Week 2- engine type and characteristics

Question 1

Why is the IC engine still so used? (3 reasons)

Answer 1

• It has a high energy density
  -It exists in convenient liquid form and infrastructure already exists.
  -Efficiency is continuously improving

Question 2

What are the four main ways that IC engines can be described?

Answer 2

By ignition, by stroke, by cooling and by cylinder arrangement

Question 3

What are the four strokes in a four-stroke engine?

Answer 3

Intake stroke, compression stroke, expansion (power) stroke, and the exhaust stroke

Question 4

What are the two strokes in a two-stroke engine?

Answer 4

Compression and power stroke

Question 5

What is the intake stroke (four strokes)?

Answer 5

The piston moves down, this causes a mixture of fuel and air into the combustion chamber due to pressure difference.

Question 6

What is a compression stroke?

Answer 6

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.

Question 7

What is the power/expansion stroke?

Answer 7

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.

Question 8

What is the exhaust stroke?

Answer 8

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.

Question 9

Draw the indicator diagrams for pv and p-angle for a four-stroke cycle- gasoline and pv for diesel engine
.

Answer 9

Answer on canvas

Question 10

What engine type uses spark ignition? What engine type uses compression ignition?

Answer 10

Gasoline = spark ignition
Diesel = compression ignition

Question 11

What are some of the main influences on the combustion system of a gasoline engine (spark-ignition)?

Answer 11

Whether the combustion is wall guided, spray-guided or air-guided

Question 12

What is the equation for pressure rise rate?

Answer 12

dp/ d(angle ) = pc-pb /(angle c- angle b)

Question 13

Draw the angle for pressure against the angle for diesel engines.

Answer 13

On onenote

Question 14

Draw the graph for motor IC engine characteristics for diesel engines and gasoline engines, rpm against torque, power and fuel consumption

Answer 14

On onenote

Question 15

Why is engine efficiency lower with higher fuel consumption at low engine speeds?

Answer 15

Due to: Poor mixing of fuel and air,poor combustion,relatively higher losses. Various losses also occur at the high engine speeds.

Question 16

What are the advantages of both diesel and gasoline engines?

Answer 16

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

Question 17

What are the advantages of a two-stroke engine and a four-stroke engine?

Answer 17

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

Question 18

Which engine requires a higher cylinder pressure diesel compression or gasoline?

Answer 18

Diesel compression.

Question 19

What is the air-fuel ratio like in diesel engines?

Answer 19

usually lean- high amount of air compared to fuel

Question 20

What is a throttle what does it do in gasoline engines?

Answer 20

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.

Question 21

What engine types need a throttle and what engine types don’t?

Answer 21

Gasoline needs a throttle, diesel engines don’t

Question 22

How do diesel engines respond to a demand for power, as shown by pressing the accelerator?

Answer 22

The fuel intake is increased, this leads to differing pressures which can cause the air intake to also increase.

Question 23

Which engine type typically has a greater operating speed in rpm?

Answer 23

Gasoline engines. (6500 rpm common)

Question 24

What is the air-fuel ratio like for gasoline engines?

Answer 24

Usually stoichiometric

Question 25

Name the four main moving components in the engine. Draw them.

Answer 25

Piston, crankshaft, valve train and connecting rod.

Question 26

What 3 main parts are in a piston assembly?

Answer 26

Piston, piston ring and gudgeon pin

Question 27

Why are piston rings required?

Answer 27

They seal with minimum ‘blow by’ (combustion gases passing piston rings) and friction losses

Question 28

Draw a conrod and its parts.

Answer 28

Correct drawing should include:
-small end
- rod
-big end

Question 29

How is conrod defined? How important is conrod length?

Answer 29

Conrod is defined by the ratio of crank and conrod length. It is vital in engine design as it affects overall deisgn

Question 30

Is a crankshaft cheap or expensive to make?

Answer 30

Very expensive, one of the most costly.

Question 31

What is the most common failure of the crankshaft?

Answer 31

Bending failure, due to oscillations and vibrations- which is an issue for multi-cylinder engines

Question 32

How are highly stressed crankshafts manufactured?

Answer 32

They are usually drop-forged.

Question 33

How are lower-stressed crankshafts manufactured?

Answer 33

Casting is used.