381 Setting top gear

To determine the maximum vehicle speed, the engine brake power curve is superimposed onto the power requirement curve which can be plotted from the sum of both the rolling (Rr) and air (Ra) resistance covering the entire vehicle's speed range (Fig. 3.31).

The total resistance R opposing motion at any speed is given by:

where Cr = coefficient of rolling resistance W = gross vehicle weight (kg) CD = coefficient of aerodynamic resistance (drag) A = projected frontal area of vehicle (m2) V = speed of vehicle (km/h)

,31 Forces opposing vehicle motion over its speed

range

,31 Forces opposing vehicle motion over its speed

The top gear ratio is chosen so that the maximum road speed corresponds to the engine speed at which maximum brake power is obtained (or just beyond) (Fig. 3.32).

Gearing is necessary to ensure that the vehicle speed is at a maximum when the engine is developing approximately peak power.

Thus

Linear wheel speed : -dN

Final drive gear ratio Gf

Linear road speed

0.06

where GF = final drive gear ratio N = engine speed (rev/min) d = effective wheel diameter (m) V = road speed at which peak power is developed (km/h)

Example A vehicle is to have a maximum road speed of 150km/h. If the engine develops its peak power at 6000 rev/min and the effective road wheel diameter is 0.54 m, determine the final drive gear ratio.

0.06 vdN

Fig. 3.32 Relationship of power developed and road power required over the vehicle's speed range
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