Influence of Compression Ratio on the Performance Characteristics of a Spark Ignition Engine
Chapter One
Aim and Objectives
The aim of the research is to determine experimentally and theoretically, the influence of the compression ratios on the performance characteristics of a spark ignition engine.
The specific objectives of this research are as follows
- To determine experimentally the influence of compression ratio on:
- brake power
- brake mean effective pressure
- brake thermal efficiency
- specific fuel
- To test the level of agreement of theoretical predictions with derived performance characteristics equations to predict theoretically, the influence of compression ratio on performance characteristics, a to d in (i)
CHAPTER TWO
LITERATURE REVIEW
A lot of research has been done to improve the performance of internal combustion engines. Below are some of the past works.
Review of Related Past Works
Asifet at. (2008) conducted a research on performance evaluation of a single cylinder four stroke petrol engine. In the research, the actual size of the engine parameters like the bore, stroke, swept volume, clearance volume, compression ratio and engine speed were recorded and computed. Based on the actual size of the engine parameters, the indicated horse power, brake power, and friction horse power were determined and were found to be 1.54, 1.29 and 0.25 respectively. The mechanical efficiency and the thermal efficiency were also calculated and were found to be 83% and 20.5% respectively. The fuel consumption per hour was found to be 0.8 litre/hour while the fuel consumption per distance traveled was found to be 60 km/litre.
Owoade (1971) conducted a research on the performance of a variable compression ratio SI engine at full throttle. The research was conducted for compression ratios of 8, 9, 10 and It was observed that a compression ratio of 10.5 gave the maximum brake horsepower.
Above this value (compression ratio of 10.5), there was a decrease in brake horse power at each ignition setting. Also, the values for the thermal efficiency for both the hypothetical and actual engine increased with the compression ratio
Yuh and Tohru (2005) carried out a research on the effect of higher compression ratio in two-stroke engines. The results showed that the actual fuel consumption improved by 1- 3% for each unit increase in the compression ratio range of 6.6 to 13.6. It was observed that the rate of improvement was smaller as compared to the theoretical values. The discrepancies were mainly due to increased mechanical and cooling losses, short- circuiting at low loads and increased time losses at heavy loads. Power output also improved, but the maximum compression ratio was limited due to knock and the increase in thermal load. In addition, the investigation covered the implementation of higher compression ratio in practical engines by retarding the full-load ignition timing.
Abdel and Osman (1997) performed an experimental investigation on varying the compression ratio of spark ignition engines working under different ethanol-gasoline fuel blends.. The result showed that the engine indicated power improved with the percentage addition of the ethanol in the fuel blend. The maximum improvement occurred at 10% ethanol-90% gasoline fuel blends. Chaiyotet al. (2005) conducted an experimental study on influence of compression ratio on the performance and emission of compressed natural gas (CNG) retrofit engine. For the engine testing, CNG dedicated engine was tested at 4 compression ratios of 9, 9.5, 10, and 10.5. The engine testing schematic diagram is shown in Figure 2.1
From the result of the engine testing, performance of the CNG dedicated engine was much more than original diesel engine. Moreover, the trend of engine power increased when compression ratio was increased. For the fuel consumption, the CNG dedicated engine had higher specific fuel consumption than original diesel engine. The diesel engine had the lowest specific fuel consumption around 136g/kWh but CNG dedicated engine had the lowest specific fuel consumption at compression ratio of 9.5 around 155g/kWh. It was shown that the existing diesel engine can be retrofitted and switches to use CNG as alternative fuel which results on money saving, more power output and clean emissions.
Jan-Ola et al. (2002) carried out a research on the compression ratio influence on maximum load of a Natural Gas HCCI engine.A Volvo TD100 truck engine was used for the experiment. The engine was controlled in a closed-loop fashion by enriching the Natural Gas mixture with Hydrogen. The first section of the paper illustrated and discussed the potential of using hydrogen enrichment of natural gas to control combustion timing. Full-cycle simulation was carried out and compared to some of the experimental data and then used to enhance some of the experimental observations dealing with ignition timing, thermal boundary conditions, emissions and how they affect engine stability and performance.
CHAPTER THREE
MATERIALS AND METHODS
Description of Test Engine
In this section, a description of the testengineis given,and the major specifications.The experimental procedures andtheoretical determination of the performance characteristicsare also presented.
Experimental Set-up of the Ricardo Variable Compression Ratio Engine
The experimental set-up consists of the engine, the dynamometer and the instrumentation unit from where the readings are taken.
CHAPTER FOUR
RESULTS AND DISCUSSION
Equations used for calculating the derived values of brake power, indicated thermal efficiency, specific fuel consumption and the brake mean effective pressure are as modelledin appendix A and B using Engineering equation solver (EES) computer programme.
The Table of values for the experimental readings, calculated values from experimental readings, improvement in the performance characteristics from the increase in compression ratios, theoretical values and the error between the experimental and theoretical values are giving in this section.
CHAPTER FIVE
SUMMARY, CONCLUSIONSAND RECOMMENDATIONS
Summary
This research work can be summarised as follows:
- Investigation of the effect of increase in compression ratio of a Ricardo four stroke single cylinder petrol engine was conducted. A range of compression ratios of 5, 6, 7, 8 and 9, and engine speeds of 1100 to 1600 rpm in increments of 100 rpm were utilised. The basic parameters that were measured directly from the engine are the torque arm, brake load, time taken for the engine to consume 50ml of the fuel and the manometer readings for the air flow rate. The experimental values for the engine torque, brake power, brake mean effective pressure, brake thermal efficiency and specific fuel consumption were calculated. Improvements in the brake power, brake thermal efficiency, brake mean effective pressure and specific fuel consumption were evaluated when the compression ratio is increased from 5 to 6, 6 to 7, 7 to 8 and 8 to 9. The results shows that the brake power improves by 1.34%, brake thermal efficiency improves by 8.49%, brake mean effective pressure improves by 1.29% and the actual fuel consumption reduces by 7.75% averagely. The performance characteristics graphs for variation of the experimental brake power, brake thermal efficiency, brake mean effective pressure and specific fuel consumption with compression ratio were obtained.
- Theoretical performance characteristics of the engine were obtained. Errors in the performance characteristics were evaluated and
Table 5.1.Grand averages ofthe percentage error
The theoretical values were compared with experimental values. The grand averages of the percentage error between the two values ranges from 0.18 to 2.29 % for the brake power, 0.43 to 9.89 % for the brake thermal efficiency, 0.69 to 11.17 % for the specific fuel consumption and 0.21 to 2.3 % for the brake mean effective pressure, as shown in Table 5.1. The discrepancies between the theoretical and experimental values for all the parameters were due to the values of K which was obtained experimentally. K varies with compression ratio and the engine speed.
Conclusions
The general conclusions drawn from the results of this research work are as follows:
- Increase in compression ratio on the Ricardo variable compression ratio engine increases the brake power, brake thermal efficiency, brake mean effective pressure and reduction in the specific fuel consumption. It means that higher compression ratios make it possible to improve the performance characteristics of spark ignition
- Increase in the engine speed affects the volumetric efficiency more than when the compression ratio is increased. Thus, compression ratio has minimal effect on the volumetric efficiency.
- The optimum compression ratio corresponding to the maximum brake thermal efficiency and brake power is 9.
- The small values of the percentage errors between the theoretical and experimental values show that there is agreement between the theoretical and experimental performance characteristics of the engine.
Recommendations
The performance characteristics of a four stroke variable compression ratio engine in terms of the brake power, brake thermal efficiency, brake mean effective pressure and specific fuel consumption have been investigated with increase in the compression ratio. Further research should be conducted to include a numerical modeling of combustion in a spark ignition variable compression ratio engine.
REFERENCES
- Abdel Rahman, A.A. and Osman, M.M.(1997). Experimental investigation of varying compression ratio of spark ignition engine working under different Ethanol- Gasoline fuel blends, International Journal of Energy Research, Kuwait University. Vol. 21, issue 1, pp. 31-40.
- Andreas, B. (2003). Torque Modeling and Control of a Variable Compression Engine. Performed in Vehicular System.Reg nr: LITH-ISY-EX-3421
- Asif, A.M., Jandool, K., Saeed, A.S. and Ali, A.C. (2008).Performance Evaluation of a Single Cylinder Four Stroke Petrol Engine.Farm machinery institute, National Agricultural Research Centre, Islamabad, Pakistan.
- Chaiyot, D. and Somrat, K. (2005). Experimental Influence of Compression Ratio on to Performance and Emission of Compressed Natural Gas Retrofit Engine, King Mongkut’s Institute of Technology North Bangkok.
- Eastop, T.D. and McConkey, A. (1995).Applied Thermodynamics For Engineering Technologists.ELBS with Longman, 5th Edition pp. 420-439.
- Haresh, K. and Swagatam.(2008). Comparison of Spark Ignition (SI) and Compression Ignition (CI) Engines. 2011 Bright Hub Inc. http://www.brighthub.com/engineering/mechanical/articles/1547.
- Heywood J. B. (1988). Internal combustion Engines Fundamentals. McGraw Hill Book Company.
- Hillier, V.A.W. and Pittuck, F.W. (1978).Fundamentals of Motor Vehicle Technology.
- Hutchinson and Co (Publisher) Ltd, London. pp. 44.
