Announcement

Collapse
No announcement yet.

Con-rods stresses: inertial+combustion forces

Collapse
This is a sticky topic.
X
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Con-rods stresses: inertial+combustion forces

    After having heard, from different sources, that some tuners were considering to reduce turbo boosts in the higher rpm to prevent con-rod failures, I decided I wanted to look a little more closely the theory, because the thing wasn't actually convincing me too much.
    Having had some time to spend, yesterday, I decided to look a little more deeper in the "con-rod stresses" related problems, with some (in my opinion) interesting reults.
    I've collected some data coming from Aprilia's Development Center testing over two of theyr engines: a "street" 650cc single cylinder, and a "racing" 600cc single cylinder.

    Following is a jpg of the significant data for those engines.
    Dimensions are of course different, but with the right proportion and from a "quality" point of wiew, they are comparable to our engines.
    Attached Files
    Last edited by Pisobiker; 16 August 2003, 22:43.

  • #2
    I've collected all the data and have developed a rough parallel to a "turbocharged" engine with same dimensional characteristics.

    Let's go in detail:

    The significant forces that concur to define rod stresses are basically two: INERTIAL and COMBUSTION forces.

    INERTIAL FORCES are the ones produced by the reciprocating movement of the piston and part of the rod. In extence it's the energy taken to make them accelerate in one direction, slow down to zero then reaccelerate in opposite direction.
    Under those conditions from 0 (zero) canckshaft degree (TDC) to 180° rod is "pulled", from 180° (BDC) to 360° rod is "compressed". These kind of forces, as well as friction losses, raise with the sqare of speed.

    Following is my 1st excell graph that came out:

    comparison between inertial forces at 7.000 rpm for
    - sandard
    - racing
    - standard at 4.000 rpm
    (in this case turbocharging or not does not make any difference)

    1st interesting poin is that, with ~30% weight loss in the reciprocating masses, inertial forces dropped of more than 40%.
    Attached Files

    Comment


    • #3
      Combustion forces are the ones produced in the "active" stroke of engine after ignition (including forces previously produced over piston to compress fresh charge in the compression stroke).
      Under those conditions from 180° (BDC) to again 180° (both the compression and combustion strokes) rod is "compressed". Other forces such as suction in the intake stroke (or, in the case of forced induction engine, positive work over the piston done by intake boost) are, in comparison with the others, too small thus not taken into account.
      Combustion forces are proportional to volumetric efficiency, intake pressure and compression ratio (CR); so they will be max where total volumetric efficiency is max (peak torque rpm).

      Following are my 2nd and 3th excell graphs:

      2nd graph:
      comparison of combustion forces at 4.000 rpm (supposed peak torque) for
      - std. engine athmospherical
      - same engine but turbocharged at 2.8 bar absolute (reduced of a % that takes into account all volumetric, entrapment, blowby and whatever losses caused by forced induction).
      Attached Files

      Comment


      • #4
        3th graph:
        same as before but at 7.000 rpm
        Attached Files

        Comment


        • #5
          At this point I was able to put inertial and combustion forces together to have a "RESULTANT" of forces giving the stress over conrod all through the 4 strokes (720 crankshaft degrees).
          This is my last excell graph:

          comparison among resultant forces for the turbocharged engine conrods
          - at 4.000 rpm and 2.8 bar abs. intake pressure
          - at 7.000 rpm and 2.8 bar abs. intake pressure
          - at 7.000 rpm and 2.4 bar abs. intake pressure
          Attached Files

          Comment


          • #6
            Now it's time to collect results.
            resultant of forces graph gives following informations:

            1) Peak force over conrods is produced in the combustion stroke; it is a "compressing one" (a "+" force) caused by combustion forces + a "pulling" one (a "-" force) produced by inertial forces. But since combustion forces decrease slightly when increasing rpm (due to higher volumetric and "entrapment" losses) and inertial forces increase a lot when increasing rpm =>
            at SAME INTAKE PRESSURE (of 2.8 bar abs. of the example), given we have a device (turbo) capable to run same pressure at both rpm, stress over conrod at 7.000 rpm IS SIGNIFICANTLY LESS than the one at 4.000 rpm.
            This means that if a conrod is supposed to run such a pressure at 4.000 rpm, there is no phisical reason why it should not run even higher at 7.000 rpm.


            2) Looking at Inertial forces differences between STANDARD and RACING (with sensibly lower reciprocating masses) is easy to guess how greater responce the racing engine would have and it's capability of running at higher rpms, with consequential gain in peak power.



            Marco


            P.S. any comment or addition welcome.

            Comment


            • #7
              Hey Marco, this confirms what I was intuitively thinking and hoping but had no way of proving it.... cool! It's nice to have an engineer with us guys, even though I know motronics, I'm still far from being an engineer unfortunately.... so actually, if Mark's rods stand 1.9 bar (and they do) at 4500 (or thereabouts), there's no reason they won't stand 1.6 at 7000 provided the turbo can do it, right? How much HP can his turbo support at that boost level?

              Dangit Mark, we're going to have to work on a 10-injector Motronic for you to make 550+ HP


              Cheers,


              Mihnea
              '98 Silver Grey 2.7T S4, dual 2.75" turbo back exhaust with no cats, FMIC, BIG injectors, 10 Bar motorsport fuel pump, modded RS4 turbos, tubular exhaust manifolds, custom intakes, RS4 MAF, Bilstein PSS9, H&R ARBs, 19" BBS CH, custom remap, 511 BHP/505ft-lbs

              www.MRCTuning.com
              info@MRCTuning.com
              Workshop Directions

              Comment


              • #8
                This stuff is brilliant Marco, its posts like this that make this forum is a great place to be, mmmm........ how much power? .

                Cheers, Mark.

                P.S. I hope you have 'time to spend' more often to do stuff like this
                Mark - Modded

                Comment


                • #9
                  Thanks guys.

                  From the data collected it seems pretty clear that from a "con-rod" point of view, you could run at least same torque as peak up to redline. To achieve that, boost would probably need to be slightly increased in the upper range to compensate for higher overall friction losses and intake flow pressure losses through pipes and IC. Must see if turbocharger is able to flow that much.
                  Not to go "out of topic", il try to find some answer on this last problem in the other thread (the "T3/T4" one).

                  Marco

                  Comment


                  • #10
                    Marco is trying to keep me awake all night trying to understand the theory here. Gimme electronics anyday :-)

                    I'm not a proper mechanical engineer, and maybe I haven't understood all the reasoning here on a 1st read, but if we are considering the 'resultant' or average forces acting on a conrod over the full engine cycle then I could intuitively see how this works BUT isn't it the instantaneous pushing and pulling forces on a conrod which causes it to bend ?

                    Uhhh lets see if I can explain this confusion better...

                    Lets say at 'normal boost' pressure at whatever speed the conrod is pushed by a force of X Newtons on one part of the cycle then pulled by a force of Y Newtons in the next cycle. Surely if X or Y exceeds the mechanical limit of the rod then it will be damaged - I can see how the rod is subject to a force of X or Y at any one time, so saying that it averages out to a force X minus Y on a whole cycle is correct but it doesn't reflect the maximum force it was subjected to.

                    I'm confused - I'm going to open a bottle of Bushmills and drink heavily for an hour or two and then try to think about this some more - maybe it will click... Or maybe I will just get a sore head in the morning.

                    This is good stuff Marco - its just too late for me man

                    Paul

                    ops:
                    Paul Nugent
                    Webmaster http://S2central.net
                    Administrator http://S2forum.com

                    1994 S2 Coupe ABY - aka Project Lazarus
                    2001 A6 allroad 2.5TDi - family tank
                    2003 S4 Avant 4.2 V8 - daily burble

                    Purveyor of HomeFries and Exclusive agent for Samco hose kits (S2/RS2)

                    There are only 10 kinds of people that understand binary - those that do, and those that don't

                    Comment


                    • #11
                      No, Paul.
                      Maybe it wasn't so clear but the "resultant" of forces is not an average. It represents exactly the real forces acting on the conrod during the whole 2 turns on crankshaft, degree by degree, as a sum of pushing and pulling forces. So the "peak" you see in the resultant graph is exactly the maximum load on the conrod for the given examples.
                      If you make an average of the forces you would get a single value representative of the whole cycle (2 crankshaft turns).
                      Anyway let's see If I can better explain:

                      If at a given crank degree (lets say after TDC in the active stroke) inertial forces would tend to "stretch" the conrod (because its reciprocating mass+piston mass it's beeng accelerated downward) combustion forces will tend to "compress" (from small end through the piston) pushing it downword. So the real force that conrod actually "feels" is the "sum" in that specific moment of the two forces. Being those one positive and one negative, resultant will be less than the greater of the two.


                      Hope what I say is clear. Doing my best.

                      Marco

                      Comment


                      • #12
                        Guys,i'd suggest that you have a look at some books written by Dave Vizard of Mini fame.This guy is a highly skilled Automotive Engineer and very famous in the Tuning world.......I once met him about 12 years ago at a show. His Mechanical Engineering knowledge of modifying Engines is incredible
                        S2 Avant......fully loaded :rock: :bow: Slipper Wagons Rule !!!!!

                        Comment


                        • #13
                          Originally posted by Audi Parts Guru
                          Guys,i'd suggest that you have a look at some books written by Dave Vizard of Mini fame.This guy is a highly skilled Automotive Engineer and very famous in the Tuning world.......I once met him about 12 years ago at a show. His Mechanical Engineering knowledge of modifying Engines is incredible


                          Is this an elegant way to tell me I'm totally wrong, man?

                          :nana:



                          I'm definitly not infallible so if there are things not clear or mistakes just tell.
                          Comments above are not meant to be the "engine bible". I was just "thinking loud" shearing with the ones of you that may be interested.



                          Marco

                          Comment


                          • #14
                            Originally posted by Pisobiker


                            Is this an elegant way to tell me I'm totally wrong, man?

                            :nana:



                            I'm definitly not infallible so if there are things not clear or mistakes just tell.
                            Comments above are not meant to be the "engine bible". I was just "thinking loud" shearing with the ones of you that may be interested.



                            Marco
                            Definately not,it's just that his books make for some excellent reading.The guy is a complete genius.......aka Vizard the Wizard
                            S2 Avant......fully loaded :rock: :bow: Slipper Wagons Rule !!!!!

                            Comment


                            • #15
                              Im new here so treat me gently..LOL..I used to build LOT or Rally engines when i was in my early 20's.None were tirbo charged but |i used to use very high CR's and dependant on engine size etc very high revs.

                              i found a cheap way to rods that were VERY strong.I used ones out of deisel engines.

                              Comment

                              Working...
                              X