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View Full Version : Do I just not know engines???


tiedeman
08-09-2004, 08:49 PM
Ok, you see my other thread in regards to the lawn boy. Well, the one that I am working one right now doesn't have a blade on it. My old man (dad), told me that I need to have that blade on before I can start the engine because it acts as a counter balancer. I have never heard of that before. I am just stupid for not knowing that? I guess what I am saying is, should I have known that?

dvmcmrhp52
08-09-2004, 08:54 PM
Counter balance?
Might be once it is running,but It doesn't have anything to do with it running...................

Green Care
08-09-2004, 09:00 PM
Don't feel bad never heard to that also.

GeeVee
08-09-2004, 09:07 PM
Is it a two srtoke?

Lets just say it might need it for (weight) much like a flywheel....

tiedeman
08-09-2004, 09:23 PM
Originally posted by dvmcmrhp52
Counter balance?
Might be once it is running,but It doesn't have anything to do with it running...................

I think that is what he was basically getting at. The fact that once it starts to run

MJLsLawnCareNmoreLLC
08-09-2004, 09:31 PM
You can start it, but the engine wont run right without the blade on it. Small mower engines dont have a large enough flywheel or some dont even have one. So you need the weight of the blade to keep the engine running normally.

tiedeman
08-09-2004, 09:33 PM
Originally posted by MJLsLawnCareNmoreLLC
You can start it, but the engine wont run right without the blade on it. Small mower engines dont have a large enough flywheel or some dont even have one. So you need the weight of the blade to keep the engine running normally.

ahh that makes sense

MJLsLawnCareNmoreLLC
08-09-2004, 09:38 PM
I try............

Steve9
08-09-2004, 09:55 PM
Actually it doesnt make sence at all to me. The engine is bolted to the deck, the blade does nothing but create a very slight resistence. Nothing even measurable.

SWD
08-09-2004, 10:01 PM
Yes, it is measuralbe-particularly to a small displacement two-stroke motor. Reciprocating mass is needed in order for the motor to translate the kinetic energy developed during the internal combustion process - no mass/or insuffcient mass = poor or no run operation. This is particularly important on two-stroke motors wherein a run away situation is a very real possibility.
Think about two-cycle outboard boat motors, start one of those motors out of the water with a flush type rinsing muffler and rev it up. A run away is a very distinct possibility despite having a greater mass of flywheel.
You father was partially correct, counter balance to some limited extent, counter weight to a very necessary extent.

Westbrooklawn
08-09-2004, 10:03 PM
Slight resistance??? Try sticking your hand under the deck when running and see how much "resistance" the blade exerts. The blade does in fact at like a flywheel, and the inertia it creates is needed to make the motor run right. Of course we are talking about a single blade mower with the blade connected directly to the motor shaft.

barnard
08-09-2004, 10:12 PM
Some techumsehs with lite aluminum flywheels are extremely hard to start without a blade on them . They will backfire and yank the rope from your hands. The blade does indeed act as a flywheel.

gvandora
08-09-2004, 10:13 PM
Think about the resitance in terms of pushing a car on flat ground. The wheels of the car reduce friction to the extent that a car could be moved by a person. That is a similar situtation to the blade. The only resistance the blade has is friction with air, which a low speeds is neglible. The difficult part is starting up the car. It requires a great deal of energy to accelerate the vehicle. Now imagine tying one end of a rope to the bumper and the other around your waist. Imagine you can only push the car in spurts and when you're not pushing the car, its yanking you along. That is how single cylinder engine with a flywheel behaves. Power only when combustion occurs, otherwise you're waiting for the crankshaft to spin around and ready for another power stroke. Speed the process up sufficiently and it smoooths out.

I know I'm missing a few pieces here with inertia and all... but I think this might make the flywheel idea clear.

Steve9
08-09-2004, 10:33 PM
Thats not resistance....take a blade and spin it on a screw driver and tell me how hard it is to spin...It will spin with negligible resistance. Now cutting grass is when its tough

MJLsLawnCareNmoreLLC
08-09-2004, 11:40 PM
Originally posted by SWD
Yes, it is measuralbe-particularly to a small displacement two-stroke motor. Reciprocating mass is needed in order for the motor to translate the kinetic energy developed during the internal combustion process - no mass/or insuffcient mass = poor or no run operation. This is particularly important on two-stroke motors wherein a run away situation is a very real possibility.
Think about two-cycle outboard boat motors, start one of those motors out of the water with a flush type rinsing muffler and rev it up. A run away is a very distinct possibility despite having a greater mass of flywheel.
You father was partially correct, counter balance to some limited extent, counter weight to a very necessary extent.

Very well put. I tried to aviod going into the longer explantion ,but he is right.

Dont believe us, take your trimmer and run it with no line, then with the proper length(with the gaurd and cutter on), then with like 2 foot lines. Notice the higher rpm and faster acceleration with no line. Also the very low rpms and slow acceleration. Different scenario but similar concept.

fixer67
08-10-2004, 12:29 AM
A 4 stroke engine ONLY gets power from 1 stroke in four=Intake-compression-POWER-exhaust. You have to have some momentum to take the engine though the 3 un-powered strokes of the engine. On a 2 stroke you get power on every other stroke=intake/exhaust-compression/power so you have to have enough momentum to take it though the un-powered stroke. You get the momentum from the inertia of the mass of the flywheel or blade.

sdwally
08-10-2004, 09:10 AM
The blade is calculated into extra flywheel mass. Without the extra mass of the blade engine speed with be higher. Potentialy cause engine damage due to over speeding. The mass is needed also to help smooth out how the engine runs. It also is quite possible to shear the flywheel key especially if the key is aluminum.
Before governor systems were used large flywheels were used to control engine speed. On small engines that use an air vane governor the mass of the flywheel and blade also help control engine speed, since response time in the governor system has a large governor droop(response is within about a change of 300 to 900 rpm). Mechanical governors have a governor droop of about 400rpm.

ikesleeping
08-10-2004, 10:29 AM
The weight mass is needed when the engine is put under load to help the piston momentum carry the piston thru to the next firing cycle. The inertia of the weight ,mass helps overcome a load on the engine,such as turning a different length of trimmer string,cutting grass etc. I used the terms weight ,mass to avoid the argument that they aint the same thing.My 2 cents....

geogunn
08-10-2004, 12:42 PM
Originally posted by fixer67
A 4 stroke engine ONLY gets power from 1 stroke in four=Intake-compression-POWER-exhaust. You have to have some momentum to take the engine though the 3 un-powered strokes of the engine. On a 2 stroke you get power on every other stroke=intake/exhaust-compression/power so you have to have enough momentum to take it though the un-powered stroke. You get the momentum from the inertia of the mass of the flywheel or blade.




don't confuse "strokes" with "revolutions" of the crankshaft.

a four stroke engine fires on every other crankshaft revolution.

a two stroke engine fires on every crankshaft revolution.

GEO :)

jbell113
08-10-2004, 05:52 PM
All four stroke engines that the blade attachs to the crankshaft have aluminum flywheels. You dont want to start it without the blade cuz ur blade is ur counter weight. If you start it without the blade it could cause it to over speed and cause major engine damage. Mowers like this have aluminum fly wheels because if it was a cast iron flywheel like a go cart engine it would not have the power it needed to cut the grass.

dvmcmrhp52
08-11-2004, 10:13 PM
I'm interested in the result here Tiedeman,
I will have lernt something if it turns out to be the case.
Let us know what you come up with.................