The IC Engine: Why… By: Matthew King

Outline A Short History Background Why… How do we compare engines and their cycles?

History The internal combustion engine was first conceived and developed in the late 1800’s The man who is considered the inventor of the modern IC engine and the founder of the industry is pictured to the right….Nikolaus Otto (1832-1891). Otto developed a four-stroke engine in 1876, most often referred to as a Spark Ignition, since a spark is needed to ignite the fuel air mixture.

History The impact on society is quite obvious, all most all travel and transportation is powered by the IC engine: trains, automobiles, airplanes are just a few. The IC engine largely replaced the steam engine at the turn of the century (1900’s) Another important cycle is the Diesel cycle developed by Rudolph Diesel in 1897. This cycle is also known as a compression ignition engine.

Background on IC Engines “An internal combustion is defined as an engine in which the chemical energy of the fuel is released inside the engine and used directly for mechanical work, as opposed to an external combustion engine in which a separate combustor is used to burn the fuel.”1 “IC engines can deliver power in the range from 0.01 kW to 20x10^3 kW, depending on their displacement.”2

Background on the Otto Cycle The Otto Cycle has four basic steps or strokes: 1. An intake stroke that draws a combustible mixture of fuel and air into the cylinder 2. A compression stroke with the valves closed which raises the temperature of the mixture. A spark ignites the mixture towards the end of this stroke. 3. An expansion or power stroke. Resulting from combustion. 4. An Exhaust stroke the pushes the burned contents out of the cylinder. To the right is an idealized representation of the Otto cycle on a PV diagram. http://www.rawbw.com/~xmwang/javappl/ottoCyc.html

Why… The Otto cycle IC engine has remained fundamentally unchanged, besides slight improvements, for over 100 years. Its’ popularity has continually increased because… Relatively low cost Favorable power to weight ratio High Efficiency Relative simple and robust operating characteristics Improvements are mainly lower emissions and higher fuel efficiency Low cost: due to manufacturability, and relative to other choices

Comparing Engines…. mep= work done per unit displacement volume Or average pressure that results in the same amount of indicated or brake work produced by the engine Scales out effect of engine size Two useful types: imep and bmep imep: indicated mean effective pressure -the net work per unit displacement volume done by the gas during compression and expansion bmep: brake mean effective pressure -the external shaft work per unit volume done by the engine

BMEP Based on torque: (4 stroke) (2 stroke)

Compare… Brake specific fuel consumption (bsfc) Measure of engine efficiency They are in fact inversely related, so a lower bsfc means a better engine Often used over thermal efficiency because an accepted universal definition of thermal efficiency does not exist Wdot b= brake power, rate at which work is done or power output of the engine Tou, torque, work done per unit rotation of the crank N=rotational speed of engine in rad/sec

bsfc bsfc is the fuel flow rate divided by the brake power We can also derive the brake thermal efficiency if we give an energy to the fuel called heat of combustion or, qc Bsfc is a valid measure of efficiency provided qc is held constant. Thus two different engines can be compared on a bsfc basis provided that they operate on the same fuel.

Compare… Volumetric Efficiency, ev The mass of fuel and air inducted into the cylinder divided by the mass that would occupy the displaced volume at the density ρi in the intake manifold Note it’s a mass ratio and for a 4 stroke engine For a direct injection engine The factor 2 accounts for the two revolutions per cycle The intake manifold density is used as a reference condition instead of the standard atmosphere, so supercharger performance is not included. For 2 stroke cycles a parameter related to volumetric efficiency called the delivery ratio is defined in terms of air flow only and the ambient air density instead of the intake manifold density. It is desirable to maximize volumetric efficiency of an engine since the amount of fuel that can be burned and power produced for a given engine displacement is maximized. (size and weight) The VE depends on intake manifold configuration, valve size, lift, and timing.

Other comparisons… First law analysis- energy conservation For a system open to the transfer of enthalpy, mass, work, and heat, the net energy crossing the control surface is stored into or depleted from the control volume Second Law Analysis – entropy conservation This approach takes into account the irreversibility that occurs in each process Another outcome of this analysis is the development of the usefulness of each type of energy (exergy)

References 1. Internal Combustion Engines, Colin R. Ferguson, John Wiley & Sons, 2001 2. Engines An Introduction, John L. Lumley, Cambridge University Press, 1999