Four-valve scooter engines

Seems like more scooters these days have 4-valve engines. The Cygnus does, there’s a 125 SYM that does and doubtless there is or will be soon a Kymco that does as well.

Do four valves make a significant difference in these small engines? People say the Cygnus is pretty zippy. As I understand it, having four valves gives more power in the upper rev. range so I presume that works well with scooters’ continuously variable transmission that keeps the engine revs within a fairly narrow range.

Yes, because nozzle area is proportional to cylinder bore and number of valves, no matter how small the bore.

Long version:

There aren’t really very many ways to get more power from a conventional naturally aspirated 4-stroke engine of given displacement. It can made to rev faster and the thermal and volumetric efficiencies (VE) can be increased. With current metallurgies, thermal efficiency (TE) has reached a plateau. There are also problems controlled oxides of nitrogen when combustion takes places on hotter surfaces.

The amount of air the cylinder head can flow will depend on port volume and nozzle area (the size of the valve openings). Opening the ports will improve mid and high rpm torque while reducing it at low engine speeds due to reduced velocity. With only two valves in the cylinder the maximum diameters will be limited by the cylinder bore diameter and valve included angle. Of course the bore/stroke ratio can be increased, but there’s again a trade off in low rpm torque in a very oversquare design.
A very high included angle between the valves will eventually require a DOHC system to operate the valves because a single cam between the two will need excessively long rocker arms to reach them. More weight, inertia and the rev ceiling is reduced. Nozzle area is thus quite limited with two valves. Also, as the valves get larger, heavier and more difficult to control. That either calling for a reduction in operating speed (counterproductive) or a beefier valvetrain (less efficient). If the valve diameter is limited, you have to open it longer and/or higher. But, overlap is limited due to emissions concerns (don’t want half-burned hydrocarbons in the exhaust gas). There are also limits on how early the intake valve may open due to flow reversion (exhaust phase not finished yet) and how late the exhaust valve may close (loss of dynamic compression ratio, reverse flow). Other side effects of a hot cam like this be poor idle quality, and poor torque at low to midrange speeds.
Another problem with a big valve, high included angle design is the shape of the combustion chamber. While a hemispherical design offers reduced surface area for it’s volume, the piston crown will have to be aggressively domed to acheive a useful compression ratio. That’s heavy and that again limits operating speed.
Opening the valves higher instead of longer is another solution, but there are mechanical limits to how high the valve may lift in relation to duration. Of course modern two-valve motors utilize roller cam followers to allow higher valve acceleration, but there are still quite finite limits.

Options:

Increase the number of cylinders. Small cylinders have a better ratio of nozzle area to piston area, have smaller lighter components which can spin faster. VE is improved a little and the engine makes similar torque at higher rpm which = more power.

Increase the VE by optimising the combustion chamber shape and raising the compression ratio. Works, but limits nozzle area and thus VE and operating speed.

Use desmodromic valve operation which relies on mechanical levers to control the valves rather than springs. This allows much higher valve lift for given duration and also decreases parasitic losses in the valve train, improving midrange torque. Try and catch a Ducati on a mountain road and you’ll know what I mean.

Use a valve control system to either vary the valve lift, duration or timing, or a combination of these. VTEC, VVTI etc. Allows the use of more aggressive valve motion at high revs while mitigating some of the effects at low and midrange speeds.

Use mulitple valves (up to nine valves per cylinder has been tried) to increase nozzle area and reduce the weight of individual components. Allows mild timing and lift to be used while improving flow over that of a single valve and also higher operating speeds.

Haha! Hsiadogah comes up trumps. I wouldn’t really expect any less from a man who can say;

[quote=“hsiadogah”]IIRC,
VTEC1 activates at 6,700rpm.
VTEC2 activates at 6,300rpm.
VTEC3 activates at 6,300rpm in 1st gear through 5th, and 6,700rpm in 6th gear. This model also has the LED tail lights.[/quote](bold added for emphasis)

So desmodronic or VTEC oojamaflippery is presumably unecessary in a scooter because the continuously variable transmission keeps the revs within their optimal, narrow band.

great post/reply Hsiadogah!.. thanks… :notworthy: :bravo:

[quote=“hsiadogah”]
Use desmodromic valve operation which relies on mechanical levers to control the valves rather than springs. This allows much higher valve lift for given duration and also decreases parasitic losses in the valve train, improving midrange torque. Try and catch a Ducati on a mountain road and you’ll know what I mean.[/quote]

:bravo: for sure… go and explain to your local grease monkey that you’d like him to put down the bin-lang for sec and install some desmo valves in your Joy 90cc… :smiley: :smiley: (BTW: Ducati’s are easy to catch on mountain roads… all you need is a 1200cc flat twin, 2 x 600cc cylinders, 2 sparks and 4 valves (intake/outlet dia. 36/31mm) per cylinder = 115Nm of torque at 5000rpm… bye bye Ducati… doesn’t hurt that around here most Ducati pilots are eedjits either :wink: )

Another question for Hsiadogah or Plasmatron; do you think this new small 4-valve engines are any less reliable? Are they just the old engines with another couple of valves stuck on?

I guess they will run hotter but that should be alright with decent oil, right?

[quote=“joesax”]
So desmodronic or VTEC oojamaflippery is presumably unecessary in a scooter because the continuously variable transmission keeps the revs within their optimal, narrow band.[/quote]:oops:

Well, desmo operation is a bit redundant with a multiple valve setup because the valves are so small and light that a spring can do the job quite effectively. If you have a larger cylinder bore, it’s easier to decrease valve size again by adding another (for say 5 total) instead of doing a desmo system.

VTEC Oojamaflippery (that’s probably a Honda trademark :wink:) helps when power output demands start heading into the <100hp/liter range because the rev range is going to extended further, port size has to increase along with it, and then midrange torque starts to suffer as a result of decreasing air velocity in the intake tract*. There will also be the factor of weight, complexity and expense. It might be cheaper and simpler to build a 125cc twin with a conventional head than a single with a VTEC type system, and it might be more compact as well. Remember that the variable valve control schemes implemented on SOHC engines this far have had quite poor results, since it’s difficult to control the intake and exhaust timing separately…

You’re right to point out that a scooters CV transmission can avoid the need for oojamaflippery by never asking the engine to run in the low rpm range. Eventually you might need to fill the torque curve by such means, but probably not until the public is demanding 125 scooters that make 30ps and go 170kmh. They’ll probably all be riding fuelcell powered devices by then, and we’ll be discussing the various merits of pole-wound motors, four quadrant drives and flux capacitors. :stuck_out_tongue:

*I forgot. There is another way to fill out the torque curve in the midrange without sacrificing high rpm VE, a variable length intake manifold.

All this really points out two major problems. One is the stubborn clinging to cylinder capacity that government uses as the yardstick by which it levies taxes and issues licenses. The other is the public insistence of using specific output (hp/liter) to judge an engine. If more representative measures were used, designers would be under less pressure to build small capacity engines which are complex and short-lived, and make their power at rpm levels so high they make hummingbirds dizzy.
Tax should be based on air consumption. Performance should be judged on brake specific fuel consumption or hp/unit volume. Size (displacement) shouldn’t matter. :wink:

[quote=“joesax”]Another question for Hsiadogah or Plasmatron; do you think this new small 4-valve engines are any less reliable? Are they just the old engines with another couple of valves stuck on?

I guess they will run hotter but that should be alright with decent oil, right?[/quote]No, all things being equal there shouldn’t be any loss of reliability. One hand you have a few more components that might fail. On the other, they are smaller and lighter and thus have less inertia. That makes them easier to move around and it takes less energy to do, which improves the reliability of the cam drive system (chain and tensioners).
Obviously the more air an engine burns the more heat it’s going to make and the more the engine needs to reject as waste. It’s easy to increase the cooling surface area to compensate for that.
I would be more worried that they screwed up the design of the oil supply to the cam bearings or something mundane like that. It happens all the time, regardless of the number of valves.

I’m not sure about the new 4v motor, but I know that the Duke/Dink 125/150 engine was designed by a German company. SYM’s R&D department sticks to what it’s good at, annual graphics updates.

Fascinating stuff. Thankyou.

[quote=“hsiadogah”]You’re right to point out that a scooters CV transmission can avoid the need for oojamaflippery by never asking the engine to run in the low rpm range. Eventually you might need to fill the torque curve by such means, but probably not until the public is demanding 125 scooters that make 30ps and go 170kmh.[/quote]Some of the European scooters modified for drag racing can do that. I’m sure you’re right that the power/transmission characteristics once modified render them no good for regular public use such as getting the groceries.

[quote=“hsiadogah”]They’ll probably all be riding fuelcell powered devices by then, and we’ll be discussing the various merits of pole-wound motors, four quadrant drives and flux capacitors. :p[/quote]Please don’t wait until then to start the discussion! I have to confess that I’m not a die-hard petrol head. I’m all for reducing emissions etc. I was taken several times to the Centre for Alternative Technology at Machynlleth, Wales, when I was a kid and I think all that stuff is great.

We might have to change the forum title though. What sound do electric vehicles make? Not much.