D
duivendyk
Guest
I have been mulling over how to come up with an inexpensive Dyno setup to test engines (or bikes with engines).but I am not 100% sure what the the objective is.First I will discuss the engine case.
The purpose of a Dyno is to put the engine under load at differents speeds and measure (or compare) the output relative to to a previous condition for test purposes.It appears to me that the best way to accomplish this would be to let the engine drive a electrical generator,and measure it's output.If the efficiency of this generator is known (or can be measured).The engine output can be measured by measuring the generator output
It fortunately so happens that car generators run at similar speeds as the small engines we are dealing with,that is at 2-3 times car engine speed.They are typically rated at 100 Amp max. at 12.5 V or a power of 1.25 kW, which corresponds to 1.7 HP at 1200 rpm,and higher power at higher speeds (the output voltage is roughly proportional to rpm)
This suggests connecting the engine with some sort of coupling device to the generator,loading the generator with a resistor to present a load to the engine and measuring the power output of the generator to arrive at the engine power for a particular speed.The power the generator delivers can be controlled by varying the rotor current,which controls the generator output voltage.If the load resistance is known the power output can be readily computed from P= V*V/ (740*R) , P stands power in HP V*V stands for Voltage squared.I know this looks intimidating to the math challenged,so I will give an example,lets assume the generator output voltage is 20V and R=1 Ohm, so we get V*V= 400 and P= 400/740 or 0.54 HP.This is the generator output,if the efficiency of the generator is known we can find the engine output.Lets assume it is 80 %,then the engine output works out to be 054/0.80=.67 HP.One fly in the ointment could be that we need a low Ohm resistor(s) that can handle quite a lot of power,lets say in the order of 2000 W (2.7 HP) without going up in smoke.
The best solution is to use a custom made resistor made of resistance wire and put it in a waterfilled container and use evaporation cooling (steam),this is not dangerous or uncomfortable with proper grounding (and may be venting), the voltages are relatively low ,below 50V dc.There has to be enough water in the container to submerge the resistor of course.The best value still needs to be determined,since it depends on numerous factors.
There is another problem that may be more difficult to grasp,as I mentioned previously the generator output increases roughly with the rotational speed squared which implies the the power it absorbs from the engine increases also in the same ratio.for instance at twice the engine speed the power absorbed would go up fourfold,unless either the rotor excitation current was reduced or the load resistor increased (or both).We don't want to mess with the resistor unless we have to,that leaves the rotor current control.We know that a particular output voltage is related to a certain output,lets say 1HP,it turns out to be 27 V (with our 1 Ohm resistor).So if we manage to regulate the generator output to 27V with the rotor current control,we've got it made!.This voltage control is what car generators are made to do by the ( nowadays usually built in ) voltage regulator, of course they are set up to regulate the generator output to 14.5 V for the car battery not to some other voltage of our choosing.There are a number of solutions to this problem,with an external regulator as found in older cars it's not difficult,with built-in ones it get's a bit messier.You have to tear into the thing and know what you are doing.
Assuming now we are set up to require the generator to be regulated to a particular voltage.We crank up the throttle until the engine reaches the speed at which that voltage is reached.So we are basically looking for the rpm at which a particular power is produced,that may be at full throttle or less,we may not get there,then you will have to dial in a lower target voltage (power)
Adapting this system to whole bike measurements,obviously the rear wheel must be put in contact somehow with a roller that now drives a generator,instead of being driven by an engine.Exactly the inverse situation.So a rollerdrive system could be adapted.The challenge would be to keep it from slipping at the higher powers while not overloading bearings and the like.I am an electrical engineer and leave such matters to more qualified members
I hope to have made matters reasonable clear to interested parties and invite your comments and suggestions.Let me make clear that I have no intention in building any such Dyno test system myself ,but would be prepared to assist others who might be so inclined.I think there is a definite need and I am convinced that it would be relatively inexpensive to construct
The purpose of a Dyno is to put the engine under load at differents speeds and measure (or compare) the output relative to to a previous condition for test purposes.It appears to me that the best way to accomplish this would be to let the engine drive a electrical generator,and measure it's output.If the efficiency of this generator is known (or can be measured).The engine output can be measured by measuring the generator output
It fortunately so happens that car generators run at similar speeds as the small engines we are dealing with,that is at 2-3 times car engine speed.They are typically rated at 100 Amp max. at 12.5 V or a power of 1.25 kW, which corresponds to 1.7 HP at 1200 rpm,and higher power at higher speeds (the output voltage is roughly proportional to rpm)
This suggests connecting the engine with some sort of coupling device to the generator,loading the generator with a resistor to present a load to the engine and measuring the power output of the generator to arrive at the engine power for a particular speed.The power the generator delivers can be controlled by varying the rotor current,which controls the generator output voltage.If the load resistance is known the power output can be readily computed from P= V*V/ (740*R) , P stands power in HP V*V stands for Voltage squared.I know this looks intimidating to the math challenged,so I will give an example,lets assume the generator output voltage is 20V and R=1 Ohm, so we get V*V= 400 and P= 400/740 or 0.54 HP.This is the generator output,if the efficiency of the generator is known we can find the engine output.Lets assume it is 80 %,then the engine output works out to be 054/0.80=.67 HP.One fly in the ointment could be that we need a low Ohm resistor(s) that can handle quite a lot of power,lets say in the order of 2000 W (2.7 HP) without going up in smoke.
The best solution is to use a custom made resistor made of resistance wire and put it in a waterfilled container and use evaporation cooling (steam),this is not dangerous or uncomfortable with proper grounding (and may be venting), the voltages are relatively low ,below 50V dc.There has to be enough water in the container to submerge the resistor of course.The best value still needs to be determined,since it depends on numerous factors.
There is another problem that may be more difficult to grasp,as I mentioned previously the generator output increases roughly with the rotational speed squared which implies the the power it absorbs from the engine increases also in the same ratio.for instance at twice the engine speed the power absorbed would go up fourfold,unless either the rotor excitation current was reduced or the load resistor increased (or both).We don't want to mess with the resistor unless we have to,that leaves the rotor current control.We know that a particular output voltage is related to a certain output,lets say 1HP,it turns out to be 27 V (with our 1 Ohm resistor).So if we manage to regulate the generator output to 27V with the rotor current control,we've got it made!.This voltage control is what car generators are made to do by the ( nowadays usually built in ) voltage regulator, of course they are set up to regulate the generator output to 14.5 V for the car battery not to some other voltage of our choosing.There are a number of solutions to this problem,with an external regulator as found in older cars it's not difficult,with built-in ones it get's a bit messier.You have to tear into the thing and know what you are doing.
Assuming now we are set up to require the generator to be regulated to a particular voltage.We crank up the throttle until the engine reaches the speed at which that voltage is reached.So we are basically looking for the rpm at which a particular power is produced,that may be at full throttle or less,we may not get there,then you will have to dial in a lower target voltage (power)
Adapting this system to whole bike measurements,obviously the rear wheel must be put in contact somehow with a roller that now drives a generator,instead of being driven by an engine.Exactly the inverse situation.So a rollerdrive system could be adapted.The challenge would be to keep it from slipping at the higher powers while not overloading bearings and the like.I am an electrical engineer and leave such matters to more qualified members
I hope to have made matters reasonable clear to interested parties and invite your comments and suggestions.Let me make clear that I have no intention in building any such Dyno test system myself ,but would be prepared to assist others who might be so inclined.I think there is a definite need and I am convinced that it would be relatively inexpensive to construct
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