Internal Combustion Engines Engine Operating Characteristics : 2

1

Internal Combustion Engines

Engine Operating CharacteristicsLecture 2

Internal Combustion Engines Engine Operating Characteristics : 2

2

Overview

• In this lecture we will learn about:– The measurement of engine torque and

power.– The relation between power, torque and

speed.– Air/Fuel ratio and equivalence ratio.– Specific fuel consumption and its relation with

different parameters.– Various definitions of the efficiency.

Internal Combustion Engines Engine Operating Characteristics : 2

3

Torque & Power• The most common method for measuring power

is by using dynamometers.• A dynamometer absorbs the engine power by

braking it and measuring the force required to do so.

• Different types of dynamometers exist: friction, hydraulic, electromagnetic or electrical.

Internal Combustion Engines Engine Operating Characteristics : 2

4

Torque & Power• A turning moment around the shaft (Torque)

can be measured from the reaction on the dynoand so power can be calculated:

P = 2 Π T * N

Internal Combustion Engines Engine Operating Characteristics : 2

5

Torque & Power• It can be seen that the maximum power is

linked with the maximum speed and maximum torque.

• Engines can be designed to achieve power through high torque or high speed.

• Heavy duty vehicles are designed on a high torque / low speed basis.

• While passenger cars are designed to yield their maximum power at high speeds.

Internal Combustion Engines Engine Operating Characteristics : 2

6

Torque & Power• Torque starts decreasing

after reaching a maximum because of the decreasing ability of the engine to take a full charge at high speeds.

• Brake power also starts to decrease at high speeds while indicated power increases. This is due to the increasing friction losses which become the dominant factor.

Internal Combustion Engines Engine Operating Characteristics : 2

7

Air/Fuel Ratio• Ratio of air mass to fuel mass used in

combustion:A/F = ma / mf

• Ideal (Stoichiometric) A/F ratios are around 15:1

• If more fuel is added, the mixture is called a rich mixture (A/F < 15).

• If excess air is added, the mixture is called a lean mixture (A/F > 15).

Internal Combustion Engines Engine Operating Characteristics : 2

8

Air/Fuel Ratio

• Gasoline engines usually run with A/F in the range of (12-18).

• Diesel engines usually run very lean (18-70).

• Equivalence ratio: φ = (A/F)ideal / (A/F)actual

– Rich mixture: φ >1– Lean mixture: φ <1

Internal Combustion Engines Engine Operating Characteristics : 2

9

Specific Fuel Consumption• Fuel consumption can be expressed in terms of

(l/100 km). • This method is dependant on the drive cycle

used. For example:– Urban Cycle: 10 l/100 km– Extra Urban Cycle: 5.5 l/100 km– Mixed Cycle: 7 l/100 km

• A more scientific way to evaluate fuel consumption is by using the term specific fuel consumption (sfc).

Internal Combustion Engines Engine Operating Characteristics : 2

10

Specific Fuel Consumption

• The ratio of the rate of fuel flow to the engine power:

sfc = fuel consumption /Power

• sfc is usually given in units of kg/kW.hr.• It can be given as the ifsc (using indicated

power) or more commonly as the bsfc(using brake power).

Internal Combustion Engines Engine Operating Characteristics : 2

11

Specific Fuel Consumption• The bsfc initially

decreases with increasing speed due to the relative decrease of heat losses.

• The curve then reaches a minimum and then starts to increase as the brake power starts to drop due to friction losses.

Internal Combustion Engines Engine Operating Characteristics : 2

12

Specific Fuel Consumption

• bsfc & rc: – bsfc decreases with the increase of the

compression ratio due to the increase of the thermal efficiency.

• bsfc & A/F ratio: – bsfc reaches a minimum near stoichiometry.

• bsfc & engine size:– bsfc decreases with increasing engine size

Internal Combustion Engines Engine Operating Characteristics : 2

13

Combustion Efficiency• A measure of the combustion quality or

the fraction of fuel that burns:ηc = Q / mf (C.V.)

– Q: Heat released/cycle– mf : mass of fuel/cycle– C.V.: Calorific value of the fuel

• Combustion efficiency is usually over 95% and in many cases can be assumed to be equal to 1.

Internal Combustion Engines Engine Operating Characteristics : 2

14

Thermal Efficiency• The ratio of the generated work to the heat

released from the fuel:ηt = W / Q

ηt = W / (ηc mf C.V.)

• It can be related to the compression ratio and the thermodynamics of the cycle.

• It can be represented as indicated or brake thermal efficiency.

Internal Combustion Engines Engine Operating Characteristics : 2

15

Volumetric Efficiency• Ratio of the actual mass of inhaled mixture

in one cylinder / cycle to the theoretical mass that could be inducted in the cylinder:

ηv = ma / ρ Vd

• It can reach 90% at wide open throttle conditions and drops dramatically when throttle is closed in S.I. engines.