an unconventional question
an unconventional question
(OP)
I'm currently designing an unconventional type of transmission. An engine drives a chain system. But it's essential that there is a "spring damper" between the engine (driving sprocket) and the driven sprocket (load). This will most likely take the form of an actuator/spring that the chain (under tension) "runs over" like a hill. So the actuator can "give" to limit the tension of the chain or the jerkiness to the load.
Now, imagine an engine running at 2000rpm and is delievering 200 ft lb of torque (which isn't constant in reality). At 200 ft lb, assume the actuator needs to apply a force of 100 lb to "hold up" the tensioned driving chain.
If the actuator force is too large, then it will fail as a damper. Let's say a shock load is applied yet the actuator force is more than 100 lb, then if the engine is still outputing 200 ft lb of torque, then the actuator will not "give", so what would happen in this case? Will the engine stall? If the shock load is too large (requires more than 200 ft lb of torque), the engine might stall right? But what if the shock load is only slightly greater than? Even if the engine does not stall, how will the efficiency be affected?
If the actuator force is too small, then it will "give" unncecessarily and this will affect the efficiency right? Since some work is done on compressing the actuator.
But the actuator force is hard to control since the engine output isn't even constant due to the nature of the internal combustion engine.
What is a solution to this? I want to system to be smooth and efficient. (when subjected to slight shock load)
Or am I worrying too much because an engine won't stall that easily as long as the shock load is small and isn't long lasting? Sometimes when I mow my lawn the the grass is too dense I can hear the engine rpm drop due to the increase in load and then rev up again as long as I quickly move to a lighter area. But will the efficiency go down in this case?
Thanks
Now, imagine an engine running at 2000rpm and is delievering 200 ft lb of torque (which isn't constant in reality). At 200 ft lb, assume the actuator needs to apply a force of 100 lb to "hold up" the tensioned driving chain.
If the actuator force is too large, then it will fail as a damper. Let's say a shock load is applied yet the actuator force is more than 100 lb, then if the engine is still outputing 200 ft lb of torque, then the actuator will not "give", so what would happen in this case? Will the engine stall? If the shock load is too large (requires more than 200 ft lb of torque), the engine might stall right? But what if the shock load is only slightly greater than? Even if the engine does not stall, how will the efficiency be affected?
If the actuator force is too small, then it will "give" unncecessarily and this will affect the efficiency right? Since some work is done on compressing the actuator.
But the actuator force is hard to control since the engine output isn't even constant due to the nature of the internal combustion engine.
What is a solution to this? I want to system to be smooth and efficient. (when subjected to slight shock load)
Or am I worrying too much because an engine won't stall that easily as long as the shock load is small and isn't long lasting? Sometimes when I mow my lawn the the grass is too dense I can hear the engine rpm drop due to the increase in load and then rev up again as long as I quickly move to a lighter area. But will the efficiency go down in this case?
Thanks





RE: an unconventional question
RE: an unconventional question
RE: an unconventional question
Wang,
Most shock loads are overcome by the inertia of the rotating parts of the engine (usually including a flywheel.) The easiest way to reduce shock loads is to "overcome/absorb" them in a flywheel.
If your intention is to protect the engine or the chain from these shock loads, then the torsional damper already mentioned will help and so would your idea. Don't worry about efficiency as in a dynamic system you'll get back the work when the spring decompresses. A variable rate spring would give you some damping at a range of output torques.
Might be easier to build some flexibility into your chassis/frame or use rubber engine mounts. cheers, derek
Join us at http://groups.yahoo.com/group/opensourcecars
RE: an unconventional question
Assuming you aren't looking for rapid acceleration, a flywheel could be external on an idler shaft (jackshaft) between engine and load.
Jay Maechtlen
RE: an unconventional question
This shouldn't be too big of a problem if the "flexibility" I need is very miniscule (like around only 0.1mm). That means if a loaded chain under tension can yield around 0.1mm (due to the spring), then the problem will be solved.
So maybe I can use some sort of spring that offer a range of force in a very short distance (around 3mm), by using varible constant spring or by some type of gearing.
Then the spring stiffness will ensure that it constantly matches the engine torque, (actually, the force of the spring is a lot smaller than the chain tension because of the angle of the set up). But it can "yield" 0.1mm if needed and still keep the chain tension approx. the same. (without the risk jerking the chain or sputtering the) engine)
Just hope the "spring going up or down" won't be felt by the operator.
I think this is the simplest and the cheapest to solve my problem. Do you think so?
RE: an unconventional question
Imagine a long skinny driveshaft that can twist or "wind up" under greater torque, and unwind a bit under lesser torque.
Now, the coupler mentioned several posts above does that kind of thing, by letting the elastomeric cushions act as springs.
I was partly joking, but also suggesting something with much more angular deflection than a typical coupling can provide.
Again- it just averages the demand, lets the engine handle it more easily. The suggested flywheel accomplishes the same thing by storing energy (and momentum) for immediate delivery.
If you are protecting the engine, either method will work.
If you are protecting the load, then you need the spring method.
cheers
Jay
Jay Maechtlen
http://home.covad.net/~jmaechtlen/
RE: an unconventional question
What's the difference between this and engine braking?
In engine braking, the vehicle will slow down. Is this energy lost or is it stored in the engine momentum?
RE: an unconventional question
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: an unconventional question
RE: an unconventional question
No. The engine will be "forced" (bad choice of words,IMO) to a higher rpm range AND this action will definately slow the vehicle if no added power is present to offset the speed change. Only if the engine is accelerated to the matching rpm will the speed NOT change.
"Engine braking" is simply reducing power output below that necessary to maintain desired speed and/or dropping to a lower gear (higher ratio) without adding compensating power---it's a matter of degree, the more you back off the power, the more "engine braking".
The IC engine in common use in transportation IS an air pump, but NOT an air compressor. The energy lost due to "engine braking" is lost to atmosphere. Added engine braking can be had by shutting off the ignition source and OPENING the intake throttle to maximise the pumping loss.
In any case, the energy is lost to friction, heat, pumping loss, etc.
Rod
RE: an unconventional question
I understand there is friction and heat loss. I'm just not very sure on what you mean by "pumping loss". Because when the piston go up and compress the air in the cylinder, it will take energy. But then this energy will be gained back when the air decompresses (normally during the combustion stroke). So it's like a spring. Of course you can't gain back 100% of the energy that went into the compressing (air) stroke, is this the "pumping loss" you are talking about?
Or does the air valve actually opens (I thought it don't) after the air get compressed so all those compressed air run out into the atomsphere and the energy that goes into compressing the air get lost to the atomsphere and none of it can be retrieved again?
RE: an unconventional question
Visualize the two sprockets, and the length of chain running between each sprocket. There would be two straight lengths of chain. Just apply strong tensioners on each straight section. Make them equal in strength, and of course, design so they don't cause mutual interference, yet could handle the wide range of motion required. You may have damping issues, but that's something that could probably be addressed by applying dampers to each tensioner.
Of course, there has to some slack in the chain, they cannot run in a straight line between sprockets. Shock loads would pass directly between sprockets.
RE: an unconventional question
All those add up to 'engine braking'.
Compression-Ignition engines sometimes use a device to increase engine braking. It is commonly called a "jake brake" but there is probably an official technical term as well. I will leave the explanation for others, lest I mangle it!
regards
Jay
Jay Maechtlen
http://home.covad.net/~jmaechtlen/
RE: an unconventional question
has a good explanation
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: an unconventional question
Also used is an exhaust brake which is a throttle in the exhaust pipe, simply a butterfly to increase back pressure.
Truck
RE: an unconventional question
If the Engine 1 has the jakebrake device that regulates the exhaust valve so that the compressed gas are dissipated into the atomsphere. The energy that went into compressing the gas in the compressing stroke is not retured to the piston/engine. It is lost to the atomsphere. This provides for very significant "engine braking". Because the energy of the moving vehicle is used to compress the air, which is dissipated into the atomosphere.
If Engine 2 doesn't have the jakebrake device, the compressed gas will merely decompress and return the energy back to the piston like a spring. And in the absence of friction and other losses, There will be essentially no engine braking. Other than the fact some energy from the vehicle will accelerate the engine to a higher rpm and the vehicle will slow down due to this. But overall, there is NO NET LOSS of energy to the outside.
Right?
RE: an unconventional question
A SI engine, which does not throttle the exhaust or
a CI engine, which does not throttle the intake---no. You still have intake and/or exhaust pumping loss.
All this is moot :-( as there is no frictionless, read that, "perpetual motion" engines.
Rod
RE: an unconventional question