THERMAL ENGINEERING (ME 2301 ). M.R.SWAMINATHAN Assistant Professor Department of Mechanical Engineering Anna University Chennai Chennai-25. DIESEL CYCLE .
THERMAL ENGINEERING(ME 2301 )
Department of Mechanical Engineering
Anna University Chennai
The diesel cycle is the ideal cycle for CI (Compression-Ignition) reciprocating engines. The CI engine first proposed by Rudolph Diesel in the 1890s, is very similar to the SI engine, differing mainly in the method of initiating combustion.
In diesel engines, ONLY air is compressed during the compression stroke, eliminating the possibility of auto-ignition.
Diesel engines can be designed to operate at much higher compression ratios, typically between 12 and 24.
The fuel injection process in diesel engines starts when the piston approaches TDC and continues during the first part of the power stroke.
Therefore, the combustion process in these engines takes place over a longer interval.
Because of this longer duration, the combustion process in the ideal Diesel cycle is approximated as a constant-pressure heat-addition process.
This is the ONLY process where the
Otto and the Diesel cycles differ.
Ideal Cycle for CI Engines
Thermal efficiency of Ideal Diesel Cycle
Under the cold-air-standard assumptions, the efficiency of a Diesel cycle differs from the efficiency of Otto cycle by the quantity in the brackets.
The quantity in the brackets is always greater than 1. Therefore, hth,Otto > hth, Dieselwhen both cycles operate on the same compression ratio.
Also the cuttoff ratio, rc decreases, the efficiency of the Diesel cycle increases.
DUAL / LIMITED PRESSURE CYCLE
BRAYTON CYCLE – GAS TURBINE
The open gas-turbine cycle can be modeled as a closed cycle, as shown in the figure below, by utilizing the air-standard assumptions
BRAYTON CYCLE - PROCESSES
12 Isentropic compression (in a compressor)
23Constant pressure heat addition
34Isentropic expansion (in a turbine)
41Constant pressure heat rejection
The highest temperature in the cycle occurs at the end of the combustion process, and it is limited by the maximum temperature that the turbine blades can withstand.
Burning exact quantity of Air/Fuel mixture at right time and place produces more power output
Moreover more economy and lesser emissions
External Combustion Engine
A device where the working fluid is obtained indirectly
e.g Steam or Locomotive boiler
Coal is first burnt and then the heat energy obtained from coal is utilised to transform water to steam which is the working fluid.
So the name External combustion engine just to differentiate from an I.C. engine
Internal combustion engines are classified according to
CLASSIFICATION OF I.C ENGINES
Near the end of the compression stroke, fuel is injected into the cylinder. After a short delay, the fuel ignites spontaneously, a process called auto ignition.