Week 2 : Basic Calibration of Single cylinder SI-Engine

AIM :

• To run the single cylinder SI engine setup at 1800 rpm and list down parameters such as air flow rate, BSFC, BMEP, In cylinder pressure.
• To run the same setup at 3600 rpm and to increase the output by 10%

INTRODUCTION:

A spark-ignition engine (SI engine) is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug. This is in contrast to compression-ignition engine, typically diesel engines, where the heat generated from compression together with the injection of fuel is enough to initiate the combustion process, without needing any external spark.

WORKING OF THE SI ENGINE:

The working cycle of both spark-ignition and compression-ignition engines may be either two-stroke or four-stroke.

A four-stroke spark-ignition engine is an otto cycle engine. It consists of following four strokes: suction or intake stroke, compression stroke, expansion or power stroke, exhaust stroke. Each stroke consists of 180 degree rotation of crankshaft rotation and hence a four-stroke cycle is completed through 720 degree of crank rotation. Thus for one complete cycle there is only one power stroke while the crankshaft turns by two or more revolutions.

Air–fuel ratio (AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process. The combustion may take place in a controlled manner such as in an internal combustion engine or industrial furnace, or may result in an explosion (e.g., a dust explosion, gas or vapour explosion or in a thermobaric weapon).

The air-fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how much-unwanted pollutants are produced in the reaction. Typically a range of fuel to air ratios exists, outside of which ignition will not occur. These are known as the lower and upper explosive limits.

In an internal combustion engine or industrial furnace, the air-fuel ratio is an important measure for anti-pollution and performance-tuning reasons. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich. Ratios lower than stoichiometric are considered "rich." Rich mixtures are less efficient, but may produce more power and burn cooler. Ratios higher than stoichiometric are considered "lean." Lean mixtures are more efficient but may cause higher temperatures, which can lead to the formation of nitrogen oxides. Some engines are designed with features to allow lean-burn. For precise air-fuel ratio calculations, the oxygen content of combustion air should be specified because of different air density due to different altitude or intake air temperature, possible dilution by ambient water vapour, or enrichment by oxygen additions.

In theory, a stoichiometric mixture has just enough air to completely burn the available fuel. In practice, this is never quite achieved, due primarily to the very short time available in an internal combustion engine for each combustion cycle. Most of the combustion process is completed in approximately 2 milliseconds at an engine speed of 6,000 revolutions per minute. (100 revolutions per second; 10 milliseconds per revolution of the crankshaft - which for a four-stroke engine would mean typically 5 milliseconds for each piston stroke). This is the time that elapses from the spark plug firing until 90% of the fuel–air mix is combusted, typically some 80 degrees of crankshaft rotation later. Catalytic convertors are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion.

1.Run the case at 1800 rpm and list down important parameters

• air flow rate
• BMEP
• BSFC
• In-cylinder pressure

The model is imported from tutorials > modelling applications > engine_performance > 1cylSi-final.gtm

1. AIR FLOW RATE:

        Air flow rate: 24.6 kg/h

2. BRAKE MEAN EFFECTIVE PRESSURE:

        Break mean effective pressure: 9.5 bar

3. BRAKE SPECIFIC FUEL CONSUMPTION:

        Brake specific fuel consumption: 239.2 g/kw-h

4. IN-CYLINDER PRESSURE:

       In-cylinder pressure: 48.89 bar

RUNNING THE ENGINE AT 3600 RPM:

        Brake power output: 16.7 KW

INCREASING THE POWER OUTPUT BY 10%:

The output power can be increased by changing the various parameters,

• By increasing the bore
• By increasing the stroke
• By increasing bore, stroke and connecting rod
• By increasing inlet pressure

• In case 1, RPM is changed from 1800 to 3600 RPM.
• In case 2, bore is changed from 86 to 90 mm.
• In case 3, stroke is changed from 86 to 96 mm
• In case 4, bore is changed from 86 to 88 mm, stroke is changed from 86 to 90 mm and connecting rod length is changed from 175 to 180mm.
• In case 5, intake pressure is changed from 1 to 1.1 bar.

RESULTS:

By changing the inputs as specified above, the brake power obtained is increased by 10% at a constant RPM of 3600.

• In case 1, the brake power obtained is 16.7 KW.
• In case 2, the brake power obtained is 18.4 KW.
• In case 3, the brake power obtained is 18.6 KW.
• In case 4, the brake power obtained is 18.3 KW.
• In case 5, the brake power obtained is 18.9 KW.

CONCLUSION:

• By increasing the cylinder volume by changing the bore and stroke more burning of air and fuel can takes place.
• By increasing the connecting rod length, better torque can be obtained.
• By increasing the intake pressure better combustion can be obtained, But increasing the intake air pressure requires turbocharger or supercharger which increases the cost of an engine.

The above steps increases the power output of the engine by 10%.