Announcement

Collapse
No announcement yet.

Cam profile mania

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #31
    No need to leave all the fun to Brian if you have a H&M do some experiments and try to advance the science just a little at least.

    Comment


    • #32
      Originally posted by Lew Hartswick View Post
      ...... I think it would be an interesting thing for "Brian" to do. :-)
      ...lew...

      Yep, no pressure there.... nope... none at all....

      Thanks for the good laugh when I read that part
      Chilliwack BC, Canada

      Comment


      • #33
        My B&S 280000series has burnt its starter motor contactors over its life. ( it is about 16 years in service !)
        About 8 years ago i learned on internet that the valve lash has to be set within spec in order to decompress during starting.
        So now I set the lash every year or so to in spec ( both 0.13mm =0.005 inch by the B&S maintenance sheet)
        That certainly helps the starter to crank over in 20 Farenheit, , but both rockers wear loose every year.
        There is no liquid oil up in the rocker box.
        I would appreciate any comments on how the decompression works, about the lack of oil in the rocker box,
        and,... "how much longer will she last" ?
        Thanks
        Last edited by wombat2go; 12-28-2016, 10:53 PM.

        Comment


        • #34
          Some good points there BC Rider.
          One thing to keep in mind. Gasoline is a fast burning fuel, so the majority of the expansion takes place shortly after ignition. I don't know but I would guess that there is not much pressure left once the piston gets to the lower end of it's travel. So opening the exhaust valve early should not affect the operation much.
          Only my observation - no facts to back it up.
          Larry - west coast of Canada

          Comment


          • #35
            Originally posted by Cuttings View Post
            Some good points there BC Rider.
            should not affect the operation much.
            Only my observation - no facts to back it up.
            Search " Otto Cycle " for pressures and efficiencies.
            One ref here is: Eastop and McConkey: "Applied Thermodynamics for Engineering Technologists"

            Comment


            • #36
              Originally posted by Cuttings View Post
              Some good points there BC Rider.
              One thing to keep in mind. Gasoline is a fast burning fuel, so the majority of the expansion takes place shortly after ignition. I don't know but I would guess that there is not much pressure left once the piston gets to the lower end of it's travel. So opening the exhaust valve early should not affect the operation much.
              Only my observation - no facts to back it up.
              More to the point, the piston does not move much for the last bit of rotation toward BDC, so the mechanical advantage is horrible, and there is little lost in terms of power from opening during the time the piston is not moving much, So if there is any advantage to be had from early opening, you may as well go for it.
              CNC machines only go through the motions.

              Ideas expressed may be mine, or from anyone else in the universe.
              Not responsible for clerical errors. Or those made by lay people either.
              Number formats and units may be chosen at random depending on what day it is.
              I reserve the right to use a number system with any integer base without prior notice.
              Generalizations are understood to be "often" true, but not true in every case.

              Comment


              • #37
                Good understanding of it JT, and I might add the benefits of proper evacuation go beyond just that stroke esp. when higher RPM's are engaged, now the lower part of the stroke is really piss poor - so cracking the exhaust valve open way ahead of time and giving the gasses a head start and ALSO getting them moving in a fury will help the next exhaust stroke evacuate better esp. in multi-cyl. engines with header/collector systems,

                you can see how this snowballs, now throw in the fact that many intake manifolds are also tuned to come on in a specific range,,, it's possible to exceed atmospheric pressures "stacking" up behind the intake valves backside in a pneumatic ram effect, couple this to what's going on in the exhaust side of it and there is allot of thought that goes on in normally aspirated engines that put out some respectable HP's

                for back in the day the B-16 honda V-tec was the highest HP per CC car engine ever in mass production... they got the job done by going from a two valve per cylinder down low - to opening up all four at higher RPM's

                4 valve heads actually flow too good down low, so if the cams profile is somewhat radical the gasses lack the momentum and will lack allot of previous mentioned details...
                by creating a 2 to 4 valve head you can have some very radical profiles and not have to pay the price of the engine falling on it's face down low, but up high watch out, now your cooking,
                I just have the D 1600 V-tec it only controls the dominant valve - the intake - exhaust are two valve throughout the range,,, My computer controlled hydraulic actuated intakes go from 1 valve to 2 @ 4,700 rpm's

                I think the B-1600 actuates at 5,200 it has a redline of 8,200 rpms and stock created 160 hp out of 1600cc that's back in 1993 and very easy to get 200 out of them with just a few bolt on components...
                Last edited by A.K. Boomer; 12-29-2016, 02:02 AM.

                Comment


                • #38
                  Originally posted by J Tiers View Post
                  More to the point, the piston does not move much for the last bit of rotation toward BDC, so the mechanical advantage is horrible, and there is little lost in terms of power from opening during the time the piston is not moving much, So if there is any advantage to be had from early opening, you may as well go for it.
                  Eggzactly.....



                  Cuttings, you mentioned that the burn occurs quickly. And yes, that is certainly the case. But the chamber is starting on the compression stroke with nearly a full stroke worth of fuel and air because the intake valve let's it fill as much as it likes with only a slight reduction from ambient pressure. And after it burns there's more volume thanks to the heat and production of the gaseous byproducts. So there would still be a pretty fair degree of pressure over ambient at the bottom of the stroke if the exhaust valve didn't open early enough to vent it down close to ambient before the piston reaches BDC.

                  This and the reasoning noted above about the rotational and linear combinations is why there's little to be lost and much to be gained by opening early. How early? I commented above that I was spouting armchair engineering. And that is the case. But I can't help but feel that the Webster cam is likely opening a touch earlier than it needs to open. Which is why I was suggesting the other range of values.

                  I'm pretty sure though that the Kerzel timing is NOT a good profile. At least not in terms of a free rotating sort of timing. Carrying the pressure past BDC with a valve that does not open until well into the exhaust stroke is not only inefficient use of the pressure but it's also going to be harder on the bearing surfaces of the piston pin, conrod and the entire lower end. Either that or we need to let the exhaust open a little before BDC and then close well before TDC. In which case it'll compress the chamber for the last portion of the rise and the inlet valve won't be drawn open until the complementary angle on the other side of TDC. So we'd have an overly short intake period.

                  But perhaps that is the intent? It would be nice to talk to the designer and see what he had in mind with that short a timing. And where it was intended to be centered and the resulting open and close times.
                  Chilliwack BC, Canada

                  Comment


                  • #39
                    We should bear in mind that in the hit and miss engine an induction stroke does not immediately follow the exhaust stroke except when starting and under full load.

                    For a 'nice' model we want an engine that will consistently miss for a few cycles.

                    Comment


                    • #40
                      That's true. We only get an intake stroke after the exhaust valve is allowed to ride on the cam through the pushrod.

                      But it stands to reason, or at least seems so to me, that we still want good efficiency from that one power stroke so it speeds the engine up enough to coast for a good number of coasting revs.
                      Chilliwack BC, Canada

                      Comment


                      • #41
                        Originally posted by BCRider View Post
                        But it stands to reason, or at least seems so to me, that we still want good efficiency from that one power stroke so it speeds the engine up enough to coast for a good number of coasting revs.
                        With hybrid cars, the hit-and-miss energy storage idea is back.
                        Last week I was in Osaka, travelling in the latest Toyota Prius station wagon ( I forget the name),
                        about size of a mid range suv in USA.
                        The consumption was about 23 kilometre/litre over the week. ( approx 54 US miles/gallon )
                        The gasoline motor could certainly be heard starting and stopping.
                        The owner was pleased with the car, the only downside she commented was that it was a bit slow
                        pulling up the ramps to the freeways around Osaka.

                        Comment


                        • #42
                          There's so many factors to consider when trying to dial in any engine - even the slow moving ones,

                          bore and stroke can radically change the way you want an engine cam'ed,
                          long stroke and small bore creates greater piston speeds and more of a pressure "stall out" at at the bottom of the stroke, so even a lower speed engine may benefit greatly by getting the gasses on their way before BDC by at least a little amount.

                          compression ratio's and porting play just as large of a role,

                          what's incredible is how well NA engines work at higher RPM's given the fact that air has mass and it does take it time to get from point A to point B,
                          at your normal shift point of a smaller auto-engine the exchange between expelling the spent gasses and getting the new charge in is happening at a rate of 1/100th of a second! that's 1/200ths of a second to either evacuate the chamber or re-fill it,
                          You can see why duration becomes very beneficial due to the delay factor, you can see why even though an intake valve is left open LONG after BDC the fuel and air mix will continue to rush into the chamber even though the piston is actually traveling back upwards a great deal...
                          It also explains the larger overlap period, spent gasses still leaving in a fury out the exhaust even though it's at the end of it's cycle well after TDC and yet the intake got cracked open before TDC - with a pneumatic ram effect behind it and the exhaust well established and still dragging everything out with it - it's all just pulsating flow dynamics in delay and at work trying to play "catch up"

                          Comment


                          • #43
                            A lot of that optimizing is fairly irrelevant to an engine at low speed that is essentially supposed to "putt-putt" nicely and is not really expected to work for a living.

                            The primary requirement is for it to "hit" once, and coast many....

                            So, yes, it needs to accelerate well on one hit, or at least to add enough energy to the flywheel in one hit to actuate the governor. As a practical matter, that means accelerating, since the governors are not very precise. So the lightest flywheel setup that gets it through several revs with enough energy to go over compression for the power stroke is the best. It seems that Brian's has that, as more mass did nothing useful.

                            But that is not a guarantee, because there is the other variable of RPM. A lighter flywheel may need to run faster to get over compression. Lightening may just require an increase of rpm.

                            The real issue is the ability of the engine to speed up the flywheel sufficiently on one hit. In many ways, the RPM is not too relevant so long as it hits and coasts with reasonable timing.

                            Some of that may be a design factor, a short stroke is not really good for that, a long stroke seems likely to be better. Long stroke tends to be slower rpm, which is what most hit and miss full size engines are. My Jaeger is rated something like 600 rpm at full power. That's a slow idle for many engines. I'd have to measure or look it up, but IIRC the Jaeger (Hercules model E) has a stroke that is considerably longer than the bore.

                            I do not recall what the Kerzel is as far as bore and stroke. It is known that they CAN do a very creditable hit and miss.
                            CNC machines only go through the motions.

                            Ideas expressed may be mine, or from anyone else in the universe.
                            Not responsible for clerical errors. Or those made by lay people either.
                            Number formats and units may be chosen at random depending on what day it is.
                            I reserve the right to use a number system with any integer base without prior notice.
                            Generalizations are understood to be "often" true, but not true in every case.

                            Comment


                            • #44
                              I just ran across an interesting article about valve timing on a hit and miss engine with atmospheric intake valve.

                              In step 6 I think there is a mistake and should read bottom dead center instead of top dead center.
                              Larry - west coast of Canada

                              Comment


                              • #45
                                Re step 6;

                                I got the same impression. But if we forget about following the rotational direction and are simply looking for the points of valve movement then it makes sense. He's aiming for the valve to either start or finish at TDC depending on which way you're jogging the engine rotation. But the key being that he wants the last/first motion to occur at TDC. Then as a secondary thing he's saying to check to see that the motion starts at or really near to BDC as a check that the cam's dwell is centered through that full up stroke. The check for movement right at TDC is very like what Brian described in post #1. But it sort of assumes that the lobe duration is long enough that the mirror movement will occur at or very close to BDC. Other than that it doesn't consider the cam's duration at all other than to infer that it should have a duration of pretty well 180°.
                                Chilliwack BC, Canada

                                Comment

                                Working...
                                X