by "RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
Published 12/30/2017 at 22:50
by "RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
Published 12/30/2017 at 22:50
Tags: FIAT 126
; FIAT 500
; Engines
STARS: 2
So, last evening and this evening I’ve been spending my free time watching this two-part series on how to reassemble a classic FIAT 500/FIAT 126 engine (yes, I am obsessed enough that I’m watching 2 hours of video on reassembling the engine for a car I don’t even have yet :P):
!!! UNKNOWN CONTENT TYPE !!!
I didn’t clue in until now, but I noticed something as he installed the pistons - the crankshaft - the pistons don’t move in opposite, but move together with a counterweight opposite!
This boggled my mind! Now, I know how engines work, but I don’t know a lot about the
design
side of engines....is this a common design for small, inline two-cylinder engines? I always thought one would fire and then be travelling down as the other was travelling up to it’s firing point, this way they would be balancing each other out without needing a counterweight (and as it is an even number of pistons, it would run smoothly...right?)? The way the 500/126 engine is designed, both pistons are travelling together even though one is on it’s firing stroke and the other is on it’s exhaust stroke...very odd to me!
Anyone else find this an odd and interesting engineering choice? The more I research and learn about these little cars, the more enamored I become! :)
"BaconSandwich is tasty." (baconsandwich)
12/30/2017 at 22:39, STARS: 1
That does seem interesting. I’d be curious to hear why it is done that way.
"Clown Shoe Pilot" (csp)
12/30/2017 at 23:14, STARS: 0
The way it’s built, you get a power stroke every 360° of crank rotation. If the conrod journals were offset it’d run lumpy as hell.
"RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
12/30/2017 at 23:21, STARS: 0
But aren’t 2-cylinder bike engines of a similar size (650cc) offset and they run OK? (Though I know not a lot of them are directly inline).
"bob and john" (bobandjohn)
12/30/2017 at 23:25, STARS: 1
Smoother power. One cylinder fires each time it goes down
"Clown Shoe Pilot" (csp)
12/30/2017 at 23:26, STARS: 0
Hmmm... indeed. I never took my cb500f engine apart but I looked it up and indeed the conrods are 180° apart.
So... yeah. I have no idea.
"Aremmes" (aremmes)
12/30/2017 at 23:28, STARS: 4
It’s done that way to phase the power pulses evenly at 360°. Theoretically one could swing one of the throws around 180° and make the corresponding cylinder fire immediately after the other one, resulting in avery long pulse that lasts roughly 360°, but that leaves an equally long duration without any power. That would require a larger flywheel to dampen the pulses and store enough energy for the compression strokes, which would in turn make the engine less responsive to throttle inputs.
"RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
12/30/2017 at 23:33, STARS: 0
Haha! Exactly! :P When I thought about it, I couldn’t think of a super strong reason either!
I mean, if they were offset, they would be theoretically pretty much perfectly balanced as each piston/conrod would weigh the same, right?
"RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
12/30/2017 at 23:36, STARS: 0
Thank you! Excellent explanation! :D
DO bike engines, in fact work this way as well? I know the 500/126 aren’t bike engines, but they are a similar size....I always thought most 2-cyl bike engines were offset, but bike designers DO want quick throttle inputs, so wouldn’t they work better not offset then...? :S
"RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
12/30/2017 at 23:37, STARS: 0
Same!
"RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
12/30/2017 at 23:38, STARS: 0
Yup, I get that...I guess I just don’t understand HOW that makes it smoother compared to being offset? Forgive my simple mind :P
"bob and john" (bobandjohn)
12/30/2017 at 23:59, STARS: 1
the counter weight can be made heavy enough to counter the 2 pistons risingf and falling in sync. the rocking motion need them to either at even intervals to be canceled out.
also easier on the bearings as there is less thrust loading on it from the crank being offset and wanting to rotate.
"punkgoose17" (punkgoose17)
12/31/2017 at 00:21, STARS: 1
All 4 stroke engines are like this. The 2 cylinders will be at opposite stages of the cycle. So inline, V, and boxer engines the pistons will be top dead center at the same time and bottom dead center at the same time. One will be at the beginning of the power stroke and the other at the beginning of the intake stroke. An inline 4 will have 2 top dead center and 2 bottom dead center.
P.S. Thanks for sharing this video. I am going to watch it now.
"punkgoose17" (punkgoose17)
12/31/2017 at 00:30, STARS: 0
The connecting rods are 180 degrees apart because the cam spins at 1/2 the speed of the crankshaft.
"Aremmes" (aremmes)
12/31/2017 at 00:42, STARS: 1
Motorcycle engines do not need to produce as much torque at low RPM as car engines, so they’re designed for high RPM performance. This tends to smooth out their lumpiness.
"RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
12/31/2017 at 07:01, STARS: 0
Yup, knew the point of the counterweight as that’s their intended purpose, but the point about thrust and ease on the bearings is definitely a good one! Thanks! :)
"RallyDarkstrike - Fan of 2-cyl FIATs, Eastern Bloc & Kei cars" (rallydarkstrike)
12/31/2017 at 07:09, STARS: 0
Yup! Knew this about Boxer 4s and Inline 4s - but it sortof adds to what my original point was, in a way!
In a Boxer or Inline 4, there are always two pistons equally balancing out two other pistons at any given time (i.e. two up and two down). That’s why I thought that in an Inline 2 that the pistons would be balancing each other out at the top and bottom of their strokes with one up and one down where in a 4 there would be two at the top of their stroke and 2 at the bottom of their stroke...if that sentence made sense? :P
"shop-teacher" (shop-teacher)
12/31/2017 at 08:14, STARS: 1
That does seem odd.
"Sachi" (sauvyasachi)
12/31/2017 at 11:48, STARS: 1
The pistons move together in order to achieve even firing intervals, which helps with smoother power delivery. As you pointed out, this does create an engine that would be nearly impossible to balance, but in a heavy vehicle like a car torque inconsistencies are much more apparent to the driver. On the other hand, most vibrations can be absorbed by big squishy motor mounts.
On motorcycles, it’s the other way around. Motorcycle engines often double as a structural component, so big squishy motor mounts aren’t an option. This is why most modern motorcycles prioritize rotational balance over even power delivery. Many motorcycles use engines with 180 degrees, 90 degrees or even 76 degrees between the crank throws, often resulting in an engine that fires like a V Twin. Wikipedia has a good explanation for this.
"Clown Shoe Pilot" (csp)
12/31/2017 at 13:14, STARS: 0
Yes, that’s true, but the power stroke still happens at uneven intervals of crank rotation. A full cycle for a single cylinder requires 720° of rotation, so if both cylinders were at TDC together, you could fire them alternating every 360°. Instead, you’ve got them 180° apart so you get power at 0°, then at 180° and then nothing for the next 540° of crank rotation. Alternatively, you get power at 0° and 540° and then power AGAIN at 0°, so you’re still only 180° apart between any 2 power strokes and you’ve always got 540° of nothing going on.