**4. Combustion process in the marine diesel engines and formation of the air pollutants**

#### **4.1. Combustion stoichiometry**

here is for uniflow scavenging. It is necessary to compress the scavenging air by the turbo‐ charger in order to scavenge the cylinder. The principal scheme of a turbocharged marine diesel engine is shown in Figure 6 as in [12]. Turbocharger (TC) is composed of the compressor (C) driven by the gas turbine (T) that receives its power from the heat energy of the exhaust gases flowing through. The compressor and the turbine are directly coupled and they are built together in a common housing. The gas turbine, usually one-stage axial type, is located after an exhaust gas receiver (EGR) which collects the exhaust gases from all the engine cylinders. The one-stage centrifugal-type compressor feeds compressed air through scavenging air cooler

**C T**

**TC**

TC

Air Exh.

gases

*pEGR TEGR*

DE

at 15 °C, marine diesel

at 15 °C and heavy fuel oil (HFO) with

at 15 °C as in [13]. ISO 8217 fuel standard for marine

s-1, is a measure for the fluidity of the fuel at 50 °C.

*qEGE*

*pT TT*

**SAR EGR**

Marine diesel engines use three types of liquid fuels standardised by ISO 8217 fuel standard

distillate fuels also defines the values of the essential properties of each type of the fuel. The

The viscosity of a fuel decreases with increasing temperature. The moment the fuel leaves the injectors, the viscosity must be within the limits prescribed by the engine manufacturer in order to obtain an optimal spray pattern; otherwise, it will lead to poor combustion,

for marine distillate fuels as gas oil (GO) with max. density of 890 kg/m3

*pSA qf*

**Diesel engine**

into scavenging air receiver (SAR) that supplies all the engine cylinders.

*pR TR*

Fuel

**Figure 6.** Principal scheme of turbocharging marine diesel engine

oil (MDO) with max. density from 890 to 900 kg/m3

most important properties of marine fuels are as follows:

max. density from 920 to 1010 kg/m3

**•** Kinematic viscosity, expressed as mm2

deposit formation and energy loss.

**3.3. Types of fuel oils for marine diesel engine**

*qAC*

176 Current Air Quality Issues

*pAC TAC*

> The aim of the combustion stoichiometry is to determine the required amount of air and fuel in order to achieve complete combustion. A stoichiometric mixture contains the exact amount of fuel and oxidiser, so that after combustion is completed, all the fuel and oxidiser are consumed to form combustion products. This ideal stoichiometric mixture approximately yields the maximum flame temperature, as all the energy released from combustion is used to heat the products. As in references [14], combustion stoichiometry for a general hydrocarbon

fuel (CαHβOy) can be expressed by equation (1) and it can be applied only for singlecomponent hydrocarbons (HC):

$$a\mathcal{C}\_a H\_\beta \mathcal{O}\_y + \underbrace{\left(a + \frac{\beta}{4} - \frac{y}{2}\right) \cdot \left(\mathcal{O}\_2 + 3.76 \mathcal{N}\_2\right)}\_{\text{stabchonette al amount}} \rightarrow a\mathcal{C} \mathcal{O}\_2 + \frac{\beta}{2} H\_2 \mathcal{O} + 3.76 \cdot \left(a + \frac{\beta}{4} - \frac{y}{2}\right) \tag{1}$$

Typical approaches for multiple-component hydrocarbon fuels develop the stoichiometric combustion using the general principle of atomic balance, making sure that the total number of C, H, N and O atoms is the same in the products and the reactants (e.g. multiple-component mixture of a 95 % methane (CH4) and 5 % hydrogen (H2)):

$$0.95\text{CH}\_4 + 0.05\text{H}\_2 + 1, 925\text{(O}\_2 + 3.76\text{N}\_2) \to 0, 95\text{CO}\_2 + 1, 95\text{H}\_2\text{O} + 7, 238\text{N}\_2\tag{2}$$

If less air than the stoichiometric amount is used, the mixture is described as rich fuel or rich mixture, and if excess air is used, the mixture is described as lean fuel or lean mixture. For this reason, it is appropriate to determine the amount of the combustible mixture using one of the following methods: a)Fuel-air ratio (FAR), b) equivalence ratio (Φ) and c) percent excess air ( % AE).

**a.** *Fuel-air ratio (FAR)* or f is the actual ratio of fuel mass mf and air mass ma and it is expressed as

$$f = \frac{m\_{\text{f}}}{m\_{\text{a}}} \tag{3}$$

and it is usually bounded by 0 and ∞. For a stoichiometric mixture, equation (3) becomes

$$\left.f\_s = \frac{m\_f}{m\_a}\right|\_{ST} = \frac{M\_f}{\left(\alpha + \frac{\beta}{4} - \frac{y}{2}\right) \cdot 4,76 \cdot M\_a} \tag{4}$$

where Mf is the molar mass of fuel and Ma molar mass of air which is approximately 28.96 kg/ kmol. The stoichiometric mixture fuel-air ratio of the most hydrocarbon fuels is bounded by 0.05 and 0.07. Air-fuel ratio (AFR) is reciprocal of FAR and it is expressed as AFR=f -1.

**b.** *Equivalence ratio (Φ)* is the actual ratio of fuel-air ratio f to the stoichiometric fuel-air ratio *fs*:

s , *as a f m f m* F= = (5)

and its value is bounded by 0 and ∞. *Φ* < 1 is a lean mixture; *Φ* = 1 is a stoichiometric mixture; and *Φ* > 1 is a rich mixture. The fuel in the combustion process must be mixed with a greater amount of air than in stoichiometric mixture because it is not possible to bring the ideal amount of air to each fuel molecule in order to mix them perfectly so that complete combustion is achieved. In the combustion analysis, an alternative variable lambda (*λ*) is often used by engineers. Lambda is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio defined as

$$\mathcal{A} = \frac{AFR}{AFR\_s} = \frac{1}{f \;/\; f\_s} = \frac{m\_{\text{a}}}{m\_{\text{as}}} = \frac{1}{\Phi} \tag{6}$$

**c.** *Percent excess air* (% AE) is the amount of air in excess of the stoichiometric amount and it is defined as

$$\%EA = 100 \cdot \frac{m\_{\text{a}} - m\_{\text{as}}}{m\_{\text{as}}} = 100 \cdot \left(\frac{m\_{\text{a}}}{m\_{\text{as}}} - 1\right) \text{ \textdegree \%} \tag{7}$$

#### **4.2. Combustion process in marine diesel engine**

fuel (CαHβOy) can be expressed by equation (1) and it can be applied only for single-

( 2 2 22 )

3,76 3,76 4 2 <sup>2</sup> 4 2 *<sup>y</sup> y y CHO*

Typical approaches for multiple-component hydrocarbon fuels develop the stoichiometric combustion using the general principle of atomic balance, making sure that the total number of C, H, N and O atoms is the same in the products and the reactants (e.g. multiple-component

If less air than the stoichiometric amount is used, the mixture is described as rich fuel or rich mixture, and if excess air is used, the mixture is described as lean fuel or lean mixture. For this reason, it is appropriate to determine the amount of the combustible mixture using one of the following methods: a)Fuel-air ratio (FAR), b) equivalence ratio (Φ) and c) percent excess air

**a.** *Fuel-air ratio (FAR)* or f is the actual ratio of fuel mass mf and air mass ma and it is expressed

f a *<sup>m</sup> <sup>f</sup>*

and it is usually bounded by 0 and ∞. For a stoichiometric mixture, equation (3) becomes

*f f*

*m M*

= = æ ö

4,76 4 2

*<sup>a</sup> ST <sup>a</sup>*

+- × × ç ÷ è ø

where Mf is the molar mass of fuel and Ma molar mass of air which is approximately 28.96 kg/ kmol. The stoichiometric mixture fuel-air ratio of the most hydrocarbon fuels is bounded by

**b.** *Equivalence ratio (Φ)* is the actual ratio of fuel-air ratio f to the stoichiometric fuel-air ratio

, *as a*

0.05 and 0.07. Air-fuel ratio (AFR) is reciprocal of FAR and it is expressed as AFR=f -1.

s

*f m f m*

*<sup>m</sup> <sup>y</sup> <sup>M</sup>* b a

0,95 0,05 1,925 3,76 0,95 1,95 7,238 *CH H O N CO H O N* 4 2 2 2 22 2 ++ + ® + + ( ) (2)

æ ö æ ö + +- × + ® + + × +- ç ÷ ç ÷ è ø

b

<sup>1444442444443</sup> è ø (1)

aa

 b

*<sup>m</sup>*<sup>=</sup> (3)

F= = (5)

(4)

stoichiometric air amount

mixture of a 95 % methane (CH4) and 5 % hydrogen (H2)):

*s*

*f*

*O N CO H O*

component hydrocarbons (HC):

a

b

a b

178 Current Air Quality Issues

( % AE).

as

*fs*:

Process of fuel combustion is comprised of the following steps: entry of fuel jet into the combustion chamber, disintegration of the jet into droplets, decomposition of larger droplets into smaller, droplet heating, droplet evaporation, mixing of fuel vapour with the surrounding air, simultaneous auto-ignition of fuel mixture in several places, continued evaporation of the droplets and burning around (diffusion combustion), formation of soot during combustion in an area near droplets, temperature drop and slowing reaction due to expansion in the cylinder.

While the combustion temperature is still high, it is necessary that the soot particulate finds their reactants (oxygen) to complete combustion reaction. The phases until the simultaneous ignition of fuel mixture represent a delayed auto-ignition in several places which can be defined as the time or engine crank angle that elapses from the beginning of fuel ignition to the auto-ignition of the mixture.

A good spatial distribution of fuel affects the proper and economical operation of the engine. To achieve a good spatial distribution of fuel, it must be injected at a rate of about 150–400 ms-1, which requires a pressure of over 80 MPa.

Dispersion quality is determined by the injection speed, fuel surface tension, fuel viscosity, density of air in the cylinder, turbulence and cavitation in the nozzle. Better turbulence, mixing with air and combustion can be achieved by better penetration and propagation of jet fuel.

In marine diesel engine, injection is performed by injectors with the nozzles that direct the fuel into the cylinder space. Under the influence of aerodynamic forces of compressed air, fuel jet expands and breaks down into small droplets. The quality of fuel atomisation is defined by a mean diameter of the droplets and their uniformity. Better fuel dispersion is achieved with the smaller diameters of the nozzle holes, greater injection pressure and higher compression pressure inside the cylinder. The combustion process in a diesel engine can be divided into four phases (see Figure7).

**Figure 7.** Phases of the combustion process

The first phase, 'ignition delay, curve C-D', defines the period from the beginning of injection until the ignition starts and has an impact on the pollutant formation. This period defines fuel atomisation, evaporation, mixing and the reaction beginning. At sufficiently low turbulence, local flame fronts are created and produce high temperature without soot.

The second phase, 'uncontrolled combustion, curve D-E', is a homogeneous phase of com‐ bustion. At this stage, there are sudden ignition and combustion of the already prepared fuel mixture during the delayed ignition phase. Combustion begins simultaneously in several places and conducts intensively, and there is a sudden increase in the pressure and the temperature.

The third phase, 'partially controlled combustion, curve E-F', is diffusion combustion when the fuel droplets vaporise from the surface. Evaporated fuel is mixed with air, and combustion speed is limited by the rate of fuel evaporation and the speed of creating fuel mixture.

The fourth phase, 'after burning, curve F till the end', is the final part of the combustion and it takes about half of the total combustion time duration. During that phase, reaction slows due to expansion and decrease amounts of reactants, and a part of soot that is created during combustion leaves the cylinder as portion of emissions.

#### **4.3. Adiabatic flame temperature**

smaller diameters of the nozzle holes, greater injection pressure and higher compression pressure inside the cylinder. The combustion process in a diesel engine can be divided into

The first phase, 'ignition delay, curve C-D', defines the period from the beginning of injection until the ignition starts and has an impact on the pollutant formation. This period defines fuel atomisation, evaporation, mixing and the reaction beginning. At sufficiently low turbulence,

The second phase, 'uncontrolled combustion, curve D-E', is a homogeneous phase of com‐ bustion. At this stage, there are sudden ignition and combustion of the already prepared fuel mixture during the delayed ignition phase. Combustion begins simultaneously in several places and conducts intensively, and there is a sudden increase in the pressure and the

The third phase, 'partially controlled combustion, curve E-F', is diffusion combustion when the fuel droplets vaporise from the surface. Evaporated fuel is mixed with air, and combustion speed is limited by the rate of fuel evaporation and the speed of creating fuel mixture.

local flame fronts are created and produce high temperature without soot.

four phases (see Figure7).

180 Current Air Quality Issues

**Figure 7.** Phases of the combustion process

temperature.

One of the most important features of a combustion process is the highest temperature of the combustion products that can be achieved. The temperature of the combustion products will be the highest when there are no heat losses to the surrounding environment and when all energy released from combustion is used to heat the products. Constant pressure adiabatic temperature calculation, using a mean specific heat capacity method, can be performed for the lean and the rich combustion mixture as in [14]:

**a.** *For a lean mixture (Φ < 1):*

$$T\_{\rm AFT} = T\_{\rm R} + \frac{\Phi \cdot f\_{\rm s} \cdot LHV}{\left(1 + \Phi \cdot f\_{\rm s}\right) \cdot \overline{c}\_{p,\rm P}}\tag{8}$$

where TR represents the temperature of the reactants, i.e. fuel which has the compression temperature (T2) after injection and ignition delay, and *c*¯ *<sup>p</sup>* is an average specific heat capacity of the mixture.

#### **b.** *For a rich mixture (Φ > 1):*

$$T\_{ATP} = T\_R + \frac{f\_s \cdot LHV}{\left(1 + \Phi \cdot f\_s\right) \cdot \overline{c}\_{p,P}} \tag{9}$$
