Heavy flywheel to improve idling and low rpm torque - Fuelly Forums

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Old 11-15-2007, 02:13 AM   #1
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Heavy flywheel to improve idling and low rpm torque

The recent post about fuel consumption of an unloaded engine at different RPM had me thinking about ways to improve idle consumption.
A well known way to improve idling and low rpm torque is to use a heavier flywheel. At least in theory, this modification is easy. We could probably achieve very low idle RPMs that way and potentially improve the glide phase of a P&G cycle. More low rpm torque could also mean improvements to the pulse phase depending on speed.

The question is how low the engine is mapped. It seems modern engine like to prevent stalling automatically and this may effectively prevent low rpm experiments.

Anyone seen any work on this?
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Old 11-15-2007, 02:45 AM   #2
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Interesting. In the Dream Engines thread I hypothesized that a lighter flywheel would be better for p&g.

Why? Well, in p&g you are either accellerating as efficiently as possible (fast but not too fast), or you are idling or at 0 rpm. You don't normally hold a steady off-idle rpm, so you will always be accelerating a heaver flywheel up to speed, then losing that extra energy you spent as the engine coasts back down towards idle/0 rpm. A lighter flywheel would take less energy to accelerate up to speed, and thus you would lose less energy when you let the RPMs drop (the drop in RPM represents kinetic energy lost).

I'm pretty certain that for EOC it would hurt matters, but if you idle in your P&G then there would be some breakeven point where the extra fuel it may save on idling is lost to less efficient acceleration.
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Old 11-15-2007, 03:00 AM   #3
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skewbe -

If you could design your own transmission from scratch, could you have multiple flywheels that attach/disattach at certain speeds so that you could have the optimum flywheel weight for city or highway driving?

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Old 11-15-2007, 03:42 AM   #4
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It is possible, I don't see the returns from such an effort being much though, compared to the effort. Depends if you EOC or where that breakeven point I mentioned is, hypothetically speaking of course.

I have found some more verbiage regarding flywheel weight:
http://www.racetep.com/flywheel.html

"Most manufacturers make the stock flywheel very heavy. This makes the engine very smooth and enables it to retain energy at part throttle cruise and up long grades for better fuel economy.

An Aluminum flywheel goes completely the other direction with almost no weight. This lets the engine rev up very quickly and allows the engine to work easier due to the reduced weight spinning around on the back of the crank. This is fine as long as you are at full throttle and wide open throttle all the time like in a Road Race or Drag Race only car. As soon as you let off the gas the engine RPM drops instantly and the car slows down. In a road car this causes surging and bucking at cruise speeds and poor driveability and clutch engagement."

I read this as lighter is definately better for p&g, but too light and smooth starts get harder to perform. They then go on to advocate lightening the stock flywheel as a compromise.


Here is a review of a metro with an aluminum flywheel, he likes
http://www.teamswift.net/viewtopic.php?t=14518
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Old 11-15-2007, 07:36 AM   #5
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If I apply this to the VX, which is engineered for MPG...

Honda actually put a lighter flywheel on this model from the factory.

From looking at it and comparing it to other honda flywheels, it's about 5-6lbs lighter with weight carved out of the engine-side of it to take weight out (not an extreme amount lighter, such as with an aluminum flywheel, but it is less than the other honda factory flywheels). I imagine with a small motor (such as the 1.5L in the VX), eliminating whatever extra weight to spin out of it, will help the engine work less, and get up to speed and RPM with greater ease.
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Old 11-15-2007, 07:57 AM   #6
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I'll let you guys know how it goes with my lightened flywheel in my VX this spring. The only problem is that I'm throwing in a freshened up d15z1 at the same time. I can report on idle and acceleration for you guys though. I'm shooting to bring the OEM flywheel down from it's 15-18lbs OEM weight. Which is 3-5lbs. lighter than the other model Civics' flywheels
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Old 11-15-2007, 08:06 AM   #7
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After reading up on this a bit, I think I'll abandon the idea. More inertia to the flywheel will make the engine idle lower and smoother with resulting lower fuel consuption. However, the notion that there will be more low rpm torque seems to be a placebo effect. What happens is that the flywheel stores a lot of energy that gets used up when a car is shifted up between gears. It then feels like the engine produces more torque initially. In reality, energy is spent spinning up the flywheel instead of accelerating the car (which was said above by skewbe).

So unless, there is something else going on within the cylinders/combustion that make an engine produce more torque when there is more inertia in the system, this will not work because we'll throw away the stored flywheel energy during the glide phase. Constant speed wouldn't suffer though.
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Old 11-15-2007, 12:19 PM   #8
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A lighter flywheel helps with acceleration and deceleration. A heavier flywheel would hurt acceleration but enhance steady state speed...momentum.
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Old 11-15-2007, 12:31 PM   #9
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However, the notion that there will be more low rpm torque seems to be a placebo effect. What happens is that the flywheel stores a lot of energy that gets used up when a car is shifted up between gears.
It's not a placebo - it really does apply more torque (conditionally). But only while the clutch is engaging. Once the tranny and engine are mated together, torque out from engine = torque in to transmission. In theory, it should also increase the engine brake coasting distance as you're adding more angular momentum to combat compression...

As you said, it stores energy... But it can release that energy faster. Of course, that stored energy came from somewhere

I've heard of doing this for off road purposes... but that's about it... quick google search.
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Old 11-15-2007, 03:56 PM   #10
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Originally Posted by DarbyWalters View Post
A lighter flywheel helps with acceleration and deceleration. A heavier flywheel would hurt acceleration but enhance steady state speed...momentum.
Hmmm.... Well, I don't think it works out quite like that. Depending on application, I've heard both heavy and light flywheels help out w/ acceleration. Heavy'uns are used for drag racing since they can store more energy in there before dumping the clutch and going, and light ones are used for cars that make more power up high and are in something like road racing, where they need to rev quicker in order to make power out of a corner or something.
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