Hybrid Electric-Biodiesel Buses: Cleaner, Fuel-Saving Alternative for Public Transportation

July 10, 2015

Print This Article Share it With Friends

by Milsa Vijayadharan* (Advanced Biofuels USA) Hybrid Electric-Biodiesel Buses used in California, Washington and Michigan Model Lower GHG Emissions Options for Cities Everywhere

Public transportation plays an important role in green and sustainable practices. Public transportation use means fewer cars on the road and a cleaner environment. Agencies operating public transportation systems are encouraged to use alternative fuels instead of high sulfur petroleum diesel to comply with federal air quality regulations and to utilize financial assistance offered by federal or state legislation to purchase clean fuel buses. Other reasons to choose alternative fuels are to reduce tailpipe emissions and improve local air quality, and to increase use of fuel derived from local sources.

University of Michigan Hybrid Electric Biodiesel-Powered Bus

Toxic emissions- combustion of standard petroleum diesel fuel produces pollutants that adversely affect the quality of air where transit vehicles operate frequently. Major pollutants in diesel exhaust are particulate matter (PM), oxides of Nitrogen NOx, hydrocarbons and non-methane hydrocarbons.

Green House Gas Emissions- Transit vehicles emit important greenhouse gases carbon dioxide and methane. These gases affect climate and are considered responsible for global warming.

Biodiesel- Biodiesel is mono-alkyl esters of long chain fatty acids. Biodiesel is a renewable fuel made from a diverse mix of feedstock such as recycled cooking oil, soybean oil, and animal fats through a chemical process called transesterification. By transesterification glycerin is separated from the fat or vegetable oil. Biodiesel and glycerin are created by the process. Glycerin is used in soap and other cosmetic products.

Biodiesel is non toxic, biodegradable, and produces less air pollutants than petroleum based diesel. Fuel grade biodiesel must meet ASTM D6751 specifications. Biodiesel is the only alternative fuel to have fully completed the health effect testing requirements of the 1990 Clean Air Act Amendments.

Renewable Diesel- Renewable diesel, also called Green Diesel, is chemically the same as petroleum diesel but is produced from recently living biomass. Biodiesel has different chemical and physical properties than petroleum diesel and renewable diesel.

San Francisco Municipal Transportation Agency

Hybrid Electric Vehicles (HEV)-As more and more fleets are showing interest in acquiring alternative fuel vehicles, hybrid electric vehicles are becoming a promising option to reduce greenhouse gas emissions, reduce carbon foot prints and increase fuel efficiency. HEV’s combine a conventional internal combustion or compression ignition engine with an electric propulsion system. The following technologies help HEV’s achieve better fuel efficiency:

a)Regenerative Braking System- In a conventional vehicle when brakes are applied kinetic energy of the vehicle is converted into heat as the result of the friction between the brake pads and wheels, this heat energy is wasted. The amount of energy wasted this way depends on how often, how hard and for how long the brakes are applied. A regenerative braking system is an energy recovery mechanism which slows a vehicle by converting its kinetic energy into another form of energy which can be stored or used immediately.

When an electric vehicle is operated, energy from the batteries flows to the wheels through an electric motor. When regenerative brakes are applied, the whole process is reversed. Electronic circuits cut the power to the motor. Kinetic energy and the vehicle’s momentum makes the wheels turn the motor. The motor now acts like a generator and produces electricity which flows to the batteries. So the energy that is normally wasted during braking is returned to batteries which gets charged and can be used when the vehicle is started again.

Thus, a regenerative braking system in a hybrid electric vehicle helps to convert kinetic energy back into electrical energy that can be stored in batteries for reuse to propel the vehicle. This type of system is best used in city driving where stop-and-go traffic is common. Hybrid electric systems recover energy normally lost during braking and stores that energy in batteries. This energy is then used to improve fuel economy and vehicle performance or to run the vehicle with electric power only.

b)Idle off- Idle off is a feature that is used to turn the vehicles conventional engine off when it is stopped ata stoplight or in traffic to save fuel. The battery powers the air conditioner and other accessories and the electric motor powers the vehicle to start again and switches to the conventional engine when more power is needed for acceleration.

c)Electric only drive- This feature allows the vehicle to operate entirely on electricity at low speed saving fuel.The conventional engine is used at higher speed, when it is most efficient.

All these technologies make hybrid electric vehicles best suited for city driving. When biodiesel is used in these hybrid electric vehicles the environmental benefits are even more.

Hybrid electric-biodiesel buses- Buses are the backbone of many public transport systems. The majority of bus fleets in the US are comprised of diesel powered buses, which emit high amounts of exhaust emissions. Health impacts of diesel buses are important because they run in densely populated areas. Particulate matter from diesel fueled vehicles can enter into lungs and pose serious health risks.

Hybrid electric-biodiesel buses have proven to be an excellent choice for city bus fleets because of low emissions and increased efficiency in city driving due to regenerative braking systems, idle off, electric only drive features.

Following are examples of some of the agencies that are using hybrid electric-biodiesel buses.

San Francisco Municipal Transportation Agency (SFMTA)- SFMTA has purchased 61 new biodiesel-electric hybrid buses.

SFMTA’s Biodiesel hybrid bus funds include but are not limited to: Federal Transit Administration grants, AB644 bridge tolls, Proposition K and Proposition B funds. Total purchase contract is $68 million.

SFMTA’s hybrid buses run on B20, a blend of diesel and biodiesel. The biodiesel used by SFMTA is made from recycled oil, fat and grease. Used oil, fat and grease from San Francisco restaurants are used as feedstock to produce biodiesel. Thus the fuel goes from fryer to fuel tank in San Francisco.

SFMTA sees no performance difference when biodiesel is used in these buses instead of regular diesel. No mechanical problem has been encountered specifically due to biodiesel usage. A beneficial difference is that fuel consumption with hybrid technology is reduced by 25%.

Biodiesel can gel in cold weather but it’s never freezing in San Francisco so there is no cold weather performance issue with B20 blend.

The main issue reported is with the storage of biodiesel because biodiesel, like food products, can get contaminated easily. Unlike petroleum diesel whose composition is the same everywhere, biodiesel can vary from batch to batch which in turn results in greater need for quality assurance.

Intercity Transit- Intercity transit is the public transportation provider in Thurston County Washington. Intercity Transit began operating hybrid biodiesel-electric buses in 2010. The agency chose to use biodiesel in hybrid buses to contribute to improving the ecological environment of the Puget Sound Area of Washington. The Agency uses biodiesel from various feedstocks as long as they meet the state's ASTM specifications and BQ-9000 delivery standards.

The agency uses B20 except in the winter months when they switch to B5. They have seen rare issues of gelling in the vehicles that sit outside for a few days in extreme temperatures. However if gelling occurs the vehicles are equipped with pre heaters to deal with this issue.

In Thurston County, the use of 1 micron filters at the diesel pumps has eliminated most of the maintenance issues associated with biodiesel use. Washington rarely faces prolonged sub freezing temperatures so there is no chance of gelling of stored fuel. To be extra careful they store their fuel in underground storage tanks.

The agency’s current biodiesel price is $3.8749 per gallon which includes the $ 1.00 per gallon Federal Blender’s Rebate.

University of Michigan- The University of Michigan transports over 6.9 million passengers annually. The University purchased biodiesel electric hybrid buses to reduce emissions. The first hybrid electric-biodiesel buses were received in December of 2011 and were placed in operations in early 2012.

The University chose biodiesel to promote sustainability within their operations. RKA and Wacker oil are their biodiesel suppliers. They use B20 in summer and B5 in winter. The operations faced gelling issues very early on when using B20 in winter so they started using B5 in winter. Currently, they are not having any gelling issue with biodiesel. They also faced fuel filter contamination issue in the past. They are now using fuel filter heater as a precautionary measure. Their current biodiesel price is $2.27 a gallon.

Biodiesel problems and solutions

1) Cold weather performance- Biodiesel tends to gel in colder temperatures. Conventional diesel gels, too, in cold weather but biodiesel gels in much higher temperatures compared to conventional diesel.

Good fuel management can help engines run smoothly all winter long. Cold weather performance of biodiesel depends on the type of feedstock they are made from. Cold flow properties of biodiesel derived from vegetable oil, used oil or grease, coconut oil, palm oil and animal fat vary greatly. Animal fats, palm oil and coconut oil are more highly saturated so they have higher cloud points. (Cloud point is the temperature at which wax like crystals appear, these crystals then adhere forming pockets similar to honey comb trapping liquid resulting in fuel gelling.)

Biodiesel derived from animal fat freezes at high temperature; biodiesel derived from recycled grease freezes at slightly lower temperatures than that derived from animal fat. Biodiesel derived from plant oils has the lowest freezing temperature.

Fuel quality is critical to successful cold flow operations. A good quality biodiesel fuel must meet national standard ASTM D 6751. Blending biodiesel with diesel that has been treated with cold weather additives will improve cold weather performance. Block and filter heaters can be used to prevent biodiesel from gelling. Electronic heaters powered by the vehicle's battery can be used inside the fuel tank to heat up fuel and get it flowing. In-line fuel heaters can also be used to stop fuel from gelling. Storing vehicles indoors also helps to prevent biodiesel from gelling.

2) Fuel filter clogging- Biodiesel is an excellent solvent. Petroleum diesel leaves deposits in fuel liners, tanks, and delivery systems. Biodiesel can dissolve these deposits which results in fuel filter clogging and stops fuel from getting into the injector system.

Improperly processed biodiesel can also contribute to filter clogging. Vehicle operators using biodiesel must be prepared to change fuel filters and should carry a spare fuel filter. More frequent filter changes may be needed until the system has been cleared of all the deposits left by petroleum diesel.

3) Water Contamination- Although biodiesel is immiscible with water it can absorb water at a molecular level as a result of inappropriate processing. The presence of water can also be as a result of storage tank condensation. The presence of water will decrease the heat of combustion which will result in loss of power, it may also lead to corrosion of fuel system components, and presence of water also promotes the growth of microbe colonies.

Some of the remedies for biodiesel contamination are as follows:

Underground storage tanks are useful in preventing water contamination of biodiesel.

Tanks should be maintained properly to avoid any water leaks into the fuel, all tank caps should be in place and in good condition. Keeping the tanks full will prevent condensation.

Prolonged exposure of the fuel to light should be avoided because it can induce algae growth.

Tanks should be tested regularly for microbes and water. Biocides can be used to prevent microbial contamination.

Biodiesel should be purchased from a reputable fuel distributor.

Conclusion- Public transit can carry larger numbers of passengers and can take cars off the road, reducing emissions and the amount of energy consumed. Public transit agencies historically relied on diesel buses. Today public transit agencies are at the forefront of adopting environmentally efficient vehicles.

Hybrid buses reduce emissions significantly and are highly efficient. Regenerative braking system, idle off, and electric only technologies make hybrid electric vehicles ideal for city driving and an excellent choice for transit agencies.

Use of biodiesel in hybrid buses adds to the environmental benefits as biodiesel is biodegradable and non toxic. Biodiesel blends help to reduce exhaust emissions by petroleum diesel. Many transit agencies across the country have adopted hybrid electric-biodiesel buses.

From the experience of some of the transit agencies it seems the main problems associated with biodiesel use are gelling, fuel filter clogging and contamination.

Using good quality biodiesel ensures better cold weather performance. The type of feedstock used to make biodiesel also affects cold weather performance. By determining the appropriate feedstock that can be used to derive biodiesel for a particular area will be helpful to solve cold weather problems associated with biodiesel use, particularly in areas with prolonged sub freezing temperatures.Block and filter heaters also can be used to solve gelling problems.

Another issue is fuel filter clogging, vehicle operators can be trained to replace fuel filter to solve this issue. By storing fuel tank underground and with proper maintenance and keeping the tanks full will help to avoid contamination.

As state and local policies discourage use of fuels and technologies that are harmful to the environment, the benefits of hybrid electric-biodiesel buses should serve to expand their popularity and use around the country.

READ MORE (Thurston Talk) and MORE (Intercity Transit) and MORE (Diesel Technology Forum) and MORE (Yahoo!Finance)

*Milsa Vijayadharan has a masters degree in Chemistry and experience working in scientific laboratories. As she keeps up with developments in advanced biofuels, she occasionally writes articles and adds posts to the Advanced Biofuels USA web site library.