Fuels used in IC engine
In IC engines, the chemical energy contained in the fuel is converted into mechanical power by burning (oxidizing) the fuel inside the combustion chamber of the engine.
As a result of the chemical reactions which occur inside the cylinder, heat is released. The fuel-air mixture (the working fluid before combustion) must stay in the cylinder for a sufficient time so that the chemical reactions can be completed.
Fuels used in IC engines
• During combustion these bonds are broken and new bonds are formed with oxygen atoms, accompanied by the release of chemical energy. Principal products are carbon dioxide and water vapour.
Fuels also contain small amounts of
O2 , N2 , S , H2O.
The carbon atoms can be arranged as, a straight chain or as branched chain compounds.
Straight chain group (normal paraffins)
• shorter the chain, stronger the bond
not suitable for SI engines – high tendancy for autoignition
• According to the value of “n” in the formula, they are in gaseous (1 to 4), liquid (5 to 15) or solid (>16) state.
Branched chain compounds (isoparaffins)
when four or more C atoms are in a chain molecule it is possible to form isomers – they have the same chemical formula but different structures, which often leads to very different chemical properties.
example : iso-octane
2. 2 .4 trimethyl pentane.
• Also called cycloparaffins.
• saturated hydrocarbons which are arranged in a circle have stable structure and low tendancy to autoignite compared to alkanes (normal paraffins).
• can be used both in SI-engines and CI-engines
low heat content and high density (740 – 790 kg / m3).
CH2=CH.CH2.CH2.CH3 called pentene-1
• Olefinic compounds are easily oxidized, have poor oxidation stability .
• Low heat content and density in the range 620 – 820 kg / m3.
• They are based on a six-membered ring having three conjugated double bonds.
• Aromatic rings can be fused together to give polynuclear aromatics, PAN, also called polycyclic aromatic hydrocarbons, PAH
simplest member is benzene.
• Can be used in SI-engines, to increase the resistance to knock.
• Not suitable for CI-engines due to low cetene number.
• Low heat content and high density in the range 800 – 850 kg / m3.
• The OH group which replaces one of the H atoms in an alkane, gives these compounds their characteristic properties.
• Specific heating value is lower than gasoline (42 – 43 MJ/kg).
• methanol (19.7 MJ/kg) and ethanol (26.8 MJ/kg) for air-fuel mixture s.h.v. is comperable with gasoline (MJ/kg-mixture at stoichiometric mixtures).
• Other alcohol groups such as dihydric and trihydric alcohols are not used as a fuel in IC engines.
• can be obtained from natural gas – has near and long-term potential.
• has high octane quality (130 RON, 95 MON).
• can be used in low-concentration (5-15 %) in gasoline to increase octane number of the mixture.
• has high octane number – can be used in low–concentrations in gasoline
• due to storage and transportation problems they are not widely used.
• reduce volumetric efficiency and power output of engine (5 – 10 %).
• have low tendancy to knock and low emissions.
LPG (liquefied petroleum gas)
• provides good mixture with air – cleaner combustion.
• has excellent cold weather performance
low sulphur content.
• high octane number (propane 111 RON and 100 MON).
• lower density and lower heat content of LPG versus gasoline (23.5 MJ/liter for propane and 32 MJ/liter for gasoline).