PART 3 - Electronic Dynos
Story and Photos by John Copeland
Well, we're back, and this month we're going to talk some more about engine dynos and how they can be a part of your karting program. As karting gets more and more sophisticated, the need to be able to evaluate individual components, set-up changes, or engine adjustments becomes critical. Unless you have a track in your backyard where you can test anytime, there's almost never enough practice time at the track to do any detailed analysis of changes. Oh sure, you can change gears and check out the differences on the tach and the stopwatch, but with multiple turns, other karters on the track, and the dozens of other variables within a single lap, it can be hard to tell just what you gained and what you lost that combined into that particular stopwatch reading. A good dyno, properly used, is a testing advantage of epic proportions. No serious engine builder can afford to be without one these days. Not that a dyno can give you all the answers. There is still no substitute for track testing, but the dyno can definitely help you sort things out in before going to the track. The more you know when you get to the racetrack about what changes have what effects, the more productive you can make your track time. Over the years I've seen plenty of things that looked promising on the dyno turn out to be so-so on the track. There have been lots of things that worked on the dyno that just didn't work on the track. But, I've never seen anything that didn't work on the dyno, actually work on the track! That's important. Sometimes knowing what not to spend your track time working on is as important as anything.
Last time we talked about how engine dynos measured engine torque output by comparing it to a known resistance called the "load". And we discussed the most commonly used methods of applying that load, by pumping water or oil and restricting the output of the pump. By measuring the output pressure of the pump, or by measuring the twisting force on the pump housing, we can accurately measure the torque output of the engine at a particular RPM. But, as we saw, these 'fluid-brake" dynos have some shortcomings. Wouldn't it be nice if you could load the engine without fussing with all those hoses and valves? And what if you didn't have to worry about the fluid heating up and changing your readings? A tall order? It sure is, but the benefits are well worth it. What I talking about is using Electricity to load the engine. The concept couldn't be easier. By replacing the pump in the fluid-brake dyno set-up with a Direct-Current generator, we can accomplish the same effect, but with many advantages. Those of you who have generators that you take to the track already know that, when your generator is running and you plug something in and turn it on, the generator lugs down for a moment while it works harder to power whatever you're using. Or start up a gasoline powered arc welder and strike an arc and listen to that baby chug! Or, for that matter, think about how, when you turn on the headlights in your car or truck, the alternator pulls the engine speed down a little. The more output you demand of the generator, the harder the engine driving it has to work. Are you beginning to get the picture?
An electronic dyno uses the same principle, but with some interesting and beneficial twists. By driving the generator with your kart engine, we're generating electricity. The more electricity we generate, the harder the kart engine has to work, just like the fluid-brake dynos. But here it gets a little tricky. Just how do we get the generator to crank out more juice? One way is by drawing more current from the generator, like turning on the headlights of the car in that example. By varying the amount of electrical load on the generator, we can vary the mechanical load on the kart engine driving the generator. Let me inject a caution here; a generator of sufficient size to properly load a kart engine will put out a lot of electrical current. We're talking in the neighborhood of 50,000 to 60,000 watts. Mishandling this sort of current can hurt you. It can kill you. And switching these kinds of electrical loads on and off to vary the load on the engine can be dangerous, even with sophisticated equipment. Fortunately, there is another alternative. The output of a generator is not only governed by the amount of load, it can also be controlled by electrically exciting the field coils of the generator. That is, by controlling how much current is directed through the coils in the stationary, or field, coils in the generator housing, we can leave a constant high load on the output cables of the generator and still vary the load by dialing the current in the field coils up and down. On our dyno, we provide a constant load on the output by running the output cables to a bank of 4,500 watt resistors, cooled in a water tank. With 68,00 watts of resistance on the line all the time, all we have to do is adjust the current going to the field coils to control the load on the engine.
"That's all well and good", you say," but how does that measure the engine's output?" Well, now that we can control the load on the engine electrically, the electronic dyno works pretty much like a fluid brake unit. By mounting the generator on a set of precision bearings that allow the generator housing to rotate slightly, we can measure the twisting force, the torque, on the generator housing. Sound familiar? We could measure this torque with a spring scale, or with a hydraulic cylinder and a pressure gauge like the Stuska units do. But in keeping with the totally electronic scheme, we use a very sensitive electronic load cell on a torque arm.
Now I don't want to minimize the complexity of designing and building a dyno that is controlled this way. As I said before, we are dealing with some very dangerous output currents. Every precaution must be taken to safely manage this much electrical power. And the control circuitry required to safely and accurately vary the current going to the field coils is sophisticated. I am deeply indebted to my long time friend, dedicated karter, and electronic genius Jerry Culp, who designed and built our dyno and who has presided over it's redesign and improvement over the years. Without the help of someone who fully understands the intricacies of this sort of electronic control, this sort of project is extremely dangerous, and hopelessly complicated.
As I mentioned before, there are some real advantages to a totally electronic dyno. Certainly the elimination of the problems associated with pumping water or oil, the cooling issues, the nagging leaks and puddles, etc, are attractive. But there are also lots of less obvious pluses. Other than keeping tabs on the condition of the bearings, there is virtually nothing to wear out. And a generator large enough to absorb the output from a 2 cycle engine is a very beefy item. It requires a substantial structure to mount and support it, so the entire assembly will likely be stout enough to withstand the most vigorous continuous usage. You'll remember that we talked about how important it was for a good dyno to be user friendly? One really nice feature of an electronic dyno like we've described is that, by flipping a switch, you can turn the generator into an electric motor and use it to start the engine! Briggs, Yamaha, 820, or whatever, with this sort of setup you have push-button, one man electric starting. For intensive dyno work, it doesn't get much more user friendly than that. It's really not as tricky as it sounds. Take 110 volt AC current out of any wall outlet with sufficient capacity, pass it through a rectifier to convert it to DC current, step up the voltage through a transformer and feed it to the output leads of the generator. Voila! Your generator is now a DC motor and, with the current to the field coils turned all the way up, it will start your engine quite easily! A simple switching circuit to reverse the routing of the input and output leads of the generator/motor is all it takes. Here's another word of caution, however. Once the kart engine is running on the dyno, you must reduce the current going to the field coils to zero before switching the input/output circuit over to generator mode. Failure to do so will create a brief, but spectacular, dispute between the dyno and the local utility company. Lots of smoke and blue fire. Trust me, it's best to avoid this.
So let's assume that you've got your engine up and running on the dyno stand. How do you take good data? Like with any other dyno, you'll need to get the engine cleared out and running well, well into it's normal operating RPM range. Then, by smoothly adjusting the load, you move the engine through the desired RPM range, recording the torque readings as you go. Here's a place where automatic data collection becomes a real advantage. If your dyno set-up dictates manual data collection, you'll need to stabilize the engine at each desired RPM setting long enough to write down the torque reading. It's helpful to get the CHT and EGT readings too. With practice, this can be done quickly and efficiently. You don't want to just hold the engine at a given RPM for any length of time. At lower RPMs it builds too much heat, and at high RPMs it's just too hard on everything. Besides, it's not natural; it's not the way your engine runs at the track. Automatic data collection allows you to move your engine smoothly through the desired RPM range, not only minimizing temperature changes and the output changes they may cause, but also more closely simulating the action of the engine on the racetrack. Whether you're using a fluid brake dyno or an electronic unit, smooth, consistent control of the load is critical to accurate automatic data collection. For those of you who are anxious to eliminate every possible bit of error in your readings, you'll want to take date while you pull the engine down from maximum RPMs down to minimum, then reverse the procedure and take the RPMs back up. That way any inertia in the dyno itself, which might raise the torque readings on the way down, will be cancelled by the opposite inertial effect on the way back up. Next time we'll talk more about the ins and outs of automatic data collection and the pitfalls of using electronics (even your faithful Digatron) around the dyno.
Just a few more items for you to consider. A useful dyno facility, whether it's one commercially made or one you build yourself, will need some important support systems. If you're going to be running 2 cycles, you'll have to provide adequate cooling air for the engine. Even a 4 cycle doesn't really like running hard in dead air. And the rule for cooling fans is Bigger is definitely better. You can't imagine how much air you'll have to blow over a Yamaha to keep it at anything approaching normal operating temps when it's on the dyno. We've gone through several different set-ups trying to keep engine heat from limiting what we could do. Our current system uses a 1 ½ HP 220Volt motor driving a high speed industrial squirrel-cage fan to deliver 8600 cubic feet per minute at 55 miles per hour! And, while that's fine for a Yamaha, for a high horsepower engine like a 135cc Controlled or a 100cc Open, it's barely adequate. And whether you'll be testing 2 cycles or 4 cycles, you'll have to get rid of the exhaust. Again, a high capacity, high speed blower will do the job. And again, don't underestimate the amount of flow you'll need. Without the ability to keep the air around the dyno clean, neither you nor your engine will be able to work effectively for long. Pull too much exhaust gas into your carb intake, and your data is worthless. Pull too much exhaust into the dyno operator and, well.......
As I said before, next time we'll look at automatic data collection, then we'll try to analyze what the dyno data is trying to tell you. We'll look at what sort of changes in output you should see when you change various factors on the engine and how to interpret the results. Finally, we'll look at how you can make educated adjustments at the track based on fundamentally sound data from the dyno. See you next time.