Can you elaborate on this please? A 4 cylinder, two stroke, V-4 with the DH's crank will have a combustion event every 90*, as opposed to a flat four 4 stroke's every 180*. Seems it should run relatively smooth....
vibration in recip engines is very complex and not "obvious".....here's a Cliff Notes version of some issues.
-- a piston on a crank is similar to an unbalanced washing machine.
-- as the crank rotates, the CG of the piston/pin/counterweight combination is not centered on the crank and the CG path is not circular .
-- H engines (boxers) have
mostly self-canceling "washing machines" BUT they are not perfectly balanced until H8
-- H engines tend to "rock" from side to side (shake your head "no")
-- V engines are statically unbalanced until 90-degree V8 (4x90) BUT a 90-V8 is still
dynamically unbalanced due to unequal "mass lengths" between "top" and "bottom" positions (TDC and BDC).
-- with the exception of certain V12's, all "V" engines have centers of rotational mass that are not on the centerline of the crank
V4's are horrible....surpassed only by the infamous Harley single-pin V2.
on a scale of 1-10 (1 being harsh) the following engines are
V2 (single-pin/harley) = 1
1-cyl (lawnmower, etc.) = 2
V4 (90*) = 2
I4 (w/o balance shafts) = 3
V6 (90*) (w/o balance shafts) = 4
V6 (72*) (w/o balance shafts) = 5
single row radial = 6
H4 (lycoming, subie, etc.) = 6
V8 (90*) = 7
H6 (lyco, subie, etc.) = 8
double row radial = 8
I6 (bmw, caterpillar, etc.) = 9
V12 (only if 2xI6 on same crank) = 9
turbine; electric = 10+
vibration patterns in recip engines are well known yet each engine design will exhibit unique vibration profiles.
there are 2 types of PRIMARY vibes in a recip engine:
pulse vibe and
dynamic vibe (i emphasize "primary" b/c the engine gurus know there are many other vibes in a recip engine).
"stroke" will affect "pulse" vibe in that an engine with more pulses per revolution will be smoother. 8 cyl vs 4 cyl, etc. thus a 2-stroke will be "smoother" than an equivalent 4-stroke
all other things being equal.
for
engines themselves dynamic balance (rotational vibration) is FAR more important than "pulse" vibration (EXCEPT for aircaft propellers...more on that later). all engine designers must decide the best compromise between power, smoothness, size, weight, compactness, complexity, fuel efficiency, emissions, cost, etc.
V4's have very strong "wobbles" along the "nod" axis (nod your head)and "wag" axis (wag the tail).
a 2-stroke V4 will be "smoother" than a 4-stroke V4, but the dynamic properties do NOT change. a 2-stroke V4 spinning at 2500 RPM will have the same "pulse" smoothness as a 4stroke V4 spinning at 5000 RPM
in terms of rotational balance, diesel is similar to gas (V4 = V4)
in terms of pulse balance, diesel is worse than gas (pressure spikes)
adding a prop to the crank complicates things enormously....the crankshaft "rings" bc it acts like a torsional spring as it winds and unwinds with every pulse. the energy "pumped" into the prop gets reflected back to the crankshaft like a wave bouncing off the wall of a pool. if the prop is metal, it also "rings" at certain frequencies as the blades bend and recoil with every power pulse. (this is why changing to electronic ignition and/or high compression pistons results in different operating limits for the same prop).
the bottom line: any vibes from the
original configuration (V4, V8, diesel pulses, etc.) are magnified by the propeller and reflected back into the engine as larger vibes .... harsh engine + prop = harsher vibes
when the whole mess is bolted to a metal airframe with its own "ring" signature, one can see how the vibe issue becomes amazingly complicated.
