Internal Combustion Engines and Intake Stroke - Assignment Example

Internal Соmbustiоn Engines (РV diagram) Name: Course: Lecturer: Institution: City & State: Date: Introduction In a four-stroke engine, the piston must complete four separate strokes (intake, compression, power stroke and exhaust stroke). This occurs in two crankshaft revolutions that are separate, which gives one thermodynamic cycle. Two crankshaft rotations result into one power cycle, which starts at the TDC (top dead center). When the piston is at farthest end from the crankshaft allows for the full power cycle (Srinivasan 162). Thus, when the piston moves from the TDC to the BDC it completes one power cycle. Engine start and Intake stroke This is the piston introduction stroke. During this stroke, the piston moves to the cylinder bottom from its top, thus altering the cylinder volume to increase thus leaving a space in the cylinder (Gupta 497),. Thus, air/fuel is forced into the cylinder for the petrol engine while only air enters the cylinder diesel engine (Stone 154). This develops the volumetric efficiency of the engines and thus the intake valve closes. Therefore, both intake and exhaust valves are now closed. Figure 1: start of the stroke (Srinivasan 160) Figure 2: intake stroke (Srinivasan 160) Compression stroke Intake and exhaust valves are closed at this stroke. The piston moves back to the cylinder top forcing the air/fuel mixture or air into the combustion chamber. Air or air/fuel mixture temperature rises due to compression (Onka 564). Figure 3: compression stroke (Srinivasan 161) Power stroke This starts the cycle second revolution and the piston is still at the cylinders’ TDC. This is the ignition start stroke. In Petrol engine, air/fuel is ignited by spark plug while for diesel engine; fuel is ignited by the heat generated. This creates pressure, which forces the piston to the bottom dead center (Srinivasan 162). Figure 4: power stroke (Srinivasan 162) Figure 5: sparking (Srinivasan 162) Exhaust stroke Exhaust valve opens as the piston moves to the top dead center. This forces out the spend air-fuel mixture from the engine through the exhaust valve (Srinivasan (a) 163). The exhaust stroke completes the engine power cycle for the four-stroke engine. Figure 6: exhaust stroke (Srinivasan 163) Four-stroke diesel engine A four-stroke diesel engine is one that initiates fuel ignition by heat of compression. These engines use diesel fuels that have low volatility and this make it hard start when the engine is cold. Thus, diesel engines use glow plug for ignition, referred to as Cetane rating. This makes enable the engines to have high compression ratio, thus developing a higher thermal efficiency compared to petrol engines. The diesel engine uses highly compressed air to ignite the fuel at the compression stroke (Srinivasan (a) 164). Diesel engines are of three types, the small type, and medium and large diesel engine type. Moreover, in the diesel engine the fuel controls power, thus there is enough air to ignite the fuel during low power running of the engine. Four stroke petrol engine In four-stroke petrol engine, air-fuel is pre-mixed prior to ignition, where spark plug ignites the fuel in the compression stroke. Petrol engines, there is spark ignition that ignites the fuel-air mixture in the compression stroke of the power cycle. Petrol engines used to have air and fuel pre-mixed in the carburetor prior to ignition (Gupta 234). However, this now is done electronically through a fuel injection air/fuel mixing. Petrol engines have an ignition coil controlled by the engine control unit, which ignites the air-fuel mixture at the compression stroke. Difference between diesel and petrol four-stroke engine Power: Essentially, diesel engines have a higher thermal efficiency thus they have a higher power than the petrol engines (Srinivasan (B) 288). Ignition: in the petrol engine, air and fuel is pre-mixed prior to ignition, while in the diesel engine fuel is ignited by compressed air (Srinivasan (B) 288). Diesel engines spend less fuel than the petrol engines when performing an equal task. This is due to a higher expansion ratio of the diesel engine. Moreover, petrol engines have lower fuel conversion efficiency compared to the diesel engines, where, diesel engines convert 45%of the fuel while petrol engine convert about 30% to mechanical energy (Onkar 564). Diesel engines do not require a high voltage for ignition thus they are more reliable and they well fit in damp environments. In addition, the diesel engine does not have spark plugs or ignition coil, which can interfere with radio frequencies (Srinivasan (B) 289). Compression ratio of an engine is the proportion of the volume measured in the combustion chamber to the smallest from the largest volume. Therefore, piston shifting between the BDC and the TDC allows for determination of the compression ratio (Onkar 563). Due to higher thermal efficiency in the diesel engine, they tend to have higher compression ratio than the petrol engine. Results and discussion Petrol engine Table 1: power developed by the petrol engine diameter of piston 0.08 m length of stoke 0.1 m       crank throw 0.05 m con rod length 0.15 m             compression ratio 7.5   volume 0.502655 liters fuel 0.9 units power 1.865329 units Figure 7: PV diagram for the petrol engine For an ideal Otto cycle (Mike 200), During the compression stroke: Compression ratio, and  In the power stroke:  and  During combustion stroke:  and  Work per cycle,  Thus power developed by the engine,  Where: Cps= cycles per second W= work done P= power T= temperature F= fuel/air ratio p= pressure  specific heat capacity  specific heat ratio  = compression ratio Q= fuel heating value As the Otto cycle equations relate, the PV diagram for the petrol engine clearly indicates the power cycle for the engine. Therefore, there is a higher pressure developed in the compression stroke, while allowing for low pressure in the power stroke (Srinivasan (a) 163). This low pressure developed during the power stroke, pushes the piston to BDC to allow for exhaust stroke. Thus this develops low heating value on the fuel fig8. Figure 8: heating value of the petrol engine Diesel engine Figure 9: fuel switched Changing the fuel heating value for the engine and switching the engine fuel changes the PV diagram. The entire system power and compression strokes adiabatic and thus the engine operate at constant pressure and volume (Srinivasan (B) 147). Thus, it gives a higher amount of the engine power after switching the fuel. Figure 10: fuel switched heating value Switching the engine fuel and maintaining the fuel in the engine alters the heating value of the fuel. This is because; diesel requires a higher heating value to ignite (Mike 217). Thus, it develops a higher compression ratio than the petrol engine. Conclusion There exist a major difference between petrol and diesel engines. This majorly forms the main discrepancies that exist between the two types of four stroke engines. Moreover, the differences created between the engines create a difference in power developed by each engine and the amount of work that each engine can do (Stone 127). Diesel engines have a higher thermal efficiency resulting from the high compression ratio of the engine to ignite fuel. In addition, petrol engines burn fuel less efficient and faster making the engine less strong (Srinivasan (a) 164). Diesel engines burn about 45% of the fuel and this is turned to mechanical energy, which provides the engine power. References Gupta, H.N.2006, Fundamentals of Internal Combustion Engines, PHI Learning Pvt. Ltd, New Delhi. Mike, P. 2011, Thermodynamics for Dummies, John Wiley & Sons, New York. Onkar, S. 2006, Applied Thermodynamics, New Age printing press, New Delhi. 564 Stone, R. 2012, Introduction to Internal Combustion Engines, Palgrave Macmillan Limited, New York. Srinivasan, S. 2001, Automotive Engines, Tata McGraw-Hill Education, New York. 160-164 Srinivasan, S. 2001, Automotive Mechanics, 2E, Tata McGraw-Hill Education, New York. Read More