Estimating Cylinder Head Airflow/Internal Combustion Engine

[m899nyc]

Can someone please help me? I'm looking for an equation to estimate the actual airflow (in cubic feet per minute) through a cylinder head. Theoretically, an intake valve should be open for one quarter of a minute (one out of four strokes), but this of course varies with cam profile, etc. Also, the intake valve doesn't spend all of it's time open at full lift. If the maximum lift is .550", it spends some time at .100", .200", .350", etc. What kind of equation could reasonably estimate the airflow of a cylinder head given the cam lift, duration, and head flow numbers? Thanks!

 

[GregLocock]

The term you are looking for is volumetric efficiency. Since it has a pretty much direct relationship with the torque produced by the engine, engine builders have been keen to maximise it, or at least pay attention to it, since the year dot.

As such, valve lift opening doesn't matter, as it is already enough. That is nobody throttles their engine via valve lift, unless they mean to.

The trick is more to do with intake duct tuning, and there is no simple equation for that.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[m899nyc]

Well, what I am looking for is a way to estimate the various horsepower capacity of different cylinder heads.

We know equations for how much fuel is required for a given power output, and how much air is required to burn that fuel.

This calculator attempts to estimate horsepower from intake flow:

http://www.wallaceracing.com/calcafhp.php

, but many have argued that it overestimates the amount of time the valve is open.

Is there no way to determine how long it takes for the camshaft to open the valve and, given flow number, how much air should theoretically flow through it?

 

[GregLocock]

Thinking about it I may have overstated the case, since undoubtedly people do fit larger valves, and alter the valve timing in the hope/expectation of more power.

The curves you are looking for do exist, but then you have to ask how much relevance a steady state flow rate has to the dynamic situation in the engine.

I'd start with an engine design book, say Heywood who has a decent section on it, and SAE papers.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[kingnero]

the "Modern engine tuning" book by A. Grahem Bell also has some technical background info, maybe/probably you can get what you need out of that book.

 

[SomptingGuy]

If you want to estimate airflow through a running engine, you'll need something more sophisticated than steady-state estimates. There are many tools out there dedicated to predicting this: some free; some not; some trivial; some complicated.

- Steve

 

[Tmoose]

I was thinking the BOSCH Automotive handbook had/has some nominal mean gas velocity for peak volumetric efficiency, and a similar number appears in old NACA papers, but I can not find it now.

I remember when 4 valve motor cycles were becoming popular in drag racing. An oft heard tale was The max power 2 valve cam timings were "too much" for the 4 valve engines.

 

[BrianPetersen]

Maybe it will help if you (original poster) explain WHY you are attempting to do what you are asking. What is it that you are trying to accomplish?

The displacement of the engine x RPM x volumetric efficiency x a units correction factor x a fudge factor = the horsepower. The cam timing and the manifold arrangement and a whole bunch of other factors affect the volumetric efficiency, and it will be a difficult-to-quantify function of RPM.

At the minimum cross-sectional area of the intake runner (inside the head), it will usually be found that at peak power RPM, the maximum gas speed (approx halfway down the intake stroke - just crunch the numbers based on the cylinder displacement and port dimensions, it will be close enough for this purpose) will be somewhere near 0.5 mach (i.e. half the speed of sound). At higher engine RPM than whatever corresponds to this, torque will drop off (i.e. if you ask for more than mach 0.5, you're only gonna get mach 0.5). This is if the intake system and cam timing and port shape and runner lengths etc are properly designed. There are plenty, and I mean PLENTY, of ways of getting this wrong - and that's why your run-of-the-mill econobox engine doesn't match the BMEP of a Formula 1 engine. Just because you hypothetically get this one particular parameter in a correct range, doesn't mean everything else will be where it needs to be.

Cam profiles are usually selected to limit valve acceleration and jerk to within acceptable bounds and to limit the possibility of valve spring problems. What's best for gas flow might not necessarily be good for valve springs. A pushrod engine will have a different set of limitations than a DOHC direct bucket actuation system. With a flat tappet lifter, the point of contact between the cam lobe and the lifter must not ever reach the outer diameter of the lifter. This is NOT simple and there is no simple magic formula to spell it all out.

 

[shawnpeter]

check out Dynomation from Motion Software.

also, do a search for jason fletcher 5.0, he's got a spreadsheet on his website for calculating all kinds of stuff with engines.

bear in mind that VE is not constant, and a lot of cheap simulation/design software bases their horsepower/torque calculations off a VE of 100%.