Internal Combustion Engines. Allan T. Kirkpatrick

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rel="nofollow" href="#ulink_cc9d9285-99d9-5f26-b2e3-df0bcb3e1cb3">Figure 1.10 Performance map of bmep and bsfc versus mean piston speed for an automotive spark‐ignition engine.

The specific fuel consumption and engine efficiency are inversely related, so that the lower the specific fuel consumption, the greater the engine efficiency. Engineers use bsfc rather than thermal efficiency primarily because a more or less universally accepted definition of thermal efficiency does not exist. We will explore the reasons why in Chapter 04. Note for now only that there is an issue with assigning a value to the energy content of the fuel. Let us call that energy the heat of combustion ; the brake thermal efficiency is then

(1.24)

      Inspection of Equation (1.24) shows that bsfc is a valid measure of efficiency provided is held constant. Thus, two different engines can be compared on a bsfc basis provided that they are operated with the same fuel.

      Air–Fuel and Equivalence Ratios

      Since internal combustion engines require both a fuel and an oxidizer for the combustion process, another engine parameter is the air–fuel ratio, AF, expressed on a mass or a mass flow‐rate basis.

      (1.25)

      The reciprocal of the air–fuel ratio is the fuel–air ratio, FA:

      (1.26)

A dimensionless measure of the fuel–air ratio is the equivalence ratio, , which is the ratio of the actual fuel–air ratio to the stoichiometric fuel–air ratio. The word is from the Greek, meaning ”element measure.” A stoichiometric reaction of a hydrocarbon (HC) is defined such that the fuel burns completely and the only products are carbon dioxide () and water (O).

      (1.27)

      The equivalence ratio is used to characterize the fuel–air mixture composition. If the mixture is stoichiometric, if the mixture is lean, and if the mixture is rich.

      Example 1.2 Engine Performance Parameters

      A six‐cylinder, four‐stroke automobile engine is being designed to produce 75 kW at 2000 rpm with a bsfc of 260 g/kWh and a bmep of 9.2 bar (920 kPa). The engine is to have equal bore and stroke, and will be fueled with a stoichiometric mixture of gasoline and air with an air–fuel ratio (AF) of 15.27. Gasoline has a heat of combustion = 44,510 kJ/kg. (a) What is the design displacement volume and bore ? (b) What is the mean piston speed at the design point? (c) What are the cycle average fuel flow and air flowrates and the fuel consumption per cycle per cylinder? (d) What is the work per cycle per cylinder? (e) What is the thermal efficiency?

      Solution

      1 The displacement volume isMost automobile engines have a 90–100 mm bore and stroke.

      2 The mean piston speed is

      3 The cycle average fuel consumption rate per cylinder isso the mass of fuel injected per cylinder per cycle isand the cycle average airflowrate is

      4 The brake work per cycle per cylinder is

      5 The brake thermal efficiency is

      Engine Kinematics

Assuming a flat piston top, the instantaneous cylinder volume, , at any crank angle is

      (1.28)

      where is the instantaneous stroke distance from top dead center:

      By reference to Figure 1.6

      (1.29)

      If the instantaneous volume is nondimensionalized by the volume at bottom dead center, , then the nondimensional volume is

      (1.30)

      We define a nondimensional parameter, , the ratio of the crankshaft radius to the connecting rod length , as

      (1.31)

      The range of for the slider‐crank geometries used in modern engines is about 0.25 to 0.33.

      Therefore, the nondimensional piston displacement is

(1.32)

      and the nondimensional cylinder volume

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