On the basis of the actual data available for the engine working, we can find the ideal cycle work and its efficiency. The actual work produced or developed in the engine cylinder will be less than the ideal work calculated from the ideal cycle.
The amount of actual work obtained depends upon where it is measured. In Fig. 25.1, for example- the work that is done in the cylinder can be measured with the use of a device or instrument called an indicator. This work, called the indicated work, is equal to the work of the fluid, which is the work obtained by the use of thermodynamic properties of the fluid. This indicated work, represented by W1, is relevant only for a reciprocating engine.
We also have means of measuring the work on the shaft. This work is called shaft work or brake work and is represented by WB. Since the shaft drives further machines, the work can similarly be measured further along as desired. In electric power generation, the shaft drives a generator, the output of the generator—being electricity, is called overall work or combined work represented by WK. This work is easily measured with electrical instruments.
Indicator and Indicator Card of IC Engine:
Indicators used in steam engine or internal combustion engines shown in Figs. 25.2. The different parts are shown on the diagram. Indicator is mounted on the cylinder.
Steam or fluid (air and gases) enters through the connection to the cylinder and acts on a small piston, whose motion is opposed by the indicator spring. The greater the pressure from the cylinder, the more the piston moves. The stylus is connected to the piston of the indicator. The stylus is actuated through an appropriate straight line linkage and the indicator card is traced. The card is attached to the drum, which moves in phase (the motion is reduced in magnitude) with the engine piston. No indicator card is shown on the drum.
A widely useful means of studying the performance of reciprocating engines is the indicator card or indicator diagram. It is the diagram giving pressure volume variation during the cycle.
Indicator shown in Figs. 25.2 is used for slow-speed engines. Other types of indicators that eliminate the piston in Figs. 25.2 and its inertia are used for high speed engines. For a reciprocating engine, an indicator card is shown in Fig. 25.4.
The area of the indicator card or diagram is found by using an instrument known as the planimeter.
Although the indicator diagram is a closed pressure volume diagram, it is not a thermodynamic cycle. The indicator diagram shows the details of what is necessary for the work process of a cycle.
The events in a power cycle are:
(i) Inlet valve opens at the point of admission (A) and fluid flows in until the end of stroke, where the inlet valve closes.
(ii) Compression of the fluid takes place at B and pressure rises and volume decreases.
(iii) Heat is added at C. Pressure increases, temperature increases.
(iv) Expansion of the products takes place at D. DE is expansion.
(v) At E—the point of release where the exhaust valve opens and the fluid (gases) begins to flow out of the cylinder.
Since the indicator card gives a record of the pressure on the piston and the volume behind the piston, areas enclosed within the indicator cards or diagrams represent work, known as indicated work W1. This work includes the effects of getting the working substance into and out of the cylinder.
A simple planimeter used to determine the area of the indicator diagram is shown in Fig. 25.5.
Point F is fixed. Point P is moved around the boundaries of the area to be measured. Wheel D turns with the movement of P, and after the complete outline has been traced, the magnitude of the area may be read from the dial G and the vernier E.
Mean Effective Pressure (Pmi), Indicated Power (IP) of IC Engine:
The indicator card is taken by an indicator. Figure 25.2 shows that it is fitted with a spring of a certain scale or strength. If the strength of the spring is given as 10 bar, in case of an indicator, we mean that the vertical movement of 1 cm of the stylus indicates a pressure of 10 bar. Therefore the actual heights of the indicator card multiplied by the scale of the indicator spring will be the actual pressure in bar or N/m2.
By definition, mean effective pressure is that constant pressure, when acting on a piston for the complete stroke, produces the work of the cycle. From the indicated diagram, we get the indicated work W1. We therefore can write-
W1 = Actual area of indicator diagram
= Stroke volume x Mean effective pressure.
To get indicated mean effective pressure MEP or pmi, find the area of the indicator card in square centimeter (cm2), divide this area by the length of the card in cm arid obtain the average height of card in (cm). This average height multiplied by the scale of the indicator spring is the average pressure called the indicated MEP (pmi).
∴ Indicated Mean Effective Pressure pmi, is given by-
The actual power developed inside the cylinder of the engine can be found with the help of indicator diagram and is known as Indicated Power (IP).
Once the indicated mean effective pressure is obtained, we can determine the indicated power (IP) of the engine, as given below-
Pmi = Indicated mean effective pressure (bar)
d = diameter of the cylinder (m)
L = Stroke of the piston (m)
N = Speed of the engine (r.p.m.)
A = Area of cylinder (m2)
The engine size is given by two numbers, say 6 x 8 cm, where the first number is always the bore or diameter and the second one the stroke; 6 cm diameter, 8 cm stroke.
Brake Power (BP) of IC Engine:
The portion of the power developed inside the cylinder is lost in friction (crankshaft and bearings, gears, valve mechanisms and other moving parts. Some of the power developed is used for running the fuel pumps water circulating pumps, lubricating oil-pumps, governors etc. The net power available at the crankshaft for doing useful work is known as shaft power or generally called Brake Power (BP).
The name brake—work or brake power came about because in the first determinations the power output was dissipated in the friction of a brake. Brakes are-still used for this purpose over suitable ranges of power and speed.
Brake Mean Effective Pressure – BMEP of IC Engine:
It is a fictitious but very useful concept. If we know or determine mean effective pressure from the indicator diagram, we can calculate or determine the indicated power IP from the engine cylinder dimensions and speed of the engine.
If we represent brake work per cycle on P-V diagram, Fig. 25.11, by a rectangle whose base is the same as that for indicated diagram, then the ordinate or vertical side of this rectangle will be called as brake mean effective pressure BMEP or pmb.
Friction Power – FP of IC Engine:
To allow for energy losses due to friction, engineers generally use efficiency numbers. If the frictional loss is caused by the rubbing together of two solid parts, with or without lubricant, what happens is that work is expended to excite the molecules in the vicinity of the surfaces being rubbed, raising their temperature.
This is to say that work is converted into internal energy of the parts. Thus the frictional energy eventually appears as heat—heat dissipated into the atmosphere, which is such an immense reservoir that it does not notice the difference except locally.
If the frictional loss is that caused by fluid friction, the substance has more internal energy at the end of the process that it would have had in the absence of friction. If the substance also gets hotter than its environment, at least a part of the frictional energy eventually leaves as heat to the surroundings. In all events, frictional energy, in effect, is eventually spilled into the surroundings, and always at the expense of work. Let Ef represent the friction losses in system, then we get,
WI = WB + Ef
Ws = shaft work
Ef = frictional loss in the bearing etc.
If Q = 0, the energy equation is
h1 + K1 = h2+K2+Ws + ef
The mechanical efficiency hmech is a number that tells us the mechanical losses in the machine. For the reciprocating engine of any type that delivers work.
Remember that engine efficiency is a dimensionless number or ratio. The difference [(IP) – (BP)] or [ W1 – WB] represents the loss due to mechanical friction of the moving parts of the engine. This loss expressed in Power, is called the Friction Power written as FP.
∴ FP = (1 – ηmech) IP
Mechanical efficiency is not a fixed number characteristic of machine. It depends on the operating conditions, especially output, speed and lubrication.
Air Standard Efficiency and Relative Efficiency of IC Engine:
(i) Air Standard Efficiency:
Air standard efficiency is the efficiency of an engine working on standard, ideal cycle. Air standard efficiency of an engine working on Otto cycle is given by-
(ii) Relative Efficiency:
Relative efficiency is defined as the ratio of thermal efficiency to the air standard efficiency. For this, thermal efficiency may be either brake thermal efficiency and indicated thermal efficiency. Accordingly, relative efficiency will be based on either BP or IP.
Volumetric Efficiency of IC Engine:
Other factors remaining the same, the power obtained from an engine that draws in air and fuel depends upon the mass of combustible mixture drawn into the cylinders—given a mixture with the correct air fuel ratio and anything that reduces the mass of fuel entering the engine reduces the power output.
(1) The suction pressure is less than atmospheric; therefore the mass of gas (m = PV/RT) is less than if atmospheric pressure were maintained.
(2) The internal surfaces and passages of the engine are relatively hot, so that the mixture is heated as it passes into the cylinder. According to Charle’s law, the increase in temperature further reduces the mass of mixture that the given displacement can contain.
(3) The unpuged gases in the clearance space or volume of the real engine are at a pressure above atmospheric at the end of exhaust stroke and must expand during the suction stroke to the intake pressure before a new charge begins to enter.
(4) The pressure of the atmosphere decreases with altitude so that the mass of mixture drawn in at high altitudes is less than at sea level.
The mass of charge brought into the cylinder is sometimes defined in terms of volumetric efficiency. Thus, volumetric efficiency of an IC Engine is defined as the ratio of mass of air drawn into the engine to the mass of air in displacement or stroke volume at inlet pressure and temperature conditions.
Here standard conditions are the pressure and temperature at intake, say, in the test room. In a particular engine, volumetric efficiency is affected by the speed. As the speed increases, the volumetric efficiency decreases because of the greater throttling effect at higher speeds.
The value of the volumetric efficiency can be made greater than unity by the use of a supercharger.
Heat Balance of IC Engine:
It is clear or evident of the second law of thermodynamics that all the heat released in the engine cylinders is not converted into useful work and an appreciable part of it is lost in different forms e.g. in exhaust gases, cooling water etc.
The power output and the efficiency of an engine can be estimated and improved upon only with a thorough understanding of the influence of various engine parameters on the losses.
The losses are chiefly of three types:
(i) Heat Losses,
(ii) Friction Losses and
(iii) Pumping losses.
The sum of friction and pumping losses is commonly known as mechanical losses.
Engine power output = Heat input x Thermal Indicated efficiency x Mechanical efficiency
A study of the heat distribution in a given engine will give sufficient indication as to how efficiently the engine is working and the general distribution of heat in an IC engine is shown below.
To draw up the heat balance sheet, the engine will be run at constant speed and load and the following quantities must be measured for the period of test:
1. Fuel Consumption
2. Indicated Power IP
3. Brake Power
4. Quantity of water circulated for cooling
5. The temperatures of the cooling water before entering and after leaving the engine cylinder jacket.
6. Quantity of exhaust gases and its temperature.
Generally the heat balance sheet will be on minute basis and percentage basis.
Calculations for the items that are to be included in the heat balance sheet are given below:
Generally, for the good performance, the various percentages given above are found as:
(i) Heat equivalent to BP……. 25 – 30%
(ii) Heat equivalent of FP……. 5-8%
(iii) Heat to cooling water……. 30%
(iv) Heat in exhaust gases……. 30%
(v) Heat unaccounted for……. 8 – 10%
Generally we are using exhaust gas calorimeter for finding the product (mg x Cpg). Otherwise we should know the air to fuel ratio so that mass of the gases will be calculated. In that case, the specific heat will be assumed as 1.1 kJ/ kg.K.
Exhaust gas calorimeter is a simple heat exchanger through which water is circulated and temperatures of water at inlet and outlet are recorded. Exhaust gases are passed through this calorimeter and again temperatures at inlet and outlet are recorded Fig. 25.13.
Quantities to be Measured While Testing an Engine:
While performing a test on an engine, it is required to measure few quantities in addition to quantities given or indicated by the indicating instruments.
The quantities that are to be measured are as follows:
1. Air Flow Measurement:
The satisfactory measurement of the rate of flow of air is more difficult. It is essentially the measurement of the rate of flow of a compressible fluid, complicated by the fact that the flow is pulsating due to cyclic nature of the engine. The usual method of damping out the pulsation is to fit an air box of suitable volume to the engine with an orifice in the side of the box remote from the engine. The pressure difference across the orifice is measured by means of a suitable manometer.
The capacity of air box is 500 to 600 times the swept volume of the engine. The pressure difference should be limited to about 150 mm to make the compressibility effect of air negligible.
d = Diameter of orifice in mm
A = Area of orifice in mm2
Cd = Coefficient of discharge
h = Pressure difference in mm of water from U-tube manometer
H = Head causing flow of air in mm of air
C = Air velocity in m/sec
pd = Density of air under atmospheric condition in kg/m3
w = Density of water in kg/m3
∴ Head causing the air flow = H
2. Fuel Consumption:
Knowledge of the fuel consumed by an engine and time it takes to consume this fuel is essential when assessing the qualities of the engine. For petrol and diesel engines, the fuel is run through a special measuring device. This can take the form of a reservoir of fuel of known quality and the time for the engine to consume this measured quantity of fuel. Several means are available for measurement of fuel consumption and one simple arrangement.
The measuring vessel consists of two bulbs of known capacity in series, the capacities being measured between marks on the connecting capillary tubes. The fuel level falls quickly past the marks on the small box capillary tube. The fuel from the tank or measuring vessel passes through a three way valve to the engine. Provision must be made to allow the fuel to fill the measuring bulbs upto the level in the tank. The time is taken for the consumption of a known volume of fuel and thus the rate of fuel consumption can be determined.
The other arrangement to measure the volume of fuel supplied to the engine.
There are other types of instruments available called flow meters which give the rate of flow directly and the calibration can be made in litres per hour or kg/ hour as required. The volume flow can be converted in mass flow by multiplying fuel density.
In many automotive laboratories and for air craft testing, flow meters are used. As the flow increases through the meter, the float rises and the area between the float and the tapered graduated tube proportionately increases. Since flow rate and area of flow are directly related to each other, and advantage of this type of flowmeter is that the graduations are linear (calibrated in kg per hour) and the instrument can be used for a wide range of flows with good accuracy.
The flow-meter has the advantage of indicating the fuel consumption at any instant. But when, as is most usual, the average or total fuel consumption is desired, it is necessary to obtain an average reading made during the test.
3. Engine Speed:
The approximate speed of an engine is found by using a tachometer. However, such an instrument, even if highly accurate, given only- instantaneous speed and speed variations. For test purposes, the average speed of the engine is determined by using a positive driven counter, with the total number of revolutions of the engine counted during a time period of test.
The average speed of the engine is found by dividing the total number of revolutions shown on the counter by the total time the counter was engaged. Pulling the lever a starts the stop watch b and closes the circuit containing the counter c. The instantaneous speed is indicated by a voltmeter d, which is generated to read rpm.
4. Water for Jacket and Exhaust Gas Calorimeter:
As in case of fuel quantity measurement, for water measurement nowadays, rotameter is the best method that can be utilised. Very rough or approximate method to measure the quantity of water used for engine jacket cooling and also for exhaust gas calorimeter, is to collect the water for the specific time in bucket and measure with either spring balance or measuring cylinder.