Impact of Compressed Natural Gas (CNG) Fuel Systems in Small Engine Wood Chippers on Exhaust Emissions and Fuel Consumption

The projected increase in the availability of gaseous fuels by growing popularity of household natural gas (NG) filling stations and the increase in the production of gaseous biogas-derived fuels is conducive to an increase in the use of NG fuel. Currently, natural gas in various forms (compressed natural gas (CNG), liquefied natural gas (LNG)) is popular in maritime, rail and road transport. A new direction of natural gas application may be non-road mobile machines powered by a small spark-ignition engine (SI). The use of these engines in the wood chippers can cause the reduction of machine costs and emissions of harmful exhaust gases. In addition, plant material chippers intended for composting in bio-gas plants can be driven by the gas they are used to produce. The biogas can be purified to bio-methane to meet natural gas quality standards. The article presents the design of the natural gas supply system, which is an upgrade of the Lifan GX 390 combustion engine spark ignition engine (Four-stroke, OHV (over head valve) with a maximum power of 9.56 kW), which is a common representative of small gasoline engines. The engine is mounted in a cylindrical chipper designed for shredding branches with a maximum diameter of up to 100 mm, which is a typical machine used for cleaning work in urban areas. The engine powered by CNG and traditionally gasoline has been tested in real working conditions, when shredding cherry plum (Prunus cerasifera Ehrh. Beitr. Naturk. 4:17. 1789 (Gartenkalender4:189–204. 1784)). Their diameter was ca. 80 mm, 3-metere-long, and humidity content ca. 25%. The systems were tested under the same actual operating conditions, the average power generated by the drives during shredding is about 0.69 kW. Based on the recorded results, it was found that the CNG-fuelled engine was characterized by nitrogen oxides (NOx) emissions higher by 45%. The other effects of CNG were a reduction in carbon dioxide (CO2), carbon monoxide (CO) and hydrocarbon (HC) emissions of about 81%, 26% and 57%, respectively. Additionally, the use of CNG reduced fuel consumption by 31% and hourly estimated machine operating costs resulting from fuel costs by 53% (for average fuel price in Poland: gasoline: 0.99 EUR/L and CNG: 0.71 EUR/m3 on 08 November 2020). The modernization performed by the authors ensured the work of the drive unit during shredding, closer to the value of stoichiometric mixtures. The average (AVG) value of the air fuel ratio (AFR) for CNG was enriched by 1.2% (AVG AFR was 17), while for the gasoline engine the mixture was more enriched by 4.8% (AVG AFR was 14). The operation of spark-ignition (SI) combustion engines is most advantageous when burning stoichiometric mixtures due to the cooperation with exhaust aftertreatment systems (e.g., three-function catalytic converter). A system powered by CNG may be beneficial in systems adapting to operating conditions, used in low-power shredding machines, whose problem is increased HC emissions, and CNG combustion may reduce them. The developed system does not exceed the emission standards applicable in the European Union. For CO emissions expressed in g/kWh, it was about 95% lower than the permissible value, and HC + NOx emissions were 85% lower. This suggests that the use of the fuel in question may contribute to tightening up the permissible emission regulations for non-road machinery.


Introduction
Natural gas (NG) is an alternative fuel within the meaning of the European Union Directive (2014/94/UE), as it is an alternative for energy sources derived from crude oil [1].
It is used in many forms for driving internal combustion engines, e.g., NG, compressed natural gas (CNG) and liquefied natural gas (LNG) [2,3]. The main component of NG is methane (CH 4 ) (usually ≥90%), in addition, small amounts of ethane, propane, butane and other organic and mineral compounds may be present [4]. Due to the relatively high amount of CH 4 in NG (e.g., in Poland it is 98%), the designs with methane supply are also worth analysing.
The use of NG for driving internal combustion engines is an innovative development trend in many industry branches. Mainly in maritime and rail transport in the form of LNG and road transport in the form of CNG. Some of the work in this area of knowledge is directed towards the replacement of traditional fuels (gasoline or diesel) by CNG. The second group of ongoing activities explores the impact of NG blends added to traditional fuels. Another group of researchers are analysing the impact of hydrogen H 2 blends in CNG.
Studies on the emission of harmful exhaust gas compounds in public transport buses fuelled with diesel or CNG under actual operating conditions have shown that CNG buses may have a reduction of CO 2 by 6% and NO x by 90% [5]. On the other hand, they are characterized by higher emissions of CO by 45% and total hydrocarbon (THC) by 156% [5]. The research on the fleet of vehicles adapted to waste collection in urban areas has shown that vehicles fuelled with CNG have reduced CO 2 emission and energy consumption [6] in comparison to traditional fuels (gasoline, diesel oil). Comparative research of the SI engine fuelled with gasoline and CNG showed that the application of CNG reduced the driving power by about 12-30% [7][8][9][10][11][12][13]. One aspect is the reduction in the volumetric efficiency of CNG-fuelled engines due to the larger percentage of air in CNG by volume. Lower torque and power were reported as a result of lower or leaner fuel (CNG) injection per cylinder. Average brake mean effective pressure and brake-specific fuel consumption (bsfc) for CNG were lower than that of gasoline owing to the higher heating value of CNG [7]. This property also exhibited an increase in the net thermal efficiency of the engine [14]. Furthermore, the emissions of harmful HC combustion gases were lower by 57%, CO by 43%, CO 2 by 19% and NO x by 20%. Additionally, the authors demonstrated that the engine operation on CNG fuel deteriorates the lubricating oil properties [7]. The research carried out in China on automotive vehicles has shown that compressed (CNG) and liquefied (LNG) gaseous fuels allow to reduce the emission of carbon to the atmosphere by about 10% in comparison to gasoline vehicles [15]. A review of research conducted on vehicles in road transport shows that the use of CNG can bring benefits in terms of reducing toxic emissions. However, this reduction does not have to apply to all the emitted exhaust gas compounds, and these parameters may change with the operating time and the fuel system wear level [2,3].
Natural gas in the form of liquid LNG is the fuel of choice for long-distance maritime transport and heavy rail transport, where it is considered a low-emission fuel [16]. The use of this fuel requires special fuel supply facilities [16,17] as it is a cryogenic fuel with a boiling point of approximately 120 K (−153 • C) [16]. LNG is a popular fuel for sea vessels due to its low cost and environmental friendliness [18]. This fuel in maritime transport is a transitional fuel in the decarbonisation of maritime transport [19]. In ship engine designs, the use of LNG fuel may limit NO x [20]. However, the level of NO x emission is strongly related to the efficiency level of the drive unit [20]. Other studies on LNG-powered ships show a reduction in fuel consumption, CO 2 and NO x emissions compared to traditional fuel (diesel), while at the same time causing higher HC emissions than traditional fuels [21]. Tests of engines with LNG fuel installations show that they can reduce exhaust emissions.
Tests are also carried out on diesel and CNG blends for diesel particulate matter emissions (diesel share between 70% and 85%). Depending on the operating conditions, the use of CNG additives reduced engine smoke from 10% to 92% and the total number of particulates from 30% to 40% [22]. Other research on compression-ignition (CI) engines shows that NG additives reduced CO 2 emissions from 17% to 20%, but increased CO and HC emissions at the same time. In addition, it also reduced NO x emissions by 60% to 82% and smoke by 23% to 39% [23]. Innovative research directions on the supply of CNG spark-ignition internal combustion engines are directed towards hydrogen admixtures, creating HCNG fuels with hydrogen addition [24]. Research shows that the right proportion of hydrogen and CNG can reduce the emission of harmful exhaust gas compounds (CO, CO 2 , HC, NO x ) [25] or keep it at a convergent level [26] in relation to petrol or CNG engines while maintaining similar torque characteristics [25].
The term "small engines" concerns spark ignition (SI) engines in terms of their use in wood chippers with a power not exceeding 19 kW [27,28]. Regulations concerning nonroad mobile equipment with compression-ignition (CI) engines apply to engines without defined restrictions of the power of the drive unit. However, they introduce a division into various research cycles, which depend on the engine power [27]. Among the innovative applications in these drive units, electronic fuel supply systems can be included [29][30][31], as the liberal regulations concerning exhaust emissions, e.g., in the European Union (Regulation 2016/1628/EU), allow the use of carburetor power systems [28]. Other innovations relate to adaptive systems for operating conditions combined with systems regulating the supply of fuel-air mixture to the combustion chamber [32][33][34][35][36] or alternative fuels (e.g., liquefied petroleum gas (LPG)) [37][38][39][40][41][42][43], ethanol [44,45], methanol [46]). The authors noted the lack of articles describing the modernization of the design enabling the use of CNG fuel in small spark ignition (SI) engines. The authors' solution is designed in such a way that it may constitute a modernisation module of the drive units available on the market. Commercial small engines characterized by the most advanced fuel supply system, electronically controlled, is a model produced by Honda: iGX 390. This unit in Europe is about four times more expensive than engines produced by Chinese and Indian manufacturers under a Honda license, with the traditional fuel supply system (carburetor) [47]. Emissions regulations in Europe are not conducive to a reduction in this type of power unit. That is why the authors have undertaken works to modernise a unit that does not require an electronic control unit by adapting the carburetor to gaseous fuels. Such a solution is characterised by lower costs and, according to the authors, it is more ready to be implemented on the market. The authors performed tests of engines powered by gasoline and CNG in real working conditions. "Gasoline" refers to non-lead petrol as a flammable liquid that is mainly used as a fuel in most spark-ignited internal combustion engines, with a value of 95 octane number. They have compared the emissions of harmful exhaust gases, referred them to emission regulations, determined fuel consumption and assessed the operating costs in terms of the cost of the fuel used. They described the modernisation and determined its costs.
The tests of the emission of harmful exhaust gas compounds are described in the global trend of testing the impact of machines on the environment and surroundings under real working conditions. Such tests were carried out for high power shredding machines (180 kW-600 kW) [48][49][50] and low power shredding machines (less than 10 kW) [32,43]. The authors see the possibility of increasing interest in and applications for the development of gaseous fuel supply systems in internal combustion engines, to which the increasingly common biogas plants [51] (increase in types of biodegradable materials [52][53][54]) and household filling stations [55] may contribute.

Materials and Methods
During the research a Red Dragon RS-100 wood chipper was used (Remet CNC Technology Sp. Z O.O., Kamień, Poland), which is presented in Figure 1a. This machine can be considered a typical one, which is commonly used for grinding wood material in urban areas. The technical specification of this chipper is presented in Table 1. The chipper with the factory settings (A) was driven by the spark-ignition (SI) engine Lifan GX390 (License: American Honda Motor Company, Inc., Torrance, CA, USA), 9.5 kW (Figure 1b). The tested drive unit technical specification is shown in Table 1, while its characteristics concerning fuel consumption as well as power and torque are presented in Figure 2a. This engine efficiency characteristics were determined on the basis of input and output energy and are presented in Figure 2b. As a result of the carried out works, the original machine was modernized to the second version (B). In version (B) the gasoline-powered carburetor was replaced by a carburetor system adapted to the use of natural gas. The modified, natural gas-powered carburetor is presented in Figure 1c. The carburetor adapted to natural gas could be supplied in two configurations: from a household gas installation-At a pressure not lower than 0.01 MPa (Figure 1f). In the case of the installation available to the authors, the gas pressure was 0.002 MPa, therefore the fuel supply system was replaced by a system adapted to CNG (Figure 1e). Such a solution involved the need to expand the gas from the cylinder in which it was stored at a pressure of 20 MPa. The gas expansion from 20 MPa to 0.6 MPa led to freezing of the first-stage regulator, so a gas heater was used between the cylinder's valve and the regulator to ensure proper operation of the first-stage regulator. Then a second stage regulator was used to reduce gas pressure from 0.6 MPa to 0.01 MPa. The gas pressure of 0.01 MPa made it possible to supply enough fuel to achieve the operating conditions ensuring full power of the combustion engine. The authors selected this value by checking the composition of the fuel-air mixture. Namely, if, as the load increased, the mixture became poorer and the introduced fuel dose control in the carburetor did not change the AFR value, it meant that the pressure on the second-stage regulator reduced the required fuel dose. If under full load the engine could operate with the enriched mixture, it meant that the amount of fuel supplied is not limited by the second-stage regulator. In the group of small engines, a CNG fuel supply system is an innovative solution and it is commercially available as a modernization possibility. The most essential components in combination with their costs required for the modernization of the gasoline engine are presented in Table 2. The designs (A) and (B) were both adjusted before the tests following the rules of regulating the tested carburetors for stoichiometric mixtures (AFR). For gasoline-powered engine (A) the AFR value was 14.7:1, while for CNG-powered design (B) it was 17.2:1. As part of the research, the emission level of selected harmful exhaust gas compounds was compared, which included: carbon dioxide CO 2 , carbon monoxide CO and hydrocarbon HC as well as nitrogen oxides NO x . This required conducting experiments on both designs (A) and (B) in real operating conditions. The tests concerned shredding branches with a maximum diameter of up to 100 mm. The size of the wood material used resulted from the strength of the cutting mechanism and the safety of machine operation. Table 3 presents the characteristics of the fuels used to power the engines during the tests. Table 1. Technical specification of Red Dragon RS-100 wood chipper and Lifan GX390 engine.

Red Dragon RS-100 wood chipper Parameter Characteristic
Cutting mechanism type Cylindrical Cut branches maximal diameter 100 mm Cut branches length (mechanism has 4 knives) 140 ± 50 mm Average mass flow rate [35] 0.66 t/h Average volumetric flow rate [35] 3.5 m 3 /h

System Components and Service Cost
Carburetor € 46 CNG gas regulator with pressure gauge (second-stage) € 7 CNG high-pressure gas regulator (first-stage) € 20 Installation hose € 6 Gas heater € 75 10 l gas tank € 70 Working time spent on the modernization of the design and system adjustment 1 h * € 23 *-based on market prices in Poland for first half of 2020. Natural gas composition at a CNG refuelling station in Poland (% by volume) The cherry plum timber was selected for the tests (Prunus cerasifera Ehrh. Beitr. Naturk. 4:17. 1789 (Gartenkalender 4:189-204. 1784)). During the experiment branches with a diameter of ca. 80 mm and a length of 3 m were shredded. Their moisture content was checked before and during the tests and was about 25%-A METTLER TOLEDO HR83 moisture analyzer (Mettler Toledo, Columbus, OH, USA) was used for this purpose. Branches of similar dimensions were selected in order to standardize the conditions of the experiment. This provided a similar machine load and uniform shredding time, which for this kind of branch was about 4.5 s [26]. Shredding of such material generates a relatively high load (work near maximum permissible load values) on the cutting mechanism. The tested branches can be considered typically representative of hardwood species in accordance with the Janka classification [61]. The Janka hardness test measures the resistance of a sample of wood to denting and wear. It measures the force required to drive a steel drive 11.28 mm (0.444 in) diameter halfway into a sample of wood [62,63]. During the experiment, the branches were delivered to the chipper by one experienced operator from a pile 1 m away.
In order to confirm the convergence of the machine working conditions when driving with different fuel supply systems, the parameters of drive unit load and speed values were recorded. Diagram of the test station is presented in Figure 3. The station consists of a tested engine connected via a 1:1 ratio link transmission with a torque measuring shaft (by Pracownia Elektroniki Roman Pomianowski, Poznań, Poland) recording the torque and speed. Due to the fact that the torque measuring shaft should not be loaded with a bending torque, which may be caused by the link transmissions, countershafts have been mounted between the link transmission and the torque measuring shaft. The countershaft behind the torque measuring shaft is connected to the transmission (5:1), which transmits the torque in accordance with the ratio used in the factory machine. The recorded torque value describes the engine load after considering the efficiency of the system between the torque measuring shaft and the engine. The torque measuring shaft is mounted according to the manufacturer's recommendations by means of clutches with a certain degree of flexibility. This is to ensure safe operation. In the presented system there are mechanical losses caused by friction in rolling elements, elastic couplings' susceptibility and belt transmission efficiency. The influence of twisting of countershafts has been omitted in the tests, because their rigidity is much higher than the aforementioned elements. The total mechanical efficiency of the system is given by formula (1): where: η b is the efficiency of rolling bearings, η c is the efficiency of the clutch, η bd is the efficiency of the link transmission. The mechanical power P 1 in the system as measured by the torque measuring shaft is related to the mechanical power P 2 on the motor shaft as described by Equation (2): The mechanical power values can be defined as relationships (3): where: M 1 is the torque measured by the torque measuring shaft, ω 1 is the angular speed measured by the torque measuring shaft, n 1 is the speed measured by the torque masuring shaft, M 2 is the torque developed by the engine, ω 2 is the engine angular speed and n 2 is the engine speed. After considering the slip, using the ξ (4) belt slip rate: and Equations (2) and (3) the relationship of torques can be written as (5): The values used in the calculation are shown in Table 4. Axion RS+, a typical portable emissions measurement system (PEMS) from Global MRV, was used to test exhaust emissions. Table 5 presents its technical specification. In the research emission, levels of hydrocarbon (HC), carbon monoxide (CO), carbon dioxide (CO 2 ) and nitrogen oxides (NO x ) were analyzed. Fuel consumption was determined on the carbon balance [68]. The measuring device used in the experiments carried out the measurement of concentrations expressed in vol.% or ppmv. As a result, more measurable emissions were determined. The emission value was calculated on the basis of the measured concentrations of the tested chemical compounds and the measurement of the air mass supplied to the combustion chamber by measuring the pressure in the inlet manifold.

Work Conditions
Comparison of exhaust emissions and fuel consumption under real work conditions should be made under similar operating conditions. During the tests, the torque value M 1 (Figure 4) and rotational speed n 1 ( Figure 5) were recorded. Based on these measurements, the measured values were recalculated using relationships (2), (3) and (5), and then, according to Equation (6), the power P 2 generated by the drive during the shredding process was determined ( Figure 6). Changes in these values as a function of time are due to system loading. The torque value can be interpreted as the torque generated by the internal combustion engine; however, it should not be interpreted as the torque value necessary for shredding the branches (from which the cutting force can be calculated [69][70][71]), as the measurement takes place in front of the chipper flywheel, which increases the cutting force.  Figure 9, Figure 10 show the results of the hourly emissions of harmful exhaust gas compounds measured during the experiments (CO, CO 2 , HC, NO x respectively). They also contain fuel consumption level of both gasoline-powered and CNG-powered engines. The mentioned characteristics are a function of time and were recorded during the chipping process. Higher changes in the instantaneous emission value for an CNG-powered engine can be observed for all harmful compounds. These instantaneous changes (visible as sudden jumps in value) result from differing engine loads and are caused by supplying branches to the chipper feeding channel. It can be concluded that emissions from the CNG-powered engine shows a greater susceptibility to variation in load.

Composition of the Fuel-Air Mixture
Air-fuel ratio (AFR) characteristics concerning the duration of shredding processes is shown on Figure 11. It is impossible to compare directly the gasoline-powered and CNG-powered engines. This is due to different AFR values for the stoichiometric mixtures of these fuels, which are 14.7 and 17.2 for gasoline and CNG, respectively. Nevertheless, the tested engines individual nature of work can be observed. Under the operating conditions, during tests, the gasoline-powered engine had a rich mixture and maintained it at a relatively stable level, differing from the average AFR value (AVG ≈ 14) by about 10%. On the other hand, the CNG-powered engine had a slightly enriched mixture (AVG ≈ 17), which was also maintained at a relatively stable level. However in the second case, the average AFR value deviated by about 2.5% from the stoichiometric mixture. This indirectly contributes to the higher efficiency of this engine, as shown in Figure 2b, because the supplied fuel was almost completely burned and converted into energy. Figure 11. Air-fuel ratio (AFR) characteristics for the engine powered by gasoline and CNG.

Fuel Consumption and Efficiency
Measuring fuel consumption is an important aspect in assessing the cost of machine use. The results of fuel consumption tests are presented in Figure 12. It can be noted that an engine running on gasoline is less susceptible to fuel consumption characteristics oscillation than an engine running on CNG. Additionally, the input energy supplied to the engine can be determined from the fuel consumption ( Figure 13). Based on the information on the input and output energy determined from the power measurement (Figure 6), the efficiency of the drive units during the chipping processes ( Figure 13) can be calculated. It can be noticed that the CNG-powered engine has an average efficiency of about 5.1% under the tested operating conditions, and the gasoline-powered engine was characterized by an efficiency of 3.8%. The CNG-powered engine is approximately 34% more efficient than the gasoline-powered engine.

Discussion
In order to control efficiently the drive unit and to evaluate the reaction of the tested design to changes in the operating conditions, the emission of the generated exhaust gas compounds as a time function should be measured. The results may show areas that need improvements in the design. The results of exhaust emission tests over a specified period of time should also be considered important. These characteristics are presented in Figure 14 along with the average values. They provide information on the influence of selected machines on global or local exhaust emissions, thus allowing their comparison. Such a comparison (for gasoline and CNG) is presented in Figures 14 and 15 on a percentage scale calculated according to Equation (7): where: x A is the selected value for the gasoline fuel supply system, x B is the selected value for the CNG fuel supply system. Equation (7) is written in such a form that drops in the value are positive and increases in the value are negative.  The use of CNG installations reduced harmful emissions of exhaust gases: CO by 81%; CO 2 by 26%; and HC by 57% compared to the engine with the classic gasoline carburetor system. The benefits obtained are consistent with the results of other researchers who have modernized internal combustion engines in other branches of industry, which are usually characterised by a much higher capacity and are subject to different exhaust emission regulations, most often more stringent, which was connected with higher technical advancement of these drive systems. Among the available test results one can observe the reduction of some chemical compounds and the growth of others. A common effect of the change of fuel from gasoline to CNG, coinciding with the results of the authors' research, is the reduction of CO 2 [5][6][7]72], CO [7,73] and HC [7,[72][73][74][75]. CNG is composed of lighter hydrocarbons, and the ratio of hydrogen to carbon in CNG is much higher than in gasoline. This affects the combustion process in the cylinder, reducing the proportion of incomplete combustion, hence lower CO and HC emissions. On the other hand, the reduction of CO 2 is mainly related to the reduction in fuel consumption. The disadvantage of the solution applied by the authors is the increase in NO x emissions by 45%. Organic NO x emissions after a fuel change is characteristic for CI engines [5,73,75,76]; however, they are often characterised by an increase in other combustion components. In SI engines an increase in NO x is noticeable with a decrease in other emissions [72,73]. The increase in NO x emissions in a CNG-fuelled engine is due to the higher maximum temperatures in the engine cylinder during combustion. As described above, it is also the cause of the reduced emission of incomplete combustion products HC and CO. Such a phenomenon can be reduced by enrichment of CNG with hydrogen. Tests of the SI engine powered by HCNG have shown the reduction of all 4 emissions at the same time (CO, CO 2 , HC, NO x ) [7], hence this may be another direction of development of the tested design. Natural gas and hydrogen mixture combustion should intensify the lean-burn traits as well as lower some of the harmful emissions, such as CO, HC and CO 2 . The main issue, however, is the expected increase in NO x emissions [77,78]. Employing an NG/H 2 mixture as a fuel can be profitable as it will take advantage of the positive aspects of hydrogen combustion. At the same time, the application of such a mixture does not require substantial alterations to the already existing natural gas engines [77,79]. The use of hydrogen inclusion as a complement to natural gas will boost the lean-burn limit to the hydrogen's extended flammability range. Lean-burn ability enhances the thermal efficiency while at the same time decreasing the combustion temperature and the emission of NO x [77,80,81]. It also decreases the knock coincidence, which is a phenomenon that can lead to serious damage to the SI engine. The tested drive unit was analysed by the authors in another study-after modernisation of the fuel supply system supplying liquefied petroleum gas (LPG). The drive unit tested there, fuelled with LPG under real operating conditions, was characterised in relation to its gasoline equivalent, only by the reduction of CO and NOx emissions, out of all analysed parameters (CO, CO 2 , HC, NO x ) [43].
Comparing indirectly the systems tested (LPG and CNG), it can be seen that the use of CNG allowed to reduce a larger number of exhaust gas compounds (CO, CO 2 , HC) emitted.
An attempt to relate exhaust emission results to EU legislation [27,28] requires their determination in g/kWh. The calculation of such a value is possible by measuring the power generated by the drive unit during shredding processes (Figure 6), the average value of which for the engines tested can be assumed to be around 0.69 kWh. The emission value expressed in g/kWh has been referred to emission regulations. The acceptable limits for the harmful compounds in exhaust gas emission for the tested group of drive units according to the provisions in force in the European Union from 2019 are presented in Table 6 [27]. These engines are subject to the NRS-vr/vi-1b category, according to Stage V emission standards for non-handheld spark ignition SI engines below 56 kW (NRS). Figure 16a shows that the exhaust emissions from the engines under test are far from exceeding the emission limits for CO and HC + CO 2 , the emissions of which are controlled during laboratory emission tests. Not all working machines and vehicles tested under real operating conditions are characterised by such emission parameters and many tests show that under real operating conditions the emission limits are exceeded [82][83][84][85]. The tests of the design described in the article showed that the emissions from an engine powered by gasoline were lower by about 43% for CO and 49% for HC + NO x , while for an engine with a CNG installation the emissions were lower than the permissible limits, for CO by 94% and for HC + NO x by 85% (Figure 16b).  Another effect of using CNG installations is a 32% reduction in fuel consumption ( Figure 14). Such an effect is also noticeable by other scientists [72]. It is also often associated with the loss of power and torque characteristics (Figure 2), and dynamics of the examined drive unit [86,87]. In the analysed design, the authors observed a reduction of maximum power by 23% and torque by 15%. At the same time, these changes did not affect the shredding process and did not change the characteristics of the machine in the area of works and tests (the load due to the strength of the cutting mechanism was 20% lower than that allowed by the manufacturer, who expressed it in the diameter of the branches to be shredded and their hardness-hardwood species [61] to 100 mm in diameter). The reduction of fuel consumption after the use of CNG, and at the same time the reduction of power and torque of the engine, is related to the reduction of the volumetric efficiency of the CNG-fuelled engine, due to the greater share of air during CNG combustion (AFR CNG 17.2: 1, AFR gasoline 14.7:1). The lower fuel consumption expressed in g/s is reliable, but it is difficult to interpret the cost of machine use. Firstly, gasoline is sold in EUR/L (0.99 EUR/L in Poland on 11 November 2020 [88]), while CNG in EUR/m 3 (0.71 EUR/m 3 in Poland 11 November 2020 [89]). The authors made the necessary calculations to facilitate the interpretation of the machine operating costs resulting from fuel consumption and determined the hourly cost of machine operation for the system powered by gasoline at about 2.02 EUR/h, while during operation with CNG at 0.95 EUR/h. The operating cost of the wood chipper under test, resulting from fuel consumption, is 53% when running on CNG compared to using gasoline.
The cost of modernizing an engine is around EUR 247, with the average cost of a new engine at EUR 270 (Table 2), so it can almost double its value. However, on the other hand, with an 8 h operating mode and savings of about EUR 1 per hour, the financial outlay incurred may be returned after about 31 days. With the average cost of a machine of EUR 1300, the addition of modernisation costs will increase the cost of machine purchase by 19% and the modernisation performed will then constitute about 16% of the machine cost.
The adaptation of IC engines to combustion of gaseous fuels may become a contemporary scientific trend as to the use of biogas [90][91][92]. Biogas obtained from the decomposition of, e.g., sewage [93], organic municipal waste [94,95], animal faeces [96], agricultural and food industry waste [97,98] and plant materials [96,97,99], can be used by plants processing these materials to propel their machinery and equipment [100,101]. Biogas is a mixture of gases characterized by a varied composition, but it can be purified to bio-methane and then meet the quality standards of natural gas [102]. Biogas can be considered as a renewable energy source because the cycle of carbon in its production and consumption cycle is closed and does not involve net carbon dioxide emissions [103]. Such work may be carried out in the future. However, it should be noted that biogas may have a methane content that is half that of natural gas.

Conclusions
Small engine modernization, by changing the fuel supply system and the fuel itself to an alternative such as CNG, can bring both environmental and economic benefits. Comparative research of a wood chipper in urban areas with two fuel supply systems: gasoline and CNG, has shown that the use of CNG fuel has reduced the emission of harmful exhaust compounds: CO (by 81%), CO 2 (by 26%), HC (57%). However, it has contributed to an increase in NO x emissions (by 45%). According to the authors, NO x emissions can be reduced by using hydrogen (H 2 ) admixtures to CNG, but this requires new tests. The value of emissions from a CNG-powered unit was significantly lower than those allowed in the European Union (CO by 94%, HC+NO x by 85%), which may be a suggestion to the legislators to introduce more restrictive provisions. The engine after the application of CNG had lower power and torque, but this did not prevent the proper implementation of the shredding process. The CNG engine had a 32% lower fuel consumption expressed in g/s and, combined with the lower CNG fuel price, also had a lower hourly cost of machine operation resulting from the fuel price (by about 53%). The cost of upgrading a CNG engine has almost doubled its value. The system used in the wood chipper increased its purchase cost by 19%. However, with an 8 h operating mode and demonstrated lower exploration costs resulting from the modernization (about 1 EUR/hour), it is returned after about 30 days. Moreover, the reduction in fuel consumption and pollutant emissions affects the local environment (reduction of air pollution in urban areas, which are highly susceptible to exceeding the permissible levels of air pollution), global environment (reduction of the greenhouse effect) and machine operators, who are most vulnerable to the negative impact of combustion engines. The engine powered by CNG fuel in a wood chipper seems to be a favourable solution to be used in adaptive drive control systems [32,36], as it is characterised by relatively low HC emissions. On the other hand, the adaptive systems powered by gasoline reduce the emission of CO, CO 2 and NO x , but emit more HC to the atmosphere [32].