Time To Start Losing Weight!?

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Weight Reduction Key to Fuel Economy Gains, Researcher Says
By William Diem
WardsAuto.com, Oct 17, 2006 12:29 PM

PARIS – Meeting Europe’s future fuel-efficiency rules will require reversing the trend of cars getting heavier, a researcher says.

Improvements on the order of 10% in aerodynamics or rolling friction provide only a 3% improvement in fuel economy, while a 10% reduction in weight generates a 7% gain in efficiency, Lino Guzzella, a researcher with the Swiss engineering university ETH Zurich, says in a presentation at a recent academic conference here.

While gasoline engines can reach an efficiency near 36% at full load, they operate most of the time at part loads of 30% or less. On the European driving cycle, a typical modern diesel operates at 20% thermal efficiency, and a gasoline engine runs at 18%, Guzzella says.

A 1,764-lb. (800-kg) car – the weight of the original ’74 Volkswagen Golf – would achieve Europe’s 2012 goal of 120 g/km of carbon dioxide with an engine of 17% thermal efficiency in the European test cycle, which most gasoline engines currently meet. At 2,646 lbs. (1,200 kg), the weight of the Golf V launched in 2003, an engine would be required to hit 23% thermal efficiency.

Reducing engine power is another potential answer to future efficiency, Guzzella says. A typical 3,307-lb. (1,500-kg) fullsize car such as the Peugeot 407 requires a 150-hp engine to accelerate 0-62 mph (100 km/h) in 10 seconds, which he says is the de facto requirement of the market today.
Higher mass (left scale) means more thermal efficiency (bottom scale) required to meet CO2 rules.

However, that same fullsize car uses an average of only 9 hp on the European driving cycle, with peak output of 46 hp, he says.

For 20 or 30 years, Guzzella says, liquid fossil fuels will remain the principal fuel source of transportation because they are energy dense. Diesel fuel, with 2 kW-hours of energy per kilogram, is the densest form of energy outside of nuclear, and gasoline is only 15% below that.

Hydrogen, natural gas and batteries are far behind, he says, but if lithium-ion doubles its current potential from 200 watt-hours/kg, it could start to compete with compressed natural gas. And if batteries get three or four times better, they will knock fuel cells out of competition, because “there are too many losses converting hydrogen to electricity,” he says.

Meanwhile, Guzzella expects the best fuel efficiency from vehicles with two sources of power: one optimized for steady state operation and the other for acceleration. Such vehicles are likely to be hybrids, in which an internal combustion engine is used to replace energy lost by the vehicle as it moves. It also could be a fuel-cell vehicle with supercapacitors or a traction battery.
ETH Zurich Technical University students gather around their PAC II vehicle.

Guzzella and another presenter, Ralph Beck of the German technical university RWTH in Aachen, argue fuel efficiency also could be improved if the car knew where it was going ahead of time.

Having a global positioning satellite navigation system onboard, and knowing about stop signs, slopes and speed limits, would allow an engine control unit to adjust its settings in anticipation of a change in traffic and road conditions. Fuel savings could reach 3%, Beck says.

Savings could be higher for vehicles following the same route every day. In a simulation of a hybrid vehicle crossing a mountain, Guzzella says, the battery could discharge further than normally allowed because the car knows it is about to begin a descent that will recharge it.

Guzzella’s ETH university has put all those ideas into practice. The PAC II fuel-cell vehicle, which won the Shell Eco-Marathon in France in 2005 by achieving the equivalent of 9,023 mpg (3,836 km/L) of gasoline, had two electric motors in its drive wheel, one for steady state and one for acceleration. And the control unit was programmed for the curves and slopes of the Nogaro Grand Prix track where the competition took place.

Most of the academic conference focused on how to build software models that would speed development of new engines, but presentations also revealed nuggets of automotive insight:

* On its current 1.5L diesel, Renault SA uses software calculations to estimate air flow instead of an air-flow meter, which saves money and weight.
* In 1892, the first diesel had a maximum thermal efficiency of 26%. Today it is at 45%.
* “Why should we have SI (spark ignition) and CI (combustion ignition)?” asks Anna Stefanpoulou of the University of Michigan. “We should call it all TI, Thermal Ignition. Temperature is so important.”