California dreaming
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Against the background of California's stringent forthcoming environmental legislation, Volvo has developed a hybrid-power Environmental Concept Truck which shows the way to meeting the challenge for a "Zero Emission" distribution wagon.
According to the old saying, whatever California does today the rest of the US does tomorrow. Arid what America does tomorrow, Europe does two years later.
When it comes to diesel engine emissions legislation the pattern is obvious. The current standards drawn up first by the California Air Resources Board (CARB), then by the US Environmental Protection Agency (EPA), have clearly influenced those from the European Commission.
The connection certainly isn't lost on the engine manufacturers that compete in all three markets. Cummins UK marketing support manager Peter Griffen notes: "Whatever California comes out with we have to comply, so we'll be in a good position to follow it through to EPA and European legislation."
Volvo's director of corporate communications and former engine development project manager, Anders johannesson agrees: "They're roughly on the same level—although certification is not the same so you can't compare figures directly—but we have the same engine generation to fulfil the requirements in the US and Europe." It's probably why Volvo should refer to the influence of California's legislators and their decision to apply Zero-Emission legislation for a specific proportion of passenger cars sold there in 1988, when it recently unveiled its futuristic Environmental Concept Truck (ECT) (CM 24-30 August). For ZeroEmission read battery power.
But a battery-powered car is one thing a battery-powered truck is another. "With today's battery technology such trucks will be extremely heavy and expensive...and substantially less efficient," believes Volvo's vice-president, global product planning Torsten Dahlberg. "But if you can accept a truck that
during a specific distance my 25 to 30kmcan run at zero emissions, and with low emissions for the rest, the weight of the batteries can be reduced—but a lot of development remains before a commercially viable vehicle is available," he says.
The ECT is Volvo's first tentative step towards that vehicle. While only a technology demonstrator it has a number of engineering ideas that could well appear on environmentally friendly trucks of the 21st Century.
The ECT's "Series Hybrid" drivetrain is based around a gas turbine with an integrated high-speed generator that either provides direct power to an electric motor mounted on the back axle, or charges the vehicle's battery pack.
There's nothing new about powering a truck by gas turbine. However, in the past, in a multispeed operation, turbines have not been able to deliver the flexibility or economy required.
But used as a constant-speed generator in a hybrid electric truck like the ECT, a turbine begins to come into its own, especially in terms of efficiency and emissions.
The ECT turbine runs on ethanol, although it could easily use almost any other gas or liquid fuel. Burning ethanol produces between 9095% lower levels of nitrogen oxides (N0x) and significantly fewer particulates (see chart) than a normal diesel, as the turbine's combustion takes place under constant pressure.
Other advantages
significantly to the greenhouse effect—any CO, produced by its combustion can easily be absorbed by vegetation.
What the hybrid gas turbine can't deliver is the kind of efficiency found in today's diesel engines: its efficiency rating at full load is only 32%.
According to Volvo Truck Great Britain's sales engineering manager Ray Cattley: "Depending on the configuration, a diesel engine's efficiency can be as high as 46%—and 44% is quite normal on a typical heavy truck engine, although on smaller ones like Volvo's D6 Euro-2 inline six it's around 42% peak."
4111 -So there will have to be some significant improvements before alternative fuels and driveline combination can match diesel engines like-for-like."
One way of raising the gas turbine's efficiency would be to find more temperature-resistant materials for the power turbine. Truck manufacturers have looked hard at the use of ceramics—but they've still to overcome problems of mechanical strength. The ECT gas turbine operates at speeds of up to 70,000rpm and at temperatures of up to 1,025°C in the combustion chamber before the turbine.
However, turbines still have a lot going for them, especially when it comes to operating in sensitive environments.
While diesel engines have a higher overall efficiency, in constant stop/start city traffic they're frequently working under low load so their efficiency rating drops.
But with a Series-Hybrid powertrain, like that used in Volvo's ECT, energy can be stored in the batteries and used for acceleration, allowing the gas turbine to be optimised for a lower peak power and kept operating close to its maximum efficiency rating. As a result the overall efficiency of the ECT driveline can, in city traffic, match the equivalent diesel driveline. And while diesel-engined trucks will never make it into a "zero-emission" environment, a hybrid truck can switch to battery power and continue its journey The ECT is also very quiet. At the recent rollout ceremony at Gothenburg, it whispered around the test track with little more than a gentle whine from the turbine. So night-time deliveries would not be a problem.
With present battery technology for an electric truck to have any kind of practical operating range in a zero-emission zone (like a city centre) it needs a large battery pack In the case of Volvo's 15-tonne GVW ECT that pack weighs two tonnes in order to satisfy a 25km operating range.
However, the ECT's Nickel Metal Hydride (NLVII-I) batteries, developed by Varta, impose a far lower environmen tal burden than lead-acid and nickel-cadmium batteries, offer a longer service life, are maintenancefree and can be recycled. Despite the eco-friendly nature of battery power, Volvo readily admits that "powering a truck by batteries alone is not a viable option using today's technology."
Hence its ECT hybrid which can operate as a zero emission battery-powered truck in sensitive city-centre areas, before reverting to the gas turbine to power the electric motor at all other times.
ECT's environmental credentials also include brake linings which are free from organic components, while its ultra-low rolling resistance Michelin tyres have a tread section free from any environmentally hazardous oil additives, Lower rolling resistance means less energy is needed to drive the ECT forward.
A hybrid driveline also allows flexibility in the positioning of components. As power transmission to the traction motor is electric there's no gearbox or propeller shaft so the turbine and batteries can be mounted almost anywhere on board. The result is a truck with a far lower frame height that's ideal for city delivery work.
Whatever ECT is, it certainly isn't a production model, although parts of it could well appear on trucks of the future. For Volvo, ECT remains a project developed "against the background of California's forthcoming stringent environmental legislation". Perhaps it's more a case of "What Sweden does today:
0 by Brian Weatherley