EESC - USP

DEMAC - UNESP

Mercedes-Benz do Brasil S.A.

Although there are new sources of alternative fuel, such as, vegetable oils, sugar cane, among others, the petrol is still largely used, supporting the control of its consumption. Generally speaking, the manufacturing process as well as the fuel distribution represent an inherent cost to products and services which are aimed at commercial vehicles, making the fuel consumption an essential item to the design of road vehicles.

Another fact which reinforces the importance of a reduction on the fuel consumption is related to some ecological aspects. The smaller the consumption, the fewer the amount of pollution gases left in the atmosphere. As a result, there can be a smaller impact on the environment, as well as on the human health.

During the design of road vehicles, the various items (drag coefficient, operational conditions, tire, building characteristics, engine, transmission system) that affect the fuel consumption must be evaluated in order to show a cost analysis.

The accuracy of fuel consumption computations is only as good as the accuracy of the available specific fuel consumption map (engine loading, power and engine revolutions); it is, therefore, essential that a representative diagram for the engine is used for the calculations.

Figure 1 shows two specific fuel consumption maps (petrol and Diesel engine).

Figure 1 - Specific fuel consumption maps [1]

In the fuel consumption analysis the driving schedules can be used for a comparison and the determination of the consumption of several vehicles. According to the country, there are specific patterns which are used in the fuel consumption tests. Such driving schedules, which are used for road tests, can be registered in the developed computational system in order to determine the fuel consumption.

It is presented below the description of some pattern driving schedules as well as a formulation the determination of the energy and consumption needed during such schedules.

The ECE cycle is illustrated in the figure 2 through a diagram of velocity versus time. For such cycle, the average velocity is 18.7 km/h and the maximum one is 50 km/h. The shift gear happens at 15 km/h from the first to the second one and at 35 km/h from the second to the third one.

Figure 2 - ECE urban driving schedule

Figure 3 illustrates the two EPA cycles. The urban EPA cycle is composed by eighteen stopping points with a maximum velocity of 91.5 km/h and an average velocity of 38.4 km/h in 11.99 traveled kilometers with the time of 1,373 seconds. The EPA road cycle is composed by a single stopping point with the maximum velocity of 96.8 km/h and an average the time of 765 seconds. The shift gear points are: from the first to the second one at the velocity of 24 km/h, from the second to the third one at 40 km/h, from the third to the fourth one at 64 km/h and from the fourth to the fifth one at 73.6 km/h. During the decelerating the gears stay engaged until the vehicle stops.

Figure 3 - EPA highway and urban driving schedules [3]

In Brazil the "GEIPOT" (Brazilian Transportation Planning Company) rules the fuel consumption test of urban buses. The vehicle must travel with its nominal load in horizontal surface pavement and according to an operational cycle presented in figure 4 the fuel consumption should not exceed 50 liters in 100 kilometers. The operational cycle specified in figure 4 aims to determine the average consumption of the vehicle in urban route. The maximum velocity of the cycle is 40 km/h consisting of periods of acceleration, constant velocity and deceleration, simulating a stopping point with the cycle total time of 60 seconds. During the deceleration, the gears stay engaged until the vehicle stops.

Figure 4 - GEIPOT urban driving schedule

For the determination of the consumption with driving schedule, it is divided according to the kind of movement, that is, idle vehicle with disengaged engine, vehicle accelerating, vehicle in constant velocity, vehicle shift gear and vehicle decelerating with the engine engaged. For the urban cycle GEIPOT, the following types of spaces exist: vehicle accelerating up to 40 km/h in 18 s, in this space the shift gears are not explicit, but they follow the change strategy adopted by the planner; space in constant velocity of 40 km/h in 17 s; space with the vehicle decelerating, in which the velocity varies from 40 to 35 km/h in 4 s, with the engine engaged in the gear that it was at the velocity from 40 km/h; space with strong decelerating and variation of velocity from 35 to zero km/h in 4 s, with the engine staying engaged in the previous gear; space with idle vehicle with disengaged engine, in which the driver places the first march for the repetition of the cycle. This urban cycle simulates the vehicle accelerating and decelerating with stops in traffic lights, jams and bus stops.

It was developed a computational system that simulates a vehicle developing, in the best possible way, a certain driving schedule with the following strategies for each space.

Wempty is the empty weight, W it is the total gross weight and Red.Cam(j) it is the reduction in the relationship j of the gearshift.

So, the acceleration of the vehicle can be expressed by the equation

Ftot is the total engine force,

Frtot is the total force of resistance,

When the vehicle is accelerating, a strategy of shift gears is adopted, in the maximum rotation of the engine, or in other specified as rotation of green range end. For the urban cycle GEIPOT, illustrated in figure 4, it is noticed that the vehicle should accelerate from 0 to 40 km/h in 18 s. In this space, the vehicle possibly will shift gears. Then, two approaches can be used: the first one, in which the shift gears happen with the time of change the same to zero, without changing in the profile of the urban cycle, and the second one, in which it is considered a stagger in the profile of the urban cycle, in which the vehicle proceeds with the shift gears within a certain time of change. In this second case, there is the disadvantage of building a profile for each vehicle or of adopting a new fixed profile with stagger, in which other vehicles would shift gears in different rotations from the ones chosen as strategy. Otherwise they would not manage to go on in this profile.

b) Constant velocity. For the vehicle traveling in constant velocity, both the available potency and the spent one are the same, that is, the potency in the axis of the engine (Pt) is used in the losses of the transmission system and in order to overcome the resistance forces. Therefore, for a constant velocity Ftot = Frtot. The value of Ftot is expressed by the equation (3) and it is relate to the potency Pt through the equation (6):

Ftot = Pt*1.000*3.6*Ren/v (6)

Therefore, the value of the necessary potency to supply the spent potency is

Pt = Ptotg(v) = Frtot(v)*v/(3.6*1,000*Ren), where (7)

Frtot(v) is calculated according to the equation (4). With the value of Ptotg(v) and the value of the rotation for a velocity v, the interpolation in the diagram of the engine map is done, being determined the specific consumption. For each value of Ptotg(v) a value of Pt should exist in the consumption map, because if the value of Pt in the maximum load is still smaller than the value of Ptotg(v), the vehicle will not stay in constant velocity and it will be decelerated.

For each rotation n and respective velocity v, the necessary potency Ptotg(v) is determined and, through the engine map, the specific consumption bs[g/kWh] is calculated. Figure 5 illustrates the interpolation process for the bs determination.

Figure 5 - Engine specific fuel consumption map

The rotation range, between the minimum and the maximum one, is used in the calculations of the velocity and respective bs. Therefore, there is the determination of curves bs[g/kWh] constants or B[l/100km] in function of the velocity, where

The distance traveled by the vehicle has influence in the calculation of the consumption accumulated in the driving schedule. Therefore, for each new space of the driving schedule, a new average consumption is calculated by the expression Bmed[l/100km] = Bacum[l] * 100 * 1.000/sacum[m], where and .

The analyzed vehicle is a Mercedes-Benz bus denominated OH1421L with 4x2 traction system. The main characteristics of the vehicle are presented below, according to information given by the vehicle manufacturer:

- model: OH1421L;

- number of rearward axles: 1;

- traction system: 4x2;

- tires: 10R20;

- dynamic rolling radius: 509 mm;

- coefficient of air resistance: 0.80;

- air density: 1.225 kg/m³;

- coefficient of rolling resistance: 0.008 constant;

- measure of grade: 0%;

- projected vehicle area: 6.5 m²;

- transmission: S5/680, efficiency: from 0.98 a 1.00 according to numerical ratio;

- ratio of final drive: 5.857;

- efficiency of final drive: 0.92;

- weight of loaded vehicle: 16,000 kgf;

- weight of empty vehicle: 10,000 kgf.

- density of the fuel (Diesel): 838,6 g/l;

- engine: OM366LA velocity range: from 1,000 to 2,600 min^-1. Maximum torque = 635 Nm in 1,820 min^-1 (engine loaded = 100% ) and partial engine loaded (torque, specific fuel consumption and engine revolutions) at 110%, 100%, 85%, 65%, 45%, 20% e 2% (values reduced for conditions in accordance test NBR5484 [8]).

The results described in the table 1 are for the vehicle traveling according to the urban cycle GEIPOT. This cycle is subdivided in spaces, in which the vehicle uses a coefficient of inferior adhesion to the available maximum adhesion coefficient. In table 1, the cycle is subdivided in five spaces and the acceleration is calculated according to the given profile, that is, with the time of shift gears being zero. In table 1, the non staggered approach in the acceleration was used, that is, considering the time of shift gears as zero. According to NAVARRO [7] this approach without stagger is advantageous for adapting to several types of vehicles, unless some can not manage to reach the velocity of 40 km/h in 18 s. According to study developed by the author, the consumption alteration in relation to the cycle with stagger is small.

Table 1 - Urban fuel consumption for driving schedule without stagger in the acceleration

The systematic of measurement for the simulated operation cycle GEIPOT was accomplished through the repetition of blocks of 30 passages by the cycle. The results are considered coherent within a maximum variation of 7%. The average experimental value of the consumption for the cycle-pattern GEIPOT is 2.46 km/l = 40.6 l/100km, while the theoretical value for the same cycle is of 40.06 l/100km. The percentile difference among the experimental and theoretical values is of -1.3%.

The theoretical model for the calculation of fuel consumption for a vehicle in a driving schedule was implemented in a computational system, obtaining the fuel consumption in each phase of the driving schedule traveled by a certain vehicle. Such driving cycles are generally presented as norms, and they are widely used to vehicles traveling in urban territory. The use of driving schedule without stagger is profitable concerning to the versatility of several kinds of vehicles and, also, due to the fact that the change of fuel consumption in relation to driving schedule with stagger is small. The theoretical and experimental results present an accomplished correlation, validating the theoretical model developed.