Explain "Displacement" to a numpty please ?

[grober]

[YOUTUBE HD]6BCgl2uumlI[/YOUTUBE HD]

 

[John]

I've always understood rotary engines are much more likely to fail than piston - but why is that?

Is it the seals break down or just that mechanical action is not a good proposition long term...

 

[whitenemesis]

IIRC it is the seals that give out. The motion is very smooth, without the 180deg change of direction associated with 'normal' combustion engines.

 

[IanMSpencer]

I've always understood rotary engines are much more likely to fail than piston - but why is that?

Is it the seals break down or just that mechanical action is not a good proposition long term...

It is the seal at the tip of the cylinder, it is doing a very tricky job. In a traditional engine the piston rings do the same task and they follow a simpler path (and there can be more than one).

 

[grober]

Here's how to rebuild one! ---FINIKY doesn't do it justice!


[YOUTUBE HD]b-gybobzSA8[/YOUTUBE HD]

 

[MancMike]

I owned a rotary engine and it was a wonderful intoxicating thing.

They get a bad rep because they're not the kind of engines you can just service at their service intervals. You need to treat it like your baby. If you're the kind who doesn't pop the bonnet at least once a week it's not going to end well. If you're the type who only replaces components when they totally fail, rather than when they're past their best, it's going to end badly...

They need looking after, but the rewards are well worth it.

They usually fail early through either apex seal failure or rear stationary bearing failure. Both are due to poor lubrication, which can be improved greatly by an owner who cares. It's a complicated subject discussed heavily on rotary car forums like the RX8 owners club where I'm a member.

Wrong oil, thrashing it before the oil is up to temperature, poor ignition coils, etc can all contribute to early engine failure, but if looked after properly (also using 2 stroke oil as premix boosts the lubrication of the apex seals and a thicker grade oil that clings to the stationary bearing better) will help the engine live as long as a piston counterpart.

If both are treated badly, I'd suggest a piston engine would usually outlive the rotary, but you'll get far more smiles per gallon with the rotary than the equivalent power output piston engine. They're really an enthusiast only engine, which is why they're seen as poor engines by non-enthusiasts who don't look after them, and you won't convince a rotary aficionado that there's anything better.

 

[Bellow]

When would a piston ever under normal operation be nearing TDC when a valve is still open?

You answered your own question previously...

if the cam belt/chain snaps.

If the cam drive fails.
Valves aren't fully open (during normal operation) until the piston is around the mid stroke point. An aggressive inlet cam though can have the valve close to out-accelerating the descending piston around the TDC position. Pistons spend much longer (in real time) around the TDC and BDC positions than is commonly realised.

I've always understood rotary engines are much more likely to fail than piston - but why is that?

As other post have mentioned, their weak spot is the apex seal. Paradoxically though, some of the micro-light fraternity prefer rotaries as being more fail-safe. Their reasoning is that even if the seals fail in flight, the engine will continue to run. A recip on the other hand when it fails.....

 

[Bellow]

I thank you all

Thing that confused me was "stroke"

IIRC GM 2.2 Engine was developed in Europe but was deemed unsuitable for the US market so the stroke was changed making the engine 2.4 L but retained similar power output - why would that be ?

Typically the American preference is for strong low end delivery. This isn't in itself 'torquier' (though the additional 50cc of cylinder capacity will deliver more torque) but reflects the tendency to describe an engine as 'torquey' merely because its output is biased toward lower rpm. And the reason this happens....

Lengthening stroke compared to bore give more torque.... lazy slow engine

...is, as is hinted at above, because a greater cylinder volume has to be filled - but can only be filled (and emptied) via the valves that fit within the cylinder's diameter. Thus the valves are relatively small and hit their flow peak earlier and the engine's output is at lower rpm. The engine makes no more power, because the valves cannot flow more air than they previously did atop the 2.2l block. Thus the power remains the same but (usually) at a reduced engine speed.
That is (broadly speaking) the characteristics of an undersquare engine (a square engine being one where the bore and stroke are dimensionally the same).

Wider bore compared to stroke is more power.... revvy quick engien

Which bring us to oversquare engines as in the above. Revvy because they have large cylinder diameters which can accomodate large valves which breathe well at high rpm, and because short strokes reduce piston speed which is often a limiting factor with regards to rpm.
(The large valves though mitigate against good torque output at lower rpm - which is another story altogether, ie, volumetric efficiency).

In short, an engine bore and stroke combination is selected to endow the engine with the characteristics best suited to the market in which it will be sold.

 

[prprandall51]

Here is the 1 litre BMW motorbike engine at 14000 rpm:

http://www.youtube.com/watch?v=c8hqoE1_7bA

Those little valves are flying.

 

[Bellow]

Those little valves are flying.

Aren't they just?!!!

Another feat of engineering that never ceases to amaze me is hearing a 125cc cylinder two stroke at the same (14000) rpm and absolutely unwavering in its 'note'. That despite the absolute efficiency and repeatability cycle to cycle being achieved not by tightly controlled mechanical valving but sound waves rattling up and down a steel pipe. Simple sophistication!