Someone needs to calculate the power of the propulsion unit in order to calculate the car tax. It is important for some to independently calculate the compressor engine power. It is important for someone to know exactly the power of the machine in order to compare it with the one that was declared. In general, power calculation and engine selection are two inseparable processes.
These are not the only reasons why motorists try to independently calculate the engine power of their cars. This is quite difficult to do without the necessary formulas for calculation. They will be given in this article so that each motorist can calculate for himself how much is the real engine power of his car.
Introduction
There are at least four common methods for calculating the power of an internal combustion engine. In these methods, the following parameters of the propulsion unit are used:
- Turnovers.
- Volume.
- Torque.
- Effective pressure inside the combustion chamber.
For calculations, you need to know the weight of the car, as well as the acceleration time to 100 km / h.
Each of the following formulas for calculating engine power has a certain error and cannot give a 100% accurate result. It is always worth considering when analyzing the data.
If you calculate the power according to all the formulas that will be described in the article, you can find out the average value of the real motor power, and the discrepancy with the actual result will be no more than 10%.
If you do not take into account the various scientific subtleties associated with the definition of technical concepts, then we can say that power is the energy generated by the motor unit and converted into torque on the shaft. In this case, the power is a variable value, and its maximum value is achieved at a certain shaft rotation speed (indicated in the passport data).
In modern internal combustion engines, maximum power is achieved at 5.5-6.6 thousand revolutions per minute. It is observed at the highest average effective pressure in the cylinders. The magnitude of this pressure depends on the following parameters:
- fuel mixture quality;
- completeness of combustion;
- fuel loss.
Power, as a physical quantity, is measured in watts, and in the automotive industry it is measured in horsepower. The calculations described in the methods below will yield results in kilowatts, then they will need to be converted to horsepower using a special converter calculator.
Power through torque
One way to calculate power is to determine the dependence of motor torque on the number of revolutions.
Any moment in physics is a product of force on the shoulder of its application. Torque is the product of the force that the engine can develop to overcome the load resistance on the shoulder of its application. It is this parameter that determines how quickly the motor reaches its maximum power.
Torque can be defined as the ratio of the product of the working volume to the average effective pressure in the combustion chamber to 0.12566 (constant):
- M = (V working * P effective ) / 0.12566, where V working is the working volume of the motor [l], P effective is the effective pressure in the combustion chamber [bar].
Engine turns characterize the speed of rotation of the crankshaft.
Using the values of torque and engine speed, you can use the following formula for calculating engine power:
- P = (M * n) / 9549, where M is the torque [Nm], n is the shaft rotation speed [rpm], 9549 is the proportionality coefficient.
The calculated power is measured in kilowatts. To convert the calculated value to horsepower, you need to multiply the result by a proportionality coefficient of 1.36.
This method of calculation consists in using only two elementary formulas, therefore it is considered one of the simplest. True, you can do even easier and use the online calculator, in which you need to enter certain data about the car and its propulsion system.
It is worth noting that this formula for calculating engine power allows you to calculate only the power that is obtained at the engine output, and not the one that really incomes from the wheels of the car. What is the difference? While power (if you imagine it as a stream) reaches the wheels, it experiences losses in the transfer case, for example. Secondary consumers like an air conditioner or generator also play a significant role. We cannot but mention the loss of overcoming the resistance to lifting, rolling, as well as aerodynamic resistance.
Partially, this drawback is compensated by the use of other calculation formulas.
Power through engine displacement
It is not always possible to determine engine torque. Sometimes car owners do not even know the value of this parameter. In this case, the power of the propulsion unit can be found using the volume of the motor.
To do this, you need to multiply the volume of the unit by the frequency of rotation of the crankshaft, as well as the average effective pressure. The resulting value must be divided by 120:
- P = (V * n * P effective ) / 120 where V is the engine displacement [cm 3 ], n is the crankshaft rotation speed [rpm], P effective is the average effective pressure [MPA], 120 is a constant, and the proportionality coefficient.
So the calculation of the engine power of the car using the volume of the unit.
Most often, the effective P value in gasoline engines of a standard sample varies from 0.82 MPa to 0.85 MPa, in forced engines - 0.9 MPa, and in diesel units the pressure value is in the range from 0.9 MPa to 2.5 MPa .
When using this formula to calculate the real motor power, to convert kW to l. C., it is necessary to divide the resulting value by a factor equal to 0.735.
This calculation method is also far from the most complex and takes a minimum of time and effort.
Using this method, you can calculate the power of the pump motor.
Power through airflow
The power of the unit can also be determined by the air flow. True, this calculation method is available only to those car owners who have an on-board computer installed, which allows you to record the air flow at 5.5 thousand revolutions in third gear.
To obtain an approximate engine power, it is necessary to divide the flow rate obtained under the above conditions into three. The formula is as follows:
- P = G / 3, where G is the air flow rate.
This calculation characterizes the engine under ideal conditions, that is, without taking into account transmission losses, third-party consumers and aerodynamic drag. Actual power is lower than calculated by 10 or even 20%.
Accordingly, the air flow rate is determined in laboratory conditions on a special stand on which the car is mounted.
The readings of the airborne sensors are highly dependent on their contamination and calibration.
Therefore, calculating engine power based on data on air flow is far from the most accurate and efficient, but it is quite suitable for obtaining approximate data.
Power through the mass of cars and acceleration time to "hundreds"
Calculation using the weight of the car and its acceleration speed to 100 km / h is one of the simplest methods for calculating the real engine power, because the mass of the car and the declared acceleration time to “hundreds” are the passport parameters of the car.
This method is relevant for engines running on any type of fuel - gasoline, diesel fuel, gas - because it only takes into account acceleration dynamics.
When calculating, it is worth considering the weight of the vehicle with the driver. Also, in order to maximally approximate the result of calculations to the real one, it is worth taking into account the losses spent on braking, slipping, as well as the reaction speed of the gearbox. The type of drive also plays a role. For example, front-wheel drive cars lose about 0.5 seconds at the start, rear-wheel cars - from 0.3 seconds to 0.4 seconds.
It remains to find a calculator on the network to calculate the power of the car through the acceleration speed, enter the necessary data and get an answer. It makes no sense to give the mathematical calculations that the calculator makes, because of their complexity.
The result of the calculations will be one of the most accurate, close to real.
This method of calculating the real power of the car is considered by many to be the most convenient, because car owners will have to make a minimum of effort - measure the speed of acceleration to 100 km / h and enter additional data into the automatic calculator for the purity of the experiment.
Other types of engines
It is no secret that engines are used not only in automobiles, but also in industry and even in everyday life. Engines of various sizes can be found in factories - they drive shafts - as well as in household appliances like an automatic meat grinder.
Sometimes it is required to calculate the real power of such engines. How to do this is described below.
It is worth noting immediately that the calculation of the power of a 3-phase engine can be done as follows:
- P = M torque * n, where M torque is the torque and n is the shaft rotation speed.
Induction motor
An asynchronous unit is a device whose feature is that the frequency of rotation of the magnetic field created by its stator is always greater than the frequency of rotation of its rotor.
The principle of operation of an asynchronous machine is similar to the principle of operation of a transformer. The laws of electromagnetic induction are applied (the time-varying flux linkage of the winding induces an EMF in it) and Ampere (an electromagnetic force acts on a conductor of a certain length along which current flows in a field with a certain value of induction).
An induction motor generally consists of a stator, a rotor, a shaft and a support. The stator includes the following main components: winding, core, housing. The rotor consists of a core and a winding.
The main task of an induction motor is the conversion of electrical energy, which is supplied to the stator winding, into mechanical energy, which can be removed from a rotating shaft.
Induction motor power
In the technical field of science, three types of power are distinguished:
- full (indicated by the letter S);
- active (indicated by the letter P);
- reactive (indicated by the letter Q).
The total power can be represented as a vector, which has a real and imaginary part (it is worth recalling the section of mathematics associated with complex numbers).
The real part is the active power that is spent on useful work such as rotating the shaft, as well as heat.
The imaginary part is expressed by reactive power, which takes part in the creation of a magnetic flux (indicated by the letter F).
It is magnetic flux that underlies the principle of operation of an asynchronous unit, a synchronous motor, a direct current machine, and also a transformer.
Reactive power is used to charge capacitors, create a magnetic field around the chokes.
Active power is calculated as the product of current with voltage and power factor:
Reactive power is calculated as the product of current with voltage and power factor shifted in phase by 90 °. Otherwise, you can write:
The value of the total power, if you remember that it can be represented as a vector, can be calculated by the Pythagorean theorem as the root of the sum of the squares of the active and reactive power:
If we calculate the formula for the total power in general form, it turns out that S is the product of current and voltage:
The power factor cosφ is a quantity numerically equal to the ratio of the active component to apparent power. To find sinφ, knowing cosφ, you need to calculate the value of φ in degrees and find its sine.
This is a standard calculation of motor power in current and voltage.
Power calculation of a 3-phase asynchronous unit
To calculate the useful power on the stator winding of an asynchronous 3-phase motor, multiply the phase voltage by the phase current and the power factor, and multiply the obtained power value by three (by the number of phases):
- P stator = 3 * U f * I f * cosφ.
Power calculation e. an engine having an active nature, that is, power that is removed from the engine shaft, is produced as follows:
- P output = P stator - P loss .
The following losses occur in an induction motor:
- electrical in the stator winding;
- in stator core steel;
- electric in the rotor winding;
- mechanical;
- additional.
To calculate the power of a three-phase motor in a stator winding having a reactive nature, it is necessary to add the three components of this type of power, namely:
- reactive power spent on creating a stator winding scattering stream;
- reactive power spent on creating the dispersion flow of the rotor winding;
- reactive power spent on creating the main stream.
Reactive power in an induction motor is mainly spent on the creation of an alternating electromagnetic field, but part of the power is spent on the creation of dissipation flows. Scattering streams weaken the main magnetic flux and reduce the efficiency of the asynchronous unit.
Current power
The power calculation of an induction motor can be carried out using current data. To do this, follow these steps:
- Apply power to the engine.
- Using an ammeter, measure the current in each turn.
- Calculate the average current value based on the measurements made in the second paragraph.
- Multiply the average current by voltage. Get power.
Power can always be calculated as the product of current and voltage. It is important to know which values of U and I should be taken. In this case, U is the supply voltage, this is a constant value, and I can vary depending on which winding (stator or rotor) the current is measured, so it is necessary to choose its average value.
Dimensional Power
The stator has many different components, one of which is the core. To calculate engine power using dimensions, the following steps should be performed:
- Measure the length and diameter of the core.
- Calculate the constant C, which will be used in further calculations. C = (π * D * n) / (120 * f), where D is the core diameter, n is the shaft rotation speed, f is the voltage frequency (most often it is an industrial frequency of 50 Hz).
- Calculate the power P by the formula P = C * D 2 * l * n * 10 -6 , where C is the calculated constant, D is the core diameter, n is the shaft rotation speed, l is the core length.
It is better to make all measurements and calculations with maximum accuracy so that the calculation of the power of the electric motor is as close as possible to reality.
Traction power
The power of an induction motor can also be determined using the value of the traction force. To do this, you need to measure the radius of the core (the more precisely, the better), fix the speed with which the unit shaft rotates, and also measure the traction force of the engine with a dynamometer.
All data must be substituted in the following formula:
- P = 2 * π * F * n * r, where F is the traction force, n is the shaft rotation speed, r is the core radius.
The nuance of an induction motor
All the above formulas, which are used to calculate the power of a three-phase motor, allow us to make an important conclusion that the motors can be of different sizes, have different rotational speeds, but in the end have the same power.
This allows designers to create engine models that can be used in a wide variety of conditions.
DC motor
A DC motor is a machine that converts electrical power received from direct current into mechanical power. Its principle of operation has little to do with an asynchronous machine.
The DC motor consists of a stator, an armature and a support, as well as contact brushes and a collector.
A collector is a device that converts alternating current into direct current (and vice versa).
To calculate the useful power of such an aggregate, which is spent on any kind of work, just multiply the EMF of the armature by the armature current:
As you can see, the calculation of the power of a DC motor is much simpler than the calculations made in an induction motor.