame propagation have been carried out applying advanced optical measurement techniques. The results enable a targeted optimization of the combustion process with respect to engine efficiency and exhaust emissions. The benefits of an increase in fuel injection pressures up to 100 MPa are discussed. The investigations have been mainly performed in a single cylinder research engine featuring a spray-guided injection process with central injector location. The considered operating range includes part load conditions with both stratified and homogeneous mixture preparation as well as full load operation with homogeneous mixture preparation. The results obtained from the research work are very promising and hence a 4-cylinder production engine is modified to operate with a suitable injection system with rail pressures in the range of 50 MPa to 60 MPa.</div><div class="htmlview paragraph">Additionally, irregular combustion events in the full load regime such as knocking combustion and low speed pre-ignition have been investigated in a 4-cylinder DISI production engine. The results support the development of mitigation strategies for both engine knock and low speed pre-ignition (LSPI).</div></div>

<div class="section abstract"><div class="htmlview paragraph">Improving vehicle fuel economy is a central part of efforts toward achieving a sustainable society. An effective way of accomplishing this is to enhance the engine thermal efficiency. Mitigating knock and reducing engine heat loss are important aspects of enhancing the thermal efficiency. Cooled exhaust gas recirculation (EGR) is regarded as a key technology because it is capable of achieving both of these objectives. For this reason, it has been adopted in a wide range of both hybrid vehicles and conventional vehicles in recent years.</div><div class="htmlview paragraph">In EGR equipped engines, fast combustion is regarded as one of the most important technologies, since it realizes higher EGR ratio. To create fast combustion, generation of strong in-cylinder turbulence is necessary. Strong in-cylinder turbulence is achieved through swirl, squish, and tumble flows. Specifically high tumble flow has been adopted on a number of new engines because of the intense effect of promoting in-cylinder turbulence. However, the application of high tumble flow incurs the reduction of intake mass flow rate. Toyota considers that increasing engine output (performance) is also important to achieve a “Fun to Drive” characteristic for customer satisfaction. So the development of technologies that generate tumble flow effectively while maintaining high intake flow coefficient is essential.</div><div class="htmlview paragraph">This paper describes the key requirements for engine design to generate high tumble flow with high flow coefficient by using both simulation and experiment. It is important that the air flow of tumble is straight and runs along the wall of the combustion chamber near the exhaust valves for high tumble flow efficiency. As a result, better performance can be achieved by changing the layout of the valve angle and the intake port. The generated tumble flow improved by 10%.</div></div>

<div class="htmlview paragraph"><i>The rapid trend in recent years toward higher octane fuels and high-performance engines has suggested a reappraisal of the ability of current laboratory knockrating methods to define the antiknock behavior of commercial gasolines in modern multicylinder passenger car engines. Road antiknock performance data have been obtained on 91 premium gasolines in five 1957 test cars and three cars equipped with experimental 11-to-1 compression ratio engines. In addition, 16 prototype gasolines were studied in the three high compression ratio cars.</i></div> <div class="htmlview paragraph"><i>The data were analyzed by establishing a linear relationship between the road ratings and the laboratory data to determine the relative importance of each variable in enhancing the accuracy of the prediction equation. Of the many fuel laboratory variables, combinations of Research and Motor octane number, fuel composition and volatility were investigated as predictors of road performance.</i></div> <div class="htmlview paragraph"><i>For any given combination the results were rather consistent from car to car, but varied greatly with car speed. Research octane number was found to be the best single predictor of low-speed road performance (2000 rpm), and Motor octane number the best single predictor of high-speed performance (3000 rpm and higher).</i></div> <div class="htmlview paragraph"><i>If two laboratory variables are used as predictors, a combination of Research and Motor octane numbers serves best for predicting low-speed road performance. At high speed, the combination of Motor octane number and percent aromatics was found to be the best two-variable predictor.</i></div> <div class="htmlview paragraph"><i>Low-speed correlations based on Research and Motor octane number were not improved when olefin content was included as a third predicting variable. At high speed, however, the use of a hydrocarbon-type variable in addition to Research and Motor octane numbers resulted in a significant reduction of the standard error of prediction.</i></div> <div class="htmlview paragraph"><i>The use of more than three variables as predictors did not improve the accuracy of prediction at low speed and improved the accuracy at high speeds to a degree which is not considered to be of practical value.</i></div> <div class="htmlview paragraph"><i>An increase in Research octane number was found to be of more importance than an increase in Motor octane number in improving low-speed road ratings. At high speeds, however, the contribution of Motor octane number was decidedly greater than that of Research octane number.</i></div> <div class="htmlview paragraph"><i>When Research and Motor octane numbers were held constant, an increase in aromatic content or a decrease in olefin content improved the high-speed road rating of a fuel</i></div>

<div class="section abstract"><div class="htmlview paragraph">Partially Premixed Combustion (PPC) has the potential of simultaneously providing high engine efficiency and low emissions. Previous research has shown that with proper combination of Exhaust-Gas Recirculation (EGR) and Air-Fuel equivalence ratio, it is possible to reduce engine-out emissions while still keeping the engine efficiency high. In this paper, the effect of changes in intake pressure (boost) and EGR fraction on PPC engine performance (e.g. ignition delay, burn duration, maximum pressure rise rate) and emissions (carbon monoxide (CO), unburned hydrocarbon (UHC), soot and NOX) was investigated in a single-cylinder, heavy-duty diesel engine. Swedish diesel fuel (MK1), RON 69 gasoline fuel and 99.5 vol% ethanol were tested. Fixed fueling rate and single injection strategy were employed.</div><div class="htmlview paragraph">For diesel and gasoline PPC, in those conditions with a predominantly mixing-controlled combustion process, increasing EGR or reducing intake boost could increase ignition delay and shorten burn duration. Combustion started later. Peak heat release rate found in premixed burn phasing was higher but occurred much later before TDC, which is beneficial to reduce maximum pressure rise rate level. Lowering boost pressure and intake oxygen further more resulted in more premixed combustion due to longer ignition delay. A large portion of the heat release occurred after TDC and the pressure rise rate was lower.</div><div class="htmlview paragraph">For ethanol PPC, due to long ignition delay, there was mainly premixed combustion and it had much shorter burn duration compared to diesel and gasoline PPC. Less EGR was beneficial for reducing pressure rise rate as a result of lower peak heat release rate after TDC timing and slower burn. Ignition delay peaked at moderate intake boost pressure level and decreased with both lower and higher boost.</div><div class="htmlview paragraph">Decreased EGR / increased intake boost pressure reduced emissions of CO, UHC and soot, with corresponding improvements in combustion efficiency and gross indicated efficiency, particularly at relatively low oxygen concentration. Generally, high EGR was necessary to inhibit NO<sub>X</sub> emission.</div></div>