Work Output and Efficiency Measurement of Diesel Engines

Saka Oluwadamilola110404085

Mechanical EngineeringUniversity Of Lagos

An Industrial Visit to:Mantrac Nigeria Limited.

2, Billings Way, Oregun Industrial EstateOregun, Lagos

I. ABSTRACT

This paper discusses the diesel engine as a reciprocating compression-ignition internal combustion engine. It

examines the principles of operation of the diesel engine, and the thermodynamic principle of the diesel cycle. It

discusses the components of the diesel engine and how they work together to convert chemical energy in fuel to

useful work. The operational characteristics of the diesel engine are also considered. Finally, it accesses the

advantages and applications of the diesel engine over gasoline engines.

II. INTRODUCTION

Over the years, engineers have developed on knowledge derived from scientific research and experiments. One

principal aim of engineers is to seek how to efficiently and safely harness all sources of energy, using tools and

machines to provide meaningful work. The application of such knowledge for manufacturing and productive

processes is known as technology. Technology is therefore an essential ingredient in industrialization and

civilization. One key element in evaluating development in nations is the level of technological advancement. This

cannot be determined until the method and quality of work done is measured.

For example, lifting barrels off ships by pulling ropes on pulley with hand implies a very low level of

technology, compared to automatically actuated weight lifting machines controlled from a desk in a control room.

Such mechanical devices that make work done easier and faster are commonly called machines. Machines are

mechanical devices with moving parts, used to perform a task, especially one that would otherwise be done by hand.

Machines are usually powered by an engine, or even engines in larger-scale machines. Engines are machines

built for converting energy into motion or doing useful mechanical work. The energy is usually supplied in the form

of a chemical fuel, such as oil or gasoline, steam, or electricity, and the mechanical work is most commonly

delivered in the form of rotary motion of a shaft. When energy is supplied in the form of a chemical fuel to an

engine, such an engine is called a combustion engine. Combustion refers to the chemical reaction of burning fuel

with an oxidizer, to supply the heat that drives the engine.

Depending on the type of engine employed, different performance characteristics including rates of efficiency

are attained. This report focuses on the diesel engine, its performance characteristics and industrial applications.

III. THEORY

A. ENGINES

Engines are machines for converting energy into motion or mechanical work. The energy is usually supplied in the

form of a chemical fuel, such as oil or gasoline.

Engines can be classified in so many ways, according to the form of energy they utilize, the type of motion of

their principal parts, the place where the exchange from chemical to heat energy takes place, the method by which

the engine is cooled, the position of the cylinders of the engine, the number of strokes of the piston for a complete

cycle, the type of cycle, and the use for which the engine is intended. When engines are classified according to the

place where the exchange from chemical to heat energy takes place, they are divided into two:

(i) External combustion engines

(ii) Internal combustion engines

1. EXTERNAL COMBUSTION ENGINES

An external combustion engines is a heat engine where an internal working fluid is heated by combustion of an

external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the

mechanism of the engine produces motion and usable work. The fluid is then cooled, compressed and reused or

dumped, and cool fluid pulled in.

In external combustion engines (such as steam power plants), heat is supplied to the working fluid from an

external source such as a furnace, a geothermal wall, a nuclear reactor, or even the sun. That is, the fuel in these

engines is burned outside the cylinder. External combustion offers several advantages. FIrst: a variety of fuels can be

used as a source of thermal energy. Second, there is more time for combustion, and thus the combustion process is

more complete, which means less air pollution and more energy extraction from the fuel. Third, these engines

operate on closed cycles, and thus a working fluid that has the most desirable characteristics (stable, chemically

inert, high thermal conductivity) can be utilized as the working fluid.

2. INTERNAL COMBUSTION ENGINE

These are engines where the energy is provided by burning a fuel within the system boundaries. The internal-

combustion engine is also known as the piston engine. The concept of the piston engine is that a supply of air-and-

fuel mixture is fed to the inside of the cylinder where it is compressed and then burnt. This internal combustion

releases heat energy which is then converted into useful mechanical work as the high gas pressures generated force

the piston to move along its stroke in the cylinder. It can be said, therefore, that a heat-engine is merely an energy

transformer.

To enable the piston movement to be harnessed, the driving thrust on the piston is transmitted by means of a

connecting-rod to a crankshaft whose function is to convert the linear piston motion in the cylinder to a rotary

crankshaft movement. The piston can thus be made to repeat its movement to and fro, due to the constraints of the

crankshaft crankpin's circular path and the guiding cylinder.

Internal combustion engine can be categorized into two based on the method of ignition of the fuel supplied.

They can be (i) Spark Ignition engines, or (ii) Compression Ignition engines. Spark Ignition engines use spark plugs

to generate little sparks that ignite the air-fuel mixture. However in compression ignition engines, the air mixture is

compressed till its temperature reaches its ignition temperature. That is, the heat of compression is sufficient to heat

the fuel.

B. THE DIESEL ENGINE

The Diesel Engine is a high-efficiency compression-ignition engine invented by Rudolf Diesel in Germany in 1893.

It was born out of Diesel desire to conserve the amount of energy wasted by the dominant heat engine compared to

the Carnot ideal energy conversion cycle. Diesel's first engine ran on coal dust and used a compression pressure of

1500 psi to increase its theoretical efficiency. Just like gasoline engines, diesel engines are, in principle, energy

converters that convert chemically bound fuel energy into mechanical energy (effective work) by supplying the heat

released by combustion in an engine to a thermodynamic cycle.

A diesel engine is similar to the gasoline engine used in most cars. Both engines are internal combustion engines,

meaning they burn the fuel-air mixture within the cylinders. Both are reciprocating engines, being driven by pistons

moving laterally in two directions. The majority of their parts are similar. Although a diesel engine and gasoline

engine operate with similar components, a diesel engine, when compared to a gasoline engine of equal horsepower,

is heavier due to stronger, heavier materials used to withstand the greater dynamic forces from the higher

combustion pressures present in the diesel engine.

The greater combustion pressure is the result of the higher compression ratio used by diesel engines. The

compression ratio is a measure of how much the engine compresses the gasses in the engine's cylinder. In a gasoline

engine the compression ratio (which controls the compression temperature) is limited by the air-fuel mixture

entering the cylinders. The lower ignition temperature of gasoline will cause it to ignite (burn) at a compression ratio

of less than 10:1. The average car has a 7:1 compression ratio. In a diesel engine, compression ratios ranging from

14:1 to as high as 24:1 are commonly used. The higher compression ratios are possible because only air is

compressed, and then the fuel is injected. This is one of the factors that allow the diesel engine to be so efficient.

Another difference between a gasoline engine and a diesel engine is the manner in which engine speed is

controlled. In any engine, speed (or power) is a direct function of the amount of fuel burned in the cylinders.

Gasoline engines are self-speed-limiting, due to the method the engine uses to control the amount of air entering the

engine. Engine speed is indirectly controlled by the butterfly valve in the carburetor. The butterfly valve in a

carburetor limits the amount of air entering the engine. In a carburetor, the rate of air flow dictates the amount of

gasoline that will be mixed with the air. Limiting the amount of air entering the engine limits the amount of fuel

entering the engine, and, therefore, limits the speed of the engine. By limiting the amount of air entering the engine,

adding more fuel does not increase engine speed beyond the point where the fuel burns 100% of the available air

(oxygen). Diesel engines are not self-speed-limiting because the air (oxygen) entering the engine is always the

maximum amount. Therefore, the engine speed is limited solely by the amount of fuel injected into the engine

cylinders. Therefore, the engine always has sufficient oxygen to burn and the engine will attempt to accelerate to

meet the new fuel injection rate. Because of this, a manual fuel control is not possible because these engines, in an

unloaded condition, can accelerate at a rate of more than 2000 revolutions per second. Diesel engines require a

speed limiter, commonly called the governor, to control the amount of fuel being injected into the engine.

Unlike a gasoline engine, a diesel engine does not require an ignition system because in a diesel engine the fuel is

injected into the cylinder as the piston comes to the top of its compression stroke. When fuel is injected, it vaporizes

and ignites due to the heat created by the compression of the air in the cylinder.

1. MAJOR COMPONENTS OF THE DIESEL ENGINE

Diesel engines are similar in appearance to, and have many of the same components as, spark-ignition engines.

Diesels have one or more cylinders (usually four, six, or eight). Pistons inside the cylinders are connected by rods to

a crankshaft. As the pistons move up and down in their cylinders, they cause the crankshaft to rotate. The

crankshaft’s rotational force is carried by a transmission to a drive shaft, which turns axles, causing the wheels to

rotate. Because a diesel engine compresses air inside the cylinders with greater force, the engine block, pistons,

connecting rods, crankshaft, and other components must be stronger than those of a gasoline engine with the same

power output. As a result, diesel engines tend to be heavier and more expensive to manufacture. A diesel engine also

needs a more powerful starter motor, which turns the crankshaft to initiate ignition. It often has an extra battery to

provide power to the starter motor. To properly understand the operation of a diesel engine, it is important to

describe the operation of the major components of a diesel engine.

THE CYLINDER BLOCK

The cylinder block is a single unit made from cast iron. The engine block is the lower part of the engine that houses

the cylinders, pistons, and crankshaft. The components of other engine systems are in a way bolted or attached to the

engine block. In a liquid-cooled diesel engine, the block also provides the structure and rigid frame for the engine's

cylinders, water coolant and oil passages, and support for the crankshaft and camshaft bearings. The cylinder block

is as a skeleton in the body in of the diesel engine. It provides rigidity for the engine as a whole and, connectivity for

the engine components.

Different Cylinder blocks in Diesel engines

THE PISTON AND PISTON RINGSA piston is a solid cylinder or disk that fits tightly inside a hollow cylinder and slides back and forth. The fit is loose

enough to allow the piston to move, but tight enough that virtually no air or fluid in the cylinder can leak past it. The

piston transforms the energy of the expanding gases into mechanical energy. The piston rides in the cylinder sleeve.

Pistons are commonly made of aluminum or cast iron alloys. To prevent the combustion gases from bypassing the

piston and to keep friction to a minimum, each piston has several rings around it.

A piston cylinder cut to show its internal structure

CONNECTING RODThe connecting rod connects the piston to the crankshaft. The rods are made from drop-forged, heat-treated steel to

provide the required strength. Each end of the rod is bored, and the large bore end of the rod is split in half and

bolted to allow the rod to be attached to the crankshaft. It is a very principal component of the engine as it is

responsible for transmitting motion from the piston in the different cylinders to the crankshaft.

Different connecting rod arrangements

CRANKSHAFTThe crankshaft transforms the linear motion of the pistons into a rotational motion that is transmitted to the load.

Crankshafts are made of forged steel. The forged crankshaft is machined to produce the crankshaft bearing and

connecting rod bearing surfaces. The crankshaft is drilled with oil passages for lubrication that allow the engine to

feed oil to each of the crankshaft bearings and connection rod bearings and up into the conditioning rod itself.

A Crankshaft Design

The crankshaft transforms the reciprocating motion of the piston into rotary motion. In multi-cylindered engines the

crankshaft has one offset portion, called a crankpin, for each connecting rod, so that the power from each cylinder is

applied to the crankshaft at the appropriate point in its rotation. This offset converts the reciprocating motion of the

piston into the rotary motion of the crankshaft. The amount of offset determines the stroke (distance the piston

travels) of the engine. Crankshafts have heavy flywheels and counterweights, which by their inertia minimize

irregularity in the motion of the shaft. An engine may have from 1 to as many as 28 cylinders. The connecting rods

also have bearings inserted between the crankshaft and the connecting rods. The bearing material is a soft alloy of

metals that provides a replaceable wear surface and prevents galling between the crankshaft and connecting rod.

An assembly showing the arrangement of the piston, connecting rod and crankshaft

FLYWHEELThe flywheel is located on one end of the crankshaft and serves three purposes. First, through its inertia, it reduces

vibration by smoothing out the power stroke as each cylinder fires. Second, it is the mounting surface used to bolt

the engine up to its load. Third, on some diesel engines, the flywheel has gear teeth around its perimeter that allow

the starting motors to engage and crank the diesel engine.

CYLINDER HEADSA diesel engine's cylinder heads perform several functions. First, they provide the top seal for the cylinder bore or

sleeve. Second, they provide the structure holding exhaust valves (and intake valves where applicable), the fuel

injector, and necessary linkages. A diesel engine's heads are manufactured in one of two ways. In one method, each

cylinder has its own head casting, which is bolted to the block. This method is used primarily on the larger diesel

engines. In the second method, which is used on smaller engines, the engine's head is cast as one piece (multi-

cylinder head).

VALVESDiesel engines have two methods of admitting and exhausting gasses from the cylinder. They can use either ports or

valves or a combination of both. Ports are slots in the cylinder walls located in the lower 1/3 of the bore. When the

piston travels below the level of the ports, the ports are "opened" and fresh air or exhaust gasses are able to enter or

leave, depending on the type of port. The ports are then "closed" when the piston travels back above the level of the

ports. Valves are mechanically opened and closed to admit or exhaust the gasses as needed. The valves are located

in the head casting of the engine. The point at which the valve seals against the head is called the valve seat. Most

medium-sized diesels have either intake ports or exhaust valves or both intake and exhaust valves.

TIMING GEARS, CAMSHAFT, AND VALVE MECHANISM

In order for a diesel engine to operate, all of its components must perform their functions at very precise intervals in

relation to the motion of the piston. To accomplish this, a component called a camshaft is used. A camshaft is a long

bar with egg-shaped eccentric lobes, one lobe for each valve and fuel injector. Each lobe has a follower, and as the

camshaft is rotated, the follower is forced up and down as it follows the profile of the cam lobe. The followers are

connected to the engine's valves and fuel injectors through various types of linkages called pushrods and rocker

arms. The pushrods and rocker arms transfer the reciprocating motion generated by the camshaft lobes to the valves

and injectors, opening and closing them as needed. The valves are maintained closed by springs. As the valve is

opened by the camshaft, it compresses the valve spring. The energy stored in the valve spring is then used to close

the valve as the camshaft lobe rotates out from under the follower. Because an engine experiences fairly large

changes in temperature (e.g., ambient to a normal running temperature of about 190°F), its components must be

designed to allow for thermal expansion. Therefore, the valves, valve pushrods, and rocker arms must have some

method of allowing for the expansion. This is accomplished by the use of valve lash. Valve lash is the term given to

the "slop" or "give" in the valve train before the cam actually starts to open the valve.

The camshaft is driven by the engine's crankshaft through a series of gears called idler gears and timing gears. The

gears allow the rotation of the camshaft to correspond or be in time with, the rotation of the crankshaft and thereby

allow the valve opening, valve closing, and injection of fuel to be timed to occur at precise intervals in the piston's

travel. To increase the flexibility in timing the valve opening, valve closing, and injection of fuel, and to increase

power or to reduce cost, an engine may have one or more camshafts.

Typically, in a medium to large V-type engine, each bank will have one or more camshafts per head. In the

larger engines the intake valves, exhaust valves, and fuel injectors may share a common camshaft or have

independent camshafts. Depending on the type and make of the engine, the location of the camshaft or shafts varies.

The camshaft(s) in an in-line engine is usually found either in the head of the engine or in the top of the block

running down one side of the cylinder bank. On small or mid-sized V-type engines, the camshaft is usually located

in the block at the center of the "V" between the two banks of cylinders. In larger or multi-camshafted V-type

engines, the camshafts are usually located in the heads.

2. PRINCIPLES OF OPERATION OF THE DIESEL ENGINE

To convert the chemical energy of the fuel into useful mechanical energy all internal combustion engines must

go through four events: intake, compression, power, and exhaust. How these events are timed and how they occur

differentiates the various types of engines. All diesel engines fall into one of two categories, two-stroke or four-

stroke cycle engines. The word cycle refers to any operation or series of events that repeats itself. In the case of a

four-stroke cycle engine, the engine requires four strokes of the piston (intake, compression, power, and exhaust) to

complete one full cycle. Therefore, it requires two rotations of the crankshaft, or 720° of crankshaft rotation (360° x

2) to complete one cycle. In a two-stroke cycle engine the events (intake, compression, power, and exhaust) occur in

only one rotation of the crankshaft, or 360°.

The Four-Stroke CycleIn a four-stroke engine the camshaft is geared so that it rotates at half the speed of the crankshaft (1:2). This

means that the crankshaft must make two complete revolutions before the camshaft will complete one revolution.

The following section will describe a four-stroke, normally aspirated, diesel engine having both intake and exhaust

valves with a 3.5-inch bore and 4-inch stroke with a 16:1 compression ratio, as it passes through one complete cycle.

IntakeAs the piston moves upward and approaches 28° before top dead center (BTDC), as measured by crankshaft

rotation, the camshaft lobe starts to lift the cam follower. This causes the pushrod to move upward and pivots the

rocker arm on the rocker arm shaft. As the valve lash is taken up, the rocker arm pushes the intake valve downward

and the valve starts to open. The intake stroke now starts while the exhaust valve is still open. The flow of the

exhaust gases will have created a low pressure condition within the cylinder and will help pull in the fresh air charge

as shown in Figure 1. The piston continues its upward travel through top dead

center (TDC) while fresh air enters and exhaust gasses leave. At about 12° after

top dead center (ATDC), the camshaft exhaust lobe rotates so that the exhaust

valve will start to close. The valve is fully closed at 23° ATDC. This is

accomplished through the valve spring, which was compressed when the valve

was opened, forcing the rocker arm and cam follower back against the cam lobe

as it rotates. The time frame during which both the intake and exhaust valves are

open is called valve overlap (51° of overlap in this example) and is necessary to

allow the fresh air to help scavenge (remove) the spent exhaust gasses and cool

the cylinder. In most engines, 30 to 50 times cylinder volume is scavenged

through the cylinder during overlap. This excess cool air also provides the necessary cooling effect on the engine

parts. As the piston passes TDC and begins to travel down the cylinder bore, the movement of the piston creates

suction and continues to draw fresh air into the cylinder.

CompressionAt 35° after bottom dead center (ABDC), the intake valve starts to close. At 43° ABDC (or 137° BTDC), the

intake valve is on its seat and is fully closed. At this point the air charge is at normal pressure (14.7 psia) and

ambient air temperature (~80°F), as illustrated in Figure 2. At about 70° BTDC,

the piston has traveled about 2.125 inches, or about half of its stroke, thus

reducing the volume in the cylinder by half. The temperature has now doubled to

~160°F and pressure is ~34 psia. At about 43° BTDC the piston has traveled

upward 3.062 inches of its stroke and the volume is once again halved.

Consequently, the temperature again doubles to about 320°F and pressure is ~85

psia. When the piston has traveled to 3.530 inches of its stroke the volume is

again halved and temperature reaches ~640°F and pressure 277 psia. When the

piston has traveled to 3.757 inches of its stroke, or the volume is again halved, the

temperature climbs to 1280°F and pressure reaches 742 psia. With a piston area

of 9.616 in2 the pressure in the cylinder is exerting a force of approximately 7135 lb. or 3-1/2 tons of force.

Figure 1

Figure 2

Injection and PowerFuel in a liquid state is injected into the cylinder at a precise time and rate to ensure that the combustion pressure

is forced on the piston neither too early nor too late, as shown in Figure 3. The

fuel enters the cylinder where the heated compressed air is present; however, it

will only burn when it is in a vaporized state (attained through the addition of

heat to cause vaporization) and intimately mixed with a supply of oxygen. The

first minute droplets of fuel enter the combustion chamber and are quickly

vaporized. The vaporization of the fuel causes the air surrounding the fuel to cool

and it requires time for the air to reheat sufficiently to ignite the vaporized fuel.

But once ignition has started, the additional heat from combustion helps to

further vaporize the new fuel entering the

chamber, as long as oxygen is present. Fuel

injection starts at 28° BTDC and ends at 3° ATDC; therefore, fuel is injected for a

duration of 31°. Both valves are closed, and the fresh air charge has been

compressed. The fuel has been injected and is starting to burn. After the piston

passes TDC, heat is rapidly released by the ignition of the fuel, causing a rise in

cylinder pressure. Combustion temperatures are around 2336°F. This rise in

pressure forces the piston downward and increases the force on the crankshaft for

the power stroke as illustrated in Figure 3b. The energy generated by the

combustion process is not all harnessed. In a two stroke diesel engine, only about

38% of the generated power is harnessed to do work, about 30% is wasted in the

form of heat rejected to the cooling system, and about 32% in the form of heat is rejected out the exhaust. In

comparison, the four-stroke diesel engine has a thermal distribution of 42%

converted

ExhaustAs the piston approaches 48° BBDC, the cam of the exhaust lobe starts to

force the follower upward, causing the exhaust valve to lift off its seat. As shown

Figure 3

Figure 4b

Figure 5

in Figure 5, the exhaust gasses start to flow out the exhaust valve due to cylinder pressure and into the exhaust

manifold. After passing BDC, the piston moves upward and accelerates to its maximum speed at 63° BTDC. From

this point on the piston is decelerating. As the piston speed slows down, the velocity of the gasses flowing out of the

cylinder creates a pressure slightly lower than atmospheric pressure. At 28° BTDC, the intake valve opens and the

cycle starts again.

The Two-Stroke Cycle

Like the four-stroke engine, the two-stroke engine must go through the same four events: intake, compression,

power, and exhaust. But a two-stroke engine requires only two strokes of the piston to complete one full cycle.

Therefore, it requires only one rotation of the crankshaft to complete a cycle. This means several events must occur

during each stroke for all four events to be completed in two strokes, as opposed to the four-stroke engine

where each stroke basically contains one event. In a two-stroke engine the camshaft is geared so that it rotates at the

same speed as the crankshaft (1:1). The following section will describe a two-stroke, supercharged, diesel engine

having intake ports and exhaust valves with a 3.5-inch bore and 4-inch stroke with a 16:1 compression ratio, as it

passes through one complete cycle. We will start on the exhaust stroke. All the timing marks given are generic and

will vary from engine to engine.

Exhaust and IntakeAt 82° ATDC, with the piston near the end of its

power stroke, the exhaust cam begins to lift the

exhaust valves follower. The valve lash is taken

up, and 9° later (91° ATDC), the rocker arm forces

the exhaust valve off its seat. The exhaust gasses

start to escape into the exhaust manifold, as shown

in Figure 6. Cylinder pressure starts to decrease.

After the piston travels three-quarters of its (down)

stroke, or 132° ATDC of crankshaft rotation, the piston starts to uncover the inlet

ports. As the exhaust valve is still open, the uncovering of the inlet ports lets the compressed fresh air enter the

cylinder and helps cool the cylinder and scavenge the cylinder of the remaining exhaust gasses (Figure 7).

Commonly, intake and exhaust occur over approximately 96° of crankshaft rotation. At 43° ABDC, the camshaft

Figure 6

Figure 7

starts to close the exhaust valve. At 53° ABDC (117° BTDC), the camshaft has rotated sufficiently to allow the

spring pressure to close the exhaust valve. Also, as the piston travels past 48°ABDC (5° after the exhaust valve

starts closing), the intake ports are closed off by the piston.

CompressionAfter the exhaust valve is on its seat (53° ATDC),

the temperature and pressure begin to rise in

nearly the same fashion as in the four-stroke

engine. Figure 8 illustrates the compression in a 2-

stroke engine. At 23° BTDC the injector cam

begins to lift the injector follower and pushrod.

Fuel injection continues until 6° BTDC (17 total

degrees of injection), as illustrated in Figure 8b.

PowerThe power stroke starts after the piston passes TDC. Figure 25 illustrates the power stroke which continues until the

piston reaches 91° ATDC, at which point the exhaust valves start to open and a new cycle begins.

Figure 8 Figure 9b

3. THE DIESEL CYCLE

The diesel cycle is the ideal cycle for Combustion Ignition (CI) reciprocating engines. The CI engine was first

proposed by Rudolf Diesel in the 1890s. In CI engines (also known as diesel engines), the air is compressed to a

temperature that is above the auto-ignition temperature of the fuel, and combustion stars on contact as the fuel is

injected into this hot air. Therefore, the spark plug and carburetor which are used in gasoline engines are not needed

and are replaced by a fuel injector in diesel engines as shown below.

Gasoline engine Diesel engine

In gasoline engines, a mixture of air and fuel is compressed during the compression stroke, and the compression

ratios are limited by the onset of auto-ignition or engine knock. In diesel engines, only air is compressed during the

compression stroke, eliminating the possibility of auto-ignition. Therefore, diesel engines can be designed to operate

at much higher compression ratios, typically between 12 and 24.

The fuel injection process in diesel engines starts when the piston approached TDC and continues during the first

part of the power stroke. Therefore, the combustion process in these engines takes place over a longer interval.

Because of this longer duration, the combustion process in the ideal Diesel cycle is approximated as a constant-

pressure heat addition

Process a-b — isentropic compression

Process b-c — constant-pressure heat addition

Process c-d — isentropic expansion

Process d-a — constant-volume heat rejection

Noting that the diesel cycle is executed in a piston-cylinder device, which forms a closed system, the amount of heat

transferred to the working fluid at constant pressure and rejected from it at constant volume can be expressed as:

And

Then the thermal efficiency of the ideal Diesel cycle under the cold-air-standard assumption becomes

Let rc be the cutoff ratio i.e. the ratio of the cylinder volumes after and before the combustion process:

Therefore, the thermal efficiency is given as:

Where r is the compression ratio:

And

q in−wout=u3−u2→q in= P2 (v3−v2 )+(u3−u2 )¿ h3−h2

¿ c p (T3−T 2 )

−qout=u1−u4→qout=U 4−U1

¿ cV (T 4−T 1)

ηth , Diesel=wnet

qin

=1−qout

qin

=1−T 4−T 1

k (T 3−T 2)=1−

T 1(T 4

T 1

−1)kT 2(T 3

T 2

−1)

rc=v3

v2

ηth , Diesel=1− 1r k−1 [ r

ck−1

k (rc−1) ]r=

v1

v2

k=c p

cv

4. THE DIESEL ENGINE FUEL AND FUEL CONSUMPTION

Modern diesel engines burn a petroleum product similar to kerosene, jet fuel, and home heating oil. Diesel fuel is

less expensive to produce than gasoline since it does not require much refining. Gasoline must be refined and

reformed to high quality, low molecular weight and viscosity. It is also safer to handle. Due to lower volatility

(tendency to vaporize) and a higher flash point (the temperature at which diesel fuel ignites), it is less likely to catch

fire during an accident.

There are three grades of diesel fuel. They are classified according to the ease with which they ignite, the lowest

temperature at which the fuel will flow, and viscosity (resistance to flow). Individual grades of fuel are better suited

for certain engines and operating conditions.

For convenience, diesel fuels can be divided into two extreme categories. Light diesel fual has a molecular

weight of about 170, while heavy diesel fuel has a molecular weight of about 200. Most diesel fuel used in engines

will fit in this range.

Essentially, diesel is a lower-grade, less-refined product of petroleum made from heavier hydrocarbons

(molecules built from more carbon and hydrogen atoms). Crude diesel engines that lack sophisticated fuel injection

systems can, in theory, run on almost any hydrocarbon fuel—hence the popularity of biodiesel (a type of biofuel

made from, among other things, waste vegetable oil). The inventor of the diesel engine, Rudolf Diesel, successfully

ran his early engines on peanut oil and thought his engine would do people a favor by freeing them from a

dependency on fuels like coal and gasoline.

Considering the geometrically increasing number of automobiles and combustion engines, and the scarcity and

cost of production of crude oil and petroleum products, an alternate fuel technology must be adopted to meet the

great demand in coming decades. Another motivating factor for development of alternate fuels for diesel engines is

the concern over the emission problems of the engines.

Alcohols are an attractive alternate fuel because they can be obtained from a number of sources, both natural and

manufactured. They deliver fuel with higher quality and lesser emissions. They even have low sulfur content in the

fuel. They have not only be put into full-fledged production because of setbacks like low power content, high

tendency to corrode engine materials and their marketability. Some diesel engines which use dual fuel are starting to

appear on the market. They use methanol and a small amount of diesel fuel that is injected at the proper time to

ignite both fuels.

SPECIFIC FUEL CONSUMPTIONSpecific fuel consumption can be defined as the ratio of the rate of fuel flow into engine to the power delivered

by the engine. It can be expressed mathematically as follows:

Brake power gives brake specific fuel consumption,

indicated power gives indicated specific fuel consumption.

Other examples of specific fuel consumption parameters can be defined as follows: friction specific fuel

consumption (fsfc), indicated gross specific fuel consumption (igsfc), and indicated net specific fuel consumption

(insfc).

Where ηm = mechanical efficiency of engine

Brake specific fuel consumption decreases as engine speed increases, reaches a minimum, and then increases at

high speeds. Fuel consumption increases at high speeds because of greater friction losses. At low engine speed, the

longer time per cycle allows more heat loss and fuel consumption goes up. THe brake specific fuel consumption

decreases with higher compression ratio due to higher thermal efficiency. It is lowest when combustion occurs in a

mixture with fuel equivalence ratio near one. Brake specific fuel consumption generally decreases with engine size,

being lowest for very large engines.

5. PERFORMANCE OF DIESEL ENGINES

The operating characteristics of internal combustion engines like the Diesel engine determine their suitability in

different environments and conditions. The characteristics include the mechanical output parameters of work,

sfc=mf

¿

W¿

bsfc=mf

¿

W b

¿

isfc=mf

¿

W i

¿

ηm=W¿

b

W¿

i

=(m

¿

f

W¿

i)

( m¿

f

W¿

b)=

( isfc )( bsfc)

torque, and power; the input requirements of air, fuel, and combustion; efficiencies; and emission measurements of

engine exhaust.

Displacement VolumeThis is the volume displaced by the piston as it moves from the Botton Dead Center (BDC) to the Top Dead Center

(TDC) of the cylinder.

WorkWork is the output of the diesel engine generated by the gases in the combustion chamber of the cylinder. Work is

the result of force acting through a distance. The force due to gas pressure on the moving piston generates the work

in a diesel engine cycle.

where:

P = pressure in combustion chamber

Ap = area against which the pressure acts

φ

TDC

BDC

V D=V BDC−V TDC

¿ (π4 )B2S

W =∫F dx

=∫PA pdx

x = distance the piston moves

And

dV is the differential volume displaced by the piston, so work done can be written:

Mean Effective PressureAs it can be seen that the pressure inside the pressure inside the cylinder of an engine is continuously changing

during the cycle, an average or mean effective pressure (mep) is defined by:

or

where:

W = work of one cycle

w = specific work of one cycle

Vd = displacement volume

Compression RatioThis is the ratio of compression of the engine piston. It is expressed as the ratio of the volume at the Top Dead Center (TDC) to the volume at the Bottom Dead Center (BDC). An increase in compression ratio increases the engine power output.

Engine Volumetric EfficiencyThis is the ratio of volume of air taken into the cylinder to the maximum possible volume that can be taken into the

cylinder.

Ap dx=dV

W =∫ PdV

w=(mep ) Δv

mep= wΔv

= WV d

Δv=vBDC−vTDC

r=Cylinder Volume at BDCCylinder Volume at TDC

r=(Cylinder Volume+Cylinder Clearance Volume )Cylinder Clearance Volume

r=V s+V c

V c

=1+V s

V c

ηV =volume of air taken in the cylinderMaximum possible volume in the cylinder

ηV =V air

V c

Where ηv = volumetric efficiency

Vair= volume of air taken in the cylinder

Vc = cylinder swept volume

Engine Indicated TorqueThis is the ratio of work done to the angle for every revolution of the crank shaft.

Engine Specific Fuel ConsumptionThis is the ratio of mass of fuel consumption to engine brake power.

where:

FC = fuel consumption

Pb = brake power

Engine Thermal EfficiencyThis can be defined as the ratio of brake power to fuel power.

where:

CV = calorific value of kilogram fuel

T i=work per one revolutionangle of one revolution

T i=imep×V c

2 π×z

SFC=mass of fuel comsumptionengine brake power

SFC=FCPb

ηth=brake powerfuel power

ηth=3600 Pb

FC×CV

6. APPLICATION OF DIESEL ENGINES

The characteristics of diesel engines have different advantages for different applications. They can be applied in

engine sizes from small to large. Diesel engines are widely used as stationary power sources for electrical generation

units, pumping stations, refrigeration facilities, and factories. Heavy construction equipment, ships, locomotives,

commercial trucks, and some large pickups are powered by diesels. They are preferably utilized in engines or

machines that require greater torque than speed.

(i) Automobiles: Diesel engines have long been popular in bigger cars and have been used in smaller cars

such as super minis like the Peugeot 205, in Europe since the 1980s. Diesel engines tend to be more

economical at regular driving speeds and are much better at city speeds. Their reliability and life-span

tend to be better. Some 40% or more of all cars sold in Europe are diesel-powered where they are

considered as a low CO2 option. Mercedes-Benz in conjunction with Rober Bosch GmbH produced

diesel-powered passenger cars starting in 1936 and very large numbers are used all over the world.

(ii) Diesel Generators: A diesel generator is the combination of a diesel engine with an electric generator

(often an alternator) to generate electrical energy. Diesel generators are used in places without

connection to the power grid, as emergency power-supply if the grid fails. Ships also employ diesel

generators, sometimes not only to provide auxiliary power for lights, fans, winches etc., but also

indirectly for main propulsion.

(iii) Railroad Locomotives: Diesel engines have eclipsed steam engines in the prime mover on all non-

electrified railroads in the industrialized world. The first diesel locomotives appeared in the early 20th

century, and diesel multiple units soon after. While electric locomotives have now replaced the diesel

locomotive almost completely on passenger traffic in Europe and Asia, diesel is still today very

popular for cargo-hauling freight trains and on tracks where electrification is not feasible. Most

modern diesel locomotives are actually diesel-electric locomotives, the diesel engine is used to power

an electric generator that in turn powers electric traction motors with no mechanical connection

between diesel engine and traction.

(iv) Ships and Submarines: In merchant ships and boats, the same advantages apply with the relative safety

of diesel fuel an additional benefit. The German pocket battleships were the largest diesel warships,

but the German boats of the Second World War were also diesel craft. Conventional submarines have

used them since before World War I, relying on the almost total absence of carbon monoxide in the

exhaust. American World War TT diesel-electric submarines operated on two-stroke cycle.

(v) Non-Road Diesel Engines: Diesel engines are also applied in the manufacture of mobile equipment and

vehicles that are not used as the public roadways such as construction equipment and agricultural

tractors. Diesel engines are used to power crawler tractors used for heavy pulling, pushing or adverse

terrain conditions. These tractors move on heavy, metal tracks that form a loop around large geared

wheels; the wheels drive the metal tracks, and the tracks distribute the weight over a wide area.

ADVANTAGES AND DISADVANTAGES OF DIESEL ENGINESDiesels are the most versatile fuel-burning engines in common use today, found in everything from trains and

cranes to bulldozers and submarines. Compared to gasoline engines, they're simpler, more efficient, and more

economical. They're also safer, because diesel fuel is less volatile and its vapor less explosive than gasoline. Unlike

gasoline engines, they're particularly good for moving large loads at low speeds, so they're ideal for use in freight-

hauling ships, trucks, buses, and locomotives. Higher compression means the parts of a diesel engine have to

withstand far greater stresses and strains than those in a gasoline engine. That's why diesel engines need to be

stronger and heavier and why, for a long time, they were used only to power large vehicles and machines. While this

may seem a drawback, it means diesel engines are typically more robust and last a lot longer than gasoline engines.

Pollution is one of the biggest drawbacks of diesel engines: they're noisy and they produce a lot of unburned soot

particles, which are dirty and hazardous to health. But since diesels are more efficient, they typically use less fuel,

produce fewer carbon dioxide emissions, and contribute less to global warming. Diesel engines tend to cost more

initially than gasoline engines, though their lower running costs and longer operating life generally offsets that.

IV. VISIT TO MANTRAC NIGERIAMantrac Nigeria Ltd. is the sole authorized dealer for Caterpillar Products in Nigeria. Mantrac Nigeria Ltd.

distributes and supports the full range of CAT high speed diesel engines, with ratings available from 54 to 13,600 hp

(40 to 10,000 kW). They also supply Caterpillar generator sets (10 kVA to 220 kVA) that provide electrical power

systems, giving both primary and standby power for a variety of uses. Mantrac Nigeria Ltd. provides ready-to-use

services to clients covering all stages of the power systems project including system design, engineering, testing,

installation, long term maintenance and repair.

It was a firsthand experience as I was opportune to see different sections in the factory where diesel engines and

generators are repaired, maintained and assembled. A section was responsible for repair and maintenance of the

cylinders and cylinder block, another section was responsible for the engine assembly, while the last section was

responsible for digital testing and simulation.

PICTURE SHOOTS FROM FIELD TRIP TO MANTRAC NIGERIA LIMITED.

A 6-Cylinder Engine Block

A Camshaft with fault watermarks

A Piston and Connecting Rod assembly

A V-shaped 8-cylinder Diesel EngineTop-view of V8 engine showing Rocker Arm

V. ConclusionThe higher efficiency of the diesel engine (due to higher compression ratios) over the gasoline engine earns

diesel engines a first choice wherever the disadvantages of diesel engines are of less importance. Diesel engines are

recommended for applications requiring large power or torque, since they burn fuel more completely as compression

ignition engines.

The Diesel engine would fare more in coming decades when crude oil and petroleum products would have

become very scarce. The demand for fuel for internal combustion engines would be met by diesel engines since they

can even run normally with vegetable oils, adding a little diesel oil for ignition.

References

[1] Gilbert Gedeon, P.E., Diesel Engine Fundamentals, Continuing Education and Development, Inc. New York

[2] Klaus M. and Helmut T., Handbook of Diesel Engines, ISBN 978-3-540-89082-9, Springer, Germany, 2010

[3] Yunus A. C., Michael A. B., Thermodynamics: An Engineering Approach, 5th Edition, Mc-Graw Hill, New York, Page 500

& 501

[4] “Diesel engine - Wikipedia, the free encyclopedia” Diesel Engine [online encyclopedia], URL:

http://en.wikipedia.org/wiki/Diesel_engine [cited 21 April 2014].

[5] “Diesel generator - Wikipedia, the free encyclopedia” Diesel Generators [online encyclopedia], URL:

http://en.wikipedia.org/wiki/Diesel_generator [cited 15 March 1998].