Liquid cooling — yea or nay?
The great aviators of yesteryear flew behind one. Eddie Rickenbacker pushed the throttle of one when he flew with the Hat in the Ring Squadron. Wilbur and Orville's Wright Flyer was powered by one. Dick Rutan and Jeana Yeager had one in the Voyager — and, except for a four-minute period, it ran continuously for more than nine days. We're talking about liquid-cooled engines.
In fact, until Charles Lindbergh made his famous single-engine solo flight in an airplane powered by an aircooled Wright J-5 radial engine, airplanes with liquid-cooled engines held most of the altitude and speed records. Yet today only a few production airplane engines are liquid cooled. In 2002, the Extra 400, a composite-construction, pressurized, retractable, high-performance four-place airplane that's manufactured in Germany (see " Extra 400: Classy Business," June 2000 Pilot) is the only airplane currently in production that's powered by a liquid-cooled engine. This is in spite of the fact that the same liquid-cooled engine that pulls the Extra through the ether has been certified for nearly 20 years.
RAM Aircraft, of Waco, Texas, markets a liquid-cooled engine retrofit for Cessna 414A twin-engine airplanes. For nearly a decade RAM replaced both 310-horsepower aircooled engines with new 350-hp Teledyne Continental TSIOL-550 engines on these pressurized twins. Until he retired five years ago, Jack Riley Jr. was the chief engineer and owner of RAM.
RAM's liquid-cooled Chancellors
When asked why his company spent the time and money to develop a liquid-cooled engine STC, Riley has this to say: "RAM always tried to achieve automotive-type durabilities in our engines. Even though we continued to fine-tune our engines and kept a tight rein on quality, we still had to absorb warranty costs dealing with cracked cylinders and cracked cases (in aircooled engines). We could have doubled our income if we got rid of the warranty costs.
"I can't stand doing things substandard, so when Continental came to us with a proposal that we develop a program to install their liquid-cooled engines on Cessna twins we were interested," says Riley. Teledyne Continental Motors had developed a liquid-cooled version of its successful big-bore 550 series of engines and was eager to sell its new engine to airplane manufacturers. Retrofitting the new engine to an existing airframe was the first step in the process.
From 1988 to the end of 1996 RAM converted 37 Cessna 414A Chancellors. It was an expensive proposition. "The conversion took 1,100 man-hours, but it eliminated the headaches associated with shock cooling and cylinder temperature management," says David Seesing, RAM special products sales manager. "Once we got the bugs worked out, the owners got an airplane that could climb to 30,000 feet and cruise at 238 knots."
Teething problems
"When TCM first sent us the engines they were certified with Mobil AV-1 oil and Prestone antifreeze," says Riley. Neither of these products worked well for aircraft applications. Mobil AV-1, an all-synthetic oil, was later removed from the market.
"By the third year we had figured out what was causing our problems, but by then the program had gotten off to a bad start," says Riley.
This bad start, coupled with a very low production rate of piston-powered twins in the late 1980s, put a damper on TCM's hoped-for liquid-cooled engine sales. Plus, reviews from the field have been mixed.
C.A. Parks of Chattanooga, Tennessee, owns one of the later RAM-converted 414As — serial number 32.
"We've been real happy with it. It's been a good airplane with very few problems. It's great to be able to get up and down and keep that CHT right where it's supposed to be," he says.
Parks is the kind of customer every manufacturer likes to hear from — the happy one who feels he made a good decision. Unfortunately not every owner is so positive. One owner, referred to AOPA Pilot by RAM, is on his third set of cylinders in yet another attempt to find a solution for oil-fouled spark plugs.
Dick Rutan, the Voyager, and the IOL-200
The liquid-cooled rear engine of the Rutan Voyager, which set a world record in a nine-day nonstop flight around the world in 1986, was pivotal in the success of the flight.
"The Voyager took off with a much smaller payload because of the fuel efficiency of the IOL-200 engine," says Rutan.
The IOL-200 is more than a Teledyne Continental O-200 (most commonly used to power the Cessna 150 series of airplanes) with water jackets installed on the cylinders and an add-on fuel injection system. According to Ron Wilkinson, who oversaw the liquid-cooled engine programs for TCM, this engine was developed by TCM to fulfill a U.S. government contract calling for a small engine that could drone along at very high altitudes while using very little fuel. TCM met these goals by designing new cylinders with high-swirl combustion chambers and converting to liquid cooling. The revolutionary (for light airplane engines) design resulted in a very efficient engine that could maintain its power output while bettering existing engine fuel consumption figures by 8 to 10 percent.
"I leaned the rear engine of the Voyager to 150 degrees [Fahrenheit] lean of peak initially, then I fine tuned it. I ran it that way for over 700 hours and never had any problems," says Rutan.
The Voyager engine was very efficient. The high-swirl combustion chambers thoroughly mixed, or swirled, the incoming fuel-air mixture so that the molecules of fuel and air were evenly distributed throughout the combustion chamber. Even mixing and distribution of the fuel-air mixture is critical for efficient operation.
The fuel-air mixture needed to support combustion varies from between 8 parts air to one part fuel to 18 parts air to one part fuel by weight. The "best power" range of engine operation occurs when the mixture is between a 12-to-1 and 16-to-1 air-to-fuel ratio. When uneven mixing or distribution of the fuel-air mixture occurs in the combustion chamber, the result is localized leaning within the combustion chamber. High pressures and temperatures within the combustion chambers result in uncontrolled combustion, or explosive burning, during the combustion cycle. This is termed detonation and always lowers power output. In extreme cases it can cause engine damage. High-swirl combustion chamber technology lessens the possibility of detonation. It also allows the engine manufacturer to increase compression ratios.
The compression ratio is the ratio of the volume of space in each cylinder when the piston is at the bottom of its stroke to the volume of the space when the piston is at the top of its stroke. The Voyager engine ratio was 11-to-1. Production aircooled airplane engines typically top out at approximately 8.5-to-1. The liquid-cooled TSIOL-550 engine cited above has a compression ratio of 7.5-to-1. The lower compression ratio in the TSIOL-550 engine is a design change that's incorporated when these engines are turbocharged. The lower compression ratio offsets the effect of decreased detonation margins caused by the high inlet air temperatures from the turbocharger compressor. Increasing the compression ratio is desirable because it results in a lower specific fuel consumption (defined as pounds of fuel burned per hour per horsepower) and better thermal efficiency (defined as the ratio of heat converted to useful work to the heating value of the fuel consumed).
Naturally a question arises — if high compression ratios help utilize fuel better, why aren't the compression ratios in production engines higher? More efficient engines would mean that airplanes wouldn't have to carry so much fuel; therefore, they could carry more payload. The simple answer is that none of today's aircooled high-performance engines utilize high-swirl combustion chamber technology.
Liquid Cooled Air PowerIn 1997 Liquid Cooled Air Power appeared at EAA AirVenture in Oshkosh with liquid-cooled cylinder assemblies bolted onto two of the most popular Lycoming engines — the 180- to 200-hp, four-cylinder 360 series and the 235- to 300-hp, six-cylinder 540 series.
Company President and CEO Bob Atkins offered a set to Rutan for installation on Rutan's VariEze kitbuilt airplane. Rutan installed the cylinders and flew the plane to Oshkosh, averaging 7.5 gallons per hour of fuel consumption.
Rutan again: "We required one-third of the air that is needed to cool aircooled engines — this led to less cooling drag. I'm a real strong advocate of liquid cooling in aircraft engines." In spite of his enthusiasm for liquid cooling, Rutan was, on the whole, disappointed and later removed the cylinders.
Atkins tells why, "After last year's efforts at the Sun 'n Fun and Oshkosh fly-ins, and some flight-test time, we decided to go to revision three." Atkins and Rutan learned that water cooling the existing cylinder technology didn't make a big enough difference to make the change worthwhile. When the tolerances are tightened up, and a more efficient combustion chamber is incorporated, Atkins thinks his liquid-cooled cylinders will be winners.
Rutan was forced — and it's likely that he was kicking and screaming — to return to the current state of general aviation engines when he had to reinstall the aircooled cylinders on his Lycoming IO-360 engine. "I just had to pay $920 to get two cylinders reworked after 420 hours. The cylinder head temperatures never got over 340 degrees, but I couldn't keep the valves from overheating," says Rutan.
That's the dilemma. The current crop of aircooled general aviation engines is finely balanced on a tightrope between the need for dependable (nondetonating) power and the costs of running cylinders at elevated cylinder head and exhaust gas temperatures. Aggressive leaning elevates temperatures while not leaning increases direct operating costs. Rutan's (aircooled) engine man told him that he could pay now (by running the engine rich) or he could pay later (in cylinder repair costs).
Loose tolerances
Having been taught in ground school that the standard temperature lapse rate is 3 degrees F for every 1,000-foot gain in altitude, many pilots mistakenly think that high cylinder head temperatures are only problematic during low-altitude, high-ambient temperature operations. Actually, high-power operations at high altitudes are more likely to nudge the upper range of temperature limits because the existing air molecules that carry away the heat from cylinder cooling fins (and oil cooler fins) are less densely packed than at lower altitudes.
Because of the wide range of operating temperatures they're subjected to, aircooled engines are built with wide (loose) tolerances in order to accommodate the variances of thermally driven expansion and contraction of the aluminum parts such as the piston and cylinder head, and steel parts such as the cylinder barrel. Only aircooled aircraft engines have to contend with cylinder head temperatures that can range from below zero to 525 degrees F. The wide range of operating temperatures and the strenuous duty cycle (65 to 100 percent power output almost continuously) present a dilemma to the designers: how to design a dependable, efficient engine that can tolerate extremely wide operating parameters and environments. Without a dependable method of stabilizing cylinder expansion and contraction the only solution is to build in loose (and therefore not too efficient) tolerances.
Poor ring sealing caused by the wide combustion ring end gaps lets high-temperature, lead-contaminated combustion gases leak down past the rings. This puts a heavy burden on the lubricating oil, and it becomes contaminated in short order. Another result of ring leakage is elevated crankcase pressures. As these pressures are vented, oil fog within the crankcase is also carried overboard, increasing oil consumption.
Because of the difficulty in providing an adequate flow of cooling air over every part of each cylinder, high-power operations in high-altitude, high-temperature conditions can result in cylinders becoming out of round, a distortion that lessens the ability of the rings to prevent combustion pressure leakage into the crankcase.
Liquid cooling controls cylinder temperatures
Many pilots easily grasp that liquid cooling an aircraft engine will eliminate shock cooling, a term describing stresses in the cylinder metallurgy caused by rapid temperature changes. There are larger advantages. Controlling and stabilizing the cylinder temperature range allows the engineers to tighten up the engine by designing in closer tolerances. The result is an engine that runs cooler, is more efficient, and is less likely to break down from heat and lubrication distress. Tighter tolerances also result in less oil usage.
"The rear engine of the Voyager used one quart every 50 hours; my aircooled engine uses one quart every five hours," says Rutan.
One harbinger of the future for general aviation is in the ever-expanding fleet of homebuilt airplanes for personal use. More than 15 percent of the registered airplanes in the United States are amateur built. An increasing number of homebuilders are experimenting with liquid-cooled engines. For a peek into the future in this arena, go to your favorite Internet search engine and start a search for "liquid-cooled airplane engines."
If piston-powered engines are going to continue to be the most reasonably priced powerplant for general aviation airplanes, high-technology combustion chambers and liquid cooling, combined with advances in fuel delivery and ignition monitoring systems, should take these engines well into the next century.
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