311 Resistance to vehicle motion

To keep a vehicle moving, the engine has to develop sufficient power to overcome the opposing road resistance power, and to pull away from a standstill or to accelerate a reserve of power in addition to that absorbed by the road resistance must be available when required.

Road resistance is expressed as tractive resistance (kN). The propelling thrust at the tyre to road interface needed to overcome this resistance is known as tractive effect (kN) (Fig. 3.1). For matching engine power output capacity to the opposing road resistance it is sometimes more convenient to express the opposing resistance to motion in terms of road resistance power.

The road resistance opposing the motion of the vehicle is made up of three components as follows:

1 Rolling resistance

2 Air resistance

3 Gradient resistance

Rolling resistance (Fig. 3.1) Power has to be expended to overcome the restraining forces caused by the deformation of tyres and road surfaces and the interaction of frictional scrub when tractive effect is applied. Secondary causes of rolling resistance are wheel bearing, oil seal friction and the churning of the oil in the transmission system. It has been found that the flattening distortion of the tyre casing at the road surface interface consumes

Vehicle speed |km/h)

Fig. 3.1 Vehicle tractive resistance and effort performance chart

40 60 so

Vehicle speed |km/h)

Fig. 3.1 Vehicle tractive resistance and effort performance chart more energy as the wheel speed increases and therefore the rolling resistance will also rise slightly as shown in Fig. 3.1. Factors which influence the magnitude of the rolling resistance are the laden weight of the vehicle, type of road surface, and the design, construction and materials used in the manufacture of the tyre.

Air resistance (Fig. 3.1) Power is needed to counteract the tractive resistance created by the vehicle moving through the air. This is caused by air being pushed aside and the formation of turbulence over the contour of the vehicle's body. It has been found that the air resistance opposing force and air resistance power increase with the square and cube of the vehicle's speed respectively. Thus at very low vehicle speeds air resistance is insignificant, but it becomes predominant in the upper speed range. Influencing factors which determine the amount of air resistance are frontal area of vehicle, vehicle speed, shape and streamlining of body and the wind speed and direction.

Gradient resistance (Fig. 3.1) Power is required to propel a vehicle and its load not only along a level road but also up any gradient likely to be encountered. Therefore, a reserve of power must be available when climbing to overcome the potential energy produced by the weight of the vehicle as it is progressively lifted. The gradient resistance opposing motion, and therefore the tractive effect or power needed to drive the vehicle forward, is directly proportional to the laden weight of the vehicle and the magnitude of gradient. Thus driving up a slope of 1 in 5 would require twice the reserve of power to that needed to propel the same vehicle up a gradient of 1 in 10 at the same speed (Fig. 3.1).

3.1.2 Power to weight ratio

When choosing the lowest and highest gearbox gear ratios, the most important factor to consider is not just the available engine power but also the weight of the vehicle and any load it is expected to propel. Consequently, the power developed per unit weight of laden vehicle has to be known. This is usually expressed as the power to weight ratio.

i.e. Power to weight Brake power developed ratio Laden weight of vehicle

There is a vast difference between the power to weight ratio for cars and commercial vehicles which is shown in the following examples.

Determine the power to weight ratio for the following modes of transport:

a) A car fully laden with passengers and luggage weighs 1.2 tonne and the maximum power produced by the engine amounts to 120 kW.

b) A fully laden articulated truck weighs 38 tonne and a 290 kW engine is used to propel this load.

lOOkW/tonne 7.6kW/tonne.

3.1.3 Ratio span

Another major consideration when selecting gear ratios is deciding upon the steepest gradient the vehicle is expected to climb (this may normally be taken as 20%, that is 1 in 5) and the maximum level road speed the vehicle is expected to reach in top gear with a small surplus of about 0.2% grade-ability.

The two extreme operating conditions just described set the highest and lowest gear ratios. To fix these conditions, the ratio of road speed in highest gear to road speed in lowest gear at a given engine speed should be known. This quantity is referred to as the ratio span.

Road speed in highest gear i.e. Ratio span =---——--

Road speed in lowest gear

(both road speeds being achieved at similar engine speed).

Car and light van gearboxes have ratio spans of about 3.5:1 if top gear is direct, but with overdrive this may be increased to about 4.5:1. Large commercial vehicles which have a low power to weight ratio, and therefore have very little surplus power when fully laden, require ratio spans of between 7.5 and 10:1, or even larger for special applications.

An example of the significance of ratio span is shown as follows:

Calculate the ratio span for both a car and heavy commercial vehicle from the data provided.

Type of vehicle
















vehicle (CV)



Car ratio span

39 975

3.1.4 Engine torque rise and speed operating range (Fig. 3.2)

Commercial vehicle engines used to pull large loads are normally designed to have a positive torque rise curve, that is from maximum speed to peak torque with reducing engine speed the available torque increases (Fig. 3.2). The amount of engine torque rise is normally expressed as a percentage of the peak torque from maximum speed (rated power) back to peak torque.

_ . Maximum speed torque

Peak torque

1000 2000 Engine speed (rev/min)

Fig. 3.2 Engine performance and gear split chart for an eight speed gearbox

The torque rise can be shaped depending upon engine design and taking into account such features as naturally aspirated, resonant induction tuned, turbocharged, turbocharged with intercooling and so forth. Torque rises can vary from as little as 5 to as high as 50%, but the most common values for torque rise range from 15 to 30%.

A large torque rise characteristic raises the engine's operating ability to overcome increased loads if the engine's speed is pulled down caused by changes in the road conditions, such as climbing steeper gradients, and so tends to restore the original running conditions. If the torque rise is small it cannot help as a buffer to supplement the high torque demands and the engine speed will rapidly fade. Frequent gear changes therefore become necessary compared to engines operating with high torque rise characteristics. Once the engine speed falls below peak torque, the torque rise becomes negative and the pulling ability of the engine drops off very quickly.

Vehicle driving technique should be such that engines are continuously driven between the speed range of peak torque and governed speed. The driver can either choose to operate the engine's speed in a range varying just below the maximum rated power to achieve maximum performance and journey speed or, to improve fuel economy, wear and noise, within a speed range of between 200 to 400 rev/min on the positive torque rise side of the engine torque curve that is in a narrow speed band just beyond peak torque. Fig. 3.2 shows that the economy speed range operates with the specific fuel consumption at its minimum and that the engine speed band is in the most effective pulling zone.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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  • alberta
    What are the various resistant to motion of an automobile vehicle?
    2 years ago
  • olli
    What are various resistance automobile overcome?
    2 years ago
  • leon
    What are the registance motion of vehicle?
    2 years ago
    What is gradient resistance?
    2 years ago

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