Joined
·
862 Posts
permalink
WHY OWN / USE A DYNO?
Every racer / mechanic understands that, without horsepower, cars go nowhere. As dynamometers are the only tool specifically designed to measure engine horsepower, it's no surprise that top racers / mechanics want their own. This examines things to consider before selecting and using these expensive tools.
Like most test equipment, a dynamometer (or dyno for short) helps isolate and quantify a particular parameter (in this case the engine's power output) from overall vehicle performance. Why do you need to do that? Racers (that don't dyno) often rationalize "I only test on the track ...where it counts"! They infer that power output is good if lap / quarter times are low. But, that fails to isolate the contribution of a sharp driver good handling vehicle from a strong engine!
Many hot-up modifications only help at high rpm, actually reducing power down low. Even with days of track testing you might condemn some new mod unless you test a bunch of other changes too. What if you need to match the fuel mixture / ignition too? Add up those exponentially increasing combinations, and thoroughly track testing stretches in to years! Dynamometer owners get pointed in the right direction with just a couple of 20-second "runs".
Using a dynamometer also helps you avoid discounting "insignificant" 1-% gains from modifications. Just because you can't "feel" a single 1-% power increase does not mean you want to forego ten such tricks! Combining small improvements is how pros win trophies. Ten 1-% tricks ad up to 10-%.
WHY DO I NEED TO DYNO?
I’ll assume you are a serious engine builder and want to start in-house dynamometer testing. What do you need? First, to measure engine torque, your dynamometer system must provide a load. Automotive engineers refer to this loading device as an absorber or a "brake" (since early dynamometer absorbers used a drum and band brake to load the engine). Absorbers do not actually absorb the power. Rather, they convert it to another form of energy, like heating water or air.
LOAD TYPES
Currently there are several commercially available absorber choices for engines.
DC GENERATOR
Professional engineers, with Fortune 500 budgets, often use electric DC generators or retarders with computer controlled field excitation to load and regulate their engines. The engine's power is typically dissipated as heat in the armature area or wired to remote heating elements. If the test engine's operating rpm is low enough, it can be directly coupled to the armature with a short drive shaft. 6,000+ rpm engines will need a gear reduction drive to match them to these low rpm generators / retarders.
The main advantage of electric generator / retarder systems is that they can be readjusted anywhere from zero load to full load in microseconds. This allows the engineer to regulate engine speed within a couple of rpm (even while changing throttle settings). Then you still need to the data acquisition system. If your engine runs at high rpm, you need the required gear reduction. Reduction transmissions add still more, complexity, and parasitic drag.
The DC generator or retarder dynamometers have another shortcoming. They have a high polar moment of inertia. That's a fancy way of saying that the generator's or retarders armature feels like a giant flywheel to a small engine. High inertia means a lot of horsepower is required to accelerate the armature or rotors. Likewise, a lot of stored horsepower will be returned when dropping down in rpm. This really skews the test data whenever rpm is changing. So, while generator or retarder dynamometers are great for steady state and ramp control, they are not ideal for testing rapid acceleration transient conditions on small engines.
EDDY CURRENT
Eddy current brakes are similar in operational characteristics to electric DC generator / retarder absorbers. The main difference is that the eddy current brake does not actually generate electricity. Rather, you use an electrical power supply to charge its electromagnetic coils. The brake's input shaft spins a metallic rotor inside that resulting magnetic field. When the dyno operator increases the current supply to the coils, the rotor shaft becomes harder for the test engine to turn. Like the DC generator, an eddy current brake's advantage is its lightning fast response to the controlling computer's loading instructions.
These eddy current brakes dissipate the engine's power as heat input to the rotor. This rotor must be cooled or it will eventually melt. Air-cooled eddy current brakes have cooling fins on a big iron rotor, making them look like automotive disk brake rotors. These big rotors have too much flywheel mass though, and dominate the rotating inertia of a typical kart dynamometer installation.
Water-cooled eddy current brakes are available that have significantly lower rotating inertia (at least compared to air cooled eddy current and DC generator systems).
BRAKE TYPE
Let's look at lower cost absorbers. The simplest and earliest forms of brakes were just that, brakes. A rotating drum with a friction brake pad was used to apply drag at the engine's output shaft. These looked like old truck brakes. To measure torque, some sort of calibrated scale linkage was inserted at the brake pad anchor points to display the applied drag load. Problems with friction brakes included much difficulty in accurately regulating the load and brake pad cooling.
HYDRAULIC PUMP
A more controllable load device is the hydraulic oil pump. These are occasionally seen on low rpm, moderate horsepower engine dynamometers. A positive displacement oil pump acts as the brake, and an adjustable oil discharge orifice valve sets the load. They can have a lower inertia than the DC generator and eddy current units if the pump is small, but sometimes require gear reduction units and couplings. Like many absorbers, the oil pump units convert a test engine's power into a fluid's temperature rise. Since the oil can't be just freely discharged, a cooling system (typically an oil to water heat exchanger) must be used to keep the oil's temperature within safe limits.
WATER BRAKE
When low cost, low inertia, high rpm limits, and race engine horsepower capacity are all requirements, the most prevalent choice for an absorber is the water brake. These have been the favorite of professional automotive engine builders for decades. Water brakes are another form of hydraulic pump absorber. These pumps typically have one or more vane'd rotors spinning in between pocketed stator housings. Load is controlled by varying the level of water in the brake with adjustable inlet and/or outlet orifices. Raising this water level increases the rotational drag of the pump's rotor, applying more resistance to the engine turning it. Interestingly the water brake is, by design, a very inefficient pump. It uses up your engine's horsepower output by making "instant hot water"! Since the discharged hot water is clean, it can either be allowed to just run off, or it can be air cooled and recirculated.
The power capacity vs. size of water brakes is startling. A water brake can be made to handle over 3000 continuous Hp at 12,000 rpm! By comparison a 3000 hp eddy current brake would need multiple retarders for this power rating and is only good to 5,000 rpm. It is no wonder that water brakes are virtually the only choice for testing 2,000+ horsepower drag car engines. Modern water brakes have low enough weight and inertia that they can be directly mounted on the engine's crankshaft. Direct mounting eliminates the inertia and parasitic drag of drive shafts, u-joints, pillow block bearings, etc.
All of the above absorbers can be controlled manually by the operator (with a simple knob), or under computer control. Manual valve water brake load control is not as responsive as the electric DC generator or eddy current controls but, with good electronic servo valve controls, you can close the gap a lot.
INERSIA DYNO
In discussing the pros and cons of various absorbers I keep mentioning problems with high inertia. To illustrate just how much power flywheel energy can mysteriously "absorbed" let's "talk about" a crude, dirt-cheap dynamometer with no brake at all! This will be an "inertia dynamometer" because the engine's power output will go into "winding up" a heavy flywheel.
This example uses a flywheel that is large, in relationship to the engine, so accelerating the combination from idle to peak rpm takes several seconds. A fast data acquisition system logs the time periods and rpm changes. From that information we calculate the torque and horsepower the engine supplied to accelerate that known flywheel mass. The formula for determining the torque is:
Torque = JM * rpm per second / 9.551
Where JM represents the Polar Moment of Inertia of our inertia dyno's flywheel.
If we don't know the Polar moment of Inertia for the flywheel (and our flywheel has a constant thickness cross-section) we can calculate it with the formula:
JM = (W * r ^2) / 32.16 / 2
Where W represents the flywheel weight in pounds and r is its radius in feet.
Once you have the torque, it is easy to calculate the horsepower with the standard formula:
Hp = Torque * rpm / 5252
Keep in mind that the rpm in the last formula must be the average rpm during the sampling period.
Say our example uses a 10-pound flywheel, 8" in diameter (thus it would have a Polar Moment of Inertia of .017 foot-pounds-second2). If the engine was able to accelerate this flywheel from say 4,800 rpm to 5,200 rpm in 2/10 of a second (a rate of 2,000 rpm per second) that would represent a torque of 3.6 pound feet. Since our above example had an average rpm of 5,000, it produced 3.4 Hp during the test. That's all here is to it. Unfortunately, inertia dynamometers alone are useless for doing the steady state testing needed for methodical development of porting, pipes, etc. You cannot adjust the load to hold the engine at a given rpm point, it must always be accelerating. Still, inertial testing is handy for working out acceleration and drivability problems.
The real reason for the above math exercise is to illustrate how much power it took to accelerate that small flywheel. If you buy an absorber with a polar moment of inertia in the same rage as our flywheel example above, don't expect to perform sweep acceleration testing. Even accelerating at just 200 rpm per second would consume 10-% of our sample engine’s power! Fortunately, high end computerized data acquisition systems provide composition algorithms to back out the effects of absorber (and crank-train) inertia from acceleration data. On a high inertia dynamometer, compensation is required even for fairly low rate sweep testing.
see part 2
__________________
WHY OWN / USE A DYNO?
Every racer / mechanic understands that, without horsepower, cars go nowhere. As dynamometers are the only tool specifically designed to measure engine horsepower, it's no surprise that top racers / mechanics want their own. This examines things to consider before selecting and using these expensive tools.
Like most test equipment, a dynamometer (or dyno for short) helps isolate and quantify a particular parameter (in this case the engine's power output) from overall vehicle performance. Why do you need to do that? Racers (that don't dyno) often rationalize "I only test on the track ...where it counts"! They infer that power output is good if lap / quarter times are low. But, that fails to isolate the contribution of a sharp driver good handling vehicle from a strong engine!
Many hot-up modifications only help at high rpm, actually reducing power down low. Even with days of track testing you might condemn some new mod unless you test a bunch of other changes too. What if you need to match the fuel mixture / ignition too? Add up those exponentially increasing combinations, and thoroughly track testing stretches in to years! Dynamometer owners get pointed in the right direction with just a couple of 20-second "runs".
Using a dynamometer also helps you avoid discounting "insignificant" 1-% gains from modifications. Just because you can't "feel" a single 1-% power increase does not mean you want to forego ten such tricks! Combining small improvements is how pros win trophies. Ten 1-% tricks ad up to 10-%.
WHY DO I NEED TO DYNO?
I’ll assume you are a serious engine builder and want to start in-house dynamometer testing. What do you need? First, to measure engine torque, your dynamometer system must provide a load. Automotive engineers refer to this loading device as an absorber or a "brake" (since early dynamometer absorbers used a drum and band brake to load the engine). Absorbers do not actually absorb the power. Rather, they convert it to another form of energy, like heating water or air.
LOAD TYPES
Currently there are several commercially available absorber choices for engines.
DC GENERATOR
Professional engineers, with Fortune 500 budgets, often use electric DC generators or retarders with computer controlled field excitation to load and regulate their engines. The engine's power is typically dissipated as heat in the armature area or wired to remote heating elements. If the test engine's operating rpm is low enough, it can be directly coupled to the armature with a short drive shaft. 6,000+ rpm engines will need a gear reduction drive to match them to these low rpm generators / retarders.
The main advantage of electric generator / retarder systems is that they can be readjusted anywhere from zero load to full load in microseconds. This allows the engineer to regulate engine speed within a couple of rpm (even while changing throttle settings). Then you still need to the data acquisition system. If your engine runs at high rpm, you need the required gear reduction. Reduction transmissions add still more, complexity, and parasitic drag.
The DC generator or retarder dynamometers have another shortcoming. They have a high polar moment of inertia. That's a fancy way of saying that the generator's or retarders armature feels like a giant flywheel to a small engine. High inertia means a lot of horsepower is required to accelerate the armature or rotors. Likewise, a lot of stored horsepower will be returned when dropping down in rpm. This really skews the test data whenever rpm is changing. So, while generator or retarder dynamometers are great for steady state and ramp control, they are not ideal for testing rapid acceleration transient conditions on small engines.
EDDY CURRENT
Eddy current brakes are similar in operational characteristics to electric DC generator / retarder absorbers. The main difference is that the eddy current brake does not actually generate electricity. Rather, you use an electrical power supply to charge its electromagnetic coils. The brake's input shaft spins a metallic rotor inside that resulting magnetic field. When the dyno operator increases the current supply to the coils, the rotor shaft becomes harder for the test engine to turn. Like the DC generator, an eddy current brake's advantage is its lightning fast response to the controlling computer's loading instructions.
These eddy current brakes dissipate the engine's power as heat input to the rotor. This rotor must be cooled or it will eventually melt. Air-cooled eddy current brakes have cooling fins on a big iron rotor, making them look like automotive disk brake rotors. These big rotors have too much flywheel mass though, and dominate the rotating inertia of a typical kart dynamometer installation.
Water-cooled eddy current brakes are available that have significantly lower rotating inertia (at least compared to air cooled eddy current and DC generator systems).
BRAKE TYPE
Let's look at lower cost absorbers. The simplest and earliest forms of brakes were just that, brakes. A rotating drum with a friction brake pad was used to apply drag at the engine's output shaft. These looked like old truck brakes. To measure torque, some sort of calibrated scale linkage was inserted at the brake pad anchor points to display the applied drag load. Problems with friction brakes included much difficulty in accurately regulating the load and brake pad cooling.
HYDRAULIC PUMP
A more controllable load device is the hydraulic oil pump. These are occasionally seen on low rpm, moderate horsepower engine dynamometers. A positive displacement oil pump acts as the brake, and an adjustable oil discharge orifice valve sets the load. They can have a lower inertia than the DC generator and eddy current units if the pump is small, but sometimes require gear reduction units and couplings. Like many absorbers, the oil pump units convert a test engine's power into a fluid's temperature rise. Since the oil can't be just freely discharged, a cooling system (typically an oil to water heat exchanger) must be used to keep the oil's temperature within safe limits.
WATER BRAKE
When low cost, low inertia, high rpm limits, and race engine horsepower capacity are all requirements, the most prevalent choice for an absorber is the water brake. These have been the favorite of professional automotive engine builders for decades. Water brakes are another form of hydraulic pump absorber. These pumps typically have one or more vane'd rotors spinning in between pocketed stator housings. Load is controlled by varying the level of water in the brake with adjustable inlet and/or outlet orifices. Raising this water level increases the rotational drag of the pump's rotor, applying more resistance to the engine turning it. Interestingly the water brake is, by design, a very inefficient pump. It uses up your engine's horsepower output by making "instant hot water"! Since the discharged hot water is clean, it can either be allowed to just run off, or it can be air cooled and recirculated.
The power capacity vs. size of water brakes is startling. A water brake can be made to handle over 3000 continuous Hp at 12,000 rpm! By comparison a 3000 hp eddy current brake would need multiple retarders for this power rating and is only good to 5,000 rpm. It is no wonder that water brakes are virtually the only choice for testing 2,000+ horsepower drag car engines. Modern water brakes have low enough weight and inertia that they can be directly mounted on the engine's crankshaft. Direct mounting eliminates the inertia and parasitic drag of drive shafts, u-joints, pillow block bearings, etc.
All of the above absorbers can be controlled manually by the operator (with a simple knob), or under computer control. Manual valve water brake load control is not as responsive as the electric DC generator or eddy current controls but, with good electronic servo valve controls, you can close the gap a lot.
INERSIA DYNO
In discussing the pros and cons of various absorbers I keep mentioning problems with high inertia. To illustrate just how much power flywheel energy can mysteriously "absorbed" let's "talk about" a crude, dirt-cheap dynamometer with no brake at all! This will be an "inertia dynamometer" because the engine's power output will go into "winding up" a heavy flywheel.
This example uses a flywheel that is large, in relationship to the engine, so accelerating the combination from idle to peak rpm takes several seconds. A fast data acquisition system logs the time periods and rpm changes. From that information we calculate the torque and horsepower the engine supplied to accelerate that known flywheel mass. The formula for determining the torque is:
Torque = JM * rpm per second / 9.551
Where JM represents the Polar Moment of Inertia of our inertia dyno's flywheel.
If we don't know the Polar moment of Inertia for the flywheel (and our flywheel has a constant thickness cross-section) we can calculate it with the formula:
JM = (W * r ^2) / 32.16 / 2
Where W represents the flywheel weight in pounds and r is its radius in feet.
Once you have the torque, it is easy to calculate the horsepower with the standard formula:
Hp = Torque * rpm / 5252
Keep in mind that the rpm in the last formula must be the average rpm during the sampling period.
Say our example uses a 10-pound flywheel, 8" in diameter (thus it would have a Polar Moment of Inertia of .017 foot-pounds-second2). If the engine was able to accelerate this flywheel from say 4,800 rpm to 5,200 rpm in 2/10 of a second (a rate of 2,000 rpm per second) that would represent a torque of 3.6 pound feet. Since our above example had an average rpm of 5,000, it produced 3.4 Hp during the test. That's all here is to it. Unfortunately, inertia dynamometers alone are useless for doing the steady state testing needed for methodical development of porting, pipes, etc. You cannot adjust the load to hold the engine at a given rpm point, it must always be accelerating. Still, inertial testing is handy for working out acceleration and drivability problems.
The real reason for the above math exercise is to illustrate how much power it took to accelerate that small flywheel. If you buy an absorber with a polar moment of inertia in the same rage as our flywheel example above, don't expect to perform sweep acceleration testing. Even accelerating at just 200 rpm per second would consume 10-% of our sample engine’s power! Fortunately, high end computerized data acquisition systems provide composition algorithms to back out the effects of absorber (and crank-train) inertia from acceleration data. On a high inertia dynamometer, compensation is required even for fairly low rate sweep testing.
see part 2
__________________