Acoustic Analysis and Investigation of the Effect of Diesel-Bioethanol Fuel Blends on the 1/3 Octave Band Noise Level of a Four-Stroke Diesel Engine

Document Type : Research Paper

Authors

1 Institute of Manufacturing Engineering and Industrial Technologies, Dez.C. , Islamic Azad University, Dezful, Iran

2 Department of Agricultural Science and Engineering, SR.C., Islamic Azad University, Tehran, Iran.

3 Department of Agricultural Machinery Engineering, Faculty of Agriculture,Tarbiat Modarres University, Tehran, Iran

4 Department of Agricultural Science and Engineering, SR.C., Islamic Azad University, Tehran, Iran

5 Department of Agro-Technology, College of Abouraihan, University of Tehran, Tehran, Iran,

Abstract

The noise generated by diesel engines is one of the most important harmful physical factors, causing numerous problems for both the driver and the work environment. This study investigates the effects of diesel-bioethanol fuel blends and engine speed on the sound pressure level(SPL) of a engine at the bystander position. The tests were conducted in a controlled environment in accordance with international standards. Acoustic signals were acquired using a sound level meter and analyzed within the 20 to 20,000 Hz frequency range using 1/3 octave bands. The results indicated, the dominant acoustic peak occurred in the 80 Hz center band, which corresponds to the engine's combustion frequency (85 Hz at 1700 RPM). The influence of speed on the SPL was significantly more substantial than that of fuel composition. An increase in speed resulted in a 2 to 10 dB rise across most frequencies, whereas altering the fuel blend, had a comparatively minor effect (less than 5 dB). The observed frequency variations were primarily attributed to the combustion process, valve operation, and mechanical impacts of engine components. Although the addition of bioethanol had a negligible effect on the overall noise level, detailed frequency analysis revealed alterations in the engine's acoustic profile. The analysis of variance of the overall sound level with A-weighting, showed that the main effects of fuel blend and speed, and the interaction effect of the variables speed × different fuel blends on that are significant at the 1% level, and the engine speed variable had the greatest effect.

Keywords

Main Subjects

Introduction

Although diesel engines are superior to gasoline engines in terms of thermal efficiency and fuel economy, they are problematic regarding noise pollution. Given the detrimental effects of noise, it is imperative to identify its sources and mitigate their occurrence as much as possible. Internal combustion engines rank among the most significant sources of noise in industrial and agricultural settings. Engine noise itself originates from various sources, the most fundamental of which is combustion noise. The sudden pressure rise from combustion is transmitted to the engine structure, and the resulting noise propagates through the air. Considering the extensive environmental advantages and renewability of bioethanol, coupled with the widespread use of diesel tractors in Iranian agriculture which exposes operators to persistent noise, the necessity for comprehensive research in this area becomes evident. To date, no systematic study has investigated the impact of diesel-bioethanol fuel blends on the acoustic profile of tractor diesel engines using a precise 1/3 octave band analysis. Therefore, this research not only seeks to propose a solution for reducing noise pollution and improving operator working conditions but also, by addressing this knowledge gap, aims to provide reliable data to support the development and promotion of eco-friendly hybrid fuels in the agricultural sector.

Method

This study investigated the effects of diesel-bioethanol fuel blends on engine noise levels. Acoustic data from an MF399 model, four-stroke, six-cylinder tractor engine were measured and analyzed at five different speeds (1700, 1800, 1900, 2000, and 2100 rpm) and with seven distinct fuel blends (E0, E2, E4, E6, E8, E10, and E12). An acoustic microphone, positioned at the standard auditory location for bystanders, was used to capture the sound signals. All tests were conducted with four replications. The primary measured parameter was the Sound Pressure Level (in decibels) across various frequencies (in Hertz). To adhere to scientific principles and ensure the accuracy of the results, the following measures were implemented:

  • Selection of a test site compliant with relevant standards.
  • Utilization of high-precision measurement equipment.
  • Rigorous assessment of the range of influential variables.

To guarantee the reliability of the findings, all measurements were performed under controlled and standardized conditions. The engine's technical specifications, the test site details, and the experimental design matrix are presented in the subsequent sections.

Result

This study analyzes the effects of rotational speed and various fuel blends on the octave band spectrum of the sound pressure level in a four-stroke, six-cylinder diesel engine. The findings indicate that heavy engine components such as the crankshaft and connecting rods induce resonant conditions and amplify noise due to free vibrations and the transmission of inertial forces. Furthermore, impacts from valve opening and closing, turbulence in the fuel, oil, and water systems, as well as pump operation, contribute significantly to vibration and noise generation. Among these, the dominant frequencies are primarily associated with combustion and valve dynamics.

Conclusions

he results of this study clearly demonstrate that while the type of diesel-bioethanol fuel blend influences the engine's acoustic characteristics, rotational speed is the more decisive factor in determining the generated noise level. A change in speed caused variations of 2 to 10 dB in the sound pressure level across a wide range of frequencies, whereas the effect of the fuel composition was in most cases less than 5 dB. Furthermore, as the engine speed increased, the number and amplitude of the frequency peaks rose significantly, a phenomenon attributable to the intensification of combustion events, mechanical impacts from components, and fluid turbulence. This study confirms that selecting an optimal fuel blend must be done with consideration of the engine's operational range and rotational speed to achieve a reduction in noise pollution. Additionally, the crucial role of preventive maintenance—such as the timely replacement of air, fuel, and oil filters—is emphasized as a significant factor in controlling and mitigating engine noise.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authorship contribution

The authors have not used generative artificial intelligence or AI-based technologies in the writing process of their final version of the article, by adding a statement at the end of their manuscript in the original file, before the references.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors have not used generative artificial intelligence or AI-based technologies in the writing process of their final manuscript. Therefore, no such statement has been included at the end of the manuscript, before the references section.

Data availability statement

The data of the present study are available upon request from the authors.

Acknowledgements

I am grateful to the respected editor-in-chief and the esteemed reviewers for their efforts.

Ethical considerations

The authors adhered to the ethical standards in conducting and publishing this scientific study, and all of them certify this.

Conflict of interest

The authors declare no conflict of interest.

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Iranian Journal of Biosystem Engineering
Volume 57, Issue 2
July 2026
Pages 81-99

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History

  • Receive Date: 03 October 2025
  • Revise Date: 20 January 2026
  • Accept Date: 26 January 2026

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