Engine test bench
Engine test stands are used to examine engines ( electric , hydraulic or internal combustion engines) regardless of their normal operating environment. If it only concerns the basic parameters of speed and torque , from which the power can then be calculated, one generally speaks of a performance test system. If the recording of measured values is further expanded ( fuel consumption , exhaust gas , noise, vibration , temperature behavior), the term test bench is used .
In addition to engine test benches, roller dynamometers are also used for vehicles , on which the entire vehicle can be checked. Brake test benches and transmission test benches (aggregate test benches for short) are used to test passive components and require a corresponding load machine .
Usage
An engine test bench can serve two purposes:
- Investigation of the engine function. The above parameters are measured and documented. The aim is either to check for compliance with specified target values (e.g. in production) or to optimize properties such as energy consumption and environmental pollution (in the development process).
- Investigation of engine durability. In this case, the motors are operated under extreme conditions in order to eliminate weak points in motor components in a significantly shorter time than normal.
function
Traditional engine test benches allow individual working points of the engine to be tested to be set stationary, with the parameters to be measured usually being entered in a speed / torque diagram. Brakes are sufficient here as relatively simple load machines, with the help of which one can set the torque to be applied by the test object (motor).
The boundary conditions to be observed today make it necessary, at least for the engine manufacturer, to run dynamic cycles ("driving curves") on test benches that approximate the engine's realistic operating conditions. Since rapid changes in torque and speed as well as a reversal of the torque direction (overrun mode) are necessary for this, only electrical load machines are used in this case.
Construction of a test bench
The drive machine to be tested is coupled with a braking device attached to the test stand (via drive shaft , cardan shaft , possibly gearbox ). The picture shows a traditional, but now outdated test bench setup:
- Torque balance (mechanical torque measuring device)
- DC pendulum machine (loading machine )
- Tachometer generator (speed measuring device)
- Opposed piston engine ( engine to be tested)
Loading machines
The loading machine must be designed in such a way that it can absorb the full power of the test object and absorb its maximum torque; it must be suitable for the maximum speed of the test object. It must be able to generate suitable speeds and torques for test conditions in overrun mode.
Passive load machines (brakes)
- Water vortex brakes (setting the torque by changing the water flow)
- Eddy current brakes (setting the torque by changing the current and thus the magnetic field)
- Hysteresis brakes
- Magnetic particle brakes
They only allow operation in two quadrants; H. the test object drives, the load machine brakes, in both directions of rotation.
Active exercise machines
Passive as well as active types are used on the engine test bench in generator mode (test specimen drives), while the active load machines are also used in overrun mode (test specimen is driven). The torque is set with the help of complex power electronics components. The frequency converters required for asynchronous machines in particular represent a high cost factor. Nevertheless, due to their good dynamic properties and their robustness, asynchronous machines are predominantly in use today. In the past, the opportunity was often used to control DC machines with the help of a Leonard theorem.
Measurement method
The very simple measurement of the speed on the test bench is nowadays usually done digitally with the help of incremental encoders , absolute encoders or resolvers . The latter are used in particular when the system is to be controlled down to a standstill, i.e. speed 0. Traditionally, tacho generators were used.
Even today, the torque is measured in many areas using a rotating load machine ( pendulum machine ). The stator is supported by a lever arm with a defined length on a load cell . In this way, it is possible to determine the load torque from the support force (reaction torque). However, this method has the serious disadvantages that the self-aligning bearing friction reduces the measured torque and the high mass of the stator acts like a low-pass filter and thus filters out high-frequency components. The mentioned disadvantages are solved with the increasingly used co-rotating torque measuring devices. These are arranged in the drive train between the test object and the load machine and are available as shafts, flanges, hubs, flexible couplings or hardy disks , with the torque measuring flange being the most widely used in test stands due to its small size, high rigidity and, depending on the measuring principle (shear or bending), high accuracy the automotive industry. The measurement signal is transmitted from the rotor to the stator telemetrically or, more rarely, via slip rings .
The optional exhaust measurement technology includes sampling at various points in the exhaust system. The gas is provided to several analyzers for measuring gaseous pollutants , for particles and possibly for other exhaust gas components via conditioned sample gas lines.
Test bench control
The test stand is controlled by influencing the speed and torque. It depends on the position of the characteristics in the speed / torque diagram which size is preferably achieved by changes to the load machine and which by changes to the test object. When testing internal combustion engines , the speed on the load machine and the torque are preferably specified by 'accelerating' on the internal combustion engine (mutual influence). Engine test benches usually build up resonances, which make measurements considerably more difficult. This can and must be counteracted by suitable measures such as flexible connecting elements made of elastomer / metal-elastomer.
Other components
Various units are also part of the equipment of engine test stands:
- Cooling and safety devices
- Exhaust or suction systems
- In development test stands or in engine series testing, horizontally or vertically arranged docking systems for the automated coupling of fluidic and electrical lines (fuel, oil, air, cooling water, exhaust gases, electrical plugs, etc.) can reduce the set-up time to a minimum.
- Modern engine test benches are equipped with a computer-controlled automation system for processing test runs and for automatic data acquisition.
- The measurement of the pressure in the combustion chamber synchronized with the crank angle (indicating device) plays a special role in combustion engines.
- Optional extras are the equipment of the test bench as an anechoic room ( soundproofed engine test bench ), as a climatic chamber (cold, heat), altitude test bench or swivel test bench .
Categories of engine test benches depending on their field of application
- Research and development of engines, typically in the development centers of automobile manufacturers
- Optimization / tuning of common engines, typically in service centers or in motorsport
- End-of-line inspection in production lines of automobile manufacturers
- special test stands for jet engines