Thursday, December 27, 2007

Turbocharged Ethanol

While some are busy at work creating the next generation of ethanol produced from cellulosics, MIT's best and brightest are continuing to work hard for the development of the next generation engine. In the process of developing industry and technology that will benefit the environment and our energy security it may be easy to lose track of how the system needs to come together. What I mean by this is that whether or not ethanol's lower energy density results in decreased MPG than gasoline, doesn't necessarily mean that we need to live with this problem. In fact, the engine may be optimized for gasoline usage without taking into consideration the advantages that ethanol might bring.

This case is illustrated in J. B. Heywood's work titled "Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection," and can be found at The basis of the study attempts to answer the problem of knock suppression in turbocharged engines using ethanol. But first, let's back up for a second. Turbocharging in an internal combustion engine is the use of the exhaust gas from the engine to drive a wheel that compresses the air to deliver to the engine. This allows for more air to enter the engine than a naturally aspirated engine and improves on the energy-to-size ratio of the engine. In other words, using the turbocharged engine allows for the use of much smaller engines with the same or more energy and torque output of the engine. The one problem with this is that a turbocharged engine is more susceptible to engine knock. Knocking occurs when the fuel/air mixture is ignited correctly in the piston but then a second pocket of fuel ignites as well. These two countering fronts of energy create destructive interference for each other, which can range from mistiming the stroke of the engine causing a loss of power all the way to possibly destroying the engine.

Ethanol has two key components that make it a prime candidate to combat engine knock in turbocharged engines -- 1) its high octane rating, and 2) ethanol's capability for evaporative cooling of the system after ignition.

Ethanol's octane rating (115 versus 87 for normal gasoline), prevents engine knock because, by definition, ethanol's fuel has the right kind of molecules that will ignite correctly and uniformly under pressure. A second requirement in turbocharged engines is to quickly decrease the in-cylinder (charge) temperatures so as to prevent the detonation of unexploded fuel/gas mixtures. This is accomplished by ethanol's ability to quickly cool the mixture to 355K versus 383K (Heywood et. al 2006).

According the Heywood, he concludes that a turbocharged engine would allow for the size of a modern fuel-injected engine to be decreased by half. At the same time, the manifold pressure and compression ratios could be increased allowing for comparative engine performance compared to the larger gasoline fueled engines. This is all possible because of the knock suppression of an ethanol blend. Heywood calculates that with the engine downsizing and performance enhancement of the turbocharged engine, efficiency of the automobile could actually increased by 30%! This would, in effect, completely offset the calculated energy density penalty of ethanol on the MPGs of a car, (which is approximately 27%).

As a final note, I found an update on this technology today that these researchers are working closely with Ford in the production of this engine and are progressing quickly. Hopefully we will see these engines in the near future.

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