Lab Report: Four Stroke Internal Combustion Engine Simulation
Introduction
Internal combustion engines work utilizing the results of combustion as the working liquid as opposed to as heat exchange medium. The combustion is carried out in a way so as to yield high pressure gaseous products with sufficient energy to rotate a turbine or move a piston. The development of these high pressure systems presents various elements that significantly impact the formation of contaminations.
There are three essential categories of internal combustion engines: (1) the spark ignition engines that are usedto drive automobiles; (2) the diesel engines, which is are used to run heavy vehicles and heavy machinery; and (3) the gas turbine engines that are used in aircrafts because of its high pressure/weight ratio.
In a four stroke internal combustion engine, one cycle is completed in four strokes with each stroke contributing to 180° of crankshaft rotation. The four strokes of the internal combustion engine are shown in fig 1.
Suction/intake stroke:
In the suction stroke, the piston is at the top center of the engine and is about to be pushed down. the inlet valve is kept open while the exhaust valve is shut. Due to the suction created by the direction of the piston, air is drawn in and mixes with the fuel and enters the cylinder. At the end of this stroke, the inlet valve closes.
Compression Stroke
Fresh charge is sucked into the cylinder during the return stroke of the piston. Here both inlet and outlet valves remain closed. Thus, the air inside the cylinder is compressed to a critical value. Just before this stroke ends, the compressed mixture is ignited by an electric spark created between the electrodes of a spark plug that is located inside the combustion chamber. The combustion takes place when the piston is at the top center. When the fuel burns, chemical energy is converted into several other forms of energy such as heat energy and later mechanical energy that moves the piston.
Power Stroke
Exhaust Stroke
Once the power stroke finishes, the exhaust valve opens up and the piston on its way up from the bottom sweeps up the burnt gasses from the chamber. Throughout this process, the input valve remains closed and once this process is over, the exhaust valve also closes with the possibility of leaving behind trace amounts of residual gasses.
Every chamber of a four-stroke motor finishes the above four operations in two revolutions. One such revolution is of the crankshaft amid the suction and pressure strokes, and the second revolution occurs during the pressure and output strokes. Consequently for one complete cycle, there is only one power stroke while the crankshaft turns by two revolutions. The vast majority of the spark-ignition internal combustion engines are of the four-stroke kind. They are most mainstream for light motor vehicles and small type aircrafts (Mollenhauer and Tschöke, 2010).
Engine Formulas
Cylinder Swept Volume (Vc) is given by the relation:
Vc=Cylinder Area ×Stroke Length
Vc= Ac ×L=π4 × dc2×L
Where, Vc= cylinder swept volume given in litres, Ac = cylinder area measured in cm2, dc = cylinder diameter in cm, L = stroke length or the distance between the top dead center and bottom dead center in cm (Van Basshuysen and Schäfer, 2004).
Engine Swept Volume (Ve) is given by the relation
Ve=Total Cylinder's Swept Volume of the Engine
Ve= n ×Vc=n × Ac ×L=n × π4 × dc2×L
Where Ve = engine swept volume, and n = number of cylinders.
The compression ratio (r) of an engine is defined as the ration between the bottom dead center and the top dead center of the engine.
Thus, r=Cylinder volume at BDCCylinder Volume at TDC=Cylinder volume+Cylinder Clearance VolumeCylinder Clearance Volume
Or r=Vs+ VcVc=1+VsVc , where Vs = cylinder swept volume in litres
The engine volumetric efficiency (ηV) is the ratio of volume of air taken into cylinder to the maximum possible volume the cylinder can accommodate.
ηV=Volume of air taken into cylinderMaximum possible volume in the cylinder
ηV=VairVc
The engine mechanical efficiency is the ratio of the engine brake power to the engine indicated power. Mathematically, it can be shown as ηm=Engine brake powerengine indicated power=PbPi=Pi- PfPi=1- PfPi, where ηm is the mechanical efficiency, Pb is the brake power, Pi is the engine indicated power and Pf is the engine friction power.
The Otto Cycle
An ideal 4-stroke engine follows the Otto Cycle shown in Fig 2 (Mozurkewich and Berry, 1982).
Step 1-2 is an isentropic compression process where the gas is compressed at constant entropy. Step 2-3 is the combustion process where heat is increased at a constant volume causing an increase in the pressure. Step 3-4 is an isentropic expansion, or the power stroke and finally, step 4-1is the constant volume heat rejection stage.
In this cycle, Q23 = Cv(T3 – T2) and Q41 = Cv(T1 – T4). Thus, η = (Q23 + Q41)/(Q23)
This gives η=1-T4- T1T3- T2. However, T2/T1 = T3/T4 = rvγ – 1, where rv is the volumetric compression ratio. Thus, rearranging and solving these equations, we get, η=T3- T4T3 . It is clear that better the compression ratio, the better is the power output and corresponding efficiency of the engine. In real heat engines, γ does not exceed 1.4 and due to the heat transfer involved in compression and expansion, this process cannot be isentropic. Thus, if γ is taken anywhere between 1.2 and 1.3, we would have a fairly reasonable approximation of a real engine (Blair, 1999).
Method Used
A simulation of the closed cycle part of the Otto cycle of a four stroke engine was done using the mixed cycle approach.
Results
The specifications of the four stroke car engine used are given in Table 1.
The given initial parameters are listed in table 2.
The data obtained through the simulation is listed in Table 3.
The engine performance is listed in table 4. The corresponding P-V diagram is shown in fig 3.
The indicated mean effective pressure (imep) is a pseudo constant pressure that would repeat the work cycle if acted upon the piston during the power stroke. The imep is an effective parameter to compare engines and engine design since it does not depend on engine speed and size. Here, the imep was measured as 12.06 bar which suggests that the engine can be used at speeds greater than 2600 rpm (Shehata & Abdelrazek, 2008). The bmep measured is 10.25 bar, which is expected of a standard 4 stroke engine.
The brake specific fuel consumption is a parameter that measures how effectively the fuel supplied to the engine is used to generate power. A low bsfc is desireable since it means less fuel is consumed to generate a particular amount of power. Here, the value obtained is 0.233 kg/kWh which is quite good. The overall efficiency of this four stroke engine is 38.4%.
Conclusions
The engine simulation was run and the results have been tabulated and discussed. The corresponding P-V diagram has also been plotted and shown. For a four-stroke engine running at a maximum speed of 2500 rpm, the specifications obtained were in agreement with what has been reported elsewhere.
References
Shehata, M.S. and Abdelrazek, S.M., 2008. Engine Performance Parameters and Emission Reduction methods for spark ignition Engine. Engineering Research Journal, 120, pp.M32-M57.
Blair, G.P., 1999. Design and simulation of four-stroke engines. Training, 2013, pp.08-02.
Mozurkewich, M. and Berry, R.S., 1982. Optimal paths for thermodynamic systems: the ideal Otto cycle. Journal of Applied Physics, 53(1), pp.34-42.
Van Basshuysen, R. and Schäfer, F., 2004. Internal combustion engine handbook-basics, components, systems and perspectives (Vol. 345).
Mollenhauer, K. and Tschöke, H. eds., 2010. Handbook of diesel engines. Springer Science & Business Media.
