Thread: Cheap to Run

Gas-fueled engine

In engines running exclusively on LPG or CNG, the Compression Ratio may be higher, due to the higher octane rating of these fuels.

[ame="http://en.wikipedia.org/wiki/Compression_ratio"]Compression ratio - Wikipedia, the free encyclopedia[/ame]

but at 4:1 compression I'm surprised it runs at all


Martyn

4.4 crate motor running 4:1 compression - um doesn't sound quite right to me

Tell him he's dreaming

From memory LPG is 105 Octane rating, so he is quite correct, however that is only a quarter of the story as its the energy content that is relevant & LPG is almost half the weight of Diesel/ Petrol per litre- looking at Wikpedia tells the story.

LPG has a typical specific [ame="http://en.wikipedia.org/wiki/Calorific_value"]calorific value[/ame] of 46.1 MJ/kg compared with 42.5 MJ/kg for diesel and 43.5 MJ/kg for premium grade petrol (gasoline). However, its energy density per volume unit of 26 MJ/l is lower than either that of petrol or diesel.

Gasoline contains about 32.0 mj/L (9.67 kWh/L,

When burnt, diesel typically releases about 38.6 MJ/l (138,700 BTU/US gal), whereas gasoline releases 34.9 MJ/l (125,000 BTU/US gal), 10% less by [ame="http://en.wikipedia.org/wiki/Energy_density"]energy density[/ame], but 45.41 MJ/kg and 48.47 MJ/kg, 6.7% more by [ame="http://en.wikipedia.org/wiki/Specific_energy"]specific energy[/ame].



So, the energy values are LPG is 26, Diesel 38.6 and petrol either 32 or 34.9 Mj/Litre