Engine balance refers to those factors in the design, production, tuning, maintenance and the operation of an engine that benefit from being balanced. Explanation:Piston engine balancing is a complicated subject that covers many areas in the design, production, tuning and operation. The engine considered to be well balanced in a particular usage may produce unacceptable level of vibration in another usage for the difference in driven mass and remounting method, and slight variations in resonant frequencies of the environment and engine parts could be big factors in throwing a smooth operation off balance.
In addition to the vast areas that need to be covered and the delicate nature, terminologies commonly used to describe engine balance are often incorrectly understood and/or poorly defined not only in casual discussions but also in many articles on respected publications. Internal combustion piston engines, by definition, are converter devices to transform energy in intermittent combustion into energy in mechanical motion. A slider-crank mechanism is used in creating a chemical reaction on fuel with air compression and ignition), and converting the energy into rotation (expansion).
The intermittent energy source combined with the nature of this mechanism make the engine naturally probationer. Multi-cylinder configuration and many of the engine design elements are reflections of the effort to reduce vibrations through the act of balancing. Fig. 01 Items to be balanced:There are many factors that could throw an engine off balance, and there are many ways to categorize them. The following is an example of categorizing the items that need to be balanced for a smooth running piston engine. . Static Balance – Static balance refers to the balancing of weight and the location of CGI on moving parts.
Reciprocating mass – e. G. Piston and con rod weight and CGI uniformity, 0 Rotating mass – e. G. Crank web weight uniformity and flywheel egocentricity. 2. Dynamic Balance – In order for a mass to start moving or change its course in the motion, it needs to be accelerated. In order tort a mass to be accelerated, a torte is required, and that force needs to be countered (supported) in the opposite direction. Dynamic balance refers to the balancing of these forces and friction. All accelerations of a mass can be divided into two components opposing in the direction.
For example, in order for a piston in a single cylinder engine to be accelerated upward, something must receive (support) the downward force, and it is usually the mass of the entire engine that moves downward a bit as there is no counter-moving piston. This means one cause Of engine vibration usually appears in two opposing directions. Often the movement or deflection in one direction appears on a moving mass, and the Other direction appears on the entire engine, but sometimes both sides appear on moving parts, e. G. A torsion’s vibration killing a crankshaft, or a push-pull resonance breaking a chain.
In Other cases, one side is a deflection of a static part, the energy in which is converted into heat and dissipated into the coolant. Reciprocating mass – Piston mass needs to be accelerated and decelerated, resisting a smooth rotation of a crankshaft. In addition to the up-down movement of a piston, a con rod big end swings left and right on top and bottom halves of a crank rotation. Fig. 02 3. Phase balance – e. G. Pistons on 60 or 90 degree V without an offset crankshaft reciprocate with unevenly spaced phases in a crank rotation. 4 Plane balance – . G.
Boxer Twin pistons travel on two different rotational planes of the crankshaft, which Creates forces to rock the engine on Z-axis. 5. Plane balance – e. G. Boxer Twin crankshaft without counterweights rocks the engine on Z-axis. 6. Torsion’s balance – If the rigidity Of crank throws on an inline 4 cylinder engine is uniform, the crank throw farthest to clutch surface (#1 cylinder) normally shows the biggest torsion’s deflections. It is usually impossible to make these deflections uniform across multiple cylinders except on a radial engine. 7. Slide resistance balance A piston slides in a cylinder with friction.
A ball in a ball bearing also slides as the diameter of inner and outer laces are different and the distance of circumference differs from the inside and out. When a ball bearing is used as the main bearing on a crankshaft, eccentricity of the laces normally create phase imbalance in slide friction. Metal bearing diameter and width define its bearing surface area, which needs to be balanced for the pressure and the rotational speed of the load, but differing main bearing sizes on a crankshaft create plane imbalance in slide friction, 8. Rolling resistance balance – e. G.
A ball in a ball bearing generates friction in oiling on a lace, 9. Torque Balance – Torque here refers to the torque applied to crankshaft as a form of power generation, which usually is the result of gas expansion, In order for the torque to be generated, that force needs to be countered (supported) in the opposite direction, so engine mounts are essential in power generation, and their design is crucial for a smooth running engine. I o Timing/Direction of torque – The concord of a cylinder with fast-burning mixture pushes the crankshaft most ATA different angle when compared to a late-igniting or slow-burning cylinder. 1. Phase balance – e. G. Firings on a single cylinder 4 cycle engine occur at every 720 degrees in crankshaft rotation, Which is not balanced from one rotation to another. 12. Plane balance – Torque is applied to the crankshaft on the crank rotational plane where the concord is located, Which are at different distances to power take off (clutch surface) plane on inline multi-cylinder engines. 13. Plane balance – e. G. Compression on a boxer twin engine occurs at different planes on the crankshaft at different distances to clutch surface.
A single plane (single row) radial engine does not have this plane imbalance except for a short schismatic between the power generating plane where the concords are, and the power take off plane where the propeller is, 14 Phase balance – e. G. It only one cylinder of a multi-cylinder engine has a restrictive exhaust port, this condition results in increased resistance every 720 degrees on crank rotation on a 4 cycle engine. IS, Plane balance – e. G. If only one cylinder of a multi-cylinder inline engine has a restrictive exhaust port, it results in increased resistance on the crank rotational plane where that cylinder/concord is located.
Primary Balancing:The term “Primary balance” is a major source of confusion n the discussion of engine balance. Primary, “first order” or “first harmonic” balance are supposed to indicate the balancing Of items that could shake an engine once in every rotation of the crankshaft, i. E. Having the frequency equal to one crank rotation. A cylinder in 4 cycle engines fires once in two crank rotations, generating forces With the frequency Of a half the crankshaft speed, so the concept Of “half order” vibrations, is sometimes used when the discussion is on the balances on torque generation and compression.
However, it is somewhat customary to discuss only two categories, primary ND secondary, in the discussion of engine balance in which ‘Primary’ is often meant to be all nanosecond’s imbalance items lumped together regardless of frequency, and ‘Secondary’ is meant to be the effects of non-sinusoidal component of piston and concord motions inconsiderable mechanism as described below.
Fig. 03 Secondary Balancing:When a crank moves 90 degrees from the top dead center (T DC) in a single cylinder engine positioned upright, the big end up-down position is exactly at the half-way point in the stroke, but the concord is at the most tilted position at this time, and this tilt angle makes the small-end position o be lower than the half-way point in its stroke.
Because the small-end position is lower than the half-way point of the stroke at go degrees and at 270 degrees after DC, the piston moves less distance when the crank rotates from 90 degrees to 270 degrees after T DC than during the crank rotation from 90 degrees before DC to 90 degrees after DC. In other words, a piston must travel a longer distance in its reciprocal movement on the top half Of the crank rotation than on the bottom half. Assuming the crank rotational speed to be constant, this means the reciprocating movement Of a piston is faster on the top half than on he bottom half of the crank rotation.
Consequently, the inertia force created by the mass off piston (in its acceleration and deceleration) is stronger in the top half of crank rotation than on the bottom half. So, an ordinary inline 4 cylinder engine with 180 degrees up-down-down-up crank throws may look like canceling the upward inertia created by the piston pair with the downward inertia of the pair and vice versa, but in fact the upward inertia is always stronger, and the vibration caused by this imbalance is traditionally called the Secondary Vibration.
The inertia force created by this non-sinusoidal reciprocating motion is equivalent to the mass times the acceleration of change in the position, which is expressed as: Fig. 04 Inherent Balance:When comparing piston engines With different configurations in the number of cylinders, the V angle, etc. , the term “inherent balance” is used. This term often describes just categories in the above list that are ‘inherent’ in the configuration. Phase balance on reciprocating mass, and phase balance on torque generation.
In rare cases when considering a boxer twin, the categories, plane balance on reciprocating mass, plane balance on rotating mass and sometimes plane balance on torque generation are included, however. Tenements like “A flat-8 boxer engine has a perfect inherent balance” ignore these three categories (as well as plane imbalance on compression) as flats boxer configuration has inherent imbalance in these four categories by having the left and right banks staggered front to back (not positioned symmetrically in plane view) in the same manner as in boxer twin.
Reciprocating mass (Phase and Plane) Rotating mass (Plane) Torque generation (Phase and Plane) and Compression (Phase and Plane) 1. Two cylinder engine:- There are three common configurations in two-cylinder engines: parallel-twin; V- inn; and boxer twin (a common form Of flat engine). Secondary imbalance is the strongest on a parallel twin with a 360 degree crankshaft (that otherwise has the advantage Of an evenly spaced firing, and lack of plane and phase imbalances), which moves two pistons together.
Parallel tin With a 180 degree crankshaft (that has the disadvantage Of uneven firing spacing and strong imbalance) produces the vibration a half as strong and twice as frequent. In a V. Twin with a shared crank pin (e. G. Ducats ‘Login’), the strong vibration of the 3600-crank parallel twin is divided into two different directions ND phase separated by the same amount of degrees as in the V angle, with unevenly spaced firing as well as the imbalances. Fig. 05 2. Three cylinder engine:line 3 with 1200 crankshaft is the most common three cylinder engine.
They have evenly spaced firing and perfect phase balance on reciprocating mass, with and imbalances, Just like in a cross-plane V, these first order rocking couples can be countered with heavy counterweights, and the secondary balance is comparable to, or better than an ordinary inline 4 because there are no piston pairs that move together. This secondary balance advantage s beneficial for making the engine Compact, for there is not as much need for longer concords, which is one of the reasons for the popularity of modern and smooth turbo-charged inline 3 cylinder engines on compact cars.
However, the crankshaft with heavy counterweights tend to make it difficult for the engine to be made sporty (i. E. Quick revving up and down) because Of the strong flywheel effect. Fig. 06 3. Four cylinder engine:line-4, flat-4 and V are the common types of four cylinder engine. Normal inline configuration has very little rocking couples that often results in smooth middle RPM range, but the secondary imbalance, which is undesirable for high RPM, is large due to two pistons always moving together.
The rotational vibration on X-axis, which is often felt during idling, tend to be large because, in addition to the non-overlapping power stroke inherent in engines with 4 or less number of cylinders, the height imbalance on concords’ CGI swinging left and right is amplified due to two concords moving together. Intake and exhaust pulse on ordinary inline-four engines have equal 360′ spacing between the front-most and the rearmost cylinders, as well as between the middle two cylinders.
So an equal-length (longshoreman) four-into-one exhaust manifold, or two ‘Y’ pipes each merging exhaust flows from #1 and #4 cylinders, as well as and #3 cylinders are required for evenly spaced exhaust pulse. Older twin-carburetor setup often had each Carr throat feeding the front two and the rear two cylinders, resulting in uneven 1800-5400-1800-5400 intake pulse on each throat. Modern inline-four engines normally have four equal-length runners to a plenum (Which is fed by a throttle at 1800 evenly distributed frequency), or four individual throttles (at 7200 equal spacing on each throttle).
Ordinary Flat-4 boxer engines have excellent secondary balance at the expense f rocking couples due to opposing pistons being staggered (offset front to back). The above mentioned rotational vibration on X-axis is much smaller than an inline-4 because the pairs of concords swinging up and down together move at different CGI heights (different left-right position in this case). Another important imbalance somewhat inherent to boxer-four that is often not dialed out in the design is its irregular exhaust pulse on one bank two cylinders.
Please see flat-four hurdle explanation part of flat-four engine article on this exhaust requirement similar to the crosspiece V exhaust peculiarity. Fig. 07 . Five cylinder engines:line five cylinder (L S) engine, with crank throws at 72′ phase shift to each other, is the common five cylinder configuration. (Notable exceptions are Honda racing VS… And Volkswagen IVR engine. ) These typical L’S engines have evenly spaced firing and perfect phase balance on reciprocating mass, with plane imbalance On reciprocating mass, plane imbalance on rotating mass, plane imbalance on torque generation, and plane imbalance on compression.
Just like in inline 3 engines above, these first order rocking couples can be countered with heavy counterweights, and the secondary balance is memorable to, or better than an ordinary inline 6 because there are no piston pairs that move together. Compared to three and four cylinder designs, a major advantage in 4. Stroke format is the overlap in power stroke, where the combustion at every BIBB of crank rotation ensures a continuous driving torque, which, while not as much noticeable at high RPM, translates to a much smoother idle.
Modern examples such as the 2013 Audio RSI engine have undersea design, because the advantage in secondary balance allows it to have longer stroke without sacrificing the higher RPM smoothness, which is desirable for a smaller ore that results in shorter engine length. Honda GAGA also with an undersea design, was originally introduced with a balance shaft driven at the crankshaft speed to counter the wiggling vibration caused by the plane imbalance on rotating mass, but it evolved into AS Liter GAS with heavier counterweights that does not have the balancer.
Pig. 08 5. Inline six cylinder engines:line 6 normally has crank throws at 1200 phase shift to each other with two pistons at about equal distance to the center of the engine (#1 and #6 cylinders, #2 and 45, #3 and #4) always moving together, which results in superb plane balance on reciprocating mass and rotating mass in addition to the perfect phase balances.
Combined vivid the overlapping torque generation at every 120′ Of crankshaft rotation, it often results in a very smooth engine at idle. However, the piston pairs that move together tend to make secondary imbalance strong at high RPM, and the long length configuration can be a cause for crankshaft and camshaft torsion’s vibration, often requiring a torsion’s damper.
The long length of the engine often calls for a smaller bore and longer stroke for a given cylinder displacement, which is another cause for large secondary imbalance unless designed with otherwise-unnecessary long concords that increase engine height. Moreover, 4-stroke inline 6 engines inherently have (Plane imbalance on torque generation) and (Plane imbalance on compression), which are typically more or less balanced on V 12 and Flat-12 configurations.
In terms outfitting spacing, these typical inline 6 are like vivo inline 3 engines connected in the middle, so the firing interval is evenly distributed within the front three cylinders and within the back three, with equal 2400 spacing within the trio and 1202 phase shift to each other, So three-into-one exhaust manifolds n the front and on the rear three cylinders, with each of them then connected with a two-into>one pipe results in 1200 (2400 if not merged in a dual exhaust system) evenly distributed exhaust pulse.
Intake pulse, which is also important to have equal spacing for evenly filling the cylinders With the same volume and mixture Of intake charge for uniform amount of torque and uniform timing in torque generation, is formed the same way, so NON carburetors or throttle bodies on two one-into-three intake manifolds each on the front and the rear three cylinders (strictly speaking when the three runner ingest are equal) results in evenly spaced intake pulse.
Jaguar XX inline 6 had three US carburetors each serving the front two, middle two and the rear two cylinders in the later models, which resulted in unevenly distributed intake pulse at the front and the rear carburetors (the middle Carr gets an evenly spaced pulse at 3600 interval). This configuration, while resulting in higher power due to the increased total flow capacity of the carburetors than the earlier evenly- capsules twin carburetor configuration, may have contributed to the later 4. Liter version’s “rougher running” reputation compared to the legendary and . 8 Liter versions. Modern inline six engines with fuel injection (including Diesels) normally have equal length intake runners connecting the intake ports to (often protruding into) a plenum (See Inlet manifold for parts descriptions) to keep intake pulse evenly spaced, Pig. 09 6. V engines:V engines with UN-split shared crank pin can have equally spaced firing when the V-angle is at 1200 (600 or 1200 for 2-stroke).
However, the 1 200 bank angle makes the engine rather wide, so production V tend to use 600 angle with a crank pin that is offset 600 for the opposing cylinders. As offsetting he crank pin for as much as 600 no longer provides overlap in the diameter of the crank pin, the actual pin is not really an offset ‘split’ pin, but normally is completely separate in two parts With a thin crank web connecting the two individual pins. This makes the crankshaft structurally weaker, much more so than in the crankshaft with slight offset seen on the Lanai pluvial V with 10. 0 to 130 offset, so racing V engines from Carlo Chit-designed 1961 Ferreira 156 engine to Cohorts GAB for Formula One often used the 1200 bank angle to avoid this weakness, unless required by the formula as in all the 2014 – 2015 Formula One . 6 Liter turbo V engines that has 90′ bank angle according to the regulation. [8] V is compact in length, width and height, which is advantageous for rigidity and weight. The short crankshaft length mitigates the torsion’s vibration problem, and secondary balance is better than in an inline 6 because there is no piston pair that move together.
Furthermore, each bank of three cylinders have evenly spaced induction/ignition interval, so the intake/exhaust system advantage is shared with inline 3. However, these advantages come at the price of having plane imbalances on rotating mass, reciprocating mass, torque enervation, and compression. Also, the left and the right banks being staggered (for the thickness Of a concord plus the thin crank web) makes the reciprocating mass plane imbalance more difficult to be countered with heavy counterweights than in inline 3.
