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Car Dynamo Meter Guide

dynamometer or dyno for short, is an apparatus for measuring force, torque, or power. For instance, the power produced by an engine, motor or other rotating prime mover can be calculated by simultaneously measuring torque and rotational speed (RPM).

In addition to being used for determining the torque or power characteristics of a machine under test, dynamometers are employed in a number of other roles. In standard emissions testing cycles like the ones defined by the United States Environmental Protection Agency, dynamometers are used to provide simulated road loading of either the engine (using an engine dynamometer) or of full powertrain (using a chassis dynamometer). In fact, beyond simple power and torque measurements, dynamometers can be used like a part of testbed for a variety of engine development activities like the calibration of engine management controllers, detailed investigations into combustion behaviour, and tribology.

In the medical terminology, hand-held dynamometers are usually used for routine screening of grip and hand strength, and the initial and ongoing evaluation of patients with hand trauma or dysfunction. They are also used for measuring grip strength in patients where compromise of the cervical nerve roots or peripheral nerves is suspected.

In the rehabilitation, kinesiology, ergonomics realms and force dynamometers are used to measure the back, grip, arm, and/or leg strength of athletes, patients, and workers to evaluate physical status, performance, and task demands. Usually, the force applied to a lever or through a cable is measured and then converted to a moment of force by multiplying by the perpendicular distance from the force to the axis of the level.

Principles of operation of torque power (absorbing) dynamometers

An absorbing dynamometer acts like a load that is driven by the prime mover that is under test (e.g. Pelton wheel). The dynamometer should be able to operate at any speed and load to any level of torque that the test requires.

Absorbing dynamometers shouldn’t be confused with “inertia” dynamometers, which calculate power solely by measuring power required to accelerate a known mass drive roller and provide no variable load to the prime mover.

An absorption dynamometer is mostly equipped with some means of measuring the operating torque and speed.

The power absorption unit (PAU) of a dynamometer absorbs the power which is developed by the prime mover. This power absorbed by the dynamometer is then converted into heat, which usually dissipates into the ambient air or transfers to cooling water that dissipates into the air. Regenerative dynamometers, in which the prime mover drives a DC motor like a generator to create load, make excess DC power and potentially and using a DC/AC inverter can feed AC power back to the commercial electrical power grid.

Absorption dynamometers can be equipped with two types of control systems for providing different main test types.

Constant force

The dynamometer has a “braking” torque regulator which is a power absorption unit configured to provide a set braking force torque load, while the prime mover is configured to operate at any throttle opening, fuel delivery rate, or any other variable it is desired to test. The prime mover is then made to accelerate the engine through the desired speed or RPM range. Constant force test routines need the PAU to be set slightly torque deficient as referenced to prime mover output to allow some rate of acceleration. Power’s calculation based on rotational speed x torque x constant. The constant varies depending upon the units used.

Constant speed

If the dynamometer has a speed regulator (human or computer), the PAU gives a variable amount of braking force (torque) that is important for causing the prime mover to operate at the desired single test speed or RPM. The PAU braking load applied to the prime mover can be controlled manually or determined through a computer. Most systems employ eddy current, oil hydraulic, or DC motor produced loads due to their linear and quick load change abilities.

Power is calculated by rotational speed x torque x constant, with the constant varying with the output unit desired and the input units used.

motoring dynamometer acts as a motor that helps to drive the equipment under test. It must be able to drive the equipment at any amount of speed and develop any level of torque that the test requires. In common usage, AC or DC motors are used for driving the equipment or “load” device.

In most dynamometers power (P) is not measured in a direct manner. It must be calculated from torque (τ) and angular velocity (ω) values or force (F) and linear velocity (v):

{\displaystyle P=\tau \cdot \omega }or

{\displaystyle P=F\cdot v}

where

P is the power in watts

τ is the torque in newton metres

ω is the angular velocity in radians per second

F is the force in newtons

v is the linear velocity in metres per second

            Division by a conversion constant might be required, depending on the units of measure used.

{\displaystyle P_{\mathrm {hp} }={\tau _{\mathrm {lb\cdot ft} }\cdot \omega _{\mathrm {RPM} } \over 5252}}

For imperial units,

where

Php is the power in horsepower

τlb·ft is the torque in pound-feet

ωRPM is the rotational velocity in revolutions per minute

Where

PkW is the power in kilowatts

τN·m is the torque in newton metres

ωRPM is the rotational velocity in revolutions per minute

Detailed dynamometer description

Electrical dynamometer setup showing engine, torque measurement arrangement and tachometer

A dynamometer consists of an absorption (or absorber/driver) unit, and mostly has a means for measuring torque and rotational speed. An absorption unit has some type of rotor in a housing. The rotor is coupled to the engine or other equipment under test and is free to rotate at whatever speed which might be required for the test. Some means is provided to develop a braking torque which is in between the rotor and housing of the dynamometer. The means that for developing torque can be frictional, hydraulic, electromagnetic, or otherwise, according to the type of absorption/driver unit.

One means for measuring torque is mounting the dynamometer housing so that it is free to turn except as restrained by a torque arm. The housing can be made to rotate freely by using trunnions connected to each end of the housing to support it in pedestal-mounted trunnion bearings. The torque arm is connected to the dyno housing and a weighing scale is positioned to measure the force exerted by the dyno housing in attempting to rotate. The torque is the force shown by the scales multiplied by the length of the torque arm measured from the center of the dynamometer. A load cell transducer can be substituted for the scales to provide an electrical signal that is proportional to torque.

Another means to measure torque is by connecting the engine to the dynamo through a torque sensing coupling or torque transducer. A torque transducer gives an electrical signal that is proportional to the torque.

With electrical absorption units, it is possible to determine torque through measuring the current drawn (or generated) by the absorber/driver. This is mostly a less accurate method and not much practiced in modern times, but it might be adequate for some purposes.

When torque and speed signals are available, test data can be transmitted to a data acquisition system through it rather than being recorded manually. Speed and torque signals can also be recorded through a chart recorder or plotter.

Engine dynamometer


Horiba engine dynamometer Titan

An engine dynamometer measures power and torque directly through the engine’s crankshaft (or flywheel), when the engine is removed from the vehicle. These dynos do not account for power losses in the drivetrain, like the gearbox, transmission, and differential.

Chassis dynamometer (rolling road)


Saab 96 on chassis dynamometer

A chassis dynamometer, sometimes also called as rolling road, measures power delivered to the surface of the “drive roller” by the drive wheels. The vehicle is usually parked on the roller or rollers, which the car then turns, and the output measured thereby.

Modern roller-type chassis dyno systems use the “Salvisberg roller”, which helps to improve the traction and repeatability, as compared to the use of smooth or knurled drive rollers. Chassis dynamometers can be fixed or portable, and can do much more than just display RPM, horsepower, and torque. With modern electronics, quick reacting as well as low inertia dyno systems, it is now possible to get best power and the smoothest runs in real time.

Other types of chassis dynamometers are available that remove the potential for wheel slippage on old style drive rollers, attaching directly to the vehicle hubs for direct torque measurement from the axle.

Motor vehicle emissions development and homologation dynamometer test systems usually integrate emissions sampling, measurement, engine speed and load control, data acquisition, and safety monitoring into a complete test cell system. These test systems mostly include complex emissions sampling equipment (such as constant volume samplers and raw exhaust gas sample preparation systems) and analyzers. These analyzers are much more sensitive and quite faster than a typical portable exhaust gas analyzer. Response times of well under one second are common, and are needed by many transient test cycles. In retail settings it is also popular to tune the air-fuel ratio using a wideband oxygen sensor that is graphed along with the RPM.

Integration of the dynamometer control system with automatic calibration tools for engine system calibration is mostly found in development test cell systems. In these systems, the dynamometer load and engine speed are varied to many engine operating points, while selected engine management parameters are varied and the results are recorded automatically. Later analysis of this data might be used to generate engine calibration data used by the engine management software.

Because of frictional and mechanical losses in the various drivetrain parts, the measured rear wheel brake horsepower is mostly 15-20 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer.

Using the Dynamometer

  1. Getting ready: The car is positioned atop the rolling road having the drive wheels on the corresponding rollers. The vehicle’s brakes should be disengaged.
  2. Running the test: When all is set, the gas pedal needs to be pushed down. The engine roars, the vehicle’s wheels spin, and the heavy dyno barrel then turns slowly.
  3. What will be the result: The dynamometer’s barrels are hooked up to an advanced, performance-deciphering computer that absorbs and records power levels as they’re generated by the engine. The result is a list of performance scores, among them max horsepower and torque levels.

Video of the dyno test of a car

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