Starting a car on LPG at low temperatures

[Marek]

Am I correct to assume that when cranking, the pressure and temperature compensation isn't active for a dual fuel user will kick in until after cranking RPM is reached?

As temperature drops, e.g. to 5'c or lower, the fuel delivered is denser and colder and produces gas a higher pressure into the injector rail when the rail is first opened. This means the start is unsuccessful as it is too rich.

A large priming pulse would be in order, to bleed off the excess pressure and allow normal starting at normal pressure. The only problem is that there is only one place for the excess fuel to go - exactly where it isn't wanted.

Is it possible to have the option to have P & T adjustments when cranking using dual fuel? The only other workable solution would be a vent valve to bleed off excess pressure to atmosphere before applying a priming pulse - i.e. two types of priming pulse.

Either that, or one has to factor in the possibility of a too dense mix at startup and commence with a cranking to create a flood clear for the excess before cranking again to start.

kind regards
Marek


kind regards
Marek

[02plus]

My gas vapor lpg always starts on petrol. At 40 C water temperature or 30 seconds after the start, I switch to LPG with an on-off output .

[vw_chuck]

Just turn off your prime pulse at low temps in the calibration

[jsmcortina]

My gas vapor lpg always starts on petrol. At 40 C water temperature or 30 seconds after the start, I switch to LPG with an on-off output .

My daily has an LPG conversion and works like this too.

James

[Marek]

Perhaps more explanation is in order.

The gas is produced by vapourising liquid propane, which boils at ~-40'c to produce 270 times its volume as gaseous propane. To do this, the liquid propane sits behind a spring loaded rubber diaphragm which is vacuum referenced to the manifold to produce gas at (for example) 2bar.

At low temperatures, when the fuel pump opens the solenoid to allow liquid propane into the reducer, it can sometimes, rather than letting gas through in small measures to prime the fuel rail to 2bar, let a tiny amount of liquid through, or more commonly, allow the low temperature denser gas through. The result is that the fuel rail can easily be pressurised to 3-4bar.

As MS doesn't afford the user the luxury of adjusting the cranking pulsewidth for this probability, the car won't start on such a rich excess of fuel. Having a huge priming pulse isn't any help, as that simply compounds the problem.

What is needed is the abilty to either vent the fuel rail to atmosphere, or to reduce the cranking pulsewidth substantially.

You would have thought that flood clear would be another option, but I don't want to crank a high compression v12 when it is coldest as that then reduces the probability of starting later.

Correctly tuned, it starts so fast that the log shows it shows the revs run away on just the third cylinder to fire - that's just half an engine revolution. At other times, with this problem, it drains the battery horribly.

What I'd quite like is the option to have P & T adjustments applied to cranking as lpg fuel is a very different animal to petrol. As a minimum, I think I need to set up a warning light.

The reason most lpg systems have a cut-in temperature of around 40'c is that most cars only have a limited amount of coolant and the reducer is often inserted into the heater matrix plumbing. This means relatively little water has to provide the latent heat of vapourisation and at low temperatures, the reducer can easily ice up. In my case, the full engine coolant runs through the reducers and the v12 has over 20 litres of it to get through. Freezing is very unlikely, even if starting at low temperatures.

kind regards
Marek