When an aviation leviathan like Cessna makes a move, everyone else in the industry stands up and takes notice. Whether it be their recent entries into the realms of VLJ’s and LSA’s, their early commitment to all-glass panels, their surprise revelation of a proof-of-concept “Next Generation Piston”, or their curveball take-over offer of Columbia Aircraft, nothing they do goes unnoticed. Indeed, whatever they do lends instant credibility to anything that might otherwise have been considered a sideshow. Their latest announcement that a Diesel- engined Skyhawk can now be purchased off their showroom floor, has given such power plants a definite justification that may finally concretize their viability in GA. So, what’s the hoopla about Diesels?
The internal operating and design principles, and most moving parts, of diesels are virtually the same as those of their gasoline counterparts, but there are some significant differences in their workings. The air/fuel mixture used in gasoline engines requires a separate source to produce an electric current to ignite the spark plugs located in the combustion chambers of its cylinders. The air/fuel mixture in diesels is ignited by heat resulting from the piston’s compression of air into the top of the cylinder. Diesels, therefore, require no separate electric ignition source. They have no spark plugs nor carburetor either. Mechanical problems associated with these latter systems – battery, coil, distributor, magneto too – are, therefore, non-existent.
A gasoline engine will usually not have a compression ratio higher than 10 or 12.5:1. By contrast, a diesel’s compression ratio is generally in the 15 or 20:1 range, with operating temperatures upwards of 900EC. Air, at such hot temperatures, causes ignition of the fuel which is sprayed into the combustion chamber in metered quantities by a high pressure injector pump. Gasoline engines can’t operate with such high temperatures at the end of the compression stroke without the fuel pre-igniting in the cycle.
Diesel fuel is a semi-lubricant whereas gasoline tends to wash away lubricating oil inside the engine. As a result, diesels suffer less internal wear which equates to longer life between overhauls. Diesel TBO’s are generally well over the 2,000 hour mark that is so common with gasoline powerplants. Their time between inspections is longer too, meaning less downtime for the aircraft. Furthermore, their fewer moving parts means there’s less to check during scheduled maintenance, speeding up the diesel aircraft’s return to service.
Diesel engines are more efficient than gasoline engines of the same power, resulting in lower fuel consumption. Diesel fuel is denser too, and contains about 15% more energy by volume.
Thus, for a given weight of fuel in the tanks, a diesel goes further than its gasoline- engined brethren. Also, in today’s environmentally-conscious world, the increased fuel economy of diesel over gasoline means that the former produces less carbon dioxide per unit distance. Enhancements in design efficiency have eliminated emission issues that, in earlier days, gave diesels their bad reputation. Black, sooty diesel start ups, the result of unburned carbon compounds, are a thing of the past. Diesel fuel does not contain any lead (as 100 LL does), thus adding to its environmental friendliness.
Diesel fuel takes less refining and is, thus, less costly to manufacture than gasoline. That’s why it’s cheaper to buy at the pumps. With threats that 100LL will eventually disappear – and given that diesels run just fine on globally-available, and less expensive, Jet-A – operational cost factors associated to diesel engine use are hard to ignore. But diesels don’t provide all of an upside: they cost more to build, they’re heavier, they require greater vibration dampening, and they’re harder to start in cold weather.
Cessna – and Diamond, which committed to the diesel configuration before it – feel that the few “cons” won’t keep it out of the hands of future owners. The fuel is cheaper, it’s more readily available worldwide, it’s more economical, and it’s friendlier to the environment than once ever imagined. If Cessna’s commitment is anything to go by, expect a pump dispensing Jet-A to be installed at your local club soon, and a line-up of aircraft awaiting their turn to fill up.