Volumetric Efficiency of a naturally aspirated 4 cycle engine.

[DriveHard]

Ok...if allowed, this is something I learned during my undergrad at Iowa State studying internal combustion engines...

For a 2 liter engine, it is theoretically possible for the engine to "inhale" bring in two liters of clean air for every two revolutions. This would be equal to an intake efficiency of 1.0...so for every CC of displacement, it is bringing in that much air. Obviously if we have turbos, or superchargers, that ratio goes well above 1.0...and can exceed 2.0, and even more.
So...for a NA engine...what is possible?

Can a NA engine achieve 0.8, 0.9. 1.0, or even above 1.0?

 

[wwillson]

Here is an interesting thread on this topic:

2023 HPL Open House

What! no BITOG group picture. :rolleyes:

bobistheoilguy.com

 

[Cujet]

It is indeed possible to achieve a VE a bit above 100% with a 4 valve, well tuned engine. Although, this is at peak torque and not peak HP. Back in the 1980's, we would approach a VE of 104% with our 4 valve racing engines. Which were small shop 'things'. I'm quite sure some very highly developed engines can hit 115% on their own.

The Honda F20c and F22c are said to have pulled this off. Along with some from Porsche and other exotics.

By way of comparison, the angle valve Lycoming IO360 (flat 4) in my Cessna 177RG has a VE of about 70% at 200HP. It is a 2 valve, air cooled, low RPM engine with a somewhat small and restrictive intake setup. This is by design. An attempt at avoiding excess cylinder pressures.

 

[jetman]

IIRC (from reading race engine magazine articles yrs ago) the answer is yes.

Inertia of air entering the combustion camber can fill/overfill it if done correctly, again IIRC?

An example may be a NHRA ProStock engine?

 

[Cujet]

A Lycoming aircraft engine with small intake pipes and 70% VE

The engines we got 104% from, with comparative ease.

I guess it's worth noting that all that work to get high VE may only be necessary if there are rules limiting displacement. Otherwise, it's often lighter faster and more reliable to achieve HP via more displacement.

 

[CR94]

That depends on tuning intake plumbing to the engine speed, as well as minimizing intake and exhaust restriction.

... this is at peak torque and not peak HP. ...

That would be true generally, including much less exotic engines.

 

[Rmay635703]

I rarely hear things described as you do but check out the BSFC of the 2000 Honda Insight, ignoring just air ingestion .

I’m uncertain if it’s VTEC mode is spelled out.

But intake and exhaust tuning helps as do the harmonics of odd cylindered engines AKA 1,3,5 cylinders

 

[SHOZ]

The Absolute Load value in OBDII data will give you your real time VE. I've seen a freak peak of 400% on the turbo but usually peaks around 250% on the Gen Coupe 2L turbo.

 

[Jetronic]

Ok...if allowed, this is something I learned during my undergrad at Iowa State studying internal combustion engines...

For a 2 liter engine, it is theoretically possible for the engine to "inhale" bring in two liters of clean air for every two revolutions. This would be equal to an intake efficiency of 1.0...so for every CC of displacement, it is bringing in that much air. Obviously if we have turbos, or superchargers, that ratio goes well above 1.0...and can exceed 2.0, and even more.
So...for a NA engine...what is possible?

Can a NA engine achieve 0.8, 0.9. 1.0, or even above 1.0?

quite a bit above 100% is possible. 120 ish for race engines

 

[SHOZ]

Valve overlap does that I think. The V6 Yamaha SHO motors were good for 100% VE on a good one with the tuned intake runners.

 

[SubieRubyRoo]

It is indeed possible to achieve a VE a bit above 100% with a 4 valve, well tuned engine. Although, this is at peak torque and not peak HP. Back in the 1980's, we would approach a VE of 104% with our 4 valve racing engines. Which were small shop 'things'. I'm quite sure some very highly developed engines can hit 115% on their own.

The Honda F20c and F22c are said to have pulled this off. Along with some from Porsche and other exotics.

By way of comparison, the angle valve Lycoming IO360 (flat 4) in my Cessna 177RG has a VE of about 70% at 200HP. It is a 2 valve, air cooled, low RPM engine with a somewhat small and restrictive intake setup. This is by design. An attempt at avoiding excess cylinder pressures.

Yes, there are many, many factors which are easily overlooked yet play a very important role into VE. Runner length & diameter, surface finish, port volume (of head and manifold), total valve area, exhaust diameter, the list goes on and on. But the important thing to remember is that max VE will show at torque peak, but average VE is what builds a truly impressive power curve. All of the details above must be chosen to compliment the given engine size and the camshaft specs, although normally it's done by the engine size and all of the attendant details, and then the cam is chosen to optimize these factors in the given RPM usage range. It's quite the ballet of physics and mathematics!

 

[Cujet]

One factor not mentioned is fuel
cooling. Fuels can significantly cool the intake air and make it more dense. Leading to a more effective air pump.