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Old 03-21-2008, 05:35 PM   #11 (permalink)
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He's talking optimum piston speed not RPM. Some of the largest engines in the world the optimum RPM is 100 RPM and our car engines wont even run that slow. The reason our car engines have a sweet spot is cam timming and port size and length. I dont know maybe piston speed plays a role as well, its never occured to me though.

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Old 03-21-2008, 06:05 PM   #12 (permalink)
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Sorry, meant piston speed.

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Old 03-21-2008, 07:21 PM   #13 (permalink)
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Sorry, I haven't been able to review the link RH provided.

waffle: What I'm wondering is why a lower ft/s isn't better yet? Less friction, fewer explosions, etc.

randy: thanks for the explanation, I still am not fluent in what exactly mep means to efficiency.

The root of my question started with knowing my car is geared for 1000-1200 ft/s at 50-60 mph. Seems perfect right from the factory doesn't it? Yet our pet theories for increased fe mods almost always include taller gearing. WTH?
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Old 03-21-2008, 09:06 PM   #14 (permalink)
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I think it depends on the engine. HD diesels designed for efficiency have super low speed BSFC peaks compared to most vehicles. I'm guessing going too low could hurt efficiency due to the exhaust/induction system not functioning as well or maybe some sort of increase in piston ring friction due to angular forces on the crank/bearing at the speed if the engine is designed for high speed power.

That being said, I think the ft/s range already includes the idea that pumping losses of the less than stoich kind is optimized. If it isn't, generally it's way better to take a hit in some other type of efficiency because the gain in pumping losses tends to be much greater. Or, as a rule of thumb, anything below half throttle at some speed can be improved, anything above not so much.
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Old 03-21-2008, 10:04 PM   #15 (permalink)
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Well, the lower the RPM the higher the thermal losses!
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Old 03-21-2008, 10:22 PM   #16 (permalink)
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Keep in mind as well the practical side of engine production drives the makers not the theoretical aspect of engine design.

Car makers may well have the information about optimum speeds for pistons but are constrained by piston weights , rod lengths , crankshaft harmonics and a stack of other items on the list with production costs and convenience frequently being at or near the top.

An interesting discussion though.

Cheers , Pete.
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Old 03-21-2008, 10:40 PM   #17 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
Sorry, I haven't been able to review the link RH provided.
It's a large, GIF file if that helps -- probably takes a while to download.

My good friend had a Tempo for a long time (until the sub-frame buckled -- too many curbs -- a welder adopted it and it's still on the road: an '89 GL).

I noticed when I drove it that the engine sounded to be racing at highway speeds (it didn't have a tach, so ??? on the RPMs). It still managed respectable FE. Consistent problems included an alternator pulley that would sync out of alignment. Is this common? It seemed to be a robust little car. I won't mention that my Wife totalled one at 16 I won't elaborate on the details, but it was her Dad's car She has a great DR since then (after a Cavalier, then Teggy came along).

What is your RPM at 60? (long story short)?

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Old 03-21-2008, 10:57 PM   #18 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
randy: thanks for the explanation, I still am not fluent in what exactly mep means to efficiency.

Here's the graph I'm talking about.

I'm not really fluent either, but the graphs in the book RH77 linked to use that. I think Mean Effective Pressure relates to a pressure plot of the whole engine cycle, with the area inside of the curve being the effective pressure. This is easy to plot, and so has been used for describing engines since they started making them.

Quote:
Originally Posted by Frank Lee View Post
The root of my question started with knowing my car is geared for 1000-1200 ft/mn at 50-60 mph. Seems perfect right from the factory doesn't it? Yet our pet theories for increased fe mods almost always include taller gearing. WTH?
That is easier to answer. If the link above worked, you can see how little power you need vs. what's available. When you go to taller gears, the MEP goes up, pushing you up towards higher efficiency. If you could control engine size, you might go with higher piston speed, but if you can't lower should almost always be better.
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Old 03-22-2008, 07:04 AM   #19 (permalink)
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The best speed for FE in a piston is an absolute stand still.
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Old 04-11-2008, 01:40 AM   #20 (permalink)
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From the Autospeed Brake Specific Fuel Consumption link:

Quote:
Firstly, why should the SFC be lowest at middle revs? Or, to put this another way, what causes an increase in fuel used per kW at both low and high revs?

At low revs, SFC suffers because there’s increased time for the heat of combustion to escape through the walls of the cylinders and so not do useful work. At higher engine speeds, the frictional loses of the engine rise alarmingly (especially in this case with 12 cylinders!) and so the energy of combustion is again being wasted, this time in heating the oil.
Maybe this is why 1000-1200 ft/sec is better than lower speeds?

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