IC Engines overviewAir and fuel mixture flows through inlet valve and exhaust leaves through exhaust valveConvertsreciprocating motion to rotary motion using piston and crank shaftTDC; Top Dead Center: Position of the piston where it forms the smallest volumeBDC; Bottom Dead Center: Position of the piston where it forms the largest volumeStroke: Distance between TDC and BDCBore: Diameter of the piston (internal diameter of the cylinder)Clearance volume: minimum volume formedCompression ratio: ratio of maximum volume to minimum volumeVBDC/VTDCEngine displacement = (# of cylinders) x (stroke length) x (bore area) (usually given in cc or liters)MEP: mean effective pressure: A const. theoretical pressure that if acts on piston produces work same asthat during an actual cycleWnet = MEP x Piston area x Stroke

= MEP x displacement volume 4 Stoke engine

Cycle consists offour distinct strokes (processes):IntakeCompression strokePower strokeExhaust

Otto cycle

Assumptions for Air standard cycle, as describe before:Fixed amount of air (ideal gas) for working fluidCombustion process replaced by constant volume heat addition with piston at TDCIntake and exhaust not considered, cycle completed withconstant volume heat removal with piston at BDC

All processes considered internally reversible

Air-Standard Otto cycleProcess 1- 2 Isentropic CompressionProcess 2 - 3 Const. volume heat additionProcess 3 - 4 Isentropic expansionProcess 4 - 1 Constant volume heat removal

Otto Cycle- indicator diagram of otto cycle

Otto, P-V and T-S diagram

Compression ratio

From previous definition, compression ratio = Since fixed mass:

1-2 Isentropic compression

Applying First law:

U2-U1 = Q - WinQ = 0 (since, reversible adiabatic compression)Win = U2-U1

2-3 Constant volume heat addition

Applying First law:

U3-U2 = +Qin -WW = 0 (since, it is a constant volume process)

Qin = U3-U2

3-4 Isentropic Expansion

Applying First law:

U4-U3 = Q -WoutQ = 0 (rev. adiabatic expansion)Wout = U4-U3

4-1 Constant volume heat removal

Applying First law:U1-U4 = - Qout +WW = 0 (no piston work)Qout = U4-U1

Otto cycle thermal efficiency

The thermal efficiency is given by:

The specific heats are assumed to be constant.

Here y=1.4 at ambient temperature

For higher efficiency, higher compression ratios are required, as shown below.

However, increase in pressure ratios, would increase the air-fuel temperature above the temperature atwhich the mixture can auto-ignite.

This would result in 'engine-knock', reducing the performance of the engine. In order to avoid such situations,additives are generally added which increases the auto-ignition temperature.

4 Stroke CI engine

Cycle consists of four distinct strokes (processes) as in the case of SI engines, except that the spark plug isreplaced by a fuel injector- Intake- Compression stroke- Power stroke- Exhaust

Here the fuel is injected when the piston approaches TDC, ie when the air is at maximum temperature due tocompression.The combustion process starts now

The fuel is injected after the piston starts moving down The volume increases, on the other hand, the fuelevaporates to fill the volume. Thus keeping the pressure inside roughly the same.

Hence the combustion can be considered to occur at constant pressure.

Diesel Cycle

Assumptions for Air standard cycle, as describe before:- Fixed amount of air (ideal gas) for working fluid- Combustion process replaced by constant pressure heat addition- Intake and exhaust not considered, cycle completed withconstant volume heat removal with piston atBDC

- All processes considered internally reversible

Air-Standard Otto cycleProcess 1- 2 Isentropic CompressionProcess 2 - 3 Const. pressure heat additionProcess 3 - 4 Isentropic expansionProcess 4 - 1 Constant volume heat removal

Diesel T-S and P-V diagram

Three Volume Ratios

From previous definition:

Thermal Efficiency of Diesel Cycle

Given:

(1)

Process 1-2: Isentropic compression

(2)Process 2-3: Isobaric heat addition

(3)Process 3-4: Isentropic expansion

(4)Thermal efficiency

From 2, 3 and 4 all temperatures can be expressed in terms of T3.

Otto and Diesel Cycle Comparison For given rchigher thermal efficiency is obtained via higher compression ratiorv and for a given rv higherthermal efficiency is achieved by loweringthe cut-off ratio rc

However a smaller rc yields less net work per cycle, so to achieve the same power at lower rc values higherengine speeds are required.

Otto and Diesel cycle comparison

Therefore, the efficiency of the diesel cycle is less than that of the otto cycle for the same compression ration.However, the advantages of Diesel over petrol engines is that we can operate at higher compression ratioswithout auto ignition and fuel is less expensive.

Gas Turbine Power PlantsGas turbine power plants are lighter and compact when compared to power plants running on vapour cycles.The power to weight ratios are generally high for high throughout Gas turbine power plants and hence arefavoured for the aviation and also for power generation.A simple GT power plant is shown in the image below.Air is first compressedThe compressed air enters the combustion chamber where fuel is injected and burned, essentially atconstant pressureThe combustion products expand in turbine to the ambient pressure and thrown out to the surroundings.

Air Standard Brayton CycleBrayton cycle is the air standard for GT power plant.

Air is first compressed reversibly and adiabatically

Heat is added to it reversibly at constant pressure

Air expands reversibly, adiabatically in the turbine The heat is removed from the system reversibly atconstant pressure to bring it to original state

Brayton cycle therefore consists of two isobars and two reversible adiabatics (isentropics):

Air is first compressed reversibly and adiabatically

Heat is added to it reversibly at constant pressure

Air expands reversibly, adiabatically in the turbine The heat is removed from the system reversibly atconstant pressure to bring it to original state P-V, T-S diagram of ideal Brayton Cycle

1 - 2 Isentropic compression2 - 3 Constant pressure heat addition3 - 4 Isentropic expansion4 - 1 Constant pressure heat removal

Thermal efficiency:The thermal efficiency of the ideal Brayton cycle is

Since processes 1-2 & 3-4 are isentropic between the same pressures :-

Where rv is the pressure ratio

Hence, substituting in the efficiency expression

This is the efficiency for ideal Joule/Brayton Cycle.

Work Ratio

It may easily be shown from the expression,Work ratio =

And a similar approach to that above, that work ratio = What we deduce from the above equations above improvements that we might make?h is increased by :-increasing T3decreasing T4 orincreasing the pressure ratio

We also know that a high work ratio is desirable in order to minimize the effect of irreversibilities in real gasturbines. This depends on the temperature limits and the pressure ratio for constant gamma.

Consider the T-S diagram below for the ideal cycle & the dotted cycles.

T3 is usually fixed by metallurgical limits on turbine blading & T1 is the natural sink temperature for an idealcooler. The two dotted cycles show the limits of operation. Consider left hand dotted cycle. Here thepressure ratio is large & the cycle efficiency approaches the Carnot Efficiency ie T2 has been raised.

Unfortunately the net work output is approaching zero. The other dotted cycle has a reduced T2 & again network output is approaching zero. It can be shown that for an ideal cycle with fixed T1 and T3, the value of T2

for maximum work output is:

Irreversibilities and isentropic efficiencies

We shall only consider the effect of irreversibilities upon compression and expansion processes.Irreversibilities in heaters and coolers who up as pressure drops and are not considered here. The two T-S diagrams, show the effect on compression and expansion processes in general from state 1 tostate 2. These are analogous to the similar diagrams for the Rankine cycle except that they are processes ofa perfect gas. Then for the steady flow compression process:-

For the steady flow compression process:

For the steady flow expansion process:

Note that Celsius temperatures may also be used in these expressions.