I'm aware they're boxers (and therefore, supposedly, in perfect balance - not that you'd believe it if you saw an old beetle at tickover) but that doesn't explain the difference in exhaust note compared to a straight four or six.
A Porsche can emit a delightful yowl.
|
|
A four cylinder boxer engine is not in perfect balance.
|
>>A four cylinder boxer engine is not in perfect balance.
I'm sure you're right. I've been having a bit of a dither about this! I originally posted that it wasn't in balance, then thought that I had made a mistake.
The imbalance can only be a secondary moment, but, I'm struggling to visualise it.
|
|
NC - I only said that after looking it up on Wikipedia ;-)
|
|
|
"The imbalance can only be a secondary moment, but, I'm struggling to visualise it."
Not like you, NC :-)
It's easier to visualise with half the number of cylinders, e.g. a BMW bike or 2CV engine. The pistons go in and out together, but there is a slight rocking couple due to their being not directly opposite each other. This is much less pronounced with more cylinders, as anyone with a 6-cylinder Gold Wing will attest.
I've never been too sure why Beetle engines make such a racket, though. Is it to do with the cooling arrangements? My Citroen GS (also air-cooled flat four) was wonderfully smooth and quiet.
|
>>but there is a slight rocking couple due to their being not directly opposite each other.
Yes!, but, if you now go back to the 4 cylinder case, if the crank is laid out so that the other pair of cylinders are producing an equal and opposite moment does it not cancel?
|
"if the crank is laid out .."
But that would mean 1 and 4 being at opposite ends, with 2 and 3 in the middle (on the opposite side). I think there's always a bit of uneveness left with the conventional arrangement, where there is a slight offset between the two sides. Not much, I grant you, once you get to four or more cylinders (hence the popularity in small aircraft), but still there, if only mathematically...
|
But that would mean 1 and 4 being at opposite ends with 2 and 3 in the middle (on the opposite side).
As can be seen from this site
endwrench.com/images/pdfs/2.2Liter.pdf
the crank is laid out almost as described.
Finally!, I can see the cause of the imbalance.
In a 4 cylinder inline engine, there's a secondary force imbalance, because the pistons coming up move (say, 2 and 3) at slightly different speeds to those coming down (1 and 4)
To create a boxer from an inline 4, (in abstract terms of cylinder balancing!) cylinders 2 and 4 are rotated about the crank by 180 degrees.
So, the secondary force imbalance contribution from the cylinders 1 and 3 is now (almost) balanced by that from 2 and 4 as they are now on opposite sides of the crank - the ony reason why they don't fully balance in all senses is the cylinder offset, and so, there's no unbalanced force, but there is an unbalanced secondary moment.
Hurrah!
|
"the crank is laid out almost as described"
Not quite what I meant, though. I was thinking of an engine (AFAIK it doesn't exist) where the cylinders on one side operate at each end of the crank, and the 'in between pair' are on the other side, so that the secondary forces from the two 'twins' are mirrored and thus cancel each other out. It's not done for reasons of space and homegeneity, I imagine, and the benefit would be small, as the forces in question are pretty trivial anyway!
Here's another way to deal with them.. :-)
www.shaneengines.co.uk/
|
>>Not quite what I meant, though.
The Subaru has a crank which has crankpins at;
0
180
180
0
degrees. Just like an inline 4 cylinder 4 stroke engine crank
Is the engine in your mind's eye
0
180
0
180
With 1 and 4 in one bank of cylinders, and 2 and 3 in the other?
|
I was thinking more like the Subaru - two twins, back-to-back, as it were.
I have an old BMW flat twin that is not as smooth as you might think - I've often wondered if this was due to the alternate firing of the cylinders and whether it might not be better if they fired at the same time, even if that did make it sound like a big single...
|
>>I was thinking more like the Subaru - two twins, back-to-back, as it were.
Yes, however, the firing interval would double.
|
"the firing interval would double"
Really? I think we may be a cross-purposes. The only difference I'm suggesting is that the location of the front two cylinders is reversed, thus:
== 1
2 ==
3 ==
== 4
So it shouldn't affect the firing intervals, just the secondary balance. The exhaust note might be a bit odd, though!
What did you think of the Shane engine? Another worthy idea that will be consigned to the bin because of NIH (not invented here), I suspect...
Edited by J Bonington Jagworth on 10/03/2009 at 10:21
|
|
|
|
Being a flat engine, some constraints are imposed on the manifold design.
On an inline 4 engine, the equivalent would be a manifold where cylinders 1 and 2 formed one downpipe, and cylinders 3 and 4, the other. Owing to the distance across the engine, these 2 downpipes must remain independant for a longer distance than would otherwise be the case. An odd layout for an inline engine, but, almost enforced on the Subaru.
In practice, there's one short pipe, and one long one, to move the exhaust away from the centreline of the car, and I think it's this asymmetry that makes these cars sound a bit rough even when they're running right.
|
What NC says.
The actual combustion is even but the uneven distance that the sound/pressure wave travels cause the 'burble' similar to that of a V8 ticking over.
Newer Subs have been engineered to have more even manifold/exhaust lengths so you hardly notice it from the latest Legacy/Forester/Impreza.
|
"Newer Subs have been engineered to have more even manifold/exhaust lengths so you hardly notice it"
Shame!
|
Shame <<
Absolutely! I love the engine on my Forester XT, but it is annoyingly smooth. I'd almost be tempted to get the PPP fitted to get a louder exhaust note (still within new car limits).
Nothing to do with the 5.2sec 0-60 time...
|
"annoyingly smooth"
That's not something you hear every day! :-)
|
|
|
|
You can get the same effect from an inline four by making one pair of downpipes much longer than the other.
|
"one pair of downpipes much longer than the other"
That sounds rather like you've done it.. :-)
|
|
This is probably going to make me sound like a complete idiot, but don't you change the diameter of one of the pipes so that it takes the exhaust gas the same amount of time to reach the point where they join?
|
Aside from the possible sales benefits of making the car have an exhaust note appropriate to its image, I thought the subtlety of exhaust tuning was to cause the pulses of high/low pressure from each port to synchronise in such a way that the low pressure from one port assisted the high pressure peak from another?
So if the optimum lengths were therefore multiples of half-wavelengths, the set up could only be perfectly optimised for a particular engine speed?
I have certainly heard the converse on the inlet side to be the case - that large multiple carburettors improve high rev performance, but a single smaller carb gives greater torque at low speed.
Enter Number Cruncher (stage right):
|
>>only be perfectly optimised for a particular engine speed?
Yes, but the positive effect can be enjoyed over a limited rev range. This rev range can be broadened by using variable valve timing, and more so for inlet manifolds, using flap valves to change the effective manifold length.
For a somewhat dated but good non-mathematical description of the subject, see "Scientific Design of Exhaust & Intake Systems" by Smith and Morrison
In broad terms, it's a bit like organ pipes in churches - the longer pipes give low frequency, while short pipes deal most effectively with the high notes; the high engine speeds.
|
|
Amazing. This far in a Subaru thread and no one has mentioned the word 'chav' or poo-pooed the fuel consumption. Things are looking up!
|
Its great my Legacy Estate with the Boxer Diesel unit displays that unique throaty burble at times.
Not as pronounced as the petrol obviously but addictive.
I dont class myself as a chav though at fifty two years old!!!!!
It is quite amusing to see people noseying round the car or asking me questions at petrol stations etc.
They notice the bonnet scoop and assume its some ultra high power petrol and then notice the diesel badge on rear.
A look of puzzlement.
|
This thread is so far over my head...
There's a Forester near me which sounds absolutely stunning when it goes past. I love Foresters. That's my contribution.
|
Why do people say that boxer twins and fours(but not
6 from my Reading)get smoother the harder you rev them?
|
|
I spent a lot of time in, and drove for about 50 miles, a 2.5 litre Forester in New Zealand a few weeks ago. It hardly made any engine noise at all and its beat was extremely subdued, only audible really at high rpm. The car was so quiet that I thought it was a six, but it wasn't.
|
The engines are inherently 'smooth' from a vibration point of view at almost any speed, but the 'warble' sound in the exhaust will appear to get smoother as the revs rise.
This is because at low revs you hear each off-beat as an individual pulse - one per revolution of the crank (or is it every 2 revolutions?). Once you rise above a certain engine speed the individual pulses will start to become a more continuous sound (at around 20-30 beats per second to the human ear) initially as a 'buzz', and at higher rpm as more of a 'hum'.
All engines share this characteristic, but other engine noises such as valve train noise and cam-chains make less pleasant, higher pitched noises giving the effect of 'roughness' at higher rpm for some engines where there is more transmission of these noises.
|
Are you talking about a true "beat" as between the slightly different notes produced by twin exhaust pipes (or multi prop aircraft) or just the basic low exhaust note?
|
There are probably some very valid points on here but the REAL reason Subarus sound like they do is as follows:-
THE TYPE OF EXHAUST MANIFOLD FITTED! (which can be fitted due to the style of engine)
That's it guys & girls simple as that & it can be placed into 2 seperate types:-
1. The non- turbo
2. The turbo.
As mentioned before the Subaru engine is a flat four engine with 2 cylinders on each side. Unlike a traditional inline four which will have usually a single cast metal manifold that curves into the cat / exhaust, Subarus have a manifold on each side (bank of 2 cylinders)which joins at the cat / exhaust. Non-turbos have what is known as an 'equal length manifold' in that each side, from the bank of 2 cylinders, is the same length & sound slightly different to the turbo. The turbo has ' a non equal length manifold' which means the pipework is slightly longer from one bank of 2 cylinder than the other meaning the sound waves crash into each other at different speeds or pulses, as one side has a little further than the other to travel due to the slightly longer pipework. This gives the turbo that popping or thumping sound that sounds so different to other cars. Note:-In the turbos case this is before it goes into the pipework up to the turbo rather than the cat / exhaust in the non turbo. After market manifolds can be bought for non turbos to make them sound like the turbo. Larger diameter exhaust pipes just amplify this sound.
Hope this explains it a little?
Edited by ScoobyNut on 04/11/2009 at 17:22
|
so its not a factory exhaust blow then made worse by the phwat back box fitted by the clown in his backwards pointed baseball cap
oh
off to see my neighbour now then to apologise
anyone got a gnome i could borrow?
|
BB, I still have an idiot of a neighbour who has a remote control for his Scooby. Starts the engine and leaves to run in the morning, waking half the neighbourhood with the vibration.
Then when he comes home, you hear him revving his way through the local streets and then he leaves it to idle while he goes in and starts his dinner.
|
|
|
|
|
Like the old VWengines and Porsche's.
As did the Alfa Romeo Alfasud, of which I had two.Lovely cars(for their time).
|
|
|
|
