Manufacturers use bench dynamometers, other than workshop roller dynamometers
Which of the dynamometers provides data close to the real ones, those that measure by the wheels or the engine? And where do the numbers provided by the automakers come from? – Vinicius de Andrade Rossello, São Paulo (SP).
In the evolution of the design of an engine, computer simulations are made to evaluate the power that should be generated. Once the theoretical requirements are met, a pre-set of engines are built, some of which will go into evaluations of performance and durability.
The tests are performed on fixed bench dynamometers under conditions determined by the standards that define from the fuel and the temperature of the intake air to the accessories to be coupled to the engine.
Current (NBR, ISO and SAE) standards for determining net power require that the engine be tested with all the accessories required for its operation, such as air filter, injection pump, fan, alternator and complete exhaust system.
In the past, the power advertised was just gross, which was obtained without these accessories. That’s why the Dodge Charger R / T had in the 70’s 215 hp, which was gross power (by the criterion of net power, it would have 145 hp). After a number of tests that generate statistical confidence in the values of power, torque and specific consumption, among others, they are homologated for commercialization.
The engine dynamometer (bench) and the chassis dynamometer (wheel) are separate test benches for different evaluations. The power and torque numbers released by the assemblers are obtained in bench dynamometer, in which the motor is measured in isolation, outside the vehicle. In it, the cooling and exhaust systems are those of the dynamometer itself and the measurement is made directly in the engine.
In the roll or chassis dynamometer the power is measured on the wheel and there the number is less, since there are losses generated by the transmission, by the friction of the tire on the floor, by the movement of the water pump of the car or by the restriction of the exhaust of the vehicle .
The friction losses are mainly in the gearbox (between 1.4% and 2% in average) and in the differential, with 13% when the engine is longitudinal front and the traction, rear, since the cardan shaft is perpendicular to the Semi-axles. In cars with transverse front engine, the losses in the transmission are smaller.
Wheel dynamometers can be programmed to simulate air resistance or greater aerodynamic pressure, but are very sensitive: if a tire in contact with the roller is poorly calibrated, the result may already have been compromised, as the resistance of the rollers to the effort To rotate them will be changed.
It should also be remembered that two cars with the same technical characteristics may have small power differences due to the variation that normally exists between one product and another manufactured on an assembly line.