Fuel Pressure Regulator

[Kiwi_Roy]

Roy,

 

You've looked at your CO trim with a software interface?

No, I haven't done that yet, I'm sure its a long way off.

Something else that makes me think so I park in an underground garage about 20 ft away from the CO sensor that starts the ventilation fan. If I start the bike it quickly triggers the fan wheras nearby cars in the same garage can start-up and leave often without triggering it.

Re regulator connections see my next post.

 

Thanks

 

Roy

[Kiwi_Roy]

OK, now I'm convinced.

I was having a hard time understanding why connecting to the manifold would have no effect.

I work in process control design so I used a valve sizing program we use to model a hypothetical injector to see what effect changing the pressure would have.

As I see it an injector is nothing more than a simple On/Off valve followed by a fixed orifice.

As you can see from attached once you go past 20 psid the program gives a "Choked" flow warning

Choked flow is a condition where no matter how low you drop the discharge pressure the flow will not change.

At a fuel pressure of 37 psig the injector is well within choked flow condition for any senario.

The only way to change the flow rate is to use a larger orifice or hold the injector open longer.

 

I'm really sorry I wasted your time with this

 

Regards

 

Roy

Fuel Pressure Explanation.pdf

[raz]

Not a waste of my time anyway: Excellent Roy, this explains the one thing that still puzzled me! I had never heard of choked flow before.

 

You are a fresh breeze to this forum!

[luhbo]

Hold it for a moment. Is this really what is happening at the injector nozzles in my bike? The model Roy used does not apply I think. Not in this context.

 

Make the same thing again and vary Inlet Pressure and Atmospheric Pressure. The pressure loss then should be a result of the resulting flow (velocity, length of nozzle, design of flow channel and valve needle etc). Not vice versa.

 

Hubert

[Kiwi_Roy]

Hold it for a moment. Is this really what is happening at the injector nozzles in my bike? The model Roy used does not apply I think. Not in this context.

 

Make the same thing again and vary Inlet Pressure and Atmospheric Pressure. The pressure loss then should be a result of the resulting flow (velocity, length of nozzle, design of flow channel and valve needle etc). Not vice versa.

 

Hubert

Hubert,

Yes if you change the inlet pressure (as it would with the spring housing connected to inlet manifold) in the sizing program the flow will change but if you only change the outlet pressure (manifold pressure) once the pressure drop exceeds 20 psi the flow stays exactly the same.

Basicly what happens with choked flow in a liquid passing through an orifice, the pressure drops (vena contracta effect)past it's vapour pressure and vapourises which takes up more space than the liquid thus choking the flow off.

I found several quite good explanations on the web.

I like Dan Ms explanation of why Guzzi chose this approach.

My guess is they decided to simplify things and go for a known flow variable time. Obviously at one time in the past they tried changing the pressure based on manifold vacuum as it shows on the earlier document with Webber fuel injection. See earlier post

 

The pressure loss is fixed in this case by the regulator and throttle position. I'm not saying the injector is as simple as I make out but I expect the valve portion is much larger than it needs to be relative to the nozzle orifice.

Perhaps someone can find something on injector theory.

BTW, are the injectors something that needs to be replaced occasionally?

[Cliff]

When your bike is cold ( before first start of the day ), check the resistance of your oil and air temperature sensors. They should be the same and correspond roughly to this table - http://www.cajinnovations.com/MyECU/temperature.htm

 

Also check your bike's rolling resistance - are any of the disks warm?

[Dan M]

BTW, are the injectors something that needs to be replaced occasionally?

 

They don't wear much. Most injector trouble comes from deposits causing a poor spray pattern and poor atomization. Occasionally, deposits will cause the pintle to stick and it won't open at all. Cleaning usually fixes either of these issues. The least seen failure is the coil opening or shorting.

 

I agree with the others that pressure is not your issue.

The system is constant pressure & constant volume. It has to have sufficient pressure to atomize the fuel at the injector nozzle and enough volume for wide open full throttle operation. The pressure in the line does not change.

[gstallons]

OK, now I'm convinced.

I was having a hard time understanding why connecting to the manifold would have no effect.

I work in process control design so I used a valve sizing program we use to model a hypothetical injector to see what effect changing the pressure would have.

As I see it an injector is nothing more than a simple On/Off valve followed by a fixed orifice.

As you can see from attached once you go past 20 psid the program gives a "Choked" flow warning

Choked flow is a condition where no matter how low you drop the discharge pressure the flow will not change.

At a fuel pressure of 37 psig the injector is well within choked flow condition for any senario.

The only way to change the flow rate is to use a larger orifice or hold the injector open longer.

 

I'm really sorry I wasted your time with this

 

Regards

 

Roy

You are not wasting anyone's time.

When you are learning you are making the most of time. You understand the injector is a solenoid/ pintle setup that allows pressurized fuel to be sprayed into the intake.

#1 fuel pressure does make a difference on gasoline fuel injection. You clamp off the fuel return and see what happens.

#2 The pulse width modulation (controlled by the pcm) and fuel pressure/volume control the fuel amount going into the intake.

 

If you are having or think you are having trouble with your fuel injectors contact www.lindertech.com 888-809-3835. These guys can test, flow rate, clean and match/set fuel injectors.

[emry]

If our injectors suffered from choked flow we would not need a pressure regulator, not to mention the resulting spray pattern would be poor. On a similar note, many turbo/supercharged engines reference the regulator to manifold, so that as manifold pressure increases so does the fuel pressure.

[Kiwi_Roy]

#1 fuel pressure does make a difference on gasoline fuel injection. You clamp off the fuel return and see what happens.

Yes you are correct the fuel supply pressure does make a difference but the downstream pressure doesn't because it's always under choked flow conditions.

In other words if you hold the supply pressure constant, while the injector is energised the instantaneous flow rate is exactly the same at idle (greatest vacuum, greatest DP) as it is at WOT (no vacuum).

So the pressure drop changes from ~50 to 37 with no change in the instantaneios flow rate.

It's quite neat really, to increase the fuel flow you just hold the injector open longer.

If you refer to the calculation sheet I posted you will see this, the first 3 calculations warn of choked flow

[raz]

I think the wikipedia article describes it good. Apparently there are two things called choked flow, one good (the one Roy talks about) and one bad (causing cavitation and/or flushing, damaging stuff).

 

Roy, I'm curious about the other parameter: How much will the flow increase if you lower the fuel temperature to 75 and 50 degrees F respectively (keeping the pressure at 3 bar and the other figures as they were)? Could you try that with the program please? I wonder if this can explain some theories my dyno guy had regarding fuel temperature.

[Kiwi_Roy]

Could you try that with the program please? I wonder if this can explain some theories my dyno guy had regarding fuel temperature.

Yes cavitation is a problem, it destroys pumps and control valves. You may hear it hapening when you open a water valve in your house, sounds a bit like gravel going through the valve, flashing is not so bad.

Calc sheet sent to your e-mail.

Note gasoline is a very complex liquid so it's hard to model exactly but this gives some idea.

 

Regards

Roy

[gstallons]

I think the wikipedia article describes it good. Apparently there are two things called choked flow, one good (the one Roy talks about) and one bad (causing cavitation and/or flushing, damaging stuff).

 

Roy, I'm curious about the other parameter: How much will the flow increase if you lower the fuel temperature to 75 and 50 degrees F respectively (keeping the pressure at 3 bar and the other figures as they were)? Could you try that with the program please? I wonder if this can explain some theories my dyno guy had regarding fuel temperature.

 

 

Gasoline density or volume expands and contracts with temperature variation. This is why racers ran their fuel through "cool cans" before going into the carbs. This cool can had ice packed into it to chill the fuel (and causing the increased density) before it went into the carb.

[Kiwi_Roy]

When your bike is cold ( before first start of the day ), check the resistance of your oil and air temperature sensors. They should be the same and correspond roughly to this table - http://www.cajinnovations.com/MyECU/temperature.htm

 

Also check your bike's rolling resistance - are any of the disks warm?

I checked the thermistors last night, both about 4.75 kOhm

No the brakes are not getting warm

Atached is the calc Raz asked me for, basicly the higher the temperature the sooner choke flow occurs, this is because the vapour pressure is lower. The change in density is insignificant.

The change in flow about -9% for 50°F temperature rise.

I don't claim that this is accurate, gasoline is far too complex for me to calculate, it just shows what happens.

Roy

Flo Calcs RevA.pdf