Influence of compression ratio on the performance characteristics of a spark ignition engine

Thesis Overview

<p> INTRODUCTION<br>The internal combustion (IC) engine has been refined and developed over the last 100 years<br>for a wide variety of applications. In most application of power generation and in<br>transportation propulsion the power source has being the internal combustion engines. The<br>reciprocating engine with its compact size and its wide range of power outputs and fuel<br>options is an ideal prime mover for powering cars, trucks, off-highway vehicles, trains, ships,<br>motor bikes as well aselectrical power generators for a wide range of large and small<br>applications. Electricity generating sets used to provide primary power in remote locations or<br>more generally for providing mobile and emergency or stand-by electrical power utilizes the<br>IC engines (Piston engine power plant, 2005). In Germany, Dr. Nicolaus August Otto started<br>manufacturing gas engines in 1866 (Hillier and Pittuck, 1978).<br>The IC engine is a heat engine in which burning of a fuel occurs in a confined space called a<br>combustion chamber. This exothermic reaction of a fuel with an oxidizer creates gases of high<br>temperature and pressure which are permitted to expand. The defining feature of an IC engine<br>is that useful work is performed by the expanding hot gases acting directly to cause<br>movement, for example by acting on piston, rotor, or even by pressing on and moving the<br>entire engine itself (Singer and Raper, 1999).<br>1.1 Advantages and Applications of Internal Combustion Engines<br>xxix<br>Spark Ignition (SI) Engines– These are lightweight enginesoflow capital costand aresuited<br>for applications in smaller and medium sized automobiles requiring power up to about 225<br>kW. They are also used in domestic electricity generation and outboard engines for smaller<br>boats.<br>Compression Ignition (CI) Engines- These are suited for medium and large size mobile<br>applications such as heavy trucks and buses, ships, auxiliary power units (emergency diesel<br>generators in industries) where fuel economy and relatively large amount of power both are<br>required (Reaz, 2001).<br>Figure 1.1 shows the electric power generation by IC engine and Figure 1.2 is an engine<br>classification chart.<br>Figure 1.1. Electric power generation by IC engine (Piston engine power plant, 2005)<br>xxx<br>Figure 1.2.Engine classificationchart(Reaz, 2001)<br>1.2 Thermal Efficiency of ICEngines<br>There is a lot of concern nowadays about the efficiency of the internal combustion (IC)<br>engine, and a lot of research is being done to improve it, so that we can get more work output,<br>for the same amount of fuel burnt. Engineers have devised manymethods like turbo charging,<br>cam-less engines and direct fuel injection (Mohit and Lamar, 2010). The following are also<br>promising breakthrough technologies for improving the thermal efficiencies of reciprocating<br>engines (<a target="_blank" rel="nofollow" href="http://www.jsme.or.jp/English/jsme%20roadmap/N0.7)">www.jsme.or.jp/English/jsme%20roadmap/N0.7)</a>:<br>1) New combustion system for reducing oxides of Nitrogenlike pre-mixed compression<br>ignition combustion.<br>2) Friction reduced by lubricant oil.<br>3) Mechanical, Electrical and recovering thermal and kinetic energies<br>xxxi<br>4) Transfer from fossil fuel to biomass fuel<br>The fuel cell is an important breakthrough technology currently under examination. It<br>is expected to be put into practical use from 2015 to 2020.<br>The thermal efficiency of the working cycle characterizes the degree of perfection with which<br>heat is converted into work. The thermal efficiency is the ratio of the energy output at the<br>shaft to input energy from the fuel. Of all the energy present in the combustion chamber only<br>some gets converted to useful output power. Most of the energy produced by these engines is<br>wasted as heat. The average IC engine has thermal efficiency between 20 to 30%, which is<br>very low (Mohit and Lamar, 2010).<br>If we consider a heat balance sheetsby Mohits and Lamar (2010) for the internal combustion<br>engines for a spark ignition (gasoline) engine, we find that the brake load efficiency is<br>between 21 to 28%, whereas loss to cooling water is between 12 to 27%, loss to exhaust is<br>between 30 to 55 %, and loss due to incomplete combustion is between 0 to 45%.By<br>analyzing the heat balance sheet we find that in gasoline engines loss due to incomplete<br>combustion can be rather high leading to poor performance characteristics of the engine.<br>In addition to friction losses and losses to the exhaust there are other engine operating<br>parameters that affect thermal efficiency. These include the fuel calorific value, the<br>compression ratio, ô€Žô€®¼and the ratio of specific heats, (γ=ô€œ¥ô€¯‰/ô€œ¥ô€¯).<br>xxxii<br>1.3<br>Effect of Compression Ratio on the Thermal Efficiency of SI Engines<br>Compression ratio (ô€Žô€®¼) is the ratio of the total volume of the combustion chamber when the<br>piston is at the bottom dead center (BDC) to the total volume of the combustion chamber<br>when piston is at the top dead center (TDC). Theoretically, increasing the compression ratio<br>of an engine can improve the thermal efficiency of the engine by producing more power<br>output. The ideal theoretical cycle,the Otto cycle, upon which spark ignition (SI) engine are<br>based, has a theoretical efficiency, ô€ŸŸô€¯, which increases with compression ratio, ô€Žô€®¼andis given<br>by (Chaiyot, 2005).<br>ô€ŸŸô€¯= (1 – ô€¬µ<br>ô€¯¥ô€²´<br>ô€´‚ô€°·ô€°­) (1.1)<br>where, γ is ratio of specific heats, and is 1.4 for air.<br>However, changing the compression ratio has effects on the actual engine for example, the<br>combustion rate. Also over the load and speed range, the relative impact on brake power and<br>thermal efficiency varies. Therefore, only testing on real engines can show the overall effect<br>Table 1.1. Heat balance sheets for internal combustion engines<br>xxxiii<br>of the compression ratio. Knocking, however, is a limitation for increasing the compression<br>ratio (Chaiyot, 2005).<br>1.4 Statement of the Problem<br>The electricity power generation by Power Holding Company of Nigeria (PHCN)<br>amount to about 3,700 MW, which is lower than the national demand of about 10,000 MW<br>(<a target="_blank" rel="nofollow" href="http://www.sweetcrudereports.com/2011/power)">www.sweetcrudereports.com/2011/power)</a>. This implies that PHCN meets less than 50% of<br>the national demand. This has therefore necessitated establishments and families to generate<br>their own electricity using small engines. Most of these engines that are bought off-shelf (in<br>the market) are designed with a fixed compression ratio. These engines are to operate at<br>maximum thermal efficiency or lowest specific fuel consumption.<br>The thermal efficiency, ô€ŸŸô€¯ of the Otto cycle on which spark ignition engines are based is<br>given by equation (1). This implies that thermal efficiency is dependent on compression ratio<br>and ratio of specific heats. Compression ratio is a fundamental parameter in determining the<br>thermal efficiency of the engine.For spark ignition (SI) engines, the compression ratio ranges<br>from 6 to 12 (Haresh and Swagatam 2008). As a general rule, the energy in the fuel will be<br>better utilized if the compression ratio is as high as possible within the detonation free range.<br>xxxiv<br>1.5 The Present Research<br>This work attempts to investigate for a giving four stroke Spark Ignition engine, the influence<br>of compression ratio on the brake thermal efficiency, brake power, brake mean effective<br>pressure, specific fuel consumption, and the economic benefits for each unit increase in<br>compression ratio from 5 to 9; which is within detonation free range for spark ignition<br>engines.<br>The concern is for us to ensure that smaller engines such as the generators that we use in the<br>homes are fuel efficient, designed for optimum thermal efficiency within detonation free<br>compression ratios in order to reduce the cost of our supplementing electricity power supply<br>from PHCN.<br>1.6 Aim and Objectives<br>The aim of the research is to determine experimentally and theoretically, the influence of the<br>compression ratios on the performance characteristics of a spark ignition engine.<br>The specific objectives of this research are as follows<br>(i) To determine experimentally the influence of compression ratio on:<br>a. brake power<br>b. brake mean effective pressure<br>c. brake thermal efficiency<br>d. specific fuel consumption.<br>xxxv<br>(ii) To test the level of agreement of theoretical predictionswith derived performance<br>characteristics equationsto predict theoretically,the influence of compression ratio<br>on performance characteristics, a to d in (i)<br>1.7 Significance of Research<br>Adopting a higher compression ratio is one of the most important considerations regarding<br>improved fuel consumption, thermal efficiency and power output in gasoline engines. Much<br>research has been devoted to the effect of higher compression ratio in compression ignition<br>engines, but little attention has been given to spark engines because of detonation at higher<br>compression ratios. By far the most widely used IC engine is the spark-ignition gasoline<br>engine (<a target="_blank" rel="nofollow" href="http://www.personal.utulsa.edu/kenneth-weston/chapter6.pdf)">www.personal.utulsa.edu/kenneth-weston/chapter6.pdf)</a>. A four-stroke SI engine is<br>different from a four-stroke CI engine in the combustion process and in the, pressure and<br>temperature characteristics of the working gases. The compression ratio has a significant<br>effect on the thermal efficiency for the respective engine types.<br>xxxvi <br></p>