Internal Combustion Engine - Bangladesh University of ...teacher.buet.ac.bd/ronin/ME 267-ICE.pdf ·...

Internal Combustion Engine

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ME 267

Fundamentals of Mechanical Engineering



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Internal Combustion Engine (ICE)

The Internal Combustion Engine is an engine in which the

combustion of fuel and an oxidizer (typically air) occurs

inside a ‘confined space’ called a combustion chamber.

This ‘exothermic’ reaction creates gases at high

temperature and pressure, which are permitted to expand

inside that confined chamber. Thrust produced by this

expanding gas drives the engine creating useful work.


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Classification of ICE


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Classification of ICE

Spark Ignition (SI) IC Engine

Inside the combustion chamber of this type of

engine, the mixture of fuel and air is ignited by a ‘spark

plug’ to initiate the exothermic combustion reaction.

Compression Ignition (CI) IC Engine

This type of IC Engine does not have spark plug.

Inside the combustion chamber of this engine, air is

compressed to a high enough pressure and temperature

that combustion occurs ‘spontaneously’ when fuel is

injected at the end of air-compression.


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Petrol or SI Engine Diesel or CI Engine

Low noise and vibration High noise and vibration

High fuel consumption rate Fuel consumption is lower

Higher RPM and power range Higher torque at low speeds

CO2 emission is higher NOx emission is higher

Longevity is lower Higher longevity

Separate ignition system No separate ignition system

Used at light duty vehicles Used at heavy duty vehicles


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Engine Terminology


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Actual 4-stroke Engine P-V Diagram 1. Intake Stroke

2. Compression Stroke

3. Power Stroke

4. Exhaust Stroke


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Complexities in analysis of actual engine cycle

- Considering the composition change during combustion

- Calculation of friction loss, pressure and temp. gradients

- Heat loss from the cylinder

- Calculation of work required to charge and purge cylinder

Air-standard assumptions for simpler qualitative analysis

- Working fluid is only air which takes and rejects heat

- No intake and exhaust stroke

- No frictional and heat loss

Air-standard cycles

1. Otto or Petrol cycle & 2. Diesel cycle


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Air-standard OTTO Cycle (SI Engine)

a b


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Process 1-2: Isentropic Compression.

Piston: BDC TDC

Process 2-3: Constant Volume Heat Addition.

Piston: at TDC

Process 3-4: Isentropic Expansion (Power Stroke)

Piston: TDC BDC

Process 4-1: Constant Volume Heat Rejection.

Piston: at BDC


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In the p-v diagram

Area 1-2-a-b-1: Work input per unit mass during the compression

process.

Area 3-4-b-a-3: Work done per unit mass in the expansion process.

Area 3-4-1-2: Net Work Output.

In the T-s diagram

Area 2-3-a-b-2: Quantity of Heat added to unit mass of air from any

External Source.

Area 1-4-a-b-1: Quantity of Heat rejected to unit mass of air from any

External Source.

Area 1-2-3-4: Quantity of Net heat added to unit mass of air from any

External Source.


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Air-standard DIESEL Cycle (CI Engine)

Indicated Work

Given the cylinder pressure data over the operating

cycle of the engine one can calculate the work done

by the gas on the piston.

The indicated work per cycle is pdVWi

Compression

W<0

Power

W>0 Intake

W>0

Exhaust

W<0

WA > 0

WB < 0

Pi = Wi N / nR w/units: (kJ/cycle) (rev/s) / (rev/cycle)

where N – crankshaft speed in rev/s

nR – number of crank revolutions per cycle

= 2 for 4-stroke

= 1 for 2-stroke

Power can be increased by increasing:

• the engine size, Vd

• compression ratio, rc

• engine speed, N

Indicated Power

Mechanical Efficiency

Some of the power generated in the cylinder is used

to overcome engine friction. The friction power is

used to describe these losses:

Pf = Pi - Pb

Friction power can be measured by motoring the engine.

The mechanical efficiency is defined as:

m = Pb / Pi = 1- (Pf / Pi )

Mechanical efficiency depends on throttle position, engine

design, and engine speed. Typical values for car engines

at WOT are 90% @2000 RPM and 75% @ max speed.


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mean effective pressure

The MEP (mean effective pressure) is the theoretical

constant pressure that, if acted on the piston during the

power stroke, would produce the same net work as

actually developed in one cycle.

minmax VV

WnetMEP


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Otto cycle


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Diesel Cycle


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An ideal Diesel cycle with air as the working fluid has a

compression ratio of 18 and a cutoff ratio of 2. At the beginning

of the compression process, the working fluid is at 101.325

KPa, 27°C, and 1917 cm3. Utilizing the cold-air-standard

assumptions, determine- (a) the temperature and pressure of air

at the end of each process, (b) the net work output and the

thermal efficiency, and (c) the mean effective pressure.


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Internal Combustion Engine - Bangladesh University of ...teacher.buet.ac.bd/ronin/ME 267-ICE.pdf ·...

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