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 Post subject: Unbalanced engine?
PostPosted: Fri Dec 07, 2018 10:35 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 12973
Location: CT
Ok, this has absolutely nothing to do with Zs so maybe it belongs in our All Things Considered forum, but it's actually a technical question so I'll try it here and see if anyone scolds me.

I have just learned that the wonderful WW-2 Soviet T-34 tank engine had a different piston stroke on one bank than on the other?! I've never heard of such a thing. How does that happen?

The T-34 used an all-aluminum V-12 dohc 38L Diesel engine which made about 500bhp @1800 rpm. The specs are quoted as:

Displacement 38.8 L (2,367.7 cu in)
Bore 150 mm (5.9 in)
stroke 180 mm (7.1 in) left cylinder group
and
186 mm (7.3 in) right cylinder group
torque 1,591.3 lbf⋅ft).

So it was an over-square V12 with a longer stroke on one cylinder bank than on the other? If both cylinder banks shared a common crankshaft I suppose thats possible by using shorter connecting rods on one bank, but what would be the advantage of it, and wouldn't it make the engine unbalanced [different reciprocating weights]? If they lowered the cylinder block deck height on the 'short' side they could keep the cylinder pressure the same on both banks, but one side would still make more power than the other side, unless they made the 'short' side higher compression than the 'long' side, which might give the same power for both sides. AAAARRRGGGGH! :shock: Why would anyone do something so complex in the middle of a war when mass-production was the only means of survival?

Who's ever heard of such a thing?
And I thought I was confused when Audi made a 5-cylinder car engine. :roll:

https://en.wikipedia.org/wiki/Kharkiv_model_V-2

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 Post subject: Re: Unbalanced engine?
PostPosted: Sun Dec 09, 2018 2:52 pm 
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Joined: Fri Jun 23, 2017 5:34 am
Posts: 55
Interesting. I'm not a big tank or airplane motor guy, but I'm a geeky researcher of the automobile engine. I have not research this engine in particular but I can tell you the changing the rod length on a motor does not change the "stroke" but the "deck height". Which is typically described as the distance between the center of the crankshaft and the top of the piston or the block surface where you mount the head. One reasonable way to change a stroke and not change the crank is to offset the piston and the crank, in a manner where if you laid a straight edge down the cylinder wall it would miss the center line of the crank... this promotes low end torque.... There has been an insane amount of research as to when combustion is done. Thanks to faster sensors it has been figured out that the vast majority of combustion / pressure thus power is done way before 90 degrees ATDC. So alot of critics are picking apart the hole big strokes equal torque because of this fact. It always amazes me how early engineers figure this stuff out with basic free body diagrams, hands on knowledge, and butt dynos.

https://www.insightcentral.net/encyclopedia/enoffset.html


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 Post subject: Re: Unbalanced engine?
PostPosted: Sun Dec 09, 2018 4:11 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 12973
Location: CT
I agree offset makes sense, especially when you're trying to obtain the greatest possible torque advantage from any engine. And I agree about deck height. I suspect on the T-34 engine, one cylinder bank must be 'taller' than the other (the bare block would appear taller on one side).

I disagree about the rod length. A shorter stroke would absolutely require a shorter connecting rod, regardless how 'tall' or 'short' the rest of the cylinder was. Stroke is the measure of piston travel and the only things which can alter that is the length of the connecting rod or the height of the journal throw on the crankshaft.

If the T-34 engine uses a conventional crankshaft (with opposing piston rods sharing a common throw), I would expect the 'shorter' side to have 'shorter' cylinders, altho I have no guess what effect or advantage that would have on power, torque, or balance. The only other alternative I can picture is if the crankshaft had 8 separate throws, one for each cylinder, and 4 of them were longer than the other 4. I just can't imagine anyone making 80,000 T-34s with such an unnecessary complication during wartime.

Why even do that?
It's just weird to me.

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 Post subject: Re: Unbalanced engine?
PostPosted: Mon Dec 10, 2018 3:47 pm 
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Joined: Mon Jun 30, 2008 2:16 pm
Posts: 396
Location: Ansonia, CT
Very simple Frank, It's called the "VODKA" effect. :P

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 Post subject: Re: Unbalanced engine?
PostPosted: Mon Dec 10, 2018 8:08 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 12973
Location: CT
:lol:

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