Can Diesel engines be converted to run on LPG?


There are currently two ways of using LPG as a fuel for a Diesel engine;


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


Lack of Spark Ignition

Diesel engines do not have this.

Spark ignition 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 Diesel injectors were 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. Leaving the Diesel injectors in place would bring no benefit when the next factor is taken into account......

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 because two head 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, and it must be remembered that when both of these changes (provision for spark ignition and reduction of compression ratio) have been made, the engine isn't a Diesel any longer.


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 converted to run on LPG. The revised engine has to'rev' more when running on LPG because 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 has been fully reworked, the crankshaft, bearings and connecting rods (to mention but a few components) will suffer higher stresses at increased RPM necessary when running on 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 cannot be a true Diesel or even a dual fuel engine as it will have lost its higher compression ratio and the means to inject Diesel.

Conclusion

The above factors combine (along with many others not discussed here) to make actual conversion of most Diesel engines uneconomic. It would be simpler and quicker to fit a Petrol engine.


2. To mix LPG with the existing Diesel fuel before induction (Fumigation, not Conversion)


Various attempts have been made to achieve this with varying 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 the low teens of %, or thereabouts.

There are other problems to consider as well - The unmodified Diesel engine was relatively slow-revving, producing its maximum torque at lower RPM than a similar Petrol version. This is not the case when it is converted to run on Diesel and LPG mix. The revised engine has to'rev' more when running on Diesel / LPG mix because its maximum torque will have been moved higher up the rev. band. This can bring new problems of reliability and longevity. The crankshaft, bearings and connecting rods (to mention but a few components) were all designed to rev. at a lower rate. These components will suffer much higher stresses (stress increases at the square of RPM) at the increased RPM necessary to get sufficient torque when running on LPG. Mechanical breakdown may result in far less time, whilst increased wear and reduced component life are certain. Given the low overall savings achieved (to date) and the cost of the adaption ( 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.*

* Summer 2008 - We keep an open mind.

As world Diesel prices continue to rise sharply, there may be more economic benefit in mixing LPG with Diesel. We are still waiting to see a system that works well and comes with audited figures for reliability and the cost savings achieved. If such a system comes along, with UK LPG accreditation, we'll do some long-term evaluation before a decision is made to offer it to our customers.



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 classic Diesel 'Knock' can be greatly reduced. 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 also get up to its optimium temperature more quickly, whilst harmful emissions like Particulates and Carbon Monoxide can be reduced. These all appear to be benefits.

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 the new, higher levels. As a result of this apparent improvement in performance, one of the best attributes of the Diesel engine (relative longevity and reliability) is dramatically reduced by the Diesel / LPG 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. That doesn't mean that the people working on these projects should stop, we hope they succeed. Perhaps their research will continue and go on to give us all a better solution.

Overall Conclusion

Above it has been shown that niether true conversion of a Diesel engine nor the mixing (Dual fuelling, Fumigation) of LPG with Diesel give economic benefits large enough to make either choice worthwhile for the average motorist. 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, to date, is a qualified 'No'.


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