Gasoline contains about 35 MJ/L (9.7 kW·h/L, 132 MJ/US gal, 36.6 kWh/US gal) (higher heating value) or 13 kWh/kg. This is an average; gasoline blends differ, and therefore actual energy content varies from season to season and from batch to batch, by up to 4% more or less than the average, according to the US EPA. On average, about 19.5 US gallons (16.2 imp gal; 74 L) of gasoline are available from a 42-US-gallon (35 imp gal; 160 L) barrel of crude oil (about 46% by volume), varying due to quality of crude and grade of gasoline. The remaining residue comes off as products ranging from tar to naptha.
Volumetric and mass energy density of some fuels compared with gasoline (in the rows with gross and net, they are from ):
Diesel fuel 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 liquefied petroleum gas (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 (i.e. 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 combustion efficiency. Also, increased mechanical efficiency is created by a higher compression ratio through the concommitant higher expansion ratio on the power stroke, which is by far the greater effect. The higher expansion ratio extracts more work from the high pressure gas created by the combustion process. The applicable formula is PV=nRT. An Atkinson cycle engine uses the timing of the valve events to produce the benefits of a high expansion ratio without the disadvantages, chiefly detonation, of a high compression ratio. A high expansion ratio is also one of the two key reasons for the efficiency of Diesel engines, along with the elimination of pumping losses due to throtttling of the intake air flow. A high compression ratio can be viewed as a necessary evil in order to have a high expansion ratio.
The lower energy content (per litre) of LPG in comparison to gasoline is due mainly to its lower density. Energy content per kilogram is higher than for gasoline (higher hydrogen to carbon ratio). The weight-density of gasoline is about 740 kg/m³ (6.175 lb/US gal; 7.416 lb/imp gal).
Different countries have some variation in what RON (research octane number) is standard for gasoline, or petrol. In Finland, Sweden and Norway, 95 RON is the standard for regular unleaded petrol and 98 RON is also available as a more expensive option. 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 unleaded fuels can vary between 86-87 AKI (91-92 RON) for regular, through 89-90 AKI (94-95 RON) for mid-grade (European Premium), up to 90-94 AKI (95-99 RON) for premium (European Super).
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