Heat

Question 1

Briefly explain the term heat release rate. State its unit

Answer 1

rate of heat release: chronological rise of heat release during the combust..
dQ/dalpha [kJ/°ca]

Question 2

Explain, using a diagram, the influence of the rate of heat release on the cylinder pressure pattern

Answer 2

Heat F9

Question 3

Explain, using a diagram, the influence of the ignition timing on the cylinder pressure pattern.

Answer 3

Heat F8

Question 4

Draw typical rates of heat release for SI-and CI-engines respectively

Answer 4

Heat F10 F16

Question 5

Briefly explain the term ignition lag. It can be found that the ignition delay for SI-engines is significantly shorter compared to CI-engines. Why?

Answer 5

ignition lag:
SI: time difference between ignition of spark plug and combustion
CI: time between injectiong fuel and combustion

Question 6

Explain the two phases of combustion for CI-engines

Answer 6

Phase 1: 
- fast combustion of premixed mixture
- distinct peak in rate of heat release
- no formation of particulate matter
- formation if thermal NOx
Phase 2:
- slow combustion of inhomogenous mixture, diffusion flame
- local O2 deficiency-> formation of particulate matter

Question 7

Name and explain one example of modeling the heat release rate of an ICE

Answer 7

Modelling Wiebe: heat F12

(Rate of heat release integrated -> mass fraction burned

Question 8

What is meant by the term heat transfer? Name three examples of heat transfer in an arbitrary ICE

Answer 8

• heat transfer describes the transport of thermal energy due to diffs. in temp.
• stationary, 1D
• the differantial equation of heat release is the equation of a phenomenological model
  3 examples:
• hot combustion gases-piston surface
• cooling of cylinder wall
• moving hot and colg gases in/out of the comb. cham.

Question 9

State the three modes of heat transfer

Answer 9

• Conduction 𝑞´=−𝜆𝛻𝑇
• Convection 𝑞´=𝛼𝑇w−𝑇f
• Radiation 𝑞´=𝜎(𝑇14−𝑇24) Fourier’s law

Question 10

Explain the difficulty of calculating the heat transfer coefficient for engine applications.

Answer 10

Swirl (gas velocity) during induction,  at different engine stroke phases not the same
-> empirical expression of Woschni
C1 compression, combustion, exhaust
C1 Gas exchang
C2 for Di 
C2 for IDI (prechamber)

Question 11

Considering a four stroke cycle: Name the two strokes where considerable mass transfer takes place.

Answer 11

• Intake
• exhaust
• > open non stationary system

Question 12

Which kind of equations are used in order to perform calculations on the real engine process?

Answer 12

• id. gas equa. pV=mRT
• Conservation of mass dm=Sum(dm)
• Conservation of energy dU/dphi=dSum(E)/dphi