Study of knock in a high compression ratio spark-ignition methanol engine by multi-dimensional simulation

doi:10.1016/j.energy.2012.09.062

Energy

Volume 50, 1 February 2013, Pages 150-159

https://doi.org/10.1016/j.energy.2012.09.062 Get rights and content

Abstract

Methanol as an alternative fuel is considered to be one of the most favorable fuels for internal combustion engines. In this paper, knock in a high compression ratio spark-ignition (SI) methanol engine was studied by using multi-dimensional simulation. Knocking combustion under various engine operating conditions were simulated, and the effects of spark timing, EGR (Exhaust Gas Recirculation) techniques, mixture concentration and combustion chamber shape for suppressing knock in a high compression ratio spark-ignition methanol engine were studied based on the multi-dimensional simulation analysis. The results showed that it could not fully suppress engine knock only by retarding spark timing methods because of the high compression ratio, with the increase of EGR rate, the knock intensity was found to be greatly suppressed and the peak pressure was reduced and postponed. Meanwhile, knock intensity had a maximum point, rich mixture and lean mixture can help reduce knock intensity and the knock occurrence timing was delayed. Finally, in order to reduce engine knock intensity, two feasible new combustion chambers were proposed.

Highlights

► Knock in a high compression ratio spark-ignition methanol engine was simulated. ► Because the compression ratio is very high, it could not fully suppress knock solely by retarding the spark timing. ► With the increase of EGR, the knock could be effectively suppressed, the peak pressure was reduced and postponed. ► Knock intensity had a maximum point, rich mixture and lean mixture can help reduce knock intensity. ► In order to reduce engine knock intensity, two feasible new combustion chambers were proposed.

Introduction

Nowadays, with the rapidly depleting reserves of the conventional petroleum-based fuels, alternative energy resources such as methanol, natural gas and LPG are needed in order to replace the non-renewable resources. With rising petroleum prices and global warming being a dominant environmental issue, it seems that the use of alternative fuels in the future is inevitable. Our present energy supply is based on the fossil fuels, which are non-renewable energy. Given the growing world population and atmospheric environment, increasing energy demand per capita and global warming, the need for a long-term alternative energy supply is clear. Methanol is one of the best candidates for long-term, widespread replacement of petroleum-based fuels [1].

Methanol (CH₃OH) is considered to be one of the most favorable fuels for engines, for instance, 1: it can be used in a high compression ratio SI (spark-ignition) engine that could replace diesels in certain vocational applications; 2: it can be used in an inlet port injection SI engine; 3: it can be used in a high compression direct-injection stratified charge SI engine; 4: it can be used in a direct-injection SI engine; 5: it can be used in a turbocharged, port-fuel-injected, high compression ratio medium duty engine [2], [3], [4], [5], [6]. Methanol can be produced from non-petroleum feed-stocks such as coal or biomass [7]. The properties of methanol, gasoline and diesel fuels are shown in Table 1. Methanol has higher octane rating and greater heat of vaporization values as compared to gasoline, making it a suitable candidate for high compression ratio engines with larger power outputs. This is because higher octane rating allows a significant increase in the compression ratio and a higher heat of vaporization value may cool down the incoming fuel-air charge, increasing the volumetric efficiency and promoting the power output [8]. Besides the auto-ignition temperatures of alcohols are higher than gasoline which makes them safer for transportation and storage [9].

The essential task in the development of spark-ignition engines today is to further improve the thermal efficiency and engine torque. One effective way to do this is to increase the engine compression ratio limited by the constrain due to the generation of knock. So, engine knock is well known as a major barrier obstructing the further improvement of the spark-ignition engines. In order to increase the compression ratio of spark-ignited engines, the phenomenon of engine knock must be avoided. Engine knock is an abnormal combustion phenomenon which is characterized by high frequency pressure oscillations in the combustion chamber. It is generally accepted that knocking combustion is caused by auto-ignition of a portion of the end-gas prior to the flame arrival [10], [11], [12], [13]. Despite many existing investigations, knocking combustion is still a crucial topic concerning the engine design and development, with various uncertainties and unsolved questions.

Soylu [14] used a zero-dimensional, two-zone thermodynamic model to study the engine knock operating conditions for a natural gas engine, and the simulated results agreed with the measurements. Radu et al. [15] investigated the knock characteristics of LPG in a spark-ignition engine. They developed an empirical knock model which was able to predict the knock onset time with an approximation of about two crank angle degrees. Some characteristics of LPG auto-ignition in engine were also put forth. They proved that there was a close correlation between the rate of heat release by auto-ignition reactions and the knock intensity. Li and Karim [16] developed a two-zone model to predict the incidence of knock in an SI hydrogen engine. The results showed that the knock-free mixture region tends to narrow significantly with the increasing compression ratio and/or intake temperature. The presence of methane with hydrogen improves its knock limits, while the presence of carbon monoxide on the other hand hardly has any significant effect. Bika et al. [17] investigated the engine knock and combustion characteristics in a spark-ignition engine operating with varying hydrogen and carbon monoxide proportions. The results showed that higher Carbon monoxide (CO) fractions in the synthesis gas could be beneficial in suppressing knock, which gives it the potential to be used with a higher compression ratio.

It can be seen that most of the previous work have been focusing on gasoline, natural gas, LPG, or hydrogen fuels, where the knock mechanisms of alternative such as methanol has not been fully substantiated [18]. The objective of the work is to investigate the knocking combustion from a high compression ratio spark-ignition methanol engine. These results should be valuable in improving the performance and suppressing the knock of high compression ratio spark-ignition methanol engines.

Section snippets

Engine model

Three major models were needed to simulate the overall combustion and knocking process. The turbulence model was based on the k-ζ-f model. This model has been recently developed by Hanjalic et al. [19]. The authors propose a version of eddy-viscosity model on Durbin's elliptic relaxation concept, which solves a transport equation for the velocity scales ratio $ζ = \bar{v^{2}} / k$ instead of $\bar{v^{2}}$ , thus making the model more robust and less sensitive to grid non-uniformities.

The combustion model was based on

Knock simulation under the baseline condition

The baseline operating conditions are specified in Table 3. The signals of different local pressure histories are shown in Fig. 5 where the mean in-cylinder pressure curve is included. For the spark timing = −10 °ATDC case, the most severe pressure oscillations occur at positions 6 and 8, the most weak pressure oscillations occur at positions 1 and 7. Positions 6 and 8 are far away from the spark plug, so the flame propagation distance are too longer, the flame front needs to take longer time

Conclusions

Knocking combustion under various engine operating conditions were simulated, and the effects of EGR technique, mixture concentration and combustion chamber shape for suppressing knock in a high compression ratio spark-ignition methanol engine were studied based on the multi-dimensional simulation analysis. High compression ratio SI methanol engine offers the potential for a lower-cost renewable fuel alternative to the diesel engine. The results from this work are important for those

Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51176137).

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View full text

Study of knock in a high compression ratio spark-ignition methanol engine by multi-dimensional simulation

Abstract

Highlights

Introduction

Section snippets

Engine model

Knock simulation under the baseline condition

Conclusions

Acknowledgment

Fuel

Energy

Fuel

P Combust Inst

Energy Convers Manage

Int J Hydrogen Energy

Int J Hydrogen Energy

Appl Energy

Int J Heat Fluid Flow

Fuel

Appl Energy

Energy

Energy Convers Manage

Fuel

The methanol story: a sustainable fuel for the future

J Sci Ind Res India

Simulation of high efficiency heavy duty SI engines using direct injection of alcohol for knock avoidance

Combustion of a spark-ignition methanol engine during cold start under cycle-by-cycle control

Energy Fuels

Regulated emissions from a direct-injection spark-ignition methanol engine

Energy