Post on 05-Apr-2018
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Class9(Transmission systems-Gear Box)
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Requirement for a transmission system
Engine PerformanceSome fundamentals
The power developed in an engine
Brake Mean effective pressure (BMEP): The average pressure acted on the piston during
expansion stroke , which would produce the shaft work (piston work-frictional losses)
Volumetric Efficiency =
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Engine Performance
Some fundamentals
Indicated power (IP):
IP =.
2....
4 60W
wherepiis the indicated mean effective pressure (IMEP)
Brake power: (BP)
BP =.
2....
4 60W
wherepbis the brake mean effective pressure (BMEP)
We have =
If mean effective pressure and remain constant when speed increases,then both IP and BP increase indefinitely as shown in the graph
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Limiting factors
Mean effective pressure depends on compression ratio and the volumetric efficiency
When speed increases, volumetric efficiency decreases sharply due to flow chocking,
inertia effects, charge heating in the manifolds (IMEP decreases)
Mechanical efficiency is also not constant as when speed increases, frictional losses
increase (Both IMEP and BMEP Decreases)
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Engine Performance
Some fundamentals
Ideal Engine Actual Engine
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Torque is a twisting force
When engine rotates torque is available at the crank shaft
Engines typically operate over a range of 600 to about 7000 RPM
Engine provides its highest torque and power outputs unevenly across the revolution range
Torque curve of an engine:
It is a curve showing the variation of torque at the crankshaft for various crank shaft
rotation speeds
Torque curve has the same shape as the BMEP curve
Engine Performance
Some fundamentals - Torque
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Torque Curve
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When BMEP and efficiency increase, BP
increases
When BMEP falls at the same rate of speed
increase, BP remains constant
When BMEP and efficiency falls rapidly,
even though speed increases, power
decreases
Torque behaves same as BMEP
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Need of a gear box Power of engine and speed can be adjusted with throttle control
But at high speeds, torque will be less depending on torque curve of the
engine
Torque required by the wheel varies with conditions (up the hill, down the
hill, level surface, starting, accelerating, etc.)
Engine speed and power output must be adjusted to get required torque
at the wheels
Required torque can be determined and adjusted through a gear box
Tractive effort (TE); the force required to drive the wheel
TE is equal to total opposing force : vehicle moves at constant speed
TE is less than total opposing force : vehicle will decelerate
TE is greater than total opposing force: vehicle will accelerate5/7/2012 7
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Total opposing forces-components1. Aerodynamic force resistance by the air, depends on shape and speed
2. Gradient resistance a force greater than H must be applied to the wheels
at their contact to move the car upward depends only on slope- not speed
of the car if it is constant up or down the slope
3. Rolling resistance: frictional resistance at the wheels of the vehicle and the
transmission system - not significantly affected by the speed
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Total opposing forces The total resistance : aerodynamic drag + rolling resistance + gradient resistance
Rolling resistance and gradient resistance are independent of vehicle speed
OS is the speed of car in km/h, then SP is the total drag
SRRolling resistance (constant w.r.t speed)
RQgradient resistance (constant.w.r.t speed)
QPaerodynamic drag (depends on speed)
The total resistance varies with road conditions (A- level road, B- steep road, C-steeper
road)
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Law of Gears The gear ratio: ratio of no. of teeth in driven gear (receives torque) to no. of
teeth in driving gear (gives torque)
Force on driver:
Force on driven:
dis proportional to Nwhere N is no. of teeth
Hence,
Or
Torque ratio is
proportional to
Teeth ratio/gear ratio5/7/2012 10
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The engine turns the clutch with torque Te
Te Ne = Tp Np
Where Ne is the speed of the engine/clutch shaft, Np is the speed of propeller
shaft and Tp is the torque on the propeller shaft
If propeller rotates at 1/ nspeed of the clutch shaft (because of gear
reduction in the gearbox), then,
Propeller shaft torque Tp=nTe where n is the gear ratio (driven to driver)
The propeller shaft drives the wheel through final drive (differential), where
another gear reduction occurs
Ifm is the final drive gear ration, the torque transmitted to the wheels
Tw
= m
n
Te5/7/2012 11
Tractive Effort
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Tractive Effort
Tractive effort is the force required to drive the wheel
The wheel in the figure is in equilibrium
Because of Tw, the force on the axle is P1 which tends to move the wheel forward
P2 is the reaction force of P1, P3 is the opposing force on the point of contact as the
wheel is in equilibrium
P2 and P3 form a couple turning the wheel backward. Tw > this couple strength, to keep
the wheel in equilibrium (constant speed) or accelerate
P1
P2
P3
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The strength of the couple is P3 x R where R is the radius of the wheel
For equilibrium, Tw
= P3
R, i.e, m n Te
= P3
R
Tractive effort, P3 =m n
For any particular gear,
m n
is a constant K
Hence the tractive effort (TE) is proportional to the torque produced by the engine
P3 = K x T, where K depends on the wheel radius gear ratio for a particular gear and the
final drive (differential) gear ratio
Variation of TE with speed
Wheel speed = Engine speed/(n x m) and TE = Engine torque x (m x n/R)
Hence for various gear ratios n, we get various TE curve depending on the torque curve
of the engine5/7/2012 13
Tractive Effort
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Tractive effort Vs Wheel Speed
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TE is proportional to Engine Torque (for a particular gear)
Wheel speed is proportional to Engine speed (for any particular gear)
Engine Torque is dependent on Engine speed through torque curve
So, TE is proportional to the wheel speed through the torque curve of the
engine
For a particular gear we get a particular TE curve
Shape of the TE curve will be the same
If the gear is changed to a higher gear, the TE
curve also changes ( via the engine torque curve)
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Summary
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For maximum speed on the level and to be able to climb medium gradients- at least two gear ratio required
For reasonably high speeds in medium gradients and yet be able to climbsteep gradients, we need a third gear ratio, and a fourth or fifth ratio may bedesirable
In other words, at level surface, for getting maximum velocity, power isrequired at the wheel, torque and tractive efforts are not important
But to overcome gradients and inertia, TE and hence torque are important.
Hence we need a gear reductions and a gear box
To take up the highly varying road conditions hence we need various gearratios