GAS BUS TECHNOLOGY AND OPERATIONAL EXPERIENCES IN HELSINKI AREA

Transcription

1 GAS BUS TECHNOLOGY AND OPERATIONAL EXPERIENCES IN HELSINKI AREA This publication has been produced with the assistance of the European Union ( The content of this publication is the sole responsibility of Baltic Biogas Bus and can in no way be taken to reflect the views of the European Union.

2 The Baltic Biogas Bus project will prepare for and increase the use of the eco-fuel Biogas in public transport in order to reduce environmental impact from traffic and make the Baltic region a better place to live, work and invest in. The Baltic Biogas Bus project is supported by the EU, is part of the Baltic Sea Region programme and includes cities, counties and companies within the Baltic region. Authors: Matti Kytö, Pekka Rantanen & Nils-Olof Nylund Projekt Manager: Matti Kytö, VTT Technical Research Centre of Finland Date: Reviewed by: Erik Pettersson (SWECO) and Lennart Hallgren (SL Stockholm Lokaltrafik) 2

3 Contents GAS BUS TECHNOLOGY AND OPERATIONAL EXPERIENCES IN HELSINKI AREA... 1 Summary INTRODUCTION ENGINE TECHNOLOGY GENERAL LEAN-BURN TECHNOLOGY STOICHIOMETRIC TECHNOLOGY DUAL-FUEL TECHNOLOGY Fuel storage EMISSION PERFORMANCE OF NATURAL GAS AND BIOGAS (methane) Buses / engines IVECO SCANIA MERCEDES BENZ VOLVO MAN CUMMINS-WESTPORT USER EXPERIENCES IN HELSINKI AREA HELSINGIN BUSSILIIKENNE PROBLEMS DUE TO HIGH TEMPERATURE: HOLES BURNED INTO THE EXHAUST MANIFOLD EXHAUST PIPE RUPTURES INSULATION OF WIRING MELTING OR BECOMING BRITTLE SHORT LIFETIME OF LAMBDA-SENSORS, LESS THAN KM REPLACEMENT INTERVAL OF TURBOCHARGERS IS ONE YEAR TAMMELUNDIN LIIKENNE GAS BUS COSTS CONCLUSIONS REFERENCES

4 Summary Methane is known as clean engine fuel, toxic exhaust emissions especially particulate emissions are very low and nitrogen oxide emission can be kept low too quite easily. Both natural gas and upgraded biogas are mostly methane and technically equal fuels for vehicle use. Drawbacks for methane use are that a special fuel infrastructure is needed and the energy efficiency of current heavy duty gas engines is not as good as that of diesel engines. Heavy duty gas engines are in most cases spark-ignition conversions based on existing diesel engines. There is still place for optimization of those engines. Energy efficiency is the key issue but availability can be improved too. Dual fuel engines using gas as the main fuel are one step for better energy efficiency, but typical transient city bus cycles with low average load are challenging applications for dual-fuel engines. Still future natural gas engines can have clear advantages from an environmental point of view regarding both toxic and CO 2 emissions compared to diesel engines. Bus engines get the emission level label according to certain engine dynamometer tests. For modern diesel engines equipped with exhaust gas aftertreatment city centre driving is a challenging use. Exhaust temperature can be too low for the proper function of emission reduction devices. Emission measurements on heavy duty chassis dynamometer show that emissions of gas buses are low in transient city cycles too. Best diesel buses has shown very good emission performance in city cycles too, but that is not always the case. 4

5 1. INTRODUCTION Natural gas or upgraded biogas is a good fuel for spark ignited (SI) engines. Octane number is high and particulate emission is close to nil. Also the noise level of gas engines is lower compared to diesel engines. Drawbacks are that a special fuel infrastructure is needed, the operation range is shorter that with diesel vehicles and energy efficiency is not as good as that of diesel vehicles. Still advantages of gaseous fuel are clear, and therefore e.g. CNG (Compressed Natural Gas) buses are quite common in many cities. Natural gas has been clearly cheaper than diesel in many countries, and this is one prerequisite for the fleet owners to invest to gas vehicles. Heavy duty gas engines are in most cases spark-ignition conversions based on existing diesel engines. There is still place for optimization of spark ignited heavy-duty gas engines. Dual fuel engines using gas as the main fuel are one step for better energy efficiency, but typical transient city bus cycles with low average load are challenging applications for dual-fuel engines. Still future natural gas engines can have clear advantages from an environmental point of view regarding both toxic and CO 2 emissions compared to diesel engines. Both natural gas and upgraded biogas are mostly methane and technically equal fuels for vehicle. In this report CNG as term is often used but it covers compressed biogas as well. 2. ENGINE TECHNOLOGY 2.1 General Methane is well suited for spark-ignition engines due to high octane number (>120). Burning of methane does not produce soot, and particulate matter (PM) emission is very low. Specific CO2 emission (g CO2/MJ) is lower for methane than for petrol or diesel due to high H/C ratio. However, both engine efficiency and specific CO2 emissions affect tailpipe CO2 emissions. Today most heavy duty gas engines are spark ignited engines using the Otto cycle, and the advantage in fuel chemistry compared to diesel is lost due to lower energy efficiency of spark ignited engines. The most important advantage of natural gas buses is the low particulate mass emission without any exhaust gas after-treatment. For gas buses are equipped with spark ignition engines, the disadvantages are lower energy efficiency and higher need for maintenance compared to diesel buses. Higher fuel consumption has been compensated by lower gas price. Price of the bus, fuel costs and maintenance costs are variables affecting the interest to invest gas buses. Of course environmental issues affect too, 5

6 and decision makers can favour low emission vehicles by different incevtives. Many gas engines used in heavy duty vehicles are based on existing diesel engines. It is very easy to convert a petrol engine to gaseous fuels, and even the conversion of a diesel engine into a spark-ignited gas engine is relatively simple. The assumption here is that gaseous fuel is compressed methane: natural gas (CNG) or biogas (CBG). The main modifications in a conversion from diesel to gas are (Nylund 1995): Replacing of diesel injection equipment by spark plugs and injection system New pistons to lower the compression ratio Addition of gas tank and fuel system However, mastering the thermal loads and controlling the NOx emissions of the converted engine is challenging, and only OEM (Original Equipment Manufacturer) manufacturers can achieve adequate performance. The main components of a gaseous fuel system are fuel containers, pressure regulators, and the gas feed system. However, to achieve low overall exhaust emissions, advanced engine technologies and control systems have to be applied. Engines which work well in steady-state emission testing do not necessarily perform so well in real life service involving a lot of transient operation. Current regulations require engines operating on gaseous fuels to comply with on-board-diagnostics (OBD) regulations. Until now, the heavy-duty gas engines for buses have been either leanburn engines or stoichiometric engines. One manufacturer is offering a combined system (lean-stoichimetric/mix), which uses stoichiometric combustion on low load and lean-burn combustion on high load. Dual fuel gas diesels for buses are coming to market, but the technology is still undergoing refinement. Lean-burn engines are cheaper than corresponding stoichiometric engines, but NOx emissions are higher than those of stoichiometric engines. 2.2 Lean-burn technology Figure 1 shows the modification from diesel to spark-ignition gas. Either lean-burn combustion or stoichiometric combustion in combination with a three-way catalyst is applied to control NO x emissions. Figure 2 shows the operating ranges for stoichiometric and lean-burn engines. Lean-burn engines must be operated with an air excess ratio of some , close to the misfire limit, in order to keep the NOx emission as low as possible. Lean-burn combustion also means lower temperatures and lower thermal loadings compared to stoichiometric burning. 6

7 Lean-burn technology is a simple way to reduce engine out NOx emission, but without any after-treatment very low NOx levels can not be reached. The ignition energy needed for lean methane-air mixture is high, and thus misfirring and lifetime of the spark plugs are challenges for lean-burn engines. Keeping the air-fuel mixture stable especially with changing gas quality requires sophisticated engine management, e.g., closed looped control with lambda sensor. Figure 1. Conversion from diesel to spark-ignition. (Lexen 2001) Figure 2. Operating ranges for natural gas engines. (Straetmans 2005) 7

8 2.3 Stoichiometric technology Today many manufacturers (e.g. Cummins, Volvo) have switched to stoichiometric combustion, as this technology in fact provides lower emissions and better fuel efficiency especially under transient running conditions. Close-loop controlled stoichimetric engines are also less sensitive to changes in gas quality than simple lean-burn engines. As mentioned earlier thermal load is high in stoichiometric engines. There is still room for technical improvements to enhance the emission performance, efficiency, and to some extent, even the reliability of heavy-duty spark-ignited gas engines. In normal service, current sparkignited gas engines consume some 30% more energy than their diesel counterparts. New engine technologies like variable valve timing, EGR, skip-fire, direct injection etc. can help to enhance the efficiency of spark ignited gas engines, but not to the level compression ignition engines. Figure 3 shows features of Volvo s stoichiometric gas engine. Figure 3. Features of Volvo s new G9A gas engine. (Danielsson 2006) 8

9 2.4 Dual-fuel technology Recently there has been renewed interest in dual-fuel applications for vehicles. Dual-fuel diesel/gas operation could provide better fuel efficiency compared with spark-ignited gas operation, while at the same time reducing emissions over pure diesel operation. Dual-fuel would also provide the possibility to run on diesel as back-up. In dual-fuel gas engines some diesel fuel is needed to ignite the gaseous fuel, because methane as such is a poor diesel fuel. Compared to spark ignition, ignition by pilot (diesel) injection provides significantly higher ignition energy, and therefore more reliable start of combustion plus the possibility to operate on very lean mixtures. Depending on the engine and engine operation, the amount of pilot fuel (diesel) is in the range of 1 to 30 % of the total fuel amount. Dual-fuel engines are more complicated than normal spark ignited gas engines or diesel engines, and city bus operation (low average loads, transient driving) is maybe the most challenging service for a duel fuel engine using methane as main fuel. Gas can be injected either directly to the cylinder or to the inlet ports. Injection directly to the cylinder Westport Innovations has actively developed direct injection for natural gas engines to improve fuel efficiency. The direct injection systems for natural gas rely on late-cycle high-pressure injection of gas into the combustion chamber. Figure 4 shows Westport s HPDI technology. Westport has been developing HPDI in combination with liquefied natural gas (LNG) storage, and this combination is especially attractive for long haul trucks. Although the fuel is stored in liquid form, it is injected in high-pressure gaseous form. Westport is also working on glow-plug assisted direct injection for smaller engines. The CNG-DI technology also relies on late-cycle high-pressure injection of natural gas into the combustion chamber. To ignite the natural gas, a hot surface, such as a ceramic-tipped glow plug, is used in the engine. (Westport Innovations 2008) Most recently Westport has focused on heavy-duty trucks with their HPDI technology. HPDI technology is commercially offered for the 15 litre Cummins ISX engine platform, called Westport GX. In the U.S., the Westport GX is currently available in factory assembled Kenworth T800 LNG and Peterbilt 386 LNG and 367 LNG models and is offered for use in various applications including port drayage trucks, heavy-haul trucks, 9

10 refuse transfer, dump trucks, roll-offs, line-haul, and other vocational applications. Figure 4. Westport Innovations HPDI technology. (Westport Innovations 2008) Gas injection into the inlet manifold Pilot injection is also possible in combination with gas injection into the inlet manifold (indirect injection). Caterpillar used to provide dual-fuel automotive engines, and Wärtsilä is successfully using the dual-fuel concept for large stationary and marine medium-speed engines. Wärtsilä dual-fuel engines achieve 47% efficiency with less than 1% of pilot injection (Wärtsilä 2007). One problem with this premixed dual-fuel operation is that the band for optimum operation is quite narrow. On one hand, detonation (knocking) limits operation, on the other hand, misfire due to lean mixture. In UK, a company called Clean Air Power supplies dual-fuel systems for retrofitting. Clean Air Power s Dual-Fuel TM technology is described as follows (Clean Air Power 2007): Engine runs on natural gas with diesel pilot ignition At full power diesel fuel is 10% of total fuel - normal operation is up to 85% natural gas substitution Runs on 100% diesel fuel until engine coolant is at operating temperature 10

11 Under normal operation, can run solely on diesel at light loads Under normal operation, can run solely on diesel at full power Uses a standard electronically-controlled diesel engine Maintains electronic control of both gas and diesel injection Base diesel ECU (Engine Control Unit) is retained Dual-Fuel ECU controls gas operation and modifies diesel demand for the diesel engine ECU Automatically reverts to diesel only when gas supply is out of acceptable parameters Recent introduction of a combustion knock sensor protects the engine from variable gas quality The operating system in the technology automatically switches the engine from diesel to natural gas when the truck reaches optimal load, making it particularly suited to long haul heavy trucks Figure 5. Principle of dual-fuel gas diesel with inlet channel gas injection. Within the International Energy Agency s (IEA) research programme on Advanced Motor Fuels (AMF) a literature study on heavy-duty automotive gas engines was recently published. The focus was on dual-fuel engines. 11

12 One problem in the introduction of dual-fuel technology is that the European emission legislation doesn t recognize this type of engine. This means that dual-fuel vehicles put in service need special permits from the local authorities. 3 Fuel storage Methane is normally stored under pressure (typically 200 bar, compressed natural gas CNG, compressed biogas CBG). International standards are in place to secure safety of high pressure CNG components and installations. In city buses compressed methane can provide sufficient operating range. Figure 6 shows a typical tank arrangement for a bus. The total volume of the tanks can be as high as litres, corresponding to some 310 litres of diesel fuel. CNG storage is heavier than diesel fuel storage. This might reduce the carrying capacity for natural gas vehicles. The material of the tanks is typically aluminium or composite material. Methane can be stored in liquid form too, but the gas has to be chilled to -160 o and stored in a well insulated tank equipped with certain safety valves. Liquid methane takes about half of the volume compared to the compressed methane (200 bar). Figure 6. CNG tank arrangement on a bus. (Lexen 2001) 12

13 4 EMISSION PERFORMANCE OF NATURAL GAS AND BIOGAS (methane) Methane is an inherently clean burning fuel with particulate emissions close to nil, and exhaust gas toxicity is very low, too. Figure 7 shows particulate probes from VTT s measurements on heavy-duty vehicles. With natural gas no colouring of the sampling filters can be seen. NO x emissions have to be controlled either during combustion using leanburn combustion or using a combination of closed-loop stoichiometric combustion and three-way catalyst. Lean-burn engines should also be catalyst equipped to control formaldehyde and methane emissions. Optimised engines with exhaust after-treatment devices produce very low regulated and unregulated emissions both in light-duty and heavy-duty applications. Thus, methane is a very promising option in urban vehicles to reduce local pollution and harmful health effects. Figure 7. Particulate samples (filters) from three emission level diesel buses and one CNG bus (Euro 5 MAAKAASU) over Braunschweig test cycle. In , VTT carried out a comprehensive emission test program for diesel and natural gas buses. Seven buses, three diesel and four CNG vehicles (one Euro III and three EEV certified), were tested on a chassis dynamometer for emission performance. The measurements covered regulated emission components as well as a number of unregulated 13

14 emission components. VTT summarized the results as follows (Nylund et al. 2004): The results demonstrate that regarding particle mass and number emissions, the CNG vehicles, on average, are equivalent to CRT (Continuously Regenerating Trap) filter equipped diesel vehicles. The particle matter (PM) emissions of both CRT diesel and CNG vehicles were some two orders of magnitude lower compared with the baseline diesel engine. No abnormity could be found regarding the numbers of nanoparticles emitted from CNG vehicles. The formaldehyde emission of the catalyst equipped CNG vehicles was low, as well as the emission of polyaromatic hydrocarbons components (PAH). The genotoxicity of CNG emissions was extremely low, determined by the Ames mutagenicity tests and calculated as a reference value per unit of driven distance. As for NO x emissions, CNG vehicles provide similar or superior emission performance, depending on the emission certification class. The results of the study are summarized in figure 8. In Italy, Instituto Superiori di Sanita and Instituto Motori and CNR conducted a joint study on exhaust emission toxicity. Measurements were done with two engines installed to engine dynamometer. Engines were the stoichiometric IVECO CNG engine with three-way catalyst and the corresponding IVECO diesel engine. The old European ECE R49 13 mode cycle was used for testing. It means that all the measurements were done in static load points. The NO x emission of the CNG engine was as low as 0.1 g/kwh, and PM emission was below 0.01 g/kwh, both clearly lower than those of corresponding diesel engine. The interesting part of the study is the results of unregulated emissions. Particle-associated PAHs and nitro-pahs were nearly 50 times lower with spark ignited CNG engine compared to those of diesel engine and formaldehyde emissions were 20 times lower for CNG engine. A 20 to 30 fold reduction of genotoxic activity was estimated. (Turrio-Baldassarri et al. 2006) As for CO 2 emissions, natural gas has some advantage over petrol and is comparable to diesel with current technology. It has been projected that with new technology, CO 2 emissions of natural gas vehicles will be 16% lower compared with petrol vehicles and 13% lower compared with diesel vehicles (AFCG 2003). The latter projection would probably require pilotinjection engines. Biogas results in significant reductions of WTW greenhouse gas emissions. Methane is a strong greenhouse gas, and therefore unburned methane in exhaust gas has to be kept low. 14

15 Relative emissions, worst result = 100% Brauschweig cycle PAH car. Euro 3 Diesel Euro 3 Diesel +CRT PM# 100 % CNG EEV NMHC 50 % PM NOx 0 % FA NO2 Ames CO2 Figure 8. A exhaust emission comparison between diesel without after-treatment, CRT diesel and lean-burn CNG buses. Results are measured over Brunscweig transient bus cycle and the for the worst result is given the index 100.(Nylund et al 2004) Natural gas is commonly used in city buses all over the world. Several studies have been performed to measure and compare the emissions of conventional diesel buses and natural gas buses. Results from VTT s follow-up programme on buses were accounted for in figure 9. All VTT s measurements are conducted with complete vehicles running transient city bus cycles on a chassis dynamometer. This is why the results depict real emission levels in typical city traffic. There is no official emission test for complete heavy duty vehicles, only the engine has to be measured in engine dynamometer, and after passing the emission test the engine can be used in any heavy duty vehicle applications (Directives 88/77/EEC and 05/55/EC). Earlier diesel engines were tested only using certain steady state cycle (ECE-R49 / Euro I and II, ECS / Euro III onwards). At the moment engines has to be measured using both steady state (ESC) and 15

16 transient (ETC) cycles. In table 1 are the emission limits in European Transient Cycle test. Table 1. Emission limits for diesel and gas engines, ETC test, g/kwh Results from chassis dynamometer measurements (figure 9) show clearly that particulate matter emission is very low with all CNG buses. NOx emission also is low with buses having stoichiometric gas engines. A city cycle with low average load and low exhaust temperature level is not an easy application for a diesel bus with exhaust gas after-treatment, but still the best diesel buses have quite low emission levels. Figure 9 also shows approximate NOx and PM limits (in the form of boxes ) for various emission certification classes. These limits have been calculated from the engine test limits values by multiplying with a factor of 1.8. The factor is the average amount of work expressed in kwh/km measured on the crankshaft of a bus while running the Braunschweig test. Example: The NOx limit for Euro III is 5.0 g/kwh (engine test). The amount of work on the crankshaft of the engine is 1.8 kwh/h, so the distance based reference value for Euro III is 5.0 * 1,8= 9 g/km. 16

17 NOx g/km NOx and PM emissions over the Braunschweig city bus -cycle 15 Diesel EEV Euro 1 CNG EEV 12 9 EEV 3-aks EGR Euro 2 Light Const. Euro 3 calibration Euro limits (by factor 1.8) EEV SCRT EEV SCR EEV EGR 6 Euro 5 3-aks SCR Euro 3 ESC ETC 3 EEV CNG stoik. EEV Euro 5 0 0,00 0,05 0,10 0,15 0,20 0,25 0,30 PM g/km Figure 9. The nitrogen oxide (NO x ) and particulate (PM) emissions of a city buses in Braunschweig city cycle, measurements The results of the individual measurements are presented as small triangles or circles; the averages of all results by emission class are presented as large triangles (diesel buses) or circles (natural gas powered buses). 5 Buses / engines All major European manufacturers have CNG buses available. Typically all new CNG engines fulfil the EEV exhaust emission limits in engine dynamometer test. Both lean-burn and stoichiometric engines equipped buses are available. The first dual-fuel buses are entering test service, but mainly in other applications than city traffic. In the following part, the current CNG bus availability from major European manufacturers is described very shortly. The emphasis is on the engine technology. 5.1 Iveco Iveco s gas engines run on stoichiometric mixture and they are equipped with three way catalytic converters. Iveco s Irisbus gas vehicles cover the range from minibuses to articulated buses, and Iveco has gas engines from 78 kw to 228 kw. Figure 11 shows the articulated City Class CNG bus from Irisbus. 17

18 Thanks to its stoichiometric combustion technology, the exhaust emissions are very low, and the engine easily complies with the EEV label. The low noise level of natural gas Irisbus Iveco buses is an advantage too. According to Iveco s presentation, the difference between CNG vehicle and diesel vehicle noise levels is 5 db. Figure 10. Layout picture of Irisbus Iveco CNG vehicle. Figure 11. The articulated Irisbus City Class 18 CNG bus. (Straetmans 2005) 18

19 5.2 Scania Scania announced new gas engines for buses and trucks in September The gas engines are based on the 5-cylinder 9.3 liter diesel engine platform, and they are available for Scania OmniCity (low-floor) and Scania OmniLink (low-entry) buses specified for inner-city operation. Engine modifications include cylinder heads for centrally-mounted sparkplugs, a gas injection system in the intake manifold, and a specially adapted camshaft. The 199 and 224 kw (1900 r/min) engines are turbocharged lean-burn engines. Stability of the mixture is controlled by a closed-loop lambda sensor system, additionally an air mass meter is fitted in the air intake. The engine management system automatically adapts to the gas quality. Good mixture control is advantageous when running on biogas or mixes of biogas and natural gas. Even natural gas from different sources varies some degree. The gas tank installations for buses include four 320-litre aluminium tanks are mounted in a package on the roof. The operation pressure is 200 bar, the typical inner-city operating range is around 500 km. A package of eight or ten 150-litre composite tanks for roof mounting is supplied with chassis modules. 5.3 Mercedes Benz The Mercedes-Benz CITARO CNG city buses are equipped with a 185 kw turbocharged lean-burn engine. The charge air is cooled, and lean mixture, 1.2 1,55, is controlled by wideband oxygen sensor, and the system detects natural gas quality, too. An oxidizing catalytic converter / silencer system is part of the exhaust pipe. The gas tanks are made of plastic (polyethylene with carbon fibres). There are five 190 liter tanks in the standard version of the solo bus, and six in the articulated bus. Maximum number is eight, and the max gas volume is about 1500 liters. The tanks are on the roof above the front axle, see figure 12. The weight of five empty tanks is about 475 kg. (Mercedes Benz Omnibus brochure: Standard description CITARO CNG, Model C ) 19

20 Figure 12. Mercedes-Benz Citaro CNG solo bus. 5.4 Volvo Volvo has both solo and articulated CNG city buses with a 9-liter stoichiometric gas engine equipped with three way catalytic converter (figure 3). According to press release 2010/10/27 Volvo is participating in a demonstration project for methane/diesel technology using buses for regional traffic and trucks. Only a small amount of diesel fuel will be injected into the cylinders. The main fuel is biogas or natural gas, which is injected to inlet channels through injectors located near the inlet ports. The principle is shown in figure 5. Diesel fuel is needed to ignite the gas. The engine control unit monitors the engine all the time and controls the proportion of the methane and diesel. Highest proportion of gas can be achieved during stable driving at high loads. For fleet owners, the ability to run on diesel fuel only provides security. Diesel-methane buses have two fuel systems. The gas is stored in tanks on the roof in a gaseous state, as with current gas-operated buses, and refilled in exactly the same manner from the same filling stations. Volvo Buses has received its first order for 11 buses, with methane/diesel technology. The buses will be in service in July Up to 70% of the fuel is biogas or natural gas. 20

21 5.5 MAN MAN has a long history with gas vehicles. At the moment MAN offers both naturally aspirated stoichiometric engines and turbocharged leanburn/mix engines for gas buses. On moderate load and speed, the turbocharged lean-mix engine operates with stoichiometric mixture. In the high torque and/or high engine speed range the engine operates with lean mixture to reduce the thermal loads of the engine. The engine is equipped with a three-way catalyst. In lean conditions, the catalyst operates as an oxidation catalyst. Lean-burn engines operate with lambda value between depending on the engine load. MAN gas engines are based on MAN diesel engines but there is a lot of e.g. different pistons and piston rings, special features in valve area and optimized cylinder head cooling. 5.6 Cummins-Westport Cummins Westport manufactures both lean-burn and stoichiometric gas engines. Solaris uses Cummins Westport engines. Lean-burn spark-ignited turbocharged engine (5.9 l) with kw power output equipped with oxidizing catalyst can meet even EEV emission standards. Cummins Westport s newest gas engine (8,9 l ISL G) is a stoichiometric engine with cooled exhaust gas recirculation (EGR). It uses high EGR rates in combustion process to lower the cylinder temperature, still air / fuel ratio is kept stoichiometric and exhaust gases are cleaned by three-way catalyst. The engine meets EPA and CARB 2010 and Euro EEV emission standards. According to the manufacturer the fuel economy compared to previous lean-burn engines is improved by up to 5 %. 6 USER EXPERIENCES IN HELSINKI AREA 6.1 Helsingin bussiliikenne Helsingin Bussiliikenne Oy is a big Finnish bus company with over 600 buses. Of those, 82 are natural gas buses. Gas buses have been operating since 1998, and 25 % of the gas buses fleet are from that year. Another 25 % is from year 1999 and 50 % from year The first buses (Volvo) were equipped with open-loop lean-burn engines, and the emission levels measured over the Braunschweig city cycle were around 10 g/km for NOx and 0,01 0,02 g/km for particulate mass. In 2002, Volvo buses with closed-loop controlled lean-burn engines entered service. The newest buses, from 2005 onwards, are MAN buses, equipped with either natural aspirated stoichiometrics engine with 3-way catalysts or turbocharged 21

22 lean-mix engines also with 3-way catalysts. Most of the time the latter ones operate on stoichiometric fuel/air ratio, but use lean mixture in full load to lower the operation temperatures. Reliability of the gas buses should be better, the availability of gas busses has been significantly lower than for diesel busses. Especially the Volvos have had a lot of engine related problems, such as ignition problems and very short spark-plug life. About 50 % to 60 % of the total number of towings has been for gas buses, although gas buses only constitute some 15 % of of the total fleet. Gas buses have been replaced on the line twice as often as diesel busses. Expenses for the gas buses have been higher than for diesel busses. Maintenance expenditure has been up to three times higher than for diesel buses, although the manufacturer declares only 30 % higher maintenance cost. The repair shop is designed for gas buses. The principal service bay is designed for gas vehicles with half-filled fuel tanks. The express service bay is designed for full gas pressures, but only for limited number of busses. Overhauls as well as cleaning of the buses must be scheduled to be done before refuelling. Capacity of gas the gas tanks has been large enough. Quality of the gas been satisfactory, although some extra lubricating oil has been found in the gas. Most of the problems with the oil were removed after a switch to oil-free compressors. The extra oil caused problems in the gas pressure regulators and injectors. Wear of pistons, cylinders or bearings has not been different from diesel engines. Drivers criticize sluggishness and slow response to throttle opening. The cylinder heads are based on diesel engine heads, compression ratio has been lowered by changing pistons. First lean-burn buses do not work properly with original engine calibration, there is a trade-off between driveability and emissions. The biggest problem with gas engines is high temperature of the engines, which leads to many difficulties. In the first gas buses lifetime of spark plugs and injection coils was not as long as designed. Cylinder heads do not withstand the high temperatures. Several heads have been broken. In addition, valve stems wear rapidly. Already a small amount of debris causes leakage in the gas injection valves. This is not a problem while running, but is a problem after shutting down the engine. Pressure regulating valves are not operating 22

23 satisfactory. Beginning of wintertime is a critical season for the operation of the gas valves. The problems probably are due to condensated moisture in the valves and piping. The burst plates of the relief valves on the gas tanks have broken due to freezing, one leakage of gas took place in the garage. Problems due to high temperature: Holes burned into the exhaust manifold Exhaust pipe ruptures Insulation of wiring melting or becoming brittle Short lifetime of lambda-sensors, less than km Replacement interval of turbochargers is one year. 6.2 Tammelundin liikenne Tammleundin Liikenne is a smaller operator, with some 20 buses. At the moment about half of the fleet is gas buses. All the gas buses are Mercedes-Benz buses, the oldest one from year 1997 and the newest from year Compared to diesel buses service cost of CNG buses has been higher due to shorter service intervals and expensive spare parts. Lifetime of the engines is about 20 % longer due to less particles and cleaner lubrication oil, less wear in the engines. Winter operability of CNG buses has been good. Starting of CNG buses in cold conditions is easier than that of diesel buses. Temperature of the engine is higher as well as output of heat, this is a benefit in wintertime but a disadvantage in summer time. However, Tammelundin Liikenne has had no engine failures due to high temperatures in the summer time. Rubber and plastic part suffer from the high temperatures, buses must be properly serviced, e.g. coolers must be kept clean. Fuel cost has been about the same as with diesel buses. CNG is cheaper, but the higher fuel consumption nullifies the advantage of the cheap fuel. With biogas appreciation would be better than with natural gas, but currently biogas is not available. Refuelling of the buses is done at a public refuelling station next to the bus depot. Fuelling stations outside have had problems due to water in the refuelling device. Worn refuelling valves, after 500 to 600 thousand kilometres, have frozen in open position, too, although return valves have prevented major problems in these cases. 23

24 7 GAS BUS COSTS Operation costs are important for fleet owners, investment for gas buses has to be justified by economical facts too. Fuel price, subsidies or some competitive advantages can be reasons for investments. Operation costs of gas and diesel buses cannot be calculated universally, local conditions can have major influence to them. Here a short estimate is made based on situation in Helsinki area on March Basic assumptions: -price of diesel bus price of CNG bus annual mileage km -diesel price without VAT 1,10 /l -CNG price without VAT 1,00 /kg -interest rate 4 % -service life 10 years -residual value: 10 % for diesel buses and 8 % for CNG buses (less buyers for second hand gas buses) -fuel consumption: 45 l/100 km for diesel buses and 42 kg/100 k for CNG buses -maintenance costs: 0,1 /km for diesel buses and 0,15 /km for CNG buses Calculated costs using those basic assumptions are / year for diesel buses and / year for CNG buses. Share of investment costs, fuel costs and maintenance costs is plotted to figures 13 and 14, cost with basic assumptions are marked diesel and CNG1. CNG2 bar is calculated with gas price is 0,95 / kg and CNG 3 with gas price 0,90 / kg, all the other figures are kept constant. The lowest gas price decreases total annual costs under those of diesel bus. 24

25 /a Annual costs Maintenance Fuel Investment Diesel CNG 1 CNG 2 CNG 3 Figure 13. Annual costs of diesel buses and gas buses. Diesel and CNG1 bars are calculated using basic assumptions given above in the text, CNG4 is calculated with fuel consumption 39 kg/100 km instead of basic 42 kg/100 km and CNG5 with the same lower fuel consumption plus lower maintenance cost 0,12 /km instead of 0,15 /km.. 25

26 /a Annual costs Maintenance Fuel Investment Diesel CNG 1 CNG 4 CNG 5 Figure 14. Annual costs of diesel buses and gas buses. Diesel and CNG1 bars are calculated using basic assumptions given above in the text, CNG4 is calculated using gas price 0,95 /kg and CNG3 with gas price 0,90 /kg keeping all other factors constant. Figure 14 shows annual costs with basic assumption plus with better fuel economy and lowered maintenance costs. Today energy consumption of typical gas bus with spark ignited engine is in city traffic typically 30 % higher compared to diesel buses. If the difference can be lowered to 20 % annual costs are about the same as with diesel bus. If the maintenance costs are 0,12 /km instead of 0,15 /km (still 20 % higher compared to diesel bus) annual costs of a gas bus are about 2000 below those of a diesel bus. Socio-economical factors (environmental costs) are not included in this study. In the Directive 2009/33/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of clean and energy-efficient road transport vehicles is written e.g. The energy and environmental award criteria should be among the various award criteria taken into consideration by contracting authorities or contracting entities when they are called upon to take a decision on the procurement of clean and energyefficient road transport vehicles. Costs for different emissions are given in the Annex 2 of the directive. The directive has to be taken into account and fleet owners can get some advantage of low emissions e.g. in Helsinki area line bidding. 26

27 Environmental costs are included in the reports 3.1 Manual for strategy, policy and action plan How to introduce biogas buses and 6.8 Feasibility study to introduce biogas buses in Tartu, Estonia. 8 CONCLUSIONS Low exhaust emissions are a clear advantage of natural gas buses. Particulate emissions are very low regardless of the engine technology. Stoichiometric engines can deliver low NOx emissions as well. Gas buses show rather good emission stability over time, especially for PM. The best new diesel buses are rather clean when new, but VTT s measurements have shown that emission levels tends to increase over time. The more complicated exhaust gas after-treatment system, the more possibilities for a decrease in cleaning efficiency. Heavy-duty natural gas engines are at the moment mostly spark ignited otto-engines, and they suffer from lower energy efficiency compared to corresponding diesel engines due to lower compression ratio and higher pumping losses due to the need to throttle intake air. Total fuel costs comes from unit fuel price and fuel consumption. Low energy efficiency can be compensated by low fuel price. Still high energy efficiency must be the target in the development of new generation gas engines. Most probably some increase in energy efficiency can be achieved by optimizing / modifying the spark ignited engines but dual-fuel engines with gas as the main fuel would give significantly higher energy efficiency. The first dual-fuel gas buses are entering service, but first to intercity lines. City traffic with low average load and continuous transient operation is challenging for a dual-fuel engines. 27

28 REFERENCES Nylund, N-O., On the development of a low emission propane engine for heavy-duty urban vehicle applications. VTT Publications 26. Espoo p. + app. 21 p. IEA AMF Annex XXXIX: Enhanced Emission Performance and Fuel Efficiency for HD Methane Engines. Enhanced emission performance and fuel efficiency for HD methane engines. Literature study. Final report. Report AVL MTC May Scania: New gas engines for trucks and buses diesel-like performance and modular design. Scania press release 22 September 2010 Westport Innovations Set To Demonstrate Natural Gas Technology On Heavy-Duty Trucks - Product Announcement - Company Profile. Nylund, N.-O. et.al. Transit bus emission study: Comparison of emissions from diesel and natural gas buses. VTT Research Report PRO3/P5150/05. Espoo p. Kaiadi, M., Diluted Operation of a Heavy-Duty Natural Gas Engine - Aiming at Improved Effciency, Emission and Maximum Load. Lund University p. Mercedes Benz Omnibus brochure: Standard description CITARO CNG, Model C p. Directive 2009/33/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of clean and energy-efficient road transport vehicles d=85 28

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