Friday, April 8, 2011

Diesel Connie

In my previous post I averred that even if a piston engined airliner were technically feasible that aircraft manufacturers would in no way be tempted to devote the billions in development costs required. I stand by the statement but for the purposes of discussion let's examine whether or not in actual reality such a beastie could in fact be built, and be a useful commercial venture.

First of let's not delude ourselves that any conceivable putative project could ever come close to duplicating the performance and passenger carrying capacity of a jumbo jet. Not going to happen. The engines on an Airbus 380 for example develop around an order of magnitude more power than the engines on the Lockheed Constellation and the likelihood of any internal combustion engine could be built to develop 30,000 horsepower and be remotely light enough to be used in aircraft is negligible, to put it as mildly as possible.

So let's take the ol' Connie as a baseline. There are regional airliners which can carry a hundred passengers or so in similar fashion to a fully loaded Constellation so the basic airframe engineering is already in hand. These craft by no look anything like a Connie due largely to advances in aerodynamics, which is a shame considering the sensuous beauty that was the Constellation, but it could not be otherwise. Let's propose parameters that would result in these hundred passengers be carried along at about 400mph airspeed at roughly 35,000 feet of altitude. This would give our putative project enough of an advantage over the Connie to pique the interest of airlines especially if significant fuel savings can be offered as well. I estimate that this performance would require four engines that develop at least 3000 horsepower each. This is less power than developed by the Connie's original 18 cylinder Wrights but I presume that a modern design with composite components would weigh several tons less.

The engines would have to weigh less as well which would be easy if gasoline were to be used as fuel. Unfortunately this approach would hardly achieve the fuel cost savings that would make this project attractive in the first place. That essentially leaves us with Diesel engines and not only that but Diesel engines that could run on the same fuel that jet turbines do--JP-4. Current Diesels have a hard time using jet fuel but it stretches not the imagination that units could be built from scratch that could use it to its fullest fuel saving potential. I don't view this approach as optional considering the costs of establishing a different fuel delivery infrastructure for an airliner running on conventional Diesel fuel.

So Diesels are it but they would not by a very long shot have any relationship to the growling smoky beasts of heavy over the road trucks. Automotive Diesel powerplants have made amazing strides in every facet of performance to the point where they are now the preferred engine in prototype LeMans competition cars from the likes of Audi and Peugeot. Powerful quiet and thrifty they have become as the technologies of electronic engine management systems and turbo/supercharging have advanced hugely in the last two decades.

Diesels do have another big advantage, similar to those huge Wright radials, in that they can easily be designed to run in RPM ranges that match up to the required rotational speeds of propellors thus avoiding heavy and power wasting gearing systems that plague the majority of automotive engines converted to aircraft use. The trick in designing such an engine would not be in arriving at either the desired running characteristics or lowered fuel consumption but rather designing a Diesel that would be both light enough and stout enough to function in its intended role. This will require as much in the way of lightweight alloys and materials as can be marshalled but since in no wise will such an engine cost any more than a jet turbine to manufacture this approach would be valid.

The lightest most efficient configuration of this engine will likely be a V12 of 2000-2500 cubic inches of displacement with at least two large exhaust driven turbochargers. This would require a very large and expensive aluminum or magnesium engine block casting but in the low volumes contemplated this cost should not be a show stopper. There are other configurations that might be considered but a V12 has enough enough advantages that it would inevitably shake out of most anyone's risk analysis calculations. A V12 is a very smooth running engine, obviously important in an airliner, and is likely the most efficient way to get the lightest powerplant needed to do the job without risking too many technological Hail Marys. Engines of this size and power go back at least as far as WWII and even though few large aero diesels have been built current tech seems up to the task.

A V12 configuration, water cooled of course, has another advantage over the big old Wrights on the Connie and that's aerodynamic drag. Those huge radials were highly refined for their day but they spent a lot of energy just pushing their bulks through the air. Aero drag from a properly cowled water-cooled V12 would likely be much less than half of what the original Connie's engines could manage. This lowered drag of course translates directly into performance and efficiency gains.

There's little reason that the full armamentarium of the modern engine designer could not be thrown at this project. High pressure direct fuel injection, multi-valve intake configurations, low friction internals, high-strength light weight components, advanced electronic engine management systems etc. etc. could easily combine, I think, to halve fuel consumption per seat mile compared to current turbine engines--even current turbo-prop versions. So all in all it seems that it could be done but it remains vanishingly unlikely that it will be done. The huge research costs involved, and the inevitable public acceptance issues, are virtually certain to stop this sort of enterprise in its tracks. Bit a shame perhaps when one contemplates what a 48 cylinder Diesel powered airliner might sound like on takeoff.

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