Valve lift in a high speed race engine is defined by kinematics and not simply flow requirements. The flow coefficient of a poppet valve peaks approximately when the valve lift is about 25 or 30 percent of the valve diameter. So purely in terms of flow gains, there is no benefit to using more lift than that.
But most racing engines use much more lift than flow conditions require, for kinematic reasons. Using greater valve lift permits a cam profile that will achieve that max flow condition for a greater period of time with a fixed duration. Or in other words, you get to the condition of max flow quicker after the point of intake valve opening, if you employ more valve lift.
Consider this very simplified example. Say your valve needs 8mm of lift to achieve its maximum flow coefficient. If you use a cam that gives 8mm of lift, the first 4mm of valve lift are used to accelerate the valve away from its seat, and the last 4mm of valve lift are used to decelerate the valve to its point of max lift. If this cam has 300deg total duration and a simple harmonic motion, this means you would achieve a max flow condition 150deg after the valve open event.
But say you used 10mm of lift for that same valve with the same 300deg duration. The 10mm cam would achieve the max flow condition (ie. 8mm of lift) after only about 120deg after the point of valve opening, which is much quicker. Understand the difference?
As for your specific question about the actual lift dimension, I can't answer that.
Regards,
Terry
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
Can we scale up the valve lift in a high rpm motorcylce to get an estimate to that in F1?
I don't remember much about dimensional analysis... but since you said that the valve diameter determines the valve flow and considering the valves take up close to all of the space in the roof of the combustion chamber in a race engine can the ratio between the cylinder bore diameter of the motorcycle engine and F1 engine be used a factor to multiply the motorcycle valve lift?
A typical 1 liter bike's valve lift can be 9mm to 10mm of lift and around 73mm bore... so is it safe to assume that F1 has approximately 98mm/73mm x 9mm (or 10) = 12mm to 13.5mm valve lift?? sounds reasonable?
Thanks Terry, I knew that but its always good to clarify.
I took my figure from a model I made in an engine simulator, just for fun. A 2.4L F1 V8 has 98mm of bore and 39.7mm of stroke. The model lacks optimization but gave me arround 720HP at 18kRPM with valve lifts arround 18mm.
I dont remember the exactly valve diameters I used, I never searched for them and the whole model is full of estimations (its fun to estimate ) and I dont have the eng sim at hand right now. I think it was arround 40mm for adm and 38mm exhaust.
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Using pneumatic springs you still have limits on profile aggressiveness because the inertia of the moving parts is generating very high hertzian pressures that are difficult to face even with advanced surface treatment. Max lift should be around 17 for intake and 14 for exhaust.
This is a nice article, only it's NASCAR. The author used 40% of the inner valve seat diameter. So their 53mm valve with 48mm inner valve seat diameter has 19mm of lift. Keep in mind their bore and stroke is some 106mm x 83mm. It might not relate to F1 the tee but it's an Interesting paper.
40mm intakes and 38mm exhausts might be a tad too large for a 98mm bore, once you allow margins between seats and around the plug (even an 8mm or 10mm racing plug). Regardless, you're numbers are likely pretty close.
As for comparing valve lifts on a production road bike engine to an F1 engine, it's probably not a good comparison. The F1 engine uses pneumatic valve springs and does not have the fatigue life requirements that the bike engine has. With the cam profile used by the F1 engine, the bike engine's valve springs would quickly fail in fatigue. It would also be very difficult to design a coil spring that had the right balance of spring rate, seat pressure, and force at max lift without over stressing the spring wire or even producing coil bind. Most metal valve springs do not like engine speeds much above 13,000 rpm, even race engines, due to the severe vibratory and inertia issues that arise within the spring body above those driving frequencies. If you remember, that was the defacto engine rev limit in F1 during the early '90's, until the pneumatic systems were widely adopted.
Another difficult issue with a severely over square (ie. huge bore diameter and short stroke length), high compression engine like a modern F1 unit, is making sure that the valves do not hit the piston crown. This issue is compounded by the use of high valve lifts, lots of duration and overlap in the cams, and the very shallow, wide, narrow valve angle pent-roof combustion chamber shapes that result from high compression ratios in the very over-square cylinders.
n smikle, thanks for linking that paper from Gordon Blair. I used to work with one of the authors, Hans Herman, for a short while about 20 years ago. Small world, eh?
Regards,
Terry
"Q: How do you make a small fortune in racing?
A: Start with a large one!"
riff_raff wrote:
Another difficult issue with a severely over square (ie. huge bore diameter and short stroke length), high compression engine like a modern F1 unit, is making sure that the valves do not hit the piston crown. This issue is compounded by the use of high valve lifts, lots of duration and overlap in the cams, and the very shallow, wide, narrow valve angle pent-roof combustion chamber shapes that result from high compression ratios in the very over-square cylinders.
Ahaaaammmm... I was thinking about a not-that-narrow valve angle but I was wrong. Surely thats why the 40-38mm is too much. Combustion chamber shape may be prohibitive for valve inclination.
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio
"I have no idols. I admire work, dedication and competence." -Ayrton Senna
Ferrari Tipo 049 ran with 15.5/14.1 mm lift using 40.4/33.0 mm valves, TWR Hart 1030 ran 15 mm lift with peak lift at 117 deg ATDC for intake and 125 deg BTDC for exhaust using 38/32 mm valves. TWR 600 ran 17/14 mm lift with 42/33 mm valves and a 94 mm bore.
Since the 2.4l V8 engines use a slightly larger bore it's reasonable that their valves sizes are somewhat larger, and because of the the valve lifts are probably also a bit higher. But on the other hand, modern engines must survive several races.
Hertzian contact stress doesn't limit the valve lift directly, but put a limit on valve acceleration.
Supertouringcar engines have run with over 20 mm lift using smaller valve sizes than the ones mentioned above, so if high lifts are possible, they are certainly an advantage. In practical applications the maximum flow coefficient are seen at lifts higher than 0.25-0.30 inner valve seat diameters even if the valve curtain area suggests that this should not be the case.
) and I dont have the eng sim at hand right now. I think it was arround 40mm for adm and 38mm exhaust.
crazy.