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Technical Documents - Documentos Técnicos: The Four Stroke Engine

In this page, we will take a peek under the car's hood and analyze the internal combustion engine from the thermodynamic point of view. The internal combustion engine is an energy converter that converts the fuel’s internal (chemical) energy into mechanical energy. By necessity, our approach will be highly idealized and our results approximate; but this is the nature of most engineering analysis. When first analyzing a system, one wishes to distill the most important features and gain a qualitative understanding which later on may serve as a gateway to more detailed and realistic analysis.

The first internal combustion engines appeared in the second half of the 19th century. The thermodynamic process that approximates the gasoline engine is named after Nikolaus Otto, who built the first successful engine in 1876-hence the Otto Cycle.

THE ENGINE'S OPERATION

 The internal combustion engine may include one or more cylinders. For example, a car engine typically contains either 4 or 6 cylinders. Fig.1 depicts schematically one cylinder and a piston. The piston is connected through a linkage mechanism to the crankshaft. When the crankshaft is at its highest point, the piston is at the top of its stroke, and the volume confined inside the cylinder is at its minimum. Since at this point the vertical velocity of the crankshaft is zero, one refers to this state as the top dead center (TDC, in short). When the crankshaft is at its lowest point, the piston is at the end of its stroke, and the volume confined inside the cylinder is at its maximum. This is the bottom dead center (BDC). Notice that the cylinder is equipped with two valves: the intake valve and the exhaust valve. A spark plug is also located in the cylinder's head. Figure 1: A Four Stroke Engine (reproduced from R. Stone, Internal Combustion Engine ) Let's start the description of the processes taking place in the cylinder when the piston is at TDC and both valves are closed. The process is conveniently described in a diagram that depicts the pressure of the gas in the cylinder as a function of its volume. See Fig. 2. i. The induction (intake) stroke: The inlet valve opens, and the piston travels from TDC to BDC. As the piston moves, low pressure forms in the cylinder, and an air-fuel mixture at the ambient temperature and pressure is sucked into the cylinder. ii. The compression stroke: The inlet valve closes, and the piston travels from the BDC to the TDC. In this process, both the air-fuel mixture’s pressure and temperature increase. Figure 2: The gas’ pressure is depicted as a function of the volume

At some point during the compression process, the spark plug fires, ignition occurs, and the fuel combusts, raising both the temperature and the pressure of the gas even further. During the compression stroke, the piston does work on the gas in the cylinder.

iii. The expansion (power) stroke: The piston moves towards BDC while the combustion process continues. The gases push the piston. Towards the end of the power stroke, the exhaust valve opens and the combustion products start escaping.

iv. The exhaust stroke: the exhaust valve remains open, and the piston moves from the BDC to the TDC expelling the combustion products. During the expulsion process, the combustion products are at a temperature and pressure above ambient conditions. At the end of the expulsion process, the exhaust valve closes, and we are back where we started.

The engine operates in fours strokes: induction, compression, expansion, and expulsion.

The engine follows a cycle in which the start and the end points are the same. The upper loop in Fig. 2 is known as the power loop, and the lower loop is the pumping loop. We will see later that the areas confined in the power and pumping loops are, respectively, proportional to the work delivered by the engine and the work consumed in expelling the exhaust gases. The motion of the piston is dictated by the crank shaft.

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