Little diesel, big fuel savings

Photography by Chris Rose

When Premier Aircraft of Fort Lauderdale, Florida, announced a Jet-A-burning turbocharged Continental 135-horsepower diesel engine for the Cessna 172, the response came as no surprise. “When we first advertised the airplane, I had people from 18 countries contact us,” said Art Spengler, vice president of operations. Overseas markets are the primary target for diesel-engine aircraft.

Often 100LL aviation gasoline is scarce overseas, or it costs two to three times as much as the $6.40 price per gallon recently found near AOPA headquarters in Maryland. Jet-A was $5.88 on that day. That 52-cent price differential isn’t enough to send American aircraft owners screaming to a diesel-engine showroom, unless they own a flight school. In Europe, however, 100LL costs double the average price in the United States—while in one African country, the price in 2014 was $22 a gallon. Diesel engines burn far less fuel per hour than gasoline engines, but cost more to purchase—meaning it takes lots of flying to realize any savings.

Now Spengler is hearing from individuals in the United States who want a diesel-powered airplane for reasons other than cost and availability of fuel. It’s a green airplane leaving no lead in the air, and it provides a jet-smooth, low-vibration ride. Especially important is the low pilot workload (just one lever). While it carries less fuel than an avgas Cessna 172R, it gets more range, thanks to its greater fuel economy.

Diesel engines tend to be heavier than gasoline engines, and Jet-A weighs more than 100LL. For those reasons Premier Aircraft limited the fuel load to 47 gallons. With full fuel the payload is 332 pounds. With 47 gallons, you can sit up in the sky for eight hours and still have an hour of fuel left when you land.

Equally simple

Demonstration pilot Corby Hallaran likes to do the “visor test” when showing Premier’s 1997 Cessna 172R powered by the Continental CD-135 diesel engine. Once level in cruise, he brings down the sun visors to demonstrate something you can’t see in most gasoline-engine Cessna 172s—the visors aren’t shaking. Premier made it a show plane and the price reflects that. Originally priced at $289,000, it is now $259,000 and future conversions can be configured at a still lower price. The demonstrator has new paint and interior and a $70,000 Garmin panel, including a G500 flight display system, a GTN 750 touch screen GPS, and Garmin navigation and communication radios. It has other Garmin goodies as well, like a GDL 88 transceiver for ADS-B weather and traffic information. (The Garmin avionics cost more than the engine and propeller conversion kit.)

Another advantage important to operators surrounded by homeowners is that diesel aircraft are quiet. That is due partly to the engine design, and partly to the fact that noise restrictions in Europe required a sound-deadening muffler that comes with the installation kit.

From engine start, it is obvious that pilot workload for this computer-controlled engine is much lower than for a conventional Cessna 172. To start, turn on the electric fuel pump, yell “Clear,” turn on the engine master switch, wait until computers have finished a glow-plug sequence, and push the Start button. Glow plugs warm the cylinders before starting the engine and the computers—a main computer and a backup—control that process, shutting off the glow-plug light on the instrument panel to signal that the engine can be started.

Computers take over from there, even adjusting the pitch of the blades on the MT propeller and determining the fuel/air mixture. The pilot’s job is to move one lever forward to speed up and backward to slow down. There’s no mixture control and no propeller control.

In flight, the task of monitoring the engine’s health, and that of other systems, is equally simple. There are two or three indicators to monitor, including two computer readouts from the full authority digital engine control (FADEC) system. If there are green lights in those locations, fly on. If the aircraft’s alternator fails, battery power is the only thing keeping the computers, and thus the engine, going. The pilot operating handbook warns that if the main battery and alternator were to fail “simultaneously,” the engine will operate for only 30 minutes on FADEC backup battery power. Company officials say you have more than two hours of main battery power if only the alternator fails. That time can be increased by turning off all electrical devices.

A diesel engine uses compression ignition to keep going. Air is compressed in the combustion chamber without fuel. As its temperature rises, fuel is added, mixes with the air, and combusts spontaneously. Power is controlled by the amount of fuel. To turn Jet-A into a mist, you need a fuel pump capable of nearly 20,000 pounds per square inch.

How fast?

So how fast does the Skyhawk fly with only 135 horsepower? Fast enough for a flight school but slower than an individual owner might like. Hallaran suggested demonstration settings of 75 percent power, which resulted in a 5.8-gallon-per-hour fuel flow; and 65 percent, which burned 4.8 gallons per hour. That consumption is similar to a gasoline Rotax engine found in Light Sport aircraft; with a 160- or 180-horsepower gasoline engine, fuel burns would be closer to eight and 10 gallons per hour, respectively. Those gasoline engines weigh from 260 to 295 pounds, depending on the model, while the CD-135 weighs 295 pounds.

At 75 percent power I saw 109 knots true airspeed, while at 65 percent the speed was 98 to 100 knots true airspeed. Spengler said that to go faster, it is permissible to fly at a higher power setting than the factory recommendation. He flew from Texas to Florida at 8,000 feet using a power setting of 85 percent, and saw 115 knots true airspeed while burning 6.2 gallons per hour. He said the engine is approved to fly at up to 92 percent power for fastest speed, while lower settings are recommended for economy cruise.

Another diesel engine in the approval process promises more speed. The FAA is processing paperwork to approve the CD-135’s identical twin, the 155-horsepower Continental CD-155, in Piper Archers, including the new DX; Warriors; and the Cessna 172 (see “Going Global,” December 2014 AOPA Pilot). The CD-135 gained 20 more horsepower primarily by tweaking the software of the CD-155’s FADEC computers.

The biggest advantage comes from fuel savings. At the engine manufacturer’s recommended cruise setting of 75-percent “load,” as it is referred to in the pilot operating handbook, the CD-135 is producing about 95 horsepower. During climb the engine generates 132 horsepower, and I saw a climb rate of 700 feet per minute with two people aboard and a reduced fuel load.

What you’ll need

To retrofit the engine to an existing 172 you’ll need a $65,000 kit that includes the MT propeller, digital panel displays, FADEC computers, and the engine itself. (The CD-155 engine costs an additional $5,000 to $8,000 depending on the value of the euro.) That number can vary depending on the value of the euro on the day you purchase, but it does include shipping. Labor is extra. After 1,500 hours TBR (time between replacement) you’ll need to replace, not overhaul, the engine. There are efforts to increase that time, and a flight school in New Zealand has run the same engine to 3,000 hours with oversight from local airworthiness authorities. Once 1,500 hours is reached, the replacement engine will cost $40,789. By comparison, a new Lycoming 180-horsepower gasoline engine commonly used in the Skyhawk is listed on the Lycoming website at $46,000, with a cost of $29,000 for a rebuilt engine and $26,000 for an engine overhaul. You can find a Cessna 172 F through N model yourself and provide it to Premier and Premier will install it for $85,000 (that includes $20,000 labor), a price that depends on the value of the euro on the day of your transaction. You can ask Premier to find an airplane and do the conversion for a total 0f $179,000, but that price can increase depending on avionics and other choices. The process takes up to eight weeks, including three weeks for actual installation at Premier and the rest for manufacture of a custome engine kit (including propeller and FADEC) and shipping.

Four inspections of the CD-135 are required during its 1,500-hour lifetime, including removal of the gearbox after 600 hours for inspection at the factory in Germany. Most operators opt instead to replace the gearbox, getting credit for the old one and paying $3,500 to $4,500 for the replacement, rather than grounding their aircraft for a month. A gearbox is necessary to reduce the speed of an automotive engine to propeller speed. A decade ago the engine was produced by Mercedes-Benz for small cars and a light truck, and shipped to Thielert, the company that developed today’s Continental diesel engine. Parts today include a few from Mercedes suppliers, but engines are no longer shipped from Mercedes for conversion.

Diesel engines create a mechanical jolt, one that would cause cracks in metal propellers. The solution is to use composite props, such as the MT propeller with a wooden core reinforced by layers of epoxy Fiberglas, Kevlar, or carbon fiber, and sealed by several coatings of acrylic-polyurethane paint. Inside the gearbox is a torsional damper that further reduces the pulsing of the diesel engine. A flywheel was a welcome replacement for a troublesome clutch—which must be replaced during the lifetime of the engine—found in an earlier iteration of this engine, the Thielert Centurion 1.7 (see “Thielert’s Story,” at left).

My in-flight impression is of an engine that inspires confidence. I conducted two flights, one for the usual student pilot maneuvers (stalls, steep turns, slow flight) and another in formation for photos with a Piper Cherokee Six carrying photographer Chris Rose. The formation flight involved frequent excursions of the power lever (especially in outside turns as I tried to keep up with the Cherokee Six), and the engine handled power changes smoothly. I had been warned ahead of time to make smooth throttle adjustments. While broad throttle excursions would cause dramatic surging of a turbocharger in a gasoline engine, it was barely noticeable with the turbocharged CD-135.

The most serious problem Hallaran has seen occurs when the aircraft is refueled. “Be there when the fuel truck shows up,” he advises. “You don’t want them putting avgas in there.” style="font-size:0.78em;">AOPA

Email [email protected]

Premier is not alone

Other companies can also swap gasoline-powered engines for diesel engines. The Miami-based Africair group of companies (with an office in Kenya) wrote AOPA to say, “We’ve been doing it for 10 years.”

The company installs the turbocharged 135-horsepower CD-135 (formerly the Centurion 2.0) and the 155-horsepower Continental CD-155 (Centurion 2.0 S) in aircraft destined for foreign customers. The company has done more than 60 retrofits, most of them on Cessna 172 aircraft.

RedHawk Aircraft, affiliated with Redbird Flight Simulations in Austin, Texas, does conversions using the CD-135 engine mostly for Skyhawks, since flight training is the focus of the company. There are four converted Skyhawks in its Redbird Skyport training fleet and three are under an experimental leasing agreement to schools in Texas, Florida, and New Jersey. The “seasonal” agreement may be offered to the general public. Redhawk offers the complete aircraft and engine for $249,000, or $209,000 if you bring the company an airframe. —AKM

Thielert’s story

The engine has had a somewhat tortured history. The Thielert 1.7, offered in 2002, had only 1,000 hours’ time between replacement and lost the confidence of customers like Diamond Aircraft, resulting in today’s redesign.

Today’s two-liter engine was designed in 2005, but the cost of development and production sent the company into bankruptcy. Centurion was created as an agent between the bankrupt Thielert and the customer, and later was bought by the same Chinese government entity that also owns Continental Motors—China’s AVIC International Holding Company.

All assets of the Thielert company, a.k.a. Centurion (the CD-135 engine was the Centurion 2.0 back then), were placed under Technify Motors as part of the Chinese acquisition. The pilot operating handbook’s engine supplement that is used by Premiere still uses the name Thielert on its pages.

Before Continental Motors took over the diesel engine line in 2013, there had been 44 engine failures in Europe involving Thielert engines between 2003 and 2011—27 of them were the first-generation Thielert 1.7 engine, and seven of those were related to the clutch design.

Other engine problems were found to be caused by operator or repair error. Causes ranged from electrical issues to fuel pumps. All were investigated by BEA, the French civil aviation safety investigation authority. The problem engines had been installed on Diamond DA40 and DA42 aircraft, the Cessna 172, the Piper PA–28, and the Robin DR400; most were based at three French flight schools that aided in the study. “The statistics…indicate that the rate of in-flight shutdowns for Thielert engines is no greater than that of other engines in general aviation,” the report stated.

In fairness it should be noted that more than 4,000 Thielert Centurion engines have been produced. A company spokesman said the engines have a total of more than four million operating hours. —AKM

SPEC SHEET

Cessna 172R Diesel

Price as tested: $259,000

Specifications

Powerplant | Continental CD-135 turbo diesel, 135 hp

Recommended time between replacement, | 1,500 hr

Propeller | 3-blade constant speed 73.6-in dia MT Propeller, composite

Length | 26 ft 11 in

Height | 8 ft 11 in

Wingspan | 36 ft 1 in

Wing area | 174 sq ft

Wing loading | 14.1 lb/sq ft

Power loading | 18.1 lb/hp

Seats | 4

Cabin width | 3 ft 4 in

Cabin height  4 ft

Empty weight | 1,819 lb

Useful load | 631 lb

Payload w/full fuel | 332 lb

Max takeoff weight | 2,450 lb

Max landing weight | 2,450 lb

Fuel capacity | 47.6 gal (44.6 gal usable) 319 lb (299 lb usable)

Performance

Takeoff distance, ground roll | 919 ft

Takeoff distance over 50-ft obstacle | 1,775 ft

Max demonstrated crosswind component | 15 kt

Rate of climb, sea level | 595 fpm

Cruise speed/endurance w/45-min rsv, std fuel (fuel consumption) 6,000 feet  @ 75% power | 104 kt/7.3 hr   (37 pph/5.5 gph)  @ 65% power | 97 kt/8.5 hr   (32 pph/4.8 gph)

Max operating altitude | 17,500 ft

Landing distance over 50-ft obstacle | 1,335 ft

Landing distance, ground roll | 575 ft

Limiting and recommended airspeeds

V_X (best angle of climb) | 60 KIAS

V_Y (best rate of climb) | 79 KIAS

V_A (design maneuvering) | 99 KIAS

V_FE (max flap extended) | 110 KIAS

V_NO (max structural cruising) | 129 KIAS

V_NE (never exceed) | 163 KIAS

V_R (rotation) | 51 KIAS

V_S1 (stall, clean) | 44 KIAS

V_SO (stall, in landing configuration) | 33 KIAS

For more information contact Premier Aircraft, 5544 NW 23rd Avenue, Fort Lauderdale, Florida 33309; phone 800-903-8402 or 954-771-0411; www.flypas.com.

All performance figures are based on standard day, standard atmosphere, sea level, gross weight conditions unless otherwise noted.

Extra: Payload (with full fuel) is 100 to 120 pounds less than a gas-powered Cessna 172R but it burns three to four fewer gallons per hour.