Turbine wheels come in many shapes and sizes. Constructed of high temperature alloys such as Inconel or Hastaloy. This is the hot end.
April 27/98
With automotive engines finding increasing use powering homebuilt aircraft, we find many people entertaining the idea of turbocharging to boost the power to weight ratios and altitude performance of these engines. We will examine the various facts and myths regarding turbocharging as applied to light aircraft use.
History
Turbos in aircraft were first successfully applied in mass numbers during WWII. The P-38 and P-47 were the best known turbocharged fighter types and both had excellent high altitude performance.
In general aviation use as applied to air cooled opposed engines, turbocharging has a less revered reputation. Much of this has to do with the air cooled engine itself and the fact that most of these engines were never designed to be boosted to begin with. They may have been modified a bit for the turbo but with power to weight ratios being a major concern, many are structurally and thermally not up to the task. Other concerns such as turbo cooling and lubrication by inferior aviation type oils also led to premature turbo failures.
What is a Turbocharger?
A turbo consists of a centrifugal compressor connected to a turbine wheel which is spun at high rpm by the energy of the engine exhaust gasses. It is a very simple device, but the high temperatures and stresses acting on it require exotic materials in the turbine section which are somewhat expensive. A turbo will cost in the neighborhood of $800 to $1300 with the wastegate assembly.
Turbine wheels come in many shapes and sizes. Constructed of high temperature alloys such as Inconel or Hastaloy. This is the hot end.
Turbine housings are cast from a high nickel/iron alloy.
The compressor pressurizes the intake manifold to achieve higher hp or maintains sea level pressure as the aircraft climbs. This permits higher climb rates at altitude and increased cruise speeds.
Compressor wheels are cast from aluminum alloy and come in many sizes or trims. This is the cold end.
Compressor housing is cast from aluminum alloy. Shape is designed to slow the air, thus compressing it.
Cast iron center section or bearing housing contains floating sleeve bearings or more recently ceramic ball bearings. High pressure oil from engine feeds the bearing and is drained out the bottom of the housing back into the engine. Some housings are water cooled to prevent coking of oil deposits leading to bearing failure.
Elegant Simplicity
Turbocharging is the most efficient method of boosting hp with the least weight penalty known. A typical turbo installation on a light aircraft including intercooler and exhaust is around 35-50 pounds. Such an installation is capable of tripling hp at sea level or adding 50% to the naturally aspirated output up to 15,000 feet. Sea level manifold pressures can be maintained to 25,000 feet in some cases. An aircraft capable of 200 knots at sea level would true out at nearly 300 knots with a good turbo system at high altitude.
Spinning the compressor with an exhaust driven turbine is a much better way than by the mechanical means as in a supercharger. It is lighter, more reliable and more efficient and has the added advantage of having more drive energy available as the aircraft climbs due to lower backpresure, which is exactly what is needed.
Fuel Flow vs. Power
There is a common misconception that turbocharging increases fuel flows because of the backpressure of the turbine. This may be true in air cooled engines using a poorly matched turbo without intercooling because the air cooled engine cannot maintain temperature stability without adding fuel for cooling purposes. Reduced fuel flows at altitude in naturally aspirated and turbine engines is not due to some magic process, it is due to the fact that as altitude is increased, less power is produced thus less fuel is required. Many people also don't seem to realize that fuel flow is related to hp developed and that a turbo engine will develop more power at altitude than a naturally aspirated engine, obviously requiring more fuel and delivering higher speeds at the same time.
Testing done back in the '40s and '50s on turbo engines actually confirm that fuel flows were LOWER at the same BMEP with turbocharging than in a naturally aspirated engine developing the same power. This suggests that when properly applied, the turbo actually recovers more energy from the exhaust to be applied to reduce pumping losses during the compression stroke than is consumed during the exhaust stroke working against higher backpressure. With approximately 50% of the energy in the fuel going out the exhaust in the form of heat and pressure, turbocharging can recover some of this waste and put it to good use.
It was shown that turbos could achieve fuel flows 3-6% lower and turbocompounding could achieve up to 15% lower fuel flows. Cylinder head and piston cooling were shown to be the primary limitations to achieving lower BMSFC figures. We can conclude from this that as applied to liquid cooled engines with their superior thermal rejection rates, turbocharging really has few disadvantages compared to the many advantages it offers.
It should be no surprise that the latest certified liquid cooled engines from Toyota and Orenda are turbocharged. People also should remember the awesome turbocompound radials of the '50s. These engines offered extremely low BSFC and high power. It seems foolish not to turbocharge most good aircraft engines used at any kind of altitude as you are throwing away performance without it.
Turbocharged Air Cooled Engines
As mentioned earlier, turbocharged , conventional opposed air cooled engines have a less than stellar track record and many lay people blame the turbo. In fact, the fault lies with the engines it is applied to. The average Lycoming or Continental has a lot of things not going for them. Many of these engines suffer premature failure of crankcases, cylinders, heads and valves in their turbocharged iterations. Let's examine why.
Probably the leading cause of case and barrel cracking is the lack of rigidity these engines have in the cylinder/ case area. For ease of barrel replacement I assume, the designers of these engines chose to bolt the cylinders individually to the crankcase at the bottom of the cylinder using a flange. This arrangement creates a very willowy structure and with each firing impulse, combined with the sheer mass of the huge reciprocating parts used in these engines, a definite high amplitude, cyclic stress is put on these parts eventually leading to cracking. The increased gas pressures associated with turbocharging compound this problem. This design problem is actually the cause of a major portion of the high vibration levels associated with conventional aircraft engines.
The flange method puts the barrel under a tension load with every firing impulse too which is just plain stupid given the thickness of the cylinders. Modern automotive racing practice would use through studs to retain the barrels, thus putting the barrels under compression.
Cylinder head cracking and exhaust valve burning and sticking are primarily due to insufficient cooling in these areas which is exaggerated by the extra heat introduced by turbocharging. Air cooling is relatively inefficient at sinking heat off around the exhaust port and valve seat area. This problem is compounded by the use of very low compression ratios resulting in higher EGTs thus even higher heat flux in the critical exhaust port area. Typical air cooled aircraft engines run cylinder head temperatures in excess of 400 degrees on a regular basis. With aluminum alloys losing roughly 50% of their strength at this temperature it is easy to see why cracking is all too common here. The high EGT's are also detrimental to the life of the turbo's turbine section.
Valve problems are due to the same cause, temperatures being too high from inefficient heat transfer. Many factory turbo aircraft are forced to either open cowl flaps or richen the mixture at high altitude to keep temperatures within limits. With power being maintained at altitude and air density falling off, there is often insufficient mass flow available for cooling. This is rarely a problem on naturally aspirated engines as power and cooling requirements are dropping off with increasing altitude.
Not a lot of design went into the induction system on many of these engines hence fuel distribution is usually not the greatest. This contributes to engine roughness and problems with leaning. Leaning is limited by the mixture in the hottest cylinder with the others running richer than desired.
The oils used in aircraft engines are relatively crude by modern synthetic automotive standards and consequently not the best for the turbocharger. The use of these oils appears to be a result of the very loose tolerances required in an air cooled engines because of the high operating temperatures which also leads to the high oil consumption characteristic of these engines.
Finally, many certified installations never used intercooling which is simply amazing. The advantages and necessity of it was clearly understood in WWII. High charge temperatures at altitude required even more fuel for cooling and yet more cowl flap opening to control head temperatures.
Turbocharged Liquid Cooled Engines
Liquid cooled engines are a natural for turbocharging because of their higher heat rejection capabilities. Water cooling also enables the turbo center section to be water cooled which alleviates a primary cause of turbo failure- coked bearing holes from high heat and fried oil. Water cooled engines can use modern synthetic oils because of their tighter clearances. Synthetic oils with their much higher temperature capabilities are more suitable to lubricate the turbo than old fashioned, aircraft oils derived from natural base stocks. The popular Mobil 1 synthetic oil, common in the racing world, can withstand temperatures of 350 degrees continuously and even 450 degrees intermittently without significant degradation. With proper lubrication and cooling, turbochargers will easily last the life of the engine.
If we compare the Popular Subaru EJ22 and EJ25 horizontally opposed 4 cylinder engines to their certified counterparts we find many advantages especially in turbocharged trim. The Subaru has 5 main bearings instead of 3 which means that the crank is better supported. The Subaru has a higher compression ratio for higher thermal efficency. Its overhead cam, 4 valve per cylinder arrangement provides lower friction and higher volumetric efficiency than a 2 valve pushrod setup. Finally, the integrated block and head design is many times stiffer and stronger than the aircraft engine.
As long as the radiator and ducting are properly designed, cooling problems are not a concern on the liquid cooled engine.
The induction system on the Subaru has proper runner lengths to boost torque, relatively equal airflow to each cylinder and can easily use a modern digital, electronic fuel injection system to precisely meter fuel. All of these things aid in producing more power with less fuel.
Power vs. Weight vs. Manifold Pressure vs. Life vs. Cost
The EJ engines when turbocharged and equipped with a radiator, PSRU and composite propeller are slightly heavier than the popular Lycoming and Continental 360 cubic inch engines which are rated at 180- 200 hp for takeoff and around 135- 150 hp for maximum cruise. The Subaru can easily attain 200- 250 hp at 50 inches for takeoff and reliably deliver 140 to 170hp at 35-38 inches for cruise. The 3.3L EG33 six cylinder is capable of 300+ hp for takeoff and 200-250 hp in cruise making it a viable alternative to the O-540 engines. These power levels can be maintained during climb and cruise at altitude thanks to turbocharging and liquid cooling.
Projected TBO is in the 1000-1200 hour range at this time. Most people flying the EJ engines are confident that no interim work will be required during this period such as cylinder barrel replacement and valve grinding operations which are quite common on aircraft engines before reaching their stated TBO. In other words, if you change the oil and check the water, that is about all that is required during those 1000 hours. There are no magneto or even valve adjustments to worry about. Expect spark plug replacement every 100- 250 hours or so at $3 each. Overhaul costs will range from about $500 to $1500 for parts and machine work depending on condition. Labor would be in the $500 to $1200 range. We are all familiar with the costs to overhaul a certified aircraft engine.
Matching a Turbocharger
As applied to the popular Subaru EJ22 and EJ25 engines for general purpose use below 15,000 feet we can narrow the choice of turbos down a bit. Garrett T3 turbos are recommended as they are relatively cheap, reliable and easy to fix with a vast array of combinations available. The compressor side may be either a -50, -60 or super 60 wheel with a stage 3 turbine wheel in a .82 A/R housing, depending on the exact use and hp. These may be ordered with an integral wastegate and a water cooled bearing housing which are both highly recommended. These turbos are quite capable of producing enough boost to extract 250 hp at sea level and still have 175+ hp available at 15,000 feet.
The integral wastegates are generally more reliable than an external type especially on an aircraft application in which it is almost constantly bypassing exhaust. The wastegate is simply a valve used to bypass exhaust gasses around the turbine to control turbine and compressor speed and thus boost pressure. It is usually actuated automatically by a diaphragm sensing manifold pressure.
Many people have tried to use the OE turbo on auto aircraft conversions, often with limited success. The turbos on cars were not matched to operate at high altitude and continuous high power settings so they are mismatched and inefficient when used on aircraft. High exhaust back pressures and overspeeds with consequent catastrophic failure are not uncommon. In extreme cases, compressor or turbine wheel bursts can cause engine damage and failure due to part ingestion or oil loss. This is not a good idea. Use a turbo which is professionally matched for your engine and intended use by someone familiar with high altitude turbocharging.
Intercooling
Intercoolers are heat exchangers placed between the compressor and intake manifold. Their purpose is to reduce the temperature of the compressed air before it enters the engine. Whenever air is compressed, its temperature increases. In the case of a high boost turbo at high altitude, the air exiting the compressor may reach 300-350 degrees F. This lowers the mass flow of air into the engine and reduces power. It also raises EGTs and lowers the detonation limits. All of these things are detrimental to performance and engine longevity.
An effective intercooler will lower the charge temperature to within a few degrees of the ambient temperature.
Exhaust Systems
The exhaust system on turbocharged engines is critically important. High temperature materials such as stainless steel or Inconel should be used for their construction to ensure longevity. Tubing thickness should be a minimum of .060 inch to withstand the constant 1500 degree temperatures. Careful attention to thermal expansion is necessary so that cracking possibilities are minimized. Bellows or slip joints are often required at junctions. The turbo itself should never be supported by the headers. A strong, triangulated structure should support the weight of the turbo on the engine.
If possible, the primary header tubes should be equal length and mandrel bent for low restriction and maximum pulse energy with equal pulse spacing. The turbine discharge pipe should be 2.25 to 2.5 inches to minimize backpressure. A muffler is often unnecessary to reduce exhaust noise as the turbo usually does a good job of this. This helps to offset the weight of the turbo installation to some degree.
Future Developments
Watch the Aircraft section for pictures and updates on turbo installations as they become available. We are currently flying our turbocharged EJ22 powered RV6A seen elsewhere on this site.
R.F.