Thread: LPG Fumigation in diesels revisited

Originally Posted by Dougal

So crunching some numbers.

100kw of diesel power with 19% substitution.

On diesel using 230 g/kwh that's 23kg (27 litres) per hour.

With 81% diesel that's 18.6kg (21.9 litres) per hour.
LPG would be displacing 5.1 litres/hour of diesel (54.7 kW of energy) with 2334 litres/hour (atmospheric pressure) or 4.2 kg/hour of gas.

How does the price in Aussie work out for 5.1 litres of diesel vs 4.2kg of gas?

If it actually worked that way on light duty passenger type engines I would not have sold a single conversion after the first few. All the owners that keep consumption figures report at least a direct substitution of litres of diesel for litres of LPG as pumped, at around 20% average substitution. That is during cruising, not using the extra power unless needed for overtaking etc. The LPG effectively advances the diesel timing, as an accelerant (so I've read). Therefore the enhancement extracts more energy from the diesel than burning by itself. This effect diminishes quickly over 20% substitution (by liquid volume).

As as a simple test, I converted a Pajero 2.8 turbo and drove it up our local freeway incline at 100km/h in 5th. To keep power output even I engaged the cruise control and watched the EGT. Diesel only ran at 650C, diesel + LPG ran at 625C. I swapped modes 7-8 times to watch for altitude changes (150-550m).

Sorry but I don't have any dyno graphs etc for you to chew on, I just do it because it works as advertised.

BTW LPG sells for around 1/2 the price of diesel, per litre, on average over a year. Cost savings of 10% are normal and expected.

Originally Posted by Dougal

What's the litre-kg conversion for LPG again?

I've only bought lpg by the kg for my bbq.

Weight is a fairly precise way of purchasing it.

The contents of an unused 9kg cylinder would vary by around 1% for every 6deg variation in its temperature ( going on memory with that one) so the industry uses 15deg C as its benchmark. At that temp, there is 1968 litres to the tonne ( or 1.968 litres to the kilogram) so roughly half the weight of water, A bowser calibrates it & charges out @ 15 deg.
So a 100 litre tank filled in Alice Springs ( to 80%) in mid summer might take 75 litres & the same tank filled at Bright in mid winter might take 85 litres. The truth is, it takes 80 litres every complete fill but the bowser compensates for the density - therefore the energy value charged is accurate.
Here endeth the lesson