Traction and speed properties of a car significantly depend on design factors. The engine type, transmission efficiency, transmission gear ratios, mass 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.
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 grades, 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 gear ratio of the main gear depends significantly maximum speed car. The following is considered optimal. ratio main gear, in which the car develops maximum speed, and the engine - 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 road resistance 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 performance. 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 demultiplier and transfer case increase their values. However, too many gears increase the weight and complexity of the gearbox and make it difficult to drive.
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 the mass of the car leads to an increase in the forces of rolling resistance, 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 power loss 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 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 grille with a slope 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 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 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 service 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 gross weight of up to 5 tons, M 3 - buses with a total weight of more than 5 tons;
N - trucks and road trains: N 1 - with a total weight of up to 3.5 tons, N 2 - over 3.5 tons, N 3 - over 12 tons;
О - trailers and semitrailers: О 1 - with a total weight of up to 0.75 t, О 2 - with a total weight of up to 3.5 tons, О 3 - with a total weight of up to 10 tons, О 4 - with a total weight of over 10 tons.
The normative (quantitative) values of the estimated indicators for new (developed) cars are assigned in accordance with the categories.
Traction and speed properties- a set of properties that determine the possible (according to the characteristics of the engine or the adhesion of the driving wheels to the road) ranges of changes in the speed of the vehicle in the traction mode in different road conditions.
Pulling is understood as such a mode of operation of the vehicle, in which power is supplied to its wheels from the engine sufficient to overcome the resistance to movement.
The speed properties of an ATS is its ability to deliver goods with a minimum amount of time.
This performance is one of the main ones. Usually, the higher the speed properties of the ATS, the greater its performance. Vehicle speed depends on many factors: engine power, gear ratios in the transmission, rolling resistance and air resistance, total vehicle weight, brake efficiency, steering, vehicle stability on the road, suspension softness and smoothness when driving on the road. flat road, cross-country ability when driving in difficult road conditions.
Traction and speed properties of vehicles are assessed by the following indicators: technical speed, maximum speed, conditional maximum speed, acceleration intensity and dynamic factor.
Technical speed- conditional average speed during movement.
V general view the technical speed of the vehicle that has traveled the distance during the continuous movement, which includes the time of situational stops (at traffic lights, railway crossings, etc.) can be represented by the formula:
The value of the technical speed most fully characterizes the speed properties of the vehicle when driving under certain operating conditions. It depends on the design of the rolling stock, its technical condition, the degree of use of the carrying capacity, road conditions, the intensity of the traffic flow, the qualifications of the driver, the characteristics of the cargo being transported, and the organization of transportation. Enhancement technical speeds traffic is one of the important tasks in organizing the transportation of goods, since the time of delivery of goods to consumers depends on its value.
Maximum speed- the most stable speed of the vehicle in the highest gear, measured when driving along a given rectilinear horizontal section of the road.
Conditional maximum speed- the average speed of passing the last 400 m when accelerating the car on a straight measuring section of the road with a length of 2000 m.
The maximum speed defines the limit of the speed capabilities of the PBX. One of the trends in the development of the automotive industry is the improvement of traction and speed properties, as evidenced by the higher values of the maximum speed and acceleration for each new generation of cars. Maximum speed of individual modern cars, determined by their technical characteristics, reaches 200 km / h and above.
At present, the minimum limits for the values of the maximum speeds for various types of vehicles have been established. So, for road trains, the permissible maximum speed of movement on the roads of Russia should not exceed: on highways - 90 km / h;
in settlements -60 km / h; outside settlements - 70 km / h.
Acceleration intensity- the vehicle's adaptability to fast starting and acceleration (increasing the speed of movement). This indicator is especially important in urban traffic conditions, as well as when overtaking on highways.
Dynamic factor allows you to evaluate the traction qualities (the possibility of realizing the speeds) of the vehicle for the cases of movement on roads with different resistance.
D = (Ptyagi - Psoprot) / Gfull
P rods = Mkrut * PP ch transmission * PP of short transmissions * Transmission efficiency / rolling radius
PP-gear ratio
The dynamic factor of vehicles intended for operation on roads of a particular technical category should be in higher gears not lower than the value of the total road resistances on slopes permissible on the roads of this category. The highest climb with a full load for cars should be at least 35, and for road trains 18% for lowest gear... The more dynamic a car is, the faster it can accelerate and move at a higher speed.
Traction and speed properties of the car are increased by improving the design of the engine, transmission and chassis, reducing the weight of the car and improving its streamlining. A car with relatively better traction and speed properties in real road conditions has a large reserve of power, which allows it to overcome the resistance to motion (rolling resistance forces, air, lift) without reducing speed or to accelerate.
INTRODUCTION
The guidelines provide a methodology for calculating and analyzing traction-speed properties and fuel efficiency of carburetor cars with a stepped 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
The movement of a vehicle is determined by the action of traction forces and resistance to movement. 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 value of P e 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 speed crankshaft at various 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
The specified 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 torque of the engine (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.
Wheeled vehicles of any type are designed to carry out transport work, i.e. for the transportation of payload. The ability of a machine to perform useful transport work is assessed by its traction and speed properties.
Traction - speed properties are a set of properties that determine the possible characteristics of the engine or the adhesion of the driving wheels to the road, the ranges of changes in the speed of movement and the limiting intensities of acceleration of the car when it is operating in traction mode in various road conditions.
A generalized indicator by which the speed properties of a wheeled vehicle can be most fully assessed; is the average speed of movement ().
The average speed of movement is the ratio of the distance traveled to the time of "pure" movement:
where is the distance traveled;
Time of clean movement of the car.
The average speed of movement is determined by road (ground) conditions and modes of movement of the machine.
Wheeled vehicles are characterized by the alternation of movement along the main highway with movement along dirt roads, or with off-road driving.
Speed modes can be divided into two types:
movement with a steady speed;
movement with unsteady speed.
Strictly speaking, the regime of the first type practically does not exist, since there are always at least small changes in resistance to movement on any roads (ups, downs, uneven road surfaces, etc.), causing a change in the speed of the car.
The mode of movement of the machine with a steady speed can be considered as conditional. This mode should be understood as one in which the speed changes are small relative to the average speed of movement on a given section of the path. In lower gears, such modes are even more absent.
In the general case, the high-speed modes of movement of the machine are composed of the following phases:
acceleration from standstill with gear shifting from a speed equal to zero to the final acceleration speed;
uniform motion with speeds that can be taken as steady and equal to the final acceleration speed;
deceleration from a speed equal to the final speed of acceleration or steady motion to the initial speed of deceleration;
deceleration from final deceleration speed to zero speed.
Currently, the speed properties of wheeled vehicles are checked in accordance with GOST 22576-90 " Motor vehicles, speed properties. Test methods ". The same standard defines the conditions and programs of control tests, as well as a set of measured parameters.
Tests to assess the speed properties of cars and road trains are carried out under normal load on a straight section of a horizontal road with a cement-concrete pavement. Its slopes should not exceed 0.5% and have a length of more than 50 m. Tests are carried out at a wind speed of no more than 3 m / s and an air temperature of 5 ... + 25 0 C.
The main estimated indicators of the speed properties of cars and road trains are:
maximum speed;
acceleration time to the set speed;
speed characteristic "Acceleration - run down";
speed characteristic "Acceleration in a gear that provides maximum speed."
Maximum vehicle speed Is the maximum speed developed on a horizontal flat section of the road.
It is determined by measuring the travel time by a car of a measured section of the road with a length of 1 km. Before driving to the measured section, the vehicle in the acceleration section must reach the maximum possible steady speed.
The speed characteristic "acceleration - coast down" is the dependence of the speed on the path and time of the vehicle acceleration from a standstill and coast to a stop.
Speed characteristic "acceleration - run down"
a) by time b) along the way; 2.3 - acceleration 1.4 - coasting
The characteristic "acceleration - run down" the resistance to movement of the vehicle is evaluated.
Speed characteristics "Acceleration in a gear that provides maximum speed" is the dependence of the vehicle speed on the path and acceleration time when the vehicle is moving in the highest and previous gears. Acceleration starts from the minimum stable speed for a given gear by abruptly pressing the fuel pedal all the way to the stop.
Speed characteristic "Acceleration in the highest gear".
a) by time b) along the way
The acceleration time in a given section (400m and 1000m), as well as the acceleration time to a given speed, are usually set according to the "acceleration - coast down" characteristic.
For trucks, the target speed is 80 km / h, and for cars, 100 km / h.
An estimated indicator of traction properties is the maximum angle of ascent overcome by a car with a full weight when driving on a dry, hard, even surface in the lowest gear in the gearbox and control gear.
In accordance with GOST B 25759-83 “Vehicles for multipurpose purposes. General technical requirements"- the maximum angle of ascent for four-wheel drive vehicles should be - 30 0 C.
This indicator is at the same time one of the estimated indicators of the vehicle's cross-country ability.
An indirect parameter that largely determines the level of traction properties of a car is the power density.
Power density is the ratio of the maximum engine power to the total mass of a vehicle or road train:
where is the maximum engine power, kW;
The mass of the vehicle and the trailer, respectively, t.
Specific power as an indicator characterizes the power-to-weight ratio of a car or road train. This indicator is especially important when comparing cars with each other. different types, as participants in a single traffic flow, in particular, automobile convoys.
For passenger cars, the specific power ranges from 40 - 60 kW / t, for wheeled trucks - 9.5 - 17.0 kW, for road trains - 7.5 - 8.0 kW / t.
Estimated characteristics of traction and speed properties of vehicles are determined during tests or can be obtained during traction calculations.
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 limiting road traffic conditions for each type of vehicle.
Traction-speed properties and indicators are determined when calculating the traction of the vehicle. The calculation object is truck low carrying capacity.
1.1. Determination of vehicle engine power.
The calculation is based on the rated carrying capacity of the vehicle.
in kg (weight 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
, necessary for the movement of a fully loaded vehicle at a speed in given road conditions, characterizing 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 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 performance.
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.
= maximum speed of movement on the road with 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 gear.
0,95*0,975=0,93.1.2. The choice of the wheel formula of the car and the geometric parameters of the wheels.
Number and dimensions of wheels (wheel diameter
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.
