Natural gas is used for different applications such as power generation, domestic use, transportation, and as a feedstock for the production of ammonia-based fertilizers. It is the dominant alternative road transport fuel in addition to ethanol. Since natural gas contains mainly methane, biomethane or electro-methane can be used as a substitute. In road transport vehicles, methane is mostly used in compressed form (compressed natural gas, CNG, or compressed biogas, CBG) in light-duty vehicles. However, in heavy-duty long-haul vehicles the interest is also in liquefied form (liquefied natural gas, LNG, or liquefied biogas, LBG). For long distance travel, natural gas is usually shipped as LNG, and then re-gasified in coastal terminals to be injected into the natural gas grid. Biomethane and electro-methane could be produced locally. Therefore, it is not as dependent on gas grid or shipping as natural gas. In all pathways, the composition of the natural gas has high variability.

Methane is traditionally used in the Otto engine, either under stoichiometric or lean-burn conditions. In last years, other engine technologies have been developed, e.g. high-pressure direct-injection (HPDI) compression-ignition engines with dual fuel operation for methane and diesel fuel. For HDPI concept, energy efficiency is higher than for stoichiometric gas engine. On the other hand, the stoichiometric engine can control emissions efficiently with a three way catalyst; also NO_x emissions that are problematic for lean-burn natural gas engines. DF engines need to be equipped with similar aftertreatment technology as diesel engines to meet emission legislation in many regions. All natural gas engines deliver low PM emissions. Methane emissions from current state-of-the-art natural gas vehicles are in general below the emission limits. Many of the emission species have been reported to be lower for natural gas vehicles than for gasoline or diesel vehicles.

AMF work on methane covers Tasks 6, 39, 48, 51 and 57 (see right-hand bar). AMF Task 51 Key Messages pointed out that "The use of methane in transport is predicted to increase, especially in liquid form in heavy-duty road transport and in the marine sector. Methane reduces, e.g., particulate emissions, and holds a promise to reduce CO2 emission up to 20…25 %. However, almost all methane fuelled engines have methane slip that could nullify the CO2 benefit. AMF Task 51 shows that technologies exist to mitigate this problem."

• General on methane pathways
• Legislation, standards and properties
• Distribution
• Methane engines, efficiency and emissions,
• Numerical emissions

General on methane pathways

Natural gas (NG) is used as an energy source and as an automotive fuel to respond the need for reduction of oil dependence and thus to increase in the security of energy supply. Natural gas is a colorless, odorless, and clean-burning fuel that has been and is currently used for many different applications such as power generation, domestic use, transportation, and as a feedstock for the production of ammonia-based fertilizers. Natural gas is the dominant alternative road transport fuel in addition to ethanol. The global fleet of natural gas vehicles is approximately 22.4 million according to the NGV Journal (http://www.ngvjournal.com/worldwide-ngv-statistics/, entered 17.7.2016). With number of light-duty and heavy-duty vehicles, their mileages and fuel consumptions, global natural gas consumption in road transport would be in maximum 60 Mtoe. Synthetic diesel fuel is produced from natural gas by liquefaction (Gas-to-Liquid, GTL). According to the IEA Headline Energy Data 2015, global use of natural gas in the transport sector in 2013 was 96.2 Mtoe, which indicates potential use of natural gas as GTL in the range of 30 Mtoe. Additionally to convering natural gas to GTL for use in road transportation, it can be converted to liquid methanol or synthetic gasoline, or to other gaseous fuels, such as DME. Hydrogen can be produced from NG via methane reforming, and electricity can be generated at a NG -powered plant for on-road vehicles. Emphasis is needed on cost, the environmental benefits, energy use, and energy security that each fuel pathway can offer to a particular nation. In the AMF Task 48, the feasibility of the different natural gas pathways used in motor vehicles were assessed to determine the advantages and disadvantages of each option. To demonstrate how differently each factor can weigh in, case studies were conducted in six different countries spanning three continents. (AMF Task 48: Sikes et al. 2015).

Fossil methane is natural gas trapped beneath the surface of the earth into the different geological formations made of layers of sedimentary porous rock, topped by an impermeable formation that acts as a roof. The gas is extracted by drilling through the impermeable rock and the gas released is usually under pressure. After extraction NG must go through some processes, basically, to eliminate oil, water, and some other trace components from the raw extracted gas. Today unconventional sources of fossil methane are also important: 1) Shale gas is a form of natural gas trapped into shales, which are fine-grained and organic-rich rock formations. The estimation of its potential and existing reserves has substantially changed along with the progress in extraction technology, the hydraulic fracturing and the horizontal drilling techniques. 2) Coal-bed gas is a form of natural gas that found in a near-liquid state adsorbed into a solid matrix of coal. Free of H₂S, coal-bed gas contains lower amounts of heavier hydrocarbons such as ethane, propane, and butane than conventional natural gas. 3) Tight gas (or tight sandstone gas) trapped within impermeable rock and non-porous sandstone or limestone formations. Thus, its extraction is complicated. 4) Methane hydrates are solid compounds trapped within a crystal water structure, forming a solid structure similar to ice. Substantial reserves of methane hydrates have been found, but commercial-scale production of gas from these formations has not been accomplished..

The term biomethane refers to methane from a renewable origin. It is produced by the anaerobic digestion of organic matter (dead animal and plant material, manure, sewage sludge, organic waste, etc.), which is stored in air-tight tanks in order to reproduce the best possible conditions for the anaerobic microbes producing gas during the digestion process. It can also be produced by anaerobic degradation of organic matter in landfills, and this is referred as landfill gas. The raw gas is known as biogas, mainly consisting of methane and CO₂ plus some minor trace components which greatly depend on the feedstock. Biomethane is known as the upgraded form of biogas, and its final quality/composition is dependent on the operational parameters of the final use, and on the upgrading technology used. I.e. biomethane is a methane-rich gas derived from biogas. The third route to biomethane is via gasification of biomass. A great benefit of biogas/biomethane is that it can be produced from a great variety of sources: basically, all types of biomatter can be used for this purpose, such as waste. However, not all substrates behave in the same way regarding the biogas production efficiency or have the same emissions saving potential. Since methane is the main constituent of natural gas, biomethane can be used in natural gas vehicles without need for modifications.

Legislation, standards and properties

Standards for fuels are essential, as design of engines should base its work on a known fuel composition and its potential variability.

ISO 15403:2006 defines natural gas as a gas with

• more than 70% volume/mole of methane and
• a higher calorific value of 30–45 MJ/Nm³.

It also recommends limits for

• moisture and dust, 3 vol-% for both, and
• for CO₂, O₂, and H₂S a limit of <5mg/m³.

The following European specifications for natural gas and biomethane automotive market fuels were published in 2016/2017:

• EN 16723-1  Natural gas and biomethane for use in transport and biomethane for injection in the natural gas network - Part 1: Specifications for biomethane for injection in the natural gas network  (Under Approval)  
• EN 16723-2  Natural gas and biomethane for use in transport and biomethane for injection in the natural gas network - Part 2: Automotive fuel specifications (Under Approval)  

Requirements for natural gas and biomethane as automotive fuels as examples are presented in Table 1 together with the historical variations that were found in GASQUAL project for some relevant components between different pipeline gas specifications in Europe. Additionally, requirements presented in UNECE Regulation No. 83 (R83) for reference gas specifications (G20 and G25) in the emissions standards for light duty NGVs are presented, as well as the respective requirements in regulation R49 for heavy-duty engines (GR-G23 for H-gas range and G23-G25 for L-range).

Table 1. Example of natural gas and biomethane properties (historical) together with the requirements in the standards for automotive fuels. Complete requirements and standards are available from the respective organizations.

	Historical natural gas quality in Europe (GASQUAL Project)	G20/G25 (R83 rev. 4)	GR/G23 (R49 rev. 7)	Standard 2017
Methane content, mol%		99-100/84-88	84-89/91.5-93.5
Ethane, mol%			11-15/-
Balance (Inerts)		<1/<1	<1/<1
Methane number				> 65
Wobbe index, MJ/Sm3	30.8 – 54.7	47.2-49.2/38.2-40.6
Total volatile silicon (as Si), mgSi/m3				≤ 0.3
Hydrogen, wt%				≤ 2
Total sulfur, mg/Sm3	10 - 150	≤ 10/≤ 10	≤ 10/≤10	≤ 30
Hydrogen sulfide and carbon sulfide (as sulfur), mg/Sm3	2 - 20			≤ 5
Oxygen, mol%	0 - 3			≤ 1
Amine, mg/m3				≤ 10
Dew point temperature, °C	-15 - +2			≤ -2
CO2, mol%	1 - 8
Nitrogen, mol%	1.5 - 10	-/12-16	-/6.5-8.5

 

The methane number is an important property of natural gas. This value, which is calculated using an approach by the South West Research Institute, indicates the knocking resistance of the fuel; a methane number of 80 gives the same knocking behavior as a mixture of 20% hydrogen and 80% methane. EN 437:2003 specifies the test gases, test pressures and categories of appliances relative to the use of gaseous fuels. The Wobbe Index is an indicator of the interchangeability of fuel gases (the higher heating value divided by the square root of specific gravity). However, both standards are of limited use when looking at natural gas vehicles (NGVs) performances, fuel economy, emissions, and consumer’s price safeguard.

UNECE Regulations No. 83 and 49 (see Table above) define the reference the reference gas specifications, which are supposed to be representative of the different existing market qualities. An essential problem is the extent to which the reference fuels used in emission tests have properties similar to the properties of the fuel in the real-world situation. The following summary and Table 1 summarize some critical information and provides relevant correlations:

• Biomethane, particularly in its liquid form (LBM) is the nearest to the G20 reference test-gas (pure methane). Liquefaction eliminates CO₂, sulfur, and metals that are present in raw biogas.
• Over 95% of LNG typically has higher grades than G23 test-gas and high grade pipeline gas. LNG contains very little nitrogen, while G23 is generated by diluting methane with ~ 7.5% nitrogen.
• Low grade pipeline gas is simulated by G25 test-gas, which is generated by adding ~ 14% of nitrogen to methane. However, L-gas has higher contents of C2, giving higher Wobbe Index and lower methane number than G25.
• The high C2-contents of the Rich-LNG is simulated by GR test-gas by adding 13% of ethane to methane. However Rich-LNG has higher contents of C3+, giving higher Wobbe Index and lower methane number than GR

A broad conclusion is that G23, G25, and GR reference test-gas formulations are not really representative of the actual composition available in the gas-pipeline and LNG markets. In the test-gases methane is diluted with either nitrogen or ethane, while methane in actual gas is “diluted” by both ethane (and C3+) and/or inerts (N₂ and CO₂), depending on source.

Table 1. Typical NG/biomethane compositions in the NGVA Europe’s LNG Position Paper (A. Nicotra - 2012).

	LBM	High-Gas/LNG-std	Low-Gas/Rich-LNG
Methane, mol%	98	93/93	82/83
Ethane, mol%	-	5/5.5	5/13
C3+, mol%	-	-/2	-/3
Nitrogen, mol%	2	2/0.5	13/0.5
Sulfur, mg/kg	3	10/3	10/3
Wobbe index	46	46.2/47.5	43.3/49.5
Methane number	90-95	75-90/75-90	60-70/63-70
MON	135-137	124-134/124/134	113-120/115-120

 

Natural gas is a mixture of different hydrocarbons, methane being the main constituent (usually 87–97%). It can also contain some impurities such as nitrogen or CO₂. For natural gas, main variations include:

• calorific value
• methane number
• sulfur content
• content on inerts (nitrogen and carbon dioxide)
• content of higher hydrocarbons

Biomethane is upgraded from biogas, which mainly consists of methane and CO₂ plus some minor/trace components which greatly depend on the feedstock (Table 3).

Table 3. Examples of biogas compositions from different sources (Kajolinna et al. 2015).

Final quality and composition of biomethane depends on the operational parameters of the final use and on the upgrading technology used (Table 4). Depending on the source, several trace components have to be closely controlled when using biomethane as a vehicle fuel, including:

• Siloxanes (risk of abrasion and increased probability for knocking)
• Hydrogen (risk of embrittlement for the metallic materials)
• Water (risk of corrosion and driveability problems)
• Hydrogen sulfide, H2S (corrosive in the presence of water could affect after-treatment devices, and combustion products could create problems by sticking the engine valves)

Table 4. Comparison of selected parameters for common upgrading processes (Urban et al. 2008).

	PSA	Water scrubbing	Organic physical scrubbing	Chemical scrubbing
Pre-cleaning needed a	Yes	No	No	Yes
Working pressure (bar)	4–7	4–7	4–7	No pressure
Methane loss b	<3%/6–10% f	<1% / <2% g	2–4%	<0.1%
Methane content in upgraded gas c	>96%	>97%	>96%	>99%
Electricity consumption (kWh/Nm3)	0.25	<0.25	0.24–0.33	<0.15
Heat requirement (°C)	No	No	55–80%	50–100%
Controllability compared to nominal load	± 10–15%	10–100%	10–100%	50–100%

^a Refers to raw biogas with less than 500 mg/m³ of H₂S. ^b The methane loss depends on operating conditions. These figures are given by manufacturers or operators. ^c The quality of biomethane is a function of operational parameters. ^d Raw gas compressed to 7 bar.  ^f CarboTech./Quest Air. ^g Malmberg / Flotech.

 

LNG suffers from the great variety of import sources and final uses. Figure 1 shows how methane number and Wobbe index vary for imported LNG in Europe:

Figure 1. Methane number vs. Wobbe index for imported LNG in Europe. (GIIGNL 2010 / E. ON Ruhrgas).

The relation between temperature and pressure for saturated LNG is seen in the Figure 2.

Figure 2. LNG (as pure CH4) saturated pressure/temperature relation (NGVA Europe. Data from NIST).

All in all, as a consequence of the multi-use nature of NG as an energy carrier together with the different import sources, the natural gas market is characterized by a substantial variation in the gas composition. This is an important factor when using methane as automotive fuel.

Distribution

Biomethane can be produced locally and therefore distribution is different from natural gas in many respects. However, both bio-origin and fossil methane is used in compressed or liquefied state for storage and for transportation purposes.

• Compressed methane (CNG, CBG): natural gas or biomethane has been compressed after processing; mainly used for vehicles and typically compressed up to 200 bar.
• Liquefied methane (LNG, LBG): natural gas or biomethane has been liquefied after processing. Temperature is about -161.7 °C at atmospheric pressure and, when used as an automotive fuel, it can be stored in on-board cryogenic tanks (vacuum-isolated stainless-steel vessels), which have different operating pressure ranges.

Natural gas is transported over long distanced as compressed by pipeline or as liquefied by ships. Natural gas pipeline pressures in Europe typically range from 2 to 80 bar. The trend nowadays is to increase the pressure in the international connecting pipelines in order to reduce the transport cost. Pressure of the pipelines laid down at the bottom of the sea has to be sufficient, as it is not possible to install intermediate compression stations. Natural gas is transported as liquefied by ship, for example, when the distances to the consumption point are long (above 4.000 km), for example, when transporting large volumes over the seas. Usually most of the LNG gets gasified and injected into the NG grid. However, part of it can be directly used as LNG, and then usually transported by LNG road tankers.

Gaseous and liquefied natural gas pathways canot be clearly separated from each other, as many of the imported LNG is re-gasified in coastal LNG terminals so that it can be injected into the NG grid. It should be emphasized that both pathways are affected by the fact that the composition of the natural gas transported has high variability.

Figure 3 provides a visual comparison of the volumetric equivalence between diesel, CNG, and LNG for a given energy content.

Figure 3. CNG/ LNG/ Diesel energy and volume equivalence (NGVA Europe).

Odorization

A well-known practice within the natural gas sector is the addition of odorants to help identify NG in case of a leak. Historically, this has been done in different ways as practically each European country has followed its own national code/standard on this subject. For years, the most used odorants have been tetrahydrothiophene (THT) and mercaptan, both sulfur-based odorants. During the last 10-15 years, several European countries have started to run technical programs in view of the substitution of THT and mercaptans with sulfur-free odorants. Countries like Germany, in which the odorization practice is governed by the DVGW standard G 280-1 ‘Gas Odorization’, started in 1995 developing a sulfur-free odorant for gas distribution networks and, already in 2007 more than 40 gas distributors in Germany, Austria and Switzerland had changed their odorization practices with THT or mercaptans to sulfur-free odorants like Gasodor™ S-Free™. The situation is still not balanced in Europe, however, as there are still countries using THT and mercaptans when running odorization practices. The level of sulfur derived from the addition of THT and mercaptan is linked to the exact positioning of the measurement equipment, as sulfur is more concentrated the closer the measurement is performed to the odorization point. According to E.ON Ruhrgas AG (and though different amounts are used in different countries), these could be indicative values:

• Mean sulfur content before odorization: 3.5–6 mg/m³
• THT is generally adding 5–10 mg/m³ (measured as S)
• Mercaptan is generally adding 1–3 mg/m³ (measured as S)

The use of sulfur-free odorants would mean further reduction of already low sulfur content of natural gas. Sulfur is known for its negative effect on the proper functioning of engine exhaust after-treatment systems leading to reduction of the conversion efficiency over time.

Oil carry-over and water/humidity control at refueling stations

Natural gas refueling stations can either be CNG, LNG, or LCNG stations, which can offer compressed, liquefied or both types of natural gas. LCNG stations are supplied with LNG, and CNG is produced with a vaporizer. Apart from this, CNG stations can either be fed from the natural gas grid directly, or fed from LNG which is then vaporized and put under pressure in order to get it settled to 200 bar. During the compression phase in a natural gas refueling station, some contaminants like water and oil can squeeze into the final compressed gas interfering with the proper functioning of NGVs. Some of the contaminants can come from the grid-distributed gas, and some others, like oil, can come from the compressor lubricants themselves. For those stations directly fed from the grid and also for stations being fed from natural gas mobile storage units, it’s typical that the gas is processed at the refueling site in order to make two main adaptations for its use in the vehicles:

• Drying: NG must be dry enough not to cause corrosion and drivability issues when put under high pressure. Water content values of 5 mg/kg are achievable and are currently good enough to guarantee the proper operation of the vehicles and their systems.
• Filtering: there is no existing suitable method for the measurement of particles in the gas, but for the protection of NGV systems (engines and associated components) it is necessary in order to ensure proper and durable functioning. There are several CNG coalescing filter suppliers that can be used today. According to the suppliers their products are able to remove 99.995% of the aerosols in the size of 0.3 to 0.6 microns when installed in series with other pre-filters. It’s generally recommended to use two filters after the compressor (and before the storage system) and another fine-mesh filter before the CNG dispenser.

Some other factors to consider are: how good are filters in removing aerosols and what is the need to have a proper maintenance program for the filtration systems. Experience has proven that, if not controlled, these two aspects can have important negative effects to the vehicles e.g. in form of corrosion of the CNG metallic tanks. This could affect safety in the long term, drivability problems due to water precipitation in connection with the expansion cooling that occurs when the gas flows from the storage tank to the engine inlet and could create ice plugs, abrasion of the mechanical parts due to solid particles entering the system, oil deposition in the engine’s distribution system, etc.

Methane engines, efficiency and emissions

For using methane as an automotive fuel, the main engine technology to date has been the Otto engine, either under stoichiometric or lean-burn conditions of the air-fuel mixture. Nevertheless other engine technologies have also been developed, such as  dual-fuel (DF) engines based on compression-ignition engines. Some of the main differences are as follows:

• Stoichiometric spark-ignition (SI) natural gas engines: characterized by a homogeneous air-fuel mixture, the air-fuel ratio being controlled via an oxygen sensor (or lambda sensor) installed in the exhaust stream.
• Lean-burn spark-ignition NG engines: characterized by a stratified air-fuel mixture. These engines usually require indirect fuel injection or a direct fuel injection with induced turbulence. The indirect fuel injection requires the fuel to be injected in a pre-chamber designed, to keep the air-fuel mixture at stoichiometric conditions until it is suctioned into the combustion chamber. Excess of oxygen concentration in the exhaust is controlled via a linear oxygen sensor.
• Dual-fuel compression-ignition engines using methane and diesel fuel: dual-fuel engines differ from dedicated engines in their capability to burn two fuels at the same time. Dual fuel engines use diesel as the main ignition source for the natural gas-air mixture. Diesel substitution ratios can vary depending on the dual fuel engine technology and also depending on the operation of the engine itself.

Engine efficiency and CO₂e emissions - High efficiency is achieved with the concepts based on compression ignition (diesel cycle), such as high-pressure direct-injection (HPDI) DF engines for methane and diesel fuel.  Spark-ignited methane  engines,  on  an  average,  have  close  to  30%  higher  energy  consumption  compared to compression ignition engines of the same size and power according to AMF Annex 57 (Söderena et al. 2021). Energy efficiency is higher also for lean-burn engines than for stoichiometric engines. However, energy consumption of methane fuelled cars is lower than that of gasoline fuelled cars (Karlsson et al. 2008).

Carbon intensity of methane is better than that of diesel fuel because of the higher hydrogen-carbon ratio of methane (CH₄) when compared to diesel (C₁₅H₂₈) or gasoline (C₇H₁₅). This generally leads to less or comparable tailpipe CO₂ emissions with CNG as for diesel and gasoline engines depending on engine efficiency. This can be seen from the results of the AMF Task 37 (Nylund and Koponen 2012), fuel and technology alternatives for buses were studied, and from a comparison of trucks conducted in the AMF Task 57 (Figure 4). Here it is noted that tailpipe CO₂ emissions are only a part of lifecycle greenhouse gas emissions, while methane and nitrous oxide (N₂O) emissions are also important.
In AMF Task 55 (Rosenblatt et al., 2020), CNG vehicles showed lower overall CO₂ levels of CNG vehicles  than comparable gasoline counterparts. In a Swiss measurement campaign, a comparison of two identical (weight, power, transmission type) cars covered one equipped with a CNG engine and one with a gasoline engine. 

Figure 4. Trucks using alternative fuels, results from the AMF Task 57. (Söderena et al. 2021).

Methane used as a fuel may slip as methane emissions, which has high global warming potential (GWP over 100 years is 28 for methane). Methane emissions from NG fuelled vehicles can be reduced by exhaust aftertreatment devices to low levels, but not necessarily for all engine concepts. In the older studies, methane emissions from a CNG bus were around 150 mg/km (Murtonen and Aakko-Saksa 2009) and even as high as 2750 mg/km (review, Hesterberg et al. 2008). Karlsson et al. (2008) noticed that, methane represented 74% of hydrocarbon emissions at normal temperature for biomethane fuelled vehicles. In Annex 57 (Söderena et al. 2021), tested methane engines  (stoichiometric  spark-ignited and  HPDI  dual-fuel)  had  relatively  low  methane emissions,  especially  in  comparison  with  older  lean-burn  engines  and  port-injected dual-fuel engines. In the cold-start tests, methane emissions were elevated in some cases. 

Another emission species having a high GWP is nitrous oxide (N₂O) emission (GWP100 is 265). The testing in AMF Task 57 (Söderena et al. 2021) revealed that N₂O emissions can be a problem for vehicles  equipped  with  specific selective  catalytic  reduction  (SCR)  systems, depending on chemistry, however, parallel testing of Euro VI Step C and Step D vehicles confirmed that the problem can be controlled.

In AMF Task 51 (Schramm, J. 2020), a number of vehicles were tested for tailpipe methane emissions and the results indicated that the major contribution of methane originates from slip during driving. The key mechanisms behind unburned methane emissions was identified due to misfire/bulk quenching, wall quenching, crevice volumes, post oxidation and valve timing/overlap. The major problems with unburned methane formation were associated with medium speed 4-stroke dual fuel engines. Since the unburned methane emissions origin from areas near the combustion chamber walls, the sensible way forward is towards direct injection of natural gas/bio-methane in order to reduce emissions. AMF Annex 51 dealt also with methane oxidation catalyst chemistry and regeneration issues.

In AMF Task 51 (Schramm, J. 2020), addition of hydrogen to NG (so called hythane concept) in a test with a stoichiometric operated Euro 4 CNG vehicle showed significant reductions in THC (including methane) and NOx emissions. Hydrogen blending could be an interesting option for processes with diluted mixture formation (lean or EGR operation). 

CO, HC, NOx, PM and PN emissions - The different principles of engines affect the exhaust emissions and their control strategies. Capability of the stoichiometric gas engine to control emissions efficiently via a three-way catalyst (TWC) is an advantage over the compression ignition and lean-burn engines, in which the excess oxygen in exhaust prevents using TWCs to oxidize CO and HC while reducing NO_x. For the DF engine, the emission legislations (e.g. EURO V and later) require that the engine is equipped with similar after-treatment technology as diesel engines, which means installation of a selective catalytic reduction (SCR), an oxidation catalyst and a diesel particulate filter (DPF). NGVs equipped with TWCs meet EURO VI NO_x emission requirements.
Recent work in the AMF Task 57 (Söderena et al. 2021) covered methane fuelled trucks tested in Finland and Sweden. Examples of the NOx and particle number (PN) emissions are presented in Figure 5. NOx and PM emissions were generally below Euro VI limits. For tested vehicles, NOx emissions were low and NOx conformity factors (CF) passed the ISC criteria for both Step C and D vehicles. Interestingly, the Step D diesel had higher NOx emissions than the Step C vehicles. PM emissions were low for all vehicles, below 6 mg/kWh. PN emissions were also low, although slightly higher for HPDI LNG than for diesel. 

Figure 5. NOx and PN emissions, Euro VI Step D vehicles (AMF Task 37, Söderena et al. 2021).

A study with buses, AMF Task 37 reported by Nylund and Koponen (2012), showed that natural gas in combination with stoichiometric combustion and TWC delivers low regulated emissions, whereas lean-burn natural gas engines are characterized by high NO_x emissions (Figure 6). All natural gas engines, independent of combustion system, delivered low particulate matter emissions, i.e. equivalent to particulate filter equipped diesel engines.

For diesel engines certain aftertreatment devices increase tailpipe NO₂ emissions, which is also the case for lean-burn CNG engines, whereas stoichiometric CNG engines are low emitters of NO₂. The share of NO₂ of NO_x was 35–70 % when engines were equipped with NO₂ producing aftertreatment devices, but in other cases below 5% in Nylund and Koponen (2012). As an example, a mean NO₂/NO_x ratio for typical heavy-duty (EEV classified) diesel engines is in the range of 0.4 to 0.6, while typical values for respective natural gas engines is in the range of 0.01 to 0.05 (Kytö et al. 2009).

Figure 6. NO_x and PM emission results for European vehicles (Nylund and Koponen 2012).

AMF Task 39 "Enhanced Emission Performance and Fuel Efficiency for HD Methane Fuelled Engines" (Olofssen et al. 2014) investigated the development level of the methane fuelled engines for heavy duty vehicles (HDVs) and the potential to reach high energy efficiency, sustainability and emission performance. Task 39 included a literature study (Broman et al. 2010) and testing in Sweden, Finland and Canada. Vehicles tested were spark ignited (SI) dedicated gas engines and vehicles equipped with DF NG/Diesel engines. Methane used was CNG and sometimes mixed with biomethane. Testing in Canada included HPDI DF concept using LNG and diesel. The tested dedicated SI gas buses operates only on gas, while the DF concept could use variable mix of diesel and methane gas or only diesel fuel. However, the truck using HPDI-technology could only operate properly when both methane and diesel fuel is available. HDVs equipped with dedicated SI methane engines showed low emissions. The bus was well in line with Euro VI emission requirements and is considered as “best in class”. HDVs equipped with DF methane/diesel technology had difficulties in achieving the theoretical diesel replacement capacity of 75-80%, particularly at low engine loads. Furthermore, to reach Euro V/EEV and Euro VI emission levels, obviously advanced combustion control and thermal management are needed. The average ratio for diesel replacement is expected to be about 60%. 

AMF Task 39 (Olofssen et al. 2014) included also tests with retrofit systems of HD Euro III vehicles converted to use DF methane/diesel technology. The emission performance will be dramatically negatively affected, except for the PM emissions. However, a possible advantage might be to reduce operating costs for the vehicle.

AMF Task 39 highlighted that 1) NG/Diesel dual fuel concepts: Difficult to meet Euro V/VI emission standards with technology available by 2014; Suitable only for OEM applications (not for retrofitting); Diesel replacement depends on load and is lower than expected; Total GHG emissions might be higher for NG/diesel than for diesel vehicles 2) Dedicated spark ignited engines (SI):No problem to meet Euro V/EEV emission requirements; Lower engine efficiency when compared to diesel especially for lean-mix applications (18% vs. 33%); Lean-mix concept operating mostly on lambda over one.

Numerical emission results as examples are collected in Table 5 (buses, trucks) and Table 6 (cars) including emissions from older and newer vehicles using methane as fuel. Generally, when equipped with efficient exhaust aftertreatment devices, most emissions are at similar levels with diesel and methane fuelled vehicles. However, NO_x emissions tend to be still lower with TWC equipped CNG than with diesel fuelled buses. For the NGVs different combustion principles lead to differences in emissions, for example, with spark ignition engines the tailpipe NO_x and PM emissions are typically low, while for non-optimized lean-burn CNG vehicles these emissions may be elevated. Formaldehyde, acetaldehyde, 1,3-butadiene, and benzene emissions have been reported to be lower for CNG vehicles than for gasoline or diesel vehicles. Ammonia (NH₃) emission tends to be high for TWC equipped vehicles, which has been observed also for the TWC equipped CNG bus. Low levels of carcinogenic emissions, such as 4-ring polycyclic aromatic hydrocarbons, have been observed with CNG fuelled vehicles and cars. In addition, Ames mutagenicity of particulate matter has been repotedly lower for CNG vehicles when compared to diesel vehicles.

For natural gas fuelled cars, CO, HC, and NO_x emissions were low in the AMF Task 22 (Aakko and Nylund 2003). In addition, the dedicated, monofuel CNG car was quite insensitive towards ambient temperature, whereas CO and HC emissions from gasoline cars increase as ambient temperature decreases down to -7 °C.  Karlsson et al. (2008) observed quite small differences in emissions between bi-fuelled biomethane and gasoline car at -7 °C. However, difficulties in switching fuel from gasoline to biomethane (CBG) for a bi-fuel NGV after the cold start at -7 °C led to higher CO emission, but lower NO_x and PM emissions for CBG than for gasoline car at -7 °C. A study with Euro 6a cars (including AMF Task 44) using alternative fuels observed low emissions even at cold ambient temperatures (Aakko-Saksa et al.  2020).  The CNG fuelled cars were the “cleanest” of the cars studied: only methane emissions were elevated.

Table 6. Examples of emissions from heavy-duty vehicles (buses, trucks) using diesel or methane as fuel.

	Diesel w/o [mg/km]	Diesel+DPF [mg/ml] a [mg/km] b, d	CNG+TWC [mg/ml] a [mg/km] b	LB CNG ox. cat./ SM CNG [mg/km]	CNG / LNG [mg/km]
CO		600; 1400 a	4900; 17900 a		4943/490 e
HC		100; 1000 a	3500; 5300 a		898/437 e
NOx	8000; 9000 d	26000; 82000 a 9000; 20000;6000 d	7700; 8700 a	7000; 8000 d / 2000 d	139/130 e
NO2	100; 1000 d	11600 a 800; 10000; 3000 d	100 a	300 d / 50 d
N2O		<120b <40b			7/255 e
NH3		<10b 280b
PM	170; 120 d	30; 240 a 20:10;10 d	40; 20 a	10; 10 d / 5 d	2.2/1.7 e
Methane		0a <15b	2750 a 0.15b		788/432 e
Benzene	3; 3 d	0.4a <3b 1; 0.4; 0 d	0 a <0.003b	0; 0.06 d / 0 d
1,3-Butadiene	8 d	2.3a 0b 0; 0; 1 d	0 a 0b	0; 0 d/ 0 d
Ethylene		3.4a <10b	0 a 0b
Propylene		0.2a <2b	0 a <0.002b
Ethylbenzene		0.1a	1.4 a
Toluene		0.1a <5b	2.3 a <0.005b
o-Xylene		0.4a	0.4 a
m/p-Xylene		0.2a	0.9 a
Formaldehyde	37; 25 d	3.4a <13b 5; 3; 5 d	0.0 a 0.003b
Acetaldehyde		1.9a <4b	0.4 a 0b
Acrolein		0.1a 0  b	0b
	µg per distance	µg per distance	µg per distance	µg per distance
PAHs A/B	613 d	10/6 a <42/<4b  94; 90 d	<1 b 0.1/0.5 a	8; 30 d / 7 d
	krev/km	krev/km	krev/km	krev/km
Ames TA98-S9	59 d	<15 b 23; 20 d	0 b	3; 50 d / 1 d

^a Hesterberg et al. 2008   ^b Murtonen and Aakko-Saksa 2009        ^c AMF Task 22 Nylund and Aakko 2003 ^d Nylund et al. 2004 ^e AMF Task 57 Söderena et al. 2021

Table 5. Examples of emissions from cars using diesel or methane as fuel.

	Diesel+DPF a [mg/ml] a [mg/km] c	Diesel, Euro 6a -7 °C [mg/km]	CNG a [mg/ml] a [mg/km] c	CNG, Euro 6a -7 °C [mg/km]
CO	110 a	97 b	680 a	75 b
HC	20 a	15 b	950 a	98 b
NOx	820 a	463 b	290 a	201 b
PM	0 a	0.5 b	18 a	0.6 b
	+23/-7 °		+23/-7 °C
NO2		92 b		1 b
N2O		16 b		0.1 b
NH3		0.4 b		6.2 b
Methane	<10/<24 c	9 b	52/84 c	96 b
Benzene	<5/<15 c	0.2 b	0/0 c	0.1 b
1,3-Butadiene	<1/<2 c	0 b	0/0 c	0 b
Ethylene	<7/<24 c		0/0.5 c
Propylene	<4/<12 c		0/0 c
Ethylbenzene	<2/<10 c		0/0 c
Toluene	<10/<55 c		0/2 c
o-Xylene	<1/<8 c		0/0 c
m/p-Xylene	<3/<22 c		0/2 c
Formaldehyde	<5/<23 c	0.3 b	0/0 c	0.3 b
Acetaldehyde	<3/<18 c	0.4 b	0/0 c	0.1 b
Acrolein
PAHs A/B		0.0001 b		0 b
Ames TA98-S9		11 b		3 b

^a Hesterberg et al. 2008  ^b Aakko-Saksa 2020, incl. AMF Task 44  ^c AMF Task 22 Nylund and Aakko 2003

CNG delivers very low particle number emissions, almost two orders of magnitude lower than diesel technologies (Figure 7). However, diesel vehicles equipped with DPF produce particle numbers comparable to CNG. The highest particle numbers were clearly observed for diesel buses without DPF in a study by Nylund and Koponen (2012, AMF Task 39). Karlsson et al. (2008) observed lower particulate mass and number emissions for a biomethane fuelled car than for a gasoline car.

Figure 7. Particle number size distribution for a number of technology alternatives (indicative). (Nylund and Koponen 2012).

 

References

Aakko, P. and Nylund, N-O. (2003) Particle emissions at moderate and cold temperatures using different fuels. AMF Task 22. Project report PRO3/P5057/03. (download report).

Aakko-Saksa, P., Koponen, P., Roslund, P., Laurikko, J., Nylund, N.-O., Karjalainen, P., Rönkkö, T. and Timonen, H. (2020) Comprehensive emission characterisation of exhaust from alternative fuelled cars. Atmospheric Environment 236 (2020) 117643. Including AMF Task 44.

Broman, R., Stålhammar, P. and Erlandsson, L. (2010) Enhanced emission performance and fuel efficiency for HD methane engines. Literature study. AMF Task 39, Final report. AVL MTC 9913. May 2010.

Hesterberg, T., Lapin, C. and Bunn, W. (2008) A Comparison of Emissions from Vehicles Fueled with Diesel or Compressed Natural Gas. Environmental Science and Technology. Vol. 42, No. 17, 2008. 6437-6445.

Kajolinna, T., Aakko-Saksa, P., Roine, J., and Kåll. L. “Efficiency testing of three biogas siloxane removal systems in the presence of D5, D6, limonene and toluene”, Fuel Processing Technology, 139, 2015, pp. 242-247.

Karlsson, H., Gåsste, J. and Åsman, P. (2008) Regulated and non-regulated emissions form Euro 4 alternative fuel vehicles. Society of Automotive Engineers. SAE Technical Paper 2008-01-1770.

Kytö, M., Erkkilä, K. and Nylund, N-O. (2009) Heavy-duty vehicles: Safety, invironmental impacts and new technology. Summary report 2006–2008. VTT-R-04084-09. June 2009.

Murtonen, T. and Aakko-Saksa, P. (2009) Alternative fuels with heavy-duty engines and vehicles. VTT's contribution. VTT Working Papers: 128.

Nylund, N-O., Erkkilä, K., Lappi, M. and Ikonen, M. (2004) Transit bus emission study: Comparison of emissions from diesel and natural gas buses. VTT Research Report PRO3/P5150/04.

Nylund, N-O. and Koponen, K. (2012) Fuel and technology alternatives for buses. Overall Energy Efficiency and Emission Performance. AMF Task 37. VTT Research Highlights 46.

Olofsson, M., Erlandsson, L. and Willner, K. (2014) Enhanced emission performance and fuel efficiency for HD methane engines. AMF Task 39, Final report. AVL MTC Report OMT 1032, 2014. (download report, key messages).

Rosenblatt, D., Winther, K., Söderena, P., Lindgren, M., Bütler, T., Czerwinski, J., Duoba, M. and Wallner. T. (2020) Real Driving Emissions and Fuel Consumption. AMF Task 55, Final report (download report).

Schramm, J. (2020) Methane Emission Control. AMF Task 51, Final report (download report)..

Sikes, K., Ford, J., Blackburn, J. and McGill, R. (2015) Feasibility of natural gas pathways for motor vehicles – An international comparison. AMF Task 48, Final report, August 2015. (download report)

Söderena et al. (2021) Heavy-Duty VehiclesPerformance Evaluation. AMF Task 57. Final report (download report).