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Types of Gasoline Engines

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ABSTRACT

Apetrol engine (a.k.a. gasoline engine in North America) is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels.

It was invented in 1876 in Germany by German inventor Nicolaus August Otto.

Till date most vehicles run on petrol much more than any other fuel i.e. the consumption of petrol is highest among the fuels.

We know that the fossil fuels are getting scarce and with the increasing rates of the fuel, there are high concerns regarding the efficiency of the engines that we use.

“The Obama administration has required automakers to nearly double the average fuel economy of new cars and light trucks to 54.5 miles per gallon by 2025 under rules that took effect in 2012. The attention to fuel efficiency, Obama said, has resulted in lower costs, improved air quality and created jobs in the auto industry.”(source:The Guardian)

The paper deals with getting a brief idea of the types of gasoline engines (namely 2 stroke and 4 stroke), their working and most importantly will include a study of the factors that affect the fuel efficiency.

INTRODUCTION

A petrol engine (a.k.a. gasoline engine in North America) is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels.

Internal combustion engine -The combustion of fuel takes place inside the engine cylinder and heat is generated within the cylinder. This heat is added to the air inside the cylinder and thus the pressure of the air is increased tremendously which moves the piston , rotates the crank shaft and thus mechanical work is done

Spark ignition engine – a mixture of air and fuel is drawn into the engine cylinder. The spark plug produces a spark and ignites the air- fuel mixture. Such combustion is called constant volume combustion

(C.V.C.).

Based on its working cycle, it can be divided into :-

1. Four stroke cycle engine - When the cycle is completed in two revolutions of the crankshaft, it’s called four stroke cycle engine.

2. Two stroke cycle engine. - When the cycle is completed in one revolution of crankshaft, it’s called two stroke cycle engine.

WORKING PRINCIPLE OF I.C. ENGINE/ FOUR STROKE CYCLE ENGINE / TWO STROKE CYCLE ENGINE

A mixture of fuel with correct amount of air is exploded in an engine cylinder

which is closed at one end. As a result of this explosion, heat is released which causes the pressure of the burning gases to increase

This pressure forces a tightfitted piston to move down the cylinder which in turn rotates the crankshaft .

Power is utilized from the rotating crank shaft to do mechanical work. To obtaincontinuous rotation of the crankshaft ,the used gases are expelled from the cylinder, before the next ignition takes place .Fresh charge of fuel and air are admitted in to the cylinder and the piston moved back to its starting position. The sequences of events taking place in an engine constitute the

working cycle of an engine. The sequence of events taking place inside the engine are as follows-

1. Admission of air or air-fuel mixture inside the engine cylinder ( suction )

2. Compression of the air or air fuel mixture inside the engine (compression)

3. Injection of fuel in compressed air for ignition of the fuel or ignition of air- fuel mixture by an electric spark using a spark plug to produce thermal power

inside the cylinder (power )

4. Removal of all the burnt gases from the cylinder to receive fresh charge (exhaust)

Note: Charge means admitting fresh air in to the cylinder or admitting a mixture of air and fuel in to the cylinder in the case of spark ignition engines.

FOUR STROKE CYCLE ENGINE

In four stroke cycle engines the four events suction, compression, power

and exhaust take place inside the engine cylinder. They are completed in four

strokes of the piston ( i.e 2 revolutions of the crank shaft). The engine has separate valves for controlling the inlet of charge and outlet of exhaust gases. The opening and closing of the valve is controlled by cams, fitted on camshaft. The camshaft is driven by crankshaft with help of suitable gears or chains. The camshaft runs at half the speed of the crankshaft. The events that take place are

1. Suction stroke 2. Compression stroke

3. Power stroke 4. Exhaust stroke

Suction stroke

In suction stroke ,the inlet valves open and the piston moves downwards. The exaust valve remains closed and air or mixture of air and fuel is sucked in.

Compression stroke

In this stroke, both the valves are closed and the piston moves upwards. The charge taken inis compressed by the upward stroke.

The mixture is ignited by a spark plug.

Power stroke

After ignition of fuel, high amount of heat is produced which pushes the piston down. The downward

movement of the piston at this point is called power stroke. The connecting rod or pin transmits the power from the piston to the crank shaft and the crank shaft rotates. Both valves remain closed during the power stroke.

Exhaust stroke

In this stroke the exhaust valve opens and the piston moves upwards, expelling the burnt gases out. The inlet valve remains closed.

remains closed .

Thus it is found that out of four strokes, there is only one power stroke and three idle strokes in four stroke cycle engine. The power stroke supplies necessary momentum for useful work.

http://media.web.britannica.com/eb-media/72/93572-034-26C16785.jpg

TWO STROKE CYCLE ENGINE (PETROL ENGINE)

In two stroke cycle engines, all the events i.e., suction,

compression, power and exhaust are completed in two strokes of the piston (i.e., one revolution of the crankshaft).

There is no valve in this type of engine.

The gas movement takes place through holes called ports in the cylinder. The crankcase of the engine is air tight in which the crankshaft rotates.

Two stroke cycle

Upward stroke of the piston (Suction & Compression)

http://www.rc-airplane-advisor.com/images/two-stroke-engine.jpgWhen the piston moves upward it covers two of the ports which are the exhaust port and the transfer port, which are normally almost opposite to each other. As a result it traps the charge of air- fuel mixture drawn already into the cylinder. The further upward movement of the piston compresses the charge and uncovers the intake port. Following it fresh mixture is drawn through this port into the crankcase. Just before the end of this stroke, the mixture in the cylinder is ignited by a spark plug and thus, during this stroke both suction and compression events are completed.

Downward stroke (Power + Exhaust)

The burning of gases increases the pressure and forces down the piston. When the piston moves down, it closes the suction port, which traps the fresh charge drawn into the crankcase during the previous upward stroke. The further downward movement of the piston uncovers the exhaust port first and then the transfer port. Now fresh charge in the crankcase moves into the cylinder through the transfer port. The burnt out gases are driven out through the exhaust port. Special shaped piston crown deflect the incoming mixture up around the cylinder so that it can help in driving out the exhaust gases . During the downward stroke of the piston, power and exhaust events are completed.

 

THE COMPARISION (2 STROKE VS 4 STROKE)

The 2-stroke engine fires once every revolution, giving it twice the power of a four stroke, which only fires once every other revolution. The 2 stroke has a higher weight to power ratio because it is much lighter than the 4 stroke. It is less expensive because of its simpler design.

These qualities make two stroke engines very popular for a variety of uses from dirt bikes, jet skis, mopeds and small outboard motors, to lawn and garden equipment such as mowers, leaf blowers,edgers, hedge trimmers and chain saws.

Many 2 strokes are simple engines and lack valves. Thus the timing of the intake and exhaust cannot be made optimal on 2 strokes. While on the other hand ,the timing of intake and exhaust can be controlled using timing gear in 4-stroke engines. As a result they burn less cleaner than the 4 stroke.

The 2-stroke engines have faster wear and shorter engine life than a four stroke due to the lack of a dedicated lubricating system .Another thing to consider is that 2 stroke engines have lubricating oil mixed with the fuel because of which it leads to incomplete combustion of the fuel. Which in turn also affect the efficiency and often cause more pollution.

The 2 stroke engines also produce higher decibels of noise. Hence it is banned in some places.

Thus in a sense 4 stroke engines are more fuel efficient than 2 stroke engines.

FACTORS AFFECTING EFFICIENCY

Of the total heat energy released by the gasoline consumed, about 70-75% is rejected as heat without being turned into useful work ie to the crankshaft.Approx half of this rejected heat is carried away by the exhaust gases, and half passes through the cylinder walls or cylinder head into the engine cooling system, and is passed to the atmosphere via the cooling system radiator. Some of the work generated is also lost as friction, noise, air turbulence, and work used to turn engine equipment and appliances such as water and oil pumps and the electrical generator, leaving only about 25-30% of the energy released by the fuel consumed available to move the vehicle.Thus the available energy needs to be used effectively in order to gain the most out of it.

  1. Weight

One of the best ways to increase the fuel efficiency of the vehicles is to reduce down the weight. Which means replacing the heavier parts with the lighter ones .This needs to be done without compromising with the safety,performance and consumer preference of the vehicle.

The lightest material for the car bodies is carbon fibre.though it comes with a hefty price. It is currently reserved for the very expensive cars and the high end sports car industry.

Ten years ago, carbon fibre cost $150 for a pound.Now it costs around $10 a pound. On the other hand steel costs less than a dollar.

Replacing the steel components with carbon fibre ones can reduce the car weight by approx 60 percent. This 60 percent reduce in weight would decrese the car’s fuel consumption by 30 percent and would cut the emissions of greenhouse gases by 10-20 percent.

  1. Displacement

Displacementis the volume of air the engine can consume in a single revolution. It is generally denoted in litres.

The more air the engine can move in, the more fuel it can burn.

Thus a small compact bike engine will be more efficient than a truck’s engine.

  1. Compression ratio

Most gasoline (petrol) engines have the compression ratio calculated purely from the geometry of the mechanical parts(geometriccompression ratio). 10:1 (premium fuel) or 9:1 (regular fuel), with some engines reaching a ratio of 12:1 or more. The greater the compression ratio the more efficient is the engine, in principle . Higher compression-ratio conventional engines in principle need gasoline with higher octane value .High octane value gives the fuel the tendency to burn nearly instantaneously (known as detonation or knock) at high compression conditions.

  1. Fuel intake

The carburetor system is adapted for the fuel intake.

The carburetor contains jets that push the gas into the combustion chambers. The amount of fuel that can flow through these jets depends completely on the amount of air that can be pulled into the carburetor .

The main issue with obtaining the best performance using a carburetor is that it can't monitor the air to fuel ratio for each individual cylinder.

Fuel injection systems on the other hand are becoming more popular for best performance from the engines.

The main benefits to using direct injection is that the amount of fuel and air can be perfectly released and then injected into the cylinder according to the engine load conditions. The electronics used in the system will calculate this information and constantly adjust. This type of controlled fuel injection results in a higher power output, greater fuel efficiency and much lower emissions. The main issues is that these systems are sophisticated and will cost much more than a carburetor.

In the past years direct injection systems haveincreased the efficiency of the engines equipped with this fueling system up to 35%

  1. Oxygen

If there is not enough oxygen for proper combustion, the fuel will not burn completely and will produce less energy. Relatively, an excessively rich air fuel ratio will increase pollutants from the engine.

The fuel burns in three stages. Firstly, the hydrogen burns to form water vapour. Second, carbon burns to carbon monoxide and finally, the carbon monoxide burns to carbon dioxide. This last stage produces most of the power of the engine.

If all of the oxygen is consumed before this stage, engine's power is reduced.

To increase the amount of oxygen, we need to increase the air intake.

One way is through forced induced intake. A compressor can be added to force a larger charge .This can be done by mechanically driven supercharging or exhaust driven turbo charging.

  1. Mechanical Resistance

Type of resistance car manufacturers are concerned with, called mechanical resistance, which has more to do with a vehicle’s mechanical components other than the tires. Mechanical resistance is described as the amount of effort it takes to move each individual piece of the vehicle’s drivetrain.

The automatic transmissions generate more resistance than their manual counterparts.

Mechanical resistance usually accounts for about a 15 percent drop in power from what the engine produces to what actually makes it to the pavement. Which means if an engine produces 100 horsepower, then only about 85 of those horses make it to the ground.

An engine has many moving parts that produce friction. Some of these friction forces remain constant while some of these friction losses increase as engine speed increases, such as piston side forces and connecting bearing forces

Synthetic lubricants, light weight materials and tighter manufacturing tolerances can all contribute to less mechanical resistance in a vehicle.

  1. Aerodynamics

At high speeds, and on exceptionally windy days,air resistance(the forces acted upon a moving object by the air also defined asdrag) has­ a tremendous effect on the way a car accelerates, handles and achieves fuel mileage.

Engineers have developed several ways ofreducing it. For instance, more rounded designs and shapes on the exterior of the vehicle are crafted to channel air in a way so th­at it flows around the car with the least resistance possible. Some high-performance cars even have parts that move air smoothly across the underside of the car. Many also include aspoileralso known as arear wing to keep the air from lifting the car's wheels and making it unstable at high speeds.

CONCLUSION

Even after years of its discovery in the 19th century, gasoline (petrol) engines account for the most used engines currently ,much ahead of its diesel counterpart which is evident enough to denote our dependency on petrol. However petrol as we know is a limited resource. the rate at which it replenishes is way much lower than the rate at which we are consuming it .Moreover burning of the fuel causes pollution.

Keeping that in mind and following the context of my study, I feel it is a great move by president Obama to order the vehicle industry to double the gross fuel economy of their vehicles by the year 2025


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