13 replies to this topic

[GBarclay]

In one of the recent issues of Racecar Engineering, there is an article on the Cosworth F1 motor.

Mention is made that the cylinders operate at 125% volumetric efficiency. I cannot remember the rpm that was discussed but think it was in the 18 000 range.

How is more than 100% efficiency possible on a naturally aspirated motor? I can see it being possible if you have some ram air effect. I did a David Vizard (sp?) course in the late 90's and if I remember correctly motors were running at high 80's and low 90's in terms of volumetric efficiency, where did the big improvement come from?

Perhaps I am not understanding volumetirc efficiency correctly? In laymans terms, I take it to mean the actual charge volume of intake divided by the theoretical maximum charge. in a 4 litre V8 each cylinder could theoretically hold 0.5 litres when the piston is bottom dead center, so to achieve 125% efficiency the intake charge would need to be 0.625 litres. Obviously in a turbo or supercharged motor v/e's over 100% are possible depending on boost levels.

How is this possible in a naturally aspirated motor?

many thanks

Grant

[Wolf]

I think intake stacks were used for that purpose, even back in fifties- their length corresponded to engine rpms at which they'd produce 'boost' (length=velocity/frequency) by means of resonance... I think someone even experimented with/used variable length intakes which produced this effect across broader range of revs.

[desmo]

It's normal for race engines to exceed 100% VE using intake and exhaust tuning of pressure waves.

[imaginesix]

As I understand it, inertia is responsible. Just because the piston reaches BDC the air isn't all of sudden going to just stop rushing in.

[Bill Sherwood]

Yep, strong Hemholtz resonance tuning.
It's extremely violent and when utilised properly can give quite a bit more power.

[Greg Locock]

it's more organ pipe tuning (ie quarter or half wave) than helmholtz, for most intake manifolds.

even easier is to imagine the incoming charge bouncing off the back of the closing valve, and then working out its various adventures as it bounces off the various changes in section. You can get a long way with that, including the effect of tapers, and other cylinders.

http://ntrs.nasa.gov..._1993090861.pdf

for a nice paper with real data

[imaginesix]

Having said all that, isn't 125% unusually high?

[alexbiker]

Originally posted by imaginesix
Having said all that, isn't 125% unusually high?

Oh yes. Especially for an engine that spent a lot of time at 0% volumetric efficiency.

[NRoshier]

would that 0% involve lots of bangs, smoke and broken bits of hot metal?

[GBarclay]

I must apologize for getting some facts wrong. I found the course reference materials last night, and I took the David Vizard (correct spelling) course in Nov 1996.

The high 80's/low 90's VE was in reference to stock engines at the time. He described a ProStock (used in NHRA drag racing) motor in detail and stated it was in the range of 115% VE.. So in terms of efficiency gains the Cosworth motor is not that amazing, although 125% is pretty darn good (well at least when the motor is running )

That is a nice paper that Greg Locock links.

FWIW Vizard mentions stack length tuning but states it is really only at it's most efficient in a narrow rpm range, about 500 to 700 rpm, hence the move to adjustable length runners. he also covered Hemholtz resonance tuning in detail.

Sorry for the lapse in memory, sucks getting old and suffering from CRS (cannot remember sh#t).

[GeorgeTheCar]

In the old days, the days of the Bruce and Denny Show, we saw an early example of that with the staggered intake trumpets of the big block Chev's right out there in the open.

[GeorgeTheCar]

More of a question than an answer but was the original "camminess" of the DFV which made it hard to drive?

[cosworth bdg]

Originally posted by GeorgeTheCar
More of a question than an answer but was the original "camminess" of the DFV which made it hard to drive?

Was it really hard to drive, a 2L Lucas injected BDG with the same "camminess" is an easy engine to drive on the "Dyno"...................

[Chubby_Deuce]

Originally posted by NRoshier
would that 0% involve lots of bangs, smoke and broken bits of hot metal?

I think he was refering to the fact that no one is using Cosworth this year?