In-cylinder Charge Temperature Research Articles

Variable Valve Strategy Evaluation for Low-Load Operation in a Heavy-Duty Gasoline Compression Ignition Engine

By harnessing gasoline’s low reactivity for partially premixed combustion promotion, gasoline compression ignition (GCI) combustion shows the potential to produce markedly improved NOx-soot trade-off with high fuel efficiency compared to conventional diesel combustion. However, at low-load conditions, gasoline’s low reactivity poses challenges to attaining robust combustion with low unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions. Increasing the in-cylinder charge temperature by using variable valve actuation (VVA) can be an effective means to address these challenges. In this numerical investigation, VVA strategies, including (1) early exhaust valve opening (EEVO), (2) positive valve overlap (PVO), and (3) exhaust rebreathe (ExReb), were investigated at 1375 RPM and 2 bar brake mean effective pressure in a heavy-duty GCI engine using a market-based gasoline with a research octane number (RON) of 93. The total residual gas level was kept over 50% to achieve an engine-out NOx target of below 1.5 g/kWh. For a complete engine system analysis, one-dimensional (1-D) system-level modeling and three-dimensional (3-D) computational fluid dynamics (CFD) analysis were close-coupled in this study. Performance of the VVA strategies was compared in terms of in-cylinder charge and exhaust gas temperatures increase versus brake-specific fuel consumption (BSFC). The EEVO strategy demonstrated in-cylinder charge and exhaust temperature increase up to 130 and 180 K, respectively. For similar in-cylinder charge temperature gains, the ExReb strategy demonstrated 11% to 18% lower BSFC compared to the EEVO strategy. This benefit primarily originated from a more efficient gas-exchange process. The PVO strategy, due to the valve–piston contact constraint, required excessive exhaust back-pressure valve (BPV) throttling for hot residuals trapping, thereby incurring higher BSFC compared to ExReb. In addition, the ExReb strategy demonstrated the highest potential for exhaust temperature increase (up to 673 K) among the three strategies. This was achieved by ExReb’s maximum air-fuel ratio reduction from high internal residuals mass and BPV throttling. Finally, the ExReb profile was optimized in terms of the peak lift, the duration, and the location for maximizing the fuel-efficiency potential of the strategy.

Combustion and Emission Enhancement of a Spark Ignition Two-Stroke Cycle Engine Utilizing Internal and External Exhaust Gas Recirculation Approach at Low-Load Operation

Two-stroke cycle engines have always been prominent due to their distinctive advantage incorporating high power-to-weight ratio, however the drawbacks are poor combustion efficiency, fuel short-circuiting and excessive emission of uHC and CO. These problems are apparent at low-load and speed regions and are the major obstacle to their global acceptance. The deficiencies can be addressed by increasing the in-cylinder average charge temperature employing Exhaust Gas Recirculation (EGR). An experimental study is conducted to investigate the influence of utilizing EGR techniques, including Internal and External EGR, on combustion misfiring occurrence, combustion stability and exhaust emissions using a single cylinder two-stroke SI engine at idling, low and mid-load conditions. From the results, it is observed since the average in-cylinder charge temperature is increased, due to utilizing EGRs, engine’s low and mid-load irregular combustions (misfire) and exhaust emissions are remarkably supressed and almost all of misfire cycles eliminated depending on the percentage of EGRs. In terms of combustion stability, it is agreed in general the application of EGRs improves the cyclic variation of IMEP, Pmax and CA10 compared to conventional operation. However, applying Ex-EGR compared to In-EGR will deteriorate cyclic variability of IMEP and CA10.

An experimental and numerical study on diesel injection split of a natural gas/diesel dual-fuel engine at a low engine load

Natural gas/diesel dual-fuel combustion is currently one of the most promising LTC strategies for the next generation of heavy-duty engines. While this concept is not new and it has been deliberated lengthily in the past two decades, several uncertainties still exist. A major shortcoming of this concept is associated with its low thermal efficiency and high level of unburned methane and CO emissions under low engine load conditions. The present paper reports an experimental and numerical study on the effect of different injection strategies (single and two pulses injection of pilot diesel fuel) on the combustion performance and emissions of a heavy duty natural gas/diesel dual-fuel engine at 25% engine load. The results of single diesel injection mode showed that advancing diesel injection timing from 10 to 30 °BTDC reduced unburned methane and CO emissions by 62% and 61% and increased thermal efficiency by 6%; however, NOx emissions increased by 74%. In order to achieve NOx – CH4 and NOx – CO trade-off and increased thermal efficiency at low load conditions, the effect of split injection strategy was experimentally and numerically examined. The results of split injection mode revealed that split injection strategy considerably increases the in-cylinder peak pressure compared to that of single injection (10 °BTDC). The results showed also that the heat release produced by the first injection of diesel fuel considerably increased the in-cylinder charge temperature before the start of the second injection. The flame zone of the split injection mode is markedly higher than that of the single injection due to larger heat release produced during the first injection which promotes the combustion of the second one. When the first injection timing is close to the second injection timing, the MPRR of split injection mode is higher than that of single injection (10 °BTDC). However, further advancing of the first injection timing continuously decreased the MPRR. OH radical analysis showed that for advanced first injection timings (38–50 °BTDC), the overall growth rate of OH radical becomes slower and its distribution is narrower as indicated by the wider non-reactive blue zones compared with those observed at a late first injection timing in the initial stages of combustion. However, OH radicals gradually grow during last stages of combustion in the expansion stroke, indicating that a more premixed combustion takes place in these cases. For very advanced first injection timing of 55 °BTDC, the OH distribution is similar to that of the single injection mode with lower OH intensity at initial stages of combustion and they barely grow during the late expansion stroke. At this condition, the ignition of premixed mixture is mainly controlled by the second diesel fuel injection. The trade-off between NOx – CH4 and NOx – CO is achieved when applying split injection. Compared to single injection (10 °BTDC), the first injection timing of 50 °BTDC decreased unburned methane and CO emissions by 60% and 63%, respectively, and increased the thermal efficiency by 8.9%. However, NOx emissions were maintained at the same level as single injection mode (10 °BTDC).

Thermal and chemical effects of the in-cylinder charge at IVC on cycle-to-cycle variations of DME HCCI combustion with combustion-phasing retard by external and rebreathed EGR

Combustion-phasing retard is an effective method to extend the high-load limit of homogeneous charge compression ignition (HCCI) engine, but it leads to unacceptable cycle-to-cycle variation. This study investigates the factors that lead to the increase of the cycle-to-cycle variations of HCCI combustion with combustion-phasing retard using dimethyl ether (DME). The experiment was conducted in a single-cylinder engine equipped with an exhaust gas recirculation (EGR) and a two-stage exhaust cam. The experimental results show that an increase of external EGR mass is accompanied with a combustion-phasing retard. The maximum pressure-rise rate decreases with combustion-phasing retard, but the cycle-to-cycle variations of HCCI combustion becomes unacceptable. For unstable operation, a partial-burn cycle is often followed by another partial-burn cycle, however well-burn cycle occurs ever for low in-cylinder charge temperature. This is assumed that the chemical effects of the residuals have to be responsible. To clarify the chemical effects of the residuals, the computation was performed using CHEMKIN with a detailed chemical-kinetic mechanism for DME. The computational results show that adding the residuals to the in-cylinder charge enhances the autoignition sufficiently to advance the combustion phasing in proportion to the amount of the residuals, and this contributes as well to the increase of IMEP.

Turbulent flame boundary and structure detection in an optical DISI engine using tracer-based two-line PLIF technique

Design and development of new combustion system for Spark Ignition Direct Injection (DISI) engines requires thorough understanding of the flame as it develops from electric discharge and propagates across the combustion chamber. The main purpose of this work was to develop an experimental setup capable of investigating premixed and partially-premixed turbulent flame boundary and structure inside combustion chamber of a DISI engine. For this purpose the tracer-based two-line Planar Laser Induced Fluorescence (PLIF) technique was set up. In order to have a thermometry technique independent of photophysical models of dopant tracer, a specially designed Constant Volume Chamber (CVC) was utilized for quasi in situ calibration measurements. The thermometry technique was evaluated by measurements of average in-cylinder charge temperature during compression stroke for both motoring and firing cycles and comparing the results with temperature values calculated from in-cylinder pressure data. The developed technique was successfully employed to detect flame boundary and structure during combustion process in the optical engine. The present study demonstrated that as the two-line PLIF thermal images are independent of species concentration and flame luminosity they can be utilized as accurate means for flame segmentation. The proposed technique has the potential to be utilized for study of turbulent flames in non-homogeneously mixed systems.

Closed-loop control of HCCI combustion for DME using external EGR and rebreathed EGR to reduce pressure-rise rate with combustion-phasing retard

This study experimentally investigates the effects of the combustion phasing on the homogeneous charge compression ignition (HCCI) combustion, and implements a closed-loop control of HCCI combustion to reduce pressure-rise rate (PRR) with combustion-phasing retard. The experiments were conducted using dimethyl ether (DME) in a single-cylinder HCCI research engine equipped with an exhaust gas recirculation (EGR) loop for external EGR and a two-stage exhaust cam for rebreathed EGR. The results show that a maximum PRR (PRRmax) and a maximum in-cylinder charge temperature decreases with combustion-phasing retard. However, excessive combustion-phasing retard leads to unacceptable coefficient of variation (COV) of CA50 and IMEP with partial-burn and/or misfire cycles. To dampen increasing cycle-to-cycle variations around the limit of combustion-phasing retard, the closed-loop control of HCCI combustion was implemented using three feedback variables. Finally, stable stoichiometric HCCI operation could be achieved with extensive combustion-phasing retard while maintaining acceptable PRRmax with the higher level of IMEP.

Experimental investigation of direct injection charge cooling in optical GDI engine using tracer-based PLIF technique

Investigation of direct injection charge cooling effects is indispensable in design and development of new combustion systems for Gasoline Direct Injection (GDI) engines. The charge cooling can be utilized to increase engine volumetric efficiency or compression ratio. It can be employed to suppress pre-ignition of highly boosted downsized engines or knocking combustion of naturally aspirated engines. The main purpose of this work was to develop an experimental setup for quantitative measurements of charge cooling during fuel injection process inside the combustion chamber of a GDI engine with optical access. For this purpose a tracer-based two-line Planar Laser Induced Fluorescence (PLIF) technique was implemented for the measurements. A specially designed Constant Volume Chamber (CVC) was utilized for quasi in situ calibration measurement so in-cylinder charge temperature measurements can be achieved independent of the photophysical model of dopant tracer. The thermometry technique was evaluated by measurements of average in-cylinder charge temperature during compression stroke for both motoring and firing cycles and comparing the results with temperature values calculated from in-cylinder pressure data assuming a polytropic compression. The PLIF technique was successfully utilized to quantify the extend of global temperature decrease as a result of direct injection charge cooling of two injection timings of 90 and 250°CA ATDC and two injection quantities of 10 and 30mg/cycle. Test results demonstrated the capability of the two-line PLIF thermometry technique in quantitative study of direct injection charge cooling effects.

NOx formation inside HCCI engines

A multizones modeling of the combustion characteristics of the spark assisted homogeneous charge compression ignition (HCCI) engine is performed. The main aim of this work is to deduce the simulation to reveal the in-cylinder conditions prevailing during the spark assisted HCCI combustion which is difficult to obtain experimentally. Conditions such as in-cylinder burned gas temperature, NOx formation, and mean in-cylinder charge temperature and pressure conditions. The second aim of the present work is the mapping of the influence of NO doping on the incylinder conditions. Recent experimental studies have shown that NO has significant impact on hydrocarbon autoignition and the NO formed inside the burned gas can be used for autoignition control of HCCI engine combustion. The tuning of a zero-dimensional multizones phenomenological model is performed separately using experimental data for SI and HCCI operation keeping the same engine parameters, engine speed and air-fuel ratio to maintain simulation similarity. Variation of the burned gas temperatures inside the individual zones due to recompression effects and the formation of NOx inside these zones in a single engine stroke for different engine speed conditions are investigated. The mean unburned gas temperature and pressure conditions before the start of ignition are obtained for different operating conditions. The effect of NO on in-cylinder conditions is done by doping it in the order of one magnitude up.

Estimating In-Cylinder Precombustion Mixture Temperatures Using Acoustic Resonances

The air-fuel ratio of automotive engines during the warm-up period is difficult to control and contributes a substantial portion of the emissions on the EPA test cycle. High bandwidth estimation of the in-cylinder charge temperature provides opportunities for improvement in spark ignition (SI) engine control algorithms. Pressure sensor based algorithms for estimating air-fuel ratio (AFR) have been shown to be improved by bulk temperature information. This paper explores the suitability of using acoustic resonances to estimate charge temperature in the presence of an unknown AFR and without the ‘sharp’ excitation of diesel or knocking SI combustion. A technique that allows for the estimation of the precombustion average bulk charge temperature during a 1-2 ms interval based on acoustic resonance data gathered from a pressure transducer is described. A parameter estimation algorithm suitable for extracting the required frequency information from short data sets is identified. The variation in temperature estimates as a function of AFR has been explored using a computer simulation that accounts for the change in the ratio of specific heats with changing mixture strength. The performance of the acoustic resonance based temperature estimates has been evaluated by comparing them to predictions based on a polytropic compression.