How to Match the Right Dynamometer to Your Application
The first question is:
What are you testing—engines directly out of the vehicle, or the complete vehicle?
If the answer is vehicles:
- Are you testing cars, motorcycles, trucks, ATVs, go-carts, something else, or a combination of these?
- Do you need four-wheel-drive, or will a two-wheel-drive dynamometer work for you?
- Which axle do you prefer: independent or interconnected?
- Do you want only power absorption, or do you need motoring capability?
With both engines and vehicles, the dynamometer selection is essentially based on horsepower range, the type of testing being conducted, and cost.
It is important to understand that horsepower is a calculation that reflects the relationship between torque and rpm throughout the operating range of an engine. Like engines, dynos are rated for torque and rpm.
To select the right engine dyno, you must first know the estimated peak torque of each engine that will be tested and at what rpm the torque is developed. You also need to know the maximum rpm the engines can achieve. This data is usually obtained through the engine manufacturer (or designer if it is custom-built). You can make a rough guess by knowing the types of engine you will be testing (gasoline, diesel, etc.), the size of the engine (displacement), and in what application they are used (cars, trucks, motorcycles, snowmobile, industrial, racing, on-highway, off-road, etc.).
Another good factor to consider is the type of engine testing you will do and the parameters associated with the test. A test profile or schedule can be created that contains the typical horsepower and torque values of your engines at the rpm ranges you expect to see those values. Those values can then be plotted on a “Horsepower and Torque versus RPM” graph, also known as a power curve.
After gathering this information, you can select a dynamometer to match those parameters. Each series and model has a different absorption characteristic and therefore a different power curve associated with its capabilities.
Dynamometer testing is basically broken into three types:
- An acceleration or sweep test is when the engine is allowed to accelerate from a specified lower “starting” rpm to a specified upper “ending” rpm. The same concept can be applied to a deceleration test when the test starts at a high rpm and ends at a lower speed. The dynamometer controls the rate of speed at which the engine accelerates or decelerates. Sweep tests are commonly used to create a performance or power curve of an engine.
- On a steady-state test, the dynamometer will hold the engine or vehicle at a specified speed, torque, or power for a specified period of time. Step tests and Break-in tests are forms of steady state tests.
- A transient or cyclical test is when the speed of the engine or vehicle
and the applied load varies in the test cycle. The difference is that a transient test has varying load points, while a cyclical test has repeating values. A common example of a transient test is a drive cycle used in vehicle emissions testing. An example of a cyclical test is a simulation of a vehicle going around a lap. In both cases, the dynamometer will vary the load on the engine to achieve the desired results.
Although most dynamometers can handle all three types, some are better suited than others for a particular type of testing:
- Water brake dynamometer can typically handle any engine and any type of test, but they are inherently slow to respond to load changes so they may not be suitable for fast transient testing.
- Electric dynamometers generally have high inertia which can factor into how they perform in acceleration tests.
- Water brake and eddy current dynamometers cannot realistically simulate how an engine coasts down when the throttle is reduced—only a motoring or inertia simulation dynamometer can do this.
The cost of a dynamometer is an important factor. Considering that the cost of a data acquisition and control system is about the same, the focus is on the power absorption unit.
- A motoring dynamometer (AC or DC) is the most versatile of the three main types of dynamometers and is also the most expensive. The main advantage of a motoring dyno is that it not only absorbs power but also drives the engine, therefore better simulating what the vehicle would do on the road or track. The value of a motoring dyno is dependent upon the size of the motor (horsepower rating) and the auxiliary equipment required for operation (a drive unit and regenerative device).
- Most eddy current dynamometers are water cooled, requiring a minimal amount of cold water (an air-cooled eddy current requires no water). However, the physical size of an eddy current absorber is proportionate to its power absorption capability, which the cost reflects (the bigger the unit, the more it costs). An eddy current dyno has a limit to the maximum power it can handle. Most eddy current absorbers are in the 100 to 500 horsepower range, but larger ones are available.
- For the size, a water brake dynamometer is generally economical in comparison to an electric dynamometer. A water brake absorber half the physical size of an eddy current absorber can typically handle three times the horsepower at one-fourth the price. However, a water brake absorber requires water—sometimes a large amount. For low power, infrequent use (two or three times a week), and short duration testing (less than one hour in a day), you may use only a few hundred gallons of water. But for high-power engines and long test days, you will need a large volume of water or a very elaborate cooling system which can escalate the cost of the complete system. Most water brake absorbers start at 500 hp and go up from there (at a fraction of the cost of a comparable eddy current). But when the water supply system is factored in, the overall cost almost evens out.