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Methanol is a competitive low-carbon fuel for spark ignition engines. Though it has higher knock resistance than gasoline fuel, knocking combustion is still an obstacle for the application of methanol in spark ignition engines with relatively higher compression ratios. In this work, experimental investigations into knocking combustion and suppression were conducted on a single-cylinder, naturally aspirated, SI engine with a compression ratio of 15. Firstly, the knocking characteristics of the methanol engine with stoichiometric mixture under port fuel injection mode were evaluated. Results showed that knocking combustion occurred under medium and high loads. Especially, under the condition of indicated mean effective pressure (IMEP) = 0.7 MPa, some random heavy knock cycles with ultra-high knock intensity were observed. The frequency of heavy knock was 2.5% and the highest maximum amplitude of pressure oscillations reached 2.39 MPa. This heavy knock was similar to the super-knock phenomenon in boosted direct injection gasoline engines, and it can not be eliminated by retarding spark timing or adopting fuel-lean combustion. Then, the mechanism of the heavy knock was studied by thermodynamic condition analysis. Results showed that the heavy knock was triggered by pre-ignition which was induced by some hot spots in the combustion chamber. Finally, to make the best use of the charge cooling effect, the methanol direct injection strategy was applied to suppress the knocking combustion. When the engine worked with full opening throttle and adopt a single injection, the maximum knock intensity decreased to 0.43 MPa×°CA at −350 °CA ATDC start of injection which was 5.1% of the maximum value during port fuel injection mode. With the co-optimization of the start of injection and spark timing, the engine with a stoichiometric mixture could operate within the knock intensity limit at full load. Under this operating condition, the crank angle of 50% mass fraction burned located at 12 °CA ATDC and IMEP = 1.05 MPa was achieved. What’s more, adopting the optimal split injection instead of a single injection, the knock intensity was further reduced by 14.3% with a slight reduction of 0.02 MPa in IMEP.

Cooled exhaust gas recirculation (EGR) is one of the most effective methods in spark ignition (SI) natural gas (NG) engines to improve the fuel economy and reduce the nitrogen oxides (NOx) emissions. The objective of this study is to anatomize the EGR rate and temperature impact on the combustion and emissions characteristics by the experiments in a heavy duty (HD) NG engine under various spark timings. The results showed that the increasing EGR rate reduced the burning rate and extended the combustion duration. Spark timing should be optimized to copy with the retarded combustion phase. Under high EGR rate, the combustion performance could be optimized with the increase of EGR temperature. There exists a proper EGR rate in real application for the fuel consumption of the engine. Higher EGR temperature plays positive effects on fuel economy, owing to the higher combustion efficiency and lower throttle losses, and the decreasing amplitude of BSFC is 1.25% between the EGR temperatures of 333 K and 373 K. With the increase of EGR rate from 5% to 25%, NOx emissions are reduced by up to 76%, the unburned hydrocarbon (HC) and carbon monoxide (CO) emissions are increased by 34.6% and 12.9% respectively. Higher EGR temperature could alleviate the increase of HC and CO emissions, but sacrificing the NOx emissions, and the increase extent of NOx emissions is 35.3%. The results provide an insight into the quantitative comparisons of EGR rate and temperature on the NG engine performance and emissions, which promotes the utilizing EGR strategy in SI NG engines.

The closed-loop combustion control is an effective way to reduce the cycle-to-cycle variations of spark ignition (SI) engine. In this paper, a series of experimental tests were conducted to investigate the combustion performance and emissions characteristics in a six-cylinder natural gas (NG) engine. The correlation between combustion phasing and control parameters was analyzed under different EGR rates and spark timing (ST) at medium and low loads. Additionally, a prediction model on the basis of the correlation was developed employing artificial neural network (ANN) for a sensor-less closed-loop combustion control. After analyzing the results, it is found that a linear relationship exists between CA 50 and ST under various EGR rates. Engine out emissions (i.e., CO, THC, and CH4) are insensitively affected by the CA50. It is observed that the NOx emission is significantly higher in comparison to those under the higher EGR rate. As a consequence, CA50 is acting as the feedback signal for closed-loop combustion control with consideration of cylinder consistency. Moreover, the ANN model was proven to be an impressive way to predict the combustion characteristics, which can substitute the cylinder pressure sensor. Moreover, the prediction model can be applied in a sensor-less closed-loop combustion control, while the correlation coefficient value can reach up to approximately 1.00.

The particulate matter emissions have seriously negative effect on the human health, especially the ultrafine particulates and an experiment study has been conducted to evaluate the particulate emissions characteristics in a diesel/natural gas dual-fuel engine. In this paper, the particulate mass (PM) and particulate number (PN) concentration as well as particulate size distributions (PSD) characteristics under different natural gas-air mixture state operation conditions are investigated. The earlier (−500°CA ATDC, −480°CA ATDC) and later (−260°CA ATDC, −240°CA ATDC) natural gas injection timings are applied to form a homogenous and stratification natural gas-air mixture and the air fuel ratio (AFR) are maintained at a constant value during experiments. The results indicated that the natural gas-air mixture state have significant influence on the combustion process. The PN, especially ultrafine particulate emissions, is sensitive to the natural gas-air mixture state and the stratified-like mixture is benefit to reduce the PN of smaller size under low load. Moreover, in the diesel/natural gas dual-fuel engine, over 60% of the PN emissions are the ultrafine particulate while it just contributes to no more than 10% in the PM emissions.

In this study, the lean-burn characteristics of a high-compression direct injection spark ignition (DISI) methanol engine were investigated. The tests were conducted on a modified diesel engine in methanol DISI mode at three excess air ratios (1.2, 1.3, and 1.4) and injection timings from −60° to −300° crank angle (CA) after top dead center (ATDC). The engine was run at 1500 rpm, 100% throttle opening, and spark timing at maximum brake torque condition. The results demonstrated that the test engine under lean-burn condition can obtain the higher indicated thermal efficiency (ITE), but the lower indicated mean effective pressure (IMEP). The THC, CO, CO2 and NOX emissions are all reduced with the increased λ. Injection timing had a large influence on the combustion and emissions and these influences became largely in the case of retarded injection conditions. There existed an optimum injection timing of −240 °CA ATDC where the IMEP and ITE got their highest values along with the lowest coefficient of variation in indicated mean effective pressure (COVIMEP). Meanwhile, the THC and CO emissions are lowest, but the CO2 and NOX emissions highest. The methanol engine can obtain the maximum toque of 27.6 N·m, which is 25.3% more than the original diesel engine at the same speed. Meanwhile, there was no knocking throughout the entire experiment, and the maximum pressure rise rate and COVIMEP were found to be 0.4 MPa and 4.1%, and those are much less than the critical points of 1.0 MPa and 5%, respectively.

The effects of methanol injection timing (MIT) and methanol substitution percentage (MSP) on the combustion and emissions of a methanol/diesel dual-fuel direct injection engine were investigated, followed by a comparative analysis with the conventional methanol/diesel dual-fuel mode. A single-cylinder, air-cooled, naturally aspirated common rail diesel engine was modified into a dual-fuel direct injection engine with a methanol direct injection system. The engine was operated at a maximum torque speed of 2500 rpm and a mean effective pressure of 0.75 MPa. The engine performance was analyzed for different methanol/diesel fuel mixtures using four MSPs: 10%, 20%, 30%, and 40%. Meanwhile, the MIT was adjusted from −60 to −300 °CA after top dead center (ATDC). The results indicated that methanol addition and retarded MIT allowed the diesel injection timing to be properly advanced. A higher MSP increased the ignition delay (CA0-10) and decreased the combustion duration (CA10-90), leading to increases in the brake thermal efficiency (BTE), coefficient of variation of the indicated mean effective pressure (IMEP) (COVIMEP), and knock intensity (KI), along with increases in the total hydrocarbon (THC) and nitrogen oxide (NOX) emissions and decreases in the carbon monoxide (CO) and soot emissions. Additionally, for a specific MSP, the retarded MIT increased the peak cylinder pressure and decreased the maximum heat release rate. Concurrently, it decreased the CA0-10 and increased the CA10-90. Moreover, increases in the BTE, COVIMEP, and KI; decreases in the THC and CO emissions; and increases in the NOX and soot emissions were achieved using the retarded MIT.

Primary alcohol fuels are the most promising fuels for diesel engines, thanks to their low emissions and easy adaptability to engine technologies. In this paper, the effects of the addition of methanol, ethanol, and n-butanol on the combustion characteristics and performance of a common rail dual fuel engine with diesel direct injection and alcohol fuel port injection are examined, followed by a comparative analysis of the test results. The test engine was operated at the maximum torque speed of 2500 rpm, and with a mean effective pressure (IMEP) of 0.75 MPa. The engine performance was analyzed for different alcohol/diesel fuel mixtures by using five alcohol substitute percentages (ASPs): 0% (pure diesel), 10%, 20%, 30%, and 40%. The experimental results demonstrate that slower flame development and faster flame propagation can be obtained by mixing any of the three alcohol fuels with diesel, compared with the pure diesel. With an increased ASP, the coefficient of variation of IMEP (COVIMEP) and the brake thermal efficiency (BTE) decreased, and the ringing intensity (RI) first increased and then dropped. The addition of primary alcohol fuels in the dual-fuel mode can also increase the total hydrocarbon (THC) and nitrogen oxide (NOX) emissions, but the carbon monoxide (CO) and soot emissions decrease. The comparative analysis indicated that the addition of methanol has the lowest COVIMEP and RI and the highest BTE among the three alcohol fuels. Adding methanol produces the lowest CO, NOX, and soot emissions and the highest THC emissions among the three alcohol fuels.

Recent years, piston engines for unmanned aerial vehicles (UAVs) are currently undergoing a transition from gasoline to diesel or aviation kerosene. This study aims to evaluate the combustion characteristics and emissions of a two-stroke direct injection spark-ignition (DISI) engine under various compression ratios with aviation kerosene (RP-3) as fuel. The tested engine was operated under three compression ratios (7.2, 6.2, and 5.2) and four throttle openings (30%, 50%, 70%, and 100%), at a fixed speed of 4000 rpm, and with an excess air ratio of 1.0. The spark timing was adjusted to knock limited spark advance (KLSA) or maximum brake torque (MBT). The results indicate that using aviation kerosene in an spark ignition engine increases the knock intensity and power loss at medium/wide throttle openings (50%, 70%, and 100%) compared to gasoline under the original compression ratio. Decreasing the compression ratio can effectively suppress knocking and make MBT/KLSA significantly advanced. No knocking tendency is observed at a small throttle opening (30%), but lower brake power (BP) and brake thermal efficiency (BTE) are observed with a lower compression ratio. At medium/wide throttle openings, reducing the compression ratio to 6.2 can yield higher BP and BTE. However, when the compression ratio is further decreased to 5.2, the BP and BTE decreased. The HC emission level is higher and the CO and NO emissions are lower compared to gasoline at a compression ratio of 7.2. As the compression ratio is decreased, the HC and NO emissions are decreased and CO emissions are increased.

To investigate the effect of excess air/fuel ratio (λ) and methanol addition on the combustion characteristics, performance, and emissions, a heavy-duty, turbo-charged, six-cylinder natural gas engine was modified into a natural gas/methanol dual-fuel engine with a methanol port-fuel injection system. The engine speed was maintained at a constant value of 1600 r/min, the engine load was kept at a low value with the brake mean effective pressure at 0.387 MPa, and the spark timing was maintained near the optimized brake thermal efficiency (ηet). The results indicated that the addition of methanol decreased the flame development period (CA0–10) and flame propagation period (CA10–90), leading to an increased ηet and a decreased equivalent brake specific fuel consumption (BSFC), along with reduced total hydrocarbon emissions. Additionally, with an increase in λ, the burning rate of natural gas decreased owing to the decreased energy density and combustion temperature and pressure. However, ηet increased and the equivalent BSFC decreased at the lean-burn condition. The total hydrocarbon emissions increased while the nitrogen oxide emissions decreased with an increase in λ. The decreased burning rate of natural gas at the lean-burn condition can be increased by adding methanol, particularly when λ = 1.5 and 1.6. The lean-burn capability of natural gas can be improved by adding methanol.

Non-monotonic behavior of Markstein lengths of 1,3-butadiene/O2/He mixtures was studied in a constant pressure combustion chamber at initial temperature of 298 K, initial pressure of 0.5–1.5 MPa and equivalence ratios of 0.8–1.5. Markstein lengths of 1,3-butadiene/air mixtures were also measured at initial temperature of 298 K, initial pressure of 0.1–0.5 MPa for comparison. Non-monotonic behavior was also observed in both theoretical Markstein lengths and Markstein numbers of 1,3-butadiene/O2/He mixtures, but was not observed in those of 1,3-butadiene/air mixtures. The effects of factors on non-monotonic behavior were studied. It is concluded that the rapid increase of Zel’dovich number in rich condition leads to the non-monotonic behavior of Markstein numbers of 1,3-butadiene/O2/He mixtures, and further leads to the rapid increase of Markstein length. Both experimental and theoretical critical flame radius and Peclet number were obtained and showed similar non-monotonic phenomenon to Markstein lengths and Markstein numbers, respectively. It is indicated that the cellular structure will first appear near stoichiometric ratio, thus the laminar burning velocities of helium diluted mixtures under elevated pressure are easier to measure under both rich and lean conditions.

The combustion characteristics and cycle-by-cycle variations of a spark-ignition engine fueled with pure methanol, ethanol, and n-butanol were comparatively analyzed. All experiments were performed using a natural gas/alcohols dual-fuel engine with the alcohol fuel energy substitution ratio of 100%. The engine was kept under a low load with a brake mean effective pressure of 0.387 MPa. Engine speed was kept constant at 1600 r/min and air–fuel ratio (λ) was set in the range of 1–1.5 and varied in intervals of 0.1. The results showed that the pure methanol fuel yielded the highest peak cylinder pressure (Pmax) and peak heat-release rate (HRRmax) of the engine, followed by ethanol and then n-butanol. Moreover, Pmax and HRRmax decreased with the increase in λ, and the corresponding crank angles were reduced for the three primary alcohol fuels. The flame-development and flame-propagation periods of methanol were shorter than those of ethanol and n-butanol. The indicated mean effective pressure (IMEP) was distributed in a wider range with the increase in λ for the engine fueled with ethanol and n-butanol. However, for methanol, the IMEP was distributed in a relatively narrow range under all conditions of λ. When λ was increased from 1.0 to 1.5, the coefficient of variation in IMEP (COVIMEP) of methanol increased from 1.36% to 2.65%, the COVIMEP of ethanol increased from 1.71% to 10.46%, and the COVIMEP of n-butanol increased from 2.06% to 15.66%. Thus, methanol had a higher burning rate, lower cycle-by-cycle variations, and a better lean-burn capability than that of ethanol and n-butanol.

Combustion and emissions of a DI diesel engine fuelled with diesel-oxygenate blends were investigated. The results show that there exist the different behaviors in the combustion between the diesel-diglyme blends and the other five diesel-oxygenate blends as the dig- lyme has the higher cetane number than that of diesel fuel while the other five oxygenates have the lower cetane number than that of diesel fuel. The smoke concentration decreases regardless of the types of oxygenate additives, and the smoke decreases with the increase of the oxygen mass fraction in the blends without increasing the NOx and engine thermal efficiency. The reduction of smoke is strongly related to the oxygen-content of blends. CO and HC concentrations decrease with the increase of oxygen mass fraction in the blends. Unlike conventional diesel engines fueled with pure diesel fuel, engine operating on the diesel-oxygenate blends presents a flat NOx/ Smoke tradeoff curve versus oxygen mass fraction. 2008 Elsevier Ltd. All rights reserved.

Measurement of laminar burning velocity of dimethyl ether–air mixtures was taken under different initial pressures and equivalence ratios using a constant volume bomb and high-speed schlieren photography. The stretched laminar burning velocity increases with the increase of stretch rate. At equivalence ratio of 1.0, low initial pressure gives high stretched flame speed. At initial pressure less than 0.1 MPa, the stoichiometric mixture gives the higher value of stretched flame speed than those at ϕ = 1.2 and ϕ = 0.8. The Markstein numbers decrease with the increase of equivalence ratio, and this reveals that lean mixture will maintain higher stability of flame front surface than that of rich mixture in dimethyl ether–air premixed flames.

In this paper, combustion characteristics of a direct-injection spark-ignited engine fueled with natural gas–hydrogen blends under various ignition timings and lean mixture condition were investigated. The results show that the ignition timing has significant influence on engine performance, combustion and emissions. The time intervals between the end of fuel injection and ignition timing are very sensitive to direct-injection gas engine combustion. The turbulence in combustion chamber generated by the fuel jet maintains high and relatively strong mixture stratification is presented when decreasing the time intervals between the end of injection and the ignition timing, giving fast burning rate, high brake mean effective pressure, high thermal efficiency and short combustion durations. For specific ignition timing, the brake mean effective pressure and the effective thermal efficiency increase and combustion durations decrease with the increase of hydrogen fraction in natural gas. Exhaust HC concentration decreases and exhaust NOx concentration increase with advancing the ignition timing while the exhaust CO gives little variation under various ignition timings.

This study focuses on the effects of initial temperature and pressure on the propagation characteristics of laminar premixed flame of methanol–air mixtures. Spherically expanding laminar premixed flames, freely propagating from spark ignition sources in initially quiescent methanol–air mixtures, are continuously recorded by a high-speed CCD at various equivalence ratios and temperatures. The flames are then analyzed to deduce the flame speed. The stretch imposed on the spherical flame front is explored experimentally; as a consequence, the unstretched laminar burning velocities of methanol–air flames have been derived. The present measurements are compared with the experimental data reported previously, and good agreements are obtained. Combined previous results, a correlation in the form of u l = u lo ( T u / T u 0 ) α T ( P u / P u 0 ) β p has been developed to describe the dependences of initial temperature and pressure on the burning velocities of methanol–air flames. The global activation temperatures are determined in terms of the burning mass flux. And then, the Zeldovich numbers for methanol–air flames are estimated as a function of equivalence ratio. On the basis of the mass burning flux, an alternative correlation of laminar burning velocities has been proposed, and agreements can still be found in the comparison between this alternative forms and the power law correlation above.