INTERNAL COMBUSTION ENGINE (SKMV 3413)

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1 INTERNAL COMBUSTION ENGINE (SKMV 3413) Dr. Mohd Farid bin Muhamad Said Room : Block P21, Level 1, Automotive Development Centre (ADC) Tel : mfarid@fkm.utm.my

2 THERMOCHEMISTRY IC engine obtains their energy from the combustion of a hydrocarbon fuel with air. The combustion process converts chemical energy of the fuel to internal energy in the gases within the engine. This internal energy is then converted to the rotating crankshaft output by the mechanical linkages of the engine. There are many thousands of different hydrocarbon fuel components Consist mainly of hydrogen and carbon. May also contain oxygen (alcohols), nitrogen, sulfur, etc.

3 THERMOCHEMISTRY The maximum possible amount of chemical energy is released from the fuel, when it reacts with a stoichiometric amount of oxygen. Stoichiometric oxygen is just enough to convert all carbon in the fuel to CO 2 and all hydrogen to H 2 O with no oxygen left over. The balanced chemical reaction of the simplest hydrocarbon fuel (methane) burning with stoichiometric oxygen is CH4 2O2 CO2 2H2O If the isooctane is the fuel component, the balanced stoichiometric combustion with oxygen is C8H O2 8CO2 9 H2O

4 THERMOCHEMISTRY In balancing a chemical equation, molar quantities are used. m N M m N M mass number of moles molecular weight 1 kgmole of CH 4 = 16 kg 1 kgmole of O 2 = 32 kg

5 THERMOCHEMISTRY Air is used as the source of oxygen to react with fuel. Atmospheric air is made up of about 78% nitrogen (by mole) 21% oxygen 1% argon - Traces of CO 2, Ne, CH 4, He, H 2 O, etc. Nitrogen and argon are chemically neutral and do not react in the combustion process, but affect the temperature and pressure. To simplify calculations, argon is assumed to be combined with nitrogen. So, the atmospheric air can be modeled as 21% O 2 and 79% N 2.

6 THERMOCHEMISTRY For 1 mole of O 2, there are 0.79/0.21 moles of N 2. For every mole of O 2 needed for combustion, 4.76 moles of air must be supplied (1 mole of O 2 plus 0.79/0.21 moles of N 2 ). Stoichiometric combustion of methane with air is then CH 4 O2 2(3.76) N2 CO2 2H2O 2(3.76) 2 N 2 The combustion of isooctane with air is then C 8 H18.5 O2 12.5(3.76) N2 8CO2 9H2O 12.5(3.76) 12 N 2

7 THERMOCHEMISTRY The molecular weight of air 29 kg/kgmole Combustion can occurs: Stoichiometric Lean (more than stoichiometric air is present) Rich (less than stoichiometric air is present) If methane is burned with 150% stoichiometric air, the excess O 2 is found in the product. CH 4 O2 3(3.76) N2 CO2 2H2O 3(3.76) 3 N O 2 2

8 THERMOCHEMISTRY If isooctane is burned with 80% stoichiometric air, there is not enough O 2 to convert all the carbon to CO 2. So, CO is found in the products. C8H18 10 O2 10(3.76) N2 3CO2 9H2O 10(3.76) N2 5CO CO is a colorless, odorless, poisonous gas which can be further burned to form CO 2. CO is formed in any combustion process when there is a deficiency of O 2. Some of the fuel will not get burned when there is a deficiency of O 2. This unburned fuel ends up as pollution in the exhaust.

9 THERMOCHEMISTRY For actual combustion in an engine, the equivalence ratio is a measure of the FA mixture relative to stoichiometric conditions. FA act FA stoich AF stoich AF act 1, running lean, oxygen in exhaust 1, running rich, CO and fuel in exhaust 1,stoichiometric, maximum energy released FA AF m m a f m m f f m m a a mass of air mass of fuel - air ratio air - fuel ratio fuel SI normally operates with in the range of 0.9 to 1.2.

10 Example 1 Isooctane is burned with 120% stoichiometric air (theoretical air) in a small three-cylinder turbocharged automobile engine. Calculate: 1) AF ratio 2) FA ratio 3) Equivalence ratio

11 Crude oil was first discovered in Pennsylvania in Crude oil is made up almost entirely of carbon and hydrogen with some traces of other species. Crude oil Carbon : 83% ~ 87% Hydrogen : 11% ~ 14% Crude oil mixture from the ground is separated into components products by cracking and /or distillation using thermal or catalytic methods at an oil refinery. Cracking process of breaking large molecular components into more useful components of smaller molecular weight. Distillation to separate the mixtures into single components or smaller ranges of components.

12 The larger the molecular weight of component, the higher is its boiling temperature. Low boiling temperature components (smaller molecular weights) are used for solvents and fuels (gasoline). High boiling temperature components (large molecular weights) are used for tar and asphalt or returned to the refining process for further cracking.

13 Crude oil obtained from different parts of the world contains different amounts and combinations of hydrocarbon species. In US Pennsylvania crude (high concentration of paraffins, little asphalt) Western crude (high concentration of asphalt, little paraffins) Mideast crude (Lower molecular weight, almost gasoline)

14 Various components of different molecular weights will vaporize at different temperatures. Small molecular weights boiling at low temperature. Large molecular weights boiling at higher temperature. Small percentage of components that vaporize at low temperature is needed. This is to assure the starting of a cold engine. Fuel must vaporize before it can burn. Too much of this front-end-volatility can cause problems when the fuel vaporizes too quickly. Volumetric efficiency will be reduced if fuel vapor replaces air too early in the intake system.

15 Temperature-Vaporization Curve (Volatility Curve) for gasoline

16 Large percent of the fuel should be vaporized at the normal intake system temperature during the short time of the intake process. To maximize the volumetric efficiency, some of the fuel should not vaporize until late into the compression stroke. This is why some high molecular weight components are included in gasoline mixtures. If too much of this high-end volatility is included in the gasoline, some of the fuel never gets vaporized and ends up as exhaust pollution.

17 Sometimes, the way to describe a gasoline is to use 3 temperatures. 10% is vaporized 50% is vaporized 90% is vaporized The gasoline could be classified as C. There is found to be little difference in the volatility curves for a given season and location in the country. There is usually about a 5 0 C shift down in temperature on the vaporization curve for winter gasoline compared to summer.

18 Volatility Issues High Volatility gives: Good Cold Starting Good vaporization required for combustion Low Volatility gives: Good Volumetric Efficiency Prevents vapor lock fuel pump can not pump vapor

19 Hydrocarbon Components Paraffins Olefins Diolefins Acetylene Cycloparaffins Aromatics Alcohol

20 Paraffins: C n H 2n+2 HYDROCARBON FUELS

21 Olefins, C n H 2n One Double Carbon Bond

22 Diolefins, C n H 2n-2 Double Carbon Bond Acetylene, C n H 2n-2 One Triple Carbon Bond

23 Cycloparaffins, C n H 2n Single Bond Ring HYDROCARBON FUELS

24 Aromatics, C n H 2n-6 Carbon Bond Ring with Double carbon-carbon bonds

25 Alcohol Paraffins with one hydrogen atom replaced with the OH radical

Copyrighted by Gabriel Tang B.Ed., B.Sc.

Copyrighted by Gabriel Tang B.Ed., B.Sc. Unit 5: Chemical Equations and Reactions & Stoichiometry Chemistry Chapter 9: Stoichiometry 9.1: Calculating Quantities in Reactions Avogadro s Number: - a group of (6.0 10 ) molecules = 1 mole Stoichiometry: