Fuel Stand Off

[mark030358]

CAn anyone explain to me what causes fuel stand off... Would be interested to know...

cheers

Mark

[RogerFrench]

I'd like to know what it is, please?

[bill308]

Are you asking about the phenomenon of a fuel mist that will collect upstream of the cab inlet at certain rpm's?

If so, I have never personally seen this. My understanding is this can happen when a pressure pulse, originating at the intake valve or combustion chamber, comes back up the inlet tract carrying some of the inlet charge. I suspect it is caused by particular valve timing, rpm, and inlet tract geometry.

Perhaps someone more knowledgable could be more specific?

Bill

[mark030358]

Bill,
That is the phenomenom...

But what causes it..

[GrUmPyBoDgEr]

I'm no specialist on the subject but as far as I know this is caused through the gas exchange system in what is basically a fairly crude engine.
By that I mean the Otto cycle internal combustion engine with pistons belting up & down.
As previously mentioned this phenomenon can occur when certain conditions occur.
In the twincam engine the influencing factors are engine speed & engine load.
Air density (temperature/moisture content) will also play a role.
The components which influence it are the whole of the intake system before & after the carbs; the design & timing of the cams (dwell & overlap) & the size/lift of the valves.
The exhaust ports & the remaining exhaust system equally influence the gas exchange process.
To define when & where this happens is really an open book due to the fixed nature of all of the mentioned systems.
Modern engines are able to reduce the effect by introducing mechanical variables to compensate for the varying gas flow.
i.e.
1.Variable or switched length intake manifold
2. Variable Valve timing on the intake camshaft or both camshafts.
3. Variable Valve lift
And of course fuel injection provides more accurate metered fuel supply.

The cause of the phenomenon is the pulses which occur in the gas exchange system not always matching the physical, let's say "fixed tuned lengths/cross sections", of components that are there to support those ever varying pulses.

Other than that, all I can say is that its bloody complicated & difficult to get right even with mechanically variable components

Cheers
John

[elansprint71]

Very good John.


Now can you explain about a Mexican stand-off?

[GrUmPyBoDgEr]

Very good John.


Now can you explain about a Mexican stand-off?

Been there a few times but only know about Mexican blow-off
"Hey Meeester you wanna go Boys Town?"

But you wouldn't want to know about that; would you?

[Andy8421]

Bill,

The reason you haven't seen standoff is that it is only visible at full throttle and high revs. You wont see it just blipping the throttle. Search the web for videos of twincam dyno runs and the standoff is clear.

Just to expand on previous explanations;

When the piston decends and inlet valve opens, air doesn't go rushing through the carb and inlet tract - the air column takes time to accelerate. Instead a low pressure zone develops at the valve throat causing a low pressure pulse to travel back along the inlet tract at the speed of sound. This pulse exits the carb mouth and a high pressure pulse fills the gap and travels back toward the valve. In an ideal world this positive pulse reaches the valve at about the time it is closing and pushes a little more mixture into the cylinder giving a 'ram charge' effect.

The positive pulse in turn gets reflected and travels back up the inlet tract, carrying mixture with it that exits the carb mouth. This doesn't happen in isolation, subsequent valve openings cause futher pulses to form. At certain engine speeds and with certain inlet tract lengths the pulses all line up and a standing wave is formed, which is stationary relative to the carb. At this point a 'cloud' of fuel vapour forms and seems to hovver over the carb inlet.

Edit: Just found this. Look at 1:20 onward. Not standing waves, you need to get the revs just right, but clearly shows standoff effect.

http://www.youtube.com/watch?v=RSSYcVTUrT8

[GrUmPyBoDgEr]

Bill,

The reason you haven't seen standoff is that it is only visible at full throttle and high revs. You wont see it just blipping the throttle. Search the web for videos of twincam dyno runs and the standoff is clear.

Just to expand on previous explanations;

When the piston decends and inlet valve opens, air doesn't go rushing through the carb and inlet tract - the air column takes time to accelerate. Instead a low pressure zone develops at the valve throat causing a low pressure pulse to travel back along the inlet tract at the speed of sound. This pulse exits the carb mouth and a high pressure pulse fills the gap and travels back toward the valve. In an ideal world this positive pulse reaches the valve at about the time it is closing and pushes a little more mixture into the cylinder giving a 'ram charge' effect.

The positive pulse in turn gets reflected and travels back up the inlet tract, carrying mixture with it that exits the carb mouth. This doesn't happen in isolation, subsequent valve openings cause futher pulses to form. At certain engine speeds and with certain inlet tract lengths the pulses all line up and a standing wave is formed, which is stationary relative to the carb. At this point a 'cloud' of fuel vapour forms and seems to hovver over the carb inlet.

Edit: Just found this. Look at 1:20 onward. Not standing waves, you need to get the revs just right, but clearly shows standoff effect.

http://www.youtube.com/watch?v=RSSYcVTUrT8

Brillant!

And there was I talking about silly mechanical gismos to alter it.

John

[mark030358]

Gents,
interesting thanks...

Mine has it at WOT as seen on a dyno run...

cheers

Mark

[Esprit2]

The most dramatic visual example of stand-off I can recall was on the old Can-Am cars, back when the Chevy 8.1 liter aluminum V8 running Hilborn injection was the common engine. At full chat, they would create a very pronounced cloud above the air horns that didn't appear to get blown back. Something like a McLaren could zip by at 200 mph, and the cloud above the injectors just travelled right along with it as if flying in formation.

Regards,
Tim Engel