Most efficient engine type?

Well aside first then, as I find it more interesting:

I read an interesting article once about 1st and 2nd order balancing and why certain engine configurations are more balanced than others and why V8s sound like they do.

A cross-plane V8 does not always fire on both banks like I believe a flat-plane V8 does, it can reach TDC simultaneously on 2 cylinders on 1 bank, thus it's firing entirely on one cylinder bank but not on the other at times, this combined with a heavy lobed cross-plane crank-shaft (designed to keep this inbalanced engine, balanced) is one of the major reasons why V8s have the sound that they do, that sort of off-beat rumble and burble.

I don't know enough about this to go it great detail, I read an article about it once and it was very interesting, I just can't remember where it was now.


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I think purely for efficiency, as in maximum power from minimum space and capacity, it's probably a rotary engine you'ed be looking at.
 
I read an article once that said a 50cc 2 stroke engine was the smoothest engine configuration ever ( cant remember where though ) it went on and proved it using complex physics and maths, though again cant remember where I read it or when or what the maths were, but it must be true because I read it somewhere at some point.
 
BigglesPiP said:
Flat 4s are interesting too, a "flat four" like in a beetle or a plane is smooth, but inefficient since it is constantly pumping air back and fourth in the crank-case in the direction of the crank. A "boxer" like Subaru's is much rougher and unsuitable for aircraft but more powerful, partly because the air is moved back and fourth between the pistons in the direction of the con rods. Like pushing 2 beanbags back and forward through a narrow door vs shaking 2 bean bags left and right either side of the door.
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Sorry, but what are you saying is the difference between a flat-4 and a 4 cylinder boxer?
 
What about the crankshaft?

Are you saying that the Subaru's is truly horizontally opposed instead of offset like the VW? Because I don't think it is.
 
One has 2 crank journals and opposed pistons moving together, t'other has 4 crank journals and opposed pistons moving in mirror of each other.
 
Edit: I get ya. TDCs on each bank are horizontally opposed rather than diagonally.

Still, both the Subaru & VW have diagonally opposed TDCs.
 
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FWIW V6's are not naturally well balanced engines, they are quite compromised in this sense. They sound nice though, especially Italian ones.

In terms of outright power, valve area becomes the dominating factor, so given a particular displacement limit, you have more power potential by lots of very short stroke (hence larger bore) cylinders. Look at F1 engines of the past to confirm this.

In terms of efficiency, there are numerous trade-offs. A small displacement cylinder has an unfavourable area/volume ratio, so proportionally more heat from the combustion process gets absorbed by the head and cylinder walls. An engine with numerous smaller cylinders will likely have higher frictional losses than one with fewer, larger cylinders. However, a small cylinder will also have a small combustion chamber which means faster combustion which is good for thermal efficiency. Modern trends seem to show that fewer, small displacement cylinders are better for efficiency, coupled with a turbo to maintain useful power outputs.

Packaging is going to be one of the main drivers for the actual cylinder configuration, e.g. the flat four gives a low bonnet line and lower COG at the expense of width. A V configuration packs in a larger number of cylinders into a shorter engine than an in-line configuration at the cost of two cylinder heads and increased width. The VR configuration engines from VAG give some of the length benefits of a V but with the cost advantage of a single head and reduced width.
 
BigglesPiP said:
Popular in boats, sounds like a bloke with a hammer smacking a post every few seconds.
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Don't care - you wanted efficiency :P

HCCI etc will be the most efficient though, lack of pumping losses and can run lean-burn conditions.

Run it on an offset crank too to reduce sideloadings on the liner
 
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