Chassis Dynamometers Theory
Methods of measuring the power and the torque of the engine with a chassis dynamometer
The problem of testing internal combustion engines is quite complex—the appropriate measurement of power and torque requires the consideration of many factors. Depending on our objective and how the chassis dynamometer is used, three basic solutions can be identified - inertial measurement (during acceleration), measurement in a load mode (during acceleration) and steady-state-measurement (braked). The dynamometers we manufacture can operate in all these configurations - it just depends on their options and accessories. Chassis dynamometers with brake or eddy current brakes will be referred to hereafter as braked dynos. Dynamometers operating in inertial mode and not equipped with a brake will be referred to as inertial dynos. Please note that all dynamometers we offer are equipped with lifters that can lift cars off the rollers and a roller locking brake, do not confuse a roller brake with an eddy current brake.
Inertia measurement (method of measurement possible on both an inertia dynamometer and a braked dynamometer)
Inertia measurement using a chassis dynamometer involves accelerating the car on the dynamometer and then, after deceleration, waiting for the car to stop without using the brake. The engine is loaded with the weight of the rollers and the rolling resistance of the drive mechanism. The measurement time is approximately 10-30 seconds of full load and a few minutes of free rolling on the dynamometer to a full stop. Power and torque are measured as functions of the car's acceleration on the rollers (wheel power/torque) and its deceleration (loss power/torque). The sum of these two values represents the power and torque of the engine.
Inertial measurement has several key advantages over braked measurement:
- it requires a shorter full load time - so is generally safer for the engine,
- does not require any expensive system to cool the car - a medium sized cooling fan is sufficient,
- is more accurate - as the load cell and eddy-current thermal characteristics degrade the measurement accuracy when measuring under load.
Inertial measurement also has several disadvantages:
- Powerful turbocharged engines can require a high load to build up a proper boost. A dynamometer with eddy-current brake(s) does not have this problem - the load can be increased at will, only slightly degrading accuracy.
- An inertial dynamometer does not allow the car to be tested at steady-state mode.
The measurement process on an inertial dynamometer is as follows:
- The vehicle enters the dynamometer. The lifter sets the vehicle on the rollers. The rollers are unlocked.
- The car is aligned on the dynamometer axis. It is then fastened to the test bench using straps.
- The gearbox and drive ratio are tested - this can be done in several ways. The information is entered into the dyno programme. Fans blowing air are switched on.
- After ensuring that the engine temperature is already correct, the driver accelerates the car in the highest possible gear (recommended), or another gear of choice, to the maximum rpm relevant for the measurement (usually - to the so-called cut-off).
- The driver depresses the clutch pedal (disengages), leaving the gear engaged. The dynamometer slows the car down until it stops. The result of the measurement is presented on the screen. The tuner can analyse the results obtained (power, torque, mechanical and rolling resistance, wheel power, results from dynamometer accessories, etc.) as a function of rpm.
Measurement in the dynamic-load mode (method of measurement possible with braked dynamometer)
Measurement in this mode is similar to measurement in inertial mode, except that the eddy current brake simulates a higher load. The load factor is a percentage set by the user in the dynamometer software.
Measurement in dynamic load mode has several significant advantages over inertial measurement:
- The user can individually select the load depending on the type and power of the engine being measured.
- Possibility to extend the measurement time.
- Possibility to accurately load powerful turbocharged engines in order to correctly "build up" the boost.
The measurement process in dynamic-load mode proceeds as follows:
- In dynamic-load mode, it is necessary to set an additional load in the dynamometer software (i.e. to increase the apparent inertia of the dynamometer by loading the car with eddy-current brake to an extent that does not balance its torque) before starting the measurement. This allows the load to be precisely matched to the engine power, e.g. to correctly 'build' and maintain boost.
- Once the load factor has been determined, the measurement procedure is the same as in the inertial mode.
Steady-state measurement (method of measurement possible with braked dynamometer)
Measurement at steady state RPM involves balancing the driving force of the car by the eddy current brake and calculating the engine power from the load cell data. The measurement time is approximately 10 s at full load (for the speed to stabilise and the result to be read out) for each measuring point (individual speed value).
Braked steady-state measurement - advantages and faults:
Companies researching new engine control systems, fuel injection systems, developing high-performance engines buy dynamometers equipped with an eddy-current brake from us. A major advantage of such a dynamometer is the ability to tune the car in real-time at selected points. However, it is important to bear in mind the limitations - especially thermal limitations. A vehicle with a full load produces the highest amount of power for the selected RPMs, and thus the amount of heat. No fan can replace a downwind air channel at around 200 km/h - after all, a car on the motorway is already moving in such a 'channel'. The car's behaviour under full load is only repeatable for a few tens of seconds. Such a measurement mode requires a suitable test method, effective cooling and some serious work on the part of the tester. Unfortunately, the engine, as a result of such work, heats up and changes its parameters (its energy conversion efficiency decreases), despite any intensive cooling.
The measurement process in steady-state mode runs as follows:
- The operator sets the measuring points in the software at which the values (RPM/speed) will be stabilised.
- The driver accelerates the car by fully pressing down the accelerator pedal (if the measurement is to be made for full throttle). The dynamometer then loads the car automatically until the RPMs/speed are equal to the set value and the vehicle neither accelerates nor decelerates. The power value, as measured by the load cell, is presented on the screen in real-time. The tuner can make changes during the measurement and immediately see the result of his work on the screen.
What does the power and torque measurement technology of our dynamometers offer you?
Rapid information gathering
The main disadvantage of the dyno solutions currently available on the market is their sampling frequency. Dyno graphs from many companies show a few to a dozen lines, from which the shape of their characteristics is estimated. Of course, smoothing the graph with curves is also possible, but can we call this the real result? Usually, power and torque are sampled on the dynamometer every 0.3-1.0 s. And the determined result is de facto the average power and average torque over this time interval. Subsequent measurements of rapidly increasing torque (especially in turbodiesels) can give different results, depending on how the sampling ranges overlap.
A few years ago, our company invented a new torque measurement technology that has found its way into both braked and inertial dynamometers. The procedure is based on our patented TrueForce™ technology, which allows for smooth torque measurement. Given the rate of change of the values measured on the dynamometer, the measurement is made in real time and with incredible accuracy. In fact, during each second of measurement, the dynamometer's speed sensor provides up to three thousand instantaneous torque and power second information. The error in the determination of the instantaneous torque is less than 0.00001 s. What is more, such accuracy is offered in the standard version. Changes in the graphs are seamless. Why? Because engine operation and torque changes are not abrupt either, due to its high inertia. The accuracy of the measurement is so high that the graph is smooth and the line does not oscillate. However, an interruption in the measurement or any minor disturbance is mercilessly and meticulously caught. The exact engine speed at which it occurred can be pinpointed. Subtle phenomena such as momentary resonances, wheel slip or a minimal drop in torque when boost pressure is stabilised can be observed.
What's more, even single misfires reduce the engine's torque and these are also evident, despite the presence of the flywheel! In addition, the results are repeatable in many tests (of course, if we take care to keep the engine at a constant temperature and do not repeat the tests one after the other too many times) - in the range we want. And this is entirely without averaging, recalculating or using imaginary percentages and ratios. No other solution offers such accuracy and precision - you have to see it to fully appreciate the TrueForce™ system's capabilities.
The considerable accuracy of the measurement and the high repetitiveness
An advantage resulting directly from our innovative measurement method is the high repeatability. Since the revolutions of the roll are measured with extreme accuracy, the torque/power measurements and the positioning of the points on the graph are also accurate. Due to the high-quality bearings, high inertia and precise balancing, the accuracy of the measurements is also higher than in other competitive solutions. In addition, the dynamometers are calibrated using a gravity torque standard to ensure that the absolute measurement values are read appropriately.
Easily expandable and modular solutions
Anyone deciding to spend at least several thousand euros on a device such as a dynamometer would probably want to know what to upgrade to next and when their needs increase.
Our solutions are completely modular, giving the option of upgrading the inertial dynamometer to a brake version and equipping the test bench with external sensors: temperature, pressure or mixture composition. All necessary dimensions and installation conditions for selected versions can be provided in advance in the form of blueprints. The user can therefore check the adequacy of the dynamometer room in terms of possible future expansion.
Popularity and credibility of the manufacturer
Our solution has been chosen by hundreds of companies in Europe - including many well-known and popular tuner and specialist companies. We are one of the largest manufacturers of dynamometers in Europe. The graphs created by our measurement systems can be seen everywhere - on the internet, in newspapers, in the hands of people testing their cars with our modular dynamometers. Our dynamometers can be found in virtually every European country, in the USA, Canada or the Far East. The results obtained with all of them are consistent as a result of the calibration carried out in our factory.
Impeccable craftmanship to enhance the owner's prestige
Our dynamometers are made from laser-cut steel components, covered with corrugated sheet metal covers and powder-coated. The rollers are made using an edgeless technique (so the welds are not visible). They feature TractionBoost knurling, chrome plating and are balanced to an accuracy of less than 1g. Each dynamometer is equipped with a pneumatic roller locking system both during entry and exit from the test stand. The control computer is housed in a professional industrial enclosure, as is the eddy current brake controller. The entire control is housed in a unique control panel. And all necessary components are included. The purchaser's address data can be present on printouts and protocols - and the diagram can have any logo as a background.
How to buy a dynamometer? What is the price of a dynamometer?
It is worth remembering that our product line is modular. You can invest a small amount, buy the main front module (single axle dynamometer) and start working with it. The dynamometer will start to pay for itself - and then at any time - upgrade to AWD (second set of rollers), eddy current brakes and additional equipment. There is no need to buy a whole 4x4 dynamometer (AWD) or eddy current brakes at once.
First you need to decide which dynamometer will best meet your needs, select accessories and determine the possibilities for future expansion. Then you need to make sure the dynamometer room is suitable for it and, based on the design we provide, plan its location, power supply, space for the control system and anchors to secure the car. If you have already decided to purchase the unit, you order the specified version (you sign a written order, binding both parties). You pay a deposit. We agree on a delivery time for the dynamometer - lead times range from 2 weeks to 2 months - depending on the complexity of the order. You have to prepare the foundations for it, according to our plans. The assembly and initialisation of the dynamometer take one to three working days (the dynamometer is already calibrated in the factory). Your staff is trained (instructions for using the dynamo are included with the unit). You can start measuring.
You can choose a few ways of financing the purchase. The simplest is to pay the advance payment and then a surcharge to the total price at the dyno installation.
We guarantee the correct and trouble-free operation of the modular chassis dynamometer and the non-defective operation of the mechanisms for a period of two years. After the warranty period, our company offers paid inspections by authorised personnel and the possibility to extend the warranty to 3 years.