Testing,testing…
Tales from a dyno cell
Some motorists are afraid to use full throttle for long periods in case they damage the engine, whereas others employ full throttle far too often for their own good.This raises the very interesting question, “How long can an engine run at full throttle without wearing out at a very fast rate or even selfdestructing?”
Without a load, the answer is “about three or four minutes, if that long”. With a load that restricts the engine to its designed rev range, the answer is “at least 500 hours, if not more. How do I know? I’ve been there, and I’ve got the T-shirt.
My first entry into the automotive manufacturing environment was as a dynamometer technician in the old Chrysler SA assembly plant at Elsies River, Cape Town. The dyno was an hydraulic unit, connected to the engine’s flywheel by means of a short shaft. Initially, my weekly routine consisted of selecting a Chrysler Valiant engine from the assembly line, performing a running-in procedure on the dyno, and then measuring the maximum power output.
The running-in procedure consisted of starting at low revs with a small load on the dyno, and then gradually increasing both until after nine-and-a-half hours the engine was running at full throttle with the dyno load adjusted to keep the revs at 3 600 r/min. This was the speed at which maximum power was developed.
The maximum power output curve was then obtained at full throttle by taking torque readings every 500 r/min from 1 500 r/min to 4 000 r/min.These “quality control” engines had to be test-run in the condition in which they were received from the engine line. This sometimes meant that the ignition timing was too far retarded.
All would appear to be well until the full throttle run at the end, but then the complete, full-length car exhaust system we were using would turn red hot – a graphic illustration of the evils of such a condition. Ignition timing that was too far advanced would introduce pinking, but I seldom received an engine with this fault.
Both errors would reduce the power output because the correct ignition timing, in the days before clean-air legislation, was the setting that gave the most power. Afterwards, I had to strip the engine completely and award demerits for any assembly mistakes.
The most commonly occuring fault was dirt in the bearings. What happened to these engines after the test? I assembled them, marked the engine block with a spot of white paint, and sent them back to the engine line. Was that fair?
Yes, because these engines were run-in scientifically. I can say this because we connected an airfl ow meter to the engine to measure the amount of blow-by (the gas that passes the rings) and drew a graph from the readings.
It invariably showed that after about eight hours the high initial readings would settle to a steady low reading, meaning that the engine was run-in. The blow-by will stay at this level for years until the gaps between the rings, pistons and bores start to grow larger.
This dyno was also used for endurance testing. Most engine components were expected to last 480 hours at full throttle, and I completed four such runs with local components.
The schedule called for 10 hours at each 400 r/min interval, starting at 2 000 r/min and ending at 4 000 r/min. This gives 60 hours, after which time the head was torqued down, tappets and ignition timing adjusted, and the oil and oil filter changed. Eight such cycles constituted a full endurance test.
I remember the first time I had an engine on the dyno at full load. The mechanical noise of the engine going full blast next to me was such that I could not imagine the engine lasting more than an hour.
But it did, and 480 hours later when I stripped it there was very little bore wear, but some of the big-end bearings were in need of replacement. When sitting close to an engine screaming away under load, it is very easy to start imagining abnormal noises.
On a number of occasions I imagined that the engine had started to knock. Of course, the power of suggestion is such that the moment I mentioned it to my assistant, he could also hear it. On two occasions we stopped the engine, dropped the sump and inspected all the bearings, but found no cause for the knocking. When we started the engine again, the knocking was gone.
We discussed the situation and formulated a plan. A flat surface will vibrate in various modes, depending on the amount of energy causing the vibration. Any one of these patterns will result in areas of more intense vibration, flanked by areas of less intense vibration.
Our dyno cell was on the second floor of a building, and the floor vibrated quite readily while an engine was on test. We therefore took off our shoes and walked around the engine from time to time to familiarise ourselves with the level of vibration. We found that we could clearly identify lines of greater or lesser vibration.
The next time the ghost vibration started we were able to establish that the floor did not amplify any knocking, allowing us to ignore the sound. It went away as soon as we started to think of something else…
WHY TEST?
Before WW2 engine design calculations were evaluated on slide rules, every part was drawn and dimensioned by hand, and prototype components were manufactured by hand.
Today, the availability of computers has made it possible to utilise design techniques such as finite element analysis (FEA), computer aided drawing (CAD), computer aided machining (CAM) and virtual imaging (VI). This has not only speeded-up the design process but also improved the mathematical models in use so that some of them are now very close to what happens in the real world.
This begs the question: Why test so extensively? Endurance testing, whether on the road or by means of dynamometers, is still needed for the following reasons:
1. To validate design criteria. Many of the mathematical models used in the design process require the input of coefficients whose values are based on assumptions. These often change over the years as new materials become available or as production processes change. The designer will only know if his assumptions have been correct when a new design passes all the endurance tests.
2. To take care of some aspect that had been forgotten, or not treated as important enough, in the design process. Even experienced engine designers have been known to slip up occasionally, and there is nothing like a full-throttle test to bring out any weakness in a new engine.
The virtual demise of the British motor industry has been attributed to a combination of factors, but anybody who has had an extended experience of these vehicles must agree that a lack of suitable testing was one of the more glaring faults that most of those designs exhibited.
ENGINE TESTING
Engine dynamometers are used extensively by the motor industry for endurance as well as performance testing.
Most modern engine designs have been subjected to at least 500 hours of full-throttle dyno testing under varying loads, as well as a further 500 hours of running on a dyno that is programmed to simulate severe traffic conditions.
Engine dynos are also used to validate day-to-day assembly quality, as Chrysler did in the story related earlier.
BASIC DYNO TYPES
An engine’s output can only be measured by giving it work to do, and measuring the result. These days there are a number of different designs available, but they all work on the above principle.
The Prony brake was designed by the Frenchman Gaspard de Prony in 1821. It’s beautifully simple and works well at low outputs. A very basic prony dyno consists essentially of a rope wrapped around the engine’s flywheel.
One end is attached to a vertical spring-balance mounted on a frame, and a pan that can accommodate a number of known weights, such as used with an old-fashioned beam balance, is fixed to the other end.
The engine is then run while different weights are tried in the pan until a combination of weights will allow the engine to run at full throttle at the test speed required.
The spring balance will pull upwards and the weights will pull in a downwards direction. This means that the difference between these two readings will be the tangential force at the flywheel rim. This result is then multiplied by the flywheel radius to give the torque from which the power can be calculated as soon as the revs have been measured.
Over the years I have constructed a number of small Prony dynos for outputs from a 0,5 kW (electric motor) to a 5 kW Lister water pump engine. There are two problems associated with such a dyno. The rope must be kept cool, because it will overheat and may burn, and the rope must be prevented from climbing off the flywheel.
Water cooling may be effective in the first case and wooden guides, attached to the rope, will help in the second. The revs can be measured with a mechanical hand-held tachometer that engages with the small indent in the centre of an engine’s flywheel or electric motor’s pulley.
The dyno I used at Chrysler was an hydraulic Heenan and Froude unit. The first ones were designed by William Froude (1810-1879), a famous naval architect and hydraulic engineer. It’s interesting to note that he died in Simon’s Town while a guest of the Royal Navy, and is buried there.
Such a dyno consists of a cast-iron flywheel equipped with specially-shaped vanes along the side that rotates inside a water-filled casing.
The casing is equipped with similar vanes on the inside. The rotation of the vanes in the water provide a resistance for the engine to work against. The load provided by the dyno is adjusted by operating a valve that changes the level of water inside the dyno.
The rotation of the dyno flywheel in the water applies a force to the casing that is mounted in bearings so that it is free to rotate, but restricted by an arm. The end of this arm butts against a load cell that provides a reading of the force on the casing.
This reading is multiplied by the length of the restraining arm to give the torque developed by the engine. This value is combined with a rev counter reading to calculate the power output.
