Long a pioneer in the development of ethanol-powered vehicles, Scania has developed a 10.4 m long Scania hybrid bus that operates on E95. Because of the layout of the series hybrid powertrain, the bus provides as much passenger capacity as a conventional 12.2 m low-floor city bus, and the flexible design means the separate engine and powertrain modules could be fitted to other bus configurations.
The European engine and vehicle manufacturer Scania has a long history with ethanol, developing its first ethanol engine in 1916. In the late 1970s, it developed another ethanol burning powerplant for the Brazilian market as part of that country's move from petrol to alcohol fuels. Over the next two decades, the company has put more than 600 Scania ethanol buses into service in Sweden, as well as in Madrid, Spain; La Spezia, Italy and Slupsk, Poland.
Not surprisingly though, Storstockholms Lokaltrafik (Stockholm Public Transport) is by far the largest user, as it began replacing its diesel buses with renewable fuel vehicles starting in the mid-1980s.
Most recently, Scania has introduced its third-generation ethanol engine and developed a hybrid-electric bus fitted with this ethanol-capable engine. Scania has long experience in modifying diesel engines to operate on ethanol, specifically E95. Engine
modifications typically include an increase in compression ratio from 18:1 to 28:1, larger fuel-injection nozzles, modified injection timing and ethanol compatible gaskets and filters.
The new ethanol engine is an adaptation of Scania's latest 9 L, five cylinder diesel engine with air-to-air charge intercooling and exhaust gas recirculation. It has unit injectors for each cylinder and twin balance shafts located in the oil sump. It is rated 201 kW at 1900 r/min and generates 1199 Nm of torque at 1100 to 1400 r/min. The new engine meets stringent Euro 5 and EEV emissions levels while achieving the same efficiency as a conventional diesel engine,the company said.
This engine is being used in a series hybrid-electric bus, as the company believes that series hybrids are most suitable for stop-and-go urban operation where significant amounts of energy can be recovered through regenerative braking.
The engine drives a water-cooled,Voith ELVO Drive TFM generator to supply the primary power. A second, similar Voith ELVO unit powers the rear axle, supplying power for propulsion and acting as a generator during regenerative braking.
Energy from the regenerative braking and the engine/generator is stored in four 125V, air-cooled Maxwell Boostcap supercapacitor modules mounted on the roof. According to Scania, supercapacitors, designed for high output and high cycle efficiency,are much more robust than batteries in heavy-duty operation. They are also more efficient than batteries in the rapid charge and discharge cycles
characteristic of urban bus duty cycles. Unlike batteries, supercapicitors are designed to last the life of the
vehicle, in this case, 10 to 15 years.
Reduced noise was another objective with a goal of 75 dB(A) external noise at bus stops and when accelerating, as well as noise from roof-mounted systems like the air conditioning. The engine-generator is located in its own encapsulated compartment fitted with a noise-insulated wall separating the passenger compartment from the rear module. The engine-generator unit is optimized for low noise, operating at the lowest possible r/min with well matched torque curves for engine and generator, Scania said.
Air for the cooling systems is transferred through separate noise absorbing ducts. Though not yet incorporated, engine shutoff at standstill could be added, Scania said.
Since there is no mechanical connection between engine and wheels or propulsion motor, it would be easy to substitute other types of power sources diesel or petrol engines, natural or biogas engines, advanced engine technologies like Homogeneous Charge Compression Ignition (HCCI), etc., Scania said. Other possibilities include fuel cells, plug-in-hybrid or battery electric systems, which could be added in more or less plug and-play fashion. Scania said the hybrid configuration could be used in other applications, including other types of buses like double-deckers and refuse and delivery trucks.
Generator efficiency is typically over 90%, peaking at over 94%, Scania said.The engine efficiency is estimated by Scania to be between 40 and 45%.The efficiency of the propulsion motor is similar to the generator and slightly lower at very low speed.
According to Scania, by using advanced electronic control and efficient energy recycling, fuel economy is at least 25% better than a regular city bus and with similar improvements in exhaust emissions. Fueled by ethanol,fossil CO2 emissions are cut by up to 90% compared to diesel, depending on the way the ethanol is produced.
The all-aluminum body consists of four main sections. The middle passenger section can carry up to 81 passengers, with entrance and exit through large doors. The low floor bus has an entirely uncluttered floor without intruding wheel arches for easy cleaning
and for accommodating standing passengers, strollers and wheelchairs.
The passenger module provides the 10.4 m bus with similar passenger capacity to a conventional 12.2 m low-floor city bus, Scania said.
The driver sits in middle of the driver station located over the front axles and is provided a commanding view. The rear section holds the engine and powertrain.
Both front and rear sections can be fitted to buses of other lengths, as well as to double-deckers. In a series hybrid configuration, powertrain components can be located almost independently of each other, for example, for optimum weight distribution. Around 80% of the components of the hybrid bus are shared with existing Scania Omni buses.
The roof section holds the supercapacitors, air conditioning, heating and ventilation with space for gas tanks, should this option be used. Carrier Sutrak electric air conditioning is used.
Four steered wheels and lack of overhangs at both ends means excellent maneuverability on crowded city streets, Scania noted. There is no cutting of corners and no swept areas to consider in tight situations. The minimum length of a bus stop is 22 m compared to 26 m for a conventional 12.2 m city bus with similar passenger capacity.
The hybrid system is being tested this year and will be installed in 12 conventional three-axle,13.7 m Scania Omni Link city buses with ethanol engines.Two hybrid buses are scheduled to begin operating in Stockholm in early 2008 and another 10 will follow later
that year and in 2009.
Diesel progress magazine
