Number of valves

Hi,

Is there a difference in performance and fuel consumption for a "2 valves
single cylinder" and "4 valves single cylinder" engine?


YC

 

This is interesting stuff.

Someone gave me the pitch (in the early 80's) that the Suzuki TSCC as I
believe they called it, with the 4 valves when it was introduced, created a
carefully thought out amount of turbulance in the combustion chamber which
caused the flame front spread much faster. This allowed higher compression
ratios on regular gas (and I guess as you mention less ignition advance,
valve overlap, and the benifits).

There must be a trade off though. The kenetic energy which goes into making
the gases swirl at high speed gases must be created at the expense of the
density of the charge (a Bernoulli thing) (?).

Something I found interesting was that, as someone pointed out to me once, a
smaller combustion chamber requires less time for the flame front to
propogate across the charge. This does all sorts of good things: including
the things you mention above. It improves the efficiency of the engine,
indipendent of increased revs thru lower inertia of moving parts. One
reason why more cyls tend to lead to better performance.

But, it would be hard tell that to a Duck!

I have wondered about this stuff. The basic deals are work = pressure *
volume, work/time =HP. But the fact that IC engines have been around well
over 100yrs, an aside from better materials which allow them to run at
higher RPMs, we're still learning about the physics behind the rest of it.

Where will it end? Or..will it end? Will we see 300HP 200 lb liter bikes in
30 yrs? Sportbikes are awesome enough as they are at the moment

D.

 

Without trying to add to K's screed on fuel consumption.....

Performance - yes, absolutely. Four valves will flow more gas than two,
simply because there's a greater valve area. In addition, small valves
are lighter than big ones, so you have less inertia. This means that the
engine can rev higher, and more revs = more power.

Four valves also have an advantage in that you can run a slightly higher
compression ratio without getting into all sorts of knock problems, but
this is minimal. The placing of the valves allows you to maximise use of
the mixture burn pattern.

Downside? Four valve engines run slightly hotter at the top end, and
greater attention needs to be paid to cooling (see K's treatise!), but
that's easily taken care of.

Ultimately, four-valve heads allow you to have a biigger bang and more
bangs per minute, which means more power. All other things being equal,
under similar conditions as identical engines with two-valve heads,
they're more economical because they're m,ore efficient.

That said, there have been some amazingly thirsty four-valve engines
made. Honda's CB900, referred to by K, was one.

And now, introducing Yamaha's five-valve heads - not just a gimmick.
These were a real, real step forward in engine design. So much so that
Ferrari bought the rights to use them.

 

I find this interesting, because even allowing for small US rather than
Imperial gallons (a UK gallon = 4.54 litres) US fuel economy sucks.

But then fuel is relatively cheap in the US....

I run a Saab 9000. Normally aspirated two-litre built-like-a-tank
Eurobox. Does 30 miles per (Imperial) gallon, so, er, um, 26-27 US?

Wife runs a Nissan Micra - built-in-Britain sub-compact, you'd call it.
Glorious little one-litre engine, sweet as can be, cruises effortlessly
at 70-80 (needs to drop down to fourth on long upgrades, though) and
returns 40mpg (so 35-odd US).

In fact, it's more economical than my 1200 Trophy, although I daresay if
I ran my Trophy at 70-80 all day it'd be similar.

With our incredibly high fuel costs (currently over 80p/litre, so do the
math(s) yourself), economy is a big issue, But decent fuel economy and
decent performance aren't mutually exclusive - a friend runs a Lotus
Elise sports car and it's damn nearly as easy on fuel as the Micra,
because it weighs about as much as a decent pair of boots.

There's one good thing about high fuel taxation - to a degree you're
insulated from the rapid oil price rises. If the price of oil doubles,
the price at our pumps only goes up by 10% or so because so much of the
price of a gallon of petrol is actually tax.

When your economy (domestic or national) is based on fuel at a buck a
gallon, a rise to 2 bucks a gallon is a big *BIG* issue, I think.

 

As much as that?

Ahem!!!! Check the sig!

 

[snip sig containing prima facie evidence of madness]

And meeting you would change Kaybearjr's statement how, exactly?

 

The trade-off comes from the short stroke, which is the major factor
in reducing avg piston velocity and supporting higher revs... The
resultant chamber shape (a *very* squat cylinder) makes swirl more
challenging.

Make the intake ports smaller -- counter-intuitive, but the increase in
charge velocity helps a lot.

 

farfalfarfalpippik

 

This is indeed the truth. It's hard to think of more fun than you can
get out of a motorcycle and the price of a gallon of fuel.

 

Performance, yes. Fuel consumption, maybe.

Matt

 

[snip]

And the fact that while being able to flow more mixture is great for
high rpms, it's a disadvantage at low rpms. Lower intake velocities
and all that.

Of course, if you are Honda, you design the VFR to switch between
4- and 2-valves. The best of both worlds! Well, except for more
complexity. And higher maintenance costs. And no real performance
gains. Ah well.

-- Michael

 

This is anecdotal evidence only, but a few years ago I took a ride on
my Honda Nighthawk with a friend on his Harley. We tanked together,
rode together, and the next time we tanked, we had used exactly the
same amount of gas, to the tenth of a litre.

I outweigh him by maybe 40 lbs, but the Nighthawk is a way lighter bike
than the Harley.

Paul Harris

 

I'm not sure what your point is here, but residual stresses can be a good
thing. Tempered glass, for instance. The glass is quenched [lemme think,
hope I get this right ] so that there is a residual compression in the
skin and a tension in the interior. Because the glass has a higher
compressive than tensile strength, any bending moments in the glass
initially serve to relieve stress in the tension side and add compression to
the other side, which doesn't care as much.When tempered glass fails, things
get so out of balance that the pane explodes into a bunch of little bits.

Cold rolled steel has a higher yield than hot rolled. I'm sure there is a
bunch of reasons, one I think of is that cold working distorts the grain
boundaries which slip by each other when the material yields. It also leaves
residual compressive stresses in the skin, just like the glass tempering,
which anyone who has put a piece in a mill and tried to dress one side
knows. It comes out looking like a potato chip

For cyclic loading, I believe residual compressive stresses in the skin help
to reduce the effects of fatigue. I think this is why they shot peen things,
like con rods, to postpone the effects of fatigue.

Cyclic loading is an interesting subject which I don't know that much about.
Apparently some materials have a fatigue limit, a stress below which they do
n't fatigue. Some don't, like aluminum. Making a part which is subject to
vibration, lets say like a outboard motor mount for the boat, should be
grossly over built, as Al has no fatigue limit . It will fail eventually if
subjected to cyclic loading.

Are you familiar with this one....preloading a bolt not enough could lead to
eventual failure? Has to do with the bolt not feeling any stress below its
preload. If you don't preload it enough, it can become subject to cyclic
loading and potentially fail at some point. Counter intuitive, but the
truth!

D.

 

This is true: I forgot to mention this. A 4v head will be weaker at low
revs than a 2v head, all other variables being equal.

Heh. Perhaps a rotary sleeve valve is best. I think they were last used
in quantity on air-cooled radial aircraft engines - I think the Bristol
Hercules was one, but Kaybearjr would probably be able to give chapter &
verse.

 

<snip>

I think the velocity thing is most important in the carburettor. If low
RPM grunt is important, it should be an advantage to have a four
or five valve head as it will allow the same flow even with milder
cam timing.
OTOH, if you optimise the cam for maximum power at high RPM
it does not really matter if there are 2 or 8 valves, the engine will
be severely asthmatic at low revs.

Variable cam timing has the potential to do both rather than have
to chose.

The system with some valves opening only at high RPM must
have something to do with swirl (as discussed earlier in this
thread) helping to get good combustion and possibly influencing
the dynamics of pulses in the intake and exhaust systems.

 

Yes it was, also the Napier Sabre engine used in the Typhoon and Tempest.

SteveM

 

Ta. Yes, I forgot about the Sabre.