Highly turbocharged alcohol-fueled direct-injection spark-ignition (DISI) engines operated at a high compression ratio could be as or more efficient than diesel engines while also providing advantages of lower vehicle cost, lower emissions and higher power, according to a recent modeling study by Leslie Bromberg and Daniel Cohn at MIT.
In their paper, presented at the SAE 2010 Powertrains Fuels & Lubricants Meeting last week, the two investigated the potential of ethanol and methanol in heavy duty (HD), long-haul applications, examining the issues of infrastructure, engine performance, and briefly, emissions and cost implications.
Alcohol-based transportation fuels from a wide range of biomass feedstocks including agricultural, forestry, municipal and industrial waste and specially grown crops and trees can potentially offer a substantial substitute for oil derived fuel. Alcohol fuels can also be produced from natural gas or coal. Use of ethanol, methanol or alcohol mixtures could be particularly attractive for heavy duty vehicles because of the need for cleaner engines than present diesel engines and a less demanding infrastructure requirement than that for light duty vehicles.
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.—Bromberg and Cohn
Diesel engines have been substantially more efficient than gasoline (spark ignition) engines, due in part to the problem of knock in the SI engines, which limits the compression ratio and the amount of pressure boosting.
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.
The high knock-free pressure resulting from the use of direct injection in combination with stoichiometric operation with no EGR at high torque and use of high rpm operation makes possible an engine power density and a torque which are each up to 3 times those of a diesel engine having the same piston displacement. A representative number for the torque/liter capability of direct injection alcohol is 200 lb-ft/liter and that for potential engine power density is 200 hp/liter.
In their study, they found that a direct injection alcohol engine downsized by around a factor of two relative to a diesel engine could have an efficiency advantage over the diesel of about 4% for prolonged high torque operation in long haul trucks. Reforming about half the methanol or ethanol would result in capture of about an additional 5% from the exhaust, as a bottoming cycle. The lean operation could result in an additional 5% improvement in efficiency, they said. There could also be a small increase in efficiency with up-speeding and further downsizing.
They notes that their calculations did include the fuel penalty expense needed for the diesel engine aftertreatment, which when included would further increase the comparative advantage of the spark ignited engine by a few percentage points. Their calculations suggest that for long haul trucks, a 5-liter direct injection ethanol engine could be used to replace a diesel engine of 11-liter displacement and a 7-liter direct injection ethanol engine could replace a diesel engine of 15-liters displacement.
The high efficiency downsized alcohol engine approach described in this document could also be used in light duty vehicles.—Bromberg and Cohn
Fuel consumption and range. Bromberg and Cohn note that M85 (around 85% methanol, 15% gasoline by volume) has roughly 60% of the range of diesel fuel for a given fuel tank size. However, the weight reduction from the highly downsized engines could essentially compensate for the increased alcohol tank and fuel weight needed to obtain the same range as a diesel vehicle, they suggested.
They have not yet evaluated the potential use of higher alcohols, which are more energy dense than methanol or ethanol.
Sandia researchers, for example, report (also at the SAE PFL meeting) that isopentanol—a five-carbon, long-chain alcohol—has superior physiochemical properties compared to ethanol and very similar HCCI combustion properties to gasoline. (Earlier post.)
Leslie Bromberg and Daniel Cohn (2010) Alcohol Fueled Heavy Duty Vehicles Using Clean, High Efficiency Engines. (SAE 2010-01-2199)