4. Reversible Adiabatic Process
Reversible adiabatic process is also called an Isentropic
Process. It is an idealized thermodynamic process that is
adiabatic and in which the work transfers of the system are
frictionless; there is no transfer of heat or of matter and the
process is reversible. Such an idealized process is useful in
engineering as a model of and basis of comparison for real
processes.
5. Basics of 2 stroke
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Diesel
technology
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9. Octane rating
From Wikipedia, the free encyclopedia
Jump to navigationJump to searchAn octane rating, or octane
number, is a standard measure of the performance of an engine or
aviation gasoline. The higher the octane number, the
more compression the fuel can withstand before detonating. In
broad terms, fuels with a higher octane rating are used in high-
performance gasoline engines that require higher compression
ratios. In contrast, fuels with lower octane numbers (but
higher cetane numbers) are ideal for diesel engines, because
diesel engines (also referred to as compression-ignition engines)
do not compress the fuel, but rather compress only air and then
inject fuel into the air which was heated by compression. Gasoline
engines rely on ignition of air and fuel compressed together as a
mixture, which is ignited near the end of the
compression stroke using electrically activated spark plugs.
Therefore, high compressibility of the fuel matters mainly for
gasoline engines. Use of gasoline with lower octane numbers may
lead to the problem of engine knocking.[
The octane rating of a fuel is a
measurement used to indicate its
resistance to engine knock.
This is done by
altering the
composition of
the fuel (using
ethanol,
methanol,
nitromethane,
leaded fuel, etc)
10.
11.
12. BASIS OF COMPARISON OTTO CYCLE DIESEL CYCLE
Proposed By
The idea of Otto cycle was first unveiled by Nicolas Otto back then in
1876.
The diesel cycle was proposed by Dr. Rudolph
Diesel in 1897.
Lighting The Charge
Uses the spark plugs to light the charge, which is a mixture of air and
fuel.
Does not require any assistance to get ignited, this
because it has a high compression ratio that does
not allow it to depend on any plug for the charge.
Petrol/Diesel Engine
Petrol engines work on principle of Otto cycle whereby to supply the
fuel into the combustion chamber Carburetor is used.
Diesel engines work on the principle of diesel cycle
whereby fuel injectors are required to supply fuel
into the combustion chamber.
Processes
Works on four processes, which are intake stroke, compression stroke,
expansion stroke and exhaust stroke.
Works on four different processes which are
adiabatic compression, heat addition, adiabatic
expansion and then heat rejection.
Heat Addition The heat addition takes place at constant volume. Heat is added at constant pressure.
Efficiency The overall efficiency is way less than that of diesel cycle.
The overall efficiency is higher than the Otto
cycle.
Adiabatic Expansion
Adiabatic expansion takes place during the backward stroke of the
piston.
The adiabatic expansion takes place when the
heat addition is cut-off.
During Intake Stroke The charge is drawn in by the cylinders during the intake stroke.
After drawing air during the intake stroke, fuel is
injected by an injector.
Compression Ratio
It has compression ratio from 7:1 to 10:1 which is lesser than that of a
diesel cycle.
It has a comparatively higher compression ratio. It
ranges from 11:1 to 22:1
Thermal Efficiency It has a high thermal efficiency when compared to Diesel cycle.
It has a comparatively lower thermal efficiency than
that of Otto cycle.
Position Of Piston At The
Time Of Heat Addition
At the time of heat addition, the piston is at Top Dead Center (TDC).
When the piston is in the backward stroke, the heat
addition begins and last at a portion of piston
stroke (when piston is retreating).
13. Petrol engine knocking is antirotational
combustion/backpressure/vibration
due to early combustion self combustion due to high
compression ratio
Knocking (also knock, detonation, spark knock, pinging or pinking) in spark
ignition internal combustion engines occurs when combustion of some of the air/fuel
mixture in the cylinder does not result from propagation of the flame front ignited by
the spark plug, but one or more pockets of air/fuel mixture explode outside the
envelope of the normal combustion front. The fuel-air charge is meant to be ignited
by the spark plug only, and at a precise point in the piston's stroke. Knock occurs
when the peak of the combustion process no longer occurs at the optimum moment
for the four-stroke cycle. The shock wave creates the characteristic metallic
"pinging" sound, and cylinder pressure increases dramatically. Effects of engine
knocking range from inconsequential to completely destructive.
Knocking should not be confused with pre-ignition—they are two separate events.
However, pre-ignition can be followed by knocking.
14. Autoignition temperature
From Wikipedia, the free
encyclopedia
Jump to navigationJump to
searchThe autoignition
temperature or kindling point of a
substance is the lowest temperature
in which it spontaneously ignites in a
normal atmosphere without an
external source of ignition, such as a
flame or spark. This temperature is
required to supply the activation
energy needed for combustion. The
temperature at which a chemical
ignites decreases as the pressure or
oxygen concentration increases. It is
usually applied to a combustible fuel
mixture.
The ignition temperature of a
substance is the least temperature
at which the substance starts
combustion.
Substances which spontaneously
ignite in a normal atmosphere at
The flash point of a volatile material is
the lowest temperature at which its
vapours ignite if given an ignition source.
The flash point is sometimes confused
with the autoignition temperature, the
temperature that causes spontaneous
ignition. The fire point is the lowest
temperature at which the vapors keep
burning after the ignition source is
removed. It is higher than the flash point,
because at the flash point more vapor
may not be produced fast enough to
sustain combustion.[1] Neither flash point
nor fire point depends directly on the
ignition source temperature, but ignition
source temperature is far higher than
either the flash or fire point.
17. 2. The Diesel Knock Phenomena Knock, pinging or detonation are all terms that have been
widely used to describe the characteristic “metallic rattling” noise associated with abnormal
combustion in spark-ignition engines. Spark-ignition knock is caused by the spontaneous ignition
of gas ahead of the propagating flame front (the end gas) within the combustion chamber. This
spontaneous ignition results in a rapid release of chemical energy and an accompanying rapid
rise in cylinder pressure [15]. Unlike spark-ignition knock, diesel knock occurs when
injected fuel auto-ignites and combusts in the premixed stage of combustion. Whilst this
process is a normal part of diesel engine operation, various circumstances can lead to excess
quantities of fuel combusting in a premixed fashion. This situation often develops if the
parameters governing combustion lead to abnormally long ignition delay periods. As a
consequence, excessive diesel knock can often be a symptom of underlying faults such as poor
or contaminated fuels, injection system problems or unsuitable rates of alternative fuel
substitution. The rapid pressure increases associated with both spark-ignition and diesel knock
result in the propagation of high amplitude pressure waves at frequencies governed by
combustion chamber resonance. The combustion-chamber resonance frequencies are in turn
determined by the geometry and the velocity of sound within the combustion chamber [3]. Figure
1 highlights the effects of spark-ignition knock on cylinder pressure. As seen, the knock
phenomenon causes high frequency pressure fluctuations, which are recorded by the pressure
sensor. It is also seen that the amplitude of the high frequency pressure fluctuations increases as
the severity of the knock increases [15]. Whilst discussing spark-ignition knock, Heywood [15]
points out that due to the highly variable nature of engine knock, fundamental definitions of knock
intensity is extremely difficult to make. However, a method whereby cylinder pressure signals are
used to calculate an average pressure rise rate (PRR) is described. Another method, also
18. Diesel knocking The fuel which has long
ignition delay. accumulate in the engine
and undergoes explosive combustion
resulting in diesel knocking.
21. WHAT IS MEANT BY SCAVENGING and SUPERCHARGING?
Scavenging is the process in which air is forced into the cylinder which is called the scavenge air, through
ports in the cylinder liner which are called the scavenge ports, which helps to clear the cylinder from gases
of combustion.
Supercharging means an increase in air flow into the cylinders of an engine which serves to increase in
power output, in addition to being used for scavenging.
So basically what is happening is scavenging is done when the air is admitted into the cylinder under
pressure with the exhaust valves or ports open.
Supercharging is done with the exhaust valve or ports closed where the air is forced to enter the cylinder
and thereby increase the amount of air that is available for combustion. Therefore an engine is said to be
supercharged when the scavenge manifold pressure exceed the atmospheric pressure.
Always remember an engine which is supercharged, of the same give size, can develop more power than
an engine of the same size that is not supercharged.
Always remember, as explained above,
SUPERCHARGERS - Supercharger is typically driven by a belt or a gear powered by the engine.
superchargers are good because the horsepower boost is available immediately however supercharger
takes the power away from the engine.
TURBOCHARGERS - The turbocharger creates pressure using the already-burnt exhaust gas that is
coming out of the engine to turn the blades of a fan that forces the air into the engine. Turbocharger takes
a moment to "spool up" from the exhaust gas. Turbocharger does not take power away from the engine.
33. "Combustion" refers to burning fuel with an oxidizer,
to supply the heat.
Engines of similar (or even identical) configuration and
operation may use a supply of heat from other sources such
as nuclear, solar, geothermal or exothermic reactions not
involving combustion; they are not then strictly classed as
external combustion engines, but as external thermal
engines.
37. Piston is one of the most important Part of an Engine and a key player in Transmitting the Power. There are 2 types of Piston Construction
which are:-
Trunk-Type Pistons:- The CROWN, or head, of a piston acts as the moving surface that changes the volume of the content of the cylinder
(compression), removes gases from the cylinder (exhaust), and transmits the energy of combustion (power). Generally, the crown end of a
piston is slightly smaller in diameter than the skirt end. The resulting slight taper allows for expansion of the metal at the combustion end.
Even though slight, the taper is sufficient so that, at normal operating temperatures, the diameter of the piston is the same throughout. There
are variety of crown head designs , be it :- cone, recessed, dome , convex or cup etc. Their application varies according to the point of
application of engine. However, In some 2-stroke cycle engines, piston crowns are shaped with irregular surfaces which deflect and direct the
flow of gases.
The SKIRT of a trunk-type piston receives the side thrust created by the movement of the crank and connecting rod. In turn, the piston
transmits the thrust to the cylinder wall. In addition to receiving thrust, the skirt aids in keeping the piston in proper alignment within the
cylinder. Some pistons are plated with a protective coating of tin which permits close fitting, reduces scuffing, and prolongs piston life. Still
other pistons may be given a phosphate treatment to aid skirt lubrication. This process etches the surface and provides a nonmetallic, oil-
absorbent, anti-friction coating that promotes rapid break-in and reduces subsequent wear.
Most trunk-type pistons are of one-piece construction. Some trunk pistons are made of two parts and two metals; the trunk or skirt is made of
cast iron or an aluminum alloy, and the crown or head is made of steel. In some pistons of this type of construction, the crown is fitted to the
trunk with a ground joint, while in others the parts are welded together. Refer the Fig for Trunk Type Piston:-
Crosshead Pistons:- The cross-head piston is a two-piece unit with a crown that can withstand the high heat and pressure of a turbocharged
engine and a skirt specifically designed to absorb side thrust.
The crown and skirt are held together by the piston pin. The downward load on the crown pushes directly on the pin through a large slipper
bearing (bushing). The separate skirt has less thermal distortion than the crown piece and is free of downward thrust loads. It specifically
guides the piston in the cylinder, takes up side thrust, and carries the oil scraper rings. The crown carries the compression rings. Since the
crown is separate, it takes only a slight amount of side thrust and is not forced to slide sideways under the compression rings when they are
pressed hard against the bottoms of their grooves by combustion gas pressure. Lubricating oil is fed upward by pressure to cool the piston
pins and piston crown. Refer the fig for Cross-Head Piston:-
Hope it will Help :-)
38.
39.
40.
41.
42. DIFFERENCE BETWEEN CROSSHEAD TYPE AND TRUNK TYPE ENGINES
Date: 18 Jul 2017Author: meoexampreparation0 Comments
1.THE TRANSVERSE THRUST DUE TO THE OSCILLATION OF THE
CONNECTING-ROD IS TAKEN UP BY THE CROSSHEAD ASSEMBLY AND ITS
GUIDE IN THE CROSSHEAD TYPE OF ENGINES WHEREAS IN THE TRUNK
TYPE PISTON TYPE OF ENGINES THE TRANSVERSE THRUST IS TAKEN UP
BY THE PISTON SKIRT.
2.THE NATURE OF THE BEARING ASSEMBLY AT THE UPPER PART OF THE
CON-ROD. FOR CROSSHEAD ENGINES, THE UPPER PART OF THE CON-
ROD IS CONNECTED TO A CROSSHEAD ASSEMBLY CONSISTING OF
CROSSHEAD BLOCK, PINS AND SLIPPERS. THIS CROSSHEAD ASSEMBLY
IS IN TURN ATTACHED TO THE LOWER PART OF THE PISTON-ROD. FOR
TRUNK PISTON TYPE ENGINES, THE UPPER PART OF THE CON-ROD IS
CONNECTED DIRECTLY TO THE PISTON VIA THE GUDGEON BEARING
ASSEMBLY.
3.CROSSHEAD ENGINE HAVE A PISTON ROD WHILE TRUNK PISTON
ENGINE DONOT.
4.CROSSHEAD ENGINES HAVE A DIAPHRAGM THAT SEPARATES THE
CYLINDER SPACE FROM THE CRANKCASE WHILE TRUNK TYPE DONOT
HAVE.
5.CROSSHEAD ENGINE HAVE DIFFERENT CYLINDER LUBRICATION
SYSTEM WITH DIFFERENT LUB OIL THAN CRANKCASE OIL WHILE TRUNK
TYPE HAVE SAME LUB OIL AS THAT OF CRANKCASE.
43.
44.
45.
46. Piston slap ::
the characteristic sound of a
seriously worn piston in a
cylinder (usually of the engine
of a motor car)
47.
48.
49.
50.
51.
52.
53.
54. A cross head
eliminates the side
load on a piston and in
the case of a steam
engine allows the back
of the piston to be
used for another
power stroke (double
acting )
In large 2 stroke
diesels the area below
the piston can be
sealed from the
crankcase and used
for an air supply to the
ports without oil
problems.
66. Sankey diagrams are a type of flow diagram in which the width of the arrows is
proportional to the flow rate.
MAN Diesel, a renown producer of marine and power plant diesel engines, has been
working on improving fuel efficiency of its engines. Today, the fuel energy efficiency is
about 50%. The MAN Turbo Efficiency System (TES) allows to recover of heat from the
exhaust gas, which is responsible for about 50% of the energy losses.
Here is a Sankey diagram that shows the recovery of energy from exhaust gas.
72. Efficiency
The only reason a practical engineer wants to run an engine at all
is to achieve a desired output of useful work, which is, for our
present purposes, to drive a ship at a prescribed speed, and/or to
provide electricity at a prescribed kilowattage.
To determine this power he or she must, therefore, allow not only
for the cycle losses mentioned earlier but also for the friction
losses in the cylinders, bearings and gearing (if any) together with
the power consumed by engine-driven pumps and other auxiliary
machines. He or she must also allow for such things as windage.
The reckoning is further complicated by the fact that the heat
rejected from the cylinder to exhaust is not necessarily totally lost,
as practically all modern engines use up to 25 per cent of that heat
to drive a turbocharger. Many use much of the remaining high
temperature heat to raise steam, and use low temperature heat for
other purposes.
The detail is beyond the scope of this book but a typical diagram
(usually known as a Sankey diagram), representing the various
energy flows through a modern diesel engine, is reproduced in
Figure 1.4. The right-hand side represents a turbocharged engine,
and an indication is given of the kind of interaction between the
73.
74. Note that the heat released from the fuel in the cylinder is augmented by the heat value of the work done by the
turbocharger in compressing the intake air. This is apart from the turbocharger’s function in introducing the
extra air needed to burn an augmented quantity of fuel in a given cylinder, compared with what the naturally
aspirated system could achieve, as in the left-hand side of the diagram.
It is the objective of the marine engineer to keep the injection settings, the air flow, and coolant temperatures
(not to mention the general mechanical condition) at those values, which give the best fuel consumption for the
power developed.
Note also that, whereas the fuel consumption is not difficult to measure in tonnes per day, kilograms per hour or
in other units, there are many difficulties in measuring work done in propelling a ship. This is because the
propeller efficiency is influenced by the entry conditions created by the shape of the afterbody of the hull, by
cavitation and so on and also critically influenced by the pitch setting of a controllable pitch propeller. The
resulting speed of the ship is dependent, of course, on hull cleanliness, wind and sea conditions, draught and
so on. Even when driving a generator it is necessary to allow for generator efficiency and instrument accuracy.
It is normal when defining efficiency to base the work done on that transmitted to the driven machinery by the
crankshaft. In a propulsion system, this can be measured by a torsionmeter; in a generator it can be measured
electrically. Allowing for measurement error, these can be compared with figures measured on a brake in the
test shop.
