1. Engine Design and Operating Parameters
Important Engine Characteristics

2. Design Requirements
Good Engine Performance over entire operating range
Minimum Fuel consumption within this range
Cost of fuel to be kept within limits
Low Engine noise and Pollution in this range
Low Initial cost and installation costs
Reliability and durability, maintenance requirements and how they affect engine cost etc.

3. Engine Performance is defined by:
Maximum power (or max. torque) available at each speed within useful range.
Range of speed power over which engine operation is satisfactory.

4. Some definitions:
Max. rated power. Highest power engine is allowed to develop for short periods
Also referred to as “Intermittent power rating”
Normal rated power. Highest power engine allowed to develop in continuous running
Also referred to as “Continuous rating”
Rated speed. Speed at which rated power is developed

6. Geometrical Properties
Compression ratio rc = Vc+Vs/Vc
Bore:stroke = Rbs = B/L
Conn rod length/crank radius R = l/a
Now L = 2a
rc = 8-12 for SI and for CI
Rbs = for small and medium engine
= 0.5 for slow speed CI engines
R = 3-4 for small & medium sized engines
= 5-9 for large slow speed CI engines

7. Geometrical Properties (cont.)
Cylinder Volume V at any crank position θ :
V = Vc + (πB2/4)(l+a-s)
S = dist. between crank and piston pin axes
= acosθ + (l2 – a2sin2θ)2
θ is the crank angle
We can obtain an equation for V/Vc as

8. Geometrical Properties (cont.)
V/Vc = 1 + ½(rc – 1)[R+1-cosθ-(R2 – sin2θ)1/2]
Comb. Chamber surface area:
A = Ach + Ap + Asl
Ach = cyl. head surface area
Ap = piston crown surface area
Asl = sleeve surface area = πB(l+a-s)
For flat top pistons and cyl heads, Ach = Ap = πB2/4 and
Asl = (πBL/2)[R+1-cosθ-(R2 – sin2θ)1/2]

10. Mean Piston Speed:
Instantaneous Piston Velocity Sp:

11. Typical values of Mean Piston Speed
Gas flow velocities in the intake manifold and cylinder all scale with mean piston speed
Values vary between about 8 to 15 m/s
Automobile engines operate at the higher end of this range
Large marine engines operate at the lower end of range

14. Brake Torque and Power:
Indicated Work per Cycle

15. Gross indicated work per cycle Wc,ig : Work
delivered to the Piston Over the compression
and expansion only
Net indicated Work per cycle Wc,i: Work
delivered to the Piston Over the
entire four strokes.
The power per cylinder is given by

16. Mechanical Efficiency:
We have:
Pig = Pb + Pf where Pf is the friction power
Ratio of brake power to indicated power is the mechanical efficiency:
ηm = Pb/Pig = 1 – (Pf/Pig)
For modern automobile engines at WOT
ηm is about 0.9 at 30-40rev/s decreasing to 0.75 at rated speed. It decreases as engine is throttled

17. Mean Effective Pressure
Work per cycle = P.nR/N
Then Mep is given by
mep = P.nR/(N.Vd)
And in SI units:
mep (kPa) = P(kW)nR x 103
V(dm3) N (rev/s)
Mep can also be expressed in terms of torque so that

18. Mep (kPa) = 6.28nR T (N.m)
Vd (dm3)
Typical values:
NA (SI): Max. values kPa where max. torque is obtained (at about 3000 rev/min). At max. rated power, bmep values are 10-15% lower.
For turbocharged SI corresponding values are and kPa respectively

19. For NA 4-stroke Diesels: Max. bmep is 700-900 kPa
For NA 4-stroke Diesels: Max. bmep is kPa. At maximum rated power it is about 700 kPa
For turbocharged 4-stroke Diesels maximum bmep values are kPa which can rise to 1400 kPa for TC/AC. At max. rated power bmep is about 950 kPa. Two-stroke diesels have comparative performance.
Large low speed 2 stroke engines can achieve bmep values of about 1600 kPa

20. Power per unit displacement
Typical values for modern naturally aspirated gasoline engines – 50 to 65 kW/L.
Some high performance car engines, for example, BMW and Ford have values - up to 75 kW/L
Maruti 800 has a value of kW/L
The Hyundai Santro Xing, with 3 cylinders like the Maruti 800 but with a higher displacement has a value of 42.6 kW/L
Maruti Alto, with same displacement as Maruti 800, has a value of 47.4 kW/L
Maruti Swift and Esteem have a value of 50 kW/L

21. Other terms include
specific fuel consumption, (brake or indicated) and
enthalpy or thermal efficiency (brake or indicated)
Air-fuel ratio and equivalence ratio ()
A/F varies between 12 and 18 for SI ( = 1.2 to 0.8) using gasoline
A/F varies between 18 and 70 for CI using diesel fuel

22. Volumetric Efficiency:
Measures the effectiveness of the engine induction process. Used only with 4 stroke engines which have a distinctive induction process. It is given by

23. N is the engine speed in rev/unit time.
where is the mass flow rate of fresh mixture.
N is the engine speed in rev/unit time.
Vd is the piston displacement (swept volume).
ρi is the inlet density.

24. Relationship between Performance Parameters

26. From relationship, we require
High fuel conversion efficiency
High volumetric efficiency
Increasing the output of a given displacement
engine by increasing the inlet air density
Maximum fuel/air ratio that can be usefully
burned in the engine
High mean piston speed