Performance Measures for I.C. EnginesDevelopment & Evaluation

P M V SubbaraoProfessor

Mechanical Engineering Department

Define A Better Engine .….Evaluate Reaction to Optimize

the Actions ….

Cycle Performance Parameters

Net Work Transfer :

)(mvpdWnet

)(120

mvpdN

P strokefourind

This is net work done by working fluid on the piston, also called as Indicated Work.

cyclelowercycleuppernet mvpdmvpdW

)()(

Power generated by gas on piston (Indicated Power

Indicated Thermodynamic Performance

Indicative Performance:

CVm

mvpdN

fstrokefourind

)(120

James Watts Solution

Brake Power Vs Indicated Power

• Work delivered by the crankshaft is less than indicated work due to mechanical friction and parasitic loads of the engine.

• Parasitic loads include the oil pump, supercharger, air conditioner compressor, alternator, etc.

• Actual work available at the crankshaft is called brake work

lossindbrake PPP

I.C. Engine Test Rig

Air Tank

DYNAMOMETERS

• Dynamometers are used to measure brake torque and power over the engine operating ranges of speed and load.

• They do this by using various methods to absorb the energy output of the engine, all of which eventually ends up as heat.

• Some dynamometers absorb energy in a mechanical friction brake (prony brake).

• These are the simplest dynamometers but are not as flexible and accurate as others at higher energy levels.

• Fluid or hydraulic dynamometers absorb engine energy in water or oil pumped through orifices or dissipated with viscous losses in a rotor-stator combination.

• Large amounts of energy can be absorbed in this manner, making this an attractive type of dynamometer for the largest of engines.

Prony Brake Dynamometer

Hydraulic Dynamometer

Eddy Current Dynamometers

• Eddy current dynamometers use a disk, driven by the engine being tested, rotating in a magnetic field of controlled strength.

• The rotating disk acts as an electrical conductor cutting the lines of magnetic flux and producing eddy currents in the disk.

• With no external circuit, the energy from the induced currents is absorbed in the disk.

Electric Dynamometer

• One of the best types of dynamometers is the electric dynamometer, which absorbs energy with electrical output from a connected generator.

• In addition to having an accurate way of measuring the energy absorbed, the load is easily varied by changing the amount of resistance in the circuit connected to the generator output.

• Many electric dynamometers can also be operated in reverse, with the generator used as a motor to drive (or motor) an unfired engine.

• This allows the engine to be tested for mechanical friction losses and air pumping losses, quantities that are hard to measure on a running fired engine.

Factory Testing of Vehicular Engines

Driving cycle

• A probable plot of vehicle speed right from the start of the engine through its journey over a prescribed time.

• Driving cycle development depends on the specific objective to be achieved

• It varies from city to city as well as for vehicle type

• Indian driving cycle which is criteria for emission testing of vehicle needs a thorough review

Industrial (Field) Measures for Performance

X

f

P

mXSFC

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,

Specific Fuel Consumption Variations

• ISFC – indicated specific fuel consumption• BSFC - brake specific fuel consumption• PSFC – PTO specific fuel consumption• DSFC – drawbar specific fuel consumption

Indicative Mean Effective Pressure:minmax vv

pdvIMEP

Actual Fuel- Air Ratio :

actair

fuel

act m

m

A

F

,

Stoichiometric Fuel- Air Ratio :

stoair

fuel

sto m

m

A

F

,

Parameters for Performance Diagnosis

Fuel Air Equivalence Ratio:

sto

act

AF

AF

Selection of Mixture Strength