1. To truly convert the engine by reducing its compression ratio and provide it with spark ignition.

Lack of Spark Ignition

This can be provided in several ways, but they all amount to the same thing. The most effective is to remove the cylinder head and machine spark plug holes into the places where the injectors used to be sited. This is only possible if there is enough 'meat' in that position. If not, the spark plug holes may have to be placed elsewhere in the combustion chamber. The existing holes for the original Diesel injectors may either be plugged or perhaps be used for direct LPG injection.

Reduction of Compression Ratio

This must be done before the engine could be run properly on LPG or Petrol for that matter. Diesels often run at a compression ratio of roughly 16-1 (one of the reasons they are more economical) whereas Petrol and LPG engines need a ratio of around 10-1. One way of reducing the compression ratio is to fit some form of spacer inbetween the cylinder head and cylinder block mating surfaces in order to reduce the compression ratio of the engine. This may present problems of gasket sealing and note that two gaskets would have to be used. Another solution is to replace the original pistons for redesigned ones with different (deeper) crown depressions, addressing the compression ratio problem, but now the costs are beginning to escalate......

Design Limits, Stresses and RPM

Although the Diesel engine is relatively slow-revving and produces its maximum torque at lower RPM than a similar Petrol version, this is not the case when it is run on a mix of Diesel and LPG. The engine has 'rev' more when running on Diesel plus LPG as its maximum torque has been pushed higher up the rev. band. This can bring new problems of reliability and longevity. Even if the 'top end' of a Diesel is fully reworked, the crankshaft, bearings and connecting rods (to mention but a few components) will suffer higher stresses at increased RPM necessary to get sufficient torque when running on Diesel plus LPG. Mechanical breakdown may result in far less time, whilst increased wear and reduced component 'life' are certain. FinaIly, note that in all of the above cases the converted engine will NOT be dual fuel as it will have lost its means of injecting Diesel.

The above factors combine (along with many others not discussed here) to make the full conversion of most Diesel engines uneconomic.

2. To mix LPG with the existing Diesel fuel before induction (Fumigation)

Various attempts have been made to do this, and with differing amounts of success. Go LPG! have examined many systems and installations and after much consideration have come to this conclusion;

Overall, the Savings are Not Huge...

None of the LPG / Diesel mix systems examined have resulted in Diesel consumption being reduced by more than 25%. Those savings made must also have LPG costs (for the LPG that replaces some of the Diesel fuel consumed) subtracted from that 25% saving. This results in final savings of only 18% or thereabouts. Given the low overall savings achieved (to date) and the cost of the adatption ( often equal to that of an injected Petrol engine conversion) many miles would have to be covered before any real savings are realised whilst reliability has been reduced. This does not seem to be an economically viable alternative.

Benefit becomes Burden

There are some other benefits apart from saving fuel cost.

The Diesel engine becomes quieter and more responsive when using the LPG / Diesel mix. The main reason for increased smoothness and reduced noise (vibration) is that the LPG element begins its combustion before the Diesel fuel does, a result of 'detonation' due to the compression ratio being so high. The engine may get up to its optimium temperature more quickly whilst harmful emissions like Particulates and Carbon Monoxide are reduced.

Sadly, a new set of problems arise when the Driver begins to use the increased performance of an engine that wasn't designed to rev to higher levels and deliver its maximum torque in a wholly different area of the rev. band. As a result, one of the merits of the Diesel engine, its relative longevity, has been seriously reduced by the adaption.

Conclusion

On a purely fuel cost-reducing basis this adaption or fumigation system does not appear to be an attractive or useful alternative for the average Diesel motorist. It is hoped that research will continue and hopefully a better solution will be found.

With current LPG and Diesel technology as it stands, it would be more economical to simply remove the Diesel engine and fit a Petrol equivalent, which answers our original question -
'Can Diesel engines be converted to run on LPG?'

The answer is a qualified 'No'.