HEAT ENGINES:)

What is Heat Engine?

In thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". 

heat engine of a car 

Heat is transferred from the source, through the "working body" of the engine, to the sink, and in this process some of the heat is converted into work by exploiting the properties of a working substance (usually a gas or liquid).

Heat engines can be characterized by their specific power, which is typically given in kilowatts per litre of engine displacement, in the U.S. also horsepower per cubic inch. The result offers an approximation of the peak-power output of an engine. This is not to be confused with fuel efficiency, since high-efficiency often requires a lean fuel-air ratio, and thus lower power density. A modern high-performance car engine makes in excess of 75 kW/L (1.65 hp/in³).

TAKE A LOOK AT THE PARTS

PARTS OF A HEAT ENGINE:

• CAMSHAFT - a device to convert circular or rotational motion into reciprocating or oscillatory motion.
  
• CAM - a rotating component used in a mechanism in order to transform a movement from a circular to a reciprocating motion.
• INTAKE VALVE - a valve that controls the flow of fluid through an intake.
• COMBUSTION CHAMBER - is the part of an engine in which fuel is burned.
• CYLINDER BLOCK - a complex part at the heart of an engine, with adaptions to attach the cylinder head, crankcase, engine mounts, drive housing and engine ancillaries, with passages for coolants and lubricants.
• CONNECTING ROD - connects the piston to the crank or crankshaft.
• CRANKSHAFT - the part of an engine which translates reciprocating linear piston motion into rotation.
• SPARK PLUG - is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.
• VALVE SPRING - are metal bellows filled with compressed air used as an alternative to the metal wire springs used to close valves in high-speed internal combustion engines. 
• EXHAUST VALVE - a valve through which burned gases from a cylinder escape into the exhaust manifold exhaust system, exhaust - system consisting of the parts of an engine through which burned gases or steam are discharged.
• CYLINDER HEAD - sits above the cylinders on top of the cylinder block. It consists of a platform containing part of the combustion chamber (usually, though not always), and the location of the poppet valves and spark plugs.
• PISTON - is a component of reciprocating engines, pumps and gas compressors.
• CRANKCASE -  is the housing for the crankshaft.

FOUR- STROKE ENGINE

Today, internal combustion engines in cars, trucks, motorcycles, aircraft, construction machinery and many others, most commonly use a four-stroke cycle. 

The four strokes refer to intake, compression, combustion (power), and exhaust strokes that occur during two crankshaft rotations per working cycle of the gasoline engine and diesel engine.

The cycle begins at Top Dead Centre *, when the piston is farthest away from the axis of the crankshaft. A stroke refers to the full travel of the piston from Top Dead Center to Bottom Dead Center.

* dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft.

• Four-stroke cycle used in gasoline engines.The right blue side is the intake and the left yellow side is the exhaust. The cylinder wall is a thin sleeve surrounded by cooling water.

AIR INTAKE

              On the intake or induction stroke of the piston, the piston descends from the top of the cylinder to the bottom of the cylinder, reducing the pressure inside the cylinder. A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port. The intake valve(s) then close.

COMPRESSION

         With both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the fuel-air mixture. This is known as the compression stroke.

COMBUSTION OR POWER

                  While the piston is close to Top Dead Center, the compressed air–fuel mixture is ignited, usually by a spark plug (for a gasoline or Otto cycle engine) or by the heat and pressure of compression (for a diesel cycle or compression ignition engine). The resulting massive pressure from the combustion of the compressed fuel-air mixture drives the piston back down toward bottom dead center with tremendous force. This is known as the power stroke, which is the main source of the engine's torque and power.

EXHAUST

                     During the exhaust stroke, the piston once again returns to top dead center while the exhaust valve is open. This action evacuates the products of combustion from the cylinder by pushing the spent fuel-air mixture through the exhaust valve(s).

PRESSURE-VOLUME DIAGRAM

                  The idealized four-stroke  p-V diagram: the intake (A)  stroke is performed by an isobaric expansion, followed by the  compression (B)  stroke, performed by an adiabatic compression. Through the combustion of fuel anisochoric process is produced, followed by an adiabatic expansion, characterizing the  power (C)  stroke. The cycle is closed by an isochoric process and an isobaric compression, characterizing the exhaust (D)  stroke.

THERMAL EFFICIENCY OF HEAT ENGINE

         The performance of heat engine is defined by its deficiency. The thermal efficiency of heat engine is the ratio of output desired to input supplied. For heat engine the output desired is work and input is heat supplied by source.

η Heat Engine = Output desired / Input supplied = W / Q₁ 

From figure, Using first law of thermodynamics

 ? (δQ- δW) = 0

⇒ Q₁ + (Q₂) – (+ W) = 0 

⇒ Q₁ – Q₂ = W

From equations (1) and (2) 

η Heat Engine = Q₁ – Q₂ / Q₁ = 1 – Q₂ / Q₁

Example : 

A plant requires 300 MJ heat per hour in winter for heating. Heat pump is used to absorb heat from the surrounding and sends it to heat the plant. The work required to operate the heat pump is 30 MJ/Hr. Determine the COP and the heat abstracted from the surrounding.

Solution:  Given that:      Q₁ = 300 MJ / Hr

W = 30 MJ / Hr 

Q₁ – Q₂ = W 

(COP)_HP = Q₁ / Q₁ – Q₂ 

= Q₁ / W 

= 300 / 30 MJ/Hr / Mj/Hr

⇒ (COP)_HP = 10

Heat abstracted from the surrounding = Q₂ 

Q₁ – Q₂ = 30 

300 – Q₂ = 30 

Q₂ = 270 MJ / Hr

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