Engine Work With Pressure Essay

Introduction

All internal combustion engines(ICE's) are heat engines. The working of the of the automobile engine is primarily dependent on simple principles of pressure and volume (Zhou, Liu & Gao, 2004). Therefore, the work output produced by the ICE is dependent on the amount of variation of the pressure in the cylinder with volume. The engine cylinder is responsible converting force generated from fuel-air combustion into work. Different engine types have different types of combustion cylinders. These cylinders adapted to the pressure needed to produce the reciprocating motion that does work. Higher pressures require larger cylinders and are used for heavy machine applications. In a four-stroke engine, pressure varies throughout the strokes, right from the intake, compression, combustion, and exhaust strokes. The complete engine cycle is normally 720o (Lancaster, Krieger & Lienesch,1975). The most critical cylinder pressure is the one responsible for the production of power. This is the pressure that causes expansion in the cylinder during the power stroke. The cylinder pressure is a very critical factor that determines the output of the engine. Engine performance can be determined by simply knowing the variation of pressure in the cylinder.

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The pressure during the Intake stroke compression and power strokes

Absolute engine pressure is the pressure used during engine simulation. During the intake stroke when simulating how an engine works under pressure, the intake valves are opened. The piston moves down the cylinder simultaneously thus creating a partial vacuum. The highest volume is attained when the piston is at the BDC. The pressure found in the cylinder at this juncture is lower than the atmospheric pressure. Therefore, to achieve a state of equilibrium, an air-fuel mixture rushes into the cylinder. The buildup of pressure at the intake stroke is dependent on a variety of factors. These include the amount of restriction in the head and also in the runners and throttle. For example, when the cylinder is idle that is with all the throttles closed, the pressure found in the cylinder is lowest. This is to mean that there is less air-fuel mixture found in the cylinder as the intake valves closed. Therefore, less power required to do work is produced (Blair,1999). As the throttle is opened, the pressure in the cylinder during the intake build up. More fuel-air mixture is therefore ingested into the cylinder. As a result, more power is produced. During this phase, the volume decreases as pressure builds up in the cylinder. This happens when the piston starts moving towards the TDC from the BDC. The inlet valve is still open as this process occurs and therefore sometimes is taken for the cylinder pressure to exceed that of the atmosphere. During the compression of the air-fuel mixture in the cylinder, a spark is released by the spark plug. This ignites the air-fuel mixture, and there is a very high-pressure build-up in the cylinder. The average peak cylinder pressure, normally considered at the Top Dead Centre(TDC) falls in between 300psi for light-duty vehicles to around 1000psi for production engines. It can also go as high as 1800 psi for race engines. However, when the piston is at the Bottom Dead Centre(BDC), the average pressure drops to between 200 and 500psi (Alla,2002).

Pressure and work done at the exhaust Stroke

Next, to the BDC, the exhaust valves open. This causes exhaust gases to exit from the cylinder, which has a higher pressure than the surrounding atmospheric pressure. This is defined as "blow down." The action of the exhaust gases being released due to high pressure in the cylinder and also that of the rate of opening of the exhaust valves is what causes a sonic or loud sound to be produced. After this process, the cylinder goes up to the TDC and in the process expels all the remaining exhaust gases. The pressure in the cylinder after the waste gases have been released will still be higher than the atmospheric pressure, depending on the degree at which the mufflers and valves are restrictive. The higher the cylinder pressure during the exhaust stroke the higher the amount of work done by the engine to expel the waste gases. The pressure in the cylinder is always a variable depending on the stroke of the engine. The peak cylinder pressure can be defined as the maximum pressure that can be reached in the combustion cylinder. The peak cranking compression pressure, on the other hand, can be defined as the maximum pressure attained in the cylinder TDC before combustion occurs. The indicated mean effective pressure(IMEP) is defined as the average pressure in the combustion cylinder when the engine is operational (Abdul Haleem,2007). The IMEP is derived from a theoretical engine pressure known as the indicated horsepower. This is attained under frictionless conditions. As a result, the indicated horsepower does not give the actual amount of work done when power is transferred to the propeller shaft (Eriksson & Andersson, 2002). The work produced by the engine inside the cylinders is a product of the force due to the explosion of the air-fuel mixture and the stroke. This is normally referred to as the indicated specific work. However, the work transferred to the crankshaft is referred to as the brake specific work, since the engines have some connection to the brake fluid.

References

Alla, G. A. (2002). Computer simulation of a four-stroke spark ignition engine. Energy Conversion and Management, 43(8), 1043-1061.

Eriksson, L., & Andersson, I. (2002). An analytic model for cylinder pressure in a four-stroke SI engine (No. 2002-01-0371). SAE Technical Paper.

Lancaster, D. R., Krieger, R. B., & Lienesch, J. H. (1975). Measurement and analysis of engine pressure data (No. 750026). SAE Technical paper.

Abdul Haleem, S. M. (2007). Theoretical and experimental investigation of engine performance and emissions of a four strokes spark ignition engine operated with hydrogen blended gasoline (Doctoral dissertation, Ph.D. thesis, College of Engineering, Al-Mustansiriya University, Baghdad, Iraq).

Zhou, L., Liu, X. J., & Gao, Z. Y. (2004). Internal combustion engine.

Blair, G. P. (1999). Design and Simulation of Four-Stroke Engines. Training, 2010, 07-08.