Influence Of Compression Ratio On The Performance Characteristics Of A Spark Ignition Engine – Complete project material



The need to improve the performance characteristics of the gasoline engine has necessitated
the present research. Increasing the compression ratio below detonating values to improve
on the performance is an option. The compression ratio is a factor that influences the
performance characteristics of internal combustion engines. This work is a an experimental
and theoretical investigation of the influence of the compression ratio on the brake power,
brake thermal efficiency, brake mean effective pressure and specific fuel consumption of
aRicardo variable compression ratio spark ignition engine. A range of compression ratios of
5, 6, 7, 8 and 9, and engine speeds of1100 to 1600rpm, in increments of 100rpm, were
utilised. The results showsthat as the compression ratio increases, the actual fuel
consumption decreasesaveragely by 7.75%, brake thermal efficiency improves by 8.49 %
and brake power also improves by 1.34%. The optimum compression ratio corresponding to
maximum brake power, brake thermal efficiency, brake mean effective pressure and lowest
specific fuel consumption is 9.The theoretical values were compared with experimental
values. The grand averages of the percentage errorsbetween the theoretical and experimental
valuesfor all the parameters were evaluated. 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.




COVER PAGE . . . . . . . . i
TITLE PAGE . . . . . . . . . iii
DECLARATION . . . . . . . . iv
CERTIFICATION . . . . . . . . v
ACKNOWLEDGEMENTS . . . . . . . vi
ABSTRACT . . . . . . . . . vii
TABLE OF CONTENTS . . . . . . . viii
LIST OF FIGURES . . . . . . . . xiii
LIST OF TABLES . . . . . . . . xix
LIST OF PLATES . . . . . . . . xxiv
LIST OF APPENDICES . . . . . . . xxv
1.1 Advantages and Applications of Internal Combustion (IC)
Engines . . . . . . . . 2
1.2 Thermal efficiency of IC engines . . . . . 3
1.3 Effect of Compression Ratio on the Thermal Efficiency of SI Engine 5
1.4 Statement of the problem . . . . . . 6
1.5 The Present Research . . . . . . . 7
1.6 Aim and Objectives . . . . . . 7
1.7 Significant of Research . . . . . . 8
2.1 Review of Related Past Works . . . . . 9
2.2 The Four Stroke Internal Combustion (IC) Engine . . 15
2.2.1 Structure and operation of a four stroke SI engine . . 15
2.3 Engine Performance Parameters . . . . 17
2.3.1 Definition of essential parameters . . . . 18
3.1 Description of Test Engine . . . . . . 22
3.2 Experimental Set-up of the Ricardo Variable Compression Ratio Engine. 22
3.3 The Engine . . . . . . . . 24
3.4 The Fuel System. . . . . . . . 25
3.5 Repairs of the Ricardo Engine . . . . . 25
3.5.1 Repairs of the central cooling system into the laboratory . 26
3.5.2 Repairs on the electric motor . . . . 26
3.5.3 Repairs of the fuel system . . . . 26
3.5.4 Repairs of the ignition system . . . . . 27
3.5.5 Replacement of the conveyor belt for the Tachometer . 27
3.6 Variation of the Compression Ratio . . . . . 27
3.7 Experimental Procedure . . . . . . 29
3.7.1 Calibration of the Ricardo engine . . . . 29
3.7.2 Test Procedure . . . . . . 30
3.8 Calculation of Mass Flow Rate of the Fuel . . . . 31
3.9 Measurement of Air Consumption . . . . . 32
3.10 Operation of the Ricardo Engine and Measurement of Break Load 33
3.11 Theoretical Determination of Performance Characteristics . . 34
3.11.1 Calculation of torque gain/loss . . . . 34
3.11.2 Error analysis . . . . . . 36
4.1 Discussion of Results . . . . . . 48
4.1.1 Effect of varying experimental compression ratio on the engine
brake power . . . . . . . 48
4.1.2 Effect of varying experimental compression ratio on the engine
brakethermal efficiency . . . . . 48
4.1.3 Effect of varying the experimental compression ratio on
brake mean effective pressure. . . . . 49
4.1.4 Effect of varying experimental compression ratio on the
fuel consumption parameters . . . . . 50
4.1.5 Effect of varying experimental compression ratio on
the volumetric efficiency . . . . . 50
4.2 Improvement in the Engine Performance Characteristics from
Increase in the Compression Ratio . . . . 51
4.3 Comparison between the Experimental and Theoretical values . 53
4.4 Comparison between Experimental and Theoretical Performance . 80
4.4.1 Brake power . . . . . . . 80
4.4.2 Brake thermal efficiency . . . . . 81
4.4.3 Specific fuel consumption . . . . . 81
4.4.4 Brake mean effective pressure . . . . 82
5.1 Summary . . . . . . . . 83
5.2 Conclusions . . . . . . . . 85
5.3 Recommendations . . . . . . . 86
REFERENCES . . . . . . . . 87
APPENDICES . . . . . . . . 90



The internal combustion (IC) engine has been refined and developed over the last 100 years
for a wide variety of applications. In most application of power generation and in
transportation propulsion the power source has being the internal combustion engines. The
reciprocating engine with its compact size and its wide range of power outputs and fuel
options is an ideal prime mover for powering cars, trucks, off-highway vehicles, trains, ships,
motor bikes as well aselectrical power generators for a wide range of large and small
applications. Electricity generating sets used to provide primary power in remote locations or
more generally for providing mobile and emergency or stand-by electrical power utilizes the
IC engines (Piston engine power plant, 2005). In Germany, Dr. Nicolaus August Otto started
manufacturing gas engines in 1866 (Hillier and Pittuck, 1978).
The IC engine is a heat engine in which burning of a fuel occurs in a confined space called a
combustion chamber. This exothermic reaction of a fuel with an oxidizer creates gases of high
temperature and pressure which are permitted to expand. The defining feature of an IC engine
is that useful work is performed by the expanding hot gases acting directly to cause
movement, for example by acting on piston, rotor, or even by pressing on and moving the
entire engine itself (Singer and Raper, 1999).
1.1 Advantages and Applications of Internal Combustion Engines
Spark Ignition (SI) Engines– These are lightweight enginesoflow capital costand aresuited
for applications in smaller and medium sized automobiles requiring power up to about 225
kW. They are also used in domestic electricity generation and outboard engines for smaller
Compression Ignition (CI) Engines- These are suited for medium and large size mobile
applications such as heavy trucks and buses, ships, auxiliary power units (emergency diesel
generators in industries) where fuel economy and relatively large amount of power both are
required (Reaz, 2001).
Figure 1.1 shows the electric power generation by IC engine and Figure 1.2 is an engine
classification chart.
Figure 1.1. Electric power generation by IC engine (Piston engine power plant, 2005)
Figure 1.2.Engine classificationchart(Reaz, 2001)
1.2 Thermal Efficiency of ICEngines
There is a lot of concern nowadays about the efficiency of the internal combustion (IC)
engine, and a lot of research is being done to improve it, so that we can get more work output,
for the same amount of fuel burnt. Engineers have devised manymethods like turbo charging,
cam-less engines and direct fuel injection (Mohit and Lamar, 2010). The following are also
promising breakthrough technologies for improving the thermal efficiencies of reciprocating
engines (
1) New combustion system for reducing oxides of Nitrogenlike pre-mixed compression
ignition combustion.
2) Friction reduced by lubricant oil.
3) Mechanical, Electrical and recovering thermal and kinetic energies
4) Transfer from fossil fuel to biomass fuel
The fuel cell is an important breakthrough technology currently under examination. It
is expected to be put into practical use from 2015 to 2020.
The thermal efficiency of the working cycle characterizes the degree of perfection with which
heat is converted into work. The thermal efficiency is the ratio of the energy output at the
shaft to input energy from the fuel. Of all the energy present in the combustion chamber only
some gets converted to useful output power. Most of the energy produced by these engines is
wasted as heat. The average IC engine has thermal efficiency between 20 to 30%, which is
very low (Mohit and Lamar, 2010).
If we consider a heat balance sheetsby Mohits and Lamar (2010) for the internal combustion
engines for a spark ignition (gasoline) engine, we find that the brake load efficiency is
between 21 to 28%, whereas loss to cooling water is between 12 to 27%, loss to exhaust is
between 30 to 55 %, and loss due to incomplete combustion is between 0 to 45%.By
analyzing the heat balance sheet we find that in gasoline engines loss due to incomplete
combustion can be rather high leading to poor performance characteristics of the engine.
In addition to friction losses and losses to the exhaust there are other engine operating
parameters that affect thermal efficiency. These include the fuel calorific value, the
compression ratio, ??and the ratio of specific heats, (γ=??/??).
Effect of Compression Ratio on the Thermal Efficiency of SI Engines
Compression ratio (??) is the ratio of the total volume of the combustion chamber when the
piston is at the bottom dead center (BDC) to the total volume of the combustion chamber
when piston is at the top dead center (TDC). Theoretically, increasing the compression ratio
of an engine can improve the thermal efficiency of the engine by producing more power
output. The ideal theoretical cycle,the Otto cycle, upon which spark ignition (SI) engine are
based, has a theoretical efficiency, ??, which increases with compression ratio, ??andis given
by (Chaiyot, 2005).
??= (1 – ?
???) (1.1)
where, γ is ratio of specific heats, and is 1.4 for air.
However, changing the compression ratio has effects on the actual engine for example, the
combustion rate. Also over the load and speed range, the relative impact on brake power and
thermal efficiency varies. Therefore, only testing on real engines can show the overall effect
Table 1.1. Heat balance sheets for internal combustion engines
of the compression ratio. Knocking, however, is a limitation for increasing the compression
ratio (Chaiyot, 2005).
1.4 Statement of the Problem
The electricity power generation by Power Holding Company of Nigeria (PHCN)
amount to about 3,700 MW, which is lower than the national demand of about 10,000 MW
( This implies that PHCN meets less than 50% of
the national demand. This has therefore necessitated establishments and families to generate
their own electricity using small engines. Most of these engines that are bought off-shelf (in
the market) are designed with a fixed compression ratio. These engines are to operate at
maximum thermal efficiency or lowest specific fuel consumption.
The thermal efficiency, ?? of the Otto cycle on which spark ignition engines are based is
given by equation (1). This implies that thermal efficiency is dependent on compression ratio
and ratio of specific heats. Compression ratio is a fundamental parameter in determining the
thermal efficiency of the engine.For spark ignition (SI) engines, the compression ratio ranges
from 6 to 12 (Haresh and Swagatam 2008). As a general rule, the energy in the fuel will be
better utilized if the compression ratio is as high as possible within the detonation free range.
1.5 The Present Research
This work attempts to investigate for a giving four stroke Spark Ignition engine, the influence
of compression ratio on the brake thermal efficiency, brake power, brake mean effective
pressure, specific fuel consumption, and the economic benefits for each unit increase in
compression ratio from 5 to 9; which is within detonation free range for spark ignition
The concern is for us to ensure that smaller engines such as the generators that we use in the
homes are fuel efficient, designed for optimum thermal efficiency within detonation free
compression ratios in order to reduce the cost of our supplementing electricity power supply
from PHCN.
1.6 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
(i) To determine experimentally the influence of compression ratio on:
a. brake power
b. brake mean effective pressure
c. brake thermal efficiency
d. specific fuel consumption.
(ii) To test the level of agreement of theoretical predictionswith derived performance
characteristics equationsto predict theoretically,the influence of compression ratio
on performance characteristics, a to d in (i)
1.7 Significance of Research
Adopting a higher compression ratio is one of the most important considerations regarding
improved fuel consumption, thermal efficiency and power output in gasoline engines. Much
research has been devoted to the effect of higher compression ratio in compression ignition
engines, but little attention has been given to spark engines because of detonation at higher
compression ratios. By far the most widely used IC engine is the spark-ignition gasoline
engine ( A four-stroke SI engine is
different from a four-stroke CI engine in the combustion process and in the, pressure and
temperature characteristics of the working gases. The compression ratio has a significant
effect on the thermal efficiency for the respective engine types.


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