Top Gear: Series 15

[aerius]

ETA: What I'm trying to get at is that I'm not at all convinced the W-configuration is inherently inferior to a V-engine, as Aerius claims. It has more parts, yes, but it's also a proven technology that the VW-group seems content to continue supporting for its luxury vehicles. I'm not an automotive engineer, so I can't tell you what the advantage of the design is - I'd wager it has to do with smoothness and compactness for a given number of cylinders - but to discard it based on vague notions of "efficiency" and "reliability" is as stupid as saying there's no point to V6-powered sedan when a turbocharged four gets similar horsepower figures.

The advantage of the W engine is that it's a lot shorter for a given number of cylinders compared to a V engine. V-12 engines are already rather hard to package into cars, nevermind a V-16. However, the shortness leads to another problem, the crankshaft becomes very crowded and it's very hard to fit all the bearing on it and get the connecting rods from the pistons to connect to it. Basically, you're more or less cramming the same of cylinders into a bit over half the length as compared to a V engine.

With a typical V engine the connecting rods are a good 20-25mm wide, lots of space to spread the load and easy to keep oiled, and the same is true of the crankshaft bearings, they can be made nice & big to carry the load and have lots of safety margin left over. On a W engine the connecting rods are something like 13mm wide and the bearings are a lot narrower as well, this puts a lot more stress on them so they have to be made of much stronger materials and leaves a lot less of a safety margin.* It also makes the oiling arrangements absolutely critical, any hiccups in oil flow, say, if the oil pressure drops 5psi below normal for a few minutes the bearings & con-rods will die whereas a V engine wouldn't even notice it.


*edit: This is actually far worse than it appears at first glance thanks to the joys of physics and math. The problem here is that all else being equal, the stiffness of the connecting rod varies proportionally with the cube of its thickness, or in this case its width. Meaning a con-rod that's half as wide has only 1/8 the stiffness. This is monumentally bad, if the con-rod flexes too much you get scuffed pistons and severe wear of the pistons & cylinders at best, at worst the con-rod fatigues and snaps and if that happens the engine's a write-off; the piston gets launched into the cylinder head, the broken con-rod trashes the block and the vibrations from the imbalance wrecks the crankshaft. Not to mention what happens when chunks of metal go flying around inside the engine at high speed.

[Dalton]

MOD DELETE PLEASE

Maybe you should do a little more thinking before you post from now on.

[Admiral Valdemar]

Veyron is a sheikh-toy, not a true sportscar. Buying one is buying a Bugatti badge with assorted impressive engineering strapped to it. I love how the car looks and the technical specs are quite impressive, but considering that there are cars that cost 1/10 of the price and perform just as good (or even better) Veyron would have to cook, clean and give blowjobs and perform boob jobs for me to even remotely consider purchasing one.

I won't say that I dislike Veyron. But it's the automotive equivalent of HMS Titanic, only that not much people seem to notice that it ran an iceberg - a vehicle marketed as a technological marvel of the century is actually slower than cars not even considered to be a competition. Oh sure, it has cockpit switches made from titanium that cost 70 thousand in cash a piece, but it's the same as being in a bathroom with a golden flush and toilet paper made from ivory. There is just no point in it other than showing off to people who can't afford it. It's probably the little commie in me speaking, but that's how I feel it is.

True petrolheads stick to tried and true cars like Ferrari's, Porsche's, Corvettes. I would pick a 911 Turbo over a Veyron any time. Or, if we are in the same price range, an F1 McLaren.

Nitpick: RMS Titanic. Unless the ship was an undercover destroyer for the RN.

And I agree with those points. The F1 was the first true ultracar, and I'd rather have one of those than a Veyron, even if I like the Veyron's lines too.

[Marcus Aurelius]

My guess is that they keep that huge hog around mainly for marketing reasons. I can't imagine a company CEO driving around an inline four car. Or a millionaire. Or a rock star. People like that enjoy having a big number on the back of their car so they can brag about it. And a W engine is certainly more 'bling' than a V unit.

That's the most likely reason, considering that the VW group manufactures one of the most efficient turbocharged fours in the world. They certainly could design a very efficient high-performance V-8, but chose not to, since a big W engine is cooler (or hotter, actually).

[Starglider]

As I understand it, the point of the W engine is to keep the cylinder size near the efficiency optimum of 0.5 L. The notion that an eight liter V8 is more efficient than an eight liter W16 is silly, because cylinder thermodynamic properties are much more important than the number of moving parts. The Veyron engine was not made that way for the hell of it, they did it that way so they could produce the power without restoring to the extreme cylinder pressures a V8 design would require. This is essentially the same argument for having a V12 rather than a V8.

I do wonder if the Deltic E.130 design (essentially three opposed piston flat engines arranged in a triangle) would work as the basis for a supercar engine. Opposed pistons are thermodynamically more efficient than single pistons, and the Deltic design shares crankshafts (and possibly valvetrain components, if you put the valve train in the center) to make the engine lighter than a V of the same displacement (as opposed to heavier for a standard opposed design). Unfortunately no one seems to make opposed piston engines any more, perhaps there's a good reason for it that I'm not aware of.

[Meest]

As I understand it, the point of the W engine is to keep the cylinder size near the efficiency optimum of 0.5 L. The notion that an eight liter V8 is more efficient than an eight liter W16 is silly, because cylinder thermodynamic properties are much more important than the number of moving parts. The Veyron engine was not made that way for the hell of it, they did it that way so they could produce the power without restoring to the extreme cylinder pressures a V8 design would require. This is essentially the same argument for having a V12 rather than a V8.

I do wonder if the Deltic E.130 design (essentially three opposed piston flat engines arranged in a triangle) would work as the basis for a supercar engine. Opposed pistons are thermodynamically more efficient than single pistons, and the Deltic design shares crankshafts (and possibly valvetrain components, if you put the valve train in the center) to make the engine lighter than a V of the same displacement (as opposed to heavier for a standard opposed design). Unfortunately no one seems to make opposed piston engines any more, perhaps there's a good reason for it that I'm not aware of.

Does that type of engine balance itself or need complex set of balance shafts? Not sure if it works the same way as a boxer engine.

[Starglider]

I do wonder if the Deltic E.130 design (essentially three opposed piston flat engines arranged in a triangle) would work as the basis for a supercar engine.

Does that type of engine balance itself or need complex set of balance shafts?

Opposed pistons inherently have perfect primary balance, the same as boxer engines. Deltic engines could easily be configured to have perfect secondary balance as well.

[aerius]

As I understand it, the point of the W engine is to keep the cylinder size near the efficiency optimum of 0.5 L. The notion that an eight liter V8 is more efficient than an eight liter W16 is silly, because cylinder thermodynamic properties are much more important than the number of moving parts. The Veyron engine was not made that way for the hell of it, they did it that way so they could produce the power without restoring to the extreme cylinder pressures a V8 design would require. This is essentially the same argument for having a V12 rather than a V8.

The W16 has a better cylinder size from an efficiency standpoint, but that doesn't matter because everything else is completely fucked to hell by the W engine layout. The most obvious one is the intake & exhaust manifold design, it's impossible to route all those tubes & pipes at anything resembling an optimal configuration, if you can't get smooth unrestricted airflow into and out of the engine it's not going to make power regardless of how optimized the cylinder size is.

Then add in a valvetrain which requires lengthy rocker arms to actuate the valves, lots of mass there so it can't be rev'd much more than a pushrod valvetrain. This will limit the redline on the engine and its power potential as well as screwing up the turbo boost mapping, it now has to run higher pressures at lower speeds to get power without blowing the redline. Also because of those long rocker arms, you can't use an optimum camshaft profile or else you'll lose control of the valves at higher engine speeds, you can't have full valve lift and the valves can't be snapped open & shut as fast as they should be, this cuts into the volumetric efficiency of the engine and kills its power. Plus stronger valve springs are needed which increases friction.

There's a lot more stuff but the bottom line is that any W engine is a heavily compromised design compared to a V, inline, or boxer engine. Its sole advantage is that it's shorter for a given number of cylinders which makes it easier to stuff into a limited space, everything else goes down the crapper.

[aerius]

I do wonder if the Deltic E.130 design (essentially three opposed piston flat engines arranged in a triangle) would work as the basis for a supercar engine. Opposed pistons are thermodynamically more efficient than single pistons, and the Deltic design shares crankshafts (and possibly valvetrain components, if you put the valve train in the center) to make the engine lighter than a V of the same displacement (as opposed to heavier for a standard opposed design). Unfortunately no one seems to make opposed piston engines any more, perhaps there's a good reason for it that I'm not aware of.

I've never heard of this engine before, but a quick search on the 'net reveals some problems, the big one being getting the output of the engine to the transmission. If you lay it flat to get a low centre of gravity then you need to take the power off the shafts and flip it 90° to line it up with the tranny, meaning bevel gears of some sort. Ford did something similar with the T-drive concept to get the power from the engine to the tranny, problem was that the gears can only handle a limited amount of power and they had all kinds of noise & vibration issues. If you stand it up so that the power goes straight into the transmission, then it ends up having a high centre of gravity which is death for a supercar.

[Tolya]

Unfortunately no one seems to make opposed piston engines any more, perhaps there's a good reason for it that I'm not aware of.

Wrong. Subaru still does. They even introduced a diesel engine in the boxer configuration a few years ago. Im not sure about motorbike producers (basically because I don't give a crap about bikes), but since the boxer design is much more common in bikes, there are probably lots of bikes that have that type of engine. Oh, and lets not forget about Porsche, where opposed six lives and thrives. In the past Alfa Romeo had a kickass boxer engine.

As for the advantages of a boxer engine, it runs much smoother, allows for shorter cranks (reducing weight and, more importantly, inertia of the piston. Boxer engines usually rev like crazy). It is also much more compact.

As for the disadvantages, it is much more expensive to make a boxer four than an opposed four. And that is probably the reason why almost everybody sticks to the inline configuration.

[Starglider]

Unfortunately no one seems to make opposed piston engines any more

Wrong. Subaru still does. They even introduced a diesel engine in the boxer configuration a few years ago.

Boxer engines are not the same thing as opposed piston engines. The later have two (opposed) pistons in each cylinder. Normal flat (boxer) engines have one piston per cyliner and are basically V engines with a 180 degree bank angle.

As for the disadvantages, it is much more expensive to make a boxer four than an opposed four.

Again, opposed piston engines are not the same thing as boxer, inline or V engines.

[Tolya]

Boxer engines are not the same thing as opposed piston engines. The later have two (opposed) pistons in each cylinder. Normal flat (boxer) engines have one piston per cyliner and are basically V engines with a 180 degree bank angle.

Oh, I wasn't aware of that. Thanks for clarifying.

[rapidsquirrel]

Boxer engines are not the same thing as opposed piston engines. The later have two (opposed) pistons in each cylinder. Normal flat (boxer) engines have one piston per cyliner and are basically V engines with a 180 degree bank angle.

Oh, I wasn't aware of that. Thanks for clarifying.

It confused me as well, mainly due to the fact that I'm used to referring to boxer type engine as horizontally opposed engine. This of course coming from spending time around airplanes, where just about everything has a boxer engine.

[Shinova]

Im not sure about motorbike producers (basically because I don't give a crap about bikes), but since the boxer design is much more common in bikes, there are probably lots of bikes that have that type of engine.

It's V-twin or some number of inline cylinder engine, like a parallel twin or inline-4. I haven't heard about a boxer design in any relatively well-known motorcycle model. Maybe there's one out there somewhere.