Lecture 12 - Internal Combustion Engines Flashcards
Internal combustion cycles
otto cycle (atkinson cycle), diesel cycle, wankel engines,
Otto cycle
4 stroke 1 air intake 2 compression 3 Combustion 4 Exhaust
closed system the change in internal energy is
zero
efficiency of otto cycle
eta = 1 - 1/rc^n-1
Draw otto cycle with location of stroke labelled
1-2 intake stroke
6-1 exhaust stroke
What do edges become curved on a real 4 stroke engine
as combustion and exhaust process occur in a finite time - not instant
How does compression ratio effect efficiency for otto cycle
higher compression ratio, higher efficiency
Ignition in diesel cycle
compression ignition, fuel injector, injects atomized fuel after compression, fuel self ignites due to temp increase from compression
What is a glow plug
Used in diesel cycle to artificially heat the air during cold starts
Stages in standard air cycles
1-2 isentropic compression
2-3 heat added at const pressure while piston moving downwards
3-4 isentropic expansion
4-1 rejection of heat from air
Effieicny of diesel cycle
1 - (1/rc^n-1)*((alpha^n -1)/(n(alpha -1)) alpha = v3/v2
For a fixed compression ratio what is more efficient diesel or otto
diesel cycle efficiency is less than the otto
but diesel combustion method allows greater compression ratios to be used increasing the efficiency
For a fixed compression ratio what is more efficient diesel or otto
diesel cycle efficiency is less than the otto
but diesel combustion method allows greater compression ratios to be used increasing the efficiency
Typical compression ratio in diesel
18 to 24:1
Typical compression ratio in otto
10 to 13:1
What is the advantage of diesel cylce
higher torque lower in the RPM range
Why does diesel potentially have higher emissions
diesel has high energy content per unit mass due to higher density
causes higher firing temp and eventually NOx
Also leads to higher PM as some components in diesel are difficult to burn
How can diesel fuel be tuned
to reduce PM emissions (reduce aeromatic composition), cant really do for NOx as cycle is good because of high compression ratios and therefore temps
what happens in the atkinson cycle
in the compression stroke - intake valve left open for longer than in otto cycle to allow some air back out of the cylinder and into the intake manifold
What is the effect of the atkinson cycle
effective compression ratio is reduced (for short time air escaping from cylinder rather than compressed), expansion ratio is unchanged
therefore compression ratio smaller than expansion ratio
What is the atkinson cycle a modified version of
the otto cycle
Draw a PV diagram of the atkinson cycle
isentropic compression isochoric heating isobaric heating isentropic expansion isochoric cooling isobaric cooling
What is the aim of the atkinson cycle
allow pressure in the combustion chamber at the end of the power stroke be equal to atmospheric pressure thus maximum energy has been obtained from combustion fuel
What is the issue with the atkinson cycle
lower power density than otto cycle as there is less air in the cylinder under compression (as some allowed to escape)
Where is the atkinson cycle used
full electric hybrid car - electric propulsion makes up for power drop when required
With intelligent valve timing allowing the car to switch to otto cycle when more power is required
Why should you mistrust “studies”
different drivers
different routes
intended driving route
etc not comparable
Types of IC engine arrangments
in line v opposed piston horizontally opposed Radial
Other types of IC engines
Rotary - wankel rotary engine
Where are IC engines used apart from cars
old aircraft
for marine
for power
