Assignment 2
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. A four-stroke spark-ignition engine is an Otto cycle engine. It consists of following four strokes: suction, compression, expansion, exhaust. 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 revolutions.
Model:
Case 1: RPM = 1800
Adjusting RPM in the engine from by default 3600 to 1800. The model is run using the default paramters.
Air flow rate & BSFC:
Amount of air flowing per unit of time through a medium is known as air flow rate.
Brake-specific fuel consumption (BSFC) is a measure of the fuel efficiency of any prime mover that burns fuel and produces rotational, or shaft power. It is typically used for comparing the efficiency of internal combustion engines with a shaft output. It is the rate of fuel consumption divided by the power produced.
BMEP:
The mean effective pressure is a quantity relating to the operation of a reciprocating engine and is a valuable measure of an engine's capacity to do work that is independent of engine displacement. It is said a high BMEP and a low rpm, or a low BMEP and a high rpm, can equal the same power. Larger valves, ports, pipes, compression, etc. all come into play to increase the volumetric efficiency of the engine. The most effective is to increase the number of cylinders. The more efficient it is, the higher the average pressure or BMEP.
Case 2: Increase the power output by 10% at 3600 RPM.
Power output at 3600 RPM:
10% increase in power output:
By-default being a square cylinder, the size of the bore and the length of the stroke was 60mm. The length of the connecting rod was 175mm.
To achieve the 10% increase in power output, the comprssion ratio is being ket constant at 9.5 where as the length of the stroke and size of the bore had been increased to 90mm and the length of the connecting rod had been reduced to 160mm.
Hence, a 13.17% increase in the power output was achieved.
