Why bother?
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
