A thermodynamic analysis of internal combustion engine cycles,
Engine Cycles Chapter3
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Transcript of Engine Cycles Chapter3
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8. Exhaust blowdown is approximated by a
constant-volume process.
9. All processes are considered reversible.
AIR-STANDARD CYCLES
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In air-standard cycles, air is considered an ideal gassuch that the following ideal gas relationships can be used:
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AIR-STANDARD CYCLES
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Air property values during the operating cycle andexhaust flow:
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OTTO CYCLE
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OTTO CYCLE
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OTTO CYCLE
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Solution
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Solution
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Solution
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Solution
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Solution
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ENGINE CYCLE with turbocharger or super
charger
ENGINE CYCLE AT PART
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ENGINE CYCLE AT PART
THROTTLE
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EXHAUST PROCESS
The exhaust process consists of two steps: blowdown and exhauststroke. When the exhaust valve opens near the end of the
expansion stroke (point 4 in Fig. 3-6), the high-temperature gasesare suddenly subjected to a pressure decrease as the resultingblowdown occurs.
A large percentage of the gases leaves the combustion chamberduring this blowdown process, driven by the pressure differential
across the open exhaust valve.
When the pressure across the exhaust valve is finally equalized, thecylinder is still filled with exhaust gases at the exhaust manifold
pressure of about one atmosphere. These gases are then pushed
out of the cylinder through the still open exhaust valve by thepiston as it travels from BDC to TDC during the exhaust stroke.
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EXHAUST PROCESS
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EXHAUST PROCESS
P4/P7= (V7/V4)k= P4/Pex= P4/Po
P3/P7= (V7/V3)k= P3/Pex= P3/Po
Xr= (1/rc)(T4/Tex)(Pex/P4)where: rc = compression ratio
Pex =P7=Po = one atmosphere under most conditions
Tex = T7
and T4 andP4 are conditions in the cylinder when the
exhaust valve opens
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EXHAUST PROCESS
When the intake valve opens, a new charge of inlet airma enters the cylinder and mixes with the remainingexhaust residual from the previous cycle.The mixing occurs such that total enthalpy remainsconstant and:
where hex, ha, and hm are the specific enthalpy valuesof exhaust, air, and mixture, all of which are treated
as air in air-standard analysis.If specific enthalpy values are referenced to zero value
at an absolute temperature value of zero, then h = cpTand:
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EXHAUST PROCESS
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EXHAUST PROCESS
EXHAUST PROCESS
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EXHAUST PROCESS
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DIESEL CYCLE
constant pressure cycle
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DUAL CYCLE
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DUAL CYCLE
Limited pressure cycle
Thermodynamic Analysis of Air Standard Dual
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Thermodynamic Analysis of Air-Standard Dual
Cycle
The analysis of an air-standard Dual cycle is the same as
that of the Diesel cycle except for the heat input process
(combustion) 2-x-3.
Thermodynamic Analysis of Air Standard Dual
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Thermodynamic Analysis of Air-Standard Dual
Cycle
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COMPARISON OF OTTO DIESEL AND DUAL CYCLES
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COMPARISON OF OTTO, DIESEL, AND DUAL CYCLES
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EXAMPLE PROBLEMA small truck has a four-cylinder, four-liter CI engine thatoperates on the air-standard Dual cycle (Fig. 3-10) usinglight diesel fuel at an air-fuel ratio of 18.
The compression ratio of the engine is 16:1 and the
cylinder bore diameter is 10.0 cm. At the start of thecompression stroke, conditions in the cylinders are 60Cand 100 KPa with a 2% exhaust residual.
It can be assumed that half of the heat input from
combustion is added at constant volume and half atconstant pressure
Calculate:
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Solution
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Solution
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Solution
S l ti
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Solution
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Solution
S l ti
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Solution
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S l ti
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Solution
MILLER CYCLE
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MILLER CYCLE
Figure 3-14 Air-standard Millercycle for unthrottled naturally
aspirated fourstroke cycle SI
engine.
If the engine has early intake valveclosing, the cycle will be 6-7-1-7-2-
3-4-5-7-6.
If the engine has late intake valve
closing, the cycle will be 6-7-5-7-2-
3-4-5-7-6.
MILLER CYCLE
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MILLER CYCLEEXAMPLE PROBLEM 3-5
The four-cylinder,2.5-literSI automobile engine of Example
Problem 3-1is converted to operate on an air-standardMiller cycle with early valve closing(cycle6-7-1-7-2-3-4-5-6 in Fig. 3-15). It has a compression ratio of 8:1 and anexpansion ratio of 10:1.A
supercharger is added that gives cylinder pressure of
160KPa when the intake valve closes, as shown in Fig.3-15.The temperature is again 60Cat this point. The samefuel and AF are used with combustion efficiency17c =100%.
Calculate:
1. temperature and pressure at all points in the cycle2. indicated thermal efficiency
3. indicated mean effective pressure
4. exhaust temperature
MILLER CYCLE
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MILLER CYCLE
EXAMPLE PROBLEM 3-5
Solution
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Solution
Solution
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Solution
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solution
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solution
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Processes of Two stroke SI engine cycle
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Processes of Two stroke SI engine cycle
Process 3-4-5- intake, and exhaust scavenging.
Exhaust port open and intake port open:
Processes of Two stroke SI engine cycle
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Processes of Two stroke SI engine cycle
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Processes of Two stroke SI engine cycle
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Many compression ignition engines-especially largeones-operate on two-stroke cycles.
These cycles can be approximated by the air-standardcycle shown in Fig.3-17.
This cycle is the same as the two-stroke SI cycle exceptfor the fuel input and combustion process.
Instead of adding fuel to the intake air or early in thecompression process, fuel is added with injectors latein the compression process, the same as with four-
stroke cycle CI engines.Heat input or combustion can be approximated
by a two-step (dual) process .
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Processes of Two stroke CI engine
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Processes of Two stroke CI engine
cycle
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Home work #3
due to:31/10/210
3.1 , 3.6, 3.7, 3.12
,3.15,3.17
Today is the dead time for
hw #2