Forums » Grassroots Motorsports » Supercharging an engine creates less wear? 1 2

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  Jan. 7, 2011 12:42 p.m. z31maniac SuperDork

  Someone, someone PLEASE help me understand this.

  It was posted in the local section of bfc, and while searching for info I ran across a company web site (selling superchargers mind you) that posited this as well.

  Here is what was posted on bfc:

  "RPM is what kills engine parts. Typically, an unblown engine has to run up to 6-7,000 rpm to make any real power. But a blower substantially increases power and torque at much lower rpm's. Additionally an engine sees maximum load on the components at the moment the piston changes speed from going up in the cylinder to going down. There is a commonly held theory, too complicated to go into here, that increasing the combustion pressure, which a supercharger does, actually reduces this maximum load when piston travel changes from up to down. Under this theory, at comparable RPM's a blown engine is easier on parts than an unblown engine. In actuality, as long as detonation is controlled, you rarely have any engine failures with a blower."

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  Jan. 7, 2011 12:59 p.m. Otto Maddox HalfDork

  As long as detonation is controlled, you rarely have engine failures whether you have a blower or not. As for the rest, I don't know.

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  Jan. 7, 2011 1:01 p.m. z31maniac SuperDork

  I'm not talking about blowing an engine up from poor tuning. This part specifically.

  "There is a commonly held theory, too complicated to go into here, that increasing the combustion pressure, which a supercharger does, actually reduces this maximum load when piston travel changes from up to down. Under this theory, at comparable RPM's a blown engine is easier on parts than an unblown engine."

  How does increasing cylinder pressure reduce the load on pistons/rods?

  That makes no sense to me at all. When someone says it's too complicated, to me, that means they don't understand it enough to explain it, so I drop it in the "big fat pile of BS" pile and move on.

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  Jan. 7, 2011 1:21 p.m. mr2peak Reader

  It's the acceleration/deceleration that puts major stress on the bearings. Because you make power at a lower RPM you don't suffer from that as much. Plenty of stress elsewhere.

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  Jan. 7, 2011 1:32 p.m. Raze Dork

  I think they're talking about the dynamic loading and unloading of the bearings, connecting rods, pistons, wrist pins, etc as in, at higher RPM they have to change direction much faster than at low RPM, thus increased compression and tension (or shear) on whichever part we're talking about. The logic, I believe, is in the comparison of power levels, i.e. unboosted 7000 RPM vs boosted 3500 RPM engines of same displacement making identical power (rudimentary example). What is glossed over is how much increased force is exerted on the same parts at the same RPM when you go to FI instead of looking at the change in RPM vs power. Its pretty back-asswards to me and that's how I roll...

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  Jan. 7, 2011 1:35 p.m. z31maniac SuperDork

  EDIT: Raze threw in his post while I was writing mine.

  Yep, understand that. Guess I haven't broken this down enough yet.

  OK, so lets say we have X engine turning 3000 rpms, with no boost. The piston/rod sees Y load.

  Now we take the same X engine turning 3000 rpms, but running 1 bar of boost, so roughly twice as much fuel/air in the cylinder. Not only are increasing cylinder pressure just from the perspective of having twice as much "stuff" in the cylinder, but it now goes BANG! much much harder, hence it's ability to produce more power.

  SO, how does having an engine under boost, and producing more power (at the same RPM so that variable is eliminated) produce LESS LOAD on the piston/rod combination?

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  Jan. 7, 2011 1:48 p.m. Raze Dork

  z31maniac wrote: SO, how does having an engine under boost, and producing more power (at the same RPM so that variable is eliminated) produce LESS LOAD on the piston/rod combination?

  That's what I'm saying, the whole glossing over of a very complex and most likely situation specific analysis is like saying don't worry about why and airplane flies kids, it just does. I'm not buying the simplifications made given an understanding of thermo and mechanics...

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  Jan. 7, 2011 1:58 p.m. bradyzq Dork

  When you look at the cylinder pressure graph over a power stroke, supposedly the average cylinder pressure in forced induction is double (in your example of 1 bar boost), but the peak cylinder pressure is only 20-30% (or something like it) higher than atmospheric. And it is the average, not peak, cylinder pressure over the stroke that is proportional to power.

  The above is (probably badly) paraphrased from "Maximum Boost" by Corky Bell.

  That would explain and corroborate the OP quoted post.

  Cheers, Brady

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  Jan. 7, 2011 2:11 p.m. 11110000 Reader

  z31maniac wrote: SO, how does having an engine under boost, and producing more power (at the same RPM so that variable is eliminated) produce LESS LOAD on the piston/rod combination?

  It doesn't. The S/C company is probably abusing the idea of the relationship that bradyzq is referring to; FI gives a bigger bang, but (thankfully) the higher cylinder pressure is spread over more of the power stroke, resulting in a much smaller increase in peak pressures than you might think. Hence, making big power at low RPM with boost is less stressful on rods than revving an NA motor to 9K RPM.
  

  Less 'wear'? Define wear.

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  Jan. 7, 2011 2:17 p.m. imirk New Reader

  The question is: who boosts their engine, and then doesn't run it up to the point it runs out of breath? So you just end up with more power and more stress than stock. This really only makes sense if you are building your engine thinking do I want a 9K RPM Screamer or a 6K RPM Grunter? and then a properly tuned and supported (cooled) FI system might be more "reliable" aka have a longer service interval.
  

  Intellectually I am a bigger fan of turbos taking heat and energy out of the exhaust rather than taking the energy out of the crank.

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  Jan. 7, 2011 2:33 p.m. bigbens6 New Reader

  11110000 wrote:

  z31maniac wrote: SO, how does having an engine under boost, and producing more power (at the same RPM so that variable is eliminated) produce LESS LOAD on the piston/rod combination?

  It doesn't. The S/C company is probably abusing the idea of the relationship that bradyzq is referring to; FI gives a bigger bang, but (thankfully) the higher cylinder pressure is spread over more of the power stroke, resulting in a much smaller increase in peak pressures than you might think. Hence, making big power at low RPM with boost is less stressful on rods than revving an NA motor to 9K RPM.
  

  Less 'wear'? Define wear.

  This

  In theory a 500HP NA motor running 7,000 rpm could have more wear then a 500hp SC/TC motor running 5,000rpm. Rotating forces increase at a proportion exponentially to the square of engine speed, meaning going from 3Krpm to 6Krpm is actually 4 times more stress.
  as seen here: http://www.arp-bolts.com/pages/technical_design.shtml Which makes F1 cars so much more impressive at 18K rpm... but i digress.

  By revving lower and spreading the bang over more of the bang stroke, you can greatly decrease rotational forces without greatly increasing peak combustion chamber pressure

  In Corkey Bell's book "Maximum Boost" he details this notion VERY well with good graphs, I am summarizing but he theorizes you can roughly add 50% power to most applications without tickling the mechanicals of a motor, where as you can only over-rev maybe 500-1000 RPMS, which even with heads and cams prolly wont get you 50% more power and would RUIN drivability prolly.

  This is why Corky argues that turbos are superior power adders because they can provide their power in the mid range when the mechanical components are under the least reciprocating stresses. He also argues that a CF SC is possibly a poor choise as it adds boost as a square function meaning your adding the MOST boost at the highest RPM, thus adding the stress at the worse possible time.

  The other side not talked about is that our 500HP NA motor will see less stress at 5K rpm than our 500HP SC/TC motor at 5k rpms, because the SC motor is making more power at that rpm than the NA motor, but that does not necessarily mean more or less wear and tear...

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  Jan. 7, 2011 2:43 p.m. ReverendDexter SuperDork

  z31maniac wrote: There is a commonly held theory, too complicated to go into here, that increasing the combustion pressure, which a supercharger does, actually reduces this maximum load when piston travel changes from up to down.

  This part makes sense to me. During the exhaust stroke --> intake stroke transition, the load on the wrist pin bearing transfers from the rod side to the piston side. In an NA motor, after the load transfers, the piston has increased load acting against it as the increase in cylinder volume as the piston travels downward creates vacuum above the piston. In a boosted motor, there would be pressure on top of the piston, which would both provide resistance against the initial shift of the piston load on the wrist pin from rod-side to piston-side, but would also help reduce the piston-side load so long as the intake valve was open.

  During the compression-->ingition transition, if the ignition even happens at any point at or after TDC, the increased air/fuel in the cylinder will also create more backpressure against the top of the piston, again acting as a cushion as the load transfers from the rod side to the piston side of the wrist-pin.

  Now, all of this is pure speculation on my part, though, and I have no idea how much actual effect the above really has. If you make a magical unit of wear, I'll call it a Mulleton, the boost may reduce wear 1 Mulleton due to the above, but increase wear 10 Mulletons due to other added stresses.

  --There is no turd more polished than the Ford Mustang--

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  Jan. 7, 2011 2:45 p.m. Teh E36 M3 HalfDork

  BigBen provides the link- for the same power levels, super/turbo charging likely reduces wear primarily because the parts don't have to move as fast to produce the same power.
  I've also heard the idea that because there is positive pressure on the intake stroke, there is always a downward pressure on the piston/rod combination (all four strokes push down on the piston head, vice the crank/rod pulling the piston down on the intake stroke), which theoretically reduces the momentum changes on the piston/rod/crank interface. I'm not sure I buy this as a wear reducer, or that the pressure on the intake stroke is enough to create a net downward pressure on the piston.

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  Jan. 7, 2011 2:47 p.m. bigbens6 New Reader

  Teh E36 M3 wrote: BigBen provides the link- for the same power levels, super/turbo charging likely reduces wear primarily because the parts don't have to move as fast to produce the same power. I've also heard the idea that because there is positive pressure on the intake stroke, there is always a downward pressure on the piston/rod combination (all four strokes push down on the piston head, vice the crank/rod pulling the piston down on the intake stroke), which theoretically reduces the momentum changes on the piston/rod/crank interface. I'm not sure I buy this as a wear reducer, or that the pressure on the intake stroke is enough to create a net downward pressure on the piston.

  I dont think it is intended ot produce a net downward force, it is theorize that it mitigates SOME of the upward force thus reducing the tensile for on the wrist pin and rod...
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  Jan. 7, 2011 2:47 p.m. WilberM3 HalfDork

  ReverendDexter wrote: This part makes sense to me. During the exhaust stroke --> intake stroke transition, the load on the wrist pin bearing transfers from the rod side to the piston side. In an NA motor, after the load transfers, the piston has increased load acting against it as the increase in cylinder volume as the piston travels downward creates vacuum above the piston. In a boosted motor, there would be pressure on top of the piston, which would both provide resistance against the initial shift of the piston load on the wrist pin from rod-side to piston-side, but would also help reduce the piston-side load so long as the intake valve was open.

  i remember reading years ago something from mercedes' racing program where they said that their turbocharged engine could run without connecting rod caps (assuming you could keep it lubricated)

  Dave

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  Jan. 7, 2011 3:24 p.m. z31maniac SuperDork

  Yes, but on the compression stroke the piston/rod is now seeing much more "resistance" to moving up than it would in a NA motor.

  So to me it doesn't seem reasonable to assume that decreased stress during the piston/rod easiest job (moving down on the intake stroke) makes up for the increase in stress during the piston/rod more difficult compression stroke.

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  Jan. 7, 2011 3:33 p.m. bigbens6 New Reader

  z31maniac wrote: Yes, but on the compression stroke the piston/rod is now seeing much more "resistance" to moving up than it would in a NA motor. So to me it doesn't seem reasonable to assume that decreased stress during the piston/rod easiest job (moving down on the intake stroke) makes up for the increase in stress during the piston/rod more difficult compression stroke.

  BUT... (god this is complex) steel (and most metals) by nature recover from COMPRESSIVE forces easily, it does not permanently distort, it cannot however recover from TENSILE forces, so increased compressive force (on the compression and power strokes for instance) will not effect longevity unless you have a catastrophic failure like a bent rod... Once a rod it stretched however is cannot return to its original state, thus tensile forces such as those seen at TDC are much more potentially harmful then compressive like seen at BDC, so negating ANY tensile force is worth your time, that being said the forces are are talking about a MASSIVE and 10-15psi is not going to make a HUGE difference realistically. http://en.wikipedia.org/wiki/Strength_of_materials:

  Compressive stress (or compression) is the stress state caused by an applied load that acts to reduce the length of the material (compression member) in the axis of the applied load, in other words the stress state caused by squeezing the material. A simple case of compression is the uniaxial compression induced by the action of opposite, pushing forces. Compressive strength for materials is generally higher than that of tensile stress. However, structures loaded in compression are subject to additional failure modes dependent on geometry, such as Euler buckling. Tensile stress is the stress state caused by an applied load that tends to elongate the material in the axis of the applied load, in other words the stress caused by pulling the material. The strength of structures of equal cross sectional area loaded in tension is independent of cross section geometry. Materials loaded in tension are susceptible to stress concentrations such as material defects or abrupt changes in geometry. However, materials exhibiting ductile behavior(metals for example) can tolerate some defects while brittle materials (such as ceramics) can fail well below their ultimate stress.

  Generally explains it, again this is all regurgetated and summarized form my memory of corky bells book... im not an engineer or anything
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  Jan. 7, 2011 4:55 p.m. Type Q HalfDork

  It seems to me the answer is it depends. Many variables are going to affect the stresses the parts are under whether NA or forced indcution. I am sure Eric (Alfadriver) and people like him who do OEM powertrain development, spend untold hours measuring and modeling these forces to achieve.

  Tough times don't last. Tough people do.

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  Jan. 7, 2011 6:14 p.m. wlkelley3 Dork

  If I understand correctly, the less engine wear comes into play from having to accelerate/decelerate less. Say engine idle at 1k and as NA makes power at 5k. With a blower it would take ~3k to make the same power as 5k on a NA. Less forces (or change of forces) because of the less rpm change to equal the same power.

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  Jan. 7, 2011 7:01 p.m. erohslc Reader

  It's not just about bearing loads. As stated the inertial bearing loads increase as the square of the RPM. Whereas gas pressure bearing loads from combustion (BMEP, Brake Mean Effective Pressure) are approximately proportional to torque produced. With most automotive engines, inertia loads exceed gas loads above 4-5K RPM. But remember that HP is torque (basically BMEP) times speed (RPM). So it's true that a NA motor which produces 300 HP at 6K RPM has greater bearing loads than the same motor boosted which produces 300 HP at 3K RPM. But to generate 300 HP, the same motor will have generate twice the BMEP at 3K RPM as at 6K RPM. And so the stress on head gaskets, etc will be twice as great.

  Given a choice, I'd still prefer to load parts at 2x instead of 4x to make more power, but a successful result is always a compromise.

  Carter

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  Jan. 7, 2011 7:57 p.m. erohslc Reader

  In reply to bigbens6: Huhh?

  Your quote from Wiki is true, but you've emphasized the wrong bits. The concern is not the strength of the material (the metal the conrod is made from), but the strength of the component (the conrod itself).

  Beams made from most materials, and virtually all metals, are far stronger in tension than compression. Tensile strength of a beam component like a conrod is primarily limited by UTS of the material, while compression strength is determined by the physical design, ie H beam, I beam, tubular. This is elementary column theory, a member stressed in compression will fail due to buckling, a member stressed in tension will fail due to tensile fracture.

  Consider a segment of piano wire, or carbon fiber: Tensile strength: very high. Compression strength: almost zero.

  Virtually all current high performance structures seek to take advantage of this by stressing the materials in tension or shear rather than compression wherever possible.

  Tubular steel space frames use triangulation to place major loads in tension. Monocoque aluminum airframes resolve loads into shear or tension wherever possible. Carbon fiber structures utilize selective strand orientation and placement to translate compression forces into tension.

  But from a practical standpoint, the weakest part of a conrod is the bigend bearing, which must ultimately handle all of the loads, tensile and compressive.

  Seen far more bearings spun than conrods bent or broke.

  Carter

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  Jan. 9, 2011 9:05 p.m. bigbens6 New Reader

  Carter: Thats all fine and good but the discussion is specifically about adding combustion chamber pressure to counter act the tensile strain at TDC.
  Piano wire is a poor analogue for a connecting rod, while your example is sound in logic, it really does not apply, a stack of quarters has quite a bit of compressive strength but NOT tensile strength, but that too is a terrible example.
  

  While the bearings do in the end take any and all load, if oiling is up to par it is a non issue. In the end i think we are saying the same thing, and while you stated i focused on the wrong part of my wiki link, i think you focused on the wrong points in my post.
  

  The post i was referencing was not accurate, I only aimed to clarify that in the case of connecting rods, they are more apt to handle compressive forces better than tensile. You see alot more buckling failure because big power motors typically are NOT high revving in the private world, most people go more displacement or power adders over high revving, its just easier on a private budget and cheaper to run so tensile forces are relatively stagnant and combustion pressure is where the increase is seen, because it is easier for the materials to take :shrug:

  Tensile strength of a beam component like a conrod is primarily limited by UTS of the material, while compression strength is determined by the physical design, ie H beam, I beam, tubular. This is elementary column theory, a member stressed in compression will fail due to buckling, a member stressed in tension will fail due to tensile fracture.

  When you have a predetermined weight you can account for thats all fine ad good, but we are talking about ADDING power over and above current levels and its impact on wear and tear, and again the increase in forces are to the square of engine speed, where as increases in torque are pretty linear, we aren't talking about a constant set load, we are talking about constant changes in force from tensile to compressive and acceleration from those tow. Assuming the part in question does not catastrophically fail on one revolution or stroke, compressive forces that slightly surpass the parts max load are MUCH easier and create less (if any) lasting permanent damage than tensile forces.

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  Jan. 10, 2011 8:55 a.m. kb58 Reader

  I always like analogies.

  About making more power with no more wear: That comes from the fact that the force on the piston is not increased - it just lasts longer. Instead of giving someone a hard shove, you just push on them for a longer time.

  About decreasing wear on the bearings, rods, and pistons: Think of it this way. You (the piston) are running toward a wall (combustion chamber roof) and slide to a stop, then run away - over and over - which is hard on your joints. Now, put a spring (higher combustion chamber pressure) on the wall. Again, run toward the wall, but this time as you start to slide to a stop, you hit the spring, which cushions your stop and in fact starts you accelerating easier in the other direction. The result is less wear on your joints.

  All that aside, you're wise to examine very closely anything that a vendor offers as proof that they have a great product.

  Kimini, and now a mid-engine Seven, www.midlana.com

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  Jan. 10, 2011 9:51 a.m. chuckles Reader

  Just for possible discussion and FWIW, let me throw in a layperson's vague impression that, for many mechanical systems, "wear," however that is defined, is not linear with stress or load. Within a load range between zero and some determinable limit, increasing stress may not increase wear AT ALL. I can imagine several possible examples...main bearings, perhaps. Past that "no wear" limit, additional stress "suddenly" begins to cause measurable wear, probably due to distortion of the structure. Within a (usually narrow) range, this wear may be acceptable in some applications, like racing. The top end of this range is catastrophic failure.

  Does this make sense? The idea that you can double an engine's output with boost and still have it last longer than an engine that must be spun up, makes sense to me. But, I'm just guessing.

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  Jan. 10, 2011 1:15 p.m. bigbens6 New Reader

  KB:58 Yea the theory is that the boost is a cushion near TDC, how ever the net effect is questionable when you look at piston speeds and momentum vs 14 psi on a few square inches...

  Chckles: yes there are thresholds where any wear is effectively none, assuming proper oiling. Because the OIL is what wears...

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