Why would a propane engine need to "warm up" to idle low?

[evranch]

Was thinking about this while driving my 460 truck today. Most of my propane engines are governed and don't really "idle" in the same way the truck does against a closed throttle plate. They all start and run great from the moment you turn them over, but most are "idling" against a governor that maintains that idle speed.

The 460 idles beautifully once it has warmed up a bit but on a cold start has a bit of a lopey idle, and prefers a little bit of throttle opening to start reliably. Otherwise it will sometimes fire a couple times and fail to catch. But in a couple minutes it starts idling nicely, it definitely doesn't have to come up to thermostat temperature or anything.

We all know carbureted gasoline engines need to warm up a bit to get good atomization of the liquid fuel in the manifold, sometimes they even need a little choke to get going on a cold start even on a warm day.

But propane is delivered as vapour and once the air/fuel mix gets into the cylinder, shouldn't it burn just the same in a cold engine on a 30C day?
Doesn't seem to be an issue with the vaporizer needing coolant heat either to deliver enough vapour, since the engine will rev smoothly and the truck can be driven just fine.

Is this normal behaviour for a carbureted propane engine, or maybe I just need to change my spark plugs? However propane keeps them clean as a whistle so I don't see how anything would need to burn off the plugs on startup. Just wondering what the mechanism would be that requires the engine to warm up at all to run smooth.

[storm]

All engines need a little warmth to idle well, LPG is no different. In my experience coolant flow through the regulator does impact on idle quality. When an engine is first started coolant isn't flowing and the coolant in the regulator is required to vapourise -30 degree liquid LPG. Once coolant is flowing things settle down alot. When I refurbish the system on my V6 Commodore I'll be fitting an electric booster pump for the regulator that will pump coolant even before I click the key to start the engine.

[Tom68]

Was thinking about this while driving my 460 truck today. Most of my propane engines are governed and don't really "idle" in the same way the truck does against a closed throttle plate. They all start and run great from the moment you turn them over, but most are "idling" against a governor that maintains that idle speed.

The 460 idles beautifully once it has warmed up a bit but on a cold start has a bit of a lopey idle, and prefers a little bit of throttle opening to start reliably. Otherwise it will sometimes fire a couple times and fail to catch. But in a couple minutes it starts idling nicely, it definitely doesn't have to come up to thermostat temperature or anything.

We all know carbureted gasoline engines need to warm up a bit to get good atomization of the liquid fuel in the manifold, sometimes they even need a little choke to get going on a cold start even on a warm day.

But propane is delivered as vapour and once the air/fuel mix gets into the cylinder, shouldn't it burn just the same in a cold engine on a 30C day?
Doesn't seem to be an issue with the vaporizer needing coolant heat either to deliver enough vapour, since the engine will rev smoothly and the truck can be driven just fine.

Is this normal behaviour for a carbureted propane engine, or maybe I just need to change my spark plugs? However propane keeps them clean as a whistle so I don't see how anything would need to burn off the plugs on startup. Just wondering what the mechanism would be that requires the engine to warm up at all to run smooth.

Cold charge hits cold piston, cylinder wall, combustion chamber and valves, no ignition advance curve built in to compensate, the surface of all those items warm up pretty quick and idle speed comes up.

[evranch]

Makes sense, even though you don't have the liquid fuel issues or need for a choke, the combustion is still being cooled by the chamber walls, resulting in lower pressures. The governed engines compensate with more throttle opening, while the uncontrolled idle of a closed throttle plate is going to be weaker until the combustion chambers quickly reach operating temperature.

The coolant flow plus the governor is probably why my old MH44 will start and idle well at -20C even if it's stuck somewhere I can't plug it in, because I installed an auxiliary electric coolant pump to assist the block heater. That at least keeps the -20C coolant flowing through the regulator so that it can still evaporate enough propane to get it warmed up. I have electric coolant pumps on a lot of my tractors and would highly recommend them in a cold climate.