V12 - do cylinders fire in pairs or staggered?

I had forgotten about that one. There may be a couple of other variations. The one I had in mind is the Fairbanks-Morse that is opposed pistons with two cranks joined together by a vertical shaft drive. The combustion chamber was formed between the two pistons as they came together. Fuel injector in the middle. Two stroke cycle with supercharger aspiration.

Lugnut

Reply to
lugnut
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Yamaha has a cross plane crank four cylinder engine:

Check the demo at:

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Reply to
Thomas Tornblom

Yes that's the sort of thing I was thinking of. So it *is* possible - I wondered whether there was some very obvious reason why it wasn't done years ago in the early development of the engine so as to become the standard.

Reply to
Mortimer

If its a 2-stroke engine, then yes you'd want 90-degree spacing. But in a 4-stroke, each cylinder only fires every other crank rotation so you have to have pairs of cylinders moving together. The inline 4 moves the middle 2 and outer 2 together, which minimizes primary imbalance but leaves a HUGE second-order imbalance. The block tries to "bounce" up and down parallel to the cylinder bores twice per revolution, which is why so many I4 engines have balance shafts turning at twice crankshaft speed. The opposed 4 is a FAR superior layout balance-wise, but packaging constraints have made the I4 dominant in the marketplace. Personally, I gladly pay the extra fuel for a v6 or I5 just to get away from the horrors of inline-4 balance. Even an inline 3 would be better. I hate inline 4s.

Reply to
Steve

Fairbanks-Morse 2-stroke opposed piston diesel. Still in production, still in use. Its the backup generator engine on every submarine in the US fleet.

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Even more exotic was the British Napier Deltic, which has *3* cranksafts in a triangle, with pairs of pistons along each axis of the triangle.

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Reply to
Steve

Apart from a Citroen GS in the early 1980s which had a 4-cylinder boxer (horizontally opposed) engine, I've never driven anything except an inline

4, so I've nothing to compare against. 6- and 8-cylinder engines here in the UK are confined to large executive cars which are waaaaaaaaaay outside my price range.

Why is an inline 4 so bad compared with even an inline 3, and why should the

*apparently* balanced configuration of two pistons going down as two pistons come up be so bad? I'd have thought an odd number of cylinders would be unbalanced. My mother has a Daihatsu with an I3 engine, and that sounds very rough at high engine revs and the car feels as if it has the tremors at idling speed.

Do all I4s have 1 and 4 moving together and 2 and 3 moving together? Wouldn't 1+3 and 2+4 do equally well in preventing a standing wave? Would it improve or worsen the imbalance that you describe?

Reply to
Mortimer

Well, an I4 isn't a disaster in any way, so don't read too much into my comments. Pretty much any number of cylinders arranged in line can be first-order balanced (in-balance at the same frequency as the crankshaft rotation speed) with crankshaft counterweights. Its the higher-orders that get more difficult. I'm not sure what the dominant vibration for an inline-3 is, but for an inline 4 its the second order (twice the crank speed) that cannot be cancelled directly using counterweights on the crankshaft. The engine tries to "bounce" up and down in the same plane as the cylinders, but at twice the crankshaft speed. An inline 6, by the way, is perfectly naturally balanced at all harmonics. So is a

60-degree V12. V8s and v6s lie in between, their natural imbalances are higher than the 2nd order (ie. 3, 4, or more times the crank rotation speed) and so they are less noticeable than an I4. The problem with the I4 is that the primary residual imbalance is at such a low frequency that it can be felt easily. The plus side is that its very easy to correct with balance shafts, but of course that saps a little bit of engine power away. Obviously its not a big enough drawback to prevent the I4 from being one of the most common engine types in the world. Packaging counts for a lot when it comes to actually putting an engine in a car. The cool thing (to me) about flat-4s like Subaru and Porsche is that the crank is SO light weight compared to an I4 of the same size and power. It looks like a bent steel bar with no counterweights at all, until you look closely and see the VERY small counterweights at each end which cancel the vibration due to the pistons on opposite sides of the engine not being directly aligned with each other.

I'm guessing that's the same (or similar) engine to the old Geo Metro that was sold in the US. The I3 has some roughness due to simply having fewer power pulses per crank rotation, but the mechanical balance is as good or better than an I4. The other thing is that the Daihatsu engine is geared toward economy (both in manufacture and in operation) so its not very optimized for creature comforts. Personally, I like the inline

5s that GM, Mercedes, and Acura have used over the years. Good compromise between fewer cylinders for economy, and avoiding the I4 vibration issue.

I don't think that would change the 2nd order "bounce" motion at all, but it would ADD a tendency for the engine to rock back and forth at crankshaft speed (or maybe 2x speed). Pairing 1 with 4 and 2 with 3 at least keeps everything in one plane of motion.

Reply to
Steve

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