June 19, 2012 § 1 Comment
Other than the imagery of British Sporting Green (a fine color but currently not in vogue even on a diminutive Austin Healey), to most people this title evokes electric cars, fuel efficiency and hybrids. The June 18, 2012 issue of the Wall Street Journal had two pieces that prompted this post. One was on innovation in the internal combustion engine and the other on the promise of natural gas fueled cars and the hurdles therein. These were unconnected, but easily could have been, so we will take this opportunity to do so.
The article America, Start Your Natural-Gas Engines deals primarily with the potential cost advantage of displacing gasoline with cheap natural gas, with the associated gains on tailpipe emissions. Such a car would certainly be considered greener than one running on gasoline. But they correctly point out that compressed natural gas (CNG) tank will have four times the volume of a gasoline tank and weigh more due to the thick walls necessitated by the need for pressures in excess of 3000 psi.
Shifting gears now to the other article New Twists on an Old Idea which describes several new variants of the internal combustion engine designed to make them more efficient. The efficiency improvements claimed range from 15% to 30%, and some have reductions in weight. But all are radical departures from orthodoxy, a tough pill to swallow for an industry in which capital costs are very high.
Assuming that much of the driver for engine innovation is to reduce gasoline consumption and associated emissions, this could be achieved by using CNG as the fuel, the precise subject of the other article. The CNG could partially or completely replace the gasoline, or diesel for that matter, and the emissions would also be reduced. The article discusses two initiatives to improve the volumetric efficiency. Not mentioned are extremely promising ongoing research in Texas A&M University, Northwestern University and the Lawrence Berkeley Laboratories, to name just three. These three are pursuing the class of compounds known as Metal-Organic Frameworks or MOF’s, which adsorb large quantities of methane and store at conventional pressures. These are still at the laboratory scale and build upon research in inexpensive hydrogen storage. The target would be to double the volumetric efficiency of CNG in a low pressure tank. Another promising avenue being pursued is the use of a graphene variant for the same purpose. Some of these are poised for scale up so we are not talking blue sky research.
Engine Innovation: Engine innovation ought to move out of the old paradigm of improving the gasoline fueled engine. Gasoline has an inherent disadvantage with respect to cleaner alternatives such as methane, methanol and ethanol. That disadvantage is octane rating. All the three cleaner fuels have octane ratings in excess of 110 compared to 87 for regular gasoline. Higher octane rating allows the fuel to be more compressed prior to being fired. This produces more energy per unit of fuel combusted. That is the definition of efficiency. So, a simple target of more efficient engines is an engine with a high compression ratio rather than complicated radical departures such as in the Journal piece. Of course, those inventors were forced in that direction because of the aforementioned infirmity of gasoline.
So, how outlandish is the idea of ultra high compression engines (ratios in excess of 15 compared to about 8.5 for a conventional V8)? Indy race cars have compression ratios near 17 and run on methanol or ethanol. Diesel engines have compression ratios in the vicinity of 16. The investigation ought to encompass both pure natural gas use and mixtures with gasoline or diesel. In the case of the latter, the host engine is already a high compression engine, albeit not spark ignited. A high ratio of natural gas may require spark ignition. But all of this can be worked out. The key point is that innovation in engines more fruitfully ought to target utilizing the high octane rating of natural gas rather than improving the performance of that octane challenged and polluting fuel, gasoline.
If the Wall Street Journal could indulge in a do-over, the companion article to the one on natural gas vehicles ought to be one on engine innovations enabling the new, clean, domestically produced, fuel.