The power of the internal combustion engine is a key parameter that motorists often mention when discussing the dynamic characteristics of the car. It depends on this figure how fast the car accelerates to a hundred and what maximum speed it can develop. However, behind the dry numbers in the passport hides the complex physical process of converting the thermal energy of combustion of fuel into mechanical work.
The question of how engineers get these values worries not only tuning enthusiasts, but also those who want to understand the real state of their own. unit. There are several ways to determine this value: from purely mathematical calculations at the design stage to practical measurements on specialized equipment. Each method has its own errors and features that can significantly affect the final result.
It is important to understand that the characteristics declared by the manufacturer are often different from those that can be removed on a particular instance in real conditions. This is influenced by many factors, from fuel quality and the state of the intake system to atmospheric pressure and air temperature. To understand the nuances of measurements is necessary to objectively assess the capabilities of the power unit.
Physical bases and units of measurement
Before we move on to measurement methods, we need to clearly define what we are measuring. In physics. capacity It is defined as the speed of the work. In the context of an engine, this means how much work a piston group is able to do in a unit of time. The main units of measurement in world practice are Watts (W) and kilowatts (kW), but in the automotive industry historically there is a tradition to use horsepower (hp).
There are several standards for defining “horsepower,” which is often confusing. The most common metric hp, which is approximately equal to 735.5 W. In English-speaking countries, especially in the United States and Great Britain, imperial L.S. is used. (hp) which is about 745.7 watts. The difference seems insignificant, but when recalculating powerful motors, it can reach several tens of units, which significantly affects the perception of characteristics.
Do not confuse power with torque. If the power shows how fast the engine can do the job, then the torque is the force with which the crankshaft rotates. It is the torque that creates traction that allows the car to move from a place or overcome climbs. These two parameters are inextricably linked by a mathematical dependence, where power is derived from the moment and frequency of rotation.
Formula for the connection of power and moment
Power (kW) = (Torque (Nm) × Turn (rpm) / 9549. This formula allows you to calculate one parameter, knowing the other two, and is the basis for the construction of external speed characteristics of the engine.
Calculation methods for power determination
In the design phase of the engine, when the physical sample does not yet exist, engineers use theoretical calculations. They are based on the geometric parameters of the cylinders, the pressure of gases in the combustion chamber and the estimated speed of rotation of the crankshaft. This method allows predicting the potential of the design, but always requires practical testing, since the actual combustion processes are more complex than ideal models.
The main calculation parameter is average effective pressure (MEP). Knowing the engine volume and the number of clocks, you can calculate the work done in one cycle. Multiplying this work by the number of cycles per minute, you get theoretical power. However, this method does not take into account mechanical friction losses in bearings, piston rings and gas distribution mechanism.
To obtain a real picture of the theoretical power subtract losses for the engine's own needs. This includes the work of a water pump, generator, air conditioner compressor and other attachments. That is why the power removed from the crankshaft is always less than the power developed by the gases inside the cylinders.
When calculating power, it is important to take into account the efficiency (efficiency) of the engine. In modern gasoline engines, it rarely exceeds 30-35%, the rest of the energy is lost in the form of heat.
Bench tests: methodology and equipment
The most accurate way to determine the real characteristics is to test on a motor bench. The engine is removed from the car and installed on a special frame, where it is connected to the car. brake-drive. This device creates a load that simulates the resistance of movement, and allows you to measure the torque on the shaft at different revolutions.
There are two main types of brake devices: hydraulic and electric. Hydraulic brakes use the resistance of water circulating in the rotor to absorb power. Electric brakes (dynamometers) work as generators, converting mechanical energy into electrical energy, which is then dissipated in the heating elements or returned to the grid. Electrical benches are considered more accurate and allow for testing in a wide range of modes.
During the tests, the engine undergoes the procedure of "run-in" on the stand, after which the external speed characteristic is removed. Measure torque with a fully open throttle, starting with minimum stable revolutions and up to maximum. Based on these data, a graph is constructed showing the dependence of power and moment on the frequency of rotation of the crankshaft.
Differences in standards: DIN, SAE, ECE and GOST
One of the main reasons for the discrepancy between passport data and real indicators is the different measurement standards. Manufacturers have used different techniques in different countries and years, making it difficult to compare engines directly. Understanding these differences is essential when choosing a car or assessing its tuning potential.
Below is a table comparing the main power measurement standards:
| Standard | Region of application | Features of measurement | Impact of hinged |
|---|---|---|---|
| DIN 70020 | Europe (old) | Engine with full attachment equipment | Accounted for |
| SAE Gross | United States (until 1972) | Engine without hinged, straight exhaust | Not counted. |
| SAE Net | United States (after 1972) | The engine is complete as in a car | Accounted for |
| ECE R85 | Europe (modern) | Strict conditions, accounting for all losses | Accounted for |
Standard SAE GrossThe engine, used in the U.S. during the era of the “mass car”, gave the highest figures, as the engine was tested without a generator, pump, silencer and air filter. This allowed American manufacturers to claim power in 400-500 hp, although the real power on the wheels was much lower. After the introduction of environmental regulations and the transition to the SAE Net standard, passport numbers fell, although the physical engines became more perfect.
Modern European Standard ECE R85 and its predecessor DIN require all the systems necessary for engine operation, including the intake and exhaust tract. This gives the most realistic picture, but the values are lower than those of older American engines. Russian GOST is largely harmonized with European standards, which simplifies the certification of imported equipment.
⚠️ Note: When comparing engines of different years of production, be sure to specify the standard for horsepower. The figure is 300 hp. in 1965 and 300 hp. Today, these are completely different indicators of real traction and efficiency.
Power measurement on the wheeled stand (Chassis Dyno)
Unlike a motor stand where the engine is tested separately from the transmission, a wheeled stand (Chassis Dyno) measures the power transmitted directly to the wheels. The car drives on drums that simulate the road surface. This method allows you to estimate losses in the entire transmission: clutch, gearbox, driveshaft, differentials and drives.
The process of measuring on the wheel stand is as follows: the car is securely fixed with belts, sensors are connected to monitor engine parameters (temperature, boost pressure, mixture composition) and accelerate the drums in full load mode. A computer system plots power and moment graphs, taking into account the inertia of the rotating masses.
The main advantage of the method is the ability to evaluate the engine in conjunction with a specific transmission and exhaust system. But there are also difficulties. It is necessary to know exactly the loss rate in the transmission, which varies from 10% for front-wheel drive cars to 20-25% for all-wheel drive with a complex transmission. Without correct recalculation it is impossible to get the engine power on the flywheel.
☑️ Preparation for measurement on the dynostende
It is important to note that the wheeled stand can reveal problems that are invisible with other diagnostic methods. For example, slipping clutch or incorrect operation of the machine will immediately be visible by the dips on the power chart. Also, this method is ideal for assessing the effectiveness of chip tuning and improvements to the intake system.
Factors affecting the accuracy of measurements
No single measurement method gives an absolutely perfect result, as many variables influence the process. Atmospheric pressure, air temperature and humidity directly affect the filling of cylinders with oxygen. The colder and denser the air, the more fuel can be burned and the higher the power will be. This is why the standards require that the results be brought to normal atmospheric conditions.
The engine temperature also plays a critical role. An overheated motor is prone to detonation, which causes the electronics to reduce the angle of ignition ahead and reduce power. A cold engine has large heat losses and a viscous oil that increases friction. Measurements should be made only after all systems have been temperature-regulation.
The quality of the fuel used is another important factor. The octane number and the actual energy value of gasoline can vary even within the same fueling network. The use of fuel with a low octane number on forced engines will lead to ignition correction and loss of declared characteristics.
⚠️ Note: When measuring on a street stand in windy weather, the results may be distorted. The headwind creates additional resistance, and gusts from the side can disrupt the cooling system or air intake.
To obtain comparable results, all measurements must be carried out in the same weather conditions and using the same brand of fuel.
The practical value of measurements for the owner of the car
Why would a typical owner know how to measure power? Understanding these processes helps to adequately assess the condition of the vehicle. If the acceleration dynamics worsened and fuel consumption increased, this may indicate problems with the engine or transmission. Measuring power on the stand in this case becomes the best diagnostic tool that allows you to localize the malfunction.
In addition, knowledge of the actual power is necessary when making changes to the design of the car. Installation of the turbine, new exhaust manifold or flashing of the control unit shall be controlled by measurements. This allows not only to ensure the growth of performance, but also to check the reliability of the engine in new modes.
In conclusion, it is worth saying that engine power is not just a marketing figure, but a complex engineering parameter that depends on many factors. Competent approach to its measurement and analysis allows you to use the vehicle resource as efficiently as possible and enjoy its dynamics.
Why does power fall with time?
Natural wear of the piston group, coking rings, pollution of nozzles and changes in gas distribution phases lead to a gradual decrease in engine power during operation.
What is the difference between power on a flywheel and power on wheels?
Power on the flywheel is an indicator taken directly from the crankshaft of the engine. Power on wheels is always less due to losses in the transmission (friction in the gears, rolling resistance of tires). The difference can range from 10% to 25% depending on the type of drive.
Can I increase the engine power without tuning?
It is impossible to significantly increase power without interference with the design or setting of the engine. However, using quality fuel, replacing the air filter and spark plugs can help to regain lost power percentages due to wear.
How often should I check the engine power?
For a conventional civilian vehicle, special power measurements are not required. The check is relevant when buying a used sports car, after overhauling the engine or when working on chip tuning.