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Engine & fuel engineering

What are the fuel properties of LPG (Propane) and how does it interface on a fuel injected engine? by franzh
Posted: 31 Jul 05

Basic Propane (LPG) facts.

There have been several posts recently (summer 2005) about using LPG, mostly by persons with no engineering background who appear to be hobbyists.  Most of the posters do not appear to have a basic understanding of LPG properties, so I thought I would throw a few items on the table:


Propane is a hydrocarbon fuel, chemical description C3H8.
Its boiling point is -44¦F (-42¦C)
Its octane rating is 104
Latent heat of vaporization = 183 BTUÆs / lb (426 kJ/kg)
91,500 BTUÆs per gallon (25300 kJ/L)
Autoignition temperature 855¦F (457¦C)
Stoichiometric by weight = 15.5:1
Molecular weight = 44.09
Carbon % by weight = 89%
Hydrogen % by weight = 18%
Flammability limits = 2.1 û 9.6%
Viscosity at 68¦F = 0.592 BTU/lb ¦F (2.48 KJ/Kg ¦K)
Expansion rate = 270:1 (expands in volume 270 times from liquid to ambient pressure vapor)
LPG is auto-refrigerating, when pressure is reduced, it boils by absorbing heat

PropaneÆs vapor pressure (the amount of pressure required to keep LPG liquid at ambient temperatures) is zero at -44¦F (-42¦C), about 120 psig at 70¦F, about 250 psig at 125¦F, and close to 400 psig at 160¦F.
Questions arise where these temperatures may be reached, but simply said, anytime the engine is running, underhood temps can quickly reach 175¦F, making LPG boil at any pressure under 450 psig.

For example:
Considering a liquid LPG injection engine:
If the ambient temperature is 100¦F, the internal tank pressure will be about 175 psig.  If LPG is taken directly out of the tank in liquid form and plumbed to a fuel injector, it would work just fine.  Now, add in underhood temperatures and the relative pressure must be increased to at least the vapor pressure plus enough pressure to retain liquid state.  This is usually about 3 BAR (about 50 psig) plus tank vapor pressure.  The LPG in any proximity to the underhood temperatures will begin to percolate, causing a constantly varying mixed-property fuel at the injector, something that is impossible to meter or calculate.

To prevent this from happening, fuel is typically circulated through the fuel rail, at pressures 50 psig above vapor pressure.  This flushes the vapor slurry and returns it to the fuel tank, where the added heat is absorbed by the tank and storage fuel.  This allows the fuel to return to liquid state, but the side effect is that the fuel tank temperature is slowly increasing, causing the entire fuel system to increase in pressure.  I have seen fuel injector pressures at 400 psig on a very hot day on a vehicle operating in mixed mode service.

Common gasoline injectors cannot withstand these pressures, nor can they meter LPG efficiently.

LPG has no lubricating qualities, so operating a gasoline injector on vapor LPG will quickly wear the internal components.  Also, operating a gasoline fuel system is at a relatively constant pressure, whereas LPG may vary considerably.

Since LPG vapor is 270 times less dense than liquid LPG, a single fuel injector cannot meter both fuels.  Vapor will require a large orifice with high volumes and good injector control.  The weight of the internal injector components could be considerable, and moving these components quickly enough to properly meter fuel may be challenging.

In liquid mode, the injector must be precisely matched to the engine, with tiny orifices capable of opening at high pressures.  This also requires internal injector components to be strong enough to pull against the higher pressures, plus the injector must be capable of response time 270 times more precise than vapor phase.  Also, the injector programming tables must be able to operate much more precisely (reduced MS response time) than either gasoline or vapor phase.

On the fuel systems that use the gasoline PCM, the injectors are sized for liquid flow, frequently without any intercept modules, plus the fuel systems use staged flow recirculation for the fuel rails.  I have not seen any vapor service systems using a straight gasoline PCM with no intercepts to change the injector profile.

I hope this has been helpful, but it only scratches the surface.

Franz

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