ENGINE TERMINOLOGY

1. Introduction to
engine terminology
1
Vibhavari Ghadge
ADAM, Pune
12 March 2019

2. 1.Stroke and bore
Fig.1 Bore and stroke
Source: http://www.engineknowhow.com/engine-
parameters/bore-stroke-and-displacement/
 Bore – The diameter of the cylinder.
 Measured across the cylinder, parallel with the top of the
block.
 Cylinder bores vary in size, but typically range from 3–4"
(75–100 mm).
 Stroke- The distance travelled by a piston from Top dead
centre (TDC) to Bottom dead centre (BDC) in a cylinder.
 Crankshaft offset (throw) controls the piston stroke.
 Stroke varies from about 3–4" (75–100 mm).
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3. 2.Stroke to bore ratio
 Stroke/bore ratio is the ratio between cylinder bore and piston
stroke.
 In engines, the stroke to bore ratio is typically between 0.75 – 1.5
 An engine can be termed:
A. Oversquare : Bore > Stroke
B. Undersquare : Bore < Stroke
C. Square : Bore = Stroke
3
Fig.2 Stroke/bore ratio
Source: http://www.engineknowhow.com/engine-parameters/bore-stroke ratio

4. 4
A.Oversquare(bore>stroke)
1.Fast revolution.
2.Responsive at higher engine speeds.
3. Shorter stroke has less friction loss.
4. Shorter stroke reduces load on the
engine bearings.
5. Shorter stroke reduces engine height.
6.Used with a lower final drive ratio.
B.Undersquare(bore<stroke)
1.Slow revolution because of longer
stroke.
2.Good low engine speed torque.
3.Low rpm engine.
4.Good fuel economy due to lower
engine speed.
5.Used with a high final drive ratio.
C.Square(bore=stroke) :
1.Good compromise between low
rpm torque & high rpm power.
2.Provides good low speed torque
with good high speed power.
3.Used with a higher final drive
ratio.

5. 3.Cylinder Displacement
 A cylinder or piston displacement is the volume the piston displaces as it moves from BDC to TDC.
 It can be calculated by multiplying its cross-sectional area by the distance of piston travels within the
cylinder.
Cylinder Volume =(Stroke) . 𝜋
𝐵𝑜𝑟𝑒
4
2
5
Fig.3 Cylinder displacement
Source: http://www.engineknowhow.com/engine-parameters/cylinderdisplacement

6. 4.ENGINE Displacement
 Engine's displacement is the swept volume of all the pistons inside the cylinders of a
reciprocating engine in a single movement from TDC to BDC.
 Engine Displacement = Stroke . 𝜋
𝐵𝑜𝑟𝑒
4
2
. (number of cylinders)
 Engine displacement does not include the total volume of the combustion chamber.
 Units of engine displacement:
1.Cubic centimeters(CC)
2.Cubic inch displacement (CID)
3.Liters (L)
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7. 5.COMPRESSION RATIO
 The compression ratio is the ratio between the maximum cylinder volume to the minimum
cylinder volume.
 The geometric compression ratio is :
 Spark ignition engines typically have compression ratios between 7-14
 Compression ignition engines typically have compression ratios between 12-24.
 The compression volume can also be calculated from the compression ratio via:
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Fig.4 Compression ratio
Source: http://www.engineknowhow.com/engine-
parameters/compression ratio

8.  Higher compression gives greater combustion efficiency, up to a point.
 Ability to use aggressive cams
 More power
 More prone to detention
 Conservative timing
 Bad tank of gas can have fatal results to engine
High Compression Ratio
Pros and Cons
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9.  The amount of air mixed with the fuel into the cylinder is called as air-fuel ratio.
 The air-fuel ratio is an important measure for anti-pollution, performance-tuning, fuel
consumption, emissions and hardware protection reasons.
 The air-fuel ratio is calculated via:
 Petrol engines typically run between 12-18kg of air per kg of fuel.
 Diesel engines operate over a greater range, typically 18 – 70kg of air per kg of fuel.
6.Air Fuel Ratio9

10. 10 Air–fuel equivalence ratio
 Air–fuel equivalence ratio, λ (lambda), is the ratio of actual AFR to stoichiometry for a given
mixture.
λ = 1.0 is at stoichiometry
λ < 1.0 is at rich mixtures
λ > 1.0 is at lean mixtures.
λ =
𝐴𝐹𝑅
𝐴𝐹𝑅(𝑠𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑦)
 Stoichiometric is the perfect amount of air available to completely combust all of the fuel and
convert it to the final combustion products of CO2, H2O and N2.
 For the stoichiometric combustion of petrol there is ≈14.7kg of air per kg of fuel.
 AFR sensor is used to measure the Air-Fuel ratio.

11. 7.Torque
 Twisting or Turning Force.
 It is what causes an object to acquire angular acceleration.
 In automotive terms, Engine torque is the amount of force that an engine can produce.
 It is the measure of rotational effort applied on engine crankshaft by the piston.
 The torque output of an automotive engine mainly depends on its stroke-to-bore ratio, compression ratio,
combustion pressures & speed in rpm.
 Torque is Determined by Three Factors:
• The magnitude of the applied force.
• The direction of the applied force.
• The location of the applied force.
 Torque is proportional to the magnitude of F and to the distance r from the axis. Thus, a tentative formula
might be:
τ= F.r Units: N.m or lb.ft
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12. Power is the application of force over a distance in a Certain time period, or the rate of application of force over a
distance.
Power = Force × distance/time.
The force is applied around the circumference of the circle 2πr. So,
Power = F×2πr/t
Power as a function of Torque and rotational speed instead of Force and radius
Power = Torque×2π×RPS.
Where, Torque = F × r and RPS = 1/t. Also RPS = RPM/60
Power(Watt) = Torque×2π×RPM/60
Since,1hp = 746watts,
Power(hp)=Torque(Nm)×π×RPM/22,380
8.Torque and power relation12

13. 9.Volumetric efficiency
 Ratio of air volume drawn into the cylinder to the cylinders swept volume.
 Volumetric Efficiency = actual volume of air taken into each cylinder
swept volume of the cylinder
 An unboosted, spark ignition engine typically has a maximum volumetric efficiency between
80 – 90% whilst the volumetric efficiency is higher for a diesel engine.
 To increase the volumetric efficiency and the performance of a particular engine we have to
increase the mass flow of air through the engine.
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14. 14
Factors affecting Volumetric efficiency
1. The temperature at which air is coming into the combustion chamber.
2. The amount of time through which the inlet valve opens.
3. The pressure at which the air is supplied.
4. Heat dissipation from cylinder sleeves and spark plug. Then the cylinder will contain more air.

15. 15
ways to improve volumetric efficiency
 Use a turbocharger or Supercharger.
 Surface flashing removal

16. 16 10.thermal efficiency
 Thermal efficiency of an engine is the fraction of heat that is converted into work by combustion of fuel.
 The thermal efficiency for a spark ignition engine is calculated by:
Thermal Efficiencyotto % =
1
𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 (𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐻𝑒𝑎𝑡 𝑅𝑎𝑡𝑖𝑜−1)
 In a typical low compression engine, the thermal efficiency is only about 26%.
 In a highly modified engine, such as a race engine, the thermal efficiency is about 34%.
 Only 1/4th of the heat is used to power the vehicle.
 The efficiency losses are from hot gases (energy wasted and exhausted to the environment), friction of
the moving parts, and non-stoichiometric combustion (too much oxygen or too much fuel for ideal
combustion).

17. 17
11.MEAN EFFECTIVE PRESSURE
 The mean effective pressure (MEP) is the average pressure
during the power stroke, minus the average pressure during
the other three strokes.
 In fact, the MEP is the pressure that actually forces the piston
down during the power stroke.

18. 18 12.Indicated Horse Power
 The rate at which the engine can do work is measured in
horse power (HP).
 One HP is equivalent to 4500 kg m per min.
 Indicated power is the actual power developed within the
cylinder of an engine due to the combustion of fuel.
IHP =
𝑃𝐿𝐴𝑁
4500
P - means effective pressure in kg/cm2
L - stroke length in m
A - area of cylinder in cm2
N - power stroke per min (for a four stroke engine N = rpm/2
and for a two-stroke engine N = rpm)

19. 19
 BHP is the power made at the crankshaft of the engine.
 The values obtained in engine dynamometer testing is the
BHP.
 In simple terms BHP= IHP - FP
where FP is the frictional power loss.
 Frictional loss is directly proportional to square of the engine
speed.
13.Brake Horse Power

20. Thank you
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