|The diesel motor of the Cessna 182NXT. (photo by Phil Weston)
By J. Mac McClellan, EAA Director of Publications
Diesel engines were big news here at AirVenture Oshkosh. The major engine companies, particularly Continental, are going full bore to certify a range of diesels that will be compatible with existing airframes. And there are several companies still new to aircraft propulsion that are working toward aircraft diesel certification.
Cessna even announced that it was repowering the popular 182 Skylane with a diesel. That is a level of commitment to Jet A fueled engines not seen before from a major piston aircraft manufacturer.
Gasoline versus kerosene
Diesels - often called compression ignition engines - have some powerful advantages.
The most compelling reason for diesel aircraft power in most parts of the world is scarcity of avgas. And when 100LL avgas is available in many countries the costs soar way beyond $10 per gallon.
A diesel can generally also deliver better specific fuel consumption (SFC), which is a measure of power developed for each pound of fuel consumed. It's important not to compare fuel flow of a gasoline engine to a diesel in terms of gallons per hour because Jet A is denser and thus weighs more per gallon.
Even when that correction is made, though, the diesel looks good in terms of efficiency.
But there are serious challenges to overcome to make a diesel practical in airplanes. That's why diesels, even though the technology has been around for more than a century, have rarely been used in aircraft.
The greatest disadvantage of a diesel compared to a spark-ignition avgas engine has been power-to-weight ratio.
The avgas engine is lighter.
The reason is that to work diesels operate at much higher internal pressures and thus the structure of the engine must be stronger, and heavier.
As the name implies, a diesel relies on the heat of compression to ignite the fuel-air charge in the cylinder. That means the compression ratio of the diesel can be as much as double the avgas engine.
That puts a lot of stress on the entire engine, particularly the cylinder heads, pistons, connecting rods, and crankshaft.
Much of the diesel's efficiency comes from the high operating pressure. But the high pressure generates more heat, so cooling a diesel is more demanding than an avgas engine.
It's possible to make an air-cooled diesel, but liquid cooling of at least the hottest parts of the engine is more practical, and liquid cooling is heavier than air cooling.
Another significant issue with a diesel is its power pulse delivered to the propeller.
The high pressure and high energy of each firing cycle in a diesel generates enormous forces on the piston, rod, and crankshaft, and those forces are transmitted directly to the prop.
Forces on a propeller are both extreme and complicated. There are the obvious forces created by moving air to generate thrust. But there are also huge centrifugal loads caused by rotation. And then, when the airplane changes attitude, the air flowing into the propeller disk changes angle and exerts even more force on the propeller hub and blades.
When you add the pounding of the diesel power stroke to the other loads on the propeller you need a lot of extra strength - and weight.
Motor mounts for a diesel also must be more robust and able to absorb the high amplitude and low frequency vibrations of the power pulse with each piston stroke. The motor mount must absorb a high percentage of that vibration or it is transmitted to both the airframe and the propeller.
Mind over motor
None of these issues with a diesel is unsolvable.
New metal alloys and casting designs can bring the necessary strength without too large of a weight penalty.
A gearbox can be designed to absorb much of the power pulse before it reaches the propeller.
Modern fuel-injection technology can help make a diesel smoother and more efficient.
And advances in elastomeric materials make motor mounts much more effective in absorbing vibration.
What had been missing over the past decades is a very solid incentive to invest in the technology to make aircraft diesel engine with performance comparable to an avgas engine. The shortage and cost of 100LL is that incentive in most of the world, and is looming over the horizon here in the United States.
There have been more than a few diesels attempting to enter the general aviation propulsion market over the years, but so far none has had really long term success. I believe that is changing, and we saw evidence of that change here this week at AirVenture Oshkosh.