P M V Subbarao Professor Mechanical Engineering Department Respect True Nature of Substance Cycle Performance Parameters Net Work Transfer onto piston This is work done by working fluid on the piston also called as Indicated Work ID: 777152
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Slide1
20th Century Thermodynamic Modeling of Automotive Prime Mover Cycles
P M V SubbaraoProfessorMechanical Engineering Department
Respect True Nature of Substance…..
Slide2Cycle Performance Parameters
Net Work Transfer onto piston :
This is work done by working fluid on the piston, also called as Indicated Work.
Indicative Performance:
Slide3Specific Fuel Consumption
XSFC – specific fuel consumption (kg/kWh)
.
X must always be specified when reporting these values (i.e., I for indicated)
Fuel consumption of an engine reported in L/h or kg/h because these values ignore engine load. A better measure of fuel consumption is,
Slide4Specific Fuel Consumption Variations
ISFC – indicated specific fuel consumptionBSFC - brake specific fuel consumptionPSFC – PTO specific fuel consumption
DSFC – drawbar specific fuel consumption
Slide5Fuel conversion efficiency (
η
f)
With units
Also called brake thermal efficiency
Major efforts to make it higher
If power = brake power
Brake thermal efficiency
If power = indicated power (net)
Indicated thermal efficiency
Brake power measurement using dynamometer
Indicated power measurement using pressure vs crank angle indicator diagram
Slide6Indicated power
Mechanical indicator
P-
θ
diagram
Indicator diagram
θ
pressure
Firing
Non-firing
Pressure vs crank angle measurement
Motoring curve
Modern method: Electronic indicator
Piezo-electric pressure transducer for pressure, rotating disc with marked magnetic strips for
θ
Indicator drum connected to crank-shaft
Stylus movement by cylinder pressure force against calibrated spring
k*x = P*A, where k = spring constant, x = stylus movement, A = indicator piston area
Slide7Brake power
Engine
Load
Output shaft
Principle of Dynamometer
Brake Power = Load
Various methods to create the load and hence various dynamometers
R
Friction face
If frictional force = FR,Work done in one revolution against frictional force W = 2πR*FR
Torque balance,
F*b = FR*RThen work per revolution
W = 2πF*bWork per unit time (Power) Pb = W*N = 2πF*b*N
N = rev. per second of crankshaft
Classification of Dynamometers:Absorption Dynamometers – Power absorbed and usually dissipated as heat by some means. E.g. Prony brake, Rope brake, Hydraulic, Eddy current dynamometer etc.Transmission Dynamometers – Power is transmitted to an external load after measuring it on some scale. E.g. Torque meters, Electrical generators etc.
Brake and dynamometer are used interchangeably
Slide8Indicative Mean Effective Pressure:
Actual Fuel- Air Ratio :
Stoichiometric Fuel- Air Ratio :
Parameters for Performance Diagnosis
Slide9Use of MEP to calculate power
MEP
Power
For convenience
n = number of power strokes per minute
=
=
r.p.m
./2 for four-stroke
=
r.p.m
. for two-stroke
Power
MEP = P (just pressure)
By knowing typical range of MEP for a category of engines and knowing engine dimensions, rotational speed and type of cycle:
Power can be deduced
BMEP
Naturally aspirated SI engines
Turbocharged SI Engines
Naturally aspirated 4-stroke CI engines
Turbocharged 4-stroke CI engines
At Max. Power
850 – 1050 kPa
1250 – 1700 kPa
700 – 900 kPa
1000 – 1200 kPa
ME
P measures the success of engine designer in producing work for a given displaced volume
Slide10Mechanical efficiency
Brake power =
Defined in terms of gross power
Pumping work is also defined as a loss
Specific fuel consumption (
sfc
)
Lower values are desirable
Since brake power is used, also called brake specific fuel consumption (
bsfc
)
Mass flow rate of fuel to engine (kg/s)
Combustion efficiency
- Heating value of fuel
- Actual heat input to the engine from combustion
Slide11Fuel Air Equivalence Ratio:
For fuels with C, H and O
Air-fuel-ratio (AFR) =
stoichiometric air-fuel ratio
Slide12Selection of Mixture Strength
Air Standard Model
Actual Performance curve
Fuel-air Model
Slide13Selection of Mixture Strength
Air Standard Model
Actual Performance curve
Fuel-air Model
Slide14Selection of Sufficient Air : Minimization of Running Cost
14
Slide15Selection of Sufficient Air : Running Cost
Vs Capital Cost
15
Slide16Optimizing Engine Performance
Engines are most efficient at or near peak load.Efficiency drops with a reduction in torque load.At zero brake torque, all fuel energy is expended in engine friction.
Lower rated engine speeds provide lower BSFC, and at the same time reduce torque reserve – design compromise.
Slide17Outline of A Modern Engine Intake Port
Slide18Enhanced Flow Area due to Twin valves
Slide19Enhancement of Intake Flow Area using Twin Valves