Heatingvalue or energycontentHeating value of a fuel is the amountof heat released by combustion of a unit quantity of the fuel. The heatingvalue varies from higher to lower values in theorder of diesel, n-butanol, ethanol andmethanol.
Ethanol has very low heating value due to itshigher oxygen content (35% by mass) as compared to diesel (Table 2.1). Heating value ofn-butanol is between ethanol and diesel inaccordance with the amount of oxygen present init. In alcohol the heatingvalue of the fuel increases as number ofcarbon atom increases. Highermass of fuel has to be supplied forthe same heat input with ethanol and n-butanol ascompared to diesel.Density andviscosityN-butanolhas density and viscosity is about the same level asthat of diesel fuel.
Hence, there willbe no wear problems in sensitivefuel pump design due to insufficient Lubricity if it used in dieselengines. Density andviscosity both increasewith longer carbon chain. The densityand viscosity of ethanol and methanol is lower than diesel andn-butanol.Octanenumber and Self ignition temperatureResearch OctaneNumber (RON) of any fuel is generally defined asthe resistance to knock and it is MotorOctane Number (MON) which is called as its Antiknock Index(AKI). N-butanol has octane numberclose to gasoline fuel which makes it a viable fuel for SI engine application. However, octanenumber of ethanol and methanol higher thangasoline and n-butanol. Hence, they have higher knockresistance. Self-ignitiontemperature of ethanol is very high, which isone of the reasons for its better anti-knockcharacteristics.
Latent heatof evaporation and Vapour pressureLatent heatof evaporation of n-butanol is lower thanmethanol and ethanol so engine running with n-butanol fuelwill result in less cold startproblems. Volumetric efficiency ofengine increases with increase in latent heat of evaporation because of chargecooling effect. In alcohols as number of carbon atom increase vapourpressure decrease.
Vapour pressure of butanol isseveral times lower than that of gasolineand methanol. Hence, this means that cavitationand vapour lock problem will less in n-butanol. Asn-butanol has lower latent heat of evaporation and self-ignitiontemperature due to these it will have less ignition problems. Stoichiometricair fuel ratio and Mixture heating valueIn alcohols as theamount of oxygen in its molecule increase, stoichiometricair fuel ratio decrease. Stoichiometricair fuel requirement of ethanol is half that ofdiesel, hence for a given equivalence ratiohigher amount of fuel need to supply. This result in mixtureheating value of alcohols is about the same level of dieselfuel even though their calorific values are low.N-butanol stoichiometric air fuel requirement is little lower than diesel fuel. Flammabilitylimits, adiabatic flame temperature and flamespeedAlcoholshave wider flammability limits than diesel and gasoline and this helpsin lower cycle by cycle variations inalcohols as compared to gasoline.
N-butanol has loweradiabatic flame temperature than diesel which will have a positive effect inreduction in NOx emission. However, among all alcohols methanol have lowestadiabatic flame temperature. N-butanolflame velocity is in between ethanol and gasoline which willresults in fastercombustion and there by leading to higher brake thermal efficiency (BTE).Corrosivenessand Affinity towards waterN-butanol is lesscorrosive as compared to methanol and ethanol so it will cause less impact ondurability of engine components. Thereforen-butanol can be supplied through existing pipeline. Ethanol andmethanol has higher affinity towards water absorption as compared ton-butanol. 2.2.
2. Typesof butanol isomers and theirpropertiesAlcoholsare defined by presence of hydroxyl group (–OH) whichis attached to one of the carbon atoms. Butanol has 4-carbonatoms in its molecule and based on it can form either a straight chain ora branched structure and this results in differentproperties. Butanol has four isomersbased on carbon chain structure and the location of hydroxyl group (–OH). 1-butanol,also better known as n-butanol, it has astraight-chain structure and the hydroxylgroup (–OH) is attached at the terminal carbon. Sec-butanol, alsoknown as 2-butanol, is also a straight-chainalcohol but in this hydroxyl group (–OH) is attached with aninternal carbon.
Iso-butanol is a branched isomer andhydroxyl group (–OH) is attached at the terminal carbon andtert-butanol refers to the branched isomer in this hydroxyl group (–OH)is attached with an internal carbon. The mainphysical properties of butanol isomers are summarized in Table 2.2.