About three years ago, we dealt intensively with the subject of boat engines that have been converted to run on liquid gas. In BOOTE 8/08, we described the conversion of two 300 hp MerCruiser V-Eights in a Sea Ray 330 DA. These engines were equipped with carburettors and had an LPG system with vaporisers (converts the LPG into a gaseous state) and mixer, which sits on the carburettor and, depending on the switch position, only allows air to pass through for carburettor operation or a mixture of gas and air in gas operation.
However, this type of system has a weak point in engines with single-circuit cooling: for the evaporator to work properly, it needs the engine's warm cooling water. As it operates at a temperature of around 65 °C in single-circuit cooling systems, the evaporator system runs quite "cool" and there is a risk that the evaporator will freeze up at high engine speeds and no longer supply enough gas to the mixer.
In addition, the vaporiser components wear excessively. If an engine with carburettors is to be converted to LPG, it would have to be equipped with dual-circuit cooling, which works with higher cooling water temperatures in the inner circuit. Disadvantage: Most petrol engines are equipped with single-circuit cooling and must first be converted, which incurs extra costs.
For owners with new petrol engines and injection systems (multipoint), there is a more modern option. Instead of the vaporiser and mixer, special gas nozzles are used next to the petrol injection valves. Up to these nozzles, the liquid gas is not transported in gaseous form, but still in liquid form at a pressure of around 5 bar.
It only changes its state to gaseous at the nozzle outlet. This creates cold, which has a positive effect on the gas-air mixture and, together with the electronic control system, ensures optimum combustion. Naturally, these systems also have many protective mechanisms that guarantee a high level of safety.
As LPG does not have the same additives as petrol, an additive must be used to ensure valve-friendly operation, especially at high engine speeds. The conversion company "fahrmitgas.de" does this with a product called P1000, which is injected into the intake tract. Mizu, another conversion company, works with a petrol component in the LPG/air mixture. This means that not only gas is used for combustion, but also a proportion of petrol, which inherently contains the valve-protecting additives.
For which engines are these conversion kits available? In principle, all modern fuel-injected petrol engines can be fitted with them, regardless of the manufacturer. The company Mizu, which is particularly active in the Lake Constance region due to the exhaust gas regulations (which are easily met with the gas systems), has even converted 100 hp outboards.
However, small engines would make little sense due to the conversion costs. The prices for such a system, including a special LPG tank, are between around 3000 and 6000 euros. Costs that have to be recouped first. Although a litre of LPG costs less than half the price of petrol, the engine often needs a little more LPG than petrol, and the operating hours of a normal motorboat driver are around 25 hours, so the conversion only pays off after years.
Frequent drivers, on the other hand, quickly get their "confirmation" for the conversion. Incidentally, it is not always just the savings that count, but also environmental considerations, as engines that run on LPG emit fewer pollutants.
One often hears the objection that there is no good petrol station network. The network is not "intoxicating", but progress can be seen here too, and new petrol stations are constantly being opened, such as the new LPG filling station in Gaienhofen on Lake Constance.
Further information: fahr mitgas.de, Rudolf-Diesel-Str. 1, 63322 Rödemark and Mizu, Weidgang 3-5, 78247 Hilzigen.
