When engineers convert gasoline cars to run on natural gas as well, they typically install a large pressurized tank and figure that the driver will go 100 miles or more, use up the gas and then switch back to gasoline. But for cars in the United States, the idea hasn’t caught on.
Now Eric Noble of the CarLab, a California consulting company, is trying a different approach. At a recent meeting in Washington, the When engineers convert gasoline cars to run on natural gas as well, they typically install a large pressurized tank and figure that the driver will go 100 miles or more, use up the gas and then switch back to gasoline. But for cars in the United States, the idea hasn’t caught on he showed off several vehicles with relatively tiny natural gas tanks – not much bigger than the propane tank in a backyard gas grill – with an all-gas range of 55 miles or so. Unlike most bi-fuel vehicles, there is no switch to change back and forth between natural gas and gasoline. And at any given moment, the car could be running on a mixture of the two.
The biggest shift was rethinking the size of the natural gas tank. Natural gas has never worked out for private vehicles in the United States because there are few public refueling stations and the cost of conversion is high. But Mr. Noble said his new approach could make it practical by drastically cutting the cost. Mr. Noble’s idea was that rather than installing as big a natural gas tank as possible, hogging most of a vehicle’s cargo capacity and forcing a sizable investment in the tank itself, he installed a tank just about big enough for a day’s driving. As a result, his 2012 Hyundai Sonata Limited, which has a natural gas range of just 56 miles, was converted for $2,600, he said. A 2012 Mustang GT, in a shade of red that would make a fire engine blush, has a 55-mile natural gas range and cost $2,900 to convert.
The problem with a big tank, he said, is that “you couldn’t get the payback to work.” But a large tank, he said, is not necessary, because 95 percent of private cars are used to travel 45 to 55 miles per day.
“If you drop the size of the tank, you lower the cost,” Mr. Noble said.
His vehicles were meant to be paired with a home refilling unit, which the driver would use every night. Sixty percent of households already have natural gas connections, he said. A compressor that could take all night to fill a car’s tank could be relatively inexpensive, and deployment of his cars will rely on invention of a cheap, low-capacity compressor, he said. Eaton and other companies are working on such a device, aiming for a price of $500.
In fact, development of a cheap natural gas compressor has been identified by the federal government as a crucial ingredient of low-cost, clean, secure energy for transportation. Through the Advanced Research Projects Agency – Energy, known as Arpa-e, the government is giving $3.4 million to Eaton to design a radically different compressor. Engineers hope the compressor will be more durable and efficient, and cheaper to produce than those now available. In fact, at $500 the price would be one-tenth the price of conventional systems.
Part of the trick is figuring out how to remove the heat created by compression. Eaton’s innovation is to use a liquid piston – that is, a slug of liquid instead of a piece of steel. The liquid forms a good seal on the cylinder wall, and it also sucks heat out of the gas being compressed, which is important when compressing a gas at a ratio of 250:1. Cooler gas is easier to compress in the cylinder.
This is not Eaton’s first venture into a hydraulic approach. It also used a liquid piston to store energy lost in truck braking, keeping the energy as pressure and turning it back into torque when the truck accelerated. This created a hybrid that was hydraulic instead of electric. The idea caught the attention of Arpa-e.
Congress established Arpa-e as the energy version of the Defense Advanced Research Projects Agency, which gives money to high-risk, high-reward ideas that the private sector will not finance. In all, Arpa-e is sponsoring 13 natural gas storage experiments, many of which could help make natural gas vehicles more successful.
There is another twist, though. The CarLab vehicle runs on natural gas during periods when demand on the engine is low, such as during highway cruising. When the accelerator pedal is floored, the system automatically switches to gasoline, which provides more power. In intermediate conditions it burns a mixture of gasoline and natural gas. In test drives, the CarLab’s vehicles consumed 87 percent natural gas and 13 percent gasoline, and that was in very aggressive driving, Mr. Noble said.
The reason that gasoline is sometimes needed is that natural gas provides less power. That problem could be avoided by burning natural gas at a much higher compression ratio — it has an octane rating of 130 and gives more power if burned at higher pressure and temperature — but the engine remains optimized for gasoline, not natural gas.
Theoretically, the CarLab’s gas-gasoline hybrids are like the Toyota Prius or Chevrolet Volt, which use multiple energy sources to increase efficiency. But the CarLab vehicle’s small tank, which is tucked under the floor of the trunk, is a lot cheaper than the batteries the Prius and Volt carry.