Automotive Engineering Class 9

8/2/2019 Automotive Engineering Class 9

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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

Automotive Engineering Class 9

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