02-03-2013, 11:40 AM
Dynamic Analysis of the Internal Combustion Engine
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Thermodynamics Process of the Internal Combustion Engine: Review
1. The induction (intake) stroke: The inlet valve opens, and thepiston travels from TDC (state 0) to BDC (state 1). 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.
2. The compression stroke: The inlet valve closes, and the piston travels from the BDC (state 1) to the TDC (state 2). In this process, both the air-fuel mixture’s pressure and temperature increase. During the compression stroke, the piston does work on the gas in the cylinder. 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 (state 2 to state 3).
3. The expansion (power) stroke: The piston moves towards BDC (state 4) while the combustion process continues. The gases push the piston. Towardsthe end of the power stroke, the exhaust valve opens and the combustion products start escaping.
4. The exhaust stroke: As the exhaust valve opens, the pressure drops (state 4 to state 1). The piston moves from the BDC to the TDC expelling the combustion products (state 1 to state 0). During the expulsion process, the combustion products are at a temperature and pressure above ambient conditions. At the end ofthe expulsion process, the exhaust valve closes, and we are back where we started.
Dynamics: An overview
1. The pressure of the gases (air + fuel, or by products of combustion) exerts a force on the piston. Think of this force as being an “input force”, although during some parts of this cycle, we know that the gases in the cylinder are not doing positive work.
2. All the parts of the internal combustion engine have a finitemass (inertia). Thus a fraction of the input forces are “spent”on accelerating or decelerating the masses.
3. Some of the input is used to overcome the friction on the piston walls and the friction at the bearings. (Gravitational forces are insignificant for internal combustion engines).
4. The crank is coupled to the crankshaft which in turn is coupled via a power train to the wheels of the automobile. The crankshaft may also power the water pump, camshaft, power steering pump, the air conditioning compressor and other accessories. A significant part of the input force is used to drive the automobile. Since the crank has a rotary motion, the fraction of the input force used to drive the crank is effectively a moment, and is called the turning moment.