As consumers in general aviation, we want companies to deliver innovative piston engine operating efficiency improvements, because aviation fuel is our single largest cost of ownership expense after aircraft acquisition. But alas, that does not appear to be a priority of many.
GA is a laggard in advancing piston engine operating efficiency. We’ve seen significant engine efficiency gains in the automotive industry, and we should expect no less from general aviation companies. Forty-year old legacy engine designs should not be the accepted norm.
We should not accept marketing brochures from aircraft manufacturers, and articles from aviation magazines that tout maximum airspeeds with little analysis comparing the aircraft’s engine operating efficiency.
Optimized for Efficient Lean of Peak Operation
You should consider the Cirrus SR22TN, by Tornado Alley Turbo, because it’s optimized for high performance and efficient lean of peak operation.
As shown in the table below, the Cirrus SR22TN has the best Nautical Miles Per Gallon (NMPG) operating efficiency:
Maximum Cruise Speed 85% Power
| Aircraft | NMPG | GPH | KIAS |
| Cirrus SR22TN | 12.2 | 17.6 | 214 |
| Cirrus SR22T | 11.7 | 18.3 | 214 |
| Cirrus SR22 | 10.3 | 17.8 | 184 |
| Cessna Corvalis TTx | 9.8 | 24.0 | 235 |
Note: Max cruise speed based on best performance altitude
In pilot discussions I find most owners typically operate in the 70% – 75% range for cruise power settings, and not maximum cruise speed – but the engine efficiency differences between these aircraft still remain. Greater operational efficiency, even 0.7 gallon per hour (i.e. SR22TN vs SR22T), can have a significant cost of ownership benefit over a 2,000 hour Time Before Overhaul (TBO).
When properly configured, turbocharged engines are more efficient than normally aspirated engines, (i.e. SR22). Design elements, like the GAMIjector System delivers performance and efficiency. The GAMIjector System is on all SR22TN Tornado Alley Turbos , but is not on normally aspirated SR22 series from the factory, although some owners may have aftermarket installations.
In addition to the best NMPG, the Cirrus SR22TN Tornado Alley Turbonormalized has an efficient Rich of Peak (ROP) climb fuel flow, and the design also allows for Lean of Peak (LOP) climbs with cylinder head temperatures at or below 380 degrees.
Maximum Climb Power Fuel Flow
| Aircraft | ROP GPH | LOP GPH |
| Cirrus SR22TN | 35 | 17.5 |
| Cirrus SR22T | 38 | n/a |
| Cessna Corvalis TTx | 38 | n/a |
| Cirrus SR22 | 27.1* | n/a |
*Note: Sea Level, 0 pressure altitude. SR22 requires multiple adjustments to mixture during climb.
Sources:
- WhyCirrus.com, Aircraft Comparison Marketing Brochure, credit source: Cirrus Aircraft 2013
- Tornado Alley Turbo, Inc. John Paul, Sales and Technical Support Manager
- POH manuals
Better performance efficiency in cruise and climb delivers recurring savings that reduce cost of ownership. Future weblog posts will review cost of ownership between aircraft in more detail based on fuel flow differences.
Cooler Cylinder Head Temperatures
The Tornado Alley Turbo, Inc (TAT) design is based on modern high compression ratio pistons (8.5:1), which are more thermodynamically efficient than lower compression pistons. The cylinders are matched with modern computer optimized dual-intercoolers, which are a key ingredient to a more fuel-efficient and cooler running engine.
Tornado Alley Turbo says, Avidyne equipped Cirrus SR22TN will normally run cylinder head temperatures 40-50 degrees cooler than competitors, and Cirrus Perspective SR22TN will typically run 20-30 degrees cooler than the competition.
In normal climb and cruise, cylinder head temperatures (CHT) do not exceed 380 degrees. Experience shows balanced CHT’s below 380 degrees are the norm.
Lower Turbo Inlet Temperatures
The turbochargers are lubricated via the engine oil system, and are capable of +100,000 RPM. The Tornado Alley Turbo design allows for lower Turbo Inlet Temperatures (TIT), which are typically between 1580 – 1630 F. Maximum TIT is 1750 degrees F.
The Tornado Alley Turbo, Inc installation includes dual-intercoolers, dual-turbochargers, dual-wastegates with absolute pressure control and a GAMI fuel injector system on the Teledyne Continental Motor IO-550-N which delivers 310 horsepower performance all the way up to 25,000 feet MSL.
The certification altitude up to 25,000 feet MSL is lower than the turbocharger’s critical altitude. Critical altitude is the height at which a turbocharger can no longer support sea level pressure, which is not an issue for the Cirrus SR22TN.
General Aviation companies need to aggressively pursue piston engine efficiency improvements, and pilots should be advocates with an organized voice for the innovations.
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