Quote:
Originally Posted by Automobilist
Dyno don't lie...
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Well, actually, depending on the type of dyno, they don't tell the whole truth, either. Time for some basic engineering.
There are two (2) types of chassis dynamometers - inertial dynos such as the Dynojet used in this article, and load dynos such as the Mustang. Inertial dynos measure horsepower and calculate torque; load dynos measure torque and calculate horsepower.
Since an inertial dyno was used for this test, let's discuss some of its shortcomings in testing the towing performance of a diesel engine.
How Does an Inertial Dyno Work?
An inertial dyno utilizes rollers of known rotational inertia that the rear wheels of the vehicle tested ride upon. No external load is applied to these rollers; rather, the dyno measures the time required for the vehicle to accelerate the rollers from one speed to another. Rear wheel horsepower is a function of delta roller RPM divided by delta T (i.e., change in roller speed per unit of time). The change in speed actually represents two different levels of kinetic energy stored in the rollers, so the computer is really basing its horsepower calculations on the change in kinetic energy per unit of time - in other words, the more the kinetic energy of the rollers changes per millisecond, the more horsepower has been produced by the vehicle being tested to accelerate them.
What are the Weaknesses of an Inertial Dyno?
Measuring horsepower through measuring acceleration (as opposed to measuring steady state torque, as a load dyno does) does not reflect the steady state power output of an engine. An engine with higher internal rotating inertia (such as the Cummins) will test lower on an inertial dyno relative to a load dyno than will an engine with lower internal rotating inertia.
To explain, just as it takes power to accelerate the dyno's rollers, it also takes power to accelerate an engine's rotating components from starting to ending RPM on a dyno pull. The heavier and larger (greater mass and radius) the engine's rotating components, the more power will be required to accelerate them from, let's say, 1500 RPM to 3200 RPM on a dyno pull. This horsepower isn't available to accelerate the dyno rollers.
Although important for vehicle acceleration at a drag strip, the internal rotating inertia of an engine is irrelevant for steady state conditions such as holding a constant road and engine speed when climbing a hill while towing a heavy trailer. This characteristic is obviously more accurately measured with a load dyno than with an inertial (think acceleration) dyno.
Just to illustrate the above, something as minor as gear selection can affect an inertial dyno's results dramatically. I'll offer the case study of my previous 2002 Dodge 3500 dually equipped with the Cummins HO, NV5600 manual 6-speed transmission and 4.10 Dana 80 rear axle. This engine was originally rated at 245/505 at the flywheel. The engine in my truck was mildly modified for towing service with an Edge EZ, Diesel Dynamics Stage 2 fuel injectors, a low restriction air inlet system and a 4" exhaust.
We made a series of 6 dyno pulls with this truck on a Dynojet 246C inertial dyno (the same type used in this test), 3 pulls in 5th gear (direct) and 3 pulls in 6th gear (overdrive). In 5th gear, the truck averaged around 320/700 at the rear wheels, while in 6th gear, the truck produced 347/762 at the rear wheels.
So, why the difference? In both cases, the pulls started at 1500 engine RPM and ended at 3250 engine RPM (fuel shutoff). In 5th gear, the engine accelerated from 1500 to 3250 RPM more quickly - this required more internal horsepower that wasn't available to drive the rollers. In 6th gear, the overdrive ratio "loaded" the engine more such that it took longer to accelerate the rollers for the full pull, which reduced the rate of acceleration of the engine from 1500 to 3250 RPM (i.e., the engine consumed less horsepower internally).
This means that, all else being equal, a truck that can hold 6th gear on a Dynojet will produce higher numbers than one that downshifts to 5th or 4th gear. The difference, however, is primarily related to the engine's internal inertia (although turbo spoolup is a contributor) and isn't there under steady state conditions such as on a load dyno or towing up a steep grade.
A reader of an article such as the one presented would do well to keep the above in mind when trying to make sense of the numbers and what they mean to real world performance.
Rusty
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