ENGINE DESIGN CONSIDERATIONS


INTRODUCTION:
  • Engine design is generally characterized by the determination of engine performance parameters and its characteristics.
  • Internal combustion engines operate within a certain range of speed. At each speed within a fixed range there is variation of power output. From power developed to the maximum usable value.
  • The ratio of the power developed to the maximum usable power at the same speed is the LOAD.
  • The fuel consumption of the engine during this range of speed varies with the load and speed.
  • The engine performance therefore depends upon the inter-relationship between power developed, speed and fuel consumption.
Evaluation and determination of engine performance
(a)    The following factors must be considered in evaluating the performance of the engine.
                                i.            Maximum power or torque available at each speed within the useful range of speed.
                              ii.            The range of power output at constant speed for stable operation of the engine.
                            iii.            Brake specific fuel consumption at each operating condition.
                            iv.            Reliability and stability of the engine for the given range of operation.
(b)   Engine performance characteristics are determined by the following two methods
                             i.                 By using experimental results obtained from engine tests.
                              ii.              By analytical calculation based on theoretical data.
The performance of an engine is judged from the point of view of following two major factors:
(a)                Engine power
(b)               Engine efficiency
ENGINE POWER:
In general, energy flowing through the engine is expressed in three distinct terms.
(i)                 Indicated power, (ip)
(ii)               Brake power (bp)
(iii)             Friction power (fp)
Indicated Power:
The indicated power is based on indicted net work and is thus a measure of the forces developed within the cylinder.
Indicated power can be determined by:
(i)                  Measurement of forces in the cylinder.
ip = bp + fp…………………………………………………………………………(1)
(ii)       Multiplying the mass flow rate of air through the engine (kg/sec) by the net work obtained from the p-V diagram
    ip =  ma x net work …………………………………………………………..(2)
(iii)     In working with actual engine it is desirable to compute ip from a given Indicated Mean Effective pressure (Pim)
  
Where
      Pim = Indicated men Effective pressure (N/m2)
      L = Length of the stroke (m)
      A = Area of the piston (m2)
      N = Speed in revolutions per minute
      n = Number of power strokes per minute
                  N/2 for a four - stroke engine
                  N for a two - stroke engine
      K = Number of cylinders.
(iv)              Morse test
Indicated Mean Effective Pressure (Pim).
  • Due to continuous variation in pressure in the cylinder it renders it difficult for simple mathematical analysis in the computation of the ip. If an average pressure for one cycle can be used then the computation of ip becomes easier.
  • As the piston moves back and forth between TDC and BDC (Fig. 1) the process lines on the    p-V diagram indicates the successive states of the working fluid through the cycle.
  • The indicated net work of the cycle is represented by the area 1234 enclosed by the process lines for the cycle. If the area of the rectangle ABCD equals area 1234, then the vertical distance between horizontal lines AB and CD represents the Indicated Mean Effective Pressure (imep).
  • It is a mean value expressed in N/m2, which when multiplied by the displacement volume, Vs, gives the same indicated net work as is actually produced with the varying pressure.
  • Net work of the cycle (Wnet) =……………………………………  (4)
                                                ………………………………………………(5)













                        Fig. 1
  • On an actual engine the p-V diagram (called the indicator diagram) is obtained by a mechanical or electrical instrument attached to the cylinder taking into consideration the spring constant.
  • The area enclosed by the actual cycle on the indicator card may be measured by a planimeter.
  • The value of the area measured, when divided by the piston displacement, results in the mean ordinate, or indicated mean effective pressure (Pim).
Brake Power:
  • Brake power is the
§  Power actually delivered by the engine at the output.
§  It is also called
ü  Shaft power or
ü  Delivered power

Measurement of brake power:
Measurement of brake power involves the determination of the torque and angular speed of the engine output shaft. The torque measuring device is called DYNAMOMETER.
Brake power may be computed from the measurement of forces at the crankshaft of the engine.
·         This is done by attaching a power absorption device to the drive-shaft of the engine.
·         This device sets up measurable forces counter-acting the forces delivered by the engine, and the determined value of these measured forces is indicative of the forces being delivered.
·         On the basis of the geometry of a prony brake (Fig 2) a formula for determining brake power of an engine is derived.
§  Assuming the drive-shaft of the engine turns through one revolution, any point on the periphery of the rigidly attached wheel moves through a distance equal to 2πr.
§  During this movement, a friction force ƒ is acting against the wheel. The friction force ƒ is thus acting through the distance 2πr, and produces work.
Thus,
Work done during one revolution = Distance x Force
                       ……………………………………………...(6)
§  The torque rƒ, produced by the drive-shaft is opposed by a turning moment equal to the product of the length of the moment arm R and the force F measured by the scale.
                                    T = rƒ= RF ………………………………………………………..(7)
§  Work during one revolution = 2πRF
§  Power …………………………………………………(8)
Where        N = revolutions per minute of the drive-shaft. Therefore
 ………………………………………………(9)










Fig. 2   Prony brake
  • Dynamometers can be broadly classified into two main types:
(i)     Absorption dynamometers:
These dynamometers measure and absorb up power output of the engine to which they are coupled.
The power absorbed is usually dissipated as heat by some means. Examples of each of dynamometers are prony brake, hydraulic, eddy current dynamometers, fan dynamometer etc.
(ii)   Transmission dynamometer:
In transmission dynamometers the power is transmitted to the load coupled to the engine after it is indicated on some type of scale. These are also called torque meter. 
Brake Mean Effective Pressure (bmep):
  • Indicated Mean Effective Pressure (imep) may be considered to consist of the bmep and fmep, two hypothetical pressures.
§  Brake mean effective pressure is the portion of imep which produces the useful power delivered by the engine.
§  Friction mean effective pressure is that portion of imep which is required to overcome friction losses.
imep = bmep + fmep
§  Since bmep has the same relationship to bp as imep is to ip
 ………………………………………………………. (10)
§  Equation for computing ip when imep is determined from an engine indicator diagram was
                                   
§  The bp and bmep have the same relationship to one another as do ip and imep, bp can be expressed as
                                     ………………………………………………….(11)
        Where      Pmep is the mean effective pressure (N/m2)
§  The mechanical efficiency of engine can be expressed as the ratio of bmep and imep
                                     ………………………………………………….(12)
Friction power:
  • The difference between the indicated power and the brake power of an engine is known as Friction power.
                          fp = ip – bp ……………………………………………………….(13)
  • The internal losses in an engine are essentially of two kinds
§  Pumping losses
§  Friction losses
ü  Pumping losses are losses caused when during both strokes the piston must be moved against to overcome gaseous pressures ( on the underside during inlet and on the upper side during exhaust stroke)
ü  The friction loss is made up of
v  The friction between the piston and cylinder walls
v  Piston rings and cylinder walls
v  Crankshaft and camshaft and their bearings
v  Loss incurred by driving essential accessories such as the water pump, ignition unit etc.
  • Friction power is used for evaluation of indicated power and mechanical efficiency.
  • Following methods are used to find the friction power to estimate the performance of the engine.
(i)     Morse test
(ii)   Willan’s line method
(iii)  Motoring test
(iv) From the measuring of indicated and brake power
(v)   Retardation test
ENGINE EFFICIENCIES 
  • Apart from expressing engine performance in terms of power, it is essential to express in terms of efficiencies.
  • Various engine efficiencies are:
(i)     Thermal efficiency
§  Indicated thermal efficiency
§  Brake thermal efficiency
(ii)   Air-standard efficiency
(iii)  Mechanical efficiency
(iv)  Relative efficiency
(v)   Volumetric efficiency
(vi)  Scavenging efficiency
(vii)                       Charge efficiency
(viii)                     Combustion efficiency
Thermal efficiency:
  • Thermal efficiency of an engine is important, since it determines how efficiently the fuel is being used in the engine.
  • There are two cases to consider. They are
§  The indicated thermal efficiency and
§  The brake thermal efficiency.
Case I             In internal combustion engine, the thermal efficiencies are as follows:
Indicated thermal efficiency = (Energy equivalent of the ip/s) / (Energy supplied by fuel/s
                                        ………………………………………………(12)
            Where  mflow rate = mass flow rate of fuel/s
                         CV = calorific value of fuel
Brake thermal efficiency = (Energy equivalent of the bp/s) / (Energy supplied by fuel/s)
                                     …………………………………………………(13)
Case II            In the steam engines or turbines the efficiencies are as follows
Indicated thermal efficiency = (Energy equivalent of the ip/s) / (Useful energy received/s)
Air-Standard Efficiency:
  • The Air-Standard efficiency is also known as thermodynamic efficiency.
§  It is mainly a function of compression ratio and other parameters.
§  It gives the upper limit of the efficiency obtainable from an engine.
Mechanical efficiency:
  • Mechanical efficiency takes into account the mechanical losses in an engine.
  • Mechanical losses may be further subdivided into the following groups:
(i)     Friction losses as in case of pistons, bearings, gears, valve mechanisms.                With the development in bearing design and materials, improvements in gears ect, these losses are usually limited from 7 to 9 percentage of the indicated output.
(ii)   Power is absorbed by engine auxiliaries such as pump, lubrication oil pump, water circulation pump, radiator, magneto and distributor, electric generator for battery charging, radiator fan ect. These losses may account for 3 to 8 percentage of the indicated output.
(iii)  Ventilating action of the flywheel. This loss is usually below 4% of the indicated output.
(iv) Work of charging the cylinder with fresh charge and discharging the exhaust gasses during the exhaust stroke. In case of two-stroke engines the power absorbed by the scavenging pumps ect. These losses may account for 2 to 6 of the indicated output. In general mechanical efficiency of the engines varies from 65 to 85%.
Relative efficiency:
  • The relative efficiency or the efficiency ratio as is sometimes called is the ratio of the actual efficiency obtained from an engine to the theoretical efficiency of the engine cycle.
  •  Hence
            Relative efficiency = (Actual brake thermal efficiency) / (Air-standard efficiency)
  • Relative efficiency for most engines varies from 75 to 95%.
Volumetric efficiency:
  • Volumetric efficiency is a measure of the success of the charge is induced into the engine.
  • It is a very important parameter, since it indicates the breathing capacity of the engine
  • It is defined as the ratio of the actual mass of air drawn into the engine during a given period of the time to the theoretical mass which should have been drawn in during that same period of time, based upon the total piston displacement of the engine and the temperature and pressure of the surrounding atmosphere.
…………………………………………………………………………………………(14)
            Where n = is the number of intake strokes per minute
                              For a four stroke engine n = N/2
                              For two-stroke engine    n = N
                        N = is the speed of the engine in rev/min
Scavenging efficiency:
  • Scavenging efficiency (in case of two stroke engines) is defined as the ratio of the amount of air or gas-air mixture which remains in the cylinder at the actual beginning of the compression to the products of the total volume and air density at the inlet.
  • Scavenging efficiency for of the two-stroke engines varies from 40 to 95 percent depending upon the type of scavenging provided.
Charging efficiency:
  • The charging efficiency shows how well the piston displacement of a four-stroke engine is utilized.
  • Various factors affecting charge efficiency are:
(i)     The compression ratio
(ii)   The amount of heat picked up during passage of the charge through intake manifold
(iii)  The valve timing of the engine
(iv) The resistance offered to air-fuel charge during its passage through induction charge.
Combustion efficiency:
  • Combustion efficiency is the ratio of the heat liberated to the theoretical heat in the fuel.
  • The amount of liberated heat is less than the theoretical value because of incomplete combustion either due to dissociation or due to lack of available oxygen.
  • Combustion efficiency in a well adjusted engine varies from 92% to 97%
ENGINE PERFORMANCE CHARACTERISTCS
  • Engine performance characteristics are a convenient graphical presentation of an engine performance.
§  They are constructed from data obtained during actual test runs of the engine
§  They are particularly useful in comparing the performance of one engine with that of another.
  • At certain speed within the speed range of an engine, the charge induced per cylinder per cycle will be maximum.
§  At this point the, the maximum force will exert on the piston.
§  The torque (or engine capacity to do work) will also be maximum at this point.
“Thus, there is a particular engine speed at which the charge per cylinder per cycle is a maximum, and at approximately this same speed the torque of the engine will be a maximum.”

VARIABLES AFFECTING PERFORMANCE CHARACTERISTICS
These are
(a)    Combustion rate and Spark Timing:
§  The spark should timed and the combustion rate controlled such that the maximum pressure occurs as close to the beginning of the power stroke as possible.
§  The spark timing and combustion rate should be regulated in such a way that approximately one half of the total pressure rise due to combustion has occurred as the piston reaches TDC on the compression stroke.
(b)   Air-fuel ratio:
§  This ratio must be set to fulfill engine requirements.
ü  Air-fuel ratio should be set as close as possible to the best economy proportions during normal cruising speeds
ü  It should also be set as close as possible to the best power proportions when maximum performance is required.
(c)    Compression ratio:
  • An increase in compression ration increases the thermal efficiency, and is generally advantageous.
§  The compression ratio in most SI engines is limited by knock
§  Increasing compression ratio increases the friction of the engine parts particularly between piston rings and the cylinder walls.
(d)   Engine speed:
·         At low speeds, a greater length of time is available for heat transfer to the cylinder walls and therefore a greater proportion of heat loss occurs.
·         Up to a certain point, higher speeds produce greater air consumption and therefore greater ip
§  However higher speeds are accompanied by rapidly increasing ƒp and by greater inertia in the moving parts.
(e)    Mass of induced charge:
·         The greater the mass of the charge inducted, the higher the power produced
·         It is desirable to induct a charge to a maximum possible density giving the highest volumetric efficiency.
(f)    Heat losses:
·         It is evident that a large amount of the available energy is lost unused i.e. heat losses.
·         Any method which can be employed to prevent the excessive heat loss and cause this energy to leave the engine in a usable form will tend to increase engine performance.
METHODS OF IMPROVING ENGINE PERFORMANCE
Following are methods through which engine performance may be improved:
(i)     Energy supply may be increased by increasing the mass of charge entering the combustion chamber.
§  This can be accomplished by introducing superchargers (supercharging). Increasing piston displacement. This method is limited by engine weight.
§  Improvement in volumetric efficiency would increase mass of charge.
§  Higher engine speeds may be utilized, but this increases friction losses.
(ii)   The use of higher compression ratios would increase the efficiency of the engine
(iii)  Recycling of exhaust gases to be used for driving turbochargers.











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