MECHANICALFUNDA for Mechanical Engineers

What is petrol engine

The first practical petrol engine was built in 1876 in Germany by Nikolaus August Otto.
The first petrol combustion engine (one cylinder, 121.6 cm³ displacement) was prototyped in 1882 in Italy by Enrico Bernardi.

In petrol engine mainly these process included :

Suction of air
Mixing of fuel with air after breaking the liquid fuel into highly automised
Ignition of air-fuel mixture either with a spark or by self ignition after raising the temperature of air by compressing it mainly this self ignition used in diesel engine.
Burning of highly automised fuel particles which result in releasing heat energy.
Heat energy converted into kinetic energy in form of reciprocating motion. the expansion of heated gas and their forces act on the engine piston, pushing them downwards result in reciprocating motion of piston.
The reciprocating motion of piston enables the crank-shaft to rotate and finally gets converted into the rotary motion and passed it motion on the wheel.

Working principle of petrol engine (Otto Engine) explained below :

The conventional Internal Combustion Engine operates on two basic principle :

Otto Cycle
Diesel Cycle

Petrol engine works on principle of Otto cycle so now let we know what is Otto cycle how and how it works? What is Otto Cycle?

Otto cycle is also known as four stroke spark-ignition cycle. It was named after German engineer Nikolaus Otto who invented first four stroke engine.

Otto cycle constructed in four stroke cycle :

Suction Stroke :

In suction stroke piston moving downward direction and opening of inlet valve that you see in above fig stroke 1. that opening of inlet valve enter fuel and creates suction of air and fuel mixture. this stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.)

Compression Stroke :

In compression stroke begins at (B.D.C) or end of suction stroke and piston moves upward and inlet valve closed. In this stroke piston compresses the air-fuel mixture in preparation for ignition during the power stroke. Both inlet and exhaust valve are closed in this stroke.

Combustion Stroke :

Combustion stroke also known as power or ignition stroke. while piston is at (T.D.C) at the end of compression stroke the compressed fuel-air mixture is ignited by spark plug and returning piston to the (B.D.C).This stroke produces mechanical work from the engine to turn the crankshaft. before start of combustion stroke the crankshaft has completed a full 360 degree revolution.

Exhaust Stroke :

Exhaust stroke means outlet. during this stroke piston returns (B.D.C) to (T.D.C) while exhaust valve is open and the piston push the exhaust gases above them through the exhaust valve which opens during this stroke.

The Otto cycle is constructed from:

Top and bottom of the loop : a pair of quasi-parallel and isentropic processes ( Adiabatic reversible )

Left and right side of loop : a pair of parallel isochoric process ( constant volume )

The process are described by :

An Otto cycle is an idealized thermodynamics cycle that describes the functioning of a typical

Spark ignition piston engine

Process 0–1 a mass of air is drawn into piston/cylinder arrangement at constant pressure.

Process 1–2 is an adiabatic (isentropic) compression of the air as the piston moves from bottom (BDC) to (TDC).

Process 2–3 is a constant-volume heat transfer to the working gas from an external source while the piston is at (TDC). This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning.

Process 3–4 is an adiabatic (isentropic) expansion (power stroke).

Process 4–1 completes the cycle by a constant-volume process in which heat is rejected from the air while the piston is at (BDC)

Process 1–0 the mass of air is released to the atmosphere in a constant pressure process.

Efficiency of Otto Cycle :

The starting point is the general expression for the thermal efficiency of a cycle:

$\displaystyle \eta = \frac{\textrm{work}}{\textrm{heat input}} =\frac{Q_H +Q_L}{Q_H} = 1+\frac{Q_L}{Q_H}.$

The convention, as previously, is that heat exchange is positive if heat is flowing into the system or engine, so

is negative. The heat absorbed occurs during combustion when the spark occurs, roughly at constant volume. The heat absorbed can be related to the temperature change from state 2 to state 3 as:

$\displaystyle Q_H$	$\displaystyle =Q_{23} =\Delta U_{23} \qquad (W_{23} =0)$
	$\displaystyle =\int_{T_2}^{T_3} C_v dT =C_v (T_3 -T_2).$

The heat rejected is given by (for a perfect gas with constant specific heats)

$\displaystyle Q_L =Q_{41} =\Delta U_{41} =C_v (T_1 -T_4).$

Substituting the expressions for the heat absorbed and rejected in the expression for thermal efficiency yields

$\displaystyle \eta =1-\frac{T_4 -T_1}{T_3-T_2}.$

We can simplify the above expression using the fact that the processes from 1 to 2 and from 3 to 4 are isentropic:

$\displaystyle T_4 V_1^{\gamma-1} =T_3 V_2^{\gamma-1}, \qquad T_1 V_1^{\gamma-1} =T_2 V_2^{\gamma-1}$

$\displaystyle (T_4 - T_1) V_1^{\gamma-1} = (T_3-T_2)V_2^{\gamma-1}$

$\displaystyle \frac{T_4-T_1}{T_3-T_2} =\left(\frac{V_2}{V_1}\right)^{\gamma-1}.$

The quantity

is called the compression ratio. In terms of compression ratio, the efficiency of an ideal Otto cycle is:

$\displaystyle \eta_\textrm{Otto} = 1-\frac{1}{\left(V_1/V_2\right)^{\gamma-1}}=1-\frac{1}{r^{\gamma-1}}.$

Ideal Otto cycle thermal efficiency

The ideal Otto cycle efficiency is shown as a function of the compression ratio in Figure 3.11. As the compression ratio,

, increases, $\eta_\textrm{Otto}$ increases, but so does

. If

is too high, the mixture will ignite without a spark (at the wrong location in the cycle).

You can also know :

Advantages of petrol engine

Disadvantages of petrol engine

Types of Engine

What is the Engine?

An engine is a power-producing machine, which can convert the potential energy of the fuel used in an engine into heat energy and then convert it into motion.

The engine power is generated bat burning the fuel in a self-regulated and controlled combustion process.

Introduction of Engine :

Engines vary in design, but certain elements are common to all engines and are used for engine classification which is following below.

The number of cylinders
The geometry of the block
Type of ignition system used

The two major engine types of engine are spark ignition (gasoline engine) and compression ignition (diesel engine) which use different types of fuel.

Types of Engine :

The following are ways engines are classified :

Fuel Burned :

According to fuel burned mainly, two types of fuel are used such as gasoline and diesel.

Gasoline car engines use spark ignition engine.

Diesel engines use compression ignition (no spark.)

Alternate fuels such as liquefied petroleum gas (LPG), gasohol (90% gasoline, 10% alcohol), and pure alcohol are used in very limited situations.

Block Geometry :

There are four types of engine block geometry:

V-type Engine
Inline Engine
Horizontally Engine
Opposed and slant Engine

Number of Cylinders :

The number of cylinders is often used in combination with the engine block geometry which can be shown above.

Two Cylinder Engine
Three-Cylinder Engine
Four Cylinder Engine
Five-Cylinder Engine
Six Cylinder Engine
Eight Cylinder Engine
Twelve Cylinder Engine
Sixteen Cylinder Engine

Camshaft Location :

There are two types of engine classified as possible locations for the camshaft:

In the cylinder head
In the engine block.

Car engines with the camshaft in the cylinder head are labeled overhead cam (OHC) engines.

A dual overhead cam (DOHC) engine uses two camshafts, one camshaft for the intake valves, the other for the exhaust valves.

Single overhead cam (SOHC) engines use one cam for both sets of valves. Engines with the camshaft in the block, “cam-in-block”, use push rods to move the valves.

Combustion Chamber :

There are three shapes in use:

Hemispherical
Wedge
Pancake.

The hemispherical, also called “Hemi-head,” is designed with the intake and exhaust valves angled and opposing each other (as viewed looking at the engine from front to back).

The pancake design has the valves nearly vertical.

Ignition Type :

There are two types of engines classified by ignition type :

Spark Ignition
Compression-Ignition

Strokes per Cycle :

There are two types of engines classified by strokes per cycle :

Two Strokes Engine
Four Strokes Engine

The number of times the piston travels up and down during one cycle is called strokes per cycle. Four strokes such as intake, compression, power, and exhaust.

Cooling System :

There are two types of engines classified by the cooling system :

Air-cooled
Liquid-cooled

Gas Laws

The gas laws were developed at the end of the 18th century.

when scientists began to realize that relationships between the pressure, volume and temperature of a sample of gas could be obtained which would hold to a good approximation for all gases.

Gases behave in a similar way over a wide variety of conditions because they all have molecules which are widely spaced, and the equation of state for an ideal gas is derived from kinetic energy.

The earlier gas laws are now considered as special cases of the ideal gas equation, with one or more of the variables held constant.

Boyle's Law :

Boyle's Law published in 1662, states that, at a constant temperature, the product of the pressure and volume of a given mass of an ideal gas in a closed system is always constant.

It can be verified experimentally using a pressure gauge and a variable volume container. It can also be derived from the kinetic theory of gases: if a container, with a fixed number of molecules inside, is reduced in volume, more molecules will strike a given area of the sides of the container per unit time, causing a greater pressure.

As a mathematical equation, Boyle's Law is written as either:

PV=k_{1}

P_{1}V_{1}=P_{2}V_{2}\,

The statement of Boyle 's law is as follows:

The volume of a given mass of a gas is inversely related to the pressure exerted on it at a given temperature and given a number of moles.

Charles's Law :

Charles's Law or the law of volumes was found in 1787 by Jacques Charles.

It states that, for a given mass of an ideal gas at constant pressure, the volume is directly proportional to its absolute temperature, assuming in a closed system.

As a mathematical equation, Charles's Law is written as either:

V\propto T\,

V/T=k_{2}

V_{1}/T_{1}=V_{2}/T_{2}

Gay-Lussac's Law :

Gay-Lussac's Law or the Pressure Law was found by Joseph Louis Gay-Lussac in 1809. It states that, for a given mass and constant volume of an ideal gas, the pressure exerted on the sides of its container is directly proportional to its absolute temperature.

As a mathematical equation, Gay-Lussac's Law is written as either:

P\propto T\,

P/T=k_{3}

P_{1}/T_{1}=P_{2}/T_{2}

where P is the pressure, T is the absolute temperature, and k₃ is another proportionality constant.

Avogadro's Law :

Avogadro's Law states that the volume occupied by an ideal gas is directly proportional to the number of molecules of the gas present in the container. This gives rise to the molar volume of a gas which at STP (273.15 K, 100 kPa) is about 22.7 l/mol. The relation is given by

{\frac {V_{1}}{n_{1}}}={\frac {V_{2}}{n_{2}}}\,

where n is equal to the number of molecules of gas (or the number of moles of gas).

Combined and Ideal Gas Laws :

With the addition of Avogadro's Law the combined gas Law develops into the Ideal Gas Law:

pV=nRT\,

where

p is pressure

V is volume

n is the number of moles

R is the universal gas constant

T is temperature (K)

where the proportionality constant, now named R, is the universal gas constant with a value of 0.083144598 (kpa∙L)/(mol∙K). An equivalent formulation of this Law is:

pV=kNT\,

where

p is the pressure

V is the volume

N is the number of gas molecules

k is the Boltzman constant (1.381×10⁻²³ J·K⁻¹ in SI units)

T is the absolute temperature

These equations are exact only for an ideal gas which neglects various intermolecular effects.

This law has the following important consequences:

If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.

Other Gas Law :

Graham's Law states that the rate at which gas molecules diffuse is inversely proportional to the square root of its density.

Combined with Avogadro's law this is the same as being inversely proportional to the root of the molecular weight.

Henry's Law states that at constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

p=k_{\rm {H}}\,c

Dalton's Law of partial pressure states that the pressure of a mixture of gases simply is the sum of the partial pressures of the individual components. Dalton's Law is as follows

P_{total}=P_{1}+P_{2}+P_{3}+...+P_{n}\equiv \sum _{i=1}^{n}P_{i}\,

P_{\mathrm {total} }=P_{\mathrm {gas} }+P_{\mathrm {H_{2}O} }\,

where P_Total is the total pressure of the atmosphere

P_Gas is the pressure of the gas mixture in the atmosphere

and P_H₂O is the water pressure at that temperature

FHP full form

What is the full form of FHP?

Answer :

Frictional Horse Power

What does FHP mean?

The amount of power consumed by an engine in driving itself.

It includes the power absorbed in mechanical friction and in driving auxiliaries plus, in the case of four-stroke engines, some pumping power.

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What is the full form of OHV?

Answer:

Over Head Valve OR Off-Highway Vehicle

What does OHV mean?

An OHV engine sometimes called a pushrod engine.

The camshaft is placed inside the block and the valves are operated through lifters, pushrods, and rocker arms.

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What is the full form of KPL?

Answer :

Kilometre per Liter

What does KPL mean?

It is a unit of measurement of fuel consumption or fuel economy.

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What is the full form of ISFC?

Answer :

Indicated Specific Fuel Consumption

What does ISFC mean?

ISFC helps to determine which engine uses the least amount of fuel while producing high power. It is the ratio of fuel used to the amount of power produced by the engine.

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