MARINE PROPULSION DIESEL ENGINE

MES INDICATOR DIAGRAMS - Continued Compression diagram (fig. below) is taken in a similar manner to the power card but with the fuel shut off from the cylinder . The height of this curve shows maximum compression pressure. BDC TDC BDC stroke based diagram TDC Compression Diagram BDC

MES INDICATOR DIAGRAMS - Continued Draw card or out of phase diagram (fig. below) is taken in a similar manner to the power card, with fuel pump engaged but with the indicator drum 90° out of phase with piston stroke. This illustrated more clearly the pressure changes during fuel combustion. Fuel timing or injector faults may be detected from its shape. BDC TDC Crank angle BDC based diagram Crank Angle -----> Out of phase Diagram

MES INDICATOR DIAGRAMS - Continued Light or Weak spring diagram (fig. below) is again similar to the power card and in phase with the engine, but taken with a light compression spring fitted to the indicator showing pressure changes during the exhaust and scavenge to an enlarged scale. It can be used to detect faults in these operations. BDC stroke based diagram Light Spring Diagram

IRREGULARITIES IN DIAGRAMS MES Some irregularities in the shape of an indicator diagram illustrate incorrect engine operation. Indicator diagrams affected by some of the more common faults are illustrated. FUEL IGNITION TIMING Provided the fuel and engine temperatures are correct and the injector is operating normally , the time between injection of fuel and ignition (the ignition lag) is almost constant. Consequently ignition timing faults signify a corresponding timing fault in the injection. They will affect the shape of the power diagram, but are far more easily detected on an out of phase diagram. EARLY IGNITION This will cause an abnormally high peak pressure in the cylinder at about the top of the piston stroke (figure below). A heavy shock load will be transmitted to the running gear and bearings with a corresponding knocking sound. Although thermal efficiency is high and exhaust temperature reduced, the shock load and consequent vibrations may cause damage. Causes of early ignition may be incorrect fuel pump timing, broken or wrongly set injector springs, incorrect fuel condition, or overheating of parts within the cylinder.

MES IRREGULARITIES IN DIAGRAMS- CONTINUED LATE IGNITION It can be seen from figure below that this cause a low peak pressure which occurs well after top centre of the piston. Power is lost since the fuel is not burned correctly to transmit power at the most effective part of the stroke. Combustion may continue during the expansion stroke and may be incomplete, giving loss in energy produced, high exhaust temperature and smoke. Late ignition may be due to excessive injector spring setting, poor atomisation, high viscosity or poor quality fuel, fuel pump leaking or incorrectly timed, low compression , insufficient supply of combustion air, or undercooling of parts within the cyclinder.

MES IRREGULARITIES IN DIAGRAMS- CONTINUED Diagram showing early ignition Diagram showing late ignition

MES IRREGULARITIES IN DIAGRAMS- CONTINUED Diagram showing afterburning Diagram showing a leaking injector

MES IRREGULARITIES IN DIAGRAMS- CONTINUED Diagram showing a chocked injector Diagram showing low compression

MES IRREGULARITIES IN DIAGRAMS- CONTINUED Light spring diagram showing early Light spring diagram showing or late exhaust valve opening choked exhaust

MES PEAK PRESSURE INDICATOR This instrument will measure and record the maximum pressure within the cylinder. It does not identify the position within the cycle at which this occurs. The ones shown in figure below is similar in principle to the engine indicator but uses a beam type spring which is less susceptible to vibration. The indicator is attached to the indicator cock; when the cock is opened , pressure raises the indicator piston which records maximum beam deflection on a dial pressure scale . If fuel is shut off from that cylinder, the maximum compression pressure can be recorded and the ignition jump calculated.

PEAK PRESSURE INDICATOR

Combustion Monitoring Modern measuring instruments and recording equipments make it possible to monitor all necessary pressure, speeds, temperatures and even wear rates within an engine. By connecting a pressure sensing transducer (electronic indicator) to the indicator cock it is possible to record pressure in the cylinder continuously. Pressure can be plotted on an oscilloscope trace to a horizontal scale of time or corresponding crank angle (Figure below). A printout of this diagram appears comparable to an accurate, constant base scale out-of-phase diagram.

Combustion Monitoring COMBUSTION MONITORING DIAGRAM Combustion monitoring may be used to maintain efficient conditions when fuel of different quality is used. Model diagrams should be made for the engine during shop testing and initial trials in the ship; these can then be compared to subsequent working traces.

LP 2 (W3) Engine Performances Measurements INDICATED POWER Power is the rate of doing work. In linear measure it is the mean force acting on a piston multiplied by the distance it moves in a given time. The force here is the mean pressure acting on the piston. This is obtained by averaging the difference in pressure in the cylinder between corresponding points during the compression and expansion strokes. It can be derived by measuring the area of an indicator diagram and dividing it by its length. This gives naturally the indicated mean effective pressure (imep), also known as mean indicated pressure (mip). The force, hence the power obtained by this method is called indicated power ..\\..\\..\\..\\..\\July 2008\\Intro to Source: Pounder’s Marine Diesel Engines and Gas Turbines ME\\Presentation Eighth edition Materials\\For Pedagogy presentation\\Presentation2. ppt

INDICATED POWER- continued To obtain the imep or mip: refer to indicator diagram below- Typical indicator diagram (stroke based) of four-stroke Diesel Engine

INDICATED POWER- continued The area of the diagram is a measure of work done by the working fluid and can be determined by either the mid-ordinate method or by the use of a planimeter. Assume that the area of the indicator diagram = Ai mm² , and the length of the diagram = Li mm. Then, the mean height of the diagram = Ai mm Li This mean height is a measure of the mean effective pressure acting on the piston throughout its stroke and Pm = s × Ai Li Where s = stiffness of the indicator spring (N/m2)/mm Pm = indicates mean effective pressure N/m2

INDICATED POWER- continued Hence, mean force acting on the piston = Pm × A where A is the cross- sectional area of the piston ( m²). If L = length of the piston stroke (m), then Work done per cylinder = Pm × A × L If the number of working strokes per second is e, then Work done per second = Pm × A × L × e Indicated power (i.p.) per cylinder, then i.p. = Pm × A × L × e where i.p. = indicated power of engine in W.

INDICATED POWER- continued Four-stroke engine produces one power stroke (working stroke) in every two crankshaft revolutions, whilst a two-stroke engine produces one power stroke (working stroke) in every crankshaft revolution. Hence, if N = speed of rotation of crankshaft rev/min then e= 1 × N for a four-stroke engine 2 60 e=N for a two-stroke engine 60

INDICATED POWER- continued Indicated power (i.p.) per cylinder, then i.p. = Pm × A × L × N for a four-stroke engine 2 x 60 i.p. = Pm × A × L × N for a two-stroke engine 60 If the engine has C cylinders , then the total power i.p. = Pm × L × A × N × C for a four-stroke engine 2 x 60 i.p. = Pm × L × A × N × C for a two-stroke engine 60 where i.p. = indicated power of engine in W. For SI system, in which power is measured in kilowatts (kW), must devide further by1000.

LP 2 (W4) SHAFT POWER or BRAKE POWER The power measured at the output shaft of the engine is known as the shaft power, or sometimes as the brake power of the engine. The indicated power developed by the engine is the power available at the piston . This mechanical power, in the form of linear motion of the piston , is transmitted through the connecting rod and the crankshaft, to be transformed into rotary power at the output shaft. During this process some of the power is used to overcome the frictional resistance between the moving parts. Hence the power available at the output shaft is less than the indicated power.

SHAFT POWER- continued The shaft power is measured using a brake or dynamometer which provides a resistance to engine torque by opposing the rotation of the shaft. All dynamometers basically measure the torque which is available at the engine shaft, and by measuring the speed of rotation the shaft power can be determined as follows: T = engine torque available at the output shaft Nm rev/min N = engine speed 22 Work done per revolution = T × 2 × 7 (radians) Nm (rotational work) (angular distance) Work done per second = T × 2 × 22 (radians) × N Nm 1 min =W 7 60 min sec Hence s.p. = T × 2 × 22 × N 7 60 where s.p. = shaft power of engine in W. For SI system, in which power is measured in kilowatts (kW), must devide further by1000.

SHAFT POWER- continued or s.p. = Tω where s.p. = shaft power (W) T = brake torque (Nm) ω = angular speed of the shaft (rad/s) = 2 x 22 x N 7 60 where N = rotational shaft speed (rpm) or s.p. (W) = T (Nm ) x angular speed of the shaft (rad/s) = T x 2 x 22 x N 7 60

TORQUE Several different types of dynamometer are used in engine test work and the choice depends upon the speed and power of the engine. The following are some of them: 1. Rope brake (absorption dynamometer) 2. Prony brake (absorption dynamometer) 3. Water brake (absorption dynamometer) 4. Electric brake (absorption dynamometer) 5. Transmission dynamometer, i.e. torsionmeter. Absorption type dynamometer is used for torque measurement at the builder’s works or in shop test. It is measured in the ship by a torsionmeter. The brake power is normally measured with a high accuracy (98 per cent or so) by coupling the engine to a dynamometer at the builder’s works. If it is measured in the ship by torsionmeter it is difficult to match this accuracy.

TORQUE- continued Rope brake - Dynamometer T= (W-S)R Nm T= torque is also known as the twisting moment about an axis. It is equal to the product of the tangential force and the radius at which the force acts.

Strain gauge torsionmeter With this device four strain gauges are mounted onto the shaft, as shown in Figure 15.18. The twisting of the shaft as a result of an applied torque results in a change in resistance of the strain gauge system or bridge. Brushes and sliprings are used to take off the electrical connections and complete the circuit, as shown. More recently use has been made of the resistance change converted to a frequency change. A frequency converter attached to the shaft is used for this purpose: this frequency signal is then transmitted without contact to a digital frequency receiver. When a torque is applied to the shaft, readings of strain and hence torque can be made.

MECHANICAL EFFICIENCY The difference between the indicated power and the shaft power is the result of energy lost in overcoming the resistances in the mechanism of the engine. The resistances include piston friction and bearing friction. This energy loss is known as friction power loss. It is therefore possible to define the mechanical efficiency of an engine as Mechanical efficiency = output at crankshaft output at cylinders shaft power (brake) Mechanical efficiency = indicated power = bhp = kW (brake) ihp kW (indicated The mechanical efficiency of a reciprocating internal combustion engine is typically between 80% and 90%.

MEAN PISTON SPEED As the piston travels from one end of the cylinder to the other , the speed of the piston varies continuously from zero at TDC and BDC to a maximum near the middle of the stroke. The mean piston speed can be determined by dividing the distance moved by the piston in one complete crankshaft revolution by the time taken to travel that distance. Mean piston speed = 2 × stroke time for one crankshaft revolution Mean piston speed = 2 × L × N meters/sec 60 Where L = stroke in meters N = revolutions per minute

COMPRESSION RATIO r = compression ratio r = max Vol/ min Vol , max Vol = Piston Disp + Clearance Vol, min Vol = Clearance Vol r = (PD + CV) / CV, or r = PD / CV + 1

VOLUMETRIC EFFICIENCY The volumetric efficiency is defined as the ratio of the actual mass of air drawn in during the suction stroke to the mass of air which would fill the swept volume of the cylinder at atmospheric pressure and temperature or ηv = actual mass of air mass of air that would fill the swept volume at atmospheric condition (theoretical) Volumetric efficiency is an important parameter since it determine how much power the engine is capable of developing . It is important to induce as great mass of air as possible into the engine cylinder in order to burn more fuel and so maximise the power which the engine can develop.

SPECIFIC FUEL CONSUMPTION The specific fuel consumption is the fuel flow rate necessary to produce unit power from an engine . This quantity enables comparisons to be made between engines. An engine with a lower specific fuel consumption is a more economic engine to operate. Specific fuel consumption (s.f.c) = Fuel mass flow rate Shaft power = mf s.p. When evaluating this quantity it is common practise to use the fuel mass flow rate in kg/h and to state the s.f.c in kg/kWh.

THERMAL EFFICIENCY The thermal efficiency of an engine is a measure of how effectively the supply of heat energy is utilized . It may be defined as η = heat equivalent of work energy heat energy supplied The heat energy supplied depends on the calorific value of the fuel (C.V) which is the amount of the heat energy liberated when unit mass of the fuel is burned at constant volume. Thus the heat energy supplied is mf × C.V, where mf = fuel mass flow rate (mass of fuel used per hour). (Sometimes the calorific value of the fuel is known as the heating value of the fuel). The work energy may be either the indicated power or the shaft (brake) power and this result in two thermal efficiencies for an engine : Indicated thermal efficiency ηi = indicated power mf × C.V. Shaft (brake) thermal efficiency ηs = shaft power (brake) mf × C.V.

THERMAL EFFICIENCY From section 1 Thermal efficiency (Thη) is the overall measure of performance. In absolute terms it is equal to: heat converted into useful work total heat supplied Heat converted into work per hour = N kWh = 3600 N kJ where N = the power output in kW ……….. either indicated or shaft power Heat supplied = M x K (kJ/hr) where M = mass of fuel used per hour in kg ……...... mf , fuel mass flow rate and K = calorific value of the fuel in kJ/kg ………... C.V., caloric value therefore Thη = 3600N or 3600 N ……… indicated or shaft MxK thermal efficiency mf x C.V.

LP 3 (W4 & W5) Problems & Calculations Refer to Problems & calculation File Problems & calculations.ppt

















MES Section 2- Engine Performance and Testing LP 1 (W3) FAA INDICATOR DIAGRAMS & ENGINE PERFORMANCE An indicator diagaram is a pressure/volume graph taken from the pressures in the cylinder of working engine. It is plotted as a continous line showing all pressure changes during one complete engine cycle. Indicator diagrams can be a great assistance in monitoring the operating conditions and the performance of an engine. ..\\..\\..\\..\\..\\July 2008\\Intro to ME\\Presentation Materials\\For Pedagogy presentation\\Presentation2. ppt Typical indicator diagram (stroke based) of two-stroke Diesel Engine

MES INDICATOR DIAGRAMS - Continued In many modern engines sophisticated measuring devices such as an electronic engine indicator may be fitted to record cylinder pressures as an oscilloscope trace or printout. For some slow- speed engines it is possible to obtain indicator diagrams from each cylinder by using mechanical engine indicator.

THERMOD Mechanical Engine Indicator

MES Mechanical Engine Indicator



Electronic Engine Indicator

MES INDICATOR DIAGRAMS - Continued Four types of indicator diagram can be obtained. These are illustrated for a two-stroke engine. Power card is taken with the indicator drum rotation in phase with piston movement ( fig. below). The area within this diagram represents the work done during the cycle to scale. This may be used to calculate the power produced or the mean indicator pressure (MIP) for the cylinder. TDC stroke based BDC diagram Power Diagram

MES INDICATOR DIAGRAMS - Continued Compression diagram (fig. below) is taken in a similar manner to the power card but with the fuel shut off from the cylinder . The height of this curve shows maximum compression pressure. BDC TDC BDC stroke based diagram TDC Compression Diagram BDC

MES INDICATOR DIAGRAMS - Continued Draw card or out of phase diagram (fig. below) is taken in a similar manner to the power card, with fuel pump engaged but with the indicator drum 90° out of phase with piston stroke. This illustrated more clearly the pressure changes during fuel combustion. Fuel timing or injector faults may be detected from its shape. BDC TDC Crank angle BDC based diagram Crank Angle -----> Out of phase Diagram