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Archive for April, 2012

LPG – liquid petroleum gas.

Most of us have probably heard about it, let’s see how production cars are converted to it, and what is usually done wrong.

I will talk about the 4th generation LPG system, where extra holes are made in the intake manifold and an injector is added for each cylinder.
Technically the system is pretty simple…

How it works

From a mechanical standpoint we have a high pressure tank, from which the liquid gas goes through an evaporator, that takes it to it’s vaporous form. From the evaporator it is distributed to the injectors via a common rail.
After the injectors are nozzles, which are sized to a certain size, according to calculations. So the same injectors can be used for many different applications and the injected quantity is adjusted by sizing the nozzles.
The evaporator also serves as a pressure regulator for the fuel, and is usually connected to a vacuum line in the inlet manifold.

Electronically most LPG ECU‘s are fairly simple. The cars petrol injectors are re-wired through the LPG unit, and the LPG unit just gets the reading from the main ECU in regards to the injector pulse. After it gets the start of the injector pulse it can correct it to be longer or shorter. It can also switch the gasoline injectors on or off.
The LPG ECU tends to have a gas pressure and temperature sensor and also allows for correction based on LPG temperature, as the density of a vapor fuel is affected quite a bit by temperature.

The way these systems are normally installed, is that they are set up to turn on only after a certain engine temperature is reached (the car only starts on petrol), and then it is calibrated with a narrowband sensor so, that the mixture is roughly stoichiometric in the cruise area. The ignition timing is not touched in most installs, and the car feels down on power. The nozzles are also rarely big enough for the car to work correctly in the entire range.

How it should be done in an ideal world

The best way is to use the additional LPG ECU to only provide correction for fuel temperature and switching settings. The engine management should be done by the already existing ECU by facilitating “map switching” based on whether LPG is used or not. This means that a signal should be taken and fed into the ECU.

A real world example

The car I will talk about is an old Audi V8, with the PT 3.6 liter engine, 2nd generation Motronic management with dual distributors and a 4th generation “Tornado” LPG system.

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