Week 2- engine type and characteristics Flashcards
Why is the IC engine still so used? (3 reasons)
- It has a high energy density
-It exists in convenient liquid form and infrastructure already exists.
-Efficiency is continuously improving
What are the four main ways that IC engines can be described?
By ignition, by stroke, by cooling and by cylinder arrangement
What are the four strokes in a four-stroke engine?
Intake stroke, compression stroke, expansion (power) stroke, and the exhaust stroke
What are the two strokes in a two-stroke engine?
Compression and power stroke
What is the intake stroke (four strokes)?
The piston moves down, this causes a mixture of fuel and air into the combustion chamber due to pressure difference.
What is a compression stroke?
The piston moves up with the intake valve closing, increasing the pressure in the cylinder. Both valves are closed, which causes the mixture to increase in pressure to prepare it for combustion.
What is the power/expansion stroke?
The mixture ignites causing the piston to move downwards due to ignition. This downward motion is what produces power in the engine, converting chemical energy to mechanical. Both valves are closed to allow for this.
What is the exhaust stroke?
The exhaust valve opens to let out burned gases, this is done with the upwards motion of the piston. This prepares the chamber for the next intake stroke.
Draw the indicator diagrams for pv and p-angle for a four-stroke cycle- gasoline and pv for diesel engine
.
Answer on canvas
What engine type uses spark ignition? What engine type uses compression ignition?
Gasoline = spark ignition
Diesel = compression ignition
What are some of the main influences on the combustion system of a gasoline engine (spark-ignition)?
Whether the combustion is wall guided, spray-guided or air-guided
What is the equation for pressure rise rate?
dp/ d(angle ) = pc-pb /(angle c- angle b)
Draw the angle for pressure against the angle for diesel engines.
On onenote
Draw the graph for motor IC engine characteristics for diesel engines and gasoline engines, rpm against torque, power and fuel consumption
On onenote
Why is engine efficiency lower with higher fuel consumption at low engine speeds?
Due to: Poor mixing of fuel and air,poor combustion,relatively higher losses. Various losses also occur at the high engine speeds.
What are the advantages of both diesel and gasoline engines?
Gasoline advantages:
- higher power density
- lightweight
- cheaper
- less NVH
- Lower pipe emissions
-Good cold start
Diesel advantages:
-Higher fuel economy
- Can reach megawatts
- Lower fuel evaporation and fire hazard
What are the advantages of a two-stroke engine and a four-stroke engine?
2 stroke:
- higher power density
-Smoother torque output
-Simpler
- lower cost
4 stroke:
-Higher fuel economy
-lower emissions
-less lubrication requirements
-better idling
-lower NVH
Which engine requires a higher cylinder pressure diesel compression or gasoline?
Diesel compression.
What is the air-fuel ratio like in diesel engines?
usually lean- high amount of air compared to fuel
What is a throttle what does it do in gasoline engines?
A throttle allows control of the intake manifold pressure by allowing more or less air into the system. The engine reads this amount of air to allow more or less fuel into the system to conserve the air-fuel ratio. This allows for more combustion and hence more power production in the engine when the throttle is pressed.
What engine types need a throttle and what engine types don’t?
Gasoline needs a throttle, diesel engines don’t
How do diesel engines respond to a demand for power, as shown by pressing the accelerator?
The fuel intake is increased, this leads to differing pressures which can cause the air intake to also increase.
Which engine type typically has a greater operating speed in rpm?
Gasoline engines. (6500 rpm common)
What is the air-fuel ratio like for gasoline engines?
Usually stoichiometric
Name the four main moving components in the engine. Draw them.
Piston, crankshaft, valve train and connecting rod.
What 3 main parts are in a piston assembly?
Piston, piston ring and gudgeon pin
Why are piston rings required?
They seal with minimum ‘blow by’ (combustion gases passing piston rings) and friction losses
Draw a conrod and its parts.
Correct drawing should include:
-small end
- rod
-big end
How is conrod defined? How important is conrod length?
Conrod is defined by the ratio of crank and conrod length. It is vital in engine design as it affects overall deisgn
Is a crankshaft cheap or expensive to make?
Very expensive, one of the most costly.
What is the most common failure of the crankshaft?
Bending failure, due to oscillations and vibrations- which is an issue for multi-cylinder engines
How are highly stressed crankshafts manufactured?
They are usually drop-forged.
How are lower-stressed crankshafts manufactured?
Casting is used.
