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Background So a friend and I are looking at building a dynamometer. We want to do it cheap. Like maybe under $2000 cheap. It might be unattainable and we’ve hit pretty severe limitations as a result. It’s an engine brake dyno for his various Volvo projects. Professional brakes are well over $10,000, approaching $20,000. The goals are 1500 foot pounds of torque rated for at least 7,000 RPM. At max rating for both, this is 2000hp. How Does a Dyno Work? Well a dyno does the opposite of an engine. An engine produces mechanical work in the way of torque. A dyno consumes work. It usually takes that torque and converts it to heat. Some types measure acceleration of a known mass. You can even use the car itself. The other main function of a dynamometer is to measure what it’s absorbing. This is typically done with a load cell to measure force acting upon the dyno. Types of Dynamometers Dynos are typically set up in 4 different ways. The most common is a chassis dyno. You park a car on a chassis dyno and place the driving wheels on rollers that go to a load that absorbs energy. Second most common is an engine brake dyno, this attaches to the crankshaft of an engine. Next is a hub dyno. You take the wheels off a car and mount the hubs to the dyno input. The least common is a hauler dyno. You pull a dyno with a car and it loads the car through the trailer wheels. There are different ways to generate a load. In no particular order these loading methods have pros and cons, whether they be cost, size, power limitations, or accuracy. Water Brake Dynamometer  The first type is a water brake. These are one of the more common types. They use a fluid coupling to generate resistance. You control the amount of resistance through coupling size and the amount of water in the coupling. The waste energy is heated water. These are great because they’re simple and have few moving parts, very controllable, and the brake itself is pretty small. The downside is that they require cold water and some laws require the heated waste water to be recirculated which requires large coolers. Eddy Current Dynamometer  An eddy current brake is another common type of brake. These use electromagnets acting upon a non-ferrous conductive rotor to create eddy currents to add a load to an engine via Faraday’s law. If you’ve ever seen the high school experiment of dropping a magnet down an aluminum or copper tube, you’ve seen faraday’s law at work. You control the amount of loading with the current running through the electromagnets. The waste energy is either heated air or heater water if its liquid cooled. They’re great because, like the water brake, they have few moving parts. They are very accurate and have the ability to be air cooled, reducing cooling system complexity. They can also have no wearing surfaces other than bearings making service intervals very long. The downside is that they can be very large and heavy, with 1000 foot pound models weighing close to a ton. They use a lot of electricity so they are often paired with alternators/generators or use industrial 3 phase. They’re also expensive compared to other types at similar specifications. Electric Brake Dynamometer  An electric brake is a rarer type of brake. These use electric generator motors to load the dyno. They’re typically not stand alone systems and often aid other types. The waste energy is heat through resistor banks. These are very controllable though not perfectly linear loading variability. They can also be wired as motors to spin the engine to calculate internal resistance of the engine or to simulate things like going down hills. The downside is that they’re limited to relatively low horsepower and a standalone electric motor brake would need to be massively large and expensive. Hydraulic Dynamometer  Hydraulic dynos are relatively rare. They use a hydraulic pump with a restriction to generate pressure which generates heat. They are generally pretty accurate and you control them with the size of the restriction. Cooling is a simple closed loop radiator. The downsides are the weight, required maintenance and can have many moving parts. Not to mention we would then be dealing with extremely high pressure cooling systems. They are relatively rare in higher horsepower applications as well, limited by pump size and cost. Pneumatic Dynamometer Pneumatic dynos are also relatively rare. Like a hydraulic dyno, they use a pump to absorb energy. They produce high pressure air which produces waste heat through adiabatic heating. They are accurate to measure load, but can have cyclic vibration through reciprocating parts, reducing accuracy. Loading is controlled with the size of the restriction in the air line. The pumps are a large size relative to other types. They can be air cooled as well. They have downsides, including a lot of moving parts, regular maintenance is needed, and they cannot absorb large amounts of power. They can also be pretty noisy loaded. Inertia Dynamometer  (This is a disk brake dyno, but the principle is similar) The last type is an inertia dyno. These use a mass, typically a flywheel, to generate a load through transient RPM. These are often paired with other load types as they have a practical upper limit in size. They can be equipped to all types of dynamometer layout and produce very fine data. They absorb energy though angular momentum and inertia. As a result, the energy must be mitigated through other brake types. They require balancing and occasionally gearboxes to achieve the necessary inertia to be effective. They can get very heavy close to the upper limits of practicality and can be very dangerous without a scatter shield or other protections to mitigate catastrophic failure. Bearings require more maintenance than other types due to velocities and masses involved, however more expensive pneumatic, hydraulic or magnetic bearings can be used. Our Decisions So Far Our torque and budget requirements make a few types impractical right off the bat. These are electric, hydraulic, and inertia. These are way too heavy AND expensive to build to the required spec. Non-starters. So this leaves water brake, eddy current, and pneumatic. The water brake is appealing because of its size, but looking into it, there aren’t any places to buy used brakes for cheap. We looked at repurposing a torque converter, but couldn’t find a way to modify it to be variably loaded. Cooling towers are an issue for a home application as well. There is still a commercially available option to repurpose: fire engine water pumps. They can run close to 500hp. This presents infrastructure and cost problems when you start moving Olympic size swimming pools in minutes, though. We looked into eddy current heavily. For a while, it seemed like the most viable. Eddy current brakes have an application beyond dynos. They’re used as hill brakes for busses, trucks, mining equipment, and construction equipment. They’re known as electromagnetic retarders, or just electric retarders for short. They’re mounted on the drive axle or integrated into the differential. They come in all sorts of flavors from a couple hundred foot pounds to over 3000 foot pounds. They’re air cooled which is a huge draw for a home application. Cherry picking from Telma’s website, I was able to pull up the spec sheets and reverse engineer the retarder to make our own down to winding the wire for the electromagnets. We reached a point where it got too expensive to make, but if we find one at a junk yard we can repurpose it on the cheap. They’re rated to 3000 RPM so a pulley and jackshaft system is required along with a couple of retarders are needed to meet the torque requirement of 1500 foot pounds.  The last option, admittedly thought of recently, seemed viable. We need a huge air pump. Industrial pumps are far too expensive and heavy for the necessary torque. So what are we to do? Well I remembered that engines themselves are air pumps. We could take a lovely diesel V8 and reconfigure the valve train, or remove the valve train all together and install reed valves. However after calculation, a 7L diesel would only make enough compressed air to load ~80hp. So that’s where we are now. We have 3 not so great options. We have to see which path offers the least resistance to get to our goals, but it will likely need a revisit to scale down the requirements. Instead of power goals, we may just pick whichever design can be scaled up the most for the budget. I’m using this thread as a sort of void to post into. I don’t expect crazy deep engineering discussions, but more than one mind on a problem typically yields faster results. My apologies if I got anything wrong in this post, feel free to draw attention to them. I’ll post the napkin math in the next few posts. Thanks for reading my crazy ranting.
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Oct 3, 2019 02:35
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| # ? May 6, 2024 13:40 |
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GRM genius/crazy fujioko built an engine dyno at home. It's pretty slick, although all the images are broken now.
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Oct 3, 2019 02:47
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You want to page mekilljoydamnit - he has ideas on this subject
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Oct 3, 2019 03:38
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Seat Safety Switch posted:GRM genius/crazy fujioko built an engine dyno at home. It's pretty slick, although all the images are broken now. That dyno is adorable. But it is good practice in integrated circuitry, a bridge I have yet to cross. CAT INTERCEPTOR posted:You want to page mekilljoydamnit - he has ideas on this subject I tried PMing him. Doesn't look like he has it. *~*~*~*~ Blissful and starry eyed, I began looking at retarders as a relatively cheap and easy way to generate a lot of load. They’re common on dynamometers so I know as a concept they’re viable. I am unable to distinguish a design difference between an eddy current brake and electromagnetic retarder for trucks, so I decided to reverse engineer one. Telma by far had the largest selection of retarders and the most detail in specs. Looking at their website, they have two kinds. The first is one that has a driveshaft input on one side and an output on the opposite. The other has a complicated case shape and flanges to integrate into the differential. These are known as axial and focal, respectively by Telma. I could immediately eliminate focal type retarders. Looking at the axial retarders, I began populating a spreadsheet with their specs to make a neat one-glance chart to make the selection easier.  As you can see, there is a 1475 foot-pound model. So the AD7 was selected as the model to reverse engineer. Now we go to their technical documents page. We need information on layout and electrical specifications. Again, I stuffed this into a spreadsheet.   I am only currently interested in the last 3 specs on the table. I need to define the electromagnet to get identical performance. Knowing resistance means I can determine wire length. This spec is usually listed for troubleshooting to ensure there isn’t a short in a coil. Now I have to define magnet wire performance. Looking up a manufacturers tables, I again pulled all the data into a spreadsheet. This one is too large to display as there are 1677 data points. But the data included wire gauge and its sizes, density, resistance per foot. From there I was able to make some purdy graphs.    Those intercept equations are valuable for a spreadsheet calculator to determine what gauge of wire I would need to match the resistance of Telma’s retarder. I would also calculate how much it weighed and the physical attributes of the coil itself.  The rest is just plug and play equations. After which I learn that Telma’s wire is most likely 10 gauge and they use 396 feet of it. Scale measuring the iron core of the magnet, I get a minimum diameter and the length give me an outer diameter. Using the drawings, I can estimate its length. Fully defining the dimensions, I model it quickly to give me a reference and to use in future modeling of the other parts of the retarder.  This coil is the main driver for the size of the retarder. The rest of it is constrained around it. Now fully defined we can look up prices for wire. After looking at the finest the Chinese have to offer, it turns out that, oops, it's $600 just for the windings. There are two retarders needed for 1500 ft*lbs at 7000 rpm. 
um excuse me fucked around with this message at 02:40 on Oct 4, 2019 |
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Oct 4, 2019 02:33
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um excuse me posted:
He's been summoned via the AI Slack channel - he talks about this very topic in there
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Oct 4, 2019 02:54
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What the poo poo? I should have PM. I'll dig in with my thoughts in the morning - imo water brakes are much less bad than you think.
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Oct 4, 2019 02:56
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I was on mobile, poo poo's all weird to use. I couldn't tag you in the address line. But welcome! I look forward to your thoughts.
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Oct 4, 2019 02:58
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OK; at power dissipation levels you're talking about poo poo's pretty serious. But the thing to think about is that you're not running continuously - you're probably not trying to dissipate 2000hp or so for hours on end. This gives you options. Water brakes like the water not too hot to avoid flashing it to steam and there's various guidelines and stuff about input temperature and all that, but one of the guidelines I came across is 5.5 gallons per HP per hour. So if the running time is say, 5 minutes at full power, you can go on craigslist and get 4 300 gallon IBC containers for about $10-15 each. In reality you're mostly going to be doing tuning around part throttle so that'll last you for a good while, and each full throttle run will only have a bit of time at maximum power. And after a few of those, you may be making mechanical changes anyway, so leaving everything overnight to cool off is fine. Need more test time, go on craigslist and gets some more IBCs. The brake itself is ideally the most inefficient pump possible - it's basically a paddlewheel in a housing with other stationary paddlewheels and the water just kinda bangs about then drops out the bottom. I was going to make one out of a steel weldment before I found a Stuska water brake on a bankruptcy auction. If you have machining capability it's not hard. It's not the best and it's sketchy but it's doable. You're in the range of a dual rotor Stuska which is just two of what I designed back to back.  There's the CAD model of the homebrew one I came up with - I could probably whip up a BOM and drawings pretty quick. I designed it to be made with a manual lathe, mill, welders and a key slot cutter. I had, I forget, something like $50 of steel into the rotor.  There's the inside of a real Stuska, with real corrosion. My guesstimate dimensions were about right. The seals are kind of interesting but they're a catalog item at Zoro, and the bearings are nothing special. I got that and associated stuff (mounting table, pump) off a bankruptcy auction for under a thousand. At the budget level you're looking at, you're probably not doing closed loop control. This is where water brakes are nice - if you have fixed water output, the torque resistance with flow increases with RPM, which means that they inherently don't want to run away, so you can actually get away with just using a hand operated valve. You need about a gallon per minute of pump capacity for every 10hp (roughly, ish) so if you're going on the extreme cheap, look on craigslist for a gas operated trash pump. So the water brake torque capacity is determined by how much water is in there - it's not necessarily flow, you just need enough flow to keep the outlet water from getting too hot. So there's various thoughts on how to do this - Stuska puts an orifice or valve on the outlet for "power capacity" and meters how much is flowing in for torque control, Superflow does it the other way around I think. All gets to the same idea. That's a quick rambling thought. I think the water brake is the best non-terrible way (terrible would be "getting semi truck brakes and spraying the outside of the drum with water") to do this, but I'm biased by being a mechanical engineer and not an electrical one.
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Oct 4, 2019 13:02
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Might know a thing or two about mechanical engineering myself, if it isn't showing yet. The water brake is still an option, but only with a stroke of luck. I have to find a pump the correct size for roughly half the budget. Retarders have more potential for the money which is the only reason I've been putting my efforts that way. I am also revisiting the torque converter retrofit. Its usually the body that spins, but in a dyno it would have to be stationary to feed it water. Not sure it works as well with the opposite parts rotating/stationary. *~*~* Last we left our saga, we had just figured out the dimensions for the coils that are used to drive the eddy brake. While very expensive for the limited budget available, that isn’t to say that we should try and get the whole picture before eliminating it as a potential option. Onwards and upwards. This post will focus on the rotor selection process I have been going through. Unfortunately, there isn’t as much engineering involved here because the budget constrains us to work with off the shelf parts to try and cobble a solution together. In photos of retarders you can see this is a large metal disk with aggressive cooling vanes. My first thought is to use a brake rotor.   Twins, right? However, this isn’t as simple as it seems. Eddy current strength is dependent on a materials electrical resistivity, measures in ohm-meters. Luckily this is well established information. Brake rotors come in a variety of different materials. An ideal material is aluminum, with resistivity in the low x10^-8 values. But you will only find those in rare applications not suitable for the heat we want to throw at it. Cast iron and carbon steels come are resistive in the order of low x10^-7. 10x less conductive, but it is better than stainless which is about 3x worse than carbon steel. I am reasonable sure carbon steel is what Telma uses. There is a chance they could use grain oriented electrical steel, but I doubt it considering the cost for such a large part. Now we need to figure out how fast we can spin the rotor. This will just be a sanity test, using transitive logic. My car, of 155mph top speed, spins under 3000 rpm. So that is what we’ll use. Strangely, or maybe not so strangely now that we’ve look at it, the AD7 retarder we selected has the same RPM limits. We may be able to go higher with some FEA modeling. I have a list of vehicles that may contain the retarder I'm looking for, but I left it at work like an idiot. So expect that Monday.
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Oct 5, 2019 00:37
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https://www.ebay.com/itm/Stuska-42634-SL-engine-dyno-dynamometer-800-HP-rotor-450-HP-display/323935704407 Not your full target, but a starting point.
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Oct 6, 2019 15:29
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mekilljoydammit posted:https://www.ebay.com/itm/Stuska-42634-SL-engine-dyno-dynamometer-800-HP-rotor-450-HP-display/323935704407 Seems steep. Since my last post we found an 800 ft*lbs water brake for $1200 and a truck retarder for $1000. My friend is in charge of acquisitions, so I guess we'll see what he does, if anything. ~~~ Today we take a turn and will take a look at why we’re focusing on an eddy current brake. It wasn’t because it was the best option more than the other options proved impractical. This post will look at the air pump solution. Air pumps require a load to pressurize air. In principle, this makes it possible to use an air pump as the load source for a dynamometer. When it comes to air pumps vs fluid pumps, air is always bigger. You need more air to reach a certain pressure and you need larger coolers to transfer heat. Hydraulic pumps the size we needed were feasible, just not at the cost we wanted, so we considered air as a cost effective alternative. Industrial air pumps used to supply shop air for manufacturing facilities were in the range of size we needed, but like hydraulic, nowhere near the correct cost. So naturally we looked into DIY solutions. An idea comes to mind. Engines are air pumps. I mentioned this, and the ultimate conclusion in the OP. But we’ll dig into it here. It is apparently a thing to take a V8 and convert one bank of cylinders into an air compressor for an all in one gas powered unit. So what if we took one and converted all of the cylinders into air compressors and drove it through the crankshaft? It would be appropriately rated to the correct RPM range and easily handle the torque we threw at it. A diesel would be even more ideal. They compression test much higher and are capable of handling much more torque. But could it move enough air to get the power we needed? How do you figure out a compressors ability to perform work? Well there are two main considerations that need to be taken. The first is the amount of work to physically squish air into a smaller space, and the second is the thermodynamic heating done as a result of adiabatic heating. There are a few steps to get to the equation for air pumps, but it ends up looking like this:  Stolen from Engineering Toolbox. We know there is only one compression stage. The k value is accurate. The pressure is 14.7PSI, possibly lower if you consider volumetric efficiency. The outlet pressure is the same as the compression test pressure. We’ll use 250PSI for a diesel. The volume of air is based on a few factors. It requires the displacement of the engine, operating RPM, and again, volumetric efficiency. We calculate the engine operating like a two stroke because that’s the way a compressor works. For a 7L, we get 988 CFM. Fully defined, we can now plug all these values into the equation above and get…74hp. That is a lot by compressor standards but not a lot of engine damping capacity. If we extrapolate the data out to the 2000hp design requirement by changing the displacement of the engine, we get the following graph:  Following the curve across the graph, we intersect at 26772 CFM, or 184L displacement. These equations do cover another kind of air pump. One found within automotive applications as well. A turbocharger, or in our case, a centrifugal supercharger since it needs to be shaft driven. If we go to Procharger’s website and go to their racing products. We can take a look at their largest unit, the F-3X-140, and plug in data. It is rated at 4500 CFM at 60 PSI. 235hp, not bad. Still not enough, but not bad. Still need 8 of them. The supercharger chart, for funsies. 
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Oct 11, 2019 01:25
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I'm guessing something like this is way over your budget but here's an example of a turbine engine compressor being used as a dyno to load up another turbine engine: https://www.youtube.com/watch?v=XInvCEcxKCI
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Oct 12, 2019 05:35
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I'd love to find a jet compressor for this. Imagine the noise. Wouldn't have to worry about exhaust or cooling either. But you're right, way too pricey. Progress has been slow lately. We're after a retarder but they aren't too common in the United States. My requisition guy is looking overseas where they cheap enough to possibly break even on shipping to the US. New Telmas for $1500, used for $500. Seeing as the prices are tripled in the US, we just need to wait for the right one to pop up. Have a kid in the way due next month, so I really need to find a good place to put a bookmark. Getting a physical unit would give me months of work.
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Dec 7, 2019 17:35
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This stuff is too big brained for me but I know Washington state is ending their emission test program Dec 31 and the test stations have some kind of Mustang dynos in them, so there may be a bunch of surplus parts available Jan 1?
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Dec 9, 2019 05:11
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| # ? May 6, 2024 13:40 |
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um excuse me posted:The last option, admittedly thought of recently, seemed viable. We need a huge air pump. Industrial pumps are far too expensive and heavy for the necessary torque. So what are we to do? Well I remembered that engines themselves are air pumps. We could take a lovely diesel V8 and reconfigure the valve train, or remove the valve train all together and install reed valves. However after calculation, a 7L diesel would only make enough compressed air to load ~80hp. Funny you say that. Go with a DOHC engine of the highest compression ratio you possibly can find. Remachine the heads a little to put a one way valve in the spark plug opening instead of a spark plug, since you're not igniting anything hopefully. Retime the exhaust cams on both sides to open the exhaust valves during what would have been the power stroke. Connect your air intake filter to the exhaust manifolds and the intake manifold. Congratulations, you now have a two-stroke air compressor without making any custom parts, assuming the exhaust cam timing options are acceptable and won't cause PtoV interference when you retime it for ideal air intake through the exhaust manifolds during the power stroke. I'm not sure this will solve all your issues, but it may make it closer to viable. That being said, I would still lean toward eddy current as well. It has the benefit of easy measurement and control - all you need to do to control load is PWM your eddy current winding, and all you need to do to measure torque is a load cell on an arm of known length from the eddy current winding armature to the frame, RPM measurement is of course easy. At least that's how the Mustang I helped set up for extended drive testing at TF was built, there may be other ways. Check your local equipment auctions regularly as well... we got our Mustang for some crazy low price because someone else bought what was left of an electric scooter company at auction and didn't want the dyno. He knew our VP and called up and said look, if you pay for the rigging company to haul all this stuff around and give me a hundred bucks, the dyno is yours. The Washington emissions program idea is a good one.
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Dec 19, 2019 20:20
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