Do Lighter Weight Flywheels
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Senior Member
Joined: Nov 2002
Posts: 292
From: Central KY
Useless facts 4 yo azzes...
I was looking around a few months ago for a 200 amp alternator for my Pontiac. I found several companies that made them, but more importantly I found a website that explained the parasitic drain from belt driven devices, namely alternators. The key influencer was the weight of the flywheel and thus inertia and other stuff that I should have stayed awake for in physics...
For example diesel engines typically have heavier flywheels than gasoline engines. A diesel engine will use 33-50% less HP to turn the exact same alternator as a gasoline engine.
So I was thinking about how when we switch to lighter flywheels we are actually increasing the parasitic drain of our belt devices BUT we are gaining low end HP and torque... I gues it is just give and take.
I was looking around a few months ago for a 200 amp alternator for my Pontiac. I found several companies that made them, but more importantly I found a website that explained the parasitic drain from belt driven devices, namely alternators. The key influencer was the weight of the flywheel and thus inertia and other stuff that I should have stayed awake for in physics...
For example diesel engines typically have heavier flywheels than gasoline engines. A diesel engine will use 33-50% less HP to turn the exact same alternator as a gasoline engine.
So I was thinking about how when we switch to lighter flywheels we are actually increasing the parasitic drain of our belt devices BUT we are gaining low end HP and torque... I gues it is just give and take.
Senior Member
Joined: Dec 2003
Posts: 419
From: Corvallis, Oregon
I disagree. Parasite drag is a function of the drag coefficient of whatever belt driven accessory is in question and has nothing to do with rotational inertia. Diesel engines probably have larger flywheels because their compression ratios are much higher than gasoline engines, making it harder to force the piston back up in the cylinder during the compression stroke than for a gas engine. The rotational inertia in the flywheel is one place where the energy comes from to push the piston back up.
What a light flywheel will do is make the engine much more sensitive to changes in parasite drag. The engine will slow and possibly stutter if the alternator places a momentary drag on the engine from a stereo amp or high beam headlights being just turned on, etc. But rotational inertia has no effect on parasite drag. And I got a B in Physics . . . I kind of let myself get too lazy in that class.
What a light flywheel will do is make the engine much more sensitive to changes in parasite drag. The engine will slow and possibly stutter if the alternator places a momentary drag on the engine from a stereo amp or high beam headlights being just turned on, etc. But rotational inertia has no effect on parasite drag. And I got a B in Physics . . . I kind of let myself get too lazy in that class.
Senior Member
Joined: Nov 2002
Posts: 1,679
From: Colorado Springs
Originally Posted by Fluid Dynamics' date='Jan 13 2004, 08:47 PM
I disagree. Parasite drag is a function of the drag coefficient of whatever belt driven accessory is in question and has nothing to do with rotational inertia. Diesel engines probably have larger flywheels because their compression ratios are much higher than gasoline engines, making it harder to force the piston back up in the cylinder during the compression stroke than for a gas engine. The rotational inertia in the flywheel is one place where the energy comes from to push the piston back up.
What a light flywheel will do is make the engine much more sensitive to changes in parasite drag. The engine will slow and possibly stutter if the alternator places a momentary drag on the engine from a stereo amp or high beam headlights being just turned on, etc. But rotational inertia has no effect on parasite drag. And I got a B in Physics . . . I kind of let myself get too lazy in that class.
What a light flywheel will do is make the engine much more sensitive to changes in parasite drag. The engine will slow and possibly stutter if the alternator places a momentary drag on the engine from a stereo amp or high beam headlights being just turned on, etc. But rotational inertia has no effect on parasite drag. And I got a B in Physics . . . I kind of let myself get too lazy in that class.
One of my customers' motor at the shop has a 5.25 inch, 4.5 lb flywheel (410 chevy) and has no problem with an alternator, power steering, and a 5 stage oil pump (about 15 hp loss there). He's got 150 watt headlights on the front of the car too.
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