Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Automotive Effect of vibration on engine efficiency

  1. Mar 15, 2017 #1
    It has been stated in previous threads that engine inefficiencies are caused by:
    Pumping losses (induction and exhaust)
    Frictional losses (in bores, bearings, and ancilliaries)
    Thermal losses to exhaust, coolant (jacket, charge and oil), and surroundings
    Does engine vibration have an effect on efficiency and, if so, where is that loss translated in the above list?
  2. jcsd
  3. Mar 15, 2017 #2


    User Avatar
    Science Advisor
    Gold Member

    Mechanical vibration can cause problems like valve chatter which affect engine performance but main problem is usually shaking the engine to bits .
  4. Mar 16, 2017 #3
    Thank you for you response. All engines vibrate to one extent or another. Is there no data that you are aware of regarding vibration's effect on efficiency? Like through the motor mounts or heat produced by the stress and strain on reciprocating components that is not friction related?
  5. Mar 16, 2017 #4
    On the one hand, vibration helps to overcome stiction in engine parts, so that improves efficiency. On the other hand, some process takes energy from the engine to drive the vibration, so that represents a loss. On the whole, vibration is bound to be a loss, but it is usually not a major loss.
  6. Mar 16, 2017 #5
    JLD Co, Had a PM to answer so maybe I can help? There's a trend now with "barn finds" to leave vehicles as original as possible. I understand that and also understand why tires, plug wires and the like are replaced for safety. Rubber deteriorates. For some reason the harmonic balancer is usually overlooked. Actually it's usually overlooked for everything people do to their engines when it's the slow killer of engines and not always slow. Particularly on engine dyno's when you begin to recognize how it effects performance the effects of individual balancers on engines is all over the dyno data. The primary purpose of the harmonic balancer is "secondary balance" or "harmonic." It's often confused with what should be being attributed to the flywheel. Physically adding weight or taking it away at the crank has absolutely no effect on secondary balance.

    What you want to look into is engine balance. Engine balance isn't about center of gravity or the distribution of mass although that work may be required, it's about forces and coupled forces that have no direction to map or trace. They are just as important can destroy an engine without rattles or shakes. Another good vibration example are frequencies that effect engine oil. There are frequencies that will reduce oil "filming" by 50% and more. There are frequencies that impede air from the oil.

    Engine designers and improvement designers spend more time on engine balance then any other single system. Maybe not in terms of individual engineers but on a whole for an engine, yes.
  7. Mar 16, 2017 #6

    Randy Beikmann

    User Avatar
    Gold Member

    I've been working on automotive vibration for over 30 years, much of that on engines. The amount of energy involved in the actual vibrations is minuscule, because the vibrating motions are so small compared to the motions of the "working parts" like pistons.

    The way vibration can cause a reduction in power is by interfering with the engine function. A big one is caused by torsional vibration of the crankshaft (typically 3 oscillations per rev, regardless of the number of cylinders). Crank torsion is mostly caused by the large amount of oscillating torque on the crankshaft resulting from accelerating the piston mass back and forth in the cylinder. The engine has a tuned vibration absorber on the front of the crank (erroneously called a "harmonic balancer") to reduce the twisting of the crankshaft. Note that the "harmonic balancer" therefore has nothing to do with engine balance.

    Most cam drive systems are driven by a sprocket mounted to the front of the crankshaft. Because of the large flywheel inertia at the back of the crankshaft, that is where the least torsional vibration is. The front of the crank oscillates the most, so the sprocket drives the timing chain with an unsteady rotation speed. If the torsional motion is excessive, it can cause valve timing errors, and sometimes a broken chain.
  8. Mar 16, 2017 #7
    Randy's comments regarding torsional vibration are right on target.
  9. Mar 17, 2017 #8

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    way to go Randy..GREAT post..i posted a comparison of camshaft drives, gear vs chain vs belt a while back addressing a portion of this. Nascar runs belt drive to reduce harmonics and make better valve timing..
    i copied it below

    One more thing.. a 4 cylinder engine (IC) fries every 90 degree
    V6 fires every 60 deg. V8 fires every 45 deg. and when you rev a 4 cyl. you really shake things up at 7000 rpm constant. You will flex the cylinder liner at high revs if you do not take measures to re-enforce this area. Same as the top engine block deck under constant loading. Stock production engine blocks are gear for grocery getting but start revving and you got longevity problems over time. One thing not readily apparent is effect of oil under high RPM which to me is high vibration. You can really aerate oil and put a lot of bubbles in it and greatly reduce lubrication and cooling capacity. That will greatly impact on engine efficiency when you got a con rod sticking thru the block.
    Last edited: Mar 17, 2017
  10. Mar 20, 2017 #9
    Not exactly an efficiency issue however, another item worth mentioning. internal combustion engines also suffer from vibration generated by the rotating mass. L4's can be especially rough in the second order harmonics. In the performance world for a while people would disconnect the balance shafts in an effort to reduce rotating mass and gain quicker throttle response. The motors would often see reduced life but it is accepted as part of racing. Many performance shops are starting to recommend against this practice. Especially in the clutch and transmission groups as the increased vibration can induce a rattle in the clutch when disengaged which often leads to issues in that area as well.
    In either case one can view a reduced operating life as lower operating efficiency or decreased reliability as another form of lowered efficiency.
  11. Mar 21, 2017 #10
    I've been incognito for a few days, please excuse the delay in responding. Very informative and helpful. Thank you for the input.
  12. Mar 21, 2017 #11
    Very helpful, thank you for that considerate response.
  13. Mar 29, 2017 #12
    This is why light flywheels are not always a good thing.
    You can get better high RPM performance with a smooth running crank
    I wish car engines had more of a real flywheel at the front
  14. Mar 31, 2017 #13
    Say what now? A flywheel... at the front?
  15. Mar 31, 2017 #14
    I think That's what a harmonic balancer is at hart
    It's just fancy
  16. Mar 31, 2017 #15
    Well sort of. It acts differently to a flywheel though, in that the mass is decoupled from the crankshaft. The harmonic balancer is a torsion vibration damper, acting against the torsional natural frequency. If you don't have the torsional mode in the running range (ie a light and stiff crank) you can get away without it.

    Heavy flywheels will drag the torsional mode to a lower frequency. It's why the trend is to move away from them to dual mass flywheels.
  17. Mar 31, 2017 #16
    Did you mean 2nd order?
    You've got me thinking now... :P
    Why would it be 3rd order regardless of cylinders?
  18. Mar 31, 2017 #17
    Yes like the big heavy pendulum at the top of some tall building
    It robs momentum from the waves of motion and adds it back in a way that dampens resonance

    But I like the way a flywheel stores energy and my bike has a cvt
    So the RPM won't climb till I hit 45 mph so for me it's a bonus
  19. Apr 1, 2017 #18

    Randy Beikmann

    User Avatar
    Gold Member

    I worked on a NASCAR V8 engine quite a while ago, measuring the crankshaft twist (angular oscillation at the front of the crank). I was expecting 4/rev, having 8 cylinders, and it did show up. But the 3/rev was much larger.

    There are two sources of torque acting on the crank, from each cylinder: 1) pressure acting on the piston (gas pressure torque), and 2) the force necessary to accelerate the piston up and down the cylinder (inertia torque). The gas pressure torque repeats once per two rev's, so it has frequencies that are multiples of 1/2 order. The inertia torque repeats once per rev, so it has frequencies that are multiples of 1st order.

    If the crank motion from the torque acting on it were equal for every cylinder, a V8's four equally-spaced firing events per rev will make all the oscillations cancel, except for multiples of 4/rev. But that actually only happens if the crankshaft is rigid. If the frequency of the pulses is near a crank torsional mode frequency, the crank is twisting, and the twist from some cylinders is greater than for others (the front cylinders are further from the flywheel, so the twist from them is larger). Many orders that would otherwise cancel no longer do, so ANY order could excite torsion if it's at the right frequency.

    If you calculate the frequencies and amplitudes of all the orders of torque acting on the crank, 3rd order inertia torque is one that is large enough and reaches a high enough frequency to excite the first crank torsion mode frequency (around 300 Hz, which 3rd order hits at 6000 RPM). Other orders do show up in the vibration, but they are not typically as big.

    Of course, after I finished the project, I looked in Taylor's book on IC engines, and it showed a graph of a 1960's Ford V8 race engine, and it showed torsion peaks, at increasing speeds, from 5th order, then 4th order, then 3rd order. Of course 3rd order was the biggest, and now I knew why...
  20. Apr 2, 2017 #19

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    excellent Randy, most awesome...one reason i stay on here!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted