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Internal Combustion Engine
Assignment No. 2
IMPORTANT ENGINE CHARACTERISTICS
Some basic relationships and the parameters commonly used to characterize engine operation are developed. The factors important to an engine user are:
Engine performance is more precisely defined by:
Maximum rated power: The highest power an engine is allowed to develop for short periods of operation.
Normal rated power: The highest power an engine is allowed to develop in continuous operation.
Rated speed: The crankshaft rotational speed at which rated power is developed.
To evaluate the performance of an engine the following are the most important characteristics.
Thermal efficiency = Indicated energy/total energy supplied by the fuel
For Otto cycle thermal efficiency is 50-54 %
For diesel cycle thermal efficiency is 32-34 %
For dual combustion cycle thermal efficiency is 42%
Mechanical efficiency:
Part of the gross indicated work per cycle or power is used to expel exhaust gases and induct fresh charge. An additional portion is used to overcome the friction of the bearings, pistons, and other mechanical components of the engine, and to drive the engine accessories. All of these power requirements are grouped together and called friction power.
Brake power = Net power available at the out put shaft
= Indicated power – Friction power
Friction power is difficult to determine accurately. One common approach for high-speed engines is to drive or motor the engine with a dynamometer and measure the power which has to be supplied by the dynamometer to overcome all these frictional losses. The engine speed, throttl3e setting, oil and water temperatures, and ambient conditions are kept the same in the motored test as under firing conditions. The major sources of inaccuracy with this method are that gas pressure forces on the piston and rings are lower in the motored test than when the engine is firing and that the oil temperatures on the cylinder wall are also lower under motoring conditions.
Mechanical efficiency = Brake power/Indicated power
It is normally between 85-95 %
Since the friction power includes the power required to pump gas into and out of the engine, mechanical efficiency depends on throttle position as well as engine design and engine speed. Typical values for a modern automotive engine at wide-open or full throttle are 90 percent at speeds below about 30 to 40 rev/s, decreasing to 75 percent at maximum rated speed. As the engine is throttled, mechanical efficiency decreases eventually top zero at idle operation.
Indicated work per cycle:
Pressure data for the gas in the cylinder over the operating cycle of the engine can be used to calculate the work transfer from the gas to the piston. The cylinder pressure and corresponding cylinder volume throughout the engine cycle can be plotted on P-V diagram. The indicated work per cycle is obtained by integrating around the curve to obtain the area enclosed on the diagram.
With two stroke cycles this application is straightforward. With the addition of inlet and exhaust strokes for the four stroke cycle, some ambiguity is introduced as two definitions of indicated output are in common use. These will be defined as:
Gross indicated work per cycle: Work delivered to piston over the compression and expansion strokes only
Net indicated work per cycle: Work delivered to piston over the entire four-stroke cycle
Mean effective pressure:
While torque is a valuable measure of a particular engine’s ability to do work, it depends on engine size. A more useful relative engine performance measure is obtained by dividing the work per cycle by the cylinder volume displaced per cycle. The parameter so obtained has units of force per unit area and is called the mean effective pressure (mep).
Specific fuel consumption:
In engine tests, the fuel consumption is measured as a flow rate. A more useful parameter is the specific fuel consumption. It measure how efficiently an engine is using the fuel supplied to produce work:
SFC = mf / P
Where mf is flow rate and P= power out put
Low values of SFC are obviously desirable. For SI engines typical best values of brake specific fuel consumption are about 270 gm / kW and for CI engines it is about 200 gm / kW
Air/Fuel and Fuel/Air ratio:
In engine testing, both the air mass flow rate and fuel mass flow rate are normally measured. The ratio of these flow rates is useful in defining engine-operating conditions.
The normal operating range for a conventional SI engine using gasoline fuel is 11< A/F <19 and for CI engines with diesel fuel, it is 17< A/F <71
Volumetric efficiency:
The intake system – the carburetor, the throttle plate (in a spark ignition engine), intake manifold, intake valve, intake port – restricts the amount of air which an engine of given displacement can induct. The parameter used to measure the effectiveness of an engine’s induction process is the volumetric efficiency. Volumetric efficiency is only used with four-stroke cycle engine, which have a distinct induction process. It is obtained as the volume flow rate of air into the intake system divided by the rate at which volume is displaced by the piston.
Engine specific weight / volume:
Engine specific weight and bulk volume for a given rated power are important in many applications. Two parameters useful for comparing these attributes from one engine to another are:
Specific weight = engine weight / rated power
Specific weight = engine volume / rated power
Specific emissions:
Levels of emissions of oxides of nitrogen, Carbon monoxide, unburned hydrocarbons, and particulate are important engine operating characteristics.
The concentrations of gaseous emissions in the engine exhaust gases
are usually measured in parts per million or percent by volume. Normalized
indicators of emissions levels are more useful, however, and two of these
are in common use.
ENERGY TRANSFORMATION IN AN IC ENGINE:
In an Internal combustion engine when the fuel is combusted the heat energy released during combustion increases the pressure and temperature of combustion gases
Combustion gases:
N2, O2, CO2, CO, SO2, SO3
N2 takes part in combustion only at high temperature (in CI engines)
The hot gases apply pressure on the piston and move it to produce mechanical
work.
LOSES INCURED DURING THE PROCESS CYCLE OF AN ICE
During this process there are loses in the system.
Thermal Loses:
Frictional Loses:
The friction loses are introduced due to the relative motion in different components of the engine.
Dual Cycle (relationship of cutoff ratio with thermal efficiency)
Modern oil engines although still called diesel engines are more closely derived from an engine invented by Ackroyd-Stuart in 1888. All oil engines today use solid injection of the fuel; the fuel is injected by a spring loaded injector the fuel pump being operated by a cam driven from the engine crankshaft.
The heat is supplied in two parts, the first part at constant volume and the remainder at constant pressure, hence the name dual combustion. In order to fix the thermal efficiency completely three factors are necessary. These are: the compression ratio, rv = v1/v2; the ratio of pressure, k=p3/p2and the ratio of volumes, b = v4/v3
h = 1 – (kbg- 1) / [(k – 1) + g k(b - 1)]rvg-1
When b = 1 this result becomes the constant volume cycle efficiency. For k = 1, this result gives the constant pressure cycle efficiency.
The efficiency of dual combustion cycle depends not only on the compression ratio but also on the relative amounts of heat supplied at constant volume and at constant pressure.
The above equation is too cumbersome to use and the best method to calculating thermal efficiency is to evaluate each temperature round the cycle and then use equation
h = 1 – Q1/Q2
where Q1 = heat supplied
Q2 = heat rejected
GEOMETRICAL PROPERTIES OF RECIPROCATING ENGINES:
The following parameters define the basic geometry of a reciprocating engine
Compression ration = Vd + Vc
Vc
Where Vd is the displaced or swept volume and Vc is the clearance volume.
Ratio of cylinder bore to piston stroke = B / L
Ratio of connecting rod length to crank radius = R = l / a
In addition, the stroke and crank radius are related by
L = 2a
Typical values of these parameters are :rc = 8 to 12 for SI engines and rc = 12 to 24 for CI engines; B/L =.8 to 1.2 for small and medium size engines, decreasing to about .5 for large slow-speed CI engines; R=3 to 4 for small and medium-size engines, increasing to 5 to 9 for large slow speed CI engines.
The cylinder volume V at any crank position q is
V = Vc + p B2/4(l + a – s)
Where s is the distance between the crank axis and the piston pin axis and is given by
S = a Cosq + (l2 –a2 sin2q)1/2
The angle q is crank angle with above definitions can be rearranged
V/Vc = 1 + ½(rc-1)[R + 1 - Cosq - (R2 – sin2q)1/2]
The combustion chamber surface area A at nay crank position q is given by
A = Ach + Ap + p B(l + a –s)
Where Ach is the cylinder head surface area and Ap is the piston crown surface area. For flat-topped pistons, Ap = p B2/r
An important characteristic speed is the mean piston speed S’p:
S’p = 2LN
Where N is the rotational speed of the crankshaft. Mean piston speed is often a more appropriate parameter than crank rotational speed for correlating engine behavior as a function of speed. For example, gas-flow velocities in the intake and the cylinder all scale with Sp. The instantaneous piston velocity Sp is obtained from
Sp = ds/dt
The piston velocity is zero at the beginning of the stroke, reaches a maximum near the middle of the stroke, and decreases to zero at the end of the stroke.
Sp/S’p = p /2 sin q [ 1 + cosq /(R2 – sin2 q )1/2]
Resistance to gas flow into the engine or stresses due to the inertia of the moving parts limit the maximum mean piston speed to within the range 8 to 15 m/s (1500 to 300 ft/min). Automobile engines operate at the higher end of this range; the lower end is typical of large marine diesel engines.
THE AIR STANDARD CYCLE:
That cycles in which the fuel is burned directly in the working fluid are not heat engines in the true meaning of the term. In practical such cycles are used frequently and are called internal combustion cycles. The fuel is burned directly in the working fluid, which is normally air. The main advantage of such power units is that high temperatures of the fluid can be attained, since heat is not transferred through metal walls to the fluid. The fluid in an internal combustion engine may reach a temperature of as high as 2750 C. This is made possible by externally cooking the cylinder by water or air cooling; also, due to the intermittent nature of the cycle, the working fluid reaches its maximum temperature for only as instant during each cycle.
Examples of internal combustion cycles are the open cycle gas turbine unit, the petrol engine, the diesel engine or oil engine, and the gas engine. The open cycle gas turbine unit, although an internal combustion cycle, is nevertheless in a different category to the other internal combustion engines.
In petrol engine a mixture of air and petrol is drawn into the cylinder, compressed by the piston, then ignited by an electric spark. The hot gases expand, pushing the piston back, and are then swept out to exhaust, and the cycle recommences with the induction of a fresh charge of petrol and air. In the diesel or oil engine the oil is sprayed under pressure into the compressed air at the end of the compression stroke, and combustion is spontaneous due to the high temperature of the air after compression. In a gas engine a mixture of gas and air is induced into the cylinder, compressed, and then ignited as in the petrol engine, by an electric spark. To give a basis of comparison for the actual internal-combustion engine the air standard cycle is defined.
In an air standard cycle the working substance is assumed to he air throughout, all processes are assumed to be reversible, and the source of heat supply and the sink for heat rejection are assumed to be external to the air. The cycle can be represented on any diagram of properties, and is usually drawn on the p-v diagram, since this allows a more direct comparison to be made with the actual engine machine cycle which can be obtained form an indicator diagram. It must be stressed that an air standard cycle on a p-v diagram is a true thermodynamic cycle, whereas an indicator diagram taken from an actual engine is a record of pressure variations in the cylinder against piston displacement.
IDEAL CYCLE:
An ideal gas is used as working fluid under all working conditions the properties of the working medium remains constant.
Parameter of Gases:
Pressure
Volume
Temperature
Specific volume
Enthalpy
Entropy
The advantage of the ideal cycle is that maximum limiting value of a theoretical cycle can be obtained.
AIR CYCLE:
In air cycle working medium is taken to be the air
Air behaves like an ideal gas with in the working range of pressure and temperature of a working engine.
Disadvantage is that air is not combustible.
AIR FUEL MIXTURE + RESIDUAL GASES:
The assumption takes the working medium, a mixture of F/A, and residual gases and the analysis is done for a theoretical cycle.
COMPARISON WITH REAL ENGINE CYCLES:
A comparison of a real engine p-v diagram over the compression and expansion strokes with an equivalent fuel air cycle analysis is shown in figure. The real engine and the ruel air cycle have the same geometric compression ratio, fuel chemical composition and equivalent ratio, residual fraction and mixture density before compression. Midway through the compression stroke, the pressure in the fuel-air cycle has been made equal to the real cycle pressure. The compression stroke pressures for the two cycles essentially coincide. Modest differences in pressure during intake and the early part of the compression process result from the pressure drop across the intake valve during the intake process and the closing of the intake valve 40 to 60 degrees after the BC in the real engine. The expansion stroke pressures for the engine fall below the fuel air cycle pressures of the following reasons; heat transfer from the burned gases to the walls: finite time required to burn the charge; exhaust blowdown loss due to opening the exhaust valve before BC; gas flow into crevice regions and leakage past the piston rings; incomplete combustion of the charge.
These differences, in decreasing order of importance, are described below. Together , they contribute to the enclosed area on the p-v diagram for a properly adjusted engine with optimum timing being about 80 percent of the enclosed area of an equivalent fuel air cycle p-v diagram. The indicated fuel conversion or availability conversion efficiency of the actual engine is therefore about 0.8 times the efficiency calculated for the fuel-air cycle. Use is often made of this ratio to estimate the performance of actual engines from fuel-air cycle results.
HEAT TRANSFER
Heat transfer from the unburned mixture to the cylinder walls has a negligible effect on the p-v line for the compression process. Due to heat transfer during combustion, the pressure at the end of combustion in the real cycle will be lower. During expansion, heat transfer will cause the gas pressure in the real cycle to fall below an isentropic expansion line as the volume increases. A decrease in efficiency results from this heat loss.
FINITE COMBUSTION TIME
In an SI engine with spark timing adjusted for optimum efficiency, combustion typically starts 10 to 40 crank angle degrees before TC, is half complete at about 10 degrees after TC, and is essentially complete 30 to 40 degrees after TC. Peak pressure occurs at about 15 degree after TC. In a diesel engine, the burning process starts shortly before TC. The pressure rises rapidly to a peak some 5 to 10 degrees after TC since the initial rate of burning is fast. However, the final stages of burning are much slower, and combustion continues until 40 to 50 degrees after TC. Thus, the peak pressure in the engine is substantially below the fuel air cycle peak pressure value, because combustion continues until well after TC, when the cylinder volume is much greater than the clearance volume. After peak pressure, expansion stroke pressures in the engine are higher less work has been extracted from the cylinder gases.
For spark or fuel injection timing which is retarded from the optimum for maximum efficiency, the peak pressure in the real cycle will be lower, and expansion stroke pressures after the peak pressure will be higher than in the optimum timing cycle.
EXHAUST BLOWDOWN LOSS:
In the real engine operating cycle, the exhaust valve is opened some 60 degrees before BC to reduce the pressure during the first part of the exhaust stroke in four-stroke engines and to allow time for scavenging in two stroke engines. The gas pressure at the end of the expansion stroke is therefore reduced below the isentropic line. A decrease in expansion stroke work transfer results.
CREVICE EFFECTS AND LEAKAGE.
As the cylinder pressure increases, gas flows into crevices such as the regions between the piston, piston rings, and cylinder wall. The crevice regions can comprise a few percent of the clearance volume. This flow reduces the mass in the volume above the piston crown, and this flow is cooled by heat transfer to the crevice walls. In premixed charge engines, some of this gas is unburned and some of it will not burn. Though much of this gas returns to the cylinder later in the expansion, a fraction, from behind and between the piston rings, flows into the crankcase. However, leakage in a well designed and maintained engine is small (usually less than one percent of the charge). All these effects reduce the cylinder pressure during the latter stages of compression, during combustion, and during expansion below the value that would result if crevice and leakage effects were absent.
INCOMPLETE COMBUSTION:
Combustion of the cylinder charge is incomplete; the exhaust gases contain
combustible species. For example, in spark-ignition engines the hydrocarbon
emissions from a warmed up engine (which come largely from the crevice
regions) are 2 to 3 percent of the fuel mass under normal operating conditions;
carbon monoxide and hydrogen in the exhaust contain an additional 1 to
2 percent or more of the fuel energy, even with excess air present. Hence,
the chemical energy of the fuel which is released in the actual engine
is about 5 percent less than the chemical energy of the fuel inducted.
The fuel air cycle pressures after combustion will be higher because complete
combustion is assumed. In diesel engines, the combustion inefficiency is
usually less, about 1 to 2 percent, so this effect is smaller.
THERMAL EFFICIENCY OF DIFFERENT AIR STANDARD CYCLES FOR ICE’S:
DYNAMOMETER
Engine torque is normally measured with a dynamometer. The engine is clamped on a test bead and shaft is connected to the dynamometer rotor. The rotor is coupled electromagnetically, hydraulically, or by mechanical friction to a stator, which is supported in low friction bearings. The stator is balanced with the rotor stationary. The torque exerted on the stator with the rotor turning is measured by balancing the stator with weights, springs, or pneumatic means
T = F b
The power delivered by the engine and absorbed by the dynamometer is the product of torque and angular speed:
P = 2p NT
Where N is the crankshaft rotational speed. In SI units
Note that torque is a measure of an engine’s ability to do work; power is the rate at which work is done.
The value of engine power measured as described above is called brake power. This power is the usable power delivered by the engine to the load.
Types of dynamometer:
TESTING FOR INDICATED LOAD:
(MORSE TEST FOR INDICATED POWER)
Indicated power of cylinder 2 = T – T2
Indicated power of cylinder 3 = T – T3
Indicated power of cylinder 4 = T – T4
Indicated power = 2p NT Watt
ISENTROPIC COMPRESSION PROCESS:
Reversible adiabatic process is called isentropic process. Adiabatic
process is one in which no heat is transferred in or out of the system.
Thus the compression process in which no heat is supplied or extracted
from the system is said to be isentropic compression process.
ISENTROPIC EXPANSION:
The expansion process in which no heat is supplied or extracted from the system is said to be isentropic expansion process.
CONSTANT VOLUME HEAT ADDITION:
The process in which heat is supplied to the system by keeping the volume
constant is called constant volume heat addition.
CONSTANT PRESSURE HEAT ADDITION:
The process in which heat is supplied to the system by keeping the pressure
constant is called constant pressure heat addition.
COMBINED HEAT ADDITION:
In this process the heat is supplied to the system in two steps. First
at constant pressure while in second step heat is added isochorically.
This type of heat addition is employed in Dual combustion cycle.
CONSTANT VOLUME HEAT REJECTION:
In this process heat is rejected at constant volume.