When an engine is converted to run on LPG as a single fuel engine the ignition timing should be revised to give more ignition advance than the petrol engine at low speeds, and less than the petrol engine at higher speeds. This means that conventional distributors must have adjustments made to the mechanical advance mechanism to reduce the rate that advance increases with speed. Merely altering the static timing is not correct.
In the case of a dual fuel conversion the matter is complicated because the ignition timing needed for optimum performance on petrol will give too much advance at high speeds when LPG is used. The difference could be as much as 10 degrees, see graph below. This graph shows typical timing for a petrol engine tuned for petrol only, a correctly calibrated LPG setting and compares these with incorrect timing for LPG achieved when the distributor has been advanced with no adjustment for rate, "Petrol+adv".
These graphs show the following;
Ignition Switching Devices
The best way to be sure that the ignition is optimised for both fuels is to install an ignition switching device, known also as "Dual Curves". These units are wired into the ignition system and switch automatically to the LPG or petrol setting when the fuel switch is activated. They will give more initial advance than the petrol setting when the engine is running on LPG, and as speed increases they will give less advance than the petrol setting. Although these are usually pre-calibrated for a range of applications (i.e. not necessarily calibrated for a particular engine) they will give a much better approximation to the ignition requirements for engines being converted than the incorrect practice of merely altering the static timing.
Small Engines In Commercial Applications
It can be seen from the graph "LPG" that there is less advance needed at higher operating speeds for any engine running on LPG. For larger more powerful vehicles operating in city areas the engine is seldom operating at maximum power so is possibly going to be less liable to damage as a result of incorrect ignition calibration.
However smaller engines fitted to delivery vans operating in city areas will usually be operating at higher loads and frequently at maximum throttle. These applications are typically Mazda FE and MA engines in light commercial applications and they work much harder due to the weight of the vehicle and even the way in which these vehicles are driven. If they are merely given some additional static advance (as often seems to be the case) the error is exaggerated at higher speeds because there was already too much advance at high speed when the ignition was set as standard. Referring to the graph again, note the error that would occur if the LPG engine is operated on the petrol calibration at high speed. That is bad enough, but now if the static timing is advanced as per curve "Petrol+Adv" (i.e. by merely advancing the distributor) then the error is doubled!
For this reason it is even more critical that any dual fuel LPG conversion in such an application includes a dual curve ignition switching device. (There is one made by AEB and available from major LPG conversion companies).
Nevertheless it is the responsibility of the customer to ensure that the engine is not damaged by continued operation with ignition settings not optimised for the fuel being used.
LPG fuels vary far more from state to state, area to area, and from time to time than petrol. The variation is often due to the proportion of propylene in the fuel which is based mainly on propane and butane. Propylene depresses the octane number. The octane value of LPG is usually regarded to be 100 to 105 RON. However it is understood that octane levels in some cities are often as low as 96 which is the same as super grade leaded petrol. Consequently there can be abnormal combustion problems (i.e. detonation) in LPG fuelled engines where the compression ratio has been raised to take advantage of the supposedly higher octane number of LPG. Also there could be problems in these engines where the compression ratio is standard, (or slightly increased as a result of reconditioning) where the ignition has been incorrectly advanced at static only. Remember that for every degree of mechanical advance you add the initial static advance to it. In other words if the distributor mechanical advance is providing 4 degrees at 600 RPM and the static timing is 6 degrees BTDC the total advance that the engine sees at 1,200 RPM is 14 degrees. Also if the compression ratio of an engine is raised for any reason the amount of ignition advance that is needed to achieve best performance usually decreases and failure to take account of this can lead to detonation occurring.
Summary and Recommendations