Every engine has its own charecteristics, and unless they are a prefect match one or the other will be trying to driver the other, acting as a brake. If you had an arragement to detect this, the slower engine would have to speed up to match its partner. In doing so it would likely become the master, and force the other to speed up too, leading to runaway. The converse would be true if the tactic was to lower the speed of the master engine. Unless the control was renarkably accurate, and could react predictively, the engines would end up fighing each other. An ECU could probably do this, but it would most likely be far more cost effective (in terms of design, maintainance and fuel costs) simply to fit a more appropiate engine.
A single engine, but with more cylinders, would not suffer from the above as it would only have one control system operating on one crank, i.e. a closed system.
But 2 V8's (for arguments sake) running EFI with a single throttle each and commoned together as a V16 would be no different to balancing twin carbs on a single V16, or to a similar extent a single V8. Just because the cylinders are located within another block casting, but mechanically linked, it doesn't cause any real issues at all. You could argue that a twin carb engine will have lagging and leading cylinders depending on how the carbs were balanced, in fact this is done deliberately on some engines (Jag XK 4.2 and RoverV8) to provide for a smooth acceleration at low rpm's, the imbalance becoming an acceptable factor in the interests of driveability, the effect reducing to virtually nothing as full throttle is reached. Badger.
Some Cold War Russian military vehicles used a pair of engines, I think based on the diesel (originally pre-WW2 French) which powered the T34 tank. I don't think there was anything fancy in the control system, and I've a vague recollection of it involving engine/clutch/gearbox units running in parallel.
Also, at least one WW2 version of the Sherman tank used an engine made up from multiple car-engines, but integrated into one unit.
It would be different - each engine would react differently since each engine has it's own unique charteristics - a change in throttle position for one engine (even if the carbs/ECU were sapped over) would have a different effect on that engine to another one. As you observe, twin carbs are a compromise that is regarded as being acceptable. If the carbs are not balanced, even by a bit, then the compromise fails. The ideal solution would be to have one carb, which doubltless could be done but is not (finacially and probably functionally) viable. In other words, exactly the same debate, only with twin carbs the compromise is acceptable (functionally) whereas with twin engines the "imporovement" can be exceded at lower cost by choosing a more approtiate engine.
I think you might just be missing the point here, the different cylinders within a single engine can in practice operate at differing efficiency levels to their adjacent cylinders, due mainly to inlet and exhaust breathing capabilities. This is very obvious on a carb rover V8, where the outer 4 cylinders (1,2,7&8) do not get the same inlet charge volume as the other 4, mainly down to the design of the inlet manifold. If you were to bolt two theoretically identical 4-cyl engines together, then at worst you'd be no worse off charge efficiency wise than a std V8. It's been done for years in tractor-pulling circles, using nose-to-tail, chain, belt, and collector gearbox types of systems, all with reasonable success. They even join the outputs from Isotov gas turbine engines, one machine having 3 of the damn things feeding into a collector gearbox!
Re-read my last posting, some engines are indeed set up with a deliberate carb imbalance. RV8 and XK4.2 being 2 that I can quote off the top of my head, where one carb opens prior to the other. In fact, jaguar even quote differing carb lead values for auto and manual versions of the same engine with the same carbs!
Multi-carbs were traditionally used so that individual port runner gas velocity could be maintained as high as possible in the interests of mid-range torque, in addition to the engineering problems encountered when trying to make one carb feed a largeish engine from idle to max. The port runner issue still stands today, look at the length of the runners including the trumpets feeding a v8, and consider why the outer 4 are a different length to the others, in respect to my first para of this post. Badger.
I don't beleive I am missing the point at all - the V8 is a closed system, i.e. inouts to one or more cylinders have a direct efffect on the total output of the system. Bolting 2 4-cylinder eninges together is trying to run two independent closed systems together, not the same thing at all.
And how long do they last?
It's been done for years on railways too, with great success, BUT, two locomotives (of the same class) working in tandem (2 drivers, one in each) only gives a 133% power increase, and in multiple (one driver, using remote controls to the second engine) only gives a 150% increase - an exact anology to our two 4-cylinder engines bolted together.
Any details on this because as a simple minded physicist I'm pushed to see how an engine at a set revs with the pedal down does not deliver its rated torque regardless what the load looks like.
I just can't see power vanishing. Conservation of energy is a pretty basic law of the universe.
On or around Mon, 27 Jun 2005 07:24:38 +0000 (UTC), beamendsltd enlightened us thusly:
Granting these things, how does this differ from a single mutli-cylinder engine with separate carbs, such as bike engine? If I have a 2-cylinder engine with 2 carbs, and they're not balanced, one pot fires less strongly than the other, which leads to less power and possibly more vibration. Similarly only moreso for a 4-cylinder with 4 carbs.
My proposed 2xTDi would have the cranks rigidly fixed together, at least as regards the rotation of same - there might be a problem the blocks bolted together true enough to actually run a rigid crank connection, although it could probably be done. But suppose some suitable coupling which allows slight lateral misalignment but not rotational play between the 2 sides. The crankshaft is then effectively one piece. The cylinders will be supplied with fuel by 2 fuel pumps, which would have to be adjusted so as to provide the same fuel to each bank of 4 clylinders. Once having done that, I don't see that the losses should be much.
I grant that synchronising the fuel pumps absolutely would be difficult or impossible, but then by the same token it's also difficult or impossible to synchronise multiple carbs absolutely.
Finally, as to the "fit a more appropriate engine", where's the fun in that? But seriously, there's a lot to be said for a double-TDi compared wit some other type of 5-litre diesel - parts are easily available and cheap, for example.
Father's just pointed out - put 'em back-to-back and take the drive from the middle. They will counter-rotate, so they can be geared together by a brace of big sod-off gears (instead of flywheels), and the drive taken from the back of the front engine by a shaft alongside the rear one; or possibly, moutnt the rear one alongside the gearbox with a modified bellhousing to allow the gears to mesh.
Ok then, take an I4 engine with a single SU carb, bolt another identical I4 engine to it to make it a twin carb I8. Now, apart from the fact that the resultant engine is 2 blocks bolted together and a joining sleeve/spline on the now 2-piece crank(s), why is that suddenly so different from an I8 engine with a single block casting and 2 carbs and a dual-point dizzy, where each set of points feeds 4 cylinders (preventing points bounce at higher rpm's)?? Answer, it isn't!
"Corskie Supertramp", over 12 years that I know of and still pulling strong with a multiple (6) rover V8 setup. I once remember it throwing a rod out the side of a block, but that was due to it being run at too high an rpm, nothing else. One (forget its name, might be Midnight Express?) with 4 (or6) jag XK 4.2's and a V12 on top!! (Drive belt issues, but engines all ok after a good few years) Now, that's just 2 for starters that are based in the North of Scotland that I know of, there are many, many more up and down the country and on the continent, and if banking engines up together, either in series through a common crank output or in parallel via chains, belts etc didn't work or was seriously problematic, they wouldn't do it!
Sorry, I can't see the logic in that at all. If I have 2 loco's each having an individual, separate, tractive effort of say 10, then I have a total combined tractive effort of 20. There is no extra work lost to friction etc by either engine as it is still pulling its own weight along, so neither robs any tractive effort from the other! If I have 2 engines, each of
200bhp, then I have 400bhp in total if they are utilised correctly. Either engine's power output doesn't simply drop off just because it is now working alongside another! The laws of physics couldn't be true otherwise. It's fair to say, however, that adding 2 2litre 200bhp engines together to make a new single engine of 4 litres will not automatically give 400bhp, this is due to pumping losses and friction.
No, it was done as a deliberate compromise to the engine's power delivery characteristics, not as a compromise to make something work in the first place! If both carbs were opened simultaneously, the sudden torque rush was too severe when coupled with either 1. an auto when pulling away from rest or 2. when trundling along in heavy traffic and feathering the throttle at very low rpm without using (slipping) the clutch. In both cases the designers introduced an efficiency (cylinder power balance) compromise in the interests of user-friendliness and driveability, Proving that at lower power and rpm settings you really don't need to have all cylinders perfectly matched. The important things are 1. that all idle stops/mixture screws are in balance and 2. all reach full throttle at the same time. Badger.
I suggest he buys a new calculator, then. Acceleration time won't half by doubling the power, because you've added weight. It can be easily worked out by the following equation, though. Force (F) = Mass (Kg) x Acceleration (m/s2)
Austin> Father's just pointed out - put 'em back-to-back and take Austin> the drive from the middle. They will counter-rotate, so Austin> they can be geared together by a brace of big sod-off
Can you put one at each end driving each axle independently? :-)
On or around Mon, 27 Jun 2005 12:52:49 +0000 (UTC), beamendsltd enlightened us thusly:
not quite. There's no solid link between the 2 engines - in the railway engine case, depending on transmissions, it would be possible for one engine to be pulling faster than the other one was running. However, if they're both running at a power output and transmission setting suitable to run at e.g. 60 mph, then they should both contribute more-or-less equally.
Going back to the 2 engines, again. Suppose I run one engine at idel and the pother at full chat, obviously, the one at idle is doing nothing and is in fact absorbing power. But suppose one is running at 3000 rpm and the other, on the same setting, would in normal circumstances be running at
2900, the one running at 3000 will still be doing more of the work, but the second one will be contributing something significant, albeit not full power. Now suppose that I set the thing up on a dyno, open the throttle to an appropriate setting (say 2500 rpm for a TDi) I can then tweak one of the pumps/linkages 'til I get maximum output. If they run through 2 separate exhausts (which would make sense) I can also assess the smoke from each separately.
I don't doubt that it's impossible, practically and maybe theoretically, to get twice the output, but by careful setting up I reckon you should get enough to make it worth while.
On or around Mon, 27 Jun 2005 13:11:01 +0100, Steve Taylor enlightened us thusly:
nah, you can have twin turbos. Mostly on V engines. The mack V8 which they use (with one turbo) in the reanult magnum trucks puts out about 500 BHP or a bit more. The same basic engine, with 4 turbos, produces 1000BHP. Probably not for so long, though.
On or around Mon, 27 Jun 2005 16:06:36 +0100, AndyC the WB enlightened us thusly:
now yer back to the 4CV sahara. There's something to be said for that arrnegment, too - given the proper amount of practice, you could, by having twin throttle pedals side-by-side, by rocking your foot left or right bias the power to the front or rear.
The 4CV had the 2 engine slinked to one set f controls, however, AFAIK, and also had the ability to run on one engine (by putting the other in neutral) on tarmac.
VW did a twin-engine 4x4 sirocco a bit back, too - very successful in initial tests but I think it fell foul of the demise of the group B rally class.
You also get the benefit of 'limp home' mode should one engine fail and could run 2wd when you didn't need the power / traction / fuel bills of running both engines.
Didn't someone do an Audi TT with two engines driving one end each?
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