What determines the traction speed parameters. Traction-speed and fuel-economic properties of the vehicle. Vehicle speed properties

Traction and speed properties are important when operating a car, since their average speed and performance largely depend on them. With favorable traction and speed properties, the average speed increases, the time spent on transporting goods and passengers decreases, and the performance of the vehicle also increases.

3.1. Indicators of traction and speed properties

The main indicators that allow assessing the traction and speed properties of a vehicle are:

Maximum speed, km / h;

Minimum steady speed (in top gear)
, km / h;

Acceleration time (from standstill) to maximum speed t p, s;

Acceleration path (from standstill) to maximum speed S p, m;

Maximum and average acceleration during acceleration (in each gear) j max and j av, m / s 2;

The maximum overcome rise in the lowest gear and at a constant speed i m ax,%;

Length of dynamically overcome rise (with acceleration) S j, m;

Maximum pulling force on the hook (in low gear) R with , N.

V
the average speed of continuous movement can be used as a generalized estimated indicator of the traction-speed properties Wed , km / h. It depends on the driving conditions and is determined taking into account all of its modes, each of which is characterized by the corresponding indicators of the traction and speed properties of the vehicle.

3.2. Forces acting on the car when driving

When driving, a number of forces act on the car, which are called external. These include (Figure 3.1) gravity G, forces of interaction between the wheels of the car and the road (road reactions) R X1 , R x2 , R z 1 , R z 2 and the force of interaction of the car with air (reaction of the air environment) P c.

Rice. 3.1. Forces acting on a car with a trailer when driving:a - on a horizontal road;b - on the rise;v - on the descent

Some of these forces act in the direction of movement and are motive, while others are against the movement and refer to the forces of resistance to movement. So, strength R X2 in traction mode, when power and torque are supplied to the drive wheels, it is directed in the direction of travel, and the forces R X1 and R in - against the movement. The force P p - a component of the force of gravity - can be directed both in the direction of movement and against, depending on the conditions of movement of the car - on the rise or on the descent (downhill).

The main driving force of the car is the tangential reaction of the road. R X2 on the driving wheels. It results from the supply of power and torque from the engine through the transmission to the drive wheels.

3.3. Power and moment supplied to the driving wheels of the vehicle

Under operating conditions, the car can move in different modes. These modes include steady motion (uniform), acceleration (accelerated), deceleration (decelerated)

and
roll forward (by inertia). At the same time, in urban conditions, the duration of the movement is approximately 20% for the steady state, 40% for acceleration and 40% for braking and coasting.

In all driving modes, except for coasting and braking with the engine disconnected, power and torque are supplied to the drive wheels. To determine these values, consider the circuit,

Rice. 3.2. Scheme for determining powerness and torque, basefrom the engine to the drivescaffolding car:

D - engine; M - flywheel; T - trancemission; K - driving wheels

shown in Fig. 3.2. Here N e is the effective engine power; N tr - power supplied to the transmission; N count - power supplied to the driving wheels; J m - the moment of inertia of the flywheel (this value is conventionally understood as the moment of inertia of all rotating parts of the engine and transmission: flywheel, clutch parts, gearbox, cardan gear, main gear, etc.).

When the car accelerates, a certain proportion of the power transmitted from the engine to the transmission is spent on unwinding the rotating parts of the engine and transmission. These power costs

(3.1)

where A - kinetic energy of rotating parts.

Let us take into account that the expression for the kinetic energy has the form

Then the power consumption

(3.2)

Based on equations (3.1) and (3.2), the power supplied to the transmission can be represented as

Some of this power is wasted to overcome various resistances (friction) in the transmission. The indicated power losses are estimated by the transmission efficiency tr.

Taking into account the power losses in the transmission, the power supplied to the drive wheels

(3.4)

Angular velocity crankshaft engine

(3.5)

where ω to is the angular speed of the driving wheels; u t - transmission ratio

Gear ratio of transmission

Where u k - gear ratio of the gearbox; u d - ratio additional gearbox ( transfer case, divider, demultiplier); and G - gear ratio of the main transfer.

As a result of the substitution e from relation (3.5) to formula (3.4), the power supplied to the driving wheels:

(3.6)

At constant angular velocity of the crankshaft, the second term on the right-hand side of expression (3.6) is equal to zero. In this case, the power supplied to the driving wheels is called traction. Its magnitude

(3.7)

Taking into account relation (3.7), formula (3.6) is transformed to the form

(3.8)

To determine the torque M To , supplied from the engine to the driving wheels, we represent the power N count and N T, in expression (3.8) in the form of products of the corresponding moments and angular velocities. As a result of this transformation, we obtain

(3.9)

Substitute into formula (3.9) expression (3.5) for the angular velocity of the crankshaft and, dividing both sides of the equality by to get

(3.10)

With a steady motion of the car, the second term on the right-hand side of formula (3.10) is equal to zero. The moment supplied to the driving wheels is in this case called traction. Its magnitude

(3.11)

Taking into account relation (3.11), the moment supplied to the driving wheels:

(3.12)

MINISTRY OF AGRICULTURE AND

FOOD OF THE REPUBLIC OF BELARUS

INSTITUTION OF EDUCATION

"BELARUSIAN STATE

AGRARIAN TECHNICAL UNIVERSITY

FACULTY OF RURAL MECHANIZATION

FARMS

Department "Tractors and Cars"

COURSE PROJECT

By discipline: Fundamentals of theory and calculation of a tractor and a car.

On the topic: Traction-speed properties and fuel efficiency

car.

5th year student group 45

A.A. Snopkova

Head of KP

Minsk 2002.
Introduction.

1. Traction and speed properties of the car.

Traction-speed properties of a car is a set of properties that determine the possible ranges of changes in speeds of movement and the maximum intensities of acceleration and deceleration of a car when it is operating in a traction mode of operation in various road conditions.

Indicators of the tag-speed properties of the car ( maximum speed, acceleration during acceleration or deceleration during braking, traction force on the hook, effective engine power, lift overcome in various road conditions, dynamic factor, speed characteristic) are determined by the design traction calculation. It involves the determination of design parameters that can provide optimal driving conditions, as well as the establishment of limit road conditions movements for each type of vehicle.

Traction-speed properties and indicators are determined during the traction calculation of the vehicle. The object of the calculation is a light-duty truck.

1.1. Determination of car engine power.

The calculation is based on the rated carrying capacity of the vehicle.

in kg (the mass of the installed payload + the mass of the driver and passengers in the cab) or road train, it is equal from the task - 1000 kg.

Engine power

required for the movement of a fully loaded vehicle at a speed in given road conditions, which characterize the reduced resistance of the road, is determined from the dependence: where is the own weight of the vehicle, 1000 kg; air resistance (in N) - 1163.7 when moving at a maximum speed = 25 m / s; - transmission efficiency = 0.93. The rated lifting capacity is indicated in the assignment; = 0.04, taking into account the work of the car in agriculture (road resistance coefficient). (0.04 * (1000 * 1352) * 9.8 + 1163.7) * 25/1000 * 0.93 = 56.29 kW.

The unladen weight of the vehicle is related to its nominal carrying capacity by the dependence:

1000 / 0.74 = 1352 kg. - coefficient of carrying capacity of the vehicle - 0.74.

For a car with a particularly low payload = 0.7 ... 0.75.

The vehicle's load-carrying capacity significantly affects the dynamic and economic performance of the vehicle: the larger it is, the better these indicators.

Air resistance depends on the density of the air, the coefficient

streamlining of the contours and bottom (windage coefficient), frontal surface area F (in) of the car and speed mode movement. Determined by the dependence:, 0.45 * 1.293 * 3.2 * 625 = 1163.7 N. = 1.293 kg / - air density at a temperature of 15 ... 25 C.

The streamlining coefficient of the car

= 0.45 ... 0.60. Accept = 0.45.

The frontal area can be calculated using the formula:

Where: B is the track of the rear wheels, I take it = 1.6m, the value of H = 2m. The values of B and H are specified in subsequent calculations when determining the dimensions of the platform.

= the maximum speed of movement on a road with an improved surface with full fuel supply, according to the assignment it is equal to 25 m / s. the car develops, as a rule, in direct transmission, then 0.95 ... 0.97 - 0.95 Idling; =0,97…0,98 – 0,975.

Efficiency of the main transfer.

0,95*0,975=0,93.

1.2. The choice of the wheel formula of the car and geometric parameters wheels.

The number and dimensions of wheels (wheel diameter

and the mass transmitted to the wheel axle) are determined based on the carrying capacity of the vehicle.

With a fully loaded vehicle, 65 ... 75% of the total weight of the vehicle falls on the rear axle and 25 ... 35% - on the front axle. Consequently, the load factor of the front and rear driving wheels is 0.25… 0.35 and –0.65… 0.75, respectively.

; 0.65 * 1000 * (1 + 1 / 0.45) = 1528.7 kg.

on the front:

... 0.35 * 1000 * (1 + 1 / 0.45) = 823.0 kg.

I take the following values: on the rear axle - 1528.7 kg, on one wheel of the rear axle - 764.2 kg; on the front axle - 823.0 kg, on the front axle wheel - 411.5 kg.

Based on the load

and tire pressure, according to table 2, tire sizes are selected, in m (tire profile width and rim diameter). Then the estimated radius of the driving wheels (in m); ...

Estimated data: tire name -; its size is 215-380 (8.40-15); calculated radius.

INTRODUCTION

The guidelines provide a methodology for calculating and analyzing traction-speed properties and fuel efficiency carburetor cars with step mechanical transmission... The work contains parameters and specifications domestic cars, which are necessary to perform calculations of dynamism and fuel efficiency, the procedure for calculating, constructing and analyzing the main characteristics of the specified operational properties is indicated, recommendations are given on the choice of a series technical parameters reflecting design features different cars, mode and conditions of their movement.

The use of these guidelines makes it possible to determine the values of the main indicators of dynamism and fuel efficiency and to reveal their dependence on the main factors of the vehicle design, its load, road conditions and engine operating mode, i.e. solve the problems that are posed to the student in the course work.

MAIN PROBLEMS OF CALCULATION

When analyzing traction-high-speed properties of the car, the calculation and construction of the following characteristics of the car is made:

1) traction;

2) dynamic;

3) accelerations;

4) acceleration with gear shifting;

5) roll forward.

On their basis, the determination and assessment of the main indicators of the traction and speed properties of the vehicle is made.

When analyzing fuel efficiency of the car, a number of indicators and characteristics are calculated and built, including:

1) characteristics of fuel consumption during acceleration;

2) fuel-speed characteristics of acceleration;

3) fuel performance steady motion;

4) indicators of the fuel balance of the car;

5) indicators of operational fuel consumption.

CHAPTER 1. TRACTION-SPEED PROPERTIES OF THE VEHICLE

1.1. Calculation of traction forces and resistance to movement

Traffic vehicle is determined by the action of traction forces and resistance to motion. The sum of all the forces acting on the car expresses the power balance equations:

P i = P q + P o + P tr + P + P w + P j, (1.1)

where P i - indicator traction force, H;

R d, P o, P tr, P, P w, P j - respectively the resistance forces of the engine, auxiliary equipment, transmission, road, air and inertia, H.

The value of the indicator thrust force can be represented as the sum of two forces:

P i = P q + P e, (1.2)

where P e is the effective traction force, H.

The P e value is calculated by the formula:

where M e - effective engine torque, Nm;

r - radius of wheels, m

i - transmission ratio.

To determine the values of the effective torque of a carburetor engine with a given fuel supply, its speed characteristics are used, i.e. dependence of the effective torque on the crankshaft speed at different positions throttle... In its absence, the so-called single relative speed characteristic can be used. carburetor engines(Figure 1.1).

Figure 1.1. Single relative partial speed characteristic of carburetor auto engines

This characteristic makes it possible to determine the approximate value of the effective engine torque at various values of the crankshaft speed and throttle valve positions. To do this, it is enough to know the values of the effective engine torque (M N) and rotational speed of its shaft at maximum effective power (n N).

Torque value corresponding to maximum power (M N), can be calculated using the formula:

, (1.4)

where N e max is the maximum effective engine power, kW.

Taking a number of values of the crankshaft speed (Table 1.1), calculate the corresponding series of relative frequencies (n e / n N). Using the latter, according to Fig. 1.1 determine the corresponding series of values of the relative values of the torque (θ = M e / M N), after which the desired values are calculated by the formula: M e = M N θ. The M e values are summarized in table. 1.1.

Traction and speed properties of a car significantly depend on design factors. The engine type, transmission efficiency, transmission gear ratios, weight and streamlining of the vehicle have the greatest influence on the traction and speed properties.

Engine's type. A gasoline engine provides better traction and speed properties of a vehicle than a diesel engine under similar driving conditions and modes. This is due to the shape of the external speed characteristics of these engines.

In fig. 5.1 shows a graph of the power balance of the same car with different engines: with gasoline (curve N " t) and diesel (curve N " T). Maximum power values N max and speed v N at maximum power for both engines are the same.

From fig. 5.1 it is seen that Gas engine has a more convex external speed characteristic than diesel. This provides him with more power reserve. (N " h> N " s ) at the same speed, for example at speed v 1 . Consequently, a gasoline-powered vehicle can accelerate faster, climb steeper climbs, and tow heavier trailers than a diesel vehicle.

Transmission efficiency. This coefficient allows you to estimate the power loss in the transmission due to friction. Decrease in efficiency caused by an increase in friction power losses due to deterioration technical condition transmission mechanisms during operation, leads to a decrease in tractive force on the driving wheels of the car. As a result, the maximum vehicle speed and road resistance overcome by the vehicle are reduced.

Rice. 5.1. Power balance graph of a car with different engines:

N " t - gasoline engine; N " T - diesel; N " h, N " s – corresponding power reserve values at vehicle speed v 1 .

Transmission gear ratios. The maximum speed of the car depends significantly on the gear ratio of the main gear. The best final drive ratio is considered to be such that the car develops maximum speed and the engine reaches maximum power. An increase or decrease in the final drive ratio in comparison with the optimal one leads to a decrease in the maximum speed of the vehicle.

The gear ratio I of the gearbox affects the maximum resistance of the road that the vehicle can overcome with uniform movement, as well as the gear ratios of the intermediate gears of the gearbox.

An increase in the number of gears in a gearbox leads to a more complete use of engine power, an increase in the average speed of the vehicle and an increase in its traction and speed properties.

Additional gearboxes. An improvement in the traction and speed properties of a car can also be achieved by using, together with the main gearbox, additional gearboxes: a divider (multiplier), a demultiplier and a transfer case. Usually, additional gearboxes are two-stage and double the number of gears. In this case, the divider only expands the range of gear ratios, and the range multiplier and the transfer case increase their values. However, too many gears increase the weight and complexity of the gearbox and make driving more difficult.

Hydraulic transmission. This transmission provides ease of control, smooth acceleration and high cross-country ability of the vehicle. However, it worsens the traction and speed properties of the car, since its efficiency is lower than that of a mechanical stepped box gear.

Vehicle weight. An increase in vehicle mass leads to an increase in rolling resistance forces, lifting and acceleration. As a result, the traction and speed properties of the vehicle deteriorate.

Car streamlining... Streamlining has a significant impact on the traction and speed properties of the vehicle. With its deterioration, the reserve of tractive force decreases, which can be used to accelerate the car, overcome hills and tow trailers, increase the loss of power for air resistance and reduce the maximum speed of the car. So, for example, at a speed of 50 km / h, the power loss in a passenger car associated with overcoming air resistance is almost equal to the power loss due to the rolling resistance of the car when driving on a paved road.

Good streamlining of passenger cars is achieved by slightly tilting the roof of the body backward, using body sidewalls without abrupt transitions and a smooth bottom, installing a windshield and a radiator lining with an inclination and such an arrangement of protruding parts in which they do not go beyond the outer dimensions of the body.

All this makes it possible to reduce aerodynamic losses, especially when driving at high speeds, and also to improve the traction and speed properties of passenger cars.

In trucks, air resistance is reduced by using special fairings and covering the body with tarpaulins.

BRAKE PROPERTIES.

Definitions.

Braking - the creation of artificial resistance in order to reduce speed or keeping it stationary.

Braking properties - determine the maximum deceleration of the vehicle and the limit values of the external forces that hold the vehicle in place.

Braking mode - a mode in which braking torques are applied to the wheels.

Braking distances - way, passable by car from detection of interference by the driver to a complete stop of the car.

Braking properties - the most important determinants of traffic safety.

Modern braking properties are standardized by Rule No. 13 of the Inland Transport Committee of the United Nations Economic Commission for Europe (UNECE).

National standards of all UN member states are drawn up on the basis of these Rules.

A car must have several braking systems that perform different functions: working, parking, auxiliary and spare.

Working The braking system is the main braking system that provides the braking process under normal vehicle operating conditions. Working brakes brake system are the wheel brakes. These mechanisms are controlled by means of a pedal.

Parking the braking system is designed to keep the vehicle stationary. The brakes of this system are located either on one of the transmission shafts or in the wheels. In the latter case, the brakes of the working brake system are used, but with an additional control drive for the parking brake system. The parking brake system is operated manually. The parking brake actuator must be only mechanical.

Spare the braking system is used when the service braking system fails. In some cars, the parking brake system or an additional circuit of the working system performs the spare function.

Distinguish the following types of braking : emergency (emergency), service, braking on slopes.

Emergency braking is carried out by means of the service braking system with the maximum intensity for the given conditions. The number of emergency braking is 5 ... 10% of the total number of brakes.

Service braking is used to smoothly reduce the speed of the car or stop in a predetermined month

Estimated indicators.

The existing standards GOST 22895-77, GOST 25478-91 provide for the following braking performance indicators car:

j set - steady deceleration with constant pedal effort;

S t - the path traveled from the moment the pedal is pressed to the stop (stopping distance);

t cf - response time - from pressing the pedal to reaching j set. ;

Σ Р tor. - total braking force.

- specific braking force;

- coefficient of unevenness of braking forces;

Steady-State Speed Downhill V tast. when braking with a retarder;

The maximum slope h t max, on which the car is kept parking brake;

Deceleration provided by a spare brake system.

The standards for the braking properties of vehicles prescribed by the standard are shown in the table. ATC category designations:

M - passenger: M 1 - cars and buses with no more than 8 seats, M 2 - buses with more than 8 seats and a bore weight of up to 5 tons, M 3 - buses full weight more than 5 tons;

N - trucks and road trains: N 1 - with a total weight of up to 3.5 t, N 2 - over 3.5 t, N 3 - over 12 t;

O - trailers and semitrailers: O 1 - with a total mass of up to 0.75 t, O 2 - with a total mass of up to 3.5 t, O 3 - with a total mass of up to 10 t, O 4 - with a total mass of over 10 t.

The normative (quantitative) values of the estimated indicators for new (developed) cars are assigned in accordance with the categories.