Download presentation
Presentation is loading. Please wait.
1
Engines Physics 313 Professor Lee Carkner Lecture 12
2
Exercise #11 Adiabatic Adiabatic Work W = - ∫ PdV, where P = KV - W = - KV (- +1) / (- +1), but K = PV W = -PV V (- +1) / (- +1) W = PV/( -1) = -(P i V i – P f V f ) / ( -1) Monatomic gas expansion ( = 5/3) P i V i = P f V f or V f = (P i V i /P f ) (3/5) W = - [(5000)(1) – (4000)(1.14)] /(1.66667 – 1) = Diatomic gas expansion ( = 7/5) W = - [(5000)(1) – (4000)(1.17)] / (1.4 – 1) =
3
Heat and Work It is easy to convert work into heat 100 % efficient It is harder to convert heat into work Need a series of processes called a cycle to extract work from heat A machine that converts heat into work with a series of processes is called an engine
4
Efficiency Engines convert heat (Q H ) into work (W) plus output heat (Q L ) The ratio of output work to input heat is called efficiency All Q and W are absolute values
5
Waste Heat The efficiency can be written (using the first law): = (Q H -Q L ) / Q H If Q L = 0 efficiency is 100% < 1
6
Ideal and Real Efficiency Our values for efficiency are ideal Real engines have all of these problems
7
Papin’s Device - 1690
8
Newcomen’s Engine - 1705
9
Watt’s Engine - 1770
10
Engines An (idealized) engine consists of a gas (the working substance) in a cylinder that drives a piston Types of engines: External combustion Internal combustion
11
Parts of the Cycle Cycle can be broken down into specific parts In general: One involves compression One involves the output of heat Q L Change in internal energy is zero
12
Otto Engine
13
Intake stroke -- Compression stroke -- Combustion -- Power stroke -- Exhaust -- Exhaust stroke -- Isobaric compression Intake and exhaust are identical and cancel
14
Between Processes Can specify 4 points, each with its own T, V and P: 1: 2: Before heat gain (after compression) 2: 4: Before heat loss (after expression) Can relate P,V and T using our equations for the various processes Q = C V T (isochoric) TV -1 = TV -1 (adiabatic)
15
Efficiency and Temperature Q L = C V (T 4 -T 1 ) From adiabatic relations: Result: = 1 - (Q L /Q H ) = 1 - [(T 4 -T 1 )/(T 3 -T 2 )] This is the ideal efficiency
16
Diesel Engine Constant pressure maintained by adjusting the rate of fuel input Can compute in similar way, but with different expression for input heat
17
Diesel Engine Efficiency = 1 - (1/ )[(T 4 -T 1 )/(T 3 -T 2 )] Can also write in terms of compression and expansion ratios: = 1 - (1/ )[(1/r E ) - (1/r C ) / (1/r E ) (1/r C ) Real efficiency ~ 30-35 %
18
Steam Engine External high T reservoir (furnace) vaporizes water which expands doing work The idealized process is called the Rankine cycle
20
Rankine Cycle Adiabatic compression (via pump) Adiabatic expansion (doing work) Real efficiency ~ 30-40 %
22
Stirling Engine Working substance is air instead of water Expansion piston in contact with high T reservoir Real efficiency ~ 35-45%
24
Stirling Cycle Isochoric compression and expansion moving air to expansion piston Isochoric compression and expansion moving air to compression piston
25
Engine Notes Should be able to plot and compute key P,V and T
Similar presentations
© 2024 SlidePlayer.com Inc.
All rights reserved.