Engine Design and Operating Parameters Important Engine Characteristics

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.

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.

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

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 12-24 for CI Rbs = 0.8-1.2 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

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

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]

Mean Piston Speed: Instantaneous Piston Velocity Sp:

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

Brake Torque and Power: Indicated Work per Cycle

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

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

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

Mep (kPa) = 6.28nR T (N.m) Vd (dm3) Typical values: NA (SI): Max. values 850-1050 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 1250-1700 and 900-1400 kPa respectively

For NA 4-stroke Diesels: Max. bmep is 700-900 kPa For NA 4-stroke Diesels: Max. bmep is 700-900 kPa. At maximum rated power it is about 700 kPa For turbocharged 4-stroke Diesels maximum bmep values are 1000-1200 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

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 34.67 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

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

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

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.

Relationship between Performance Parameters

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