engine cycles chapter3

7/28/2019 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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Process 3-4-5- intake, and exhaust scavenging.

Exhaust port open and intake port open:



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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

engine cycles chapter3

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