Thermodynamic Principles and Engineering terms

Question 1

define the following engineering terms

mass
work
power
energy
pressure
volume
heat

Answer 1

Mass

Mass is the quantity of matter in a body and is proportional to the product of volume and density. The unit is the kilogram and the abbreviation used is ‘kg’. Large quantities are often expressed in tonnes (t)

Work

When a force applied to a body causes it to move then work has been done. When unit mass is moved unit distance then a unit of work has been done. The unit of work is the joule (J)

Power

This is the quantity of work done in a given time or the rate of doing work. When unit work is done in unit time then a unit of power has been used. The unit of power is the watt (W).

Energy

This is the stored ability to do work and is measured in units of work done, i.e. joules.

Pressure

The intensity of force or force per unit area is known as pressure, A unit of pressure exists where unit force acts on unit area. The unit of pressure is the newton per square metre. The datum or zero for pressure measurements must be carefully considered. The complete, absence of pressure is a vacuum and this is therefore the absolute zero of pressure measurements. However, acting upon the earth’s surface at all times is what is known as ‘atmospheric pressure’.

Volume

The amount of physical space occupied by a body is called volume. The unit of volume is the cubic metre, Other units are also in use, such as litre (1) and cubic centimetres.

Heat

Heat is energy in motion between a system and its surroundings as a consequence of a temperature difference between them.

Question 2

describe the process known as isothermal operation?

Answer 2

thermodynamic process in which the temperature of a system remains constant. This is an ideal, reversible process carried out at constant temperature. It follows Boyle’s law, requiring heat addition during expansion and heat extraction during compression. It is however impractical due to the requirement of very slow piston speeds. (PV=constant)

Question 3

describe the process known as adiabatic operation

Answer 3

Adiabatic operation (PV^γ = constant) (where γ = gamma = the adiabatic index Cp/Cv)

This is also an ideal and reversible process but with no heat addition or extraction and therefore the work done is equivalent to the change of internal energy. It is again impracticable due to the requirement of very high piston speeds. Adiabatic index is a measure of how much a gas will compress under pressure

Question 4

describe the process known as polytropic operation?

Answer 4

Polytropic operation (PV^n = constant)

This is close to a practical process where the value of the index n usually lies between unity and gamma. Polytropic process is a thermodynamic process that obeys the Polytropic equation of PV^n =constant. It describes the relationship between the pressure and volume of a fluid or gas during expansion or compression. This may involve heat transfer, work interaction, or both.

Question 5

what is volumetric efficiency?

Answer 5

This is a comparison between the mass of air induced per cycle and the mass of air contained in the stroke volume at standard conditions. This term is usually used to describe four-stroke engine and air compressor operation. Due to the restrictions of practical engine design typical values range between 86% and 92%.

Question 6

define the term scavenge efficiency

Answer 6

‘scavenging’ is the term used to describe the air exchange process. This is similar to volumetric efficiency but is used to describe two-stroke engines where some exhaust gas from the previous stroke may be included with the induction air at the start of compression. The ratio of trapped fresh air to the maximum theoretical amount of fresh air that could be retained in the cylinder.

Question 7

define the term mechanical efficiency

Answer 7

Mechanical efficiency is a measure of the mechanical perfection of an engine. It is numerically expressed as the ratio between the indicated power (power available from burning the fuel) and the brake power (power measured at the flywheel).

Question 8

define break thermal efficiency?

Answer 8

This is the ratio between the energy developed at the flywheel, or the output shaft of the engine, and the energy supplied from burning the fuel. Traditionally this was measured by placing a ‘load’ or ‘brake’ on the output shaft, hence the term brake thermal efficiency.

Question 9

define specific fuel oil consumption

Answer 9

SFOC is the fuel consumption per unit of energy at the cylinder or output shaft, kg/kWh (or kg/kWs), 0.38 kg/kWh would be normal for measurement at the shaft for a modern engine. However, the current general practice is for the manufacturers to quote a consumption figure measured at the cylinder and expressed in g/kWh and not kg/kWh. Therefore, a typical fuel consumption figure for a modern two-stroke diesel main engine would be quoted as being between 160 and 185 g/kWh.

Question 10

define compression ratio

Answer 10

Compression ratio (CR)

CR is a measurement of the ratio of the volume of air at the start of the compression stroke to the volume of air at the end of this stroke (measured between top dead centre (TDC) and bottom dead centre (BDC)

Question 11

define fuel air ratio

Answer 11

Its the ratio between the mass of air and the mass of fuel. Depending upon the type and quality of fuel the amount of air required to give enough oxygen to completely burn all the fuel is about 14.5 kg for each kg of fuel. However, engines supply excess air to the combustion process and therefore the actual air supplied varies from about 29 to 44 kg/kg fuel. The percentage of excess air is about 150 (36.5 kg for each kg of fuel)

Question 12

what is variable injection timing

Answer 12

VIT or Variable Injection Timing is a form of fuel pump control, enabling an engine to operate with the designated maximum cylinder firing or combustion pressure from approximately 75% power output to maximum power. This improves thermal efficiency and lowers fuel consumption.

Question 13

What are Ideal Thermodynamic Cycles?

Answer 13

Ideal Thermodynamic cycles are a series of operations carried out by a machine manipulating a substance. During the process heat and work are transferred by varying temperatures and pressures and eventually returning the system to its original state. The ideal thermodynamic cycles form the benchmark for reference against the actual performance of IC engines.

Question 14

In terms of ideal thermodynamic cycles what are two main classifications for reciprocating IC engines?

Answer 14

There are two main classifications for reciprocating IC engines: (a) spark ignition (SI) such as petrol engines and, (b) compression ignition (CI) such as diesel and oil engines

Question 15

what are the types indicator cards or otherwise known as indicator diagrams?

Answer 15

1. power or in phase card
2. out of phase card
3. light spring card
4. compression card
5. pressure derivative card

Question 16

what is the purpose of indicator diagrams?

Answer 16

allows the engineer to receive more information about the combustion process (via the draw or out of phase card), measure the cylinder power output of the engine (via the power cards), and check the cleanliness of the scavenging process (via the light spring diagram).

Question 17

if the maximum firing pressure for engine is high due to early fuel injection what is this caused by

Answer 17

Incorrect fuel timing of the cams.
Incorrect VIT setting
Leaky fuel injector

Question 18

if the engine peak pressure is too low due to late injection what is this caused by

Answer 18

Bad quality of fuel.
Fuel injector nozzle blocked.
Fuel pumps leaking.
Low fuel pressure.
Injector seized.

Question 19

engine Compression pressure is low, and the peak pressure is also too low what could be the reasons for this

Answer 19

Leaking exhaust valve.
Leak through piston rings i.e. broken or worn out piston rings.
High Liner wear.
Burnt piston crown.
Low scavenge pressure

Question 20

what is an engine electronic indicator and what are the advantages?

Answer 20

Electronic indicators

The limitations of mechanical indicating equipment have been overtaken by the need for greater accuracy and a higher level of detail as engine powers have risen and especially now as electronic combustion control has become normal practice. With outputs reaching 5500 hp per cylinder inaccuracies of ±4.0% will lead to large variations in indicated power and therefore attempts to balance the engine power by this method will only have limited success. The inaccuracies stem from the friction and inertia of mechanical equipment transmitting errors in recording as well as the inaccuracy of measuring the height of the power card. Modern practice utilises electronic equipment to monitor and analyse the cylinder peak pressures and piston position and presents the information on a display which could be part of the analyser or could be the display on a computer. The cylinder pressure is measured by a pressure transducer which is attached to the indicator cock. Engine position is detected by a magnetic pick-up in close proximity to a toothed flywheel. The information is fed to a microprocessor, where it is averaged over a number of engine cycles, before calculations are made as to indicated power.

The advantages of this type of equipment are as follows:

1. It supplies real-time dynamic operational information. Therefore, the injection timing can be measured while the engine is running, which is a more accurate method of measuring the timing – and if sensors are placed on the crankshaft at each unit as with Wärtsilä ‘intelligent combustion’ system – it allows for crankshaft twist while the engine is under load, unlike static methods which do not.
2. It can compare operating conditions with optimum performance which leads to improvements in fuel economy and thermal efficiency.
3. It produces a load diagram for the engine, clearly defining the safe operating and optimum performance zones for the engine.

4.It can produce a trace of the fuel pressure rise and fall in the high pressure lines, which is invaluable information when diagnosing fuel injection faults.

Question 21

explain the Farnboro type engine indicator

Answer 21

For high-speed engines an indicator of the ‘Farnboro’ type is often used. Maximum combustion pressure and compression pressure can be taken using a peak pressure indicator. The fuel is shut off to the cylinder being measured to obtain the compression pressure. The Farnboro engine indicator is a device used to measure and record the pressure inside a cylinder of an internal combustion engine. It consists of a spring-loaded piston connected to a pen that traces a graph on a rotating drum, showing the pressure changes during the engine’s operation. The Farnboro engine indicator works by attaching the piston assembly to the cylinder head of the engine. As the piston moves up and down, it compresses the spring and moves the pen, creating a graph of the pressure changes. The rotating drum allows for multiple revolutions of the engine, providing a complete picture of the pressure throughout the entire cycle. It measures engine performance and efficiency, by analysing pressure changes in cylinder changes can be made to adjust performance

Question 22

explain what is variable valve timing

Answer 22

VVT is one of the areas of combustion control that has had a big influence on the operation of engines. The problem with mechanical operation, using camshaft control of the inlet and exhaust valve is that it is difficult to change the cam profile and influence the time of opening and closing for different engine speeds. VVT enables the use of the Miller cycle, without the disadvantage of the very poor part load operation. If the Miller cycle is used at part load then the combustion is starved of air and a large amount of smoke it produced by the engine. The use of VVT technology overcomes this and allows the inlet and exhaust valve timing to be optimised for all engine loads. It also allows the Miller cycle to be switched on and off to match the engine operating conditions Today, there are two major types of variable valve timing systems ie cam phasing and cam changing. With cam changing, the Electrical control Unit selects a different cam profile based on engine load and speed, whereas, with cam phasing, an actuator rotates the camshaft, changing the phase angle. VVT systems use all sorts of high tech sensors, but the most important of all is the camshaft and crankshaft position sensors (which are often hall effect sensors).The ECU uses these sensors to closely monitor the relationship between the piston’s location and the valves’ positions. The crankshaft is connected to the rod and piston, and the camshaft’s lobes actuate the valve lift events. Thus, with the data from the crankshaft and camshaft position sensors, the ECU can interpret how fast the engine is rotating and the relative positions of the piston and the intake and exhaust valves. Cam Switching System This method uses two cam profiles, equipped with an actuator to swap between the profiles (usually at a specific engine speed). Cam switching can also provide variable valve lift and variable duration, however, the adjustment is discrete and not continuous. Cam Phasing
Many production VVT systems are the cam phasing type, using a device known as a variation. This allows continuous adjustment of the cam timing (although many early systems only used discrete adjustment), however, the duration and lift cannot be adjusted.

Question 23

what’s power card and light spring card used for

Answer 23

power draw card is particularly useful for compression and/or combustion fault diagnosis and the light spring diagram for the analysis of scavenge – exhaust considerations.

Question 24

Explain what is meant by octane number an what is the relationship between octane number and engine knocking and why anti knock additives, octane number and knocking are important factors?

Answer 24

The octane number is a standard measure of the performance of an engine. It is determined by comparing the knocking characteristics of a fuel to blends of two primary reference fuels: iso-octane (octane rating of 100) and n-heptane (octane rating of 0). Engine knocking, also known as detonation, occurs when the air-fuel mixture in the engine’s cylinders ignites spontaneously or unevenly, causing a knocking sound.The organic compounds present in crude oil are the source of the hydrocarbons that make up gasoline. The composition of these compounds affects the octane number of the gasoline produced. Higher octane numbers indicate a greater resistance to knocking, which is desirable in high-performance or high-compression engines. This resistance to knocking is achieved through the use of different organic compounds or additives. Therefore, the relationship between octane number, engine knocking, and organic compounds in crude oil is that the composition of organic compounds in crude oil influences the octane number of the gasoline produced, which in turn affects the likelihood of engine knocking. The amount of power that an engine develops is controlled by many factors, one of which is the compression ratio. All other things being equal, the higher this ratio the more power. The problem is that at higher compression ratios the fuel burns too quickly and too soon before TDC. This is solved by putting additives in the fuel to make it burn more slowly, notoriously in past years, lead. This is called the anti-knock additive and the effectiveness of it is indicated by the octane number, the higher that number the higher the compression ratio you can use. If you use a fuel with too low an octane number the engine will pre-ignite or ‘knock’, very harmful to the engine. High octane fuel will not give more power but allows a higher compression ratio. Therefore Pinking (knocking) and its relation to octane number are important factors as are anti-knock additives such as lead tetra-ethyl, Pb(C2H5)4.

Question 25

Explain the following with reference to CI engines

Cetane number, ignition quality, diesel knock

Answer 25

Cetane number

Cetane number is a measure of the ignition quality of diesel engine fuels as it indicates the comparative ease with which a diesel fuel will ignite in a diesel engine cylinder. The time interval between the start of injection and the start of combustion (ignition) of a fuel in a diesel engine, i.e., the fuel’s ignition delay, is measured by the cetane number. This number is a measurement of the combustion quality of diesel fuel during compression ignition. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder and has a short ignition delay period, both of which are desirable characteristics in a diesel engine.The cetane number of a diesel fuel oil is determined by comparing its combustion characteristics in a test engine with those for blends of reference fuels of known cetane number under standard operating conditions

Ignition quality

The ignition quality of diesel fuel is linked to the ignition delay time, the time between the start of injection and the start of combustion. Fuels of high ignition quality are characterized by short ignition delay and vice versa, fuels of poor ignition quality produce long ignition delay.

Diesel 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

Question 26

with reference to indicator diagrams, what is an advantage of having a electronic system over a mechanical one

Answer 26

Electronic system can easily be set up to obtain results even on high-speed engines. rather than the older mechanical ones which results would have to be taken manually.