AURLTA101 (engine fundamentals)

automotive mechanical systems

key principle of an engine

conversion of chemical energy to mechanical energy

• explosion generates force to move rotating assembly in order to generate power (crankshaft)

List the Petrol OTTO cycle and piston / valve movement

• intake - piston moving from tdc to bdc , intake valve opens at tdc and closes js after bdc

• Compression , piston moving from bdc to tdc all valves closed

• Power - piston nearly tdc , explosion forces piston back towards bdc

• exhaust - piston moves from bdc to tdc to expel gases whilist exhaust valve open

what happens to valves after the end of one 4 stroke cycle

Valve overlap: exhaust and intake valves stay open simultaneously to allow for a vacuum to be created so the cycle can be repeated

exhaust gases being expelled creating a vacuum to draw the new AFM in

key events of the otto cycle

intake - AFM drawn in as piston moves down

Compression - AFM compressed as piston moves up

Power - Ignition of afm forces the piston back down, turning the crankshaft

exhaust - as piston goes back up, exhaust gases are expelled

list the formula

calculate total displacement of a V8 engine with a stroke length of 186mm and radius of 186mm

Pi R squared X stroke length

total displacement = Ans(number of cyl)

type answer

define bore and stroke

bore - the radius of the cylinder

stroke - the length of travel of the piston (or the depth of the cyl at BDC)

list the formula

calculate the compression ratio of an engine with a swept volume of 325 and a clearance volume of 75

(swept volume plus clearance volume) divided by clearance

(325 + 75) / 75 = 5.3

5.33:1

why are compression ratios important for a car

they determine the hardness of motor components as well as the fuel economy and power output

Anatomy of Overhead valve trains

A tappet (lifter) - consists of a spring and a cap , its purpose is to transfer motion from the cam lobes to actuate rocker arms

rocker arm ,- acts as a linkage to valve lifters to valves on top

valves - open and close the ports

why are OHC better than ohv

less moving parts - no rockerarm needed

directly driven by cam gears , so more efficent

lead , lag , overlap

refers to valve positions in respect to eachother

lead - the valves open before tdc / bdc

lag - valves remain open for a short time after

overlap - valves are briefly open at the same time

what happens to the crankshafts rotation as it gets closer to tdc / bdc

the crankshaft has to travel more degrees for leverage in order to move the piston

characteristics of two stroke engines

has ports instead of traditional valves

• no camshafts

• the piston skirting seals against ports acting as a valve

• Afm is compressed twice before ignition / power

• simmilar to rotaries

Two stroke anatomy

Ports:

• intake port , trasnfers AFM into the crank case

• transfer port , transfers crankcase afm into coombustion chamber

• exhaust port , transfers expent afm out of the exhaust

two stroke cycle

up stroke , piston moving towards tdc ,compressing combustion chamber afm , drawing in crank case afm

top stroke , ignition of chamber afm , intake port convered drawing afm into crankcase

downstroke, piston forced down as a result of ignition , passing / uncovering the exhaust port , exhausting the spent gases , downward pressure compresses crank case afm to make it suitable for transfer to chamber

bottom stroke

crankcase situated afm drawn into chamber, whilist exhaust port is still opened , most gases excaped

reed valve purpose

1 way valve that lifts up whilist piston is moving up , which causes low pressure creating a vauum to let afm in

rotary engine anatomy

Eccentric shaft - serves as a crankshaft, which has eccentric lobes which allow the rotor to move in an eccentric pattern around the housing

rotor , the piston of the rotary engine , rotates and serves to push afm around the rotor housing

seals - they are on each rotors verticies and seal against the rotor housing

rotary anatomy

no camshaft (like two strokes)

has the main rotor to seal against ports (like a two stroke)

no crankshaft - has a eccentric shaft

has 3 seals to mate the surface between rotor and housing

2 spark plugs for each rotorhousing

rotary engine cycles

*cycle happens in a clockwise direction

Intake

• Afm is introduced through the intake port and is pushed by one vertex / flank of the rotor towards the right side of the housing

Compression

• at the right side of the housing , the rotor face compresses the pushed afm between the rotor face and the rotor housing - making it suitable for combustion

Expansion/ power

• compressed afm ignited twice (by leading and trailing plugs) as fuel makes it past unbornt,

• the gases force push against the rotor face, driving it clockwise towards the exhaust port

Exhaust

• the rotor face pushes the spent gases through thhe exhaust port, which is on the same side as the intake port

5 rotary characteristics (disadvantages)

challenges

• Sealing: the seals on the tip of the rotor are constantly in contact with the rotor housing, no lubrication = wear

• Thermal management: rHigh RPM = high heat , this heat can thin the oil and worsen lubrication

• Low torque as lacks mechanical leverage of conventional ice,

• rotaries get raped by knock.

5 Rotary characteristics (advantages

High volumetric efficiency

• lacking small conventional valves, they can cram more AFM through the intake port. this is good for smoother higher rpm power

Power to weight

• their engines produce alot of power for very little weight and size making most applications lighter and less cramped.

simple

• less moving parts compared to conventional engines, making them easier to assemble

Fueling (performance applications)

• can handle alternative fuels alot better, making them highly versatile

Knock

• exceptionally devestating as force is directly trasnferred onto seals, which can instantly ruin the motor, this also makes it hard to knock test a rotary when tuning

Four stroke diesel characteristics

• utilises compression ignition - a form of spontaneous combustion , ignition occurs without a spark plug

• same anatomy as piston engines, except running higher compression ratios

• valves overlap significantly less due to small clearance volume

• valves stay open for less time to build compression

• heavier than petrol counterparts as parts are more denser/ durable to handle the higher comp

• they have shorter stroke for more torque

four stroke diesel cycles

intake - intake pure air as piston moving to BDC , which is then full of air

compression , just after bdc intake valve closes to begin high compression of air

power

piston is moving towards tdc , compressing air, then just before tdc, fuel charge sprayed to mix with air , afm ignites from high heat and pressure.

after bdc, piston pushes spent afm out exhaust valve.

Engine configurations

Inline

• cylinders are lined up , block can be slanted slightly to accomodate room

V Bank

• 2 angled 90 degree banks, with each bank contain cylinders paralell to eachother

• wider and shorter compared to inline

W bank

• 2 v engines combined to form a total of 4 individual bankse-

• 2 outermost banks are typically 72 degrees apart, whereas inner and outer banks on their respective side are 15 degrees apart

• crams more cylinder into a smaller space, negating size concerns

• compactness introduces simmilar concerns to rotaries (except for sealing

boxer

flatt engines with cylinder banks 180* apart.'

• wider and lower, offering a better balanced engine and lowering the vehicles center of mass

• servicibility is compared with rotary engines, as crucial maintenance such as belt changes or gasket removal becoming tedious

Over square vs under square

Oversquare

bore diameter bigger than piston stroke,

• good for high rpm application requiring alot of speed, as crankshaft doesnt need to rotate as much to generate power

• produces less leverage as crankshaft isnt turning conrods that far , lower torque

Undersquare

bore diameter smaller than piston

• good for high torque applications

• requires more effort for crankshaft to turn the piston between dead centers.