ENGINE BALANCE: Inline 6 vs. Odd fire V6 vs. 90 degree V6 vs. 60 degree V6

what is up engine heads it's time for another engine balance video and in this one we're doing a detailed comparison of the e96 engine with different versions of its engine nemesis the v6 but before we start just a super quick little recap on primary and secondary engine balance because understanding this stuff is key to understanding all these engine balance videos now a primary engine imbalance is the result of an unbalanced mass in the reciprocating assembly of your engine in other words your pistons are usually the greatest potential source of a primary engine imbalance an engine with an odd number of pistons is going to have a primary imbalance if the mass of the odd piston isn't balanced out by the other pistons secondary engine imbalances occur because the piston travels at different speeds at the top and bottom half of the stroke it does this because it has to cover a greater distance at the top half than at the bottom half of the stroke this occurs because although the piston moves in a simple up and down motion the connecting rod does not the conrod steps out from a linear path of motion and because it does this it pulls down the piston a greater distance during the top half of the stroke a greater distance means that the piston must travel faster to cover this greater distance and because of this there's a difference in speed between the top and bottom half of the stroke and because there's a difference in speed there's also a difference in inertial forces between the top and bottom half of the stroke and this difference in inertial forces is the source of secondary engine imbalances now if you need an easier to understand a more detailed explanation of secondary engine balances then i suggest that you watch the very beginning of my first video on engine bounce and don't worry if you take a bit of time to wrap your head around this because nobody really gets it the first time also primary engine imbalances occur once per single engine revolution while secondary engine imbalances occur twice per single engine revolution and usually primary engine imbalances are greater in magnitude and more apparent than secondary engine imbalances why is engine balance even important well an unbalanced engine is going to run rough and it's going to reduce comfort also a poor engine balance is going to limit performance potential as well as efficiency and it can negatively impact the lifespan of your engine as well as associated parts like engine mounts for example now let us start with the inline six cylinder engine a beautifully simple and beautifully balanced engine configuration to see how it's so well balanced we're going to start by figuring out its firing interval to do that we use this very simple formula 720 divided by six 720 is 2 times 360. 360 degrees is of course one full single engine revolution we're doing two times 360 because we're talking about four stroke combustion engines here and four stroke internal combustion engines need two full revolutions for one completed combustion cycle one stroke is 180 degrees two strokes are 360 degrees and four strokes are of course 720 degrees 6 is simply the number of cylinders in our engine the result is 120 and this tells us that the inline six cylinder engine fires every 120 degrees of crankshaft rotation now we're going to take 120 and deduct it from 180 180 degrees is as we said one single stroke the result is minus 60. the minus here tells us that there's power stroke overlap 60 degrees of power stroke overlap to be exact power stroke or app is great because it improves engine smoothness the more post stroke overlap we have the smoother the engine so the inline six already meets two important criteria for good engine balance and even firing interval and power stroke overlap what about primary balance well the inline-six has perfect primary balance because it has an even number of pistons so the mass of each individual piston can be canceled out by the mass of another equal piston also unlike the inline three or the e95 the inline six is balanced in relation to its center of gravity and there is no rocking moment front to back like you can see in the inline three or the inline five what about secondary balance also perfect because in the e96 pistons move in pairs and no single pair is ever at the same part of the stroke as any other pair when one pair is up the other pair is down the third one's going to be in between them so the forces associated with the different speeds the different parts of the piston stroke cancel each other out but there's a problem with the inline-six engine it's very long which means it's very hard to stuff it under the hood of a vehicle especially if you want to mount it transversely so what do you do of course you cut it in half and then you mount the two banks of threes into a v shape and voila the v6 is born it's two inline threes in a v shape it's much shorter than the inline six and you can transversely mount it into pretty much anything well almost anything but now there's a new question arising because we have a v shape between the two banks the question is what kind of angle is the perfect angle for that v shape and to figure that out we're going to use again our very simple useful formula and because we have again a four stroke engine in the same number of cylinders we have the same formula and the same result 120 degrees which was our firing interval but 120 degrees is also the perfect angle between the two banks of the v in a v6 engine why well to understand that we need to take a closer look at the crankshaft so here's the crankshaft of an inline six under engine this is the crank pin for piston and rod one this is the crank pin for piston rod two this is four three and four and so on until we mount all the rods and pistons onto the crankshaft as you can see each rod gets its own crank pin the inno i6 can do this because the cylinders are mounted one after the other in a line so there's ample space for each rod to have its own crank bin unfortunately the v6 does not have this kind of luxury the cylinders sit across each other they look at each other which means you need two cylinders for one crank pin this means that rods must share a single crank pin now both the v6 and the inline six because they have the same number of cylinders have the same firing interval 120 degrees and this is also going to be the angle of separation between each individual crank pins the only difference is that in the v6 one and two three and four and five and six are going to be sharing crank pins and in the inline six nobody's going to be sharing anything it's just that one and six are going to be pointing uh in the same direction and two and five and three and four will be pointing in their respective directions so here's the typical firing interval of an inline six cylinder engine so we fire cylinder one rotate crankshaft 120 degrees then we fire a cylinder five then we rotate crankshaft under 120 degrees then we fire some of the tree and so on until we fire everything the firing interval remains the same it's always 120 degrees between the firing of each individual cylinder so that's the inline six and the 120 degrees v6 is the same so it just has a different firing interval one two three five six and here's what we do we fire cylinder one and then we rotate 120 degrees and this is where the bank angle of 120 degrees comes into play because the bank angle is the same as the firing interval and the same as the crank pin separation angle it means that once we fire cylinder number one and rotate crank 120 degrees cylinder number two is gonna be right close to tdc ready to be fired so we can fire it so we do one 120 degrees rotation we fire two 123 degrees rotation we fire three and so on until we fire everything so the 120 degrees v6 is great it enables an even firing interval and the same power stroke overlap as an inline six it's a great engine right well actually no the 120 degrees a v6 is useless and stupid it's useless and stupid because the whole point of developing a v6 engine is to make it more compact than an inline six but with 120 degrees between the cylinder banks you're actually making an engine that's almost as wide as a flat six but it's taller than a flat six so it's everything except compact it's a very bulky engine and this is why you don't really see it in mass production cars you can only see it in leman or f1 aldo recently asked martin tease their tm01 engine which seems to have a very wide angle between the two banks of the v6 although i'm not sure if it's really 120 degrees we'll see so the 120 degrees v6 is impractical how do we make it more practical more compact of course we reduce the bank angle how about 90 degrees just like a v8 the added bonus is we can make it on the same production line as a v8 we just chop off the last two cylinders well that's exactly what buick did in 1961 for the first generation of their fireball v6 engine they derived it from their 215 cubic inch v8 engine the only difference between the blocks was there was two cylinders less and the crankshaft was different of course they used the same shared pin crankshaft we just we just described in our 120 degrees uh v6 and result was a very rough odd firing engine so what is an odd firing engine well i'll tell you what it isn't it isn't even firing it wants to be even firing but it can't to see why it can't let's observe the first generation 1961 buick fireball in action so here's the representation of that engine the engine block here's the cylinder numbers and here's the firing interval and here's what each cylinder is doing at a particular moment in time as you can see the firing interval is kind of weird now to better explain things we're going to start from the end of firing for the firing interval from cylinder number two so let's fire cylinder number two and then let's rotate 120 degrees because we want to try and maintain an even firing interval so we fire two rotate 120 degrees and we should fire one next but we can't because we just skipped tdc of one we just missed our opportunity to fire cylinder number one why well because our bank angle isn't 120 degrees it's 90 degrees the bank angle dictates when opposing cylinders in a v engine are going to be seeing tdc so in a 90 degree v6 when one cylinder fires and the crankshaft rotates it needs to rotate the same amount of degrees as the bank angle for the opposing cylinder to reach tdc so let's take it from the top again we obviously can't do 120 degrees we need to fire cylinder one so let's do we fire two we rotate 90 degrees that's our back angle again and voila one is a tdc now we can fire it so we fire sounder one let's try to fire the next cylinder we should fire six next right but let's how much should we rotate well let's try rotating 90 again let's try to keep an even firing interval but guess what we can't fire anything let's try three well three is actually just going to tdc of exhaust stroke it needs another 30 degrees to reach tdc of exhaust we obviously can't fire that what about cylinder number four well this is doing intake and it finished exhaust some 60 degrees ago so we can't fire that either let's try sunder number five that is in compression right we might be able to fire that but well obviously we can't for that it's very old down in the bore as we can see and it needs another 150 degrees to reach tdc of compression what about center number six well that is pretty close to tdc of compression but it needs another 60 degrees to reach ddc off compression so what do we do well we're forced to rotate another 60 degrees to fire cylinder number six so let's do that we fire two we rotate 90 we fire one and we rotate 90 and then we rotate 160 to fire cylinder 6. and we just made a boo-boo why because our first firing interval was 90 degrees and then our next is 90 plus 60 that's 150 that means our firing interval is 90 150 90 150 which means it's odd it isn't even like 120 121 2120 an engine naturally wants to have equal spacing between each firing event if it doesn't have that it runs rough so our shared crank pin v6 which has 90 degrees between the two cylinder banks ends up being forced to have an odd firing interval because its bank angle is an equal to the naturally desired even firing interval of the engine as well as the separation angle between the individual shared crank bins as you can see what's missing from the bank angle ends up being reflected on the firing interval the missing 30 degrees from the bank angle are tossed between individual firing interval instances now the gist of all of this is that an odd firing v6 is never going to be as smooth as an even firing v6 and this is exactly what buick realized as well in 1977 so they decided to fix it and they fixed it by making a 90 degree v6 that was also even firing how did they do that well let's think about it ourselves where can we make up for the wrong bank angle what can we sacrifice that shared crank pin of course we can make up for the wrong bank angle in the shared crank pen by making it a non-shared crank and let's turn the shared room arrangement into individual rooms let's split the shared crankbait in half and offset it by how much should we offset it by whatever number of degrees is missing from that bank angle so if our bank angle is 90 that means we're missing 30 degrees and that means our offset angle between the now split crank pins should be 30 degrees as well and this angle is called the splay angle and it makes up for whatever is missing from the bank angle and it saves the day it enables a and even firing interval 90 degree v6 thanks to the split angle when we fire a cylinder in this cylinder bank and rotate the crankshaft 120 degrees the cylinder in the opposing bank is now at tdc it's ready to be fired we're no longer missing we're no longer skipping the opportunity to fire the other cylinder we now have a compact v6 that has an even firing interval and a power stroke overlap just like an inline six but smaller but we can squeeze even more benefits out of display angle thing we can make the v6 even smaller even narrower let's do a 60 degree v6 which is going to make up for it in display angle so if the bank angle is 60 it means we're missing 60 degrees more 220 so let's make a 60 degree spray angle yeah cool well i know what you're thinking look at this cross section between the offset uh crank pin has it's kind of small this means this is a weak crankshaft right well yes it is but we can fix that we can do what alf romeo did in their buso v6 engine we can put flying arms between the offset crank up pin halves and what we just got is a narrow compact v6 that has an even firing interval power stroke overlap and a strong crankshaft it's a triple it's a quadruple win so with a bit of engineering we finally managed to fix the firing interval problem while still retaining the compact nature of the v6 but what about the primary and the secondary balance well to understand that let's recall the inline three cylinder engine it has a primary imbalance in relation to its center of gravity and therefore it rocks it has a rocking moment front to back but the inline six cylinder engine has perfect primary balance how come well it's two in-line threes mirroring each other so the two inline threes in the inline six cancel each other's rocking moment out and the inline six has perfect primary balance but the v6 isn't like this it isn't there's no mirroring of inline threes instead the inline threes are in opposing banks which means that each bank of the v6 actually has the primary imbalance of an inline three cylinder engine how do we fix it balancing shafts of course because we're fixing a primary imbalance we need a single shaft rotating at the speed of the crankshaft and you can often find balancing shafts like this in the v6 engine but we don't like balancing shafts they add friction weight and cost let's do something else again let's look to the alfre mayo brusso v6 it doesn't have balancing shafts instead it uses the flywheel and the crankshaft pulley because these two things rotate at the speed of the crankshaft all you got to do is incorporate an offset weight into these two items and you get rid of your primary imbalance in fact the alpha male v6 is one of the first engines using this sort of method to get rid of a primary imbalance in a v6 engine so what about secondary balance well again let's look at an individual inline three cylinder engine and as you can see it has no problems with secondary bounds we have three cylinders three pistons but each of the pistons is in different parts of the stroke which means that they cancel the difference between uh the different inertial forces associated with different speeds at the different parts of the stroke because of the pistons are in different parts of the stroke at different times so a single in three cylinder has no issues with secondary balance and a v6 is two in line threes is opposed in opposing banks which means that neither bank has issues with secondary balance which means that the v6 doesn't have any issues with secondary balance and there you have it we have witnessed how engineering has turned the v6 from something bulky and rough to something compact and smooth that can rival the inline six cylinder engine the added bonus of the v6 is that it has a shorter more rigid block and it also has a shorter crankshaft which doesn't experience as much torsion or twisting force as the long uh crankshaft of the inline six cylinder engine although we have many modern inline six cylinder engine designs that testify to the fact that with the right engineering and the right amount of materials and the right at the right places block flexing due to the blocks length or crankshaft twisting really isn't an issue on modern inline six cylinder engines on top of this and inline six cylinder engine just needs one cylinder head as opposed to a v6 that needs two cylinder heads so yeah there you have it now that we have explored the v6 i plan to compare the inline six with the v6 uh with the vr6 and then with the for hat6 in our future engine balance video so stay tuned for that in case you want to find out how they fare against each other as always thanks a lot for watching and i'll be seeing you soon with more fun and useful stuff on the d40 channel

Channel: driving 4 answers

Views: 145,907

Rating: 4.9517627 out of 5

Keywords: inline six, v6, inline 6 vs v6, odd fire v6, 90 degree v6 vs 60 degree, 90 degree v6, 60 degree v6, buick fireball v6, splay angle, alfa romeo v6, alfa romeo busso, 2jz, ford barra, bmw s54, nissan rb26dett, nissan rb, primary engine balance, secondary engine balance, engine balance explained, inline 6, engine firing order, engine firing interval, le mans v6, f1 v6, 120 degree v6, aston martin tm01, v6 angle, v8, straight six, inline six engine, automotive, buick v6, VR38

Id: VQJ0kYFF9T4

Length: 20min 3sec (1203 seconds)

Published: Sun Feb 07 2021