Variable Length Intake

[DaveKillens]

You haven't gotten this backwards. Long intake runners help with low-engine speeds and short runners are necessary to properly fill cylinders at high rpm.

It's my engine bible. Take a look at 'Engines: An Introduction' by John Lumley (Prof. Sibley School of MAE, Cornell)

Yup, you're correct, somehow I got it bass ackwards. Higher RPM, shorter intake.
Here's a bit of history, how engineers tried to deal with tempermental engines and the torque spread. It's the old McLaren Can Am solution before variable length runners were adopted.

[mvf4sp]

Hi!

Well as stated above, the shorter the primary pipe's in an engine, the more efficient it will be at higher rpm. Longer primary pipes will bring larger volumetric efficiency at lower RPM. It´s explained by the fact that the wave travels fore and back the intake runners in a shorter period of time. The bigger amount of mass in the manifold also produces a major inertia that brings more air to be delivered inside the combustion chamber.

Using a plenum brings a second peak in the volumetric efficiency vs engine speed curve. This second peak grows in the lowest engine operation range of speed. The shorter the plenum, the bigger the peak, and also it shifts at a lower speed range. A plenum does not affect the performance of the volumetric efficiency peak in the higher RPM range. Also increasing the plenum volume shifts the first and second harmonic of the whole manifold system to the left of the spectrum of frequencies, reducing the gain of pressures.

Concluding the primary pipes in the manifold influence mainly the v.e. at higher engine speed while plenum influence the v.e at lower engine speeds.

Hope this info is helpful,

[gcdugas]

If you are ever in Vegas there is a 2002 F1 engine in the Ferrari dealer at the Wynne. I was able to "play" with the engine including moving the intake trumpets (a glorified trombone). There was a lot of friction but I assume that with the engine buzzing, the vibrations help free things up. You will also have to "play dumb" and listen to some spiel from a male model hired as a floor spokesperson. He will tell you all sorts of junk that is inaccurate. You can touch it and inspect it if you are coy and discrete about it. There was some interesting things around the exhaust as well. There is some other F1 stuff around like a broken crankshaft, some aero bits etc. One thing that suprised me was how far upstream the injectors were. And these can be seen from any of the many pictures that float around this site. My road car has the injectors closer to the action. I thought that the ideal was in-chamber injection after the exhault valves close fully. I have read that this is what they want to persue. The F1 engine's injectors seemed like stone-age plumbing to me. Sure the actual nozzles were impressive but the rest looked like a bath tub aparatus.

[mep]

Sorry whats a plenum?
Can you explain this more detailed?

[RH1300S]

A plenum is a space filled with air.

This is the box at the end of the intake duct (running from above the driver's head) and seals around the intake trumpets.

When sized and shaped correctly it can have a big influence of the quality of air getting into the engine.

[zac510]

Dave, are you sure the CanAm Chevs didn't have the porcupine inlet due to different port lengths inside the head (thus necessitating different length trumpets to obtain indentical runner lengths). The heads were later upgraded, later CanAm cars had normal equal length runners.

I hvae asked people about this before but nobody seems to be entirely sure.

[Mikey_s]

gcdugas,

Probably only Ferrari know the real answer to your question about injector placement, but I'll hazard a guess;

A key element in maximising efficiency of combustion is to have a homogeneous charge in the cylinder. High injection pressures are used to atomise the fuel; making the droplets as small as possible gives a high surface area:volume ratio enabling rapid evaporation of the fuel droplet in the inlet charge. GDI (gasoline direct injection) is something the commercial engine manufacturers have been working on for some time, so far with limited success, but again homogeneity of charge plus avoiding knock is a perpetual issue.

In any case the regulations state that the injectors must be placed in the intake;

5.7.1. The pressure of the fuel supplied to the injectors may not exceed 100 bar. Sensors must be fitted which directly measure the pressure of the fuel supplied to the injectors, these signals must be supplied to the FIA data logger.

5.7.2. Only one fuel injector per cylinder is permitted which must inject directly into the side or the top of the inlet port.

In an F1 engine the revs are so high I doubt direct injection could function efficiently unless injector pressures were extremely high, as the pressure is limited the degree of atomisation is also limited. For the engine you were looking at, situating them further out a) gives the variable intake trumpet space to move, and b) permits maximum time for the fuel to evaporate completely (even at 20k rpm).

That's my 2c on the question.

[zac510]

I thought Audi had quite well proven the benefit of GDI in the Audi R8.

If the economy figures they quote are replicated on an F1 car, these would surely outweight a slight drop in power?

Of course it is outlawed so the discussion is moot.

[Mikey_s]

Zac,

I would agree that in principle it appears to have been successful in the R8 - not a series I know much about, but key to making it work is timing of the injection charge to avoid knocking and also getting good distribution of the fuel into the cylinder to get efficient combustion. This requires immense pressures to atomise the fuel effectively.

To squirt, atomise, vapourise and homogenise the fuel approx 170 times per second is a formidable engineering challenge (even if the charge termperature is going to be high close to TDC).

As you say, in any case it becomes an academic discussion as they aren't allowed to do it.... so much for transferable technology Max!!

[Carlos]

Zac510--No the Chevy CanAm cylinderhead positively did not have the features you suggest. My PM may offer more insight.
Regards Carlos

[DaveKillens]

Dave, are you sure the CanAm Chevs didn't have the porcupine inlet due to different port lengths inside the head (thus necessitating different length trumpets to obtain indentical runner lengths). The heads were later upgraded, later CanAm cars had normal equal length runners.

I hvae asked people about this before but nobody seems to be entirely sure.

I thought about that, but these are individual intakes, and most likely equal length intakes inside the head.

[ACRO]

does anybody know what teams when introduced variable intakes? where they common also in the 2001 season or maybe even long before?

[Jersey Tom]

Anyone else out there feel variable length exhaust would be more beneficial than variable length intake?

[Birel99]

can someone please explain how variable intake effects performance? what does it do to change the power band?
also same thing for the exhaust, how does it effect the power band?
i am very skilled with 2 cycle engines and understand the exhaust side, but dont see how this would effect a 4 cycle?
thanks,

[Jersey Tom]

There's a couple ways you can go thinking about and analyzing it. I like the simple one..

Take exhaust scavenging. The idea is, at EVO (Exhaust valve opening), a positive pressure wave starts going down the primary for that cylinder, at the speed of sound in exhaust gas at whatever EGT you're at. When a pressure wave goes from a small diameter tube to a large diameter tube (as is the case in a collector), a fraction of that wave is reflected with opposite sign back up the pipe. Thus you now have a negative pressure wave coming up the primary back towards the cylinder.

The idea is to time everything such that the reflected wave comes back to the cylinder just before EVC. The negative pressure then sucks out a bit more of the exhaust gas remaining in the cylinder, increasing the volumetric efficiency at that RPM.

Same idea should apply to intake manifolds but its been my experience on high-RPM 4-stroke engines that exhaust manifold tuning plays a much bigger role than intake manifold tubing.