Bi-Fuel and LPG
A dummy's intro to automotive LPG and bi-fuel systems
Questions and comments are welcome, as always.
Why bi-fuel?
In the Netherlands, many other European countries and in Australia it's fairly common to fit cars with aftermarket LPG kits. In the Netherlands, there's a fairly good infrastructure, where about one out of every 2 or 3 fuel stations have LPG pumps.
Off-the-shelf kits
LPG kits usually consist of an LPG fuel system running off an extra tank that can be installed in the trunk, in the spare wheel well or anywhere else you may find an empty spot. LPG, although gaseous at atmospheric pressure, is stored in liquid form. The [tank] is usually a thick walled steel cylinder or donut shape tank, although composite tanks are being developed.
Pressure
Automotive LPG tanks in most countries have limiters installed that do not allow them to be filled past 80%. Along with controlled let-off by a safety valve, this prevents the tank from bursting in case of extreme heat (sun) or even fire. During normal use, it contents will be part liquid and part vapor at all times, just like in a common disposable cigarette lighter. Pressure in the tank is determined by vapor pressure of the LPG mixture in use and therefore varies with temperature.
The graph below is for propane, but quite close to actual LPG pressures.
Most "dry" LPG systems don't have a pump. The converter is fed by the pressure in the tank and then regulates pressure down to a fixed value much below in-tank pressure. "wet" systems on the other hand have an in-tank pump that pressurizes the liquid fuel much above in-tank pressure. None depend on in-tank pressure for fuel metering.
Switching
When installing LPG, the stock petrol fuel system is usually retained and kept fully functional. The driver can switch between the two fuel systems, hence a bi-fuel car.
LPGandBiFuel/Switching contains further details about switching between different fuels while driving.
(GoranJurkovic) My idea is this:
I'm driving on LPG, and when I press hard and LPG injectors reach like 95% of duty cycle, VEMS opens gasoline injectors as needed (staged injectors)... Is this possible in double configs?
On the other hand, when I switch config to gasoline, and I'm driving on gasoline, and when I press hard and gasoline injectors reach 95% of duty cycle, VEMS opens LPG injectors as needed (staged injectors)... If I get big enough injectors for gasoline, this would never happen, but it's to solve the problem of injector left open when switching configs... Is this possible on 1.1.33? Or not all the config is doubled?
Systems
LPG fuel systems come in roughly four flavors.
- The classic mechanical venturi type system. Consists of a converter that evaporates the liquid LPG and a venturi type gas/air mixer in the engine's intake. All "dry" LPG systems have a converter. The converter is heated by engine coolant to keep it from freezing up, with all that evaporation going on inside.
- Single point gas injection (either using a single large pulsed injector or a stepper motor controlled valve)
- "dry" port injection using a thick fuel rail @ 15 psi and big "injectors" that inject gaseous LPG. Early systems had problems with injector lubrication, this has been solved.
- "wet" port injection. Has a fuel rail and injectors resembling those used for petrol. The most common "wet" system over here (Vialle LPi) has standardized injectors that will support about 50hp each. The injectors are controlled by a small computer that converts the existing petrol fuel injector signal to the right pulse width for the LPi injectors. LPi runs off an in-tank pump that pressurizes the liquid LPG much more than its "standard" 150psi, to prevent cavitation in lines and injectors (do you have more info on this?). So it is a very high pressure system.
All of these systems, when installed on fuel injected cars, have some electronics in place to "fool" the standard ECU into thinking its injectors are still working (while in fact they aren't and control over fuelling has been taken over by the LPG system).
Depending on emissions regulations, LPG installers had to add more and more electronics over time, making those kits troublesome at times when development wasn't done properly. The latest generation of port injection systems works a bit better because they leave the standard ECU mostly in control of fuelling, basically only connecting different injectors to it.
LPG - origin, smell, octane
Automotive LPG (Liquid Petroleum Gas) is a waste byproduct of oil refineries. It is a mixture of mainly propane and butane, with a slight smell artificially added to make leaks a bit more obvious. Octane ratings are always over 100 and can be up to 120 depending on actual propane/butane ratio.
Beware: some lambda meters display 14.7*lambda and say it is AFR - which is not real AFR except for gasoline
Actual air/fuel mixtures (by mass) are slightly different than on petrol. Lambda 1.00 on LPG (stoich) is 15.5:1 A/F by mass. All O2 sensors still work correctly though and their readings require no adjustment. O2 sensors measure lambda, not actual AFR.
A/F meters often display a calculated AFR instead of actual lambda. When using one of those meters, you could choose to pretend you're running on petrol and simply assume an indicated 14.7:1 as stoich (actual mixture would be 15.5:1, but who cares as long as you know what to aim for). In short: Use Lambda, not AFR. AFR can be confusing.
I've found a [PDF file] that contains (near the bottom) AFR to lambda graphs for various fuels including petrol and LPG.
While gasoline hesitates to mix with air at low temperatures (=> significant cold enrichment is required), LPG mixes very well (cold enrichment value can be set small).
Why LPG? - more why-s
One could use [google] and find several sites listing pros and cons of LPG as an automotive fuel. So I will keep this section short and list only my personal reasons to use it.
- It's cheap. Dirt cheap. Where I live, it's about 1/3rd the price of petrol. Fuel consumption is a bit higher on LPG, but still. I do pay extra road tax as a penalty but it's still worthwhile.
- Emissions. If I keep NOx emissions down, only CO2 and water will come out my tailpipe. (isn't the case similar for gasoline?)
- High octane. Nice to have when playing with turbos.
- Did I mention it's cheap? I can actually floor the accelerator without EVER feeling guilty!
Using GenBoard for LPG + Gasoline control underdevelopment
- Two different fuels. This will not require two different VE maps (only asymetric intake requires dual VE-maps). It requires different injector settings (including injopen) and cold-enrichment settings.
- 8 injector drivers is just enough for a bi-fuel 4-cylinder. For an 8 cyl, 8 primary injector drivers + using the 2 extra FETs for 2 TBI injectors (or more injectors arranged in 2 banks) for the 2nd fuel requires minor firmware change (so the extra FETs can be used for inj).
- Dual ignition maps are nice for a later stage of the project. LPG will run fine on petrol ignition maps, but only a specialized map will make the most of it in both power and economy.
- Implementing switching between the two.
page TODO:
- GenBoard related info on using it for direct LPG + gasoline control
- GenBoard related info on using it as supplementary controller, when used with factory ECU (which is usually dumber, wbo2-less, but at least configured in factory): measuring injector PW of factory ECU, calculations, controlling LPG valve(s) Comment : I don't see why anyone would want to use the GenBoard in this way. It is certainly not my intention.
- needs more links
Physics basics for injector pulsewidth
Liquid
http://www.msefi.com/viewtopic.php?t=3728&highlight=propane
Looking at normal pump gasoline :
An injector injects a certain volume, proportional to the pulsewidth and the injector capacity. and depending on fuel pressure across it.
For a liquid, density depends little on temperature. The injector flowrate is proportional to the squareroot of the liquid pressure across it - but the pressure differential is usually kept the same with FPR, so the injector flowrate stays the same. Disregarding opening and closing issues here.
Fuel temperature will have some impact, because it changes the density of the liquid, but I this effect is marginal: I don't know any oem that has a fuel temp sensor.
propane/lpg-drygas injection
The lpg injector also injects a certain volume, proportional to pulsewidth and injector flow. The nr of molecules however in this volume (~density) is pressure and temperature dependent.
The vaporiser /pressure reducer holds the fuel pressure at 0.8 bar above MAP. It has a vacuum connection, just like a FPR.
All dry LPG injection systems have some kind of fuel pressure transducer and temp sensor. [ for some reason I kinda doubt the latter... will pick up some of this gear to play with soon ]
The universal gas law is: PV=nRT
where
- P= pressure (kPa)
- V= volume (m3)
- n = the nr of molecules
- R is a constant which is related to molecule size and shape: same for 1 atom (noble-gas) molecules, 2-atom molecules, and shape-dependent for bigger molecules (like LPG)
- T = temperature (in Kelvin scale)
The goal is now to derive an algorithm indicating the relationship between pulsewidth and injected nr of molecules.
The V in the formula can also be expessed as the injector flow multiplied by the pulsewidth.
Q(cc/min) * t ( millisec) = V
If I rearrange this :
n = ( P *Q*t )/R*T or
n= (X*t*P)/T
X being all the constants put together (stuffed to an enrichment value).
This indicates, to keep a lineair relationship between pulsewidth and the injected amount of fuel, the pulsewidth has to go down if pressure rises, and has to go up if temperature rises.
Now how am I going to integrate this into the fuel equation?
If the temperature doubles, the pulsewidth has to double also.
If the pressure doubles, the pulsewidth has to be divided by two.
I think it should be possible to tune the engine for a given set of
variables , say T = coolant tempareture with a warm engine ,
P = MAP at idle + 0.8 .
Then calculate how much % correction is needed, and adjust pulsewidth accordingly.
Basic fueling equation:
PW = reqFuel * MAP * VE(MAP,RPM) * gamma_enrichment
where gamma_enrichment already is a multiple of several enrichments, multiplier due to gas pressure and temp should be stuffed in or a new multiplier created:
enrichment_fueldens(fP, fT) = constant / (fT * fP)
(fT up -> PW down, fP up -> PW down)
where fT and fP are in absolute values (fT in K, fP in bar absolute?) based on some ideal conditions.
fT could be measured by a second IAT sensor stuffed in the vapor fuel rail if vapor injection systems don't all come with such a sensor already, fP either by a second MAP sensor (ugly but precise) or calculated from MAP and the constant pressure-differential (not quite precise but likely close enough)
- Bosch already developed this sensor.
TODO move these PW issues to separate page (getting long). Will take care of that after lunch...