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uia automotive
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LECTURE 5
MEC 2621-‐ INTRODUCTION TO AUTOMOTIVE ENGINEERING
By: Dr. SANISAH SAHARIN
Engine Performance
OTTO Cycle
ENGINE POWER TRAIN OPERATION
Ø CrankshaI-‐transferring power to the transmission shaI
Ø The top dead centre (TDC) of an engine refers to the crankshaft being in a position such that θ = 0º.
Ø The volume in this position is called clearance volume (VC)
Ø Bottom dead centre (BDC) refers to the crankshaft being θ = 180º.
Ø The total volume VL is maximum at bottom dead centre (BDC).
Ø What is significant of the compression raNo for the engine on vehicle performance ?
Ø Heavy vehicle needs high torque-‐ what about its engine compression raNo?
Ø Lighter passenger car needs low torque-‐ what about compression raNo for its engine?
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aL
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o A 3-litre SI V6 engine that operates on a four-stroke cycle at 3600 rpm. The compression ratio is 9.5, the length of the connecting rods is 16.6 cm, and the engine is square (b=s). At this speed, combustion ends at 20º aTDC.
o Cylinder bore and stroke length o Average piston speed o Clearance volume of one cylinder o Distance the piston has travelled from TDC at the end of
combustion o Volume in the combustion chamber at the end of
combustion.
Example
example
• The peak pressure of a SI engine rotaNng at 1500 rpm occurs 0.003 s aIer the spark, what will be the spark Nming when peak pressure is at TDC. If the inlet valve opens at 10 degrees bTDC and closes at 45 degrees aBDC, how long the inlet valve opening period is in seconds
Ø Energy input to an engine Qin comes from the combusNng fuel.
Ø Fuel is a HC.
Ø Air is used to supply the oxygen needed for this chemical reacNon.
Ø For combusNon Ø the proper relaNve amounts of air (oxygen) and fuel
must be presented.
Ø Air-‐fuel raNo (AF) are parameters used to describe the mixture raNo.
Air-‐Fuel Ra,o
Ø The mean effecNve pressure (mep) is the work done per unit displacement volume.
Ø Two useful mean effecNve pressure parameters:
Ø imep : The indicated mean effecNve pressure is the net work done per unit of displacement volume by the gas during compression and expansion.
Ø bmep: is the external sha6 work done per unit volume by the engine.
Ø Volumetric efficiency:
Ø it is defined as the mass of fuel and air induced into the cylinder divided by the mass that would occupy the displaced volume at the density (ρ) in the intake manifold.
Ø Engine power is the power that is available at the crankshaI to drive the car. Ø The unit of engine power is in kW (or hp) Ø Power is defined as the rate of work of engine
Ø P =(Force x Stroke length/Nme=F x S/t).
Ø Three types of works are normally defined in case of engine power,
Ø Indicated horse power (ihp) : is the power developed inside the combusNon chambers due to the combusNon.
o Engine power
Ø Torque is the static measurement of how much work an engine does.
Torque
Ø Power is a measure of how fast the work is being done.
Power
Ø A two-‐stroke gas engine has piston diameter of 150 mm, length of stroke 400 mm and indicated mean effecNve pressure 5.5 bar. The engine makes 120 explosions per minute. Determine the mechanical efficiency of the engine, if its bhp is 5 kW.
Example
Ø In a four-‐stroke single cylinder gas engine the indicated mean effecNve pressure is 0.46 MN/m2, the brake power 9 kW, speed 250 rpm, mechanical efficiency, η = 0.8, and bore to stroke raNo = 0.66. Calculate cylinder diameter and mean piston speed.
Example
example
• A four stroke petrol engine delivers 35.75kW with a mechanical efficiency of 80%, the fuel consumpNon of the engine is 0.4 kg per brake power hour, and the A/F raNo is 14:1.The heaNng value of the fuel is 41870 kJ/kg. Find: (a) i.p, (b) f.p., (c)ηBT , (d) ηIT , (e) fuel consumpNon per hour, (f) air consumpNon per hour.
Effect of Engine speed
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END