Effect of Compression Ratio on Performance and Emissions of a Single Cylinder Four Stroke Diesel Engine

In the present research study, the effect of different compression ratios on engine performance and emission behaviour of diesel engine was studied and optimum compression ratio was determined. The compression ratios set for study were ranging from 16 to 20 for diesel engine. The present study focuses on investigating the better compression ratio for the variable compression ratio diesel engine at variable loads.

The setup consists of single cylinder, four stroke, VCR (Variable Compression Ratio) Diesel engine connected to eddy current dynamometer for loading. The schematic diagram of experimental setup is shown below in fig.1. The compression ratio can be changed by changing the clearance volume. Setup is provided with necessary instruments and provision for interfacing airflow, fuel flow, temperatures and load measurement. The set up has stand-alone panel box consisting of air box, two fuel tanks for duel fuel test, manometer, and fuel measuring unit, transmitters for air and fuel flow measurements, process indicator and engine indicator. Rotameters are provided for cooling water and calorimeter water flow measurement.

1 Variable Compression Ratio Model: VCR, Single cylinder Engine diesel, 4 stroke, water- cooled stroke 110 mm, bore 80 mm Manual Crank Start,

3 Speed 1450–1550 rpm 5 Maximum Load 10 kg 6 Injection pressure 200 bar 7 Compression ratio range 6:1–20:1 9 Lubrication oil SAE 20W40 10 Dynamometer Type: eddy current, air cooled with loading unit 11 Load measurement Direct coupling, strain gauge, manual loading 12 Exhaust Gas Calorimeter Type shell and tube, K type thermocouple, water cooling 13 Exhaust gas analyser Model—AVL

VCR engine was allowed to run with diesel for 15 minutes to precondition the experimental setup. The cooling water was allowed at constant rate of 80 ml/s. The compression ratio was changed from 16 to 20 by changing the clearance volume of the VCR engine. An eddy current dynamometer with a load cell was connected to load the engine. In each test, the speed of the engine, manometer readings, exhaust gas emissions such as carbon monoxide, hydrocarbon, oxides of nitrogen, and carbon dioxide were recorded. From the observed measurements mass of fuel consumption, brake power, indicated power, brake thermal efficiency and specific fuel consumption were calculated. For each operating condition the performance and exhaust emission levels were measured.

Brake Thermal Efficiency curves were plotted for varying loads at different compression ratios from 16 to 20. From Fig. 1, it is seen that for the compression ratio 20, brake thermal efficiency is higher at all the loads.

Brake thermal efficiency increases with load for the entire compression ratio. Due to higher compression ratio, the temperature and pressure of intake air increases which support better combustion of the fuels. Therefore efficiency of the engine is higher at the full load.

Specific fuel consumption is the amount of fuel consumed by the engine to produce unit power per hour. SFC is an important performance indicator for internal combustion engine.

ratio. Cylinder temperature increases while increment in the load due to which combustion temperature also increases resulting in better combustion at higher loads and compression ratio. The fuel requirement for developing unit brake power comes down at higher loads and compression ratios. This is evident from the SFC noted for compression ratios 16–20.

Exhaust gas temperature is also an important parameter as it provides some qualitative information about the combustion process .Exhaust gas temperature is an indicator of the heat release rate of the fuels tested during combustion and its effective utilization to produce power. It depends on the combustion characteristics of the fuel and also on the heat losses to exhaust. Figure 4 shows the variation of exhaust gas temperature (EGT) with load for diesel engine. It can be clearly seen from the figure that exhaust gas temperature increases with the increase in load for all compression ratios. This is due to the fact that temperature after combustion becomes more as more fuel is required to be burnt at higher load condition. It can also be noted from Fig. 4 that the exhaust gas temperature increases with the decrease in compression ratio.

Unburnt hydrocarbons are emitted from CI engine due to low combustion temperature and presence of lean or rich mixture. Fuel exhaust hydrocarbons are composed of original fuel molecules and partially oxidized hydrocarbons. The curves in Fig. 5 show that CI engine emits higher HC with increase in load. At higher loads CI engine requires rich mixture, due to poor atomization and due to local rich mixture formation incomplete combustion takes place.

With increase in compression ratio, temperature and pressure of air in the combustion chember increases which enables complete combustion of fuel and lowers emmission of HC.

Oxides of Nitrogen are produced by oxidization inside the cylinder at a temperature of 2200 K. Nitrogen requires very high energy to break its triple bond. When the high combustion temperature favours oxidization NOx is produced. The effect of compression ratio on NOx emission can be observed from Fig. 6. At higher compression ratio, NOx emission is higher. The higher temperature inside the cylinder creates suitable environment for NOx emission. It has also been reported that at lower compression ratio, the premixed combustion phase is lengthened due to the longer ignition delay and eventually decreases the NOx emission.

One can note that CO2 emission increases for all the loads with the increase in compression ratio. At higher compression ratio, the combustion is improved and it leads to more amount of CO2 formation. On the other hand, poorer atomization of fuels at lower compression ratio reduces the amount of CO2 emission.

increase in load the CO emission increases because to meet maximum load condition, rich mixture is supplied to CI engine but same amount of oxygen is supplied for combustion of rich mixture resulting in higher CO emission.

It is observed from above graphs that the Brake Thermal Efficiency increases with increase in load as well as increase in compression ratio. At higher compression ratio the temperature of combustion chamber is higher so complete combustion of fuel takes place resulting in higher thermal efficiency and lower BSFC. So at CR 20 Engine performance is better. EGT is again at higher side at CR17 at all the loads. It is seen from the graphs that with increase in CR, the EGT is reduced. Also at all the compression ratios EGT increases with load due to availability of more fuel at higher loads. Fig. 5 show that CI engine emits higher HC with increase in load and emission decreases with increase in compression ratio. At higher loads, CI engine requires rich mixture, due to poor atomization and local rich mixture formation leads to incomplete combustion which further causes higher HC emission. From Fig. 6 It is evident that at higher compression ratio and higher loads NOx emission is higher. This is because at higher temperature inside the cylinder creates suitable environment for NOx emission. CO2 increases at high compression ratio and higher loads. This is because at higher compression ratio, the combustion is improved resulting in more CO2 emission. Also CO emission increases with increase in load as well as compression ratio.

Authors wish to thank the I. C. Engine Lab attendent for giving the permission for utilizing the lab facility available at GCE, Karad.

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