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Frank Lee 03-19-2008 07:52 PM

Optimum piston speed?
 
I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?

RH77 03-19-2008 08:55 PM

Quote:

Originally Posted by Frank Lee (Post 15131)
I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?

I recall the same. At the time, I did the calcs for my engine and came up with the same RPM at which my best, constant FE is supposed to occur, which it, indeed, does:

1714 RPM @ 42.5 MPH (4th gear, Full TC lockup)

Here is a link to an article on it.

The text may hold some advice on the slower speed question.

RH77

MetroMPG 03-19-2008 09:45 PM

Quote:

Originally Posted by Frank Lee (Post 15131)
Why wouldn't, say, 700-840 ft/mn be even better?

Did you suggest that specific range for a reason?

DifferentPointofView 03-19-2008 10:30 PM

is there a calculator to determine the best speed for fuel economy? if so let me at it!

Peter7307 03-19-2008 11:09 PM

Here you go;

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.
OR:
Piston speed = 2 x Stroke in inches x rpm / 720


Please keep in mind this is only a rule of thumb and specific situations will alter the "rule".
That said it is a good starting point.

The source by the way is Taylor as linked above.
A good book too since it has all the necessary info in one place and is more up to date than many others including Judd and the now classic Ricardo publications.

Cheers , Pete.

Frank Lee 03-19-2008 11:30 PM

Quote:

Originally Posted by MetroMPG (Post 15156)
Did you suggest that specific range for a reason?

I picked 700 ft/mn out of thin air, and went with a range of 20% as in the 1000-1200.

roflwaffle 03-20-2008 12:05 AM

It's a figure relating displacement to engine speed such that for relatively (+/-10%) square engines efficiency is pretty good.

Randy 03-20-2008 02:56 AM

The text linked also has the volume 1 (under 'other editions') with nearly the whole discussion on efficiency. It centers on the efficiency graphs of piston speed vs mean effective pressure. They seem to orbit around the highest efficiency point, but mainly on MEP, not piston speed. So MEP is the key more than speed. At low MEP, lower speeds seem to hurt efficiency.

The Sulzer RTA96C that showed up as the 'most powerful diesel engine' is also quite efficient. It's rated at 92 to 102 rpm, with a 2.5 m stroke, listed 8.5 m/s (1670 ft/min) piston speed but it must be average speed. The highest efficiency is the high speed, lowest horsepower rating. Cross heads don't have much in the way of side pressure problems, but surely they don't have bore size or weight limits either. They must have chosen high piston speeds for efficiency.

Duffman 03-21-2008 12:27 AM

I would think that as slow as possible piston speed would yield the highest efficiency. After pondering it for a while maybe this ideal piston speed is best used to determine the bore to stroke ratio. After all if your piston speed is below what they consider optimum, you coulda designed you motor with more stroke and less bore for the same displacement.

LostCause 03-21-2008 03:44 PM

I wonder why an optimum rpm exists? The only things I can think of are:

Increased Inertial Loads at High Rpm?
Increased Friction at High Rpm?
Varying Disparity Between Piston Speed and Flame Front?
*(I know this set a limit on piston size during WWII @ ~5.5" Bore + Stroke)
Piston Position @ Peak Pressure?
Vibration?
Combustion Gas Dwell Time in Piston
Little Magic Elves Need Time to Do Their Work

Answers are no fun without the reasons. :)

- LostCause

Duffman 03-21-2008 04:35 PM

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.

LostCause 03-21-2008 05:05 PM

Sorry, meant piston speed.

- LostCause

Frank Lee 03-21-2008 06:21 PM

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?

roflwaffle 03-21-2008 08:06 PM

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.

tasdrouille 03-21-2008 09:04 PM

Well, the lower the RPM the higher the thermal losses!

Peter7307 03-21-2008 09:22 PM

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.

RH77 03-21-2008 09:40 PM

Quote:

Originally Posted by Frank Lee (Post 15440)
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.:o

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 :rolleyes: She has a great DR since then (after a Cavalier, then Teggy came along).

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

RH77

Randy 03-21-2008 09:57 PM

Quote:

Originally Posted by Frank Lee (Post 15440)
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 (Post 15440)
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.

8307c4 03-22-2008 06:04 AM

The best speed for FE in a piston is an absolute stand still.

Frank Lee 04-11-2008 12:40 AM

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?

Duffman 04-11-2008 01:37 AM

Quote:

Originally Posted by Frank Lee (Post 18982)
From the Autospeed Brake Specific Fuel Consumption link:



Maybe this is why 1000-1200 ft/sec is better than lower speeds?

My old Thermodynamics professor use to say heat transfer is a slow process. I dont think its why.

tasdrouille 04-11-2008 08:04 AM

At least 60% of the energy in an ICE is lost as heat. Thermal losses through the cylinder walls are significant at any rpm but more so at lower rpm.

dremd 04-11-2008 10:54 AM

A few notes on Large diesels operating more efficiently at lower speeds

1) CRAZY long strokes = Higher Mean piston speeds
2) Diesel must have a different burn rate than Gasoline
3) Diesel components are forced to be stronger due to stress, heavier= stronger = more reciprocating mass= less reciprocations to reduce energy loss in reciprocations


I'm no expert here, but it is what came to mind.

}{ead$hot Zod 04-11-2008 11:04 AM

IIRC when it comes to diesels it's based mostly on torque. They best way to figure that is to get it dyno tested. Since the torque curve will always change when you mod the motor this will have to done every time you make a mod. With this knowledge you will know exactly when to shift the tranny.

Duffman 04-11-2008 11:42 AM

Quote:

Originally Posted by tasdrouille (Post 19008)
At least 60% of the energy in an ICE is lost as heat. Thermal losses through the cylinder walls are significant at any rpm but more so at lower rpm.

True about the heat losses, I dont have the source handy but the majority of the heat lost in a ICE is pumped out of the engine through the exhaust and replaced with cool intake air. The actual losses to the engine coolant is small in comparison. Thats why air cooled engines can exist.

jonr 07-12-2008 09:55 AM

No, it's roughly 1/3 lost to engine heat and 1/3 out the exhaust.

Agreed - heat loss is the problem with low piston speeds. In general, you want high piston speeds, quick burn rate, high cylinder temps and pressures, long stroke, high compression, and insulating materials or coatings inside the combustion chamber (aluminum is not your friend).

Of course you also want the engine to stay in one piece.

aerohead 07-12-2008 03:26 PM

piston speed
 
Quote:

Originally Posted by Frank Lee (Post 15131)
I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?

Boy Frank,that's a new one for me.Although many things are new to me.I was looking at BSFC maps last night and don't recall a term like that even on the charts.Rpm seems to matter.Road load is an issue.Volumetric efficiency.The oil ring,by itself,is supposed to be responsible for 10% of all engine friction.And friction is given as a square function of rpm and is borne out by the curves for internal friction.I'll look into it,but off the top of my head,I'm clueless.It's worth knowing,and it seems like there would be a context to if making such a general claim like that.Maybe there's a powerplant expert in the house?

jonr 07-12-2008 04:30 PM

You won't see piston speed on a map because the only way to change it independent of RPM is to redesign the engine.

You can build an efficient engine for 100 rpm or for 3000 rpm - and the 100 rpm one might have more friction losses.

aerohead 07-14-2008 02:14 PM

Quote:

Originally Posted by Frank Lee (Post 15131)
I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?

Sorry everybody,I took a couple days to dig through my rat's nest.Here's some stuff I culled out.The only statement citing an optimum piston speed range is from "Internal Combustion Engines",an old textbook,by Edward F. Obert.Piston speed wasn't even listed in the index,but after forraging,I found this citing:-------------------------- "...large and small diesel engines(designed to develop 125-200 bmep),attain their minimum specific fuel consumption in the range of 1,200-1,500 ft/min mean piston speeds."------------------------------------- A large-bore 100-rpm,700 hp/cylinder diesel engine for direct propeller drive was given with the lowest BSFC,@ 0.32 lb/hp-hr.------------------------------------ Friction seems to be the overriding factor for maximum BSFC.----------------------------------------- for spark-ignited gasoline engines,best BSFC occurs at point of maximum charge loading efficiency,which is basically where torque peaks,which is at about 80% load.At light load,or full load,efficiency suffers.--------------------------------------- BSFC maps showed minimums of about 0.45 lbs ( 275 grams ) per hp-hr.(0.746 kWh).-------------------------------- The Orbital Engine Co. 2-stroke is rated at 7-12% better BSFC,but they can't seem to clean up the emissions enough to use it.------------------------------------------- SAE said that an"adiabatic compound engine" operating at 1,300-1,500 degrees F,with no cooling system,and all waste energy into exhaust,could be harvested with compound turbos doing double duty,supercharging and doing shaft work with a BSFC 0.29 lb/hp-hr.------------------------------------ I guess that means a ceramic engine we've been hearing about for 20-years or so.

bryn 07-14-2008 02:43 PM

Quote:

Originally Posted by aerohead (Post 43878)
---------- for spark-ignited gasoline engines,best BSFC occurs at point of maximum charge loading efficiency,which is basically where torque peaks,which is at about 80% load.At light load,or full load,efficiency suffers.---------

so if 80% load at ~1700 rpm is 50 hp, that is only the most effecient engine speed if you need 50 hp. most of our cars might only need 15 hp to cruise down the road at 55mph.
if we change the gearing so that 15 hp is closer to 80% load, maybe down around 1000 rpm. the engine will still be more effecient even though piston speed is less than ideal.

a manual cvt and a scan guage setup would be really interesting to see

aerohead 07-14-2008 02:57 PM

gearing
 
Quote:

Originally Posted by bryn (Post 43887)
so if 80% load at ~1700 rpm is 50 hp, that is only the most effecient engine speed if you need 50 hp. most of our cars might only need 15 hp to cruise down the road at 55mph.
if we change the gearing so that 15 hp is closer to 80% load, maybe down around 1000 rpm. the engine will still be more effecient even though piston speed is less than ideal.

a manual cvt and a scan guage setup would be really interesting to see

Yeah bryn,Hucho calls it gear-matching,and says that, say ,you streamline a car,you could lose up to 60% of the mpg potential if you don't redo the gearing.That really sucks! The engine wants to see the same "load" or it falls off it's "sweet-spot".

metromizer 07-14-2008 06:25 PM

As if this wasn't confusing enough...

Piston speed isn't constant, right? The piston stops at top and bottom dead centers, therefore accelerates and decelerate in between, or every 180 degrees of crankshat rotation. So using 'average piston speed' is kinda like using average speed while calculating fuel economy: Great if you only vary a few mph, but not if your commute invovles stop-n-go traffic.

The variable of rod ratio governs max piston speed for a given crankshaft rotational position, so wouldn't average speed be the same for every rod ratio if the same rpm is used?

Acceleration and deceleration rates, dwell time where the piston is at it's slowest, burn rate and bore size. Race engine builders ofter get to a place where the piston outruns the flamefront. Phew... so much going on I get dizzy. But wait, there's more, there's cylinder filling efficiency... what is optimal for the power cycle may hurt the intake or two other cycles to the point you have 1+1 steps forward, then 2 steps backward. :confused:

I think if you fix certain parameters (bore, stroke, port and head design) every engine has it's own 'sweet spot' where it is most fuel efficient, with fifteen (a number taken out of thin air) variables that influence it. I think you can make tweaks and changes that improve on FE, but question weather simply decreasing the final drive ratio until the engine no longer accelerates is the magical answer.

On the high performance side of things, I spent hours playing with "Engine Analyser" software varying only camshaft and exhaust headers selection. Then I put the thing on the dyno :eek: :D

I concluded the whole thing is one giant compromise.

dremd 07-14-2008 06:27 PM

Quote:

Originally Posted by metromizer (Post 43969)
As if this wasn't confusing enough...

Piston speed isn't constant, right?

I typically see it rated as Mean (average) Piston Speed. Also bear in mind that rod length alters the time that the piston is near/ at TDC/ BDC. It is actually an incredibly complicated science once you get down to the tid bits.

Quote:

Originally Posted by metromizer (Post 43969)
I concluded the whole thing is one giant compromise.

I second your conclusion.

bryn 07-14-2008 07:05 PM

Quote:

Originally Posted by metromizer (Post 43969)

I concluded the whole thing is one giant compromise.

so how about a wankel rotary? just to throw one more wrench into the mix. anyone play with one of those for fuel economy?

jonr 07-14-2008 07:37 PM

Generally not very efficient - they have too much combustion chamber surface area - so the hot gases cool off before you can get work out of them.

bryn 07-14-2008 08:07 PM

Quote:

Originally Posted by jonr (Post 44000)
Generally not very efficient - they have too much combustion chamber surface area - so the hot gases cool off before you can get work out of them.

that would make sense

not to get off topic but are there any other non fourstroke engines that people are working with? is there another thread somewhere that covers different engine types?

MechEngVT 07-15-2008 08:59 AM

Quote:

Originally Posted by metromizer (Post 43969)
Piston speed isn't constant, right? The piston stops at top and bottom dead centers, therefore accelerates and decelerate in between, or every 180 degrees of crankshat rotation. So using 'average piston speed' is kinda like using average speed while calculating fuel economy: Great if you only vary a few mph, but not if your commute invovles stop-n-go traffic.

The variable of rod ratio governs max piston speed for a given crankshaft rotational position, so wouldn't average speed be the same for every rod ratio if the same rpm is used?

Using Mean Piston Speed is the effective average speed of the piston as it travels through the given stroke over the given time. It doesn't matter how wide the engine's operating range is the only factor affecting mean piston speed is the physical engine design (considered fixed) and the actual engine speed. It's similar to using mean effective pressure to talk about combustion pressure...it's not REALLY how much pressure is in the cylinder since theoretically at TDC pressure is tremendously large and drops off very quickly as the piston descends on its power stroke, but there is an average effective pressure that would complete the same amount of work as the actual combustion does and that (the MEP) is a useful design tool.

I don't think rod/stroke ratio affects maximum piston speed. It will have a *huge* affect on maximum piston acceleration and therefore wristpin loading (which will limit maximum RPM), but generally pistons accelerate from 0 at BDC to max speed halfway up the cylinder and decelerate to 0 at TDC. When piston speed is maximum the rod is perpendicular to the crankshaft throw and the piston's maximum linear speed at that engine rotational RPM is independent of connecting rod length. The higher the rod/stroke ratio the closer the piston's actual velocity approaches a sinusoidal variation, and lower ratios cause less dwell (i.e. sharper acceleration into and out of) the top/bottom centers. So succinctly, the average piston speed for a given engine RPM is independent of rod length, just as the formula quoted earlier indicates.

metromizer 07-15-2008 04:52 PM

Quote:

Originally Posted by MechEngVT (Post 44175)
Using Mean Piston Speed is the effective average speed of the piston as it travels through the given stroke over the given time. It doesn't matter how wide the engine's operating range is the only factor affecting mean piston speed is the physical engine design (considered fixed) and the actual engine speed....

I don't think rod/stroke ratio affects maximum piston speed. It will have a *huge* affect on maximum piston acceleration and therefore wristpin loading (which will limit maximum RPM), but generally pistons accelerate from 0 at BDC to max speed halfway up the cylinder and decelerate to 0 at TDC. When piston speed is maximum the rod is perpendicular to the crankshaft throw and the piston's maximum linear speed at that engine rotational RPM is independent of connecting rod length. The higher the rod/stroke ratio the closer the piston's actual velocity approaches a sinusoidal variation, and lower ratios cause less dwell (i.e. sharper acceleration into and out of) the top/bottom centers..

So succinctly, the average piston speed for a given engine RPM is independent of rod length, just as the formula quoted earlier indicates.

I think we agreed/I understand, independant of average piston speed for a given rpm.

But, I should have said piston speed (average, max, and profile) is stroke dependant, right? Also, the other key is piston speed has a profile that is rod ratio dependant. For a given rpm, the bigger the engine's stoke, the greater the piston speed. For a longer stroked engine for instance, the piston has a longer distance to travel over the same time, so piston travel in feet per minute (speed) must be greater.

In your expanding pressure wave example you used, that wave has a pressure profile. With dwell, max piston speed, the piston must have a speed profile we could graph over 180 crankshaft degrees. Short rod race engines are known for yanking the pin out of the piston boss from extreme acceleration rates, for example. Short rod race engines are also known for 'outrunning' the expanding pressure wave at very high rpms. Extreme acceleration rates are the result of high bore-to-rod angles present in a high rod ratio engine. If I think about a graph, I think about the (piston speed) magnatude is stroke dependant, and the speed profile is rod ratio dependant. The shape of the curve from TDC to where the rod is perpendicular to crankshaft throw, is rod ratio dependant.

Why do you care?

Some choices must be better for for extracting the maximum motion energy from that expanding pressure wave.

If what I've said is true, and average piston speed changes with stroke, to answer the question "what rpm do I want to acheive XX average piston speed" one needs more information... what's the stroke?

There may very well be a magic average piston speed, but not a magic rpm without considering the engine's stroke.

I would argue there must be an optimal piston speed profile (which include speed, decel and accel rates) to take the most advantage of a given pressure wave profile, but I also think there is a lot more to the puzzle. I also belive for every piston speed profile, the pressure wave profile changes.

You should want to find the best engine specific RPM for best FE, with so many variables I think you need an engine dyno to tell you that.

jonr 07-15-2008 05:18 PM

Some engine designs have used cams instead of crankshafts so they can better control the piston movement/speed profile.

Better combustion chamber shape at TDC is also an attribute of long stroke engines - a flat pancake chamber at ignition vs something a little closer to a sphere.

aerohead 07-16-2008 04:40 PM

This engine stuff is really a can of worms.I think the spirit of what Hucho is eluding to is that given whatever engine we have,it's going to have it's "sweet-spot." If we streamline the car,we move the engine out of this area of performance,and without gear-matching,we stand to lose the full benefit of the streamlining.If I don't alter anything under the hood,and switch to a lower numerical gear ratio,in theory,I should be able to restore the engine to it's plateau of efficiency.I think that's all he's saying.I will lose on acceleration,and everybody is in agreement,that it would be better to have another gear,twelve gears to be exact.Some guys at Bonneville are running series transmissions to get all kinds of gear splits,as their cam grinds and induction provide very narrow power bands,requiring very small rpm drops between gears.One day I may go this direction as I fear I may never see the full benefit of the wind-cheating.


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