Engine combustion and emission fuelled with natural gas: A review

Natural gas (NG) is one of the most important and successful alternative fuels for vehicles. Engine combustion and emission fuelled with natural gas have been reviewed by NG/gasoline bi-fuel engine, pure NG engine, NG/diesel dual fuel engine and HCNG engine. Compared to using gasoline, bi-fuel engine using NG exhibits higher thermal efficiency; produces lower HC, CO and PM emissions and higher NOx emission. The bi-fuel mode can not fully exert the advantages of NG. Optimization of structure design for engine chamber, injection parameters including injection timing, injection pressure and multi injection, and lean burn provides a technological route to achieve high efficiency, low emissions and balance between HC and NOx. Compared to diesel, NG/diesel dual fuel engine exhibits longer ignition delay; has lower thermal efficiency at low and partial loads and higher at medium and high loads; emits higher HC and CO emissions and lower PM and NOx emissions. The addition of hydrogen can further improve the thermal efficiency and decrease the HC, CO and PM emissions of NG engine, while significantly increase the NOx emission. In each mode, methane is the major composition of THC emission and it has great warming potential. Methane emission can be decreased by hydrogen addition and after-treatment technology.     

增压稀燃天然气掺氢发动机排放特性

为了研究20%掺氢比的增压稀燃天然气掺氢(HCNG)发动机的排放特性,通过对发动机进行了空燃比和点火提前角调整试验、ETC循环测试试验和加装氧化型催化器试验,获得了20%HCNG发动机的排放规律.CH4排放随着空燃比的增大先减少后增加;CO排放在高于理论空燃比后骤减;Nox排放随着空燃比的增大先增加后减少,在空燃比19～21 左右达到最大值,1600～1800r/min时最低.CO、Nox随着点火提前角的增大而增加;CH4随着点火提前角的增大略有增加,并且点火提前角越大,对CH4排放的影响越小.加装催化器后,CO、CH4的转化效率均＞90%.试验结果表明:增压稀燃和氧化型催化器相结合是天然气掺氢发动机节能减排的有效方案.     

Evaluation of acetylene as a spark ignition engine fuel

In spite of its known shortcomings as a fuel for spark ignition engines, acetylene has been suggested as a possible alternative to petroleum-based fuels since it can be produced from non-petroleum resources (coal, limestone and water). Therefore, acetylene was evaluated in a single-cylinder engine to investigate performance and emission characteristics with special emphasis on lean operation for NO_x control. Testing was carried out at constant speed, constant airflow and MBT spark timing. Equivalence ratio and compression ratio were the primary variables. The engine operated much leaner when fuelled with acetylene than with gasoline. With acetylene, the engine operated at equivalence ratios as lean as 0·53 and 0·43 for compression ratios of 4 and 6, respectively. However, the operating range was very limited. Knock-induced preignition occurred either with compression ratios above 6 or with mixtures richer than 0·69 equivalence ratio. Both the indicated thermal efficiency and power output were less for acetylene fuelling than for gasoline. Acetylene combustion occurred at sufficiently lean equivalence ratios to produce very low NO_x and CO emissions. However, when the low NO_x levels were achieved hydrocarbon control was not improved over that with gasoline. Despite the potential for NO_x control demonstrated in this study of acetylene fuelling, difficulties encountered with engine knock and preignition plus well-known safety problems (wide flammability limits and explosive decomposition) associated with acetylene render this fuel impractical for spark ignition engines.     

双点火及补气对摩托车发动机性能影响的研究

针对摩托车发动机经济性差、排放高的问题,笔者在一台加装了双火花塞和进气补气系统的单缸摩托车发动机上,在不同转速条件下,就双火花塞点火和补气对内燃机燃烧与排放特性的影响进行了试验研究。试验结果表明,采用双点火配合进气补气的方法可以有效降低发动机CO、HC排放及比燃油消耗率。在转速为3500r.min-1的条件下,采用双点火配合进气补气的发动机比燃油消耗率较原机降低约12.5%。在发动机转速高于3000r.min-1时,采用双点火也可以在一定程度上改善发动机经济性并降低CO和HC排放。但NOx排放在双点火条件下较原机略有升高。     

Effect of ethanol–gasoline blends on engine performance and exhaust emissions in different compression ratios

Renewable energy sources for the gasoline engines alcohols gain importance recently. These renewable energy sources have attracted the attention of researchers as alternative fuel due to their high octane number. In addition, these are also clean energy sources and can be obtained from the biomass alcohols with low carbon like ethanol. In this study, the effect of compression ratio on engine performance and exhaust emissions was examined at stoichiometric air/fuel ratio, full load and minimum advanced timing for the best torque MBT in a single cylinder, four stroke, with variable compression ratio and spark ignition engine.     

Emissions reductions using hydrogen from plasmatron fuel converters

Improvements in internal combustion engine and aftertreatment technologies are needed to meet future environmental quality goals. Systems using recently developed compact plasmatron fuel converters in conjunction with state-of-the-art engines and aftertreatment catalysts could provide new opportunities for obtaining substantial emissions reductions. Plasmatron fuel converters provide a rapid response, compact means to transform a wide range of hydrocarbon fuels (including gasoline, natural gas and diesel fuel) into hydrogen-rich gas. Hydrogen-rich gas can be used as an additive to provide NO_x reductions of more than 80% in spark ignition gasoline engine vehicles by enabling very lean operation or heavy exhaust engine recirculation. It may also be employed for cold start hydrocarbon reduction. If certain requirements are met, it may also be possible to achieve higher spark ignition engine efficiencies (e.g., up to 95% of those of diesel engines). These requirements include the attainment of ultra lean, high compression ratio, open throttle operation using only a modest amount of hydrogen addition. For diesel engines, use of compact plasmatron reformers to produce hydrogen-rich gas for the regeneration of NO_x absorber/adsorbers and particulate traps for diesel engine exhaust aftertreatment could provide significant advantages. Recent tests of conversion of diesel fuel to hydrogen-rich gas using a low current plasmatron fuel converter with non-equilibrium plasma features are described.     

在汽油机上实施HCCI的技术策略

均质混合气压燃(HCCI)燃烧方式，是一种克服常规柴油机和汽油机缺点、集常规汽油机和柴油机优点于一体的新概念燃烧。本文分析了汽油机实施HCCI的可行性，介绍了HCCI发动机实用化所面临的问题，提出了双工作模式的折衷方案：在中低负荷工况实施HCCI，而在大负荷工况和冷起动工况恢复常规发动机工作方式。推荐可变压缩比(VCR)方案、可变废气再循环率(EGR)方案、可变排气门关闭时刻方案，以及废气再循环滚流分层充气方案等。为尽快在汽油机上实施HCCI燃烧方式指出了技术方向。     

利用快速压缩膨胀机研究火花点火LPG发动机的燃烧过程

利用快速压缩膨胀机对火花点火LPG发动机的燃烧过程进行了模拟与测量，分析了混合气浓度，点火提前角，压缩比以及燃烧室形状等对LPG燃烧过程的影响，可供汽油机改装成汽油／LPG双燃料发动机的参考。     

Analysis of performance parameters of an ethanol fueled spark ignition engine operating with hydrogen enrichment

Concerns with the environment and energy security have increased interest in phasing out fossil fuels in the automotive industry, as it transitions from conventional internal combustion engines (ICE) to electric and fuel cell powertrains. During this transition, ethanol is of particular interest as a renewable fuel option in ICE, despite drawbacks compared to gasoline. Adding hydrogen to ethanol could remedy the disadvantages associated with ethanol, while maintaining the benefits of using renewable fuels. There is a gap in the literature of both experimental and numerical studies considering hydrogen addition in turbocharged ethanol engines. Therefore, this paper presents an experimental and numerical study of a turbocharged ethanol engine operating with hydrogen enrichment at stoichiometric conditions under boosted conditions. It was concluded that hydrogen addition allowed spark ignition engines to achieve lower brake specific energy consumption, better performance, and lower emissions. Thus, after proper calibration, a simulation model was created and shown to be a suitable tool to predict engine performance of a spark ignition engine operating with hydrogen enrichment and reduce the overall number of experimental tests needed to tune engines operating with this fuel blend. Finally, some operating strategies are recommended based on these findings.     

Experimental study of hydrogen enriched compressed natural gas (HCNG) engine and application of support vector machine (SVM) on prediction of engine performance at specific condition

The effect of excess air ratio (λ) and ignition advance angle (θ_ig) on the combustion and emission characteristics of hydrogen enriched compressed natural gas (HCNG) on a 6-cylinder compressed natural gas (CNG) engine has been experimental studied in an engine test bench, aiming at enriching the sophisticated calibration of HCNG fueled engine and increasing the prediction accuracy of the SVM method on automobile engines. Three different fuel blends were selected for the experiment: 0%, 20% and 40% volumetric hydrogen blend ratios. It is noted that combustion intensity varies with the excess air ratio and the ignition advance angle, so are the emissions. The optimal value of λ or θ_ig has been explored in the specific engine condition. Results show that blending hydrogen can enhance and advance the combustion and stability of CNG engine, and it also has some benefic influence on the emissions such as reducing the CO and CH₄. Meanwhile, a simulation research on forecasting the engine performance by using the support vector machine (SVM) method was conducted in detail. The torque, brake specific fuel consumption and NO_x emission have been selected to characterize the power, economic and emissions of the engine with various HCNG fuels, respectively. It can be seen that the optimal model built by the SVM method can highly describe the relationship of the engine properties and condition parameters, since the value of the complex correlation coefficient is larger than 0.97. Secondly, the prediction performance of the optimal model for torque or BSFC is much better than the case of NO_x. Besides, the optimal model built by the PSO optimization method has the best prediction accuracy, and the accuracy of the model obtained based on the training group with 20% hydrogen blend ratio is the best compared with those of others. The upshots in this article provide experimental support and theoretical basis for the adoption of HCNG fuel on internal combustion engines as well as the application of intelligent algorithmic in the engine calibration technology field.     

Performance,combustion and emission characteristics of a spark‐ignition engine with simultaneous injection of n‐butanol and gasoline in comparison to blended butanol and gasoline

Simultaneous injection of n‐butanol and gasoline through a new system of two injectors directing the sprays towards the back of the intake valve in a spark‐ignition engine was tried in lieu of injecting a blend of these fuels through a single injector. This system avoids the problem of phase separation, which is generally faced during the use of alcohol‐gasoline blends. Experiments were conducted on a spark‐ignition engine with this dual injection system using a fuel ratio of 1:1 (B50S) on the mass basis. High‐speed photographs indicated that the sprays from the injectors did not interfere till they reached the intake valve. Comparisons were made with pre‐blended butanol‐gasoline (B50) and neat (100%) gasoline at the best spark timing. All injection and spark parameters were controlled using a real time engine controller. Neat n‐butanol (B100) was superior only near full throttle with improved efficiency of the engine of about 1.2% (absolute). Heat release rates were observed to be higher and more advanced with B100 at wide open throttle. However, a reverse of this trend was observed at the throttle position of 15%. NO emission was also lower by 30% with B100 at wide open throttle as compared with gasoline. However, a small increase in carbon monoxide (CO) levels was observed because of lower post combustion temperatures as compared with gasoline and B50S. Simultaneous injection reduced hydrocarbon (HC) emissions by 13% to 50% as compared with B50 (blended fuel). HC emissions with gasoline and B50S were similar. Nitric oxide (NO) emission was lower with B50S as compared with gasoline; however it was higher than B50 because of better combustion. On the whole, the developed dual injection system was superior to the conventional method of blending in terms of performance, emissions and ability to change the fuel ratio as needed. B50S is suitable at all throttle positions, whereas B100 shows benefits at full throttle conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

The safe operation zone of the spark ignition engine working with dual renewable supplemented fuels (hydrogen+ethyl alcohol)

The effect of the amount of hydrogen/ethyl alcohol addition on the performance and pollutant emission of a four-stroke spark ignition engine has been studied. The results of the study show that all engine performance parameters have been improved when operating the gasoline spark ignition engine with dual addition of hydrogen and ethyl alcohol. The important improvements of alcohol addition are to reduce the NOx emission while increasing the higher useful compression ratio and output power of hydrogen-supplemented engine. An equation has been derived from experimental data to specify the least quantity of ethyl alcohol blended with gasoline and satisfying constant NOx emission when hydrogen is added. A chart limiting the safe operation zone of the engine fueled with dual renewable supplemented fuel, (hydrogen and ethyl alcohol) has been produced. The safe zone provides lower NOx and CO emission, lower s.f.c. and higher brake power compared to an equivalent gasoline engine. When ethyl alcohol is increased over 30%, it causes unstable engine operation which can be related to the fact that the fuel is not vaporized, and this causes a reduction in both break power and efficiency.     

Fuel conversion efficiency of a port injection engine fueled with gasoline–isobutanol blends

This paper describes the comparative advantages of using isobutanol as a fuel for SI (spark ignition) engines instead of ethanol. An experimental study of fuel conversion efficiency was performed on a port injection engine fueled with mixtures containing 10, 30 and 50% isobutanol blended with gasoline. Efficiency as well as performance levels were maintained within acceptable limits for all three types of fuel blends compared to running the engine on straight gasoline. These results show that isobutanol is an attractive drop-in fuel for SI engines, and can be blended with gasoline in much higher concentrations compared to ethanol, without any modifications to the fuel system or other engine components.     

Influence of hydrogen peroxide emulsification with gasoline on the emissions and performance in an MPFI engine

Spark ignition (SI) engines have been a major contributor from the transportation sector towards the increased emissions to the environment. Modifications to the SI engine like structural modifications, pre, and post-combustion treatments have been investigated in the literature. The use of oxygenated additives to gasoline fuel has been major research interest in curbing the emissions without any significant loss in engine performance. Hydrogen peroxide (H₂O₂) has not been investigated as an additive in SI engines although its effect is demonstrated for compression ignition (CI) engines. This paper aims to address this gap by ascertaining the influence of H₂O₂ concentration on SI engine emissions and performance. H₂O₂ is varied from 0 to 1.5% and the engine speed varied from 1500 to 3000 rpm by operating at a constant load. A total of 16 trials (with three replicates) is carried out. The output responses are brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), emissions of CO, CO₂, HC, and NOx. Artificial neural networks are adopted to ascertain the relation between the inputs and the output responses. Emulsifying gasoline with 1.5% H₂O₂ resulted in an average reduction of CO and HC emissions by 21.1% and 28.6% respectively with an overall average of 25.3% of reduction in the NOx. The average BTE at all engine speeds increases from 21.6% for G0 to 23.8% for G1.5 and an overall average of 10.5% reduction in BSFC is obtained. The study shows that H₂O₂ can be employed as an emulsifier to gasoline fuel, however, more rigorous studies are required to ascertain its impact, volatility, and storage.     

Experimental study of stratified lean burn characteristics on a dual injection gasoline engine

Due to increasingly stringent fuel consumption and emission regulation, improving thermal efficiency and reducing particulate matter emissions are two main issues for next generation gasoline engine. Lean burn mode could greatly reduce pumping loss and decrease the fuel consumption of gasoline engines, although the burning rate is decreased by higher diluted intake air. In this study, dual injection stratified combustion mode is used to accelerate the burning rate of lean burn by increasing the fuel concentration near the spark plug. The effects of engine control parameters such as the excess air coefficient (Lambda), direct injection (DI) ratio, spark interval with DI, and DI timing on combustion, fuel consumption, gaseous emissions, and particulate emissions of a dual injection gasoline engine are studied. It is shown that the lean burn limit can be extended to Lambda= 1.8 with a low compression ratio of 10, while the fuel consumption can be obviously improved at Lambda= 1.4. There exists a spark window for dual injection stratified lean burn mode, in which the spark timing has a weak effect on combustion. With optimization of the control parameters, the brake specific fuel consumption (BSFC) decreases 9.05% more than that of original stoichiometric combustion with DI as 2 bar brake mean effective pressure (BMEP) at a 2000 r/min engine speed. The NO_x emissions before three-way catalyst (TWC) are 71.31% lower than that of the original engine while the particle number (PN) is 81.45% lower than the original engine. The dual injection stratified lean burn has a wide range of applications which can effectively reduce fuel consumption and particulate emissions. The BSFC reduction rate is higher than 5% and the PN reduction rate is more than 50% with the speed lower than 2400 r/min and the load lower than 5 bar.     

Hydrogen–ethanol blending as an alternative fuel of spark ignition engines

The performance and pollutant emission of a four-stroke spark ignition engine using hydrogen–ethanol blends as fuel have been studied. The tests were performed using 2, 4, 6, 8, 10 and 12 mass% hydrogen–ethanol blends. Gasoline fuel was used as a basis for comparison. The effect of using different blends of hydrogen–ethanol on engine power, specific fuel consumption, CO and NO_x emission was studied. Operating test results for a range of compression ratio (CR) and equivalent ratio are presented. The results show that the supplemental hydrogen in the ethanol–air mixture improves the combustion process and hence improves the combustion efficiency, expands the range of combustibility of the ethanol fuel, increases the power, reduces the s.f.c., and reduces toxic emissions. The important improvement of hydrogen addition is to reduce the s.f.c. of ethanol engines. Results were compared to those with gasoline fuel at 7 CR and stoichiometric equivalence ratio.     

Emission components characteristics of a bi-fuel vehicle at idling condition

Natural gas (NG) represents today a promising alternative to conventional fuels for road vehicles propulsion, since it is characterized by a relatively low cost, better geopolitical distribution than oil, and lower environmental impact. This explains the current spreading of compressed natural gas (CNG) fuelled spark ignition (SI) engine, above all in the bi-fuel version, which is able to run either with gasoline or with NG. However, the aim of the present investigation is to evaluate the emission characteristics at idling condition. The vehicle engine was converted to bi-fueling system from a gasoline engine, and operated separately either with gasoline or CNG. Two different fuel injection systems (i.e., multi-point injection (MPI)-sequential and closed-loop venturi-continuous) are used, and their influences on the formation of emissions at different operating conditions are examined. A detailed comparative analysis of the engine exhaust emissions using gasoline and CNG is made. The results indicate that the CNG shows low air index and lower emissions of carbon monoxide (CO), carbon dioxide (CO₂), and total hydrocarbon (THC) compared to gasoline.     

Performance and emission assessment of a multi-cylinder S.I engine using CNG & HCNG as fuels

The present study was carried out to assess the possibility of using the HCNG in the commercially available CNG vehicles, as the available literature indicated the benefits of adding hydrogen to CNG in small percentages by volume, leading to improved combustion characteristics of CNG and yielding sizeable benefits, regarding improved engine performance and reduced engine emissions in automotive applications. In the present study, a commercially available CNG manifold carburation kit, commonly known as “sequential injection” in the market, is evaluated for its operation characteristics, on a Spark Ignited (SI), MPFI automotive engine, of a mass-produced passenger vehicle, converted for gas operation, using, gasoline, CNG, HCNG 10% and HCNG 18% as fuels. In the study, the following performance parameters, torque, power, thermal efficiency, brake specific energy consumption (BSEC), lambda, engine oil temperature, exhaust gas species were measured. After exhaustive engine testing, a comparison of engine performance emission characteristics for gasoline, CNG and HCNG 10% and HCNG 18% is presented. The engine performance using the optimized MAP tables demonstrated torque and power improvements for HCNG 10% and HCNG 18% in comparison to CNG. The torque benefits up-to 6% and power benefits up-to 4% were observed. The fuel energy consumption was measured to be reduced, and improvement in fuel conversion efficiency was also observed. Hydrogen substitution in CNG helped in reducing CO, HC, CO₂ emissions for HCNG in comparison to CNG. Increase in NO_x emission was observed for HCNG in comparison with CNG. Superior engine emission characteristics in comparison to gasoline and CNG is also demonstrated. The commercially available sequential gas manifold carburation was found to be suitable for HCNG 10% and HCNG 18%.     

Experimental investigation of varying composition of HCNG on performance and combustion characteristics of a SI engine

With rapid depletion of petroleum resources, researchers are investigating alternate fuels to meet global transportation energy demand. Gaseous fuels such as compressed natural gas (CNG) and hydrogen are of special interest because of their cleaner combustion characteristics compared to liquid petroleum based fossil fuels. However both these gaseous fuels have some technical issues when they are used as stand-alone alternate fuel in conventional spark ignition (SI) engines. CNG suffers from lower energy density and narrow flammability range whereas backfiring tendency is highly pronounced in hydrogen fueled engines. Hydrogen enriched compressed natural gas (HCNG) mixtures are observed to be good alternative to these individual fuels since these mixtures do not pose the issues experienced by the constituent fuels i.e. CNG and hydrogen. In this study, experiments were conducted in a spark ignited gas engine using various compositions of HCNG mixtures having 0, 10, 20, 30, 50, 70 and 100% (v/v) hydrogen fraction. The performance and combustion characteristics of these test fuels were compared with that of baseline CNG, in order to find an optimum HCNG mixture composition for a single cylinder gas engine. Results obtained showed that 30HCNG mixture delivered superior engine performance compared to other HCNG mixtures and baseline CNG, which is in sharp contrast to 15HCNG being advocated globally.     

Effects of intake air temperature on SI engine emissions during a cold start

This paper presents the concept of preheating the intake air to reduce cold-start emissions from gasoline engines. The effects of intake air temperature on emissions from a gasoline engine were studied by using an air heater based on spark ignition. A light-duty vehicle test of cold-start emissions was carried out at an ambient temperature of?7°C according to New European Driving Cycle for Euro 3 and Euro 4 exhaust emission legislations. The results showed that preheating the intake air could effectively reduce both hydrocarbon (HC) and carbon monoxide (CO) emissions and improve fuel economy during a cold start. During idling conditions, the key phase of the HC and CO emissions was the first 40 s. With the aid of the air heater, cold-start HC and CO emissions from the vehicle were lower than the limit values in the Euro 3 and Euro 4 regulations.