October 27, 2006
Ethanol Fuel Cooling Allows 30% Fuel Efficiency Increase
MIT researchers and some collaborators at Ford Motor Company have found a way to boost conventional gasoline engine fuel efficiency 20% to 30%.
MIT researchers are developing a half-sized gasoline engine that performs like its full-sized cousin but offers fuel efficiency approaching that of today's hybrid engine system--at a far lower cost. The key? Carefully controlled injection of ethanol, an increasingly common biofuel, directly into the engine's cylinders when there's a hill to be climbed or a car to be passed.
These small engines could be on the market within five years, and consumers should find them appealing: By spending about an extra $1,000 and adding a couple of gallons of ethanol every few months, they will have an engine that can go as much as 30 percent farther on a gallon of fuel than an ordinary engine. Moreover, the little engine provides high performance without the use of high-octane gasoline.
Given the short fuel-savings payback time--three to four years at present U.S. gasoline prices--the researchers believe that their "ethanol-boosted" turbo engine has real potential for widespread adoption. The impact on U.S. oil consumption could be substantial. For example, if all of today's cars had the new engine, current U.S. gasoline consumption of 140 billion gallons per year would drop by more than 30 billion gallons.
The $1000 per year cost beats the heck out of the thousands of dollars extra for a hybrid design. The fuel efficiency becomes comparable with diesel (better?) and without the need to switch to diesel and deal with diesel's emissions problems.
A car company that can beat others to market with this technology would gain a huge competitive advantage.
"There's a tremendous need to find low-cost, practical ways to make engines more efficient and clean and to find cost-effective ways to use more biofuels in place of oil," said Daniel R. Cohn, senior research scientist in the Laboratory for Energy and the Environment and the Plasma Science and Fusion Center (PSFC).
How does it work? The researchers inject ethanol to cool the fuel and thereby prevent premature firing (i.e. knock).
For decades, efforts to improve the efficiency of the conventional spark-ignition (SI) gasoline engine have been stymied by a barrier known as the "knock limit": Changes that would have made the engine far more efficient would have caused knock--spontaneous combustion that makes a metallic clanging noise and can damage the engine. Now, using sophisticated computer simulations, the MIT team has found a way to use ethanol to suppress spontaneous combustion and essentially remove the knock limit.
When the engine is working hard and knock is likely, a small amount of ethanol is directly injected into the hot combustion chamber, where it quickly vaporizes, cooling the fuel and air and making spontaneous combustion much less likely. According to a simulation developed by Bromberg, with ethanol injection the engine won't knock even when the pressure inside the cylinder is three times higher than that in a conventional SI engine. Engine tests by collaborators at Ford Motor Company produced results consistent with the model's predictions.
Elimination of knock enables 3 optimizations of engine design.
With knock essentially eliminated, the researchers could incorporate into their engine two operating techniques that help make today's diesel engines so efficient, but without causing the high emissions levels of diesels. First, the engine is highly turbocharged. In other words, the incoming air is compressed so that more air and fuel can fit inside the cylinder. The result: An engine of a given size can produce more power.
Second, the engine can be designed with a higher compression ratio (the ratio of the volume of the combustion chamber after compression to the volume before). The burning gases expand more in each cycle, getting more energy out of a given amount of fuel.
The combined changes could increase the power of a given-sized engine by more than a factor of two. But rather than seeking higher vehicle performance--the trend in recent decades--the researchers shrank their engine to half the size. Using well-established computer models, they determined that their small, turbocharged, high-compression-ratio engine will provide the same peak power as the full-scale SI version but will be 20 to 30 percent more fuel efficient.
The favorable economics would undermine demand for hybrids.
The ethanol-boosted engine could provide efficiency gains comparable to those of today's hybrid engine system for less extra investment--about $1,000 as opposed to $3,000 to $5,000. The engine should use less than five gallons of ethanol for every 100 gallons of gasoline, so drivers would need to fill their ethanol tank only every one to three months.
Hybrids still have an important advantage: They can capture energy lost in braking by use of regenerative braking. Hybrids also are an important step down the road toward pure electric cars. Hybrids increase the demand for better batteries and therefore are spurring a great deal of research and development to produce cheaper, longer lasting, lighter, and higher energy storage capacity batteries.
What about a hybrid with ethanol cooling? A smaller engine would reduce weight and space to allow for more batteries.
It seems that this technology, if applied to the entire feet, would have a similar effect to converting the fleet to E85 based on corn ethanol (if the EROEI of corn etoh is about 1.3). Of course, if a better process for ethanol production is used, then that changes.
Does that sound right?
I think that the point of this tech is to allow the gas engine to be smaller but still have available power for acceleration bursts etc... something that could be met by the electric motor in a hybrid. And I'm not sure how much a PHEV would benefit, as it would be designed to be on batter most of the time anyway.
The more efficient the engine the less advantageous the hybrid feature.
Suppose this ethanol injection engine increases fuel efficiency 30%. Suppose instead of spending $1000 on gasoline you then spend only $700. You save $300. Suppose that the hybrid feature would also save 30% and would do so regardless of whether ethanol injection was used. Well, once you already have an ethanol cooled superior engine the 30% savings on $700 is only $210.
But my expectation is that the combined percentage savings would be smaller than what you might get from each separately. A hybrid design can reduce the need for heavy load on an engine. The most advanced hybrid design would run the engine at a constant RPM (either off when batteries are well charged or a constant RPM) and use electric motors on each wheel. So I would not expect fuel cooling with ethanol to help in that case.
What if you added this ethanol booster to a diesel engine? I'd like to see that combined with a hydraulic booster capturing momentum from the brakes. Combined, these might make for a pretty darned fuel efficient SUV, even a big one.
I wonder if this anti-knock development makes it substantially more likely that, not just gas-battery hybrid, but battery cars will be leapfrogged to the mass market by ultracapacitor cars. If regenerative breaking is a stepping stone then why not just hybrids with ultracapacitors to more efficiently capture the energy?
This design for a gasoline engines captures much of the efficiency advantage of diesels. I do not think diesels can be enhanced in this fashion because they already are capturing the advantage that this method gives gasoline engines.
BTW, there's one other gasoline engine design improvement under development (forget the name for it) where in mid range RPMs the gasoline engine becomes a compression-fired engine just like diesels. This approach is supposed to give 80% of the fuel efficiency advantage that diesel engines provide but with gasoline as the fuel.
It would seem to me that while a constant RPM engine may not benefit much from enthanol cooling, it could benefit from higher compression ratios (just as long as its constant RPM setting is below the knock threshold). Is this true or am I missing something?
The more efficient the engine the less advantageous the hybrid feature.
I think you look at that backwards. People are buying hybrids now that have little or no hope of paying for the added cost of producing a hybrid. Mostly these buyers are white folks who have a desperate need to feel moral superiority over other white folks.
Given a hybrid, a smaller gasoline engine means less weight to push and/or more batteries to push with.
You're thinking of HCCI, Homogeneous Charge Compression Ignition. Unfortunately, it's got noise and harshness issues.
This isn't bad at all, considering the other possible uses of ethanol. Just blending it into standard fuel not only requires special vapor pressure and other properties, it makes fuel economy go down. E-85 is even worse, requiring as much as 50% more fuel volume for the same distance travelled. If ethanol is used in a separate tank for engines like this, it not only improves net economy but it also eliminates the extra refining burdens on the gasoline fraction. Last, if the ethanol doesn't have to be blended it does not have to be anhydrous and the energy required for distillation drops steeply.
The real problem is that 30% isn't anywhere near enough to cut usage as much as we need to. World oil production is following the plateau we'd expect from peak oil having just passed (despite a massive increase in drilling rigs in Saudi Arabia and prices 2.5 times the target of just a few years ago, KSA production is flat and they've announced a cut... hmmm). Cantarell (Mexico) has peaked and is crashing. Burgan (Kuwait) has peaked. If we replace vehicles at about 1/15 of the fleet per year (and new vehicles account for perhaps 10% of total mileage), a 30% cut in fuel per vehicle translates to 3%/year decrease in demand, assuming no growth in either vehicle count or miles driven.
Won't take much annual decline in production to get ahead of 3%/year cut in demand. The North Sea is seeing about 30% year-on-year, IIRC.
What we need are vehicles like plug-in hybrids which can cut fuel requirements by 80%, not 30%.
This idea sounds great to me. It would appear to require a relatively limited technology shift to be put into mass production, with a very significant benefit available on a much shorter time line than some of the more radical or complex ideas noted above.
Apart from the efficiency gains related to a high compression ratio, it avoids needlessly burning a large percentage of ethanol when it is not needed under lower load conditions, avoiding the high consumption issues related to simply running on an ethanol blend.
Combining this with a hybrid would appear to make little sense as the benefit dimishes if the motor runs at a constant high load, hence with a high ethanol consumption. The combined additional outlay would also be very negative.