|
| |
Energy content
Gasoline contains about 34.6 megajoules per litre (MJ/l) or 131 MJ/US gallon.
This is an average, gasoline blends differ, therefore actual energy content
varies from season to season and from batch to batch, as much as 4% more or less
than the average, according to the US EPA.
Volumetric energy density of some fuels compared with gasoline:
Fuel type MJ/l MJ/kg BTU/Imp gal BTU/US gal Research octane
number (RON)
Regular Gasoline 34.8 44.4 150,100 125,000 Min 91
Premium Gasoline Min 95
Autogas (LPG) (60% Propane + 40% Butane) 26.8
Ethanol 23.5 31.1 101,600 84,600 129
Methanol 17.9 19.9 77,600 64,600 123
Gasohol (10% ethanol + 90% gasoline) 33.7 145,200 120,900 93/94
Diesel 38.60 45.41 166,600 138,700 25(*)
Aviation gasoline (high octane gasoline, not Jet fuel) 33.5 46.8 144,400 120,200
Liquefied natural gas 25.3 ~55 109,000 90,800
(*) Diesel is not used in a gasoline engine, so its low octane rating is not an
issue; the relevant metric for diesel engines is the cetane number
A high octane fuel such as LPG has a lower energy content than lower octane
gasoline, resulting in an overall lower power output at the regular compression
ratio an engine ran at on gasoline. However, with an engine tuned to the use of
LPG (ie. via higher compression ratios such as 12:1 instead of 8:1), this lower
power output can be overcome. This is because higher-octane fuels allow for a
higher compression ratio - this means less space in a cylinder on its combustion
stroke, hence a higher cylinder temperature which improves efficiency according
to Carnot's theorem, along with fewer wasted hydrocarbons (therefore less
pollution and wasted energy), bringing higher power levels coupled with less
pollution overall because of the greater efficiency.
The main reason for the lower energy content (per litre) of LPG in comparison to
gasoline is that it has a lower density. Energy content per kilogram is higher
than for gasoline (higher hydrogen to carbon ratio). The weight-density of
gasoline is about 737.22 kg/m3.
Different countries have some variation in what RON (Research Octane Number) is
standard for gasoline, or petrol. In the UK, ordinary regular unleaded petrol is
91 RON (not commonly available), premium unleaded petrol is always 95 RON, and
super unleaded is usually 97-98 RON. However both Shell and BP produce fuel at
102 RON for cars with hi-performance engines, and the supermarket chain Tesco
began in 2006 to sell super unleaded petrol rated at 99 RON. In the US, octane
ratings in fuels can vary between 86-87 AKI (91-92 RON) for regular, through
89-90 (94-95) for mid-grade (European Premium), up to 90-94 (RON 95-99) for
premium unleaded or E10 (Super in Europe)
| |
|
