Powertrain

Question 1

What is the approximate thermal efficiency of IC engines?

Answer 1

The worst 30%. The best 50% In the real world, around 40%

Question 2

What is the air fuel mixture in diesel engines like?

Answer 2

Very lean. 30:1 is common

Question 3

How does power, torque and fuel consumption vary with RPM with petrol and diesel engines?

Answer 3

Question 4

Why is engine efficiency lower at lower speeds?

Answer 4

1) poorer mixing 2) poorer combustion; 3) relatively higher losses

Question 5

What are the relative ignition tempertatures of petrol and diesel?

Answer 5

450 for petrol

350 for diesel.

Question 6

What is IMEP?

Answer 6

Indicated mean effective pressure

Fictional consant pressure that would provide the same power if it acted on the pisston all the way down. It is independent of engine speed and volume

A good measure of effectiveness of the design of the engine.

Also known as net mean effective pressure

Roughly between 0.9 and 1.3 MPa

Question 7

How is conrod length defined?

Answer 7

λ=r/I

Ratio of crank and control length

Normally around 1/3

Question 8

How do you find BMEP?

Answer 8

BMEP=IMEP-FMEP

BMEP=2πTn / V_d

T=torque

n=number of revs per power stroke(normally 2)

V_d=engine displacement

Question 9

Which pressures corespond to the different areas?

Answer 9

A=gross mean effective pressure

B=pumping mean effective pressure

IMEP=A-B

Question 10

What bore/stroke ratios are used for which applications?

Answer 10

Question 11

What are the relative merrits of two-stoke and four stroke engines?

Answer 11

Question 12

What is volatility?

Answer 12

How readily a fluid becomes vapour

Question 13

What is the relative volatility, flash point and ignition temperature of petrol and diesel?

Answer 13

Petrol is more volatile and has the lower flash point. Diesel has the lower autoignition temperature.

Question 14

What is the equation for the distance between shafts of a gearbox?

Answer 14

Question 15

What is R_g?

Answer 15

Ratio of the top gear ratio to the bottom gear ratio

Question 16

What are the typical largest smallest gear ratios in different types of vehicles?

Answer 16

Top gear ratio= 0.7-1

Bottom gear ratio= 3-4.5 for passenger vehicles

5-8 for commercial vehicles

Question 17

What is the equation relating the distance between gear shafts and the number of teeth on engaging gears?

Answer 17

Z₁+Z₂=2A*cos(β)*(1/M)

M is the module

Z is the number of teeth

A is the distance between the shafts

β is the helix angle of the gears.

Question 18

What equation gives the ratio of first gear from the number of gear teeth?

Answer 18

ratio=(driven/drive)*(driven/driver)

Question 19

What equation gives the power of an engine from the torque?

Answer 19

2πTn/V_d

T is the torque

n is the number of revolutions per power stroke (2 for 4 stroke engine)

V_d is the total volume of the engine

Question 20

What is the mean piston speed?

Answer 20

stroke of engine*RPM / 30

Question 21

What is the effect of increasing the number of cyclinders and the bore of the engine?

Answer 21

When n increases for given other design conditions total engine mass and size decrease but length increases

Increasing D is commonly used to upgrade an engine design for higher power

Question 22

What are the typical engine compresstion ratios?

Answer 22

10 to 12 for gasoline

14 to 18 for diesel

Question 23

Draw the diagram that explains how the force acting on the piston becomes torque

Answer 23

Question 24

What is the valve timing clock for the best engine performance?

Answer 24

Question 25

What is the valve timing clock diagram like for a more efficient engine?

Answer 25

Question 26

What does a theoretical valve clock diagram for a petrol engine look like?

Answer 26

Question 27

What is the definition of a gear ratio ?

Answer 27

Driven gear teeth no. / driver gear teeth no.

Question 28

What is GMEP?

Answer 28

the work delivered to the piston from the compression and expansion strokes of the four stroke engine cycle. GMEP is the positive work extracted by the piston.

Question 29

What is PMEP?

Answer 29

Pumping Mean Effective Pressure, or PMEP is the work required by the piston during the intake and exhaust strokes of the four stroke engine cycle. PMEP is the negative work required by the piston to draw the fresh charge into the cylinder and to push the exhaust from the cylinder.

Question 30

What is FMEP?

Answer 30

Friction Mean Effective Pressure, or FMEP is work which is lost due to mechanical friction and parasitic losses. Mechanical friction is for example, the friction between the piston rings and the cylinder walls whilst parasitic losses are for example, the driving of the oil pump, water pump and alternator. Engine friction is largely a function of engine speed and you feel the friction of the engine as engine braking.

Question 31

How do you find mechanical efficiency?

Answer 31

BMEP/ IMEP

Question 32

What are first order reciprocating inertial forces?

Answer 32

Forces produced from reciprocating components that happen every 360 degrees of rotation

Question 33

How are crankshafts made?

Answer 33

Highly stressed crankshafts are usually drop-forged. In smaller and less highly stressed engines, casting is used (lower weight and lower cost)

Question 34

What are the different in cylinder pressures?

Answer 34

4-6Mpa for gasoline engine 6-20Mpa for diesel engines

Question 35

What is the typical gas temperature in engines?

Answer 35

\>2300k

Question 36

What is the local maximum temperature of a piston head?

Answer 36

Up to 600k

Question 37

What does an engine fuel map look like?

Answer 37

Question 38

What is the Taylor aproximation used in the derivation of normalised displacement?

Answer 38

Question 39

What is the human perception of noise with respect to frequency curve?

Answer 39

Question 40

How is a single cylinder engine balanced?

Answer 40