Airframe and Powerplant

Less fuel, same power — it must be a diesel

Replacing the avgas-fueled 230-horsepower engine with a Société de Motorisations Aéronautiques (SMA) Jet A-fueled engine of the same power will save 30 percent in operating costs. That's the claim that SMA makes to Cessna 182 owners. Based on a short flight taken in early December 2006, this claim seems reasonable. Not only will operating costs drop with the installation of an SMA SR305-230, but also powerplant management chores will become jetlike since there's only one power lever sprouting out of the newly installed power quadrant. Easier powerplant management, a longer time between overhauls (TBO), and less money spent on fuel amount to the kind of engine progress airplane owners have been anticipating for a long time.

STC at Oshkosh

At EAA AirVenture Oshkosh 2006 the FAA presented SMA with a supplemental type certificate (STC) covering the installation for its SR305-230 diesel aircraft engine in certain Cessna 182 models. The four-cylinder compression-ignition 230-horsepower Jet A-fueled engine is now being installed in airframes here in the United States, and based on a recent report from the installer, buyers are very satisfied with the engine.

SMA's diesel engine has now been installed in approximately 55 airplanes, and preliminary reports seem to indicate that the engine is living up to its billing as an easy-to-fly and easy-to-maintain engine that cuts direct operating costs by around 30 percent over comparable avgas-fueled spark-ignition engines.

The engine is a clean-sheet design. At first glance it looks pretty much like an avgas-powered engine. The four cylinders are configured in the same flat, or boxer-style, layout as aircooled engines, and the propeller is driven directly off the crankshaft without any gear-reduction drive assembly.

Displacement is 305 cubic inches spread between four cylinders, which translates to 76.25 cubic inches of displacement for each cylinder. The avgas-fueled 471-cubic-inch O-470 engine from Teledyne Continental that currently occupies the engine compartment of a Cessna 182 generates 230 horsepower, with each of its six cylinders displacing 78.5 cubic inches. This comparison emphasizes one of the claims by SMA — the SR305-230 engine will be more dependable than a comparable avgas-fueled engine because of reduced complexity and a lower parts count. Equal power from two fewer cylinders bears this out. Reliability can't help but improve over spark-ignition engines primarily because diesel engines are not dependent on ignition system components such as magnetos, ignition wires, and spark plugs to ignite the fuel.

Diesel engines ignite the fuel — which is directly injected into the combustion chamber — by compressing the air charge to a much higher degree than avgas engines do. Compression ratio is expressed by numbers that define the difference between the volume of the space within the cylinder when the piston is at the bottom of travel and the volume of space when the piston is at the top of travel. In production avgas engines this ratio is limited to values of 9.0-to-1 or less to prevent the possibility of spontaneous combustion of the fuel/air charge during the compression stroke of operation. This spontaneous combustion is termed detonation and in extreme cases can cause piston and-spark plug damage. Diesel-style fuel has low volatility. This differs from volatile avgas, which gives off explosive vapors under normal conditions. This characteristic and the fact that diesel-style engines delay fuel delivery until very late in the compression stroke prevent this phenomenon from occurring in diesels. The SMA diesel-fueled engine has a compression ratio of 15-to-1. Compressing the air charge to this degree causes a rapid increase in air-charge temperature that is sufficient to spontaneously ignite the fuel when it is injected. SMA projects the TBO to be 3,000 hours, or 50 percent greater than the avgas engine.

Since magnetos and spark plugs require attention at every inspection, the elimination of these sometimes troublesome and often exasperating — who hasn't suffered through the noisy aggravation of attempting to burn clean a lead-fouled spark plug? ignition system components — adds credence to the SMA claim that installation of the SMA diesel engine also will reduce maintenance costs.

SMA and Safran

SMA was founded in 1997 and is owned by Safran, an international technology group. It was created to specialize in the development, production, commercialization, and support of Jet A-fueled piston engines. The first flight of the SR305-230 engine was in March 1998 in a Socata TB 20 Trinidad airframe. Despite the inevitable setbacks that occur when bringing a new engine to certification, progress has been steady. August 2001 saw European certification for the engine; in less than a year the FAA had certified the engine. In September 2003 the European Aviation Safety Agency (EASA) presented SMA with an STC for installation of the SR305-230 in the Cessna 182Q and F182Q (F denotes French manufacture).

Instead of applying to the FAA for certification via the conformity agreement based on the EASA STC, SMA decided to pursue a completely independent STC for the installation in Cessna 182 airframes. This STC complies with the latest FAR Part 23 regulations. As mentioned previously, this landmark was achieved in July 2006 at that year's EAA convention. SMA has applied to extend the STC to apply to Cessna 182M, -N, -O, and -P models covering years from 1969 through 1976. Installations in the 1977 through 1980 Q models are already approved under the present STC. The company has said that work on an STC for installing the SR305-230 in Cessna 182RG airframes will begin soon. A 300-horsepower version of the engine also is being developed.

Turbocharged plus

The SMA engine differs from avgas engines in additional ways. First off, an avgas-fueled engine's speed is managed by controlling the amount of air admitted into the induction system with a pilot-controlled throttle at the carburetor. A Jet A-fueled engine has no throttle in the same sense that the avgas-fueled engine does — engine speed is varied by increasing or decreasing the amount of fuel injected into the cylinders. This free flow of induction air is one of the keys to efficient diesel-engine operations. The SMA SR305-230 is equipped with a turbocharger to ensure the free flow of combustion air. The diesel engine's voracious appetite for induction air results in manifold pressure of up to 90 inches during high-power takeoff operations.

Because of the potential for destructive pre-ignition and detonation of the fuel-air mixture in an avgas-fueled engine, temperatures and pressures within the combustion chamber have to be closely monitored and controlled, especially in high-performance engines. Three methods are used to control these temperatures and prevent detonation. The first method involves moderating the compression ratio — typical ratios are 8.5-to-1 in a normally aspirated engine. Ratios are lowered to 7.5-to-1 when the engine is turbocharged. This is the result of the elevated temperature of the compressed air entering the engine from the turbocharger, which increases the volatility of the fuel. Lowering compression ratios creates a trade-off because it lowers the thermal efficiency of the engine. The two other methods depend on pilot-controlled fuel-air-mixture manipulations. The traditional technique is to maintain very rich mixtures. Another method is to lean the mixture past peak exhaust gas temperature over to where the mixture is so lean that there's not enough heat energy in the fuel-air mixture to cause heat-related problems. This is called "running lean of peak." Although it's proven, it requires advanced engine instrumentation and pilot education.

None of these problems occurs in a turbocharged diesel engine. In fact, these engines are so free running that they have a different problem — they will continue to accelerate as more and more fuel is pumped into the cylinder combustion chambers. SMA prevents this destructive phenomenon — called diesel runaway — by utilizing an electrically controlled engine control unit (ECU) that automatically limits the maximum fuel delivered at full power to 12 gallons per hour. Fuel consumption at 64-percent power is less than seven gallons per hour.

Although diesel fuel has been mentioned throughout this article, Jet A is the only fuel that is approved for use in the SR305-230. The fuel is delivered to each cylinder from a Bosch mechanical injection pump that is mounted on the accessory case of the engine. The pump pressurizes the fuel and delivers it to individual injectors at approximately 16,000 pounds per square inch. These extremely high pressures atomize the diesel fuel, resulting in a very clean-running engine.

Performance numbers

The big news is the extraordinary fuel-consumption numbers. Brake-specific fuel consumption (BSFC) is used to indicate the efficiency of internal combustion engines. The number indicates the pounds of fuel required to produce 1 horsepower for one hour. The SMA SR305-230 has a BSFC of 0.35 pounds/horsepower/hour. This translates to five gallons of fuel per 100 horsepower. The BSFC for the avgas-fueled engine it replaces is 0.44 pounds/horsepower/hour or 7.4 gallons per 100 horsepower.

The first part of my flight in N715MA consisted of a full-power climb at 88 to 90 knots to 9,500 feet. The first surprise came when Rick Rossner, president of Tule River Aero-Industries in Porterville, California, held us at the end of the runway until the oil temperature had reached operating temperature. This took a couple of minutes even though Rossner had arrived in the same airplane an hour before. The ambient air temperature was 66 degrees Fahrenheit. The second surprise was that there was no magneto check-type runup required. The only pretakeoff tests consisted of pulling a knob mounted on the subpanel once the prop reached 1,700 rpm for a pretakeoff check of the prop governor. At all other times the propeller governor maintains a steady-state propeller rpm of 2,200. This low rpm ensures that propeller-generated airplane noise is minimized.

Aircraft performance charts state that a normal, maximum-gross-weight climb from sea level to 10,000 feet would require 14 minutes and 2.4 gallons of Jet A. Cruise performance charts show three power settings — maximum continuous, recommended cruise, and economy cruise — at standard temperature-lapse-rate temperatures and at plus 30 degrees Celsius (plus 86 degrees F) above international standard atmosphere (ISA) temperatures for the altitudes. Since this engine turbocharger system does not have a wastegate, the manifold pressure drops off as air density decreases. For instance, at 7,500-foot pressure altitude and a standard air temperature of zero degrees C (plus 32 degrees F) a recommended cruise setting of 61 inches of manifold pressure yields a true airspeed (TAS) of 129 knots while consuming Jet A at 9.5 gallons per hour. If the outside air temperatures (OATs) are higher than standard, less power is available. For instance, if the OAT at 7,500 feet is plus 30 degrees C (plus 86 degrees F), the recommended cruise setting must be reduced to 53 inches of manifold pressure, resulting in a TAS of 119 knots while burning 7.4 gallons per hour. The pilot refers to a power-setting chart, inserts the pressure altitude and the OAT, and sets the power for the desired power by adjusting the single power lever to the appropriate manifold pressure.

The third surprise took place as the power lever was pushed forward during the takeoff roll. The manifold pressure gauge wound around to 94 inches. Welcome to the world of diesel engines. Through 3,000 feet — climbing at 800 feet per minute at 88 KIAS — the turbine inlet temperature (TIT) remained low at 1,060 degrees F (571 degrees C). The TIT redline is 1,346 degrees F (730 degrees C). This low number may surprise those who are familiar with TIT redlines of 1,600 degrees to 1,700 degrees F for avgas-fueled turbo installations. This low limit is further evidence of the superior thermal efficiency of diesel engines over gasoline engines. Diesels extract more heat energy from each gallon of fuel than gasoline engines do; therefore, the exhaust stream is cooler. A side benefit includes less heat-related exhaust and turbocharger erosion than in gasoline engines.

One gallon of diesel fuel contains 15 percent more British thermal units than does a gallon of avgas. Diesel fuel also provides a couple of other side benefits. Since diesel fuel is more like an oil than a fuel, it provides very good protection against internal engine corrosion during periods of inactivity. This same characteristic promises that bladder-type fuel cells should last quite a bit longer than the same cells in gasoline-fueled engines.

Oil- and aircooling

The SR305-230 is an air- and oil-cooled engine. Ram air flows over the cylinder barrels before exiting the lower cowling through a large mechanically controlled cowl flap. Engine oil — 9-quart capacity, 7.1 in the sump — is circulated around the combustion-chamber portion of each cylinder to extract the heat produced by the high compression ratios and combustion process. The recommended oil-change interval is 200 hours. There are two large radiators under the cowling. The first provides oil cooling; the second is for turbocharger intercooling. Ram air for both coolers enters through the lower cowling. Because of the cooling-air drag required for the large radiators, it may seem like the SMA-powered 182 must not be quite as fast as a standard 182Q. Owners report that the SMA 182 is every bit as fast as their avgas-powered airplane down low and, because of the turbocharger, is capable of 145 to 150 KTAS when cruising between 9,500 and 12,500 feet.

The engine installation is very straightforward. With the exception of the requirement to cut one small hole in the firewall, the SMA firewall-forward assembly bolts onto the same points as did the Continental engine it replaces. The required hole provides an entry point for hot exhaust air off the oil cooler to enter the original hot-air manifold located on the aft side of the firewall. This hot air provides all cabin heat while yielding an additional safety benefit by completely eliminating the specter of carbon monoxide poisoning that's part of conventional single-engine-airplane cabin and carburetor heating systems.

Manual control

The SMA diesel engine with a three-blade propeller installed weighs approximately 40 pounds more than the TCM O-470 and propeller it replaces. To prevent the center of gravity from moving too far forward, the battery was relocated aft. This compromise seemed to work well. Based on my experience, N715MA flew very much like other 182s I've flown in the past. Like all 182s, when flown with two pilots, no baggage, and a moderate fuel load, a healthy dose of nose-up trim must be cranked in before landing. Despite the additional weight up front, there was no difficulty in flaring the airplane for landing.

According to Thierry Saint Loup, the North America support manager for SMA, the ECU "prevents pilot error." Although takeoff power settings of 80 to 90 inches of manifold pressure sound like extreme values, the engine is built to take them. If the ECU should ever fail, or electrical power is exhausted, the engine will maintain steady-state operation since the mechanical fuel pump is driven off the engine and is not dependent on electrical power to continue to deliver fuel. In the event of the loss of the ECU, the backup mode of engine control is exercised by pulling the throttle lever aft until it's aligned with a point on the pedestal marked by an arrow before the ECU disconnect handle is pulled aft.

This disconnects the ECU and connects the power lever directly to the fuel pump. Since the ECU has been disconnected there is no longer any pilot-error-prevention device between the power lever and the fuel pump. SMA-trained pilots are taught to refer to power and temperature charts to prevent engine damage from overboosts when the engine is in the manual control mode.

Jet A fuel won't start to gel until minus 23 degrees F (minus 30 degrees C), so the engine is suitable for all except the most extreme cold-weather operations. A cold-weather kit is also available.

Firewall-forward kits include all parts — including new upper and lower cowlings — required for modification of the airframe and installation of the engine. N715MA was equipped with a wide-chord, three-blade MT propeller. Hartzell has produced one of its new second-generation advanced composite ASC II three-blade propellers for the SMA engine.

According to Rossner, buyers will have a choice between the two props. Kit price is $78,000. Installation adds around $8,000 for a fly-away price of $86,000. These numbers may sound pretty high until they're compared with the reality of replacing the avgas engine. Firewall-forward replacement of the avgas engine with a new TCM O-470 will cost nearly $30,000 for the engine; a new three-blade prop lists for nearly $9,000; and removal and replacement of the engines — including mount overhaul, new hoses, and labor — probably will be in line with the diesel numbers, for a total of approximately $47,000.

Opting for a factory-remanufactured engine will lower this number by between $4,000 and $5,000. Selling the firewall-forward avgas engine with the propeller, all accessories, and the mount further narrows the gap. Midtime O-470 engines with two-blade McCauley propellers are worth from $12,000 to $18,000 on the used market.

Even after citing a low-end value for the used engine-propeller combination, and choosing a remanufactured engine instead of a new one, the price delta between going back to the avgas-fueled engine and installing a new Jet A-fueled SMA SR305-230 engine ciphers out to around $30,000.

What does a buyer get for the extra $30K? No longer having to worry about the future of leaded avgas, an engine with a 3,000-hour TBO, a greatly reduced possibility of internal engine corrosion, longer fuel-bladder life, less chance of fire in the event of an off-airport landing, and fewer engine management chores, as well as a reduction in operating costs of around 30 percent.

E-mail the author at [email protected].