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Propane and Natural Gas Fuel Delivery and Tuning Discussion about the care and feeding of Propane and Natural Gas Engines.

Propane and Natural Gas Fuel Delivery and Tuning

Natural Gas Engines in Cold Weather


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  #81  
Old 12-07-2018, 08:08:47 AM
cjjmw cjjmw is offline
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Default Re: Natural Gas Engines in Cold Weather

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Originally Posted by dkamp View Post
No, it needs to be vented to atmosphere external to the engine... totally ambient...

And just in case someone's wondering... no fuel gas vents through the vent- the vent's purpose is to allow the demand regulator's diaphragm to modulate properly. If it was sealed in a cavity, the pressure inside the cavity would prevent the diaphragm from moving in response to demand vacuum... on a cold day, pressure in the cavity would 'lock off' the fuel supply, and on a warm day, pressure would be so low, that it'd flow fuel with no engine operation.

If you look at the Garretson KN, you'll see that one of the features is a 'primer' button. This is generally used to purge air from a propane line when an exchangeable bottle is used... everytime you remove the fuel hose, propane in the hose wanders out, and air wanders in... when you reattach the tank, air in that hose has to get pulled out... and it takes lots of cranking to get it started up. Press the PRIME button for a little bit, and all the air will be purged out. Well, if you look closer, you'll see that the vent is not far from that purge button... and the purge button simply presses on the diaphragm.

The fuel controller works on the principle that air pressure inside the VENTURI of the carb (the part where it's narrow... where a liquid fuel's main jet would normally be found) is LOWER than ambient pressure.

Realize that a carbeurator works because the pressure drop inside the venturi LIFTS fuel up from the carbeurator bowl, because ATMOSPHERIC pressure is in the bowl, pushing DOWN on the fuel level. If you were to plug off the carbeurator's vent... or connect it to some other partial-vacuum location, then the fuel draw from the bowl would be incorrect, and worse yet, unstable... so your fuel mix would be likewise errant.

The fuel controller (zero governor, negative pressure regulator) takes place of the liquid fuel's float bowl level... and the needle and seat... in one simple component.

And as an aside note... the 'power valve'... the large adjustable valve between the fuel controller and the venturi feedpoint... has NO control of the mixture at ANY point other than full throttle. It simply limits fuel at full throttle. At any other speed, it's the fuel controller's gain adjustment, which is essentially linear, based on the difference between ambient atmospheric pressure, and the demand pressure signal it senses in the venturi.

I entirely understand the purpose of the vent and how the regulator works.

My understanding is carburetor bowls on better setups are typically vented into the air filter, so the bowl sees the same pressure the carburetor sees. Besides keeping dirt out of the bowl, this also tends to help keep the balance correct as the filter gets dirty etc.

My thoughts were if you vent the demand regulator in the same manner, it's seeing the same thing the inlet of the carburetor is vs the venturi vacuum, consistently including as filters get dirty.
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  #82  
Old 12-08-2018, 02:48:12 AM
dkamp dkamp is offline
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Default Re: Natural Gas Engines in Cold Weather

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Originally Posted by cjjmw View Post
I entirely understand the purpose of the vent and how the regulator works.

My understanding is carburetor bowls on better setups are typically vented into the air filter, so the bowl sees the same pressure the carburetor sees. Besides keeping dirt out of the bowl, this also tends to help keep the balance correct as the filter gets dirty etc.

My thoughts were if you vent the demand regulator in the same manner, it's seeing the same thing the inlet of the carburetor is vs the venturi vacuum, consistently including as filters get dirty.
Yes, I expected your reasoning on it... but the assumption that it'd keep the balance is errant, because the differential is not consequent with operation. The demand flow must be compared to ambient, not a difference downstream of filtration. IF you have that considerable of a restriction, airflow through the venturi is REDUCED, therefore the demand signal pressure is not as great.

IF you place the vent point between a restrictive filter and the venturi demand port, a restricted filter causes lower pressure at the vent point, yet less of a signal pressure occurs in the venturi.

Effectively, doing so would cause the regulator to skew mixture towards cutoff... which you don't want to do.

The demand regulator is fascinating with respect to it's mixture-management. You're feeding a known regulated fuel pressure, in known ratio, in a linear rate, entirely based on a linear pressure appearing in the venturi. Effectively, the mixture is a fixed value... and basically, stoichiometry at all points.

When learning gaseous fueling, there's a disposition that engine people have a tough time shedding... and that is 'variable mixture'.

Liquid fuel engines have a choke. In a gaseous engine, there is no need for choke... the fuel is already fully evaporated... adding more just makes for a less-powerful charge in the cylinder, rise in CO and nitric acid in the exhaust, and rapid rise in fuel cost.

The function of a choke is to provide 'additional enrichment' for cold starting. Why do we do this? Easy: Liquid fuel does not evaporate well in a cold engine... and liquid doesn't burn- the gaseous vapors burn. When you have a cylinder that has some vapors, and some liquid... the spark ignites vapors, but the unburned liquid washes down the rings and gets thrown out the exhaust valve.

(when you cold-start a liquid fuel engine, it wakes up fast, with a raspy 'braaap' to each cylinder firing. It's fast, because there's not much oil pressure load yet. After a few turns-at-speed, oil pressure comes up, and it's THICK, so the engine slows a little. That Braaappy sound... is FLAME from the overr-rich mixture coming out the exhaust ports. Why? Because you have raw fuel trying to find it's way out the exit... and it's still kinda burning... and there's a little more oxygen in there.

Once the chambers start warming up, and you get the choke pushed in, the incoming fuel is evaporating much better, so the burn cycle is starting, and finishing under closed valves.

On a gaseous engine... you'll never hear this. What comes in as vapor, burns, and as long as you haven't seriously goofed up the fuel controller, the mixture will be perfect. It'll labor a little after the first few seconds, as oil pressure has come up, but once the oil has circulated a bit, it'll labor a lot less and settle down.
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