Overview

 

Topic 1 - IC engines versus EC or CC engines

Internal combustion versus external combustion

Intermittent combustion versus continuous combustion

We will examine the conversion of chemical energy stored in our fuel, to the creation of kinetic energy of motion, or possibly fluid energy or output power as a desired product.

Topic 2 - Terminology

SI engine	Spark Ignition Engine - an IC engine where combustion of the fuel/air mixture is initiated by a spark – e.g. typical gasoline (petrol) engine whether reciprocating piston or rotary.
CI engine	Compression Ignition Engine – an IC engine where combustion is caused by compression of the fuel/air mixture – e.g. diesel, wave rotor, bubble combustion.
Reciprocating engine	An IC engine with pistons that move back and forth, in a stationary cylinder under the influence of combustion pressures and a crankshaft.
Rotary engine	An IC engine with rotors that rotate angularly in a stationary block under the influence of combustion pressures and an output shaft.  Commonly referred to as a Wankel engine, after the person who invented it.

By User:Y_tambe - User:Y_tambe's file, CC BY-SA 3.0, Link

Block	The major, stationary component of the engine, in which most of the moving parts reside.
Head	A stationary part of a reciprocating engine, forming the top of the combustion chamber and usually containing the valves that allow gasses entry and exit from the cylinder where the piston resides.
4 stroke engine	A reciprocating engine in which 4 strokes of the piston (2 up, 2 down) completes a cycle.
2 stroke engine	A reciprocating engine in which 2 strokes of the piston (1 up, 1 down) completes a cycle.
Valves	The mechanisms which seal the intake and exhaust ports allowing the flow of gasses into and out of the cylinder.
Cylinder	The volume within the engine block in which the piston travels.  May be arranged as straight, V, narrow angle V, W, flat, or radial.  V engines are shorter than straight engines and frequently easier to package.   V8 is inherently smoother running than a V6 because there are the same number of power strokes per revolution, so it is better balanced and also each cylinder is smaller for the same displacement.

 

Induction	The process by which air is induced into the engine for combustion.
Naturally aspirated induction	Air at ambient pressure is simply drawn into the engine by the suction of down-stroke of the piston
Forced induction	Air is forced into the engine at a pressure greater than ambient.
Supercharged	Intake air pressure is boosted by an external compressor, or pump, driven by power from the engine crank.
Turbocharged	Intake air is forced into the engine at a pressure greater than ambient by a compressor driven by exhaust gases.
Carburettor	A mechanism for supplying fuel to the cylinder at a pressure near ambient air pressure.
Fuel injection	A system which sprays fuel into the air entering the cylinder at a pressure far in excess of ambient.
Throttle body injection	The injectors add fuel to the air in the intake manifold, prior to it reaching the cylinder.
Multiport injection	The injectors add fuel at the entrance port to each cylinder.
Direct injection	Injectors add fuel directly into the combustion chamber of the cylinder.
Liquid cooled	Engine heat is carried away by liquid coolant flowing through water jackets around the cylinders.
Air cooled	Engine heat is cooled by air flow conducting heat away from the engine’s metal surfaces.
Crankshaft	The shaft which the pistons are connected to (by connecting rods) which transmits power out of the engine.
Camshaft	The shaft which actuates the valves in the head.
Oil pan	The pan at the bottom of the engine in which the oil resides when it is not being circulated thru the engine for lubrication and cooling.
TDC	Top Dead Centre – the position of the piston at the top of its stroke.
aTDC	after (following) Top Dead Centre – used to measure timing.
bTDC	before (prior to) Top Dead Centre – used to measure timing.
BDC	Bottom Dead Centre – the position of the piston at the bottom of its stroke.
Bore	Cylinder diameter.
Stroke	Distance the piston travels.
Cylinder displacement	Volume displaced by piston = ($\text{π}$/4)(Bore2)(Stroke)
Engine displacement	Cylinder displacement times number of cylinders.
EMS	Engine Management System – the computer controlling the engine operating system in any of the new ‘smart’ engines – colloquially referred to as ‘the black box’.

Topic 3 - Engine types

While IC engines can include Wankel (rotary) engines and a few other types, the vast majority are reciprocating piston engines.  These fall into three main categories.  The Four Stroke engine experiences our strokes of the reciprocating piston in each complete cycle, where the Two Stroke engine experiences only two strokes in every complete cycle.  Another distinction is between the standard (spark ignition) and diesel (compression ignition) engines, differentiated by whether the explosion of combustible gasses is prompted by a spark, or by the heat generated by compression of the gasses.  We will now look at the differences.

Two-Stroke engine

 

 

Topic 4 - IC Engine dimensions and parameters

Text version

 (Pulkrabek 2015: p36 fig 2-1)

Stroke = S = 2 x crank offset

                S = 2a

If S = B (i.e. Stroke = Bore) then the engine is referred to as “square”

If S>B, it is referred to as ‘under square’

If S

For a given displacement engine, being under square makes for less surface area in the combustion chamber and thus less heat loss, as well as higher piston speed and higher friction losses.  Going over square reverses these.

Most of today’s engines are near square.

Displacement Volume = V_d = volume swept by the piston during its stroke

                                            V_d = ($\text{π}$B²/4)S

Clearance Volume = V_c = volume remaining when piston is at TDC

 

Ave Piston Velocity = U_p-ave = travel/rev x rev/min

                                       U_p-ave =  2 x Stroke x rpm

                                       U_p-ave = 2SN

How do piston velocities vary between types of engines?

Type Car	Typical Max RPM	Typical Up-ave (ft/sec)
Normal street car	5000-6500	15-65
Pure race engine	14,000	115
Locomotive	400	10
Model Airplane	12,000	10

Locomotive and model aeroplane engines have similar U_p-ave, but greatly different N because of the huge difference in Stroke.

Remember   F = ma , i.e. Force = mass x acceleration.  The piston and connecting rod go from 0 to U_p-max to 0 every cycle which makes for a significant acceleration, and the higher the U_p-max the higher the acceleration, and therefore the higher the forces. 

So component life is a function of U_p-max.  This explains why race engines, which have higher speeds, have shorter lives.

The Compression ratio is a measure of how much the air which enters the engine is compressed during the compression stroke of the engine.

$\text{Compression Ratio}=\text{rc}=\frac{{\text{V}}_{\text{BDC}}}{{\text{V}}_{\text{TDC}}}=\frac{\text{Volume of Chamber at BDC}}{\text{Volume of Chamber at TDC}}$ 

${\text{r}}_{\text{c}}=({\text{V}}_{\text{c}}+{\text{V}}_{\text{d}})/{\text{V}}_{\text{c}}$  

How do compression ratios vary between types of engines?

Type engine	rc
SI street	8-11
SI race	12-16
CI	12-24

Forced induction engines usually have lower r_c  but produce more power because the final pressure is higher due to the initial pressure being boosted above ambient.

Let us consider the Work done by the engine.  Recall the basic definition of work is force times distance over which the force is applied.

$\text{Work}=\text{∫}\text{F(x)dx}=\text{∫}\text{P(x)}{\text{A}}_{\text{p}}\text{dx}$ 

Where P is combustion chamber pressure, A_P = Piston Area

But    

A_pdx is volume, so A_pdx = dV

So

$\text{W}=\text{∫}\text{P(V)dV}$ 

If we define specific work as w = W/(mass of gas),  then specific volume as v = V/(mass of gas).

Then

$\text{W}=\text{∫}\text{Pdv}$ 

There are a number of different expressions for work done in an IC engine installation.

w_i   =   Indicated Work = work occurring inside combustion chamber

w_f   =   Friction Work = work lost to friction and parasitic (driven) loads

w_b   =   Brake Work = actual work supplied to crankshaft

w_gross   =   Gross Work = work produced from compression and power strokes

w_p     =  Pump Work = work required (lost to) intake and exhaust strokes    (negative for naturally aspirated engines, positive for engines with forced induction).

w_net   =   Net Indicated Work

w_b    =   w_i   - w_f                                

w_net   =   w_gross +  w_pump

From work we can calculate Mean Effective Pressure =  mep = w/Dv  =  W/V_d

where

Dv  =  v_BDC  -  v_TDC

W = work for one cycle

w = specific work for one cycle

V_d = displacement volume

Mean effective pressure may be stated as brake (bmep), indicated (imep), pump (pmep) or friction (fmep) depending on which work is used.

Now we can consider Efficiencies

${\text{η}}_{\text{m}}=\text{Mechanical Efficiency}={\text{w}}_{\text{b}}/{\text{w}}_{\text{i}}={\text{W}}_{\text{b}}/{\text{W}}_{\text{i}}$ 

(If parasitic loads are ignored, this is typically 55%-60% at high speed where friction losses are greatest, and 85%-95% at low speed where thermal losses are greatest)

(neither r_c or B effect h_m to any significant extent)

${\text{η}}_{\text{c}}=\text{Combustion Efficiency}={\stackrel{\text{•}}{Q}}_{\text{in}}/\left({\stackrel{\text{•}}{m}}_{\text{f}}{Q}_{\text{HV}}\right)\text{ (typically 95-98\%)}$ 

Where

${\stackrel{\text{•}}{Q}}_{\text{in}}=\text{heat flow into the combustion process}$
${\stackrel{\text{•}}{m}}_{\text{f}}=\text{mass flow rate of fuel}$
$Q_{\text{HV}}=\text{heating valve of fuel}$ 

${\text{η}}_{\text{f}}=\text{Fuel Conversion Efficiency}=\stackrel{\text{•}}{W}/{\stackrel{\text{•}}{m}}_{\text{f}}{Q}_{\text{HV}}$ 

${\text{η}}_{\text{t}}=\text{Thermal Efficiency}=\stackrel{\text{•}}{W}/{\stackrel{\text{•}}{Q}}_{\text{in}}=\stackrel{\text{•}}{W}/{\stackrel{\text{•}}{m}}_{\text{f}}{Q}_{\text{HV}}{h}_{\text{c}}={\eta }_{\text{f}}/{\eta }_{\text{c}}$ 

(This can be stated as either indicated (40%-50%) or brake (30%-50%), depending on which Work is used)

${\text{η}}_{\text{m}}={\left({\text{η}}_{\text{t}}\right)}_{\text{b}}/{\left({\text{η}}_{\text{t}}\right)}_{\text{i}}$

${\text{η}}_{\text{v}}=\text{Volumetric Efficiency}$ 

$=\text{efficiency of getting air into the cylinder}$ 

$={\text{m}}_{\text{a}}/{\text{ρ}}_{\text{a}}{\text{V}}_{\text{d}}=\text{n}{\stackrel{\text{•}}{m}}_{\text{a}}/{\text{ρ}}_{\text{a}}{\text{V}}_{\text{d}}\text{N}$ 

where

${\stackrel{\text{•}}{m}}_{\text{a}}=\text{mass flow rate of air into the engine}$
${\text{m}}_{\text{a}}=\text{mass of air into the engine for one cycle}$
${\text{ρ}}_{\text{a}}=\text{air density entering the engine}$
${\text{V}}_{\text{d}}=\text{dispacement volume}$
$\text{N}=\text{engine speed}$
$\text{n}=\text{number of engine revolutions per cycle}$  

$\text{Torque}=\text{t}={\text{W}}_{\text{b}}/\text{2π}=\text{(bmep) }{\text{V}}_{\text{d}}/\text{n2π}$ 

(Brake work (Wb) is used, because torque comes only from the work supplied at the crankshaft)

$\text{Power}=\text{work/time}=\text{P}=\text{W}=\stackrel{\text{•}}{W}\text{N}/\text{n}=\text{2πNt}=\text{(mep)}{\text{A}}_{\text{p}}{\text{U}}_{\text{p-ave}}/\text{2n}$ 

$\text{Specific Fuel Consumption}=\text{sfc}={\stackrel{\text{•}}{m}}_{\text{f}}/\stackrel{\text{•}}{W}$  

(May be brake (bsfc) or indicated (isfc) depending on which work is used)

$\text{Specific Power}=\text{SP}={\stackrel{\text{•}}{W}}_{\text{b}}/{\text{A}}_{\text{p}}$ 

$\text{Output per Displacement}=\text{OPD}={\stackrel{\text{•}}{W}}_{\text{b}}/{\text{V}}_{\text{d}}$  

$\text{Specific Volume}=\text{SV}={\text{V}}_{\text{d}}/{\stackrel{\text{•}}{W}}_{\text{b}}$  

$\text{Specific Weight}=\text{SW}=\text{(engine weight)}/{\stackrel{\text{•}}{W}}_{\text{b}}$  

Fuel and air mixture

$\text{Air-Fuel Ratio}=\text{AF}={\text{m}}_{\text{a}}/{\text{m}}_{\text{f}}={\stackrel{\text{•}}{m}}_{\text{a}}/{\stackrel{\text{•}}{m}}_{\text{f}}$  

$\text{Fuel-Air Ratio}=\text{FA}=\text{1/AF}$  

$\text{Φ}=\text{Equivalence Ratio}={\text{FA}}_{\text{actual}}/{\text{FA}}_{\text{stoichiometric}}$  

 

Text version

 

Standard Air Values

Pressure = P_o = 101 Kpa = 14.7 psia

Temperature = T_o  = 298 K   = 25 C   =  537 R   =  77 F

Air density  =  $\text{ρ}$_a = 1.181 kg/m³ 

Summary

This section has presented the terminologies, work, pressure, and efficiency relationships which will permit us to analyse the operation of an IC engine in subsequent sections.

Reference and bibliography

Heywood, J. (2018) Fundamentals of Internal Combustion Engines. New York, USA: McGraw-Hill.

Proe Power Systems (2012) ‘Proe Afterburning Cycle.’ http://www.proepowersystems.com/  

Pulkrabek, W. (2015) Engineering Fundamentals of the Internal Combustion Engine.  New York, USA: Pearson.