According to the theory of a car, traction calculations are carried out to assess its traction and speed properties.

Traction calculations establish the relationship between the parameters of the car and its units on the one hand (car mass - G , transmission ratios - i, wheel rolling radius - r to etc.) and speed and traction properties of the machine: movement speed – Vi , traction force - R etc. with another.

Depending on what is specified in the traction calculation and what is determined, there can be two types traction calculations:

1. If the parameters of the machine are set and its speed and traction properties are determined, then the calculation will be verification.

2. If the speed and traction properties of the machine are set, and its parameters are determined, then the calculation will be design.

Verification traction calculation

Any task related to the determination of the traction and speed properties of a serial machine is the task of a verification traction calculation, even if this task concerns the determination of any private vehicle properties, for example, the maximum speed on a given road, the traction force on the hook, etc.

As a result of the verification traction calculation, it is possible to obtain general traction and speed properties (characteristics) car. In this case, a full verification traction calculation is performed.

Initial data of verification traction calculation. The following basic quantities should be set as the initial data for the verification calculation:

l. Weight (mass) of the vehicle: curb weight or gross weight (G).

2. Gross weight (mass) of the trailer (trailers) - G".

3. Wheel formula, wheel radii ( r o- free radius, r to- rolling radius).

4. Characteristics of the engine, taking into account losses in the engine installation.

For vehicles with hydromechanical transmission - operating characteristic engine units - hydrodynamic transformer.

5. Gear ratios at all gear stages and overall gear ratios (i ki , i o).

6. Coefficients of rotating masses (δ).

7. Parameters of the aerodynamic characteristic.

8. Road conditions for which traction calculation is made.

Verification Calculation Tasks. As a result of the verification traction calculation, the following quantities (parameters) should be found:

1. Movement speeds in given road conditions.

2. The maximum resistance that the car can overcome.

3. Free traction sips.

4. Injectivity parameters.

5. Braking parameters.

Verification charts. The results of the verification calculation can be expressed by the following graphical characteristics:

1. Traction characteristic (for vehicles with hydromechanical transmission - traction and economic characteristics).

2. Dynamic characteristic.

3. Graph of engine power usage.

4. Overclocking chart.

These characteristics can also be obtained empirically.

Thus, the traction-speed properties of a car should be understood as a set of properties that determine the possible ranges of speed changes and the maximum acceleration rates of the car when it is operating in traction mode in various road conditions.

The traction and speed properties of military motor vehicles (VAT) depend on its design and operational parameters, as well as road conditions and the environment. Thus, with a strict scientific approach to assessing the traction and speed properties of the BAT, a systematic research method is required to determine, analyze and evaluate the traction and speed properties in the driver-car-road-environment system. System analysis is the most modern method of research, forecasting and justification, currently used to improve existing and create new military vehicles (components - verification and design traction calculation). The emergence of system analysis is explained by the further complication of the tasks of improving existing and creating new technology, in the solution of which there was an objective need to establish, study, explain, manage and solve complex problems of interaction between man, technology, road and environment.

However, the systematic approach to solving complex problems of science and technology cannot be considered absolutely new, since this method was used by Galileo to explain the construction of the Universe; it was the systematic approach that allowed Newton to discover his famous laws; Darwin to develop a system of nature; Mendeleev to create the famous periodic system of elements, and Einstein - the theory of relativity.

An example of a modern systematic approach to solving complex problems of science and technology is the development and creation of manned spacecraft, the design of which takes into account the complex relationships between man, ship and space.

Thus, at present, we are not talking about the creation of this method, but about its further development and application to solve fundamental and applied problems.

An example of a systematic approach to solving problems of the theory and practice of military automotive technology is the development by Professor Antonov A.S. the theory of force flow, which makes it possible to analyze and synthesize complex mechanical, hydromechanical and electromechanical systems on a single methodological basis.

but individual elements of this complex system are probabilistic in nature and can be described mathematically with great difficulty. So, for example, despite the use of modern methods of system formalization, the use of modern computer technology and the availability of sufficient experimental material, it has not yet been possible to create a model of a car driver. In this regard, three-element (car - road - environment) or two-element (car - road) subsystems are distinguished from the general system and tasks are solved within their framework. Such an approach to solving scientific and applied problems is quite legitimate.

When completing a thesis, term papers, as well as in practical classes, students will solve applied problems in a two-element system - a car - a road, each element of which has its own characteristics and factors that have a significant impact on the traction and speed properties of the BAT and which, of course, must be taken into account.

So, these main design factors include:

The mass of the car;

Number of leading axles;

Arrangement of axles on the base of the car;

control scheme;

Type of wheel mover drive (differential, blocked, mixed) or transmission type;

Engine type and power;

drag area;

gear ratios, transfer box and main gear.

Main operating factors, affecting the traction-speed properties of the BAT, are;

Type of road and its characteristics;

The condition of the road surface;

The technical condition of the car;

Driver qualification.

To assess the traction and speed properties of military vehicles, generalized and single indicators .

As generalized indicators for assessing the traction-speed properties of the BAT, they are usually used average speed and dynamic factor . Both of these indicators take into account both design and operational factors.

The most common and sufficient for a comparative assessment are also the following single indicators of traction and speed properties:

1. Maximum speed.

2. Conditional maximum speed.

3. Acceleration time on the way 400 and 1000 m.

4. Acceleration time to set speed.

5. Speed ​​characteristic acceleration-run-out.

6. High-speed acceleration characteristic in top gear.

7. Speed ​​characteristic on a road with a variable longitudinal profile.

8. Minimum sustained speed.

9. The maximum climb.

10. Steady speed on long climbs.

11. Acceleration during acceleration.

12. Traction force on the hook. .

13. Length of dynamic climb. Generalized indicators are determined both by calculation and by experience.

Single indicators, as a rule, are determined empirically. However, some of the individual indicators can also be determined by calculation, in particular, when applying a dynamic characteristic for this.

So, for example, the average speed of movement (generalized parameter) can be determined by the following formula

where S d - the distance traveled by the car during non-stop movement, km;

t d - travel time, h

When solving tactical and technical problems during exercises, the average speed of movement can be calculated using the formula

, (62)

where K v 1 And K v 2 - coefficients obtained by experience. They characterize the driving conditions of the machine

For all-wheel drive vehicles moving on dirt roads, K v 1 \u003d 1.8-2 And K v 2 \u003d 0.4-0.45, while driving on the highway K v 2 \u003d 0.58 .

From the above formula (62) it follows that the higher the specific power (the ratio of the maximum engine power to gross weight cars or trains), the better the traction and speed properties of the car, the higher the average speed.

At present, the specific power four-wheel drive vehicles lies within: 10-13 hp/t for heavy-duty vehicles and 45-50 hp/t for command and light-duty vehicles. It is planned to increase the specific power of all-wheel drive vehicles entering the Armed Forces of the Russian Federation to 11 - 18hp/t The specific power of military tracked vehicles is currently 12-24 hp / t, it is planned to increase it to 25 hp / t.

It should be borne in mind that the traction and speed properties of the machine can be improved not only by increasing engine power, but also by improving the gearbox, transfer case, transmission as a whole, as well as the suspension system. This must be taken into account when developing proposals for improving the design of vehicles.

So, for example, a significant increase in the average speed of the machine can be obtained through the use of continuous-speed transmissions, including those with automatic gear shifting in an additional gearbox; through the use of control systems with several front, with several front and rear steered axles for multi-axle vehicles; regulators of brake vulture and anti-blocking systems; due to the kinematic (stepless) regulation of the turning radius of military tracked vehicles, etc. The most significant increase in average speeds, cross-country ability, controllability, stability, maneuverability, fuel efficiency, taking into account environmental requirements, can be obtained through the use of continuously variable transmissions.

At the same time, the practice of operating military vehicles shows that in most cases the speed of movement of military wheeled and tracked vehicles operating in difficult conditions is limited not only by traction and speed capabilities, but also by the maximum permissible overloads in terms of smoothness. Vibrations of the hull and wheels have a significant impact on the main tactical specifications and operational properties of the vehicle: the safety, serviceability and performance of the weapons and military equipment installed on the vehicle, reliability, working conditions of personnel, efficiency, speed, etc.

When operating a car on roads with large irregularities and, especially, off-road, the average speed is reduced by 50-60% compared to the corresponding indicators when working on good roads. In addition, it should also be taken into account that significant vibrations of the machine make it difficult for the crew to work, cause fatigue of the transported personnel and ultimately lead to a decrease in their performance.

MINISTRY OF AGRICULTURE AND

FOOD FOOD OF THE REPUBLIC OF BELARUS

EDUCATIONAL INSTITUTION

"BELARUSIAN STATE

AGRICULTURAL TECHNICAL UNIVERSITY

FACULTY OF RURAL MECHANIZATION

FARMS

Department "Tractors and cars"

COURSE PROJECT

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

On the topic: Traction and speed properties and fuel efficiency

car.

5th year student 45 groups

Snopkova A.A.

Head of CP

Minsk 2002.
Introduction.

1. Traction and speed properties of the car.

The traction and speed properties of a car are a set of properties that determine the possible ranges of speed changes and the limiting intensities of acceleration and deceleration of the car during its operation in traction mode in various road conditions.

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

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

1.1. Determining the power of a car engine.

The calculation is based on the nominal load capacity of the vehicle

Engine power

The dead weight of the vehicle is related to its rated load capacity by the dependence:

For an especially light vehicle = 0.7 ... 0.75.

The load-carrying capacity coefficient of a car significantly affects the dynamic and economic performance of the car: the larger it is, the better these indicators.

Air resistance depends on air density, coefficient

Car streamlining coefficient

The frontal surface area can be calculated using the formula:

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

main gear efficiency.

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

Number and dimensions of wheels (wheel diameter

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

to the front:

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

Based on load

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

Traction and speed properties are important in the operation of the car, since its 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 car increases.

3.1. Indicators of traction and speed properties

The main indicators that allow you to evaluate the traction and speed properties of the car are:

Maximum speed, km/h;

Minimum sustained 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 cf, m/s 2 ;

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

The length of the dynamically overcome rise (with acceleration) S j ,m;

Maximum hook pull (in low gear) R from , N.

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

3.2. Forces acting on a car while driving

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

Rice. 3.1. Forces acting on a car with a trailer when moving:but - on a horizontal road;b - on the rise;in - downhill

Some of these forces act in the direction of movement and are driving, others - against movement and are related to the forces of resistance to movement. Yes, power R X2 in traction mode, when power and torque are supplied to the drive wheels, it is directed in the direction of movement, and the forces R X1 and R in - against the movement. The force P p - a component of gravity - can be directed both in the direction of movement and against, depending on the conditions of the car's movement - 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 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 torque supplied to the driving wheels of the vehicle

Under operating conditions, the car can move in various modes. These modes include steady motion (uniform), acceleration (accelerated), braking (slow)

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

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

Rice. 3.2. Scheme for determining powerness and torque, supplysmoke from the engine to the leadingcar scaffolding:

D - engine; M - flywheel; T - transmission; 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 drive 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, driveline, final drive, etc.).

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

(3.1)

where BUT - kinetic energy of rotating parts.

We take into account that the expression for the kinetic energy has the form

Then the power cost

(3.2)

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

Part of this power is lost to overcome various resistances (friction) in the transmission. The specified power losses are estimated by the efficiency of the transmission tr.

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

(3.4)

Angular speed of the engine crankshaft

(3.5)

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

Transmission ratio

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

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

(3.6)

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

(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 drive wheels, imagine the power N count and N T , in expression (3.8) as products of the corresponding moments and angular velocities. As a result of this transformation, we get

(3.9)

We substitute expression (3.5) for the angular velocity of the crankshaft into formula (3.9) and, dividing both parts of the equation by to get

(3.10)

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

(3.11)

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

(3.12)

INTRODUCTION

The guidelines provide a method for calculating and analyzing the traction-speed properties and fuel efficiency of carburetor vehicles with a manual 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 these operational properties is indicated, recommendations are given for choosing a series technical parameters reflecting the design features of various vehicles, the 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 identify their dependence on the main factors of the vehicle design, its loading, road conditions and engine operation, i.e. solve the problems that are put before the student in the course work.

MAIN OBJECTIVES OF CALCULATION

When analyzing traction and high-speed properties of the car, the following characteristics of the car are calculated and constructed:

1) traction;

2) dynamic;

3) accelerations;

4) acceleration with gear shifting;

5) rolling.

On their basis, the determination and evaluation of the main indicators of the traction and speed properties of the car is carried out.

When analyzing fuel economy 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. DRIVING AND SPEED PROPERTIES OF THE VEHICLE

1.1. Calculation of traction forces and resistance to movement

The movement of a vehicle is determined by the action of traction forces and resistance to movement. The totality of all forces acting on the car expresses the force balance equations:

Р i = Р d + Р о + P tr + Р + P w + P j , (1.1)

where P i - indicator traction force, H;

R d, R 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:

Р i = Р d + Р e, (1.2)

where P e is the effective thrust force, H.

The value of P e is calculated by the formula:

where M e is the effective torque of the engine, Nm;

r - wheel radius, m

i - transmission ratio.

To determine the values ​​​​of the effective torque of a carburetor engine for a particular fuel supply, its speed characteristics, i.e. dependence of the effective torque on the crankshaft speed at various throttle positions. In its absence, the so-called unified relative speed characteristic can be used carburetor engines(fig.1.1).

Fig.1.1. Unified relative partial speed characteristic of carburetor motors

This characteristic makes it possible to determine the approximate values ​​of the effective torque of the engine at various values ​​​​of the crankshaft speed and throttle positions. To do this, it is enough to know the values ​​​​of the effective torque of the engine (MN) and the frequency of rotation of its shaft at maximum effective power (nN).

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

, (1.4)

where N e max - maximum effective engine power, kW.

Taking a number of values ​​of the frequency of rotation of the crankshaft (Table 1.1), calculate the corresponding number 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 values ​​of M e are summarized in Table. 1.1.

Introduction

Functional properties determine the ability of the car to effectively perform its main function - the transportation of people, goods, equipment, that is, they characterize the car as a vehicle. This group of properties, in particular, includes: traction-speed properties - the ability to move at a high average speed, accelerate intensively, overcome climbs; controllability and stability - the ability of the car to change (controllability) or maintain constant (stability) movement parameters (speed, acceleration, deceleration, direction of movement) in accordance with the actions of the driver; fuel efficiency -- travel fuel consumption under specified operating conditions; maneuverability - the ability to move in limited areas (for example, on narrow streets, courtyards, parking lots); patency - the ability to move in difficult road conditions (snow, mud, overcoming water obstacles, etc.) and off-road; smoothness - the ability to move on rough roads with acceptable level vibration impact on the driver, passengers and the car itself; reliability -- trouble-free operation, long service life, suitability for Maintenance and car repair. The traction and speed properties of the car determine the dynamism of movement, i.e., the ability to transport goods (passengers) at the highest average speed. They depend on the traction, braking properties of the car and its cross-country ability - the ability of the car to overcome impassability and difficult sections of roads.

Vehicle speed properties

The ability of the car to achieve high speed communication is characterized by high-speed properties. The indicator of speed properties is the maximum speed. In accordance with the equation of maximum speed on a horizontal section of the road, the equality of the traction force P t corresponds to the sum of the forces of rolling resistance R k and air resistance R v. To determine the maximum speed of the car, it is necessary to solve the force balance equation. A graphical way to solve it is shown in Fig. 1. On the graph in the coordinates speed V a - tractive force P t, four curves P t for different gears of a four-speed transmission and a curve for the sum of rolling resistance forces P k and air R v are plotted.

The point of intersection of the curve of change in the traction force P t in 4th gear with the total curve of the resistance forces P to + P in determines the maximum speed of the vehicle V max on a horizontal section.

When moving uphill, the lifting resistance force P p is added, therefore the curve P k + P in shifts upwards by the value of the lifting resistance force R pg. The maximum speed on the rise V Pmax in our case is determined by the point of intersection of the curve of change in the traction force P t in 3rd gear with the total curve of the resistance forces P k + P v + P p.

The traction force reserve res P T can be used to overcome the inertial force P and during acceleration: resP t = P and = P t - P c - P c.

Rice. one.

The value of acceleration j x , m/s 2 , is proportional to resP T and inversely proportional to the mass of the car M a, multiplied by the coefficient k j of accounting for rotating masses:

j x = res P t /M a,k j

The change in vehicle speed during acceleration is shown in fig. 2. The duration of acceleration characterizes the inertia of the car, which is proportional to the acceleration time constant T p. The value of T p is related to the maximum speed V max . During the time t \u003d T p, the car accelerates to a speed V T equal to 0.63 V max.

It turned out that the average speed of cars in free conditions coincides or is close to V T . This can be explained as follows. The difference between the maximum speed V max and the current speed V a is the speed reserve that the driver can use when overtaking. When the vehicle speed exceeds 0.63 V max , the driver begins to feel that, if necessary, he cannot increase the speed with the desired intensity. Therefore, the speed reserve res V without = V max - V T is the smallest safe reserve, and V T is the highest safe speed in free conditions.

Rice. 2.

The maximum speed V max, the safe speed V T and the acceleration time constant T p are indicators of the speed properties of the car. The safe speed V T can serve as a guide when choosing a vehicle speed in free traffic conditions. Values ​​V max , V T and T p for different models cars are given in table. 1. The acceleration time constant T p changes in proportion to the change in the mass of the car. Therefore, the intensity of acceleration truck and bus without load is much higher than with load.

Table 1.

Speed ​​properties indicators Vehicle(TC) of various categories with a gross weight

* Permitted maximum weight 3.5 ... 12 tons.

* * Permitted maximum weight over 12 tons.

The vehicle coasts out when the gear lever is moved to the neutral position. This movement is called rolling. In this case, the inertia force P and is the driving force, the equation takes the form:

P and \u003d M a j x \u003d - R K ± R p - R in

Dividing the left and right sides of the equation by M a, we obtain an expression for determining the magnitude of the deceleration during rollover J n:

J n \u003d (- R K ± R p - R c) / M a

It can be seen from the expression that the greater the mass of the car M a, the less deceleration and the longer the coasting time to a stop. The dependence of the speed V a on the time t during coasting is shown in fig. 3.

Fig.3.

As can be seen from the graph, the inertia of the car during coasting is characterized by the coasting time constant T n. Acceleration time constants T p and run-up T n are interconnected, since they depend on the mass of the car M a. The run-up time constant T n is approximately 1.5-2 times higher than the acceleration time constant T p. The more T n, the more part of the path you can coast, which is of great importance for reducing fuel consumption.