Even when cellulosic ethanol is being produced on a mass scale, it’s unlikely ever to meet all our liquid fuel needs. So, can we leverage its impact?
Three MIT specialists on engines believe we can — and that one result could be a half-sized gasoline engine which performs as well as its big brothers and is significantly more efficient.
Daniel Cohn and Leslie Bromberg are research scientists with MIT’s Plasma Science and Fusion Center, which besides conducting sophisticated experiments in nuclear fusion energy has a program in advanced engine technologies. John Heywood is a mechanical engineering professor and head of MIT’s Sloan Automotive Lab.
One of the trio’s concerns is engine knock. Knock happens when the temperature in a car’s cylinders rises to the point that some fuel burns before the flame consumes it. That in turn limits engine designs. “It’s the knock barrier,” notes Heywood, “which prevents most gasoline engines from having compression ratios higher than 10 or 11 to 1.”
High-compression engines yield more power for their size than ordinary engines, which is why they’re found in racecars. But cooling the in-cylinder gases to the point that you could boost compression would also yield another big benefit: letting carmakers turbocharge virtually any gasoline-powered vehicle.
Turbocharging is the auto industry’s version of over-packing a stiffsided suitcase. By forcing added air into the cylinder during the intake process, it lets engines burn more fuel, thus boosting power.
But since turbocharging also boosts temperatures, it too can lead to knock. “That’s why relatively few gasoline engines are turbocharged,” says Heywood.
How does ethanol help? The researchers theorized they could use it as a kind of wet blanket within the cylinder. Heywood explains:
The ethanol’s injected into the cylinder before the spark. On entering the cylinder’s hot interior, it turns into a vapor. (“To vaporize any liquid, you have to supply heat,” notes Heywood. “Ethanol’s attractive because the vaporization energy it requires is large.”) The ethanol’s cooling effect opens the door for both a high compression ratio and turbocharging.
The amount of ethanol can be modest. Cars in daily use aren’t in constant need of the power that high compression and turbocharging provide. “It’s only when you’re doing things like trying to get away quickly from a red light,” notes Heywood.
Engine designers often build in that kind of performance by adding cylinders. The changes enabled by ethanol, though, may offer a better option. Higher compression plus turbocharging would boost engine efficiencies about 10 percent. “But now you can get by with a small engine,” says Heywood, “which means you can increase your car’s efficiency by another 10 to 15 percent, so you end up with a 20-25 percent benefit overall.”
In practical terms, he adds, “that means you could have a small fourcylinder that acts like a six-cylinder or even an eight-cylinder.”
That stunning advance can’t occur until several hurdles are cleared, including designing cars with an ethanol storage-and-delivery system. For now, though, the work’s on track. The researchers, with Ford engineers, are testing an engine that uses this approach. “The results have been very encouraging,” says Heywood. “They’ve confirmed that the concept works.”