Combustion performance of bio-ethanol at various blend ratios in a gasoline direct injection engine

Bio-ethanol has the potential to be used as an alternative to petroleum gasoline for the purpose of reducing the total CO₂ emissions from internal combustion engines and this paper is devoted to the investigation of using different blending-ratios of bio-ethanol/gasoline with respect to spark timing and injection strategies. The experimental work has been carried out on a direct injection spark ignition engine at a part load and speed condition. It is shown that the benefits of adding ethanol into gasoline are reduced engine-out emissions and increased efficiency, and the impact changes with the blend ratio following a certain pattern. These benefits are attributed to the fact that the addition of ethanol modifies the evaporation properties of the fuel blend which increases the vapour pressure for low blends and reduces the heavy fractions for high blends. This is furthermore coupled with the presence of oxygen within the ethanol fuel molecule and the contribution of its faster flame speed, leading to enhanced combustion initiation and stability and improved engine efficiency.     

Homogeneous charge compression ignition of LPG and gasoline using variable valve timing in an engine

The combustion characteristics and exhaust emissions in an engine were investigated under homogeneous charge compression ignition (HCCI) operation fueled with liquefied petroleum gas (LPG) and gasoline with regard to variable valve timing (VVT) and the addition of di-methyl ether (DME). LPG is a low carbon, high octane number fuel. These two features lead to lower carbon dioxide (CO₂) emission and later combustion in an LPG HCCI engine as compared to a gasoline HCCI engine. To investigate the advantages and disadvantages of the LPG HCCI engine, experimental results for the LPG HCCI engine are compared with those for the gasoline HCCI engine. LPG was injected at an intake port as the main fuel in a liquid phase using a liquefied injection system, while a small amount of DME was also injected directly into the cylinder during the intake stroke as an ignition promoter. Different intake valve timings and fuel injection amount were tested in order to identify their effects on exhaust emissions and combustion characteristics. Combustion pressure, heat release rate, and indicated mean effective pressure (IMEP) were investigated to characterize the combustion performance. The optimal intake valve open (IVO) timing for the maximum IMEP was retarded as the λ_TOTAL was decreased. The start of combustion was affected by the IVO timing and the mixture strength (λ_TOTAL) due to the volumetric efficiency and latent heat of vaporization. At rich operating conditions, the θ_90-20 of the LPG HCCI engine was longer than that of the gasoline HCCI engine. Hydrocarbon (HC) and carbon monoxide (CO) emissions were increased as the IVO timing was retarded. However, CO₂ was decreased as the IVO timing was retarded. CO₂ emission of the LPG HCCI engine was lower than that of the gasoline HCCI engine. However, CO and HC emissions of the LPG HCCI engine were higher than those of the gasoline HCCI engine.     

Influence of ethanol blends on the combustion performance and exhaust emission characteristics of a four-cylinder diesel engine at various engine loads and injection timings

The aim of this work is to investigate the effect of ethanol blending to diesel fuel on the combustion and exhaust emission characteristics of a four-cylinder diesel engine with a common-rail injection system. The overall spray characteristics, such as the spray tip penetration and the spray cone angle, were studied with respect to the ethanol blending ratio. A spray visualization system and a four-cylinder diesel engine equipped with a combustion and emission analyzer were utilized so as to analyze the spray and exhaust emission characteristics of the ethanol blending diesel fuel. Ethanol blended diesel fuel has a shorter spray tip penetration when compared to pure diesel fuel. In addition, the spray cone angle of ethanol blended fuels is larger. It is believed that the lower fuel density of ethanol blended fuels affects the spray characteristics. When the ethanol blended fuels are injected around top dead center (TDC), they exhibit unstable ignition characteristics because the higher ethanol blending ratio causes a long ignition delay. An advance in the injection timing also induces an increase in the combustion pressure due to the sufficient premixed duration. In a four-cylinder diesel engine, an increase in the ethanol blending ratio leads to a decrease in NO_x emissions due to the high heat of evaporation of ethanol fuel, however, CO and HC emissions increase. In addition, the CO and HC emissions exhibit a decreasing trend according to an increase in the engine load and an advance in the injection timing.     

Combustion of n-butanol in a spark-ignition IC engine

Alcohols, because of their potential to be produced from renewable sources and because of their high quality characteristics for spark-ignition (SI) engines, are considered quality fuels which can be blended with fossil-based gasoline for use in internal combustion engines. They enable the transformation of our energy basis in transportation to reduce dependence on fossil fuels as an energy source for vehicles. The research presented in this work is focused on applying n-butanol as a blending agent additive to gasoline to reduce the fossil part in the fuel mixture and in this way to reduce life cycle CO₂ emissions. The impact on combustion processes in a spark-ignited internal combustion engine is also detailed. Blends of n-butanol to gasoline with ratios of 0%, 20%, and 60% in addition to near n-butanol have been studied in a single cylinder cooperative fuels research engine (CFR) SI engine with variable compression ratio manufactured by Waukesha Engine Company. The engine is modified to provide air control and port fuel injection. Engine control and monitoring was performed using a target-based rapid-prototyping system with electronic sensors and actuators installed on the engine [1]. A real-time combustion analysis system was applied for data acquisition and online analysis of combustion quantities. Tests were performed under stoichiometric air-to-fuel ratios, fixed engine torque, and compression ratios of 8:1 and 10:1 with spark timing sweeps from 18° to 4° before top dead center (BTDC). On the basis of the experimental data, combustion characteristics for these fuels have been determined as follows: mass fraction burned (MFB) profile, rate of MFB, combustion duration and location of 50% MFB. Analysis of these data gives conclusions about combustion phasing for optimal spark timing for maximum break torque (MBT) and normalized rate for heat release. Additionally, susceptibility of 20% and 60% butanol-gasoline blends on combustion knock was investigated. Simultaneously, comparison between these fuels and pure gasoline in the above areas was investigated. Finally, on the basis of these conclusions, characteristic of these fuel blends as substitutes of gasoline for a series production engine were discussed.     

Performance and emission characteristics of an SI engine operated with DME blended LPG fuel

In this study, a spark ignition engine operated with DME blended LPG fuel was experimentally investigated. In particular, performance, emissions characteristics (including hydrocarbon, CO, and NO_x emissions), and combustion stability of an SI engine fuelled with DME blended LPG fuel were examined at 1800 and 3600 rpm.Results showed that stable engine operation was possible for a wide range of engine loads up to 20% by mass DME fuel. Further, we demonstrated that, up to 10% DME, output engine power was comparable to that of pure LPG fuel. Exhaust emissions measurements showed that hydrocarbon and NO_x emissions were slightly increased when using the blended fuel at low engine speeds. However, engine power output was decreased and break specific fuel consumption (BSFC) severely deteriorated with the blended fuel since the energy content of DME is much lower than that of LPG. Furthermore, due to the high cetane number of DME fuel, knocking was significantly increased with DME.Considering the results of the engine power output and exhaust emissions, blended fuel up to 10% DME by mass can be used as an alternative to LPG, and DME blended LPG fuel is expected to have potential for enlarging the DME market.     

The potential of di-methyl ether (DME) as an alternative fuel for compression-ignition engines: A review

This paper reviews the properties and application of di-methyl ether (DME) as a candidate fuel for compression-ignition engines. DME is produced by the conversion of various feedstock such as natural gas, coal, oil residues and bio-mass. To determine the technical feasibility of DME, the review compares its key properties with those of diesel fuel that are relevant to this application. DME’s diesel engine-compatible properties are its high cetane number and low auto-ignition temperature. In addition, its simple chemical structure and high oxygen content result in soot-free combustion in engines. Fuel injection of DME can be achieved through both conventional mechanical and current common-rail systems but requires slight modification of the standard system to prevent corrosion and overcome low lubricity. The spray characteristics of DME enable its application to compression-ignition engines despite some differences in its properties such as easier evaporation and lower density. Overall, the low particulate matter production of DME provides adequate justification for its consideration as a candidate fuel in compression-ignition engines. Recent research and development shows comparable output performance to a diesel fuel led engine but with lower particulate emissions. NO_x emissions from DME-fuelled engines can meet future regulations with high exhaust gas recirculation in combination with a lean NO_x trap. Although more development work has focused on medium or heavy-duty engines, this paper provides a comprehensive review of the technical feasibility of DME as a candidate fuel for environmentally-friendly compression-ignition engines independent of size or application.     

Investigations on surrogate fuels for high-octane oxygenated gasolines

Gasoline is a complex mixture that possesses a quasi-continuous spectrum of hydrocarbon constituents. Surrogate fuels that decrease the chemical and/or physical complexity of gasoline are used to enhance the understanding of fundamental processes involved in internal combustion engines (ICEs). Computational tools are largely used in ICE development and in performance optimization; however, it is not possible to model full gasoline in kinetic studies because the interactions among the chemical constituents are not fully understood and the kinetics of all gasoline components are not known. Modeling full gasoline with computer simulations is also cost prohibitive. Thus, surrogate mixtures are studied to produce improved models that represent fuel combustion in practical devices such as homogeneous charge compression ignition (HCCI) and spark ignition (SI) engines. Simplified mixtures that represent gasoline performance in commercial engines can be used in investigations on the behavior of fuel components, as well as in fuel development studies. In this study, experimental design was used to investigate surrogate fuels. To this end, SI engine dynamometer tests were conducted, and the performance of a high-octane, oxygenated gasoline was reproduced. This study revealed that mixtures of iso-octane, toluene, n-heptane and ethanol could be used as surrogate fuels for oxygenated gasolines. These mixtures can be used to investigate the effect of individual components on fuel properties and commercial engines performance.     

Effect of n-Butane and propane on performance and emission characteristics of an SI engine operated with DME-blended LPG fuel

In this study, a spark ignition engine operated with DME-blended LPG fuel was studied experimentally. In particular, the effect of n-Butane and propane on performance, emissions characteristics (including hydrocarbon, CO, and NOx), and the combustion stability of an SI engine fuelled with DME-blended LPG fuel were examined. Four kinds of test fuel with different blend ratios of n-Butane, propane, and DME were used. The percentage of DME in the fuel blend was 20% by mass.The results showed that stable engine operation was possible for a wide range of engine loads with propane containing LPG/DME-blended fuel rather than n-Butane containing LPG/DME-blended fuel since the octane number of propane is higher than that of n-Butane. Also, engine power output, break specific fuel consumption (BSFC), and combustion stability when operating with propane containing DME-blended fuel were comparable to those values in case of pure LPG fuel operation. Engine power output was decreased and BSFC was increased with n-Butane containing DME-blended fuel due to the lower energy density of DME. To examine the effect of n-Butane and propane on emissions and fuel economy in an actual vehicle, a vehicle was tested during an FTP-75 cycle. Through the emission and fuel economy test in the FTP-75 cycle, we conclude that the differences in emission level and fuel economy were not significant according to the blend of n-Butane, propane, and DME.Considering the experimental results from the engine bench and the actual vehicle, the 20% content of DME-blended fuel, regardless of LPG type, can be used as an alternative fuel for LPG.     

Experimental investigation of prechamber autoignition in a natural gas engine for cogeneration

A novel ignition concept based on autoignition in an unscavanged prechamber is currently being developed at the Laboratory for Industrial Energy Systems (LENI). On a single cylinder test engine a series of experimental runs (CR = 8.5-14, λ = 1 − 1.6, RPM = 1150/1500 min⁻¹) have been realized with natural gas as fuel, comparing the new ignition concept to standard spark ignition. The comparison is based on fuel efficiency and exhaust emissions (CO, THC, NO_x). The feasibility of operating the engine in autoignition mode has been demonstrated, and the potential of prechamber autoignition, in particular in the lean combustion regime, is indicated by the trends in fuel efficiency and emission concentration. The resistive heating of the prechamber walls has been shown to be an effective mean to trigger ignition. The prechamber could clearly be identified as primary ignition location. A reduction of the cycle-by-cycle variations - due to mixture fluctuations - is necessary to exploit the full potential of this engine concept.     

Simultaneous reduction of NOx emission and smoke opacity of biodiesel-fueled engines by port injection of ethanol

In the present study, the detailed combustion characteristics and emissions of biodiesel-fueled engines with premixed ethanol by port injection were investigated. The experiments were carried out on a single-cylinder, four-stroke, natural aspirated direct injection engine at a fixed speed. The heat release analysis indicates that, with the introduction of ethanol fuel by port injection, the ignition timing of the overall combustion event delays remarkably, while the maximum heat release rate increases smoothly. At a leaner fuel/air mixture, the peak value of the heat release rate (HRR) increases slightly, the maximum in-cylinder gas pressure and temperature decrease, and the indicated thermal efficiency (ITE) deteriorates with the increase of ethanol proportion. While at a rich fuel/air mixture, with the increase of the ethanol proportion, the maximum HRR increases rapidly, the overall combustion event is completed at an earlier crank angle. Moreover, the maximum values of the HRR reach the peak point in a certain premixed ratio which ranges from 20% to 40%. Also, the ITE reaches the largest value at this operation point. Due to the introduction of the ethanol fuel by port injection, both the NO_x emission and smoke opacity decrease to a very low level under overall operation conditions. During the experimental points of this test, NO_x and smoke opacity simultaneously decrease about 35-85% compared to those of the neat biodiesel-fueled engines.     

Effect of spark timing and load on a DISI engine fuelled with 2,5-dimethylfuran

Currently, bioethanol leads the automotive fuel market as the main substitute for gasoline in spark-ignition engines. However, worldwide interest has been triggered in the potential of 2,5-dimethylfuran, known as DMF, since the discovery of improved production methods. Although the energy content of DMF is comparable to that of gasoline, little is known about its combustion characteristics and emissions. Therefore, this work examines the effect of DMF in a single cylinder direct-injection spark-ignition engine. The results are compared to ethanol and gasoline using the optimized spark timings for gasoline and the respective fuel. In summary, DMF produces competitive combustion and emissions qualities to gasoline, which, in some cases surpass ethanol. The two biofuels have a higher burning rate and lower initial combustion duration than gasoline. They also produce greater combustion efficiency, which helps to lower hydrocarbon and carbon monoxide emissions. These initial results highlight how DMF, which was originally only considered as an octane improver, has the potential to become a competitive renewable gasoline alternative.     

Experimental study on effects of operating conditions and fuel quality on thermal efficiency and emission performance of a 300-MW boiler unit firing Thai lignite

This paper presents the results of an experimental study on a 300-MW boiler unit fired with Thai lignite. Effects of operating conditions (excess air ratio and unit load) and fuel quality on the boiler heat losses and thermal efficiency as well as on the gaseous (CO₂, CO, NO_x and SO₂) and particulate matter (PM) emissions from the boiler unit are discussed. The boiler thermal efficiency was weakly affected by the excess air ratio, unit load and fuel lower heating value, varying from 90.3 to 92.3% for wide ranges of the above variables. In all the tests, the NO_x, SO₂ and PM emissions were below the national emission standards for these pollutants. Quite low level of the SO₂ emission was secured by the high-efficiency flue gas desulphurization system. The CO emissions of rather small values were detected only at extremely low excess air ratios. The emission rate and specific emission (i.e. per MWh of electricity produced) for NO_x, SO₂ and CO were quantified using experimental emission concentrations of the pollutants. Meanwhile, the emission characteristics for CO₂ were determined with the use of fuel-C and fuel consumption by the boiler. In addition, the emission rate and specific emission for PM were estimated by taking into account the actual fuel-ash content and fuel consumption by the boiler, as well as the effects of SO₂ adsorption by fly ash in the boiler gas ducts and overall ash-collecting efficiency of the electrostatic precipitators and flue gas desulphurization system. Elevated CO₂ and NO_x emissions from the 300-MW boiler units firing Thai lignite are of great concern.     

Combustion characteristics of coal in a mixture of oxygen and recycled flue gas

The combustion of coal in a mixture of pure O₂ and recycled flue gas is one variant of a novel combustion approach called oxy-fuel combustion. With the absence of N₂, this approach leads to a flue gas stream highly enriched in CO₂. For many applications, this flue gas stream can then be compressed and sequestered without further separation. As a result, oxy-fuel combustion is an attractive way to capture CO₂ produced from fossil fuel combustion. When coal is burned in this O₂ and CO₂ rich environment, its combustion characteristics can be very different from conventional air-fired combustion. In CETC-O, a vertical combustor research facility has been used in the past years to investigate the combustion characteristics of several different coals with this variant of oxy-fuel combustion. This included flame stability, emissions of NO_x, SO_x and trace elements, heat transfer, in-furnace flame profiles and flue gas compositions. This paper will report some of the major findings obtained from these research activities.     

CO2 capture using oxygen enhanced combustion strategies for natural gas power plants

This paper describes a series of experiments conducted with natural gas in air and in mixtures of oxygen and recycled flue gas, termed O₂/CO₂ recycle combustion. The objective is to enrich the flue gas with CO₂ to facilitate its capture and sequestration. Detailed measurements of gas composition, flame temperature and heat flux profiles were taken inside CANMET's 0.3 MWth down-fired vertical combustor fitted with a proprietary pilot scale burner. Flue gas composition was continuously monitored. The effects of burner operation, including swirling of secondary stream and air staging, on flame characteristics and NO_x emissions were also studied. The results of this work indicate that oxy-gas combustion techniques based on O₂/CO₂ combustion with flue gas recycle offer excellent potential for retrofit to conventional boilers for CO₂ emission abatement. Other benefits of the technology include considerable reduction and even elimination of NO_x emissions, improved plant efficiency due to lower gas volume and better operational flexibility.     

Experimental study on the auto-ignition and combustion characteristics in the homogeneous charge compression ignition (HCCI) combustion operation with ethanol/n-heptane blend fuels by port injection

This article investigates the auto-ignition, combustion, and emission characteristics of homogeneous charge compression ignition (HCCI) combustion engines fuelled with n-heptane and ethanol/n-heptane blend fuels. The experiments were conducted on a single-cylinder HCCI engine using neat n-heptane, and 10%, 20%, 30%, 40%, and 50% ethanol/n-heptane blend fuels (by volume) at a fixed engine speed of 1800 r/min. The results show that, with the introduction of ethanol in n-heptane, the maximum indicated mean effective pressure (IMEP) can be expanded from 3.38 bar of neat n-heptane to 5.1 bar, the indicated thermal efficiency can also be increased up to 50% at large engine loads, but the thermal efficiency deteriorated at light engine load. Due to the much higher octane number of ethanol, the cool-flame reaction delays, the initial temperature corresponding the cool-flame reaction increases, and the peak value of the low-temperature heat release decreases with the increase of ethanol addition in the blend fuels. Furthermore, the low-temperature heat release is indiscernible when the ethanol volume increases up to 50%. In the case of the neat n-heptane and 10% ethanol/n-heptane blends, the combustion duration is very short due to the early ignition timing. For 20–50% ethanol/n-heptane blend fuels, the ignition timing is gradually delayed to the top dead center (TDC) by the ethanol addition. As a result, the combustion duration prolongs obviously at the same engine load when compared to the neat n-heptane fuel. At overall stable operation ranges, the HC emissions for n-heptane and 10–30% ethanol/n-heptane blends are very low, while HC emissions increase substantially for 40% and 50% ethanol/n-heptane blends. CO emissions show another tendency compared to HC emissions. At the engine load of 1.5–2.5 bar, CO emissions are very high for all fuels. Beside this range, CO emissions decrease both for large load and light load. In terms of operation stability of HCCI combustion, for a constant energy input, n-heptane shows an excellent repeatability and light cycle-to-cycle variation, while the cycle-to-cycle variation of the maximum combustion pressure and its corresponding crank angle, and ignition timing deteriorated with the increase of ethanol addition.     

Combustion characteristics of lignite-fired oxy-fuel flames

This experimental work describes the combustion characteristics of lignite-fired oxy-fuel flames, in terms of temperature distribution, gas composition (O₂, CO₂, CO, total hydrocarbon concentration and NO) and ignition behaviour. The aim is to evaluate the flame structure of three oxy-fuel cases (obtained by changing the flue gas recycle rate) including a comparison with an air-fired reference case. Measurements were performed in Chalmers 100 kW test unit, which facilitates oxy-fuel combustion under flue gas recycling conditions. Temperature, O₂ and CO concentration profiles and images of the flames indicate that earlier ignition and more intense combustion with higher peak temperatures follow from reduction of the recycle rate during oxy-fuel operation. This is mostly due to higher O₂ concentration in the feed gas, reduced cooling from the recycled flue gas, and change in flow patterns between the cases. The air case and the oxy-fuel case with the highest recycle rate were most sensitive to changes in overall stoichiometry. Despite significant differences in local CO concentration between the cases, the stack concentrations of CO are comparable. Hence, limiting CO emissions from oxy-fuel combustion is not more challenging than during air-firing. The NO emission, as shown previously, was significantly reduced by flue gas recycling.     

车用乙醇汽油挥发性研究

分析了车用乙醇汽油中影响挥发性的组分,介绍了变性燃料乙醇对汽油挥发性及车用乙醇汽油挥发性对汽车运行和环境的影响。通过对车用乙醇汽油族组成、蒸汽压和蒸馏特性的解析,明确了催化裂化汽油、催化重整汽油、烷基化汽油、异构化汽油、直馏汽油及含氧化合物等的调合作用。车用乙醇汽油挥发性的增大使汽车的启动性能增强、驱动性能变差、发生光化学污染的可能性变大。针对环保方面提出了车用乙醇汽油调合组分偏重一些,少加或不加其他含氧化合物,增加高辛烷值调合组分调合量的建议。     

Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine

The aim of this work was to investigate the effect of narrow fuel spray angle injection and dual injection strategy on the exhaust emissions of a common-rail diesel engine. To achieve successful homogeneous charge compression ignition by an early timing injection, a narrowed spray cone angle injector and a reduced compression ratio were employed. The combination of homogeneous charge compression ignition (HCCI) combustion and conventional diesel combustion was studied to examine the exhaust emission and combustion characteristics of the engine under various fuel injection parameters, such as injection timings of the first and second spray.The results showed that a dual injection strategy consisting of an early timing for the first injection for HCCI combustion and a late timing for the second injection was effective to reduce the NO_x emissions while it suppress the deterioration of the combustion efficiency caused by the HCCI combustion.     

Clean and efficient use of petroleum coke for combustion and power generation

Petroleum coke is a challenging fuel in terms of its low volatile content, high sulfur and nitrogen content, which give rise to undesirable emission characteristics. However, the low price and increased production of petroleum coke from high-sulfur feedstocks give a powerful economic stimulus to use it for power generation. In this study two strategies are explored for the clean combustion of petroleum coke by in situ removal of CO₂ and sulfur or in the longer term for hydrogen production with CO₂ recovery. CaO sorbent is used to absorb CO₂ and SO₂ in fluidized combustor or steam gas reformer, and is regenerated in a calciner. In the calciner, petroleum coke is fired with pure oxygen to provide the heat for regenerating the sorbent and give a CO₂ stream for subsequent utilization or sequestration. The SO₂ is retained in solid CaSO₄ and would not be emitted to the atmosphere. The high carbon content and low moisture content of petroleum coke ensure high purity of the CO₂ stream. The low ash content is also important since it reduces the possibility of ash fusion in the calciner. It also reduces the heat loss and the requirement for ash disposal, and hence contributes to high overall efficiency. Simulation results show that high efficiency can be achieved with incorporation of the proposed scheme for power generation, even after the penalty of CO₂ recovery. Thus, there is a potential for the abundant available and low cost, but environmentally challenging petroleum coke to become a fuel for clean combustion and power generation.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.