Numerical analysis of performance, combustion, and emission characteristics of PFI gasoline, PFI CNG, and DI CNG engine

The paper put forwards and designs high-pressure common-rail fuel injection system based on technology characteristics of in-cylinder and direct-injection compressed natural gas (CNG) engine, and a model of high-pressure common-rail for CNG engine is built with AMEsim software. For researching the dynamic characteristics of high-pressure common-rail system of a CNG engine, the influences of speed, common rail volume, aspect ratio and rail pressure on pressure fluctuation amplitude were synthetically analyzed. At the controlling aim of pressure fluctuation amplitude, a compound controller based on cerebellar model articulation controller(CMAC) neural network and PD is presented and finally the model of control system was built with MATLAB/Simulink. The co-simulation results show that the simulation system can simulate the actual working process of the high-pressure common-rail system for CNG engine and that the used control strategy can meet the demands of flexible control of fuel injection quantity and pressure. Accordingly, the system lays the foundation for researching high-pressure direct injection CNG engine.

A study of spray development and combustion propagation processes of spark-ignited direct injection (SIDI) compressed natural gas (CNG)

Compressed natural gas (CNG) is regarded as one of the most promising alternative fuels, is widely used for the automobile engines. In order to improve the thermal efficiency of CNG engine, the direct-injection (DI) technology has been adopted. The stratified CNG mixture can be ignited by the spark plug which is installed near the injector nozzle, so the lean combustion can be subsequently achieved. For the direct-injection spark-ignition (DISI) engine, the reliable ignition is a foundational problem for the reliable operation, so it is very important to study on the spray characteristics of DISI engine. In our study, a combustion chamber is designed and a visualization system is built. The DI CNG spray's injection process was digital recorded with the schlieren optical system under different experiment conditions. The spray characteristics of the DI CNG engine were analyzed with the experimental results.

Effect of compression ratio on engine knock, performance, combustion and emission characteristics of a bi-fuel CNG engine

This paper presents the experimental results on brake power enhancement of a modified natural gas engine. Conventional natural gas engine produces 15 to 20% less brake power than that of gasoline fired engine. This lacking in compressed natural gas (CNG) engine needs to be recovered through modifications and optimization of fuel injection system to provide complete gas combustion in the engine cylinder. A multi-cylinder gasoline engine was modified to bi-fuel engine for operating in several test conditions, such as constant full and half throttle condition. Variation in power production with corresponding fuel flow rate and emission gases (such as carbon monoxides, CO; unburnt hydrocarbon, HC and nitrogen oxides, NOx) were studied by using two fuels (gasoline and CNG). Engine tune up information like variation in air fuel ratio (AFR) for lean burn operation can be known from those tests. Performance of engine was studied with both the fuels at fixed load and the corresponding fuel flow rate and emissions were measured for evaluation and optimization. The engine produced 10% higher brake power with CNG fuel as compared to that produced with gasoline fuel at full load, but at partial load gasoline fuel produced more brake power than CNG. Emission results revealed that CNG fuelled engine emits less CO and HC, showing more complete combustion than gasoline fuel. On the contrary, higher combustion temperature of CNG fuel produced more NOx than gasoline. The results of these investigations can be used to develop a new compressed natural gas (CNG), direct injection (DI) and higher efficiency engine in the near future to build an environment friendly fuel economic clean burning automotive vehicle with less emission and similar power rating like gasoline engine. Key words: Brake power, compressed natural gas-direct injection (CNG-DI), bi-fuel engine, fuel consumption, emission.

Particle number (PN) emissions from gasoline, diesel, LPG, CNG and hybrid-electric light-duty vehicles under real-world driving conditions

<div class="section abstract"><div class="htmlview paragraph">Today, passenger car makers around the world are striving to meet the increasing demand for fuel economy, high performance, and silent engines. Corporate Average Fuel Economy (CAFE) regulations implemented in India to improve the fuel efficiency of a manufacturer's fleet of vehicles. CAFE goal is to reduce fuel consumption and, by extension, the emissions that contribute to climate change. CNG (Compressed Natural Gas) engines offer several advantages that help manufacturers meet and exceed these standards.</div><div class="htmlview paragraph">The demand for CNG vehicles has surged exponentially in recent years, CNG engine better Fuel efficiency and advantage in CAFÉ norms make good case for OEM & Customer to use more CNG vehicle. CNG is dry fuel compared to gasoline. These dry fuels lack lubricating properties, unlike conventional fuels like petrol, diesel and biofuels, which are wet and liquid. Consequently, the operations and failures associated with these fuels differ. The materials and designs of engine parts, such as fuel lines, ECU, exhaust valves, and cylinder heads, vary depending on the fuel used.</div><div class="htmlview paragraph">In CNG engine most challenging issue is leakage at Valve seat and Valve face interface causing unstable combustion and power drop. This study discusses the countermeasures adopted to address the high valve face, Valve guide and valve seat wear in cylinder heads and engine valves. The investigation focuses on material, design, and manufacturing process improvements specially for Exhaust valve, supported by part-level and vehicle-level validation and testing for CNG exhaust valves and cylinder heads.</div></div>

Hydrogen is an attractive fuel source not only because it is abundant and renewable but also because it produces almost zero regulated emissions. Internal combustion engines fueled by compressed natural gas (CNG) are operated throughout a variety of industries in a number of mobile and stationary applications. While CNG engines offer many advantages over conventional gasoline and diesel combustion engines, CNG engine performance can be substantially improved in the lean operating region. Lean operation has a number of benefits, the most notable of which is reduced emissions. However, the extremely low flame propagation velocities of CNG greatly restrict the lean operating limits of CNG engines. Hydrogen, however, has a high flame speed and a wide operating limit that extends into the lean region. The addition of hydrogen to a CNG engine makes it a viable and economical method to significantly extend the lean operating limit and thereby improve performance and reduce emissions. Drawbacks of hydrogen as a fuel source, however, include lower power density due to a lower heating value per unit volume as compared to CNG, and susceptibility to pre-ignition and engine knock due to wide flammability limits and low minimum ignition energy. Combining hydrogen with CNG, however, overcomes the drawbacks inherent in each fuel type. Objectives of the current study were to evaluate the feasibility of using blends of hydrogen and natural gas as a fuel for conventional natural gas engines. The experiment and data analysis included evaluation of engine performance, efficiency, and emissions along with detailed in-cylinder measurements of key physical parameters. This provided a detailed knowledge base of the impact of using hydrogen/natural gas blends. A four-stroke, 4.2 L, V-6 naturally aspirated natural gas engine coupled to an eddy current dynamometer was used to measure the impact of hydrogen/natural gas blends on performance, thermodynamic efficiency and exhaust gas emissions in a reciprocating four stroke cycle engine. The test matrix varied engine load and air-to-fuel ratio at throttle openings of 50% and 100% at equivalence ratios of 1.00 and 0.90 for hydrogen percentages of 10%, 20% and 30% by volume. In addition, tests were performed at 100% throttle opening, with an equivalence ratio of 0.98 and a hydrogen blend of 20% to further investigate CO emission variations. Data analysis indicated that the use of hydrogen/natural gas fuel blend penalizes the engine operation with a 1.5 to 2.0% decrease in torque, but provided up to a 36% reduction in CO, a 30% reduction in NOX, and a 5% increase in brake thermal efficiency. These results concur with previous results published in the open literature. Further reduction in emissions can be obtained by retarding the ignition timing.

Compressed natural gas (CNG) engines normally operate in lean condition to take the advantage of higher efficiency and better fuel economy. Several studies have shown that gas-jet ignition with two-stage injection technique is effective to extend the lean combustible range of CNG engines. This paper investigates the effectiveness of such technique using a prototype lean burn direct injection CNG engine. The experiment was conducted at speed of 900 rpm, fuel injection pressure of 3 MPa, equivalence ratio φ=0.8, and ignition timing at top dead center. The effect of first injection timing on the test engine performance and exhaust emission was analyzed. The result shows that the first injection timing is crucial in determining the performance of the engine. First injection timings when the piston is near to bottom dead center produced relatively stable combustion. First injection timings when the piston is at midpoint produced misfire. First injection timings near the gas-jet ignition produced unstable combustion except at a certain timings which produced acceptable combustion with low hydrocarbon and carbon monoxide emissions.

Natural gas is well-suited as a fuel in the transport sector. Due to its excellent combustion characteristics, engines operating with compressed natural gas (CNG) reach high efficiency, especially if operated at lean conditions. However, CNG engine research mainly focusses on stoichiometric conditions in order to use a three-way catalytic converter for the exhaust gas after treatment system. With the objective to explore the potential of CNG engines operated at lean conditions, a turbo-charged CNG engine with high com-pression ratio is developed and optimized for lean operation. In order to increase the ignition energy, the CNG engine is equipped with scavenged pre-chambers. A specific control structure is developed, which allows to operate the engine at a pre-defined (lean) air-to-fuel ratio. Further functionalities such as the combustion placement control and algorithms to estimate the conditions inside of the pre-chamber are implemented. The first part of this paper describes this engine control structure, which is specifically developed for the lean-burn CNG engine. In the second part, the effects of pre-chamber scavenging on engine performance criteria such as the combustion stability, engine efficiency or engine emissions are analyzed. With the objective to use pre-chamber scavenging to improve engine performance, a scavenging feed-back control strategy is proposed. In order to control the ignition delay, this strategy adapts the amount of CNG injected into the pre-chamber with a linear controller or an extremum seeking algorithm depending on the air-to-fuel ratio of the main chamber.

The diesel engine converted to compressed natural gas (CNG) engine effect is lower in performance. Problem statement: The hypothesis is that the lower performance of CNG engine is caused by the effect of lower in engine cylinder pressure. Are the CNG engine is lower cylinder pressure than diesel engine? This research is conducted to investigate the cylinder pressure of CNG engine as a new engine compared to diesel engine as a baseline engine. Approach: The research approach in this study is by convert the diesel engine to multi point injection dedicated CNG engine. The engine conversion is by changed the diesel fuel to CNG fuel, changed the injection fuel system, changed the ignition system, modified piston to reduce the compression ratio and added throttle to in intake port. If the development is completed, the engine cylinder pressure is investigated. In this study, the cylinder pressure is investigated in 7 cases engine speed from 1000 to 4000 rpm with range in 500. Results: The research results are cylinder pressure and maximum pressure of CNG engine compared to diesel engine in 1000 to 4000 rpm engine speed. Conclusion/Recommendations: Effect of diesel engine converted to CNG engine is decrease the cylinder pressure. The further research is needed to find the higher performance of CNG engine.

Toxicity and mutagenicity of exhaust from compressed natural gas: Could this be a clean solution for megacities with mixed-traffic conditions?

<div class="htmlview paragraph">The increasing demand of passenger fleet with the depletion of conventional fuels has resulted into a massive research for harnessing the potentials in alternate fuels. Vis-à-vis, compressed natural gas (CNG) has been a proven clean automotive fuel and is playing a vital role towards the development of new engine concepts or in terms of retrofits. With vast CNG reservoirs present in over 90 countries worldwide, CNG as an alternate auto fuel has been drawing a greater attention.</div> <div class="htmlview paragraph">CNG has a very high octane rating and hence can be operated to run under the Highest Useful Compression Ratio (HUCR), thus seeming more applicable under both SI and CI mode. CNG engines desire a compact and turbulent combustion chamber owing to its lower laminar flame propagation characteristics and hence desire proper mixing of air and fuel as well for the flame to propagate to the end charge, making it inevitable to develop piston geometries that can develop turbulence inside the combustion chamber. Thus, in an attempt to develop a dedicated CNG engine technology based on a series of experimental work and design carried out in the recent past, the present work is a carry forward of the experimentations conducted on a single cylinder 4 stroke cycle, 100 cc gasoline engine retrofit to run on CNG with successive modifications.</div> <div class="htmlview paragraph">Three major aspects viz. the ignition voltage, compression ratio and the piston crown geometry is investigated in this report to evaluate the performances and emission characteristics on the engine and a comparison with gasoline is also made with the results obtained during the recent developmental stages. The authors found a major achievement in both performances as well as emissions on this 100 cc category of the Indian market. The performances improved by 20 - 30 % under certain cases with a considerable reduction in emissions in CNG itself and presently this system able to compete the conventional gasoline engine with reduced emissions.</div>

A comparative energy and exergy analysis was experimentally performed using a bi‐fuel compressed natural gas (CNG) spark ignition engine. The experiments were conducted for gasoline and CNG fuel at 1700 rpm and different operating loads from 5 to 30 Nm. The experiments were performed under stoichiometric air‐fuel ratio and maximum brake torque ignition timing. Quantitative and qualitative analysis were conducted using the first and second law of thermodynamics, respectively. The effect of engine operating load on various energy and exergy parameters was compared for both the fuels. Output energy with respect to engine load was found higher for CNG compared to gasoline. Engine wall heat transfer was also higher in the CNG engine case due to its high combustion chamber temperature and lower burning velocity. The difference in heat transfer energy fraction between gasoline and CNG gradually increased with engine load. The exhaust energy was found maximum in the case of CNG under low operating load and reduced to a minimum at higher engine operating load. The unaccounted energy fraction was found lower for the CNG engine and reduced with respect to engine load. CNG exhibits the highest exergy efficiency (26.80%) compared to gasoline (25.50%) at 30 Nm load. On average, a 2% higher exergy efficiency was observed with the CNG engine at all operating load conditions. This indicates CNG has a higher potential to convert chemical energy present in the fuel into useful work output. CNG shows lower exergy destruction at all operating loads compared to gasoline, and it reduces significantly at higher engine loads. In all operating loads, exergy transfer due to exhaust gas and heat transfer to the wall was higher in the case of CNG.

Since the pressure development in a combustion chamber is uniquely related to the combustion process, substantial variations in the combustion process on a cycle-by-cycle basis are occurring. To this end, an experimental study of cycle-by-cycle variation in a direct injection spark ignition engine fueled with natural gas- hydrogen blends combined with exhaust gas recirculation at relative air-fuel ratios was conducted. The impacts of relative air-fuel ratios (i.e. λ = 1.0, 1.2, 1.3 and 1.4 which represent stoichiometric, moderately lean, lean and very lean mixtures respectively), hydrogen fractions and EGR rates were studied. The results showed that increasing the relative air-fuel ratio increases the COVIMEP. The behavior is more pronounced at the larger relative air-fuel ratios. More so, for a specified EGR rate; increasing the hydrogen fractions decreases the maximum COVIMEP value just as increasing in EGR rates increases the maximum COVIMEP value. (i.e. When percentage EGR rates is increased from 0% to 17% and 20% respectively. The maximum COVIMEP value increases from 6.25% to 6.56% and 8.30% respectively). Since the introduction of hydrogen gas reduces the cycle-by-cycle combustion variation in engine cylinder; thus it can be concluded that addition of hydrogen into direct injection compressed natural gas engine employing EGR at various relative air-fuel ratios is a viable approach to obtain an improved combustion quality which correspond to lower coefficient of variation in imep, (COVIMEP ) in a direct injection compressed natural gas engine employing EGR at relative air-fuel ratios. Keywords—Coefficient of variation in indicated mean effective pressure (COVIMEP); Direct injection compressed natural gas (DI-CNG); Exhaust gas recirculation (EGR); Hydrogen fractions; Relative air-fuel ratios.