IC Engine - Testing and Performance Study

Suresh Kumar YAssociate Professor

Mechanical Engineering

Introduction: IC Engines: 4 - Stroke and 2 - Stroke engines, SI and CI Engines.

Combustion process in SI and CI Engines.

Abnormal combustion.

Testing and Performance characteristics.

Topics to be covered

Combustion in SI engine

1. Ignition lag or preparation phase2. Flame propagation3. After burning

Uncontrolled combustion (knocking in SI engine):

Stages of combustion

Octane number

Primary reference fuels – iso-octane C8H18 and n-heptane C7H16.

The engine used for this test is CFR – Cooperative fuel research variable compression ratio engine.

Rating of SI engine fuels

Combustion in CI Engine

1. Ignition delay period2. Rapid or uncontrolled combustion3. Controlled combustion4. After burningAbnormal combustion or diesel knock:

Stages of combustion

Cetane Number

Primary reference fuels – Cetane (C16H34 – c.no. 100) and α-methyl naphthalene (C10H7CH3 – c.no. 0)

The engine used for this test is CFR – Cooperative fuel research variable compression ratio engine.

Rating of CI engine fuels

1. Mean effective pressure: The hypothetical pressure which is thought to be acting on the piston through out the power stroke.

Pm = (area of the indicator diagram/length of the indicator diagram)*spring constant = as/l.

2. Indicated Power is the total power developed by the combustion of the fuel in the combustion chamber or it is the total power available inside the engine cylinder.

Work done on the piston by the gas per cycle = Pm*A*L

Engine Performance parameters

Work done per second = Pm*A*L*n/60 (watt) = Indicated Power

where n = no. of power strokes per minute or no. of explosions per min. n = N for 2-stroke enginen = N/2 for 4-storke engine

For a multi-cylinder engine Indicated Power IP = PmLAnk/60000 kW

where k = no. of cylinders.

3. Brake Power: The total power available at the output shaft = 2πNT/60000 kW.where T is the Brake torque applied on the shaft

4. Friction Power: Of the power developed by the engine a part is consumed in overcoming friction in the moving parts some in the process of inducting the air and exhausting the products of combustion from the cylinder(pumping losses). F.P = I.P – B.P

5. Mechanical efficiency: B.P./I.P = B.P./B.P+F.P. For a given engine B.P. can be measured in terms of

mean effective pressure known as brake mean effective pressure(Bmep). The Bmep maybe considered as the hypothetical mep acting on the piston which would give the measured BP if the engine were frictionless.

Bmep is the portion of Imep which produces the useful power delivered by the engine.

Friction mean effective pressure is the portion of Imep which is used to overcome friction losses

I.P. = ImepLAnk/60000 kW B.P. = BmepLAnk/60000 kW Mechanical efficiency = B.P./I.P. = Bmep/Imep. Bmep is very useful in comparing engines.

6. Specific output: = B.P. per unit piston display = B.P./AL = constant*Bmep*rpm.

7. Volumetric efficiency: = mass of air or charge actually induced/ mass of air or charge corresponding to the cylinder volume at ambient conditions = volume of air or charge induced at ambient conditions/stroke volume.For supercharged engine, the volumetric efficiency has no meaning as it is more than 1.

8. Air-Fuel ratio: Ratio of mass of air to mass of the fuel in the air fuel mixture.

9. Specific fuel consumption (SFC): ratio of total fuel consumed in kg/hr to the power developed = SFC = TFC/P kg/kWhrBSFC = TFC/BP kg/kWhrISFC = TFC/IP kg/kWhr

10. Relative efficiency = actual thermal efficiency / air standard efficiency.For Otto cycle the air standard efficiency = 1- For Diesel cycle air standard efficiency is 1- where r = compression ratio and ρ is the cut-off ratio

11. Thermal efficiency = Heat converted to useful mechanical work/chemical energy input.Brake thermal efficiency= B.P.*3600/(mf*C.V)

where mf = TFC kg/hr, C.V is the calorific value of the fuel.Indicated thermal efficiency=I.P*3600/(mf*C.V)

where mf = TFC kg/hr

Mechanical efficiency = Brake thermal efficiency / indicated thermal efficiency

12. Specific weight: Weight of the engine per brake power developed. It plays an important role in the applications such as power plant for aircrafts.

1.Speed: Using mechanical tachometer, electrical tachometer, magnetic pick-up.(pulses are produced for every revolution and are counted by the pulse counter).2. Fuel consumption: The volume flow rate per unit time is measured.3. Air consumption:

Basic measurements

The orifice plate is used to measure the air flow rate. The pressure pulsations into the engine is reduced by fitting an air tank.

The velocity of flow of air across the orifice and the mass flow rate is equal to

3. Viscous air flow meter: This meter uses an element where viscous resistance is the principal source of pressure loss and the kinetic effects are small

4. Brake Power: Dynamometers (absorption dynamometers and transmission dynamometers) are used to measure the output power. A rotor driven by the engine under test is electrically, hydraulically or magnetically coupled to the stator.Work done per revolution = 2πrF,Where F is the coupling force.r*F = P*R = T

∴B.P. = 2πNT/60000 kW

5. Absorption dynamometers: These dynamometers measure and absorb the power output of the engine to which they are coupled. The power absorbed is usually dissipated as heat. Example: Prony Brake, Rope Brake, Hydraulic Dynamometer etc.

Rope Brake/Belt Brake:

Hydraulic dynamometer:

Eddy current dynamometer: consists of a stator on which are fitted a number of electromagnets, a rotor disk mounted disk mounted on the engine shaft made of copper or steel. When the rotor rotates eddy currents are produced in the stator due to the magnetic flux set-up by the passage of field current in the electromagnets. These eddy currents oppose the rotor motions thus loading the engine.

Swinging Field DC dynamometer: Basically a DC shunt motor mounted on bearings. Can also be run as motor by suitable electric connections.

Transmission dynamometers: Also called as torque meters. Consists of a set of strain gauges mounted on the rotating shaft. The torque is measured by the angular deformation of the shaft which is indicated as strain of the strain gauge.

1. Willan’s line method

2. Morse test

3. Motoring test.

4. Difference between I.P. and B.P.

Measurement of Friction power

Willan’s line method

This method is applicable only to Compression ignition engines. In this method total fuel consumption vs. Brake power at a constant speed is plotted and the graph is extrapolated back to zero fuel consumption.

Qualitative governing. Quantitative governing. Pumping losses.

Governing of IC Engines

Applicable only to multi cylinder engines The I.P. of n cylinders is given by IPn = BPn + FP

IPn-1 is given by IPn-1 = BPn-1 + FP

Since the experiment is conducted at constant speed it is quite reasonable to assume that FP remains constant. Then, IP(nth) = BPn – BPn-1

Total IP of the engine, IPn = ∑ IP(nth)

Morse test

1. By taking an indicator diagram(P-V diagram) with the help of suitable engine indicator.

2. By measuring BP and FP separately.

Measurement of IP

Only a part of the energy supplied to the engine is transferred into useful work (BP). The rest of it is wasted. So the main components of heat balance sheet are -

1. Total heat supplied = mf*C.V kJ/hr

2. Heat equivalent to effective work of engine(BP) = BP*3600 kJ/hr

3. Heat rejected to the cooling medium = mwCpw(T0 - Ti) kJ/hr

4. Heat carried away from the engine with the exhaust gas = mgCpg(Tex-Tatm) kJ/hr

5. Heat unaccounted = losses which include radiation losses from various parts of the engine, heat lost due to incomplete combustion, heat lost due to friction, heat taken away by the lubricant etc. = (1- [2+3+4])

Heat balance sheet

Performance curves

Hook curve (consumption loop)

A - Richest mixtureE – Weakest mixtureB – Maximum Bmep D – Maximum economyC – Chemically correct or stoichiometric air-fuel ratio (14.5:1)Between B and D – Mixture strength vary for maximum power and maximum economy

1. Explain Morse test and motoring test to find friction power.

2. Why Willan’s line method cannot be used to find friction power in SI engine. (hint – pumping loss)

3. Explain exhaust gas calorimeter to measure heat in the exhaust.

4. Draw the performance curves for CI engines.

Assignment questions

Valve timing diagram

Port timing diagramFor 2-stroke engines