Tyre quality influence on fuel consumption for motor vehicles.

Mihon, Liviu ; Negoitescu, Arina ; Tokar, Adriana 等

1. INTRODUCTION

At present, the tyres that equip vehicles play a vital role in fuel economy. At normal speed travel tires are responsible for 20% consumption of fuel in cars case and 30% for trucks. If all vehicles would be equipped with tyres that reduce the rolling resistance, over 4.5 billions litres diesel and 1.5 billions litres petrol can be saved every year. Also, the C[O.sub.2] emissions reduction by over 15 million tones would be possible. The tyres low pressure can also determine the consumption increase. At a speed of 90 km/h with a tyre pressure less with 1 bar than the normal one the insecurity in traffic increases and the fuel consumption increases with 3-5%. The rolling resistance is one of the five forces that on vehicle must overcome (Anghelache&Teodorescu, 2002).

2. THE TYRES CONTRIBUTION ON FUEL

CONSUMPTION

For a given vehicle, the percentage of fuel consumption accounted for a rolling resistance depends on:

* The speed and acceleration at each instant of the considered movement,

* The vehicle's characteristics (mass, aerodynamics, internal friction, gear ratio),

* The tyres' rolling resistance coefficient.

The fuel consumption due to rolling resistance (in litres per 100 km) also depends on the engines efficiency at each instant of the considered movement.

If all these parameters are known, the contribution of each resistive force to fuel consumption may be determined for the movement in question. This determination was accomplished in the Road Vehicles Laboratory of "Politehnica" University of Timisoara for four types of movements for a 51 kW engine that equips a VW Caddy car shown in Figure 1. There are also included the result for a European NMVEG driving cycle.

[FIGURE 1 OMITTED]

From one type of movement to another, tyres with a rolling resistance coefficient of 12 kg/t determine the fuel consumption variation between 20% (motorway driving) and 30% (urban cycle). As an absolute value, the tyre's contribution varies between 1.38 litres per 100 kilometres (motorway driving) and 2.57 litres per 100 kilometres (urban cycle) (Tires and Passenger Vehicle Fuel Economy, Transportation, 2006).

In order to describe the influence of rolling resistance on the fuel consumption, driving on flat roads, the hypotheses are: the vehicle mass: 1,400 kg; the aerodynamic drag (A): 2.09 [m.sup.2]; the internal friction: 50 N; the engine power: 51 kW; the diesel fuel highest calorific value (HCV): 42.58 MJ/kg; the average gearbox efficiency: 88 % for urban driving, 95 % for other movements; the rolling resistance coefficient for "normal" tyres (NT): 12 kg/t; the rolling resistance coefficient for "eco" tyres (ET): 8.5 kg/t.

Surprisingly, the absolute savings obtained by using tyres with low rolling resistance are almost entirely independent on the movement type (Schuring, 1994).

Fuel consumption does not only depend on the resistive forces exerted on the vehicle but also on engine efficiency. To determine the quantity of fuel consumed by a vehicle due to rolling resistance, we need to know engine efficiency at each moment.

The efficiency of an engine is defined as the ratio between the power required (brake power output) and the power consumed (fuel power consumption). All the parameters determining the amount of fuel consumed as a result of each resistive force are therefore closely linked, Figure 2.

Measurements recorded on the dyno test can be used to plot a "map" of the engine's efficiency. These maps are very accurate but do not directly indicate engine consumption at each moment, Figure 3 & Figure 4.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

3. FUEL SAVINGS MADE BY REPLACING "NORMAL" TYRES WITH "ECO" TYRES

NT currently on the market have a rolling resistance coefficient of around 12 kg/t compared with 8.5 kg/t for ET. Therefore, for a 1.4 t car the rolling resistance force of a NT set is around 165 N, compared with 117 N for an ET set that represents a difference of 48 N.

The graphic shown in Figure 5 derives from the Willans lines. It clearly shows that for a given vehicle, a required reduction in force always leads to the same savings in fuel consumption, whichever the required engine speed and gear force are. That means the savings are quasi-constant whichever the speed and movement category are. For the vehicle with 51 kW engine previously described, fuel savings are around 0.28 litres per 100 km.

If the savings are expressed as a percentage, they represent between 3.2% for the urban cycle and 5.1% for driving on major and minor roads (Schuring, 1980).

If a broad spectrum of cars currently on the market is considered, we may say that lowering rolling resistance by 30% leads to fuel savings of between 3 and 6% without modifying vehicle design. However, these figures must be reconsidered for each vehicle and each category of movement.

Table 1 shows the fuel savings made by the tested car when NT are replaced with ET. These savings are independent on the category of movement and the vehicle's original consumption. If the vehicle consumes 8 l or 6 l to start with, the savings remain stable at around 0.28 l/100 km.

[FIGURE 5 OMITTED]

4. ADDITIONAL SAVINGS MADE BY OPTIMIZING THE GEAR RATIO

To understand the principle behind optimizing a vehicles gear ratio, the Willans lines are used again. It is observed that the relationship between power consumed ([P.sub.c]) and power required ([P.sub.nec]) is expressed as follows:

[P.sub.C] = a x [P.sub.nec] + b x n (1)

where n is the engine speed in rpm.

For the diesel internal combustion engines that equip the VW Caddy involved in these tests the factor "a" is around 2 and factor "b" lies between 5 and 7 (The Tyre Rolling Resistance and Fuel Savings, 2003).

It is also seen that efficiency is equal to:

[eta] = [P.sub.nec]/[P.sub.C] = 1 / a + (b x n/[P.sub.nec]) (2)

If the NT of a vehicle which engine and gear ratio were optimized for this type of tyre are removed and replaced by ET, the required power is reduced:

[P.sub.necET] < [P.sub.necNT] (3)

The manufacturer changes the gear ratio as to modify the engine speed and so the optimal efficiency will be [[eta].sub.ET] = [[eta].sub.NT].

5. CONCLUSION

These results show slightly differences in the fuel consumption savings made by "eco" tyres from one category of movement to another when expressed as absolute values (from 0.28 l/100 km to 0.30 l/100 km). These differences are due to two factors:

* The use of the full engine map in calculations rather than the simplified Willans lines;

* During a movement, the difference in fuel consumption between "eco" and "normal" tyres is only seen when power is required and not throughout the movement. If the ratios of "duration that power is required" over "duration of movement" were the same during the movements, the savings would be exactly the same.

As with all testing conducted at different time, by different laboratories, and with different equipment, some of the observed variability in rolling resistance, both across and within data sets, may belong to the testing mechanisms themselves.

6. REFERENCES

Anghelache, G. & Teodorescu, C. (2002), Testing and evaluation of the tyre like component part of motor vehicle, BREN Publisher, ISBN 973-648-027-5, Bucuresti

*** (2006) Tires and Passenger Vehicle Fuel Economy. Transportation. Research Board Special Report of the National Academies 286, ISBN 0-309-09421-6, Washington, D.C., 2006

Schuring, D. J. (1994). Effects of Tire Rolling Loss on Vehicle Fuel Consumption. Tire Science and Technology, pp.149-161, Vol. 22, No. 3, 1994

Schuring, D. J. (1980). The Rolling Loss of Pneumatic Tires. Rubber chemistry and Technology, pp. 600-727, Vol. 53, No. 3, 1980.

*** (2003) The Tyre Rolling Resistance and Fuel Savings. Societe de Technologie Michelin, 2003

Tab. 1. The fuel savings made by a passenger car when
"normal" tyres are replaced with "eco" tyres

 Movement Urban Extra-urban NMVEG

Consumption [1/100km] NT (12kg/t) 7.4 5.2 6.64
 ET (8.5kg/t) 7.12 4.92 6.38

Saving compared with 0.28 0.28 0.28
normal tyres [1/100km]

Fig. 4. Fuel consumption versus force required according to the
Willans lines for a 51 kW diesel engine

 Rolling Internal Aerodynamic
 resistance friction drag Inertia

Urban 2.57 0.97 0.76 4.14 8.44
Extra-urban 1.47 0.55 2.24 1.33 5.59
NMVEG 1.87 0.71 1.69 2.37 6.64
Major and minor
 and driving 1.56 0.59 2.19 0.78 5.13
Motorway driving 1.38 0.50 4.93 0.38 7.20

Note: Table made from bar graph.