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? |
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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 |
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is there a calculator to determine the best speed for fuel economy? if so let me at it!
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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. |
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It's a figure relating displacement to engine speed such that for relatively (+/-10%) square engines efficiency is pretty good.
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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. |
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.
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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 |
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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Sorry, meant piston speed.
- LostCause |
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? |
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. |
Well, the lower the RPM the higher the thermal losses!
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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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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 |
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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:
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The best speed for FE in a piston is an absolute stand still.
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From the Autospeed Brake Specific Fuel Consumption link:
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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.
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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. |
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.
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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. |
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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. |
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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 |
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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. |
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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.
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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? |
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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. |
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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. |
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. |
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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