Oxyfuel Combustion in IC Engines

*Jun Peng and Xiang Li*

#### **Abstract**

This chapter introduces principles, mixing, ignition and combustion and controls processes of oxyfuel combustion which aims to achieve CCS (Carbon Capture and Storage) in IC (Internal Combustion) engines. By replacing air with pure oxygen and using hot and/or cooled EGR as dilutant gas for controlling the combustion process and flame speed, the mixing and combustion process will be explained. Fuel delivery, pre-mixing arrangement between pure oxygen and dilutant gas and their influences on combustion performances will be discussed. HCCI (Homogeneous Charge Compression Ignition), water injection, etc., technologies for enhancing the combustion efficiency will be demonstrated in detail. Finally, the emission characteristics and possible implementation of practical engine operation will be described.

**Keywords:** oxyfuel combustion, pure oxygen combustion, mixing, ignition, combustion process, combustion efficiency, EGR (Exhaust Gas Recirculation), CCS (Carbon Capture and Storage)

#### **1. Introduction**

As Net Zero Carbon Emissions strategy has been implemented in some countries and will be implemented gradually throughout the world, various zero carbon technologies are being developed, such as pure Battery Electric Vehicle (BEV) and Fuel Cell Vehicle (FCV) that have achieved certain mass production, although they have a high price and low driving range. For considering heavy-duty applications and with an alternative technology for a low-cost operation, hydrogen combustion technology and CCS (Carbon Capture and Storage) technology for hydrocarbon combustion are being developed widely [1].

It has been proposed that oxygen-enriched combustion and oxyfuel combustion are efficient ways to increase engine efficiency and reduce pollutant emissions. Oxyfuel combustion uses pure oxygen for combustion instead of air [2]. Due to the absence of nitrogen in the intake charge, NOx emissions will be eliminated. As a result, carbon dioxide and water vapour are the only products of combustion. Studies to date have been mainly focused on applying oxyfuel or oxygen-enriched combustion technologies to gas turbines and coal-fired power plants. The utilization of oxyfuel combustion and CCS for IC (Internal Combustion) engines has been gaining a lot of attention during the last few years [3]. Research on oxygen-enriched combustion shows that a slight increase in oxygen reduces smoke emissions as well as the amount of CO (Carbon Monoxide) and unburnt hydrocarbons but increases the amount of nitrogen oxides (NOx). Various technologies have been used to decrease NOx and particulates, such as Exhaust Gas Recirculation (EGR) and optimum injection

strategies. Research conducted recently has drawn attention to oxyfuel and nitrogenfree combustion because of the benefits it brings to vehicles and is being used to make huge improvements to the efficiency of IC engines and to achieve zero NOx emissions. As is known, over-high peak pressures and peak pressure increases can easily appear in engine cylinders in oxygen-enriched conditions. An increase in combustion flame temperature is expected if oxygen is used instead of air. To minimize overheating problems due to overheating, it is crucial that the fuel injection flow rate should be accurately controlled in order to eliminate unexpected temperature rises in the premixed and diffusion combustion. Further, the diluent ratio and intake charge temperature have a meaningful impact on controlling in-cylinder temperature and combustion process.

Compared with other zero-carbon technologies being applied in various power and energy systems, oxyfuel combustion has the following advantages and drawbacks which should be paid attention.

*Advantages*:


#### *Disadvantages*:


#### **2. Oxygen combustion principles**

#### **2.1 Principles**

Pure oxygen getting combustion with conventional hydrocarbon fuels (liquid or gas) produces just carbon dioxide (CO2) and water steam as combustion products. As water steam can be condensed to be liquid and using a liquid-gas separator to remove liquid water, only CO2 gas is left and it can be stored, as demonstrated in **Figure 1**.

*Oxyfuel Combustion in IC Engines DOI: http://dx.doi.org/10.5772/intechopen.107155*

#### **Figure 1.**

*Schematic of an application of oxyfuel combustion engine [1].*

Because too fast reaction and too fast combustion flame propagation speed then result in too high combustion pressure increase rate and too high peak combustion temperature (although this will not produce high NOx emissions, while it lacks nitrogen in the mixture, it will need high-quality materials for the engine design), the combustion with the mixture of pure oxygen and hydrocarbon must be diluted. Basically, EGR (Exhaust Gas Recirculation) which consists of only CO2 and water can be employed. Some research also used pure CO2 or pure water (water injection).

Then, the stoichiometric reaction equation can be written as follows:

$$\rm C\_aH\_b + \left(a + \frac{1}{4}b\right)O\_2 + dilutant = aCO\_2 + \frac{1}{2}bH\_2O + dilutant \tag{1}$$

Replacing Air/Fuel Ratio (AFR) which is a very important control parameter used for conventional air-fuel combustion, Oxygen/Fuel Ratio (OFR) which should be employed for measuring the mixture for oxyfuel combustion can be defined as follows:

$$AFR = \frac{m\_{air}}{m\_{fuel}}\tag{2}$$

$$OFR = \frac{m\_{\text{oxygen}}}{m\_{\text{fuel}}} \tag{3}$$

For keeping a stoichiometric combustion (this is especially important for oxyfuel combustion while the stoichiometric combustion will save oxygen consumption) as demonstrated in Eq. (1), OFR can be derived from Eq. (1) as:

$$OFR\_{Stoi} = \frac{m\_{\text{oxygen}}}{m\_{\text{fuel}}} = \frac{\Im \left(a + \frac{1}{4}b\right)}{(\mathbf{1}\mathbf{2}a + b)}\tag{4}$$

If considering the normal air has 21% oxygen and 79% nitrogen by volumetric fraction or molar fraction, AFR for conventional combustion can also as expressed as function of OFR:

$$AFR = \frac{m\_{air}}{m\_{fuel}} = \frac{\left(m\_{\text{oxygen}} + m\_{\text{nitrogen}}\right)}{m\_{fuel}} = \frac{\left(m\_{\text{oxygen}} + \frac{7996 \times 28}{2196 \times 32} m\_{\text{oxygen}}\right)}{m\_{fuel}} = 4.29 \frac{m\_{\text{oxygen}}}{m\_{fuel}} = 4.29 \text{OFR} \tag{5}$$

As oxyfuel combustion has only water and CO2 as the combustion products (the tiny fraction of other emissions can be ignored here), EGR (Exhaust Gas Recirculation) can be utilized as the dilutant for decelerating the reaction speed. The EGR rate can be expressed as:

$$EGR = \frac{m\_{EGR}}{m\_{EGR + Oxygen + fuel}}\tag{6}$$

GFR (Gas Fuel Ratio) for reflecting the ratio of in-cylinder all gas to fuel can be expressed as:

$$GFR = \frac{m\_{oxygen + EGR}}{m\_{fuel}} = \frac{\left(m\_{oxygen} + m\_{EGR}\right)}{m\_{fuel}} = OFR + \frac{m\_{EGR}}{m\_{fuel}}\tag{7}$$

#### **2.2 Fuel, oxygen and dilutant supplies**

For oxyfuel combustion engines, there is no difference for the fuel supply from conventional IC engines. Gasoline-oriented oxyfuel combustion engines can still have PFI (Port Fuel Injection) or GDI (Gasoline Direct Injection) whatever as used as the original design.

Although diesel engines can also operate oxyfuel combustion, it is not economic in terms of oxygen cost while it is not easy to keep stoichiometric oxyfuel combustion for diesel engines.

With regard to oxygen supply, pure oxygen can be supplied with oxygen tanks for light-duty applications, in particular those mobile light-duty applications. For heavy duty application, onboard oxygen production may be needed. Oxygen can be delivered to the oxyfuel engine through the intake manifold before or after mixed with diluent gas.

As mentioned previously, an easy option for dilutant is EGR. To delivery EGR gas into the cylinder, water, CO2 and water + CO2 should have appropriate mixing quality. A mixing chamber can be considered for this purpose.

For achieving ideal thermal efficiency, water is always a better candidate than CO2 because water has a similar volumetric heat capacity as nitrogen but CO2's volumetric heat capacity is much higher than nitrogen.

#### **2.3 Combustion process and flame propagation**

Replacing nitrogen as existing in air-fuel combustion with EGR (CO2 or/and H2O) as used in oxyfuel combustion, it results in a significant influence on combustion characteristics. As volumetric heat capacities of N2, CO2 and H2O are 32.76 kJ/kmolK, 54.12 kJ/kmolK and 36 kJ/kmolK, respectively, the big increase of heat capacity from N2 to CO2 can lead oxyfuel combustion's flame speed obviously reduced under the same operating condition, compared with air-fuel combustion. Then, the combustion temperature and in-cylinder pressure will be reduced too much. As a result, oxyfuel combustion has lower thermal efficiency than air-fuel combustion if CO2 is used as the only or main diluent gas. Therefore, for obtaining a better thermal efficiency, the dilutant of oxyfuel combustion should use as less as possible CO2. As the increase of heat capacity from N2 to H2O is much smaller than that from N2 to CO2, EGR as main diluent choice for oxyfuel combustion can be treated for providing as high as possible H2O fraction.

#### *Oxyfuel Combustion in IC Engines DOI: http://dx.doi.org/10.5772/intechopen.107155*

In a DNS (Direct Numerical Simulation) study presented by Zhong et al. [4] for oxyfuel combustion with CH4 as fuel in a constant volume combustion chamber, it demonstrated the influence of CO2 and H2O on oxyfuel combustion characteristics. As shown in **Table 1**, five modelling cases were tested for Case 1—with N2 (Nitrogen) as dilutant gas (or conventional air-fuel combustion), Case 2—with H2O (steam) as dilutant gas, Case 3—CO2 as dilutant gas, Case 4—with H2O as dilutant but its chemical effects are isolated (FH2O), Case 5—with CO2 as dilutant gas also its chemical effects are isolated (FCO2). The method for isolating chemical effects of H2O in Case 4 or CO2 in Case 5 is to stop those reactions between pre-added H2O or CO2 and other species. Those H2O and CO2 produced during the combustion process are still allowed to have reactions with other species.

In **Figure 2**, it can be seen that CO2 additive (Case 3) can significantly decelerate the combustion speed and then slow down the increase of combustion temperature very obviously, compared with air-fuel combustion (Case 1). H2O additive (Case 2) does not reduce the combustion speed, compared with Case 1, and even accelerates that a little. This suggests that using pure water steam as dilutant gas in oxyfuel combustion would keep the flame speed, combustion temperature and thermal efficiency very similar to air-fuel combustion. But CO2 will provide a significant disadvantage for those.

In the same study, by isolating chemical effects of H2O (Case 4) and CO2 (Case 5), it can be found that CO2 additive even can remarkably accelerate the combustion speed. That suggests CO2's chemical effects play much bigger role than its thermal effects in decelerating the combustion speed. Water steam's chemical effects have a


#### **Table 1.**

*DNS simulation parameters and results [4].*

#### **Figure 2.**

*Temporal development of consumed fuel fraction Cf and combustion temperature (Case 1—N2, Case 2—H2O, Case 3—CO2, Case 4—FH2O, Case 5—FCO2) [4].*

reverse function, and the magnitude is very small. In **Table 1**, laminar flame speeds (SL) and other results of five cases are also listed.

In an experimental study of partially premixed CH4/oxyfuel flames in a swirl stabilised burner [5], it demonstrated that the shape and stabilisation of oxyfuel combustion flame are closely related to the oxygen concentration, and it was suggested that the oxygen fraction in the mixture of oxygen and dilutant gases should be at least similar as in air (volumetric 21%) for maintaining the stabilisation of oxyfuel combustion flames.

#### **3. Oxygen combustion engines and performances**

#### **3.1 Gasoline-oriented oxyfuel combustion**

So far, most reported oxyfuel IC engines are gasoline-oriented engines. Because gasoline engines can be easily operated under stoichiometric conditions, they will not waste oxygen, which results in a certain cost for oxyfuel combustion. As shown in **Figure 3** below, a gasoline engine modified for oxyfuel combustion which was used by Wu et al. [6] employs almost the same configuration as the core engine combustion system. The fuel delivery is conventional PFI (Port Fuel Injection), and the gas path has a pure oxygen supply from an oxygen tank. CO2 also from a tank is supplied to the intake manifold for representing EGR. For increasing water content in dilutant gas, a water injection system is fitted for directly inject water into the cylinder.

By experimental investigation, in-cylinder pressure traces are recorded for demonstrating the effects of water injection on the increase of in-cylinder pressure [7], as shown in **Figure 4**. The in-cylinder traces show that water injection can remarkably increase in-cylinder pressure during expansion stroke. If the water injection time is adjusted for the optimal crank angle, a very ideal thermal cycle efficiency can be achieved. The same researchers have carried out comprehensive for applying water injection to improve oxyfuel combustion's thermal efficiency and optimal operating and control parameters have been presented.

#### **Figure 3.** *Gasoline oxyfuel combustion engine with Direct Water Injection (DWI) [6].*

*Oxyfuel Combustion in IC Engines DOI: http://dx.doi.org/10.5772/intechopen.107155*

**Figure 4.** *In-cylinder pressure traces demonstrating the influence of DWI [7].*

In **Figure 5**, effects of water injection temperature on the thermal efficiency of oxyfuel combustion are presented. The results suggest that the increase of water injection mass will linearly increase the thermal efficiency. Meanwhile, higher water temperature will always benefit the thermal efficiency too. With a heat exchanger for allowing injected water heated from exhaust heat, the water temperature up to 200°C can be achieved [8].

Effects of intake pressure with increased water injection mass are demonstrated in **Figure 6**. The increase of intake pressure can be understood for meeting the requirement of engine power increase, while fuel amount can increase with the increase of intake pressure. As shown in **Figure 6**, increased water to total intake mass ratio can result in MEP (Mean Effective Pressure) increase with various intake pressure. The higher the intake pressure is, the bigger increase of MEP by water to total intake mass ratio provides.

#### **3.2 Diesel-oriented oxyfuel combustion**

Diesel-oriented oxyfuel combustion engines have been paid attention to because they can be operated with much heavier duty than gasoline engines. Most applications with IC engines integrated with oxyfuel combustion and CCS (Carbon Capture and Storage) are those heavy engines, such as those used in maritime sector and stationary power generation sector.

**Figure 5.** *Effects of water injection temperature on thermal efficiency [8].*

**Figure 6.** *Effects of intake pressure on MEP [8].*

Although it is difficult to operate diesel engines under stoichiometric conditions, optimal combustion system configuration and control can make the OFR (Oxygen/ Fuel Ratio) very close to the stoichiometric condition, and then, wasted oxygen amount can be limited.

Because diesel combustion needs high AFR (OFR for oxyfuel diesel combustion), the combustion process is very sensitive to oxygen concentration in the mixture. As shown in **Figure 7**, reduced oxygen concentration (when the total mixture amount in the cylinder is constant) can obviously reduce in-cylinder pressure (then definitely combustion temperature). As a result, IMEP and thermal efficiency will reduce with the same fuel injection amount [9, 10].

For reducing the cost of oxygen supply, it is always beneficial to have as low as possible OFR (Oxygen/Fuel Ratio). But reduced oxygen concentration can result in the mixture contaminated with inadequate oxygen for achieving complete combustion for diesel oxyfuel combustion. Reflected on ISFC (Indicated Specific Fuel Consumption), stoichiometric OFR with the lowest ISFC can be found with adjustable oxygen to CO2 ratio, as shown in **Figure 8**.

In **Figure 8**, LambdaO2 (relative OFR) is defined as:

$$
\Box \Lambda \text{ambda}\_{O2} = \frac{\text{OFR}\_{\text{actual}}}{\text{OFR}\_{\text{stoi}}} \tag{8}
$$

**Figure 7.** *Effects of oxygen concentration on diesel oxyfuel combustion process [9].*

In **Figure 8**, at LambdaO2 = 1.0, the combustion with stoichiometric mixture is the most economic point with regard to the cost of oxygen supply. Achieving a lower ISFC (Indicated Specific Fuel Consumption) but maintaining LambdaO2 at 1.0 will be desired. Then, lower oxygen concentration with higher total in-cylinder mass due to increased dilutant gas CO2 (green line in **Figure 8**) is a better choice than other conditions.

Intake temperature also needs carefully adjusted, while it also influences diesel oxyfuel combustion performances remarkably. As shown in **Figure 9**, higher intake temperature can reduce IMEP (Indicated Mean Effective Pressure) because higher intake temperature results in less intake gas amount, and then less fuel amount and lower IMEP.

In **Figure 10**, it shows that the reduced IMEP by increased intake temperature may be more due to the advanced ignition timing. As the intake temperature increases from 140°C (blue line in **Figure 10**) to 220°C (yellow line), the ignition timing can advance from about 355 CA (Crank Angle) to about 345 CA. Earlier ignition timing makes the heat release taking place too early before TDC will no doubt increase compression negative work, then IMEP reduces.

**Figure 8.** *Effects of relative OFR (LambdaO2) on ISFC [9].*

**Figure 9.** *Effects of intake temperature on IMEP under different oxygen concentrations [10].*

**Figure 10.** *Effects of intake temperature on ignition timing [10].*

From the viewpoint for a better mixing, late ignition timing with the same injection timing will lead long ignition delay, which can result in more homogeneous mixture with long fuel evaporation and mixing time. Then, HCCI (Homogeneous Charge Compression Ignition) combustion under oxyfuel combustion mode can be achieved. Several reported studies of diesel oxyfuel combustion [9–11] have demonstrated their explorations into HCCI oxyfuel combustion in diesel engines.

Because higher intake temperature can result in lower IMEP under the same fuel amount, ISFC with increased intake temperature has obvious increase, as shown in **Figure 11**. Those results suggest that operating parameters for oxyfuel combustion play very important role for influencing oxyfuel combustion engine performance, similar as conventional air-fuel combustion. Careful calibration should always be necessary for achieving desired engine performances.

By experimental investigation, diesel oxyfuel combustion has been examined by several report research. Reported by Kang et al. [11] for studying diesel oxyfuel

**Figure 11.** *Effects of intake temperature on ISFC [10].*

#### **Figure 12.**

*A diesel test engine modified to operate oxyfuel combustion [11].*

combustion in a test diesel engine, conventional common rail high-pressure injection system is employed, as shown in **Figure 12** below. By arranging CO2 and O2 supply from gas tanks and with direct water injection for achieving higher thermal efficiency, the effects of water injection temperature on in-cylinder pressure can be found in **Figure 13** below. From the result, it can be seen that water injection can result in a small reduction of compression negative work, a small reduction of expansion work, and an obvious increase of ignition delay and retarded ignition timing. By increasing

**Figure 13.** *In-cylinder pressure traces under different water injection temperature [11].*

#### **Figure 14.**

*Effects of water injection temperature on IMEP, COV of IMEP, ignition delay and thermal efficiency [11].*

the water temperature, it shows that expansion work can obviously increase, while the compression negative work has no obvious variation.

With the combustion analysis based on the measured in-cylinder pressure traces, the experiment work reported that the increased water temperature can benefit oxyfuel combustion performances significantly.

As shown in **Figure 14**, based on the reported operation condition, IMEP (Indicated Mean Effective Pressure) gets some increase and COV (Coefficient of Variation of IMEP) gets reduction. Finally, the increase of water injection temperature makes thermal efficiency increased.

#### **3.3 Emissions**

As discussed previously, oxyfuel will not produce NOx emissions due to the absence of nitrogen in the mixture. Although the combustion can result in uHC (unburnt Hydrocarbon) and CO emissions, those can be captured and storage with CO2 together. Therefore, generally there is not concern of emissions for oxyfuel combustion.

**Figure 15.** *Effects of intake temperature on CO emissions [10].*

#### **Figure 16.** *Effects of intake temperature on CO2 emissions [10].*

Therefore, the combustion efficiency should be always given priority over emissions for oxyfuel combustion process optimization. Reported level of CO emissions from oxyfuel diesel combustion by Mobasheri et al. [10] can be up to 0.8 kg/kgfuel under 15% of oxygen concentration and 220°C of intake temperature from less than 0.1 kg/kgfuel under 21% of oxygen concentration and 140°C intake temperature, as shown in **Figure 15** below.

Compared with CO2 level (shown in **Figure 16**) in the same exhaust gas which are about 23 kg/kgfuel and 27 kg/kgfuel under those two operating conditions, it can be estimated that incomplete combustion (just considering CO from the incomplete combustion but not including uHC) can be between 0.3% and 3.5%. This means that some amount energy is not fully released during combustion stage. As CO is mainly produced from inadequate oxygen in the mixture, increasing OFR will be necessary for diesel oxyfuel combustion, if the price of oxygen is not a big problem.

#### **4. Oxygen productions and supplies**

Supplying oxygen to oxyfuel combustion can be carried out in several ways. The first one is to use high-pressure oxygen tanks. This is more suitable for mobile and light-duty applications. The second alternative is using an oxygen generator working on the swing adsorption technique using pump compressors and air driers for producing oxygen with a purity of approximately 95%. This will be suitable for mobile but heavy-duty applications. For those heavy-duty but stationary applications, oxygen produced from water electrolysis with mass production can be considered. As green or blue hydrogen production are being developed for meeting the requirement of Net Zero Carbon Emissions, hopefully oxygen as a side product will become abundant in the near future.

Considering useful energy output of 1 kg gasoline or diesel fuel from oxyfuel combustion is about 44 MJ � 40% = 17.6 MJ. The gasoline or diesel fuel of 1 kg consists of about 0.87 kg of carbon (or 0.87/12 = 0.0725 kmol) and 0.13 kg of H2 (or 0.13/2 =

0.065 kmol). To fully burning those carbon and hydrogen, the required oxygen amount will be:

$$0.0725C + 0.065H\_2 + 0.105O\_2 = 0.0725CO\_2 + 0.065H\_2O \tag{9}$$

$$0.105 \, kmol \times \text{\textdegree 32} = \text{3.36 kg} \tag{10}$$

Therefore, 1 kg of gasoline or diesel fuel for complete combustion will require about 0.105 kmol or 3.36 kg of oxygen.

#### **4.1 Oxyfuel tank**

If the oxygen supply is supplied by commercial oxygen cylinder for lab application, the current price from BOC Company in the UK is about £2/kg. Burning 1 kg gasoline or diesel fuel and 3.36 kg oxygen will require £6.72. Using the current diesel price £2/ litre or £2.44/kg in the UK market, the cost for oxygen is nearly three times of diesel fuel price. This is too expensive if using commercial lab oxygen supplied from BOC.

By checking possible lowest price for oxygen, it is found that in China, 99.5% of oxygen for industrial applications can be down to RMB 2.0/kg, equivalent to £0.22/kg. If similar technology can be provided in the United Kingdom and further improvements can be made for mass production, the cost for oxygen supply by cylinder will be acceptable.

Using commercial oxygen supply, safety regulations for handling and use of highpressure gas cylinders must be strictly adhered to for use and installation on oxyfuel combustion engines. In this case, the EU pressure equipment directive would be applicable to the onboard use of oxygen gas cylinders. More information about this regulation can be found online [12].

If high-pressure oxygen gas cylinders are used on oxyfuel combustion engines, other components such as pressure regulators, electronic flow control valves and piping to engine are required. A flow control circuit that is controlled by the engine speed requirements must be designed and installed to provide the right amount of oxygen at the engine intake as required.

#### **4.2 Onboard oxyfuel production**

*Cryogenic Compression Cycle:* This technology can separate oxygen from air using a cryogenic compression cycle [13]. In **Figure 17**, it illustrates some major unit

**Figure 17.** *Cryogenic compression cycle system [13].*

**Figure 18.** *Schematic of a cryogenic distillation technology [14].*

operations required to cryogenically separate air into useful products. The method will at first lower the temperature of air until it liquefied. Then, liquid air is distilled, and the component gases are separated from each other. Each component that boiled off was separated and captured individually. This cryogenic distillation process was the foundation for the industrial production of high-grade oxygen, nitrogen and argon.

*Cryogenic Distillation:* As demonstrated in **Figure 18**, the cryogenic distillation technique [14] can be used when either very high-quality oxygen (>99.5%), high volumes of oxygen (≥102 tons of oxygen/day) or high-pressure oxygen are required.

**Figure 19.** *Schematic of pressure swing adsorption [15].*

Cryogenic air separators need to take more than an hour to start up. Additionally, since cryogenics can produce such a high purity of oxygen, the waste nitrogen stream is of usable quality. This can add significant financial benefits to a process integrated with a cryogenic. However, the cryogenic distillation process and equipment are very expensive and suited for large industrial oxygen production plants.

*Pressure Swing Adsorption (PSA)*: Pressure Swing Adsorption (PSA) technology, as demonstrated in **Figure 19**, is the most promising from an energy and cost point of view. The technology involves an adsorption/desorption of gases onto solid surfaces while under pressure [15]. In fact, the pressurising of the system means that the solid material can adsorb more gases than under normal atmospheric conditions. The gases are usually pressurised to about 8 bar and adsorbed on the solid materials. Different solid substrates attract different gases; hence, the oxygen can be separated from air using this technique. This pressure adsorption technique is also being investigated for adsorption and storage of CO2 from the exhaust in thermal power stations as postcombustion CCS. To produce 1 m<sup>3</sup> of oxygen with the pressure, swing adsorption technology needs about 1 kWh of energy.

#### **5. Implementation of oxyfuel combustion**

#### **5.1 Case Study I—Gasoline oxyfuel combustion with water injection**

As shown in **Figure 20**, a practical system for implementing gasoline oxyfuel combustion with water injection was developed by Wu et al. [6]. The system has a PFI fuel injection system. The gas path has a pure oxygen supply from a liquid tank. Before the oxygen is fed into the intake manifold to mix with EGR, a heat exchange utilities exhaust heat warming the oxygen flow. After the EGR manifold, exhaust gas is firstly cooled with a heat exchanger for condensing steam and then separating liquid water. The liquid water then is reheated through the coolant channel of the engine

**Figure 20.** *Gasoline oxyfuel combustion engine [6].*

*Oxyfuel Combustion in IC Engines DOI: http://dx.doi.org/10.5772/intechopen.107155*

**Figure 21.** *Gasoline oxyfuel combustion engine [16].*

block and the heat exchanger. Finally, the heated water is injected into the cylinder directly.

The advantages of this configuration can have as more as possible water and as less as possible EGR (consisting of CO2). The water temperature can also be adjusted by controller by controlling the water flow or by a bypass valve outside the heat exchanger. Then, the oxyfuel combustion performance can be maintained at satisfactory level.

#### **5.2 Case Study II—Oxyfuel combustion engine for inland boats**

This case is about a practical application of diesel oxyfuel combustion engine on an inland boat where there is possible space for installing the exhaust gas after treatment system [16, 17]. As shown in **Figure 21**, the oxygen supply has no difference from that as used in Case Study 1. Diluent gas mainly comes from raw EGR and/or treated EGR, depending on the requirement for controlling the combustion performance. Two stages of heat exchangers are employed for ensuring an adequate condensation of steam in exhaust gas. An exhaust heat recovery cycle based on Rankine cycle is integrated for increased the whole system's energy efficiency. Liquid water in condensed exhaust gas is separated with a water-gas separator; then, CO2 is compressed and stored in an onboard tank.

#### **6. Summary**

Although oxyfuel combustion has been studied on gasoline and diesel engines by a number of researchers and developers, so far, there are still very limited practical applications. The main factors for limiting oxyfuel combustion in practical engines are oxygen cost, CCS cost, necessary space for installing exhaust treatment equipment and storing CO2 onboard.

With possible technology development and increased requirement on zero carbon emissions with low cost, hopefully oxyfuel combustion engine will be prototyped and manufactured in the near future.

*Renewable Energy – Recent Advances*

#### **Author details**

Jun Peng<sup>1</sup> \* and Xiang Li<sup>2</sup>

1 University of Lincoln, Lincoln, UK

2 University of Bedfordshire, Luton, UK

\*Address all correspondence to: jpeng@lincoln.ac.uk

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Oxyfuel Combustion in IC Engines DOI: http://dx.doi.org/10.5772/intechopen.107155*

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