Diesel Cycle vs Otto Cycle

 Duel Cycle Dual cycle, or limited pressure cycle, is a thermodynamic cycle that combines the Otto cycle and the Diesel cycle. In the dual cycle, combustion occurs partly at constant volume and partly at constant pressure. It can be used to describe internal combustion engines. Dual Cycle – Processes In a dual cycle, the system executing the cycle undergoes a series of five processes: two isentropic (reversible adiabatic) processes alternated with two isochoric processes and one isobaric process: Isentropic compression (compression stroke) – The gas is compressed adiabatically from state 1 to state 2, as the piston moves from intake valve closing point (1) to top dead center. The surroundings do work on the gas, increasing its internal energy (temperature) and compressing it. On the other hand, the entropy remains unchanged. The changes in volumes and their ratio (V1 / V2) are known as the compression ratio. The compression ratio is smaller than the expansion ratio. Isochoric compression (ignition phase) – In this phase (between state 2 and state 3), there is a constant volume (the piston is at rest ) heat transfer to the air from an external source while the piston is at rest at the top dead center. This process is similar to the isochoric process in the Otto cycle. It is intended to represent the ignition of the fuel-air mixture injected into the chamber and the subsequent rapid burning. The pressure rises, and the ratio (P3 / P2) is known as the “explosion ratio”. Isobaric expansion (power stroke) – In this phase (between state 3 and state 4), there is a constant pressure (idealized model) heat transfer to the air from an external source (combustion of the fuel) while the piston is moving toward the V4. During the constant pressure process, energy enters the system as heat Qadd, and a part of the work is done by moving pistons. Isentropic expansion (power stroke) – The gas expands adiabatically from state 4 to state 5 as the piston moves from V3 to the bottom dead center. The gas works on the surroundings (piston) and loses an amount of internal energy equal to the work that leaves the system. Again the entropy remains unchanged. Isochoric decompression (exhaust stroke) – In this phase, the cycle completes by a constant-volume process in which heat is rejected from the air while the piston is at the bottom dead center. The working gas pressure drops instantaneously from point 5 to point 1. The exhaust valve opens at point 5. The exhaust stroke is directly after this decompression. As the piston moves from the bottom dead center (point 1) to the top dead center (point 0) with the exhaust valve opened, the gaseous mixture is vented to the atmosphere, and the process starts anew. Thermal Efficiency for Dual Cycle In general, the thermal efficiency, ηth, of any heat engine is defined as the ratio of the work it does, W, to the heat input at the high temperature, QH. The thermal efficiency, ηth, represents the fraction of heat, QH, converted to work. Since energy is conserved according to the first law of thermodynamics and energy cannot be converted to work completely, the heat input, QH, must equal the work done, W, plus the heat that must be dissipated as waste heat QC into the environment. Therefore we can rewrite the formula for thermal efficiency as: Therefore the heat added and rejected are given by: Qadd-1 = mcv (T3 – T2) Qadd-2 = mcp (T4 – T3) Qout = mcv (T5 – T1) Therefore the thermal efficiency for a dual cycle is: Engine Performance Engine performance is often characterized by the engine operating behavior in the speed–load domain, for example, the behavior of emissions, fuel consumption, noise, mechanical and thermal loading. Why engine performance is important ? 1) Efficient and reliable operation of the engine. 2) Helps in saving fuel and optimizing SFOC( Specific Fuel Oil Consumption. 3) Helps in predicting the necessary repairs and preventing engine failure. 4) Helps in reducing spare parts cost and increasing time between overhauls. How engine performance is measured? The most common method for measuring torque and power of an engine is through a dynamometer, or “dyno” test. This test works usually by connecting the output shaft of an engine to a set-up that applies a resistive load What improves engine performance? Increasing compression is the most productive way to increase horsepower. Build compression into your engine and you build in power. In more than a century of internal combustion, there has never been a more common sense way to make power. But be careful about how you raise compression What can affect engine performance? Factors Affecting the Engine Performance (Automobile)  Heat Transfer. The heat is exchanged in both directions between the gases and engine cylinder walls and. ...  Residual Gas. ...  Valve Resistance. ...  Valve Timing. ...  Combustion Time. ...  Incomplete Combustion. ...  Atmospheric Conditions