Engine power LPG vs. Petrol 1

[Deividas]

Hello, i'm thinking, why petrol engines (most of them) have less power, when running on lpg (sequential injection)? It's because lpg has lower energy density (lpg propane 25.3MJ/L, lpg butane 27.7MJ/L vs. petrol 34.2MJ/L)? But LPG has higher specific energy: lpg propane 49.6MJ/kg, lpg butane 49.1Mj/kg vs. petrol 46.4MJ/kg. In other forum i read, that gas displace more air in intake manifold or engine than petrol, is it psoible? And LPI (liquid propane injection) realy can solve this problem?

 

[BrianPetersen]

Propane (C3H8) is a smaller molecule than the "average" gasoline molecule, which is somewhere near 8 - 10 carbon atoms on average.

Therefore, once it is in gaseous form, a greater number of those molecules are necessary in order to reach stoichiometry ... and that displaces some of the air volume.

And ... with a gasoline engine, by the time the intake valve closes, a fair portion of the fuel is still in liquid form (displacing a negligible amount of air).

AND ... With most propane fuel systems, the fuel is already vaporized by the time it mixes with the air. The evaporative cooling possibility has been lost. Gasoline injected in liquid form before the intake system can have a significant evaporative cooling effect if the intake system is designed to make use of it (and not all of them are).

A propane molecule (C3H8) requires seven oxygen molecules to be burned completely to CO2 and H2O. Stoichiometric mixture of propane gas in air will have about 3% of the air volume displaced by propane vapor. The lack of evaporative cooling probably has around the same result on top of this.

Gasoline engines can make use of the liquid gasoline to help keep the intake valve cool. Can't do that with gaseous fuel.

Injecting propane in liquid form would allow the engine to make use of evaporative cooling.

 

[Deividas]

And if some air is displaced by propane, mixture goes richer, and not all propane is burned, and that's because engine loss power or what?

 

[dgallup]

Power is mainly a function of how much air (oxygen) an engine can pump. If some of the air is displaced by the gaseous propane then you are pumping less oxygen and the power is reduced. It's not a matter of the mixture being incorrect. There are liquid propane injection systems that do not lose power compared to gasoline. When the liquid propane vaporizes it substantially cools the intake mix keeping volumetric efficiency high.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[gruntguru]

Switching to propane with no other changes will also reduce power due to the slower burn. Propane needs additional ignition advance.

If the engine is dedicated propane (not dual fuel) further power and efficiency can be recovered by increasing CR to take advantage of propane's higher octane rating.

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