Diesel engines are the workhorses of the heavy-duty vehicle segment. Oil burners have abundant low-end torque which, coupled with inherent higher efficiency than gas engines, make them ideally suited for heavy hauling applications. But what if a turbocharged, high-compression, alcohol-fed, direct-injection engine could offer more power and emit less while lowering overall vehicle costs. Sounds intriguing, doesn't it?

Up until now, the idea of ripping out an 11.0-liter diesel engine from a semi truck and replacing it with a 5.0-liter direct-injection ethanol-fed mill seemed dreamy. On top of this, there are concerns over engine knock and durability to keep in mind. A recent modeling study conducted by Leslie Bromberg and Daniel Cohn at MIT, though, suggests that the dream of replacing a diesel monster with a wee ethanol-burner is inching closer to reality.

In their paper, presented at the SAE 2010 Powertrains Fuels & Lubricants Meeting in late October, Bromberg and Cohn wrote:
Small, very high power density, spark ignition engines which are fueled with ethanol methanol or mixed alcohols can be used as a substitute of heavy duty diesel engines, with higher engine thermal efficiency and much reduced size and weight. In this manner a 3.6 liter engine could potentially be used to replace a diesel engine with a displacement as high as 11 liters. These extreme downsizing indicates the potential of a SI, knock-free engine, operating at the same peak pressure as the diesel engine and higher engine speeds. However this aggressive downsizing may not be practical because other constraints (durability, exhaust temperatures). More modest downsizing up to 5 liters could offer a practical solution.
Engine downsizing is thought to be constrained by low-speed preignition, a condition which can lead to engine failure, but Bromberg and Cohn suggest that:
With the use of alcohol-based fuels it is possible to reduce much further the tendency of knocking in spark ignited engines...By eliminating the knock constraint, much higher compression ratios can be used. Similarly, turbocharging allows for substantial engine downsizing.
Of course, it's worth noting that although the research was conducted on heavy-duty vehicles, both Broomberg and Cohn believe that:
The high efficiency downsized alcohol engine approach could also be used in light-duty vehicles.
We're intrigued.

[Source: Green Car Congress]


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