valvetrain systems and engines?

[Reca]

SOHC = Single Over Head Camshaft, on the same camshaft you have the cams for the intake valve(s) and the cams for the exhaust valve(s), obviously intake and exhaust valves move following different laws so you need different cams for them. Although SOHC is usually adopted with 2-valve per cylinder, some engines with 3 or 4 valves per cylinders still have a SOHC and the valves are operated via rockers.

DOHC = Double Over Head Camshaft, you have a camshaft for the cams of the intake valves and a camshaft for cams of the exhaust valves. Typical solution for engines with 4 or 5 valves per cylinder and, as tempest said, it’s mandatory to modify intake and exhaust valve timing separately.

Flat is an engine with pistons all in the same plane and with the two pistons of a pair (one from the left bank and the other one from the right bank) sharing the same crank pin (hence the cranks of the same pair are at 0°). When a piston of the pair is at TDC the other one is at BDC and viceversa.

The opposed (boxer) is an engine with again all the pistons in the same plane but with one crank pin per piston (hence the two cranks of the same pair are at 180°). The result is that both pistons of each pair are at TDC contemporarily and obviously both are at BDC contemporarily.

There are also other ways of activating the valves: pneumatic (but don't know any road car that has it already)

pneumatic spring is just a substitute to the metal spring, to open the valve you still need the cam so conceptually it’s exactly the same as a standard valvetrain. Pneumatic springs are totally unnecessary in the low revving engine for road cars, isn’t a cheap system and would probably introduce problems related with the pressure in the system, a further reason for the mechanics to put hands in your engine and drain money from your wallet...
Another possibility to operate the valve is the desmodromic, completely mechanical but without springs, it eliminates the valve bounce problem and allows an extremely precise control of the valve movement and also friction reduction at low rpm. There are several different ways to do it. Famous examples of engines with desmo are the Mercedes W196 or all the Ducati bikes (MotoGP bike, Desmosedici, included). Ferrari was experimenting a desmo valvetrain for the F1 V12 in late ’80s-early ‘90s, before the introduction of pneumatic springs by Renault, the prototype should be visible at Galleria Ferrari.
I’ve seen in another thread (that I fail to find now) that someone suggested to outlaw the pneumatic springs in the attempt to limit revs. Well, desmo is just one of the possibilities they would have to overcome the problem.

[DaveKillens]

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[uzael]

What I'm waiting for is the one engine per season rule. I think that will be a great showcase of advanced technology. An engine that can generate ~800BHP for a lifetime of 20,000KM will be a real engineering achievement. There would have to be allowances made for replaceings valves, rings and a few other small pieces. But the pistons, block and crankshaft would have to l;ast for the whole season. No engine changes allowed. If it blows, you're out.

[DaveKillens]

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[Reca]

So basically, DOHC is better because it allows increased RPM, lower valve spring strength, and theoretically, better gas flow (2 valves vs one). The venerable and classic Rolls Royce Merlin engine of WW2 fame had a SOHC 4 valve V-12.

Actually the advantages you are listing don’t come from the DOHC, they come from the increased number of valves, and as your write just the following row, also with SOHC you can activate 4 valves per cylinder, hence having reduced valve spring rate and allowing theoretically better airflow due to more valve area.
On production cars the advantage of DOHC vs SOHC is mainly the lack of finger followers hence a bit more simple packaging (and the addition of a huge D O H C on a, possibly coloured, cam cover and on the rear end of the car). F1 engines, even if they are DOHC, do use finger followers anyway because given the high valve lift, you would need huge and heavy tappet follower and cam lobes. So in F1 the real advantage of DOHC vs SOHC is the possibility to have a double inclination of the valves improving the shape of the combustion chamber, fundamental given the particularly large bore/stroke ratio of those engines.

As for the main advantage of OHC vs pushrod, it's exactly the lack of the pushrods, but not just because of the reduced mass, but mainly because a long rod working in compression is never a good idea when you need a very precise control of the movement.

With all due respect tempest, it is possible to construct a SOHC engine that can perform variable valve timing, intake and exhaust independant of each other. You just build in two rockers/cam followers for each valve, and use mechanical or hydraulic pressures to switch between each follower. One follower is riding on a conservative cam design, while the other follower is riding on a more aggressive design. If memory serves me correct, Honda did it.

What you describe here is indeed the Honda SOHC VTEC. But exactly because of the packaging limitations it’s applied only on the intake (2 stages) while the DOHC VTEC can be applied to both intake (3 stages) and exhaust (2 stages).
And that without starting a discussion about VTEC being a real vvt or not. Tempest for example was probably referring to the vvt actuated via the rotation of the camshaft relative to the crankshaft. Clearly that isn’t applicable to a SOHC.

As for the Corvette engine, the guys at GM could have manufactured a world class modern engine for that automobile. But marketing and economics played a major role in the decision to go with the OHV V-8. It would have cost a lot of money to retool for a new engine and chassis, when they already had the tooling and jigs for the V-8. Additionally, the US consumers love tradition, and are loath to adopt a new concept. The traditional Corvette owner enjoys the fact that 'Vettes have fiberglass bodies, independent suspension, and a kick-ass huge V-8. Change that formula, and you may alienate a very large, loyal customer base.

Indeed. Last week I’ve read an excellent article about the recent Detroit exhibition, and the author pretty well described the inconsistency of US cars manufacturers, here an extract :
“[...] words are ‘advanced commuting’, ‘zero emission’, ‘eco-compatibility’ etc etc, facts are 7+ litres engines, 2 tons cars with 500 or 600 hp etc etc. Each year they brag about new low pollution cars coming soon and launch very clean and advanced prototypes, but we never see them on the road and the next year they are substituted by even more clean concept cars that manufacturers swear will soon be on the road [...]
Maybe for us [Europeans] is difficult to understand why instead of bragging about some marvellous projects far in the future they don’t start making cars a bit smaller, truck a bit lighter and so on. But maybe the answer is a lot more simple and it’s inside everyone. Any car lover knows how exciting is the sound of the V8 and that a Corvette Z06 or of an old Cobra are in everyone’s dreams; that’s something US car makers know better than anyone else, just, to officially admit it saying “as long as we can that’s the way we like it” isn’t politically correct.

What I'm waiting for is the one engine per season rule.

What I’m praying for is Max not reading this.

[malbeare]

DaveKillens
There is another valvetrain concept that reduces the accellerations in the valvtrain to allow higher RPM The beare head or piston valve

The piston speed of the upper piston is lower than the main piston so the wear rate is lower.
Mechanically the engine is silent as there is no tappet noise. The scotch yoke is a silent sliding action with no sudden changes in direction.

The Beare head offers an array of advantages, but it specifically offers a compact combustion chamber with a 50 per cent squish. Thus, the combustion in the center of the piston is concentrated, increasing the flame speed and the speed of combustion. In doing so the thermal stress on the piston is actually reduced.
2. An added benefit of this configuration is that it allows a higher bore stroke ratio, due to a lesser expansion of the piston. As there are no cut outs for valves, the crown of the piston can be slightly domed for higher strength and less weight. The 50 per cent squish factor keeps the edges of the piston from being exposed to the flame. By doing so, it allows the use of a gapless L shaped compression ring to be implemented right to the top of the piston. Therefore ring flutter is reduced or even eliminated.

(1) The 6-stroke engine is fundamentally superior to the 4- stroke because the head is a net contributor to, and an integral part of the power generation within the engine, unlike a cam only absorbing power.
(2) The 6stroke is thermodynamically more efficient because the change in volume of the power stroke is greater than the intake, compression, & exhaust strokes.
(3) The compression ratio can be increased because of the absence of hot spots in the combustion chamber.
(4) The rate of change in volume during the critical combustion period is less than in a 4stroke.
(5) The absence of valves within the combustion chamber allows design freedom.
(6) A one-piece engine from crankshaft to upper shaft becomes feasible. No head gasket.
(7) Fewer components, 15 per cylinder compared to 40 for a 4-stroke. Therefore the cost of manufacture is much less.
(8)Can be fitted to standard engine blocks so the market is much larger than the OEM sector, also includes the retrofit aftermarket sector.
The engine has proven to be robust on the racetrack, & have significant advantages over 4-strokes
(1) The valving is desmodromic
(2) There are no valves to drop or bounce.
(3) The rev limit is only what the bottom end can stand.
(4) Gas flow on intake increase of 20%.
(5) No possibility of engine damage if the timing belt slips or snaps
(6) the reed valves are so close to the intake ports that their tips become the virtual port opening. This achieves variable port area & variable engine demand valve timing. The tips open late & small amounts with low throttle settings & open early & fully at full throttle
(7) air assisted fuel injection has unsurpassed (5 micron with 20% air premix) fuel mixture preparation directly into the cylinder without the inhibiting poppett valve in the way, just a lovely big port. And the injector is protected from combustion.
Malbeare

[riff_raff]

BMW has their infinitely variable valve timing/lift system (Valvetronic) on all of their 5-series I6 engines. It is only on the intake valves, but it completely eliminates the need for an intake throttle butterfly. It uses the variable intake valve timing to control engine speed/load. BMW claims it improves fuel economy by up to 12%.

http://www.autospeed.com/cms/article.html?&A=1083

It's not suitable for an 18,000 rpm F1 engine though!

[DaveKillens]

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[malbeare]

riff_raff.
This concept also reduces fuel consumption and is suitable for 18,000 RPM

FUEL CONSUMPTION TEST
ROAD SPEED MPH 4STROKE RUN TIME SECONDS 100cc FUEL 6STROKE RUN TIME SECONDS100cc FUEL LOADED RPMIn 5th GEAR % LONGER RUN TIME
30 159 216 2000 35.8%
35 138 184 2500 33%
40 107 134 3000 25.2%
45 89 101 3500 13%
YAMAHA TT 500cc
Test by Malcolm Beare, Elliot Munro, Grant Guy, July 1995
The dyno used was an old motorbike dyno with the rear wheel driving a large fan with a speed readout dial. The throttle was opend enough to maintain the designated speed. So the power outputs were identicle
The sixstroke head was designed to as closely match the fourstroke as possible compression ratio , valve timing , port sizes. Not a fully optimised sixstroke much more port area is available.
and compression ratio could be higher.
The sixstroke would run happily at lower revs(1000) than the fourstroke in 5th gear. The fourstroke would pull 4000 RPM at full throttle the sixstroke 3500.
Same gearing same carburetor.
Fuel was gravity fed to the carb from a long clear tube with two level marks to indicate 100cc
To help keep a cap on power and, hence, speed, the MSMA has decided to propose a reduction in engine capacity from 990cc to 900cc. "The intention is not to reduce performance but to prevent a continuous improvement in speed and lap times," according to the press release.

2004 2007 weight changes
2 Cylinders 135 Kg 133Kg - -2Kg
3 Cylinders 135Kg 140.5 Kg +5.5Kg
4 cylinders 145 Kg 148Kg + 3Kg
5 cylinders 145Kg 155.5 Kg +10.5 Kg
6 cylinders 155Kg 163 Kg +8Kg


The proposed changes to the rules also affect the minimum weight standards, adding more weight to engines with more than two cylinders from 2007.


The proposed changes above may indicate the technical direction that some manufacturers are pursuing for the future. As Honda is the most powerful voice among the companies, it is interesting that the proposed minimum weight for five-cylinder machines, such as the Honda RC211V (and Proton KRV5), has been increased the greatest amount. This may indicate that Big Red is already working on new engine configurations and is looking to abandon the V-5.

And, as two-cylinder bikes are the only ones to get a minimum weight decrease, might we see the introduction of a 900cc MotoGP V-Twin? If so, it wouldn't be as powerful, no doubt, but it would enjoy nearly a 50-pound weight advantage over a V-5-powered machine. And, as a Twin would have a 66-pound advantage over a six-cylinder-powered bike, it looks like the rumors of a Honda V-6 will not be fulfilled.

The MSMA is also looking at perhaps reducing the 2005 rule for a 22-liter fuel tank capacity (down 2 liters from current rules) for the 2007 season.

The introduction of 4-stroke machines to MotoGP has resulted in a huge amount of newfound interest in the class. Now, with revised regulations again on the table, the series might get even more interesting.


The Testastretta engine fitted to the Ducati 998R 2002 version, the bore is 104 mm.
Unfortunately, such a large bore currently causes combustion problems with dramatically decreased efficiency.
This stems fundamentally from the need to augment the injection advance and from the worsening of the "shape factor" of the combustion chamber which, with the reduction of the bore/stroke ratio, becomes ever broader and flatter. The "shape factor" is a critical synthetic value to check a combustion chamber's good operation, and a good indicator of its compactness and "thermal efficiency".
It should be borne in mind that aspirated racing engines require rather extreme valve lift and overlap angles, therefore, cavities are made in the piston crowns to prevent contact with the half-open valves. The combustion chamber is therefore practically contained in the piston cavities, such cavities becoming bigger as the stroke/bore ratio decreases, which makes it hard to obtain the high compression ratios required by high specific power engines.

The Beare sixstroke does not have these limitations because the main lower piston does not have valve cutouts and the combustion chamber is a compact design with squish contribution from both upper and lower pistons. The shape is much more like a fist than a flat hand hence thermal efficiency is high .
Combustion chamber diameter oprox 75mm
The main piston is lighter and stronger than the 4-stroke, because the lack of cutouts allow a thinner slightly domed top
Malcolm does believe that the sixstroke 15kg weight advantage will be a major benefit for the Beare Sixstroke, much more so than the 30kg handicap enjoyed by Twins in 500cc twostroke racing. "Working on the assumption that all these four-strokes are going to make enough horsepower, 15 kilos is a lot," he says. It’s straightforward enough, the Twins will have a 10 percent weight advantage and force equals mass times acceleration, so it is a big difference.


Sixstroke Beare 900cc Vtwin MOTO GP

Bore 116.25 mm stroke 42.5 upper bore 82mm upper stroke 34mm
compression ratio 12.25 to 1
power 337HP @ 15000 RPM
torque 74.6Ft/Lbs x80% x2 = 118Ft /Lbs
piston speed at 18000 is 5019 Ft/min or 25.4965 Mtre / sec
XL engine file
Torque 101.2 NM or 74.6 Ft /Lbs discount by 20% and multiply by 2 for twin cylinder is 118 FT/ Lbs
6 port design with 3 exhaust ports leading to a rotary disk, 3 intake ports,One intake rotary disk and 2 reed valves with air assisted injectors. 2 or 4 10mm plugs per cylinder.
The port area is oprox 20% to 30% more than a 4 valve head
Results of XL file sixstroke touque calculator

Based on Dual Cycle
Total Torque
Fourstroke 62.00

Main Top
66.05 35.15 101.20

Increase in torque 63.23%




Malbeare
Things are a little more complicated than you would expect
During the intake stroke the main piston is increasing the cylinder volume while the upper piston is decreasing the volume (half of its stroke) so the net change in volume is +722cc . During the compression stroke the upper piston is still reducing volume (half its stroke) while the main piston is also reducing volume, net change -1082cc.
During the expansion stroke the upper piston is increasing the volume (half of its stroke) while the main piston is increasing volume, also net change +1082cc.
During the exhaust stroke the upper piston is increasing volume (half of its stroke) while the main piston is decreasing volume, net change -722cc
if you add all the strokes together and average them you get 900cc
The story changes again when you phase the coordination of upper and main pistons but the average remains constant.
The combustion chamber is only aprox 75 mm in diameter maybe 2 plugs will be OK
malbeare

[Reca]

One design I have always had a sweet spot for was the sleeve valve engine

Then you could be interested in knowing that between 1989 and 1993 Ferrari founded a study of the CRF (Fiat Research Centre) on the sleeve valves. They experimented on a single cylinder with a “mixed” design, sleeve at the intake and poppet exhaust valves (that solution was preferred because of the high bore/stroke ratio, hence limited area for the openings on the sides). Some results of the experimentations were encouraging, some not very much (especially on the mechanical efficiency side), anyway due to a financial crisis of 1992-93, the research was abandoned.

[riff_raff]

Variable valve timing/lift is possible with either DOHC/SOHC engines or with the classic pushrod OHV configuration. BMW has it's Valvetronic DOHC design in production. I also know that a VVT system is possible for the classic pushrod OHV V8 engine (ie. the Corvette) because I have one that works! I have a purely mechanical VVT system that fits directly onto a Chevy small block V8. It varies both valve lift and duration, from zero to infinity. It also can vary inlet and exhaust separate from each other. So I know it's possible.

As for a VVT system that is suitable for an 18,000 rpm F1 engine, FORGET IT!

[Reca]

Variable valve timing/lift is possible with either DOHC/SOHC engines or with the classic pushrod OHV configuration. [...]
I also know that a VVT system is possible for the classic pushrod OHV V8 engine (ie. the Corvette) because I have one that works! I have a purely mechanical VVT system that fits directly onto a Chevy small block V8. It varies both valve lift and duration, from zero to infinity. It also can vary inlet and exhaust separate from each other. So I know it's possible.

If you want to control separately the intake valve timing and the exhaust valve timing continuously changing the camshaft phase (= changing the relative angle between camshaft and crankshaft), hence what almost all VVT systems do, you need the intake cams to be on a camshaft and the exhaust cams on another one. SOHC doesn’t allow it, DOHC does. VTEC is a totally different thing because it uses directly different cams (different duration and lift) for the different 2 or 3 stages (non continuous) so you can also have all the cams on the same shaft and switch between them, it doesn’t involve the modification of the camshaft phase.
Obviously the same limitation as above applies to OHV, I know that GM has a VVL for V6 and V8 OHV engines but also that system needs separate camshafts for intake valves and exhaust valves, the 2 camshafts are between the cylinders, one above the other.

BMW has their infinitely variable valve timing/lift system (Valvetronic) on all of their 5-series I6 engines. It is only on the intake valves, but it completely eliminates the need for an intake throttle butterfly. It uses the variable intake valve timing to control engine speed/load.

Valvetronic isn’t a VVT, Valvetronic only controls the maximum lift of the intake valve, as you said it allows to eliminate the throttle butterfly (at idle the maximum valve lift is less than 1 mm).
Then, to modify the valve timing there’s a further system, the BMW VANOS (Double VANOS while mounted on both intake and exhaust valves), that again works changing the camshafts phase relative to the crankshaft, VANOS isn’t surely a new thing for BMW, also Valvetronic is relatively old, it was introduced in 2001 IIRC.
BTW, Double VANOS, in a slightly modified version is used also on the M engines, Valvetronic isn’t. One of the reasons is that Valvetronic at the moment operates on all the intake valves contemporarily while M engines (as F1 engines) have a single throttle butterfly per cylinder. They are controlled with an electronic management that allows some further “tricks” to help the torque distribution at the different rpm ranges, Valvetronic would prevent the possibility to use them and very likely it would also reduce the throttle responsiveness.

[Guest]

Reca,

I would agree with most of what you say. The production BMW Valvetronic system controls intake lift and duration within a range of lifts from 0.3 mm (at idle) to about 10.0 mm. The camshaft phasing (lobe center timing) is controlled by the VANOS system. The control mechanism for the Valvetronic system is a stepper motor and worm sector gear. It can achieve very rapid response times (<0.3 sec) that are more than adequate for a road car throttle. Thus the intake valving is truly "infinitely variable" and no other intake throttle device is required for SI engines. As you point out though, it requires a DOHC engine configuration.

The "VVT" system that I am working with, for the ubiquitous OHV pushrod V8 engine, can vary both intake and exhaust valve lift and duration, infinitely and independently, but cannot change the"lobe center" characteristic. That variable is obviously determined by the single (block mounted) camshaft.

My pushrod VVT mechanism is not a high rpm capable device, but should be satisfactory for 6000 rpm. It's most attractive feature is that it is a purely mechanical system, and is virtually a "bolt-in" arrangement for any OHV pushrod, small block V8. Detroit loves these engines because they are cheap to produce, make lots of power and they have lots of experience manufacturing them. All other things being equal, my "bolt-in" OHV pushrod VVT system should give about a 10% reduction in fuel consumption, but will add about 15% in cost. The only production VVT system that I have seen for OHV pushrod engines is GMs "D-O-D" lifter. It de-activates four of the eight cylinder engine's valves, during part throttle operation, thus reducing FMEP/throttle losses in their SI engines.

As for VVT systems for purely race engines, race engines spend most of their time at WOT and within a fairly narrow rpm range, so the benefits of a VVT system may not be all that great.

Regards,
Terry

[Guest]

Variable valve timing/lift is possible with either DOHC/SOHC engines or with the classic pushrod OHV configuration. BMW has it's Valvetronic DOHC design in production. I also know that a VVT system is possible for the classic pushrod OHV V8 engine (ie. the Corvette) because I have one that works! I have a purely mechanical VVT system that fits directly onto a Chevy small block V8. It varies both valve lift and duration, from zero to infinity. It also can vary inlet and exhaust separate from each other. So I know it's possible.

As for a VVT system that is suitable for an 18,000 rpm F1 engine, FORGET IT!

[DaveKillens]

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