Air Inlet Sizing Question (Fluids Theory) 4

[spectreeng]

Ignoring pulsation effects and assuming a steady inviscid incompressible flow, old Bernoulli says pressure loss is required to accelerate still air (outside the air inlet) to whatever inlet pipe flow velocity is needed for the engine at a specific RPM. Now I read volumetric efficiency being the ratio: P(cylinder)/P(manifold). So even with a fixed VE, increasing manifold pressure would be beneficial.

It seems like P(cylinder) will be closest to P(atm) when the inlet pipe is very large so the flow velocity is very small. And if using a small inlet pipe the flow velocity required is very large necessitating a larger pressure drop just to accelerate the air (still ignoring viscous effects).

In other words keeping large bores for the air paths would keep velocities down, pressures closer to atmosphere and increase manifold and cylinder density. Is this theory applicable at all? Or does it apply, but its effect are completely dominated by pulse tuning (which necessitates high flow velocities)? I'm trying to understand the theory - no this is not a homework question.

- Confused engineer

 

[Fabrico]

Not an expert in that area but pulsed tuning does not have involve high velocity air flow. 2-stroke expansion chambers are an example of pulse tuning with only moderate air velocity. A healthy pulse can be sent through air that is otherwise not moving at all.

 

[Flamefront]

It is a question of volumetric efficiency (VE). Reciprocating engines are surprising horrible and ingesting air charges. Even at Wide Open Throttle (WOT), VE at 2000 rpm is around 60%, which means that the mass of air volume ingested by a 400cc cylinder not close to 400cc. With pulse tuning you can get it to 115% or more at a particular rpm, but your question is a good one.

Imagine if you could get a VE of 100% at 2000 rpm at WOT. You could make tremendous torque, which is exactly what happens with a supercharged engine (or turbo - don't get started) with good low speed boost conditions. Once you put ~4 psi of boost into an engine you can bring the VE to 100% at all engine speeds with a Roots blower, for example, and make good bmep. For example, a 1.6L four cylinder with a normally aspirated (n/a) VE of 88% at peak but only 60% at 1000 rpm (at WOT) makes ~30 ft lbs of torque at WOT at that 1000 rpm due to the poor VE. When you run that n/a engine to its peak VE point near to 6000 rpm (VE = 88%, you make 110 ft/lbs of torque.

However, when you put a positive displacement blower on that same engine and create 4 psi at 1000 rpm, you can make 110 ft lbs of torque, matching what the n/a engine does at its best VE conditions at 6000 rpm. This is not theoretical, it is from data from years of developing engines with supercharger intaked systems.

This is a round-a-bout answer to your intuitive question: If you fill the cylinder with more mass of air, more torque can me made, all else being equal. So a bigger port is a good thing, getting your intake air charge up to high speed is only a band-aid to compensate for a resictive opening in the first place. It would be better to get 100% of the cylinder's displacement volume filled in the first place.

 

[GregLocock]

Really? In Heywood I could see only one VE graph that went as low as 60%, and that was as an extreme tuning example.

So, if I were to rephrase your argument - in a poorly designed intake and valve system it is possible to compensate for the poor design by adding some sort of compressor.

Real engineers sort the VE of the intake and valve system out as a first step, if it matters that much in the context of the application.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Like Greg said.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Flamefront]

Not so much that you are compensating for a poor design intake system with a compressor, but the example was to highlight how well an engine would behave it VE was closer to 100% all the time, supporting Spectreeng's supposition that larger intake tracts lead to improved VE, in general. The rest of the discussion needs to cover the benefit of the inertial ram effect of a column of intake air moving at high velocity (in some cases, approaching Mach 1), which more than compensates (at chosen rpm) for the delta P created by "smaller" intake passages.

The 60% value for VE for an engine at WOT at 850 rpm should be pretty representative for an OHV pushrod engine, but every engine is different. It is much worse when the throttle is closed for actual idle operation. If you calculate the amount of fuel needed to create the 20 or so hp needed (produced by, say, 6 cylinders) at idle to keep the engine rotating against its fricional and pumping losses created at 850 rpm, there is only a tiny amount of air needed to match that fuel dose (even at ~15X the fuel mass). The ratio of that mass of air to the total mass of the displacement volume of all six cylinders in two revolutions gives a terribly low VE for an engine at idle with the throttle closed. Of course, this is the definition of a "throttle", trashing the VE to keep the engine from running away, rpm-wise, against its load.

In practice, for a production passenger vehicle engine, what is the desired point of peak VE (and the related best BSFC point)? Maybe someone can comment on that. In the real world, I would imagine that it is a value sometimes related to the HFET and/or FTP tests considering the emphasis on CAFE or emissions, respectively, or it might be dictated by the marketing department's desire for high HP numbers, so it is shoved to high rpm ranges (i.e. the often unusable high rpm VTEC hp resulting from mechanically manipulated VE optimization)?

 

[patprimmer]

Ummm

The point of having a throttle is to control VE and therefore power output to match requirements.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[btrueblood]

Mmm...yeah, what pat said. Get too much torque at low rpm (aka lugging?), and will you overwhelm the oil film on the output/crankshaft bearings?

Also, your quote "The rest of the discussion needs to cover the benefit of the inertial ram effect of a column of intake air moving at high velocity (in some cases, approaching Mach 1)"

Dunno of ANY piston engine whose attached vehicle ever approached/es Mach 1, at least not intentionally. Maybe M 0.6 or so, in supercharged V12 aircraft (WWII), M0.8 or so in a dive, but attempts to reach Mach 1 ususally resulted in loss of aircraft and/or pilot.

 

[spectreeng]

So anybody want to shed any light on the original topic? Do we want pressures in the port to be as high as possible by keeping flow velocity as slow as possible, by using large ports? Or is the port size more dictated by valve area and doing back calculations to arrive at port size, then manifold size, then intake size?

 

[OP800]

I think you need to look at this from another point of view.
The cylinder displacment first and what air it will draw (assuming 100% VE.flow).
Then subtract the restrictions offered by each part of the intake tract at wide open throttle and peak rpm, to meet the required operating rpm band.
For peak overall performance you should find that the smallest intake tract sizing that will flow the required amount of air will help keep the intake velocity up for good overall cylinder filling and combustion across the rpm band.
It is not a matter of just makeing it the largest/ least pressure drop intake, because there are many paremeters that should be optimized for a 4 cycle gas engine to be flexable enough to make power over a reasonable band.
An example of the above is a 5L Ford small block v8 will only pump about 500 cfm of air at 6000 rpm minus the flow losses in the intake tract making the use of a carburator or fuel injection flow rating about 80% of 500 cfm to be practical unless it is to be a race engine.
In line with this is the cam opening profile that has a great deal to do with the airflow thru the tract as the gate keeper so to speak.
Making the cam duration longer raises the power band up to the limit of the intake tract ability to flow air if it has not already been limited by it.
So the original question can't be answered well without specifying some rules and specs.

 

[Flamefront]

First off - the Mach 1 description related to the localized flow velocity of the intake charge flowing through the curtain area of an open poppet valve. Of course, at the speed of sound you create a choked flow venturi, so Mach 1 is not the goal, but the upper limit. Maybe there was a joke in there that I missed...

On the original question, I might focus and reiterate what OP800 said: An engine has to run at too many different speeds, loads, and throttle settings to optimize it one way. Steady state generator motors can come close.

The intake velocity helps the ram effect, but it is theoretically true that if you had a perfect square wave valve flow signal (0 to 100% flow to 0 instantaneously), then you would not need a ram effect to compensate...and thus the smaller ports and their related higher velocities would be somewhat moot.

 

[btrueblood]

So, in your mind, choking the flow at the intake valve and thus achieving sonic flow will better fill the cylinder because of a "ram air" effect?

The only place "ram air" has any merit to a discussion of engine efficiency is in speaking of relative air speed (velocity difference between engine air inlet and outside "still air". The velocity you create in the intake air flow is at the expense of work done by the moving piston(s). The faster the velocity, the higher the losses due to friction/turbulence, with the actual losses scaling approximately by the square of the velocity.

 

[Flamefront]

My point exactly - it is better to have large passageways and not incurr pumping losses, but that cannot be reduced to practice even with large side and periphery ported rotary engines.

This addresses the original question. Flowing 400cc of air, for example, in ~200 crank angle degrees (0.005 seconds at 6k rpm) is going to result in pumping losses, even if the intake port was equal to the bore diameter and sucking on stationary air.

Since you have the de facto velocity anyway, it can be used to ram in more air at some rpm...

 

[Marquis]

In answer to the original poster, you're pretty much on the money.
If your only criteria is WOT full load performance, then you want as big an orafice as possible in the intake spout and as an entry to the plenum.
This will offer the least pressure drop across the intake system- or the lowest manifold depression.

So-answer to the question- problem solved...not quite- it's never quite that simple in engineering!

Ok, so what are the trade offs?

Ok, first, the bigger the intake spout/orafice the more of a challenge it is for the NVH departments to keep overall noise levels down via clever design (rather than the lazy NVH engineers method of using back pressure and manifold losses/small orafice itself!). If the intake system is of a conventional design with single plenum throttle- this must be sized for driveability- throttle progression or else there will be too agressive a "tip-in"- and on some engines the electronic throttle itself is used for idle speed control- if this is the case- this will impose a further restriction on thorttle size- due to good sealing and idle speed control/response.

In terms of tuning, in my experience a typical 6 cylinder (with usual passenger engine dimensions or air box and duct lengths/diameters) you're unlikely to see many perfomance tuning effects near the air box and ducting there. Nearer the single throttle to what many call "secondaries" on split-plenum systems you'll get some possible resonance tuning effects. And of course, at the plenum and primary intake runners tuning must be one of the primary design constraints.
With a conventional firing cross plane crank V8 -post intake plenum the tuning effects on performance are negligible and most emphasis can be placed on low pressre drop. The same with a V12. This is convenient for NVH engineers who want to place quarter wave resonators and such like to get rid of nasty booms without impacting performance.
With a 4 cylinder config, a flat plane crank V8 or engines of LESSER cylinders one must consider the intake ducting and even air box size with an eye also on both tuning effects and resonace effects as their size WILL impact the SHAPE VE or torque curve somewhere along the rev range and the effect won't ONLY be a difference in pressure drop.

On engines with individual port throttles such as the BMW M3 or M5, not only can the plenum be made very big without impacting thorttle response, the intake orafice can also be enlargened to place highest priority on low intake pressure drop without adversely impacting tip-in driveability, idle speed control or any of the other constraints mentioned above (and NVH priority may also be different on such kind of animals).

 

[boatmotorgod]

desired RPM and cam timing has important influences on cylinder filling and the approach you take. At lower RPM's there is more time to fill the cylinder, so intake inertia coupled with the fact that the intake valve closes 35 or more degrees ABDC helps to fill the cylinder. At higher RPM's there is less time so we need to lessen the restriction with a larger port, though this larger port will hurt low RPM power. Of course to achieve really high VE we start with the end of the exhaust cycle and create a vacume pulse at valve overlap to get the air started down the intake.

 

[Flamefront]

I fussed with cam timing, polishing ports, and five angle valve jobs (and some with full radii) for around 20 years, then had to admit that it all can be ignored if you put a few psi in the intake plenum. Supercharging is not everyone's favorite solution (turbo or belt driven blower), but bowing to the finicky and delicate needs of a normally aspirated intake tract is hard to get excited about once you've fixed it with a pulley, a belt, and a blower. Six or more psi of boost makes everyone happier, and cam timing can be optimized for charge capture.

An Eaton engineer once told me that the perfect engine in his circle is recogized as the old Iron Duke GM four cylinder (Fiero, et al.) - a cheap pushrod unbreakable cast iron engine mated to an Eaton Roots blower pushing 10 psi of very linear boost. The cheapness of the engine offset the cost of the blower, and the net was a lighter weight, easier to package engine that made over 250 hp (with a torque curve similar to a 5.3L Vortec V-8) and great fuel economy at part throttle, and lived for 100k+ miles. I bristled at his comment the first time I heard it, with my history being in European DOHC cross flow alloy engines, but he was spot on for many points. Who needs DOHC or Vtec or Five Valve Heads, or variable plenums, or any of it if you have forced induction?

Not a new idea. When the best brains in the world were making the best piston engines for WWII fighter planes, it all was done then. Supercharging, Nitros, High octane fuel, sleeve valves,, etc. All we've really done this then is improve the materials and the control systems...(save for the Wankel).

 

[patprimmer]

A very clear, precise and well thought out answer from Marquis

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[fojo]

Flamefront -

Just a note here so that no one gets led astray - I frequent the biggest Fiero forum, and people are always asking about supercharging the Duke. The overwhelming consensus is that the bottom end won't hold up under anything more than a negligible amount of boost (3-4 lbs.)Block, rods and crank all get thumbs down. I wonder if the higher boost was just speculation on the Eaton engineer's part, or based on experience?

I definitely agree with the basic point (small engine/considerable amount of boost). GM ran a 2-liter Ecotech at Bonneville that put out what, 1000 hp? But it's got a stout bottom end.

 

[Flamefront]

Fojo

I think you are corret - The mention of the Iron Duke engine was more of a philosophical comment on taking a cheap high volume production pushrod engine and making s/c power rather than going the DOHC Vtec route - cheaper and better in the end.

The duty cycle for Park Avenue Eaton superchargers (data from ten years ago) was something under 15%, so most of the time the customer did not need the hp the brochure sold them on. This is the core of the justification for his comment - give them a four, and "blow" it into an eight when they (occassionally) need it. In today's high gas price world, it makes even more sense - the four would have lower pumping and frictional losses at cruise than an eight.

Having said that, however, it would be an interesting exercise to compare the frictional bmep of a four cylinder at 3000 rpm (making ~50 hp) for a highway cruise condition versus a Vortec 5.3L V-8 loping along at 1600 rpm for the same power. It might come down to pumping losses being the major player. However, again, the well-executed GM Displacement-on-Demand (the new one, not the V-8-6-4!)is claimed to be only worth 7% fuel economy, and addresses pumping losses only.

The Ecotec engine is an impressive piece - we're just getting one up on a dyno for some combustion studies. Nice to see a cam chain again, plus lots of other clever touches.

 

[Viper488]

Speaking of today's high gas price world, would you believe a V10 Viper turbocharged to over 1000hp can get almost 30mpg on the highway with its' OE 3.07 axle ratio and a .5 overdrive 6th gear?

10 litre/450hp Vipers typically get highway mpg numbers well into the 20's when burbling along in 6th gear. A turbo essentially not adding any pressure at 6th gear hwy speeds isn't going to reduce those numbers.

.Now if there was only some govt program to 'fund' privately owned vehicles being 'upgraded' for better MPG to stave off greenhouse effect, I could get my car TT'd at govt expense