SUBARU ENGINES

The Package

Before shipping our engines, each one is configured on a test stand exactly as it would be in the customers airplane, then test run while simultaneously performing a diagnostic check. The engines arrive on a pallet already mounted to the engine mount.

We understand that building an experimental aircraft is a tremendous challenge with more work than most builders could ever imagine. By the time the aircraft is complete, the convenience of purchasing an engine that simply bolts in and hooks up, certainly could be appreciated.

When buying a traditional air cooled aircraft engine, the price does not include the purchase or fabrication of engine mount, and exhaust system, baffling or a host of other parts such as hoses, ducts, clamps etc.

When rebuilding an aircraft engine you are dealing with the tedious process of cleaning it and tearing it down, all the while hoping the extremely expensive components such as the crank shaft, camshaft and cylinders will pass inspection. Then you go on to rebuild the alternator, magnetos sand starter. when all is said and done, you still have an old engine in design as well as components, possibly rebuilt several times previously.

The 2,457 cc boxer Subaru based aircraft engine produces 165 horsepower at 5600 rpm and 300 LB-ft of torque at 4000 rpm. 75% cruising power is at 4,200 rpm.

The 2.5 four-cylinder engine is a horizontally-opposed engine. This boxer engine has been perfected to capitalize on all the advantages of this design.

The modern SUBARU engines manufactured by Fuji Heavy Industries in Japan, a company involved in aerospace, is a very logical choice for an aircraft builder looking for a modern engine for his or her modern airplane.

The basic layout of the engine block and cylinders is identical to that of the traditional aircraft engines in that they are horizontally opposed. The main difference being that the SUBARU is liquid-cooled and the others are cooled by air. Air cooled engines are forced to run within a large temperature range, making it necessary to provide large internal clearances. The SUBARU, being liquid cooled, maintain exact tolerances and tight clearances within the engine for efficient operation and long life. A thermostat provides stable engine temperatures not depending on power output, aircraft speed, outside temperature or altitude.

Subaru boxer engines have a "built-in" feature that contributes to enhanced engine life and reduction of unwanted airframe vibration. A Subaru based aircraft engine is inherently balanced. The pistons are opposite from one another and because of the engine’s unique firing order, vibration caused by the combustion process cancels before it can cause undue stress on reciprocating parts such as the crankshaft and its bearings.

Cylinder Block/Crankcase

The engine’s crankcase is made from lightweight aluminum alloy die casting. More expensive than sand casting, die casting eliminates excess metal and reduces porosity through a better grain structure in the metal. This also has the added benefit of increasing the strength of the crankcase and its rigidity.

One reason for the boxer engine’s excellent rigidity is that the crankshaft is fully supported by the case. In an in-line engine the crankshaft is affixed to the crankcase by five main bearing caps bolted to the crankcase in a distance of only eleven inches (this puts a main bearing every two inches). Combustion forces in an in-line engine are vertical and put tremendous loads on the crankcase and its main caps.

A boxer engine does not use main bearing caps to hold its crankshaft in place. Instead, the crankshaft is located between both case halves. In effect half of the engine supports one side of the crankshaft while the other half of the engine supports the other side of the crankshaft.

In addition to the entire crankcase controlling piston forces there are five main bearings in which the crankshaft rotates. While most in-line four cylinders also have five main bearings, keep in mind that the overall length of the boxer engine is only 12". The crankshaft is much shorter and is better supported than in a typical in-line four cylinder engine. Because the crankshaft is shorter, it is also lighter allowing a freer revving engine.

The crankshaft receives special machining on the corners of its journals and webs consisting of a rolled-edge design which further enhances strength. This same machining is used on the crank pins and webs. Another feature of the engines bottom end include copper-over-lead alloy bearings for better wear and longer life.

This Subaru based aircraft engine use a full-flow type lubrication / filtration system. The oil pump is directly driven by the engine’s crankshaft. An oil separator is used at the rear of the right cylinder to control oil mist in the blow-by gasses.

The oil filter is conveniently located for easy maintenance. Oil capacity is 5.7 quarts. Oil viscosity is 5W-30.

Pistons and Rings

The pistons ride in cast-iron liners (dry sleeves) which are cast into the block for long life and low friction. The piston skirt has a "slipper" design to reduce weight and sliding friction. A molybdenum coating is applied to the skirt to further reduce friction.

A boxer engine uses piston pin offset to control cylinder wear. This offset compensates for the thrust forces generated by combustion. Interestingly one side of the engine requires an opposite offset direction from the other. The pistons in cylinder one and three (the pilot's side) have their offset in the lower direction while the pistons in cylinders two and four (co pilot’s side) have their offset in the upper direction. This way thrust force is controlled and engine life is increased.

To help the engine rev easier low mass pistons are used. And to help increase low speed torque the piston deck height is comparatively large. The piston face has been designed to allow the 9.7:1 compression ratio without the risk of detonation. Finally, three piston rings are used on each piston. Two rings control compression and the third is a slit-type oil-control ring. To further control oil consumption the top ring has an inner-bevel design and the second ring uses an interrupt design.

Cylinder Heads and Valves

Positioning the valves at a 30-degree included angle allows for a compact cylinder head design. The heads (there are two in a boxer engine) are made from lightweight cast-aluminum alloy for quick even warm-up with the crankcase and to aid in combustion-chamber cooling efficiency. In addition the water jacket is designed to further improve cooling efficiency. This allows a more advanced initial ignition timing setting without fear of engine knock.

The combustion chamber is a compact, center-plug, pent roof type that has a wide "squish" area for more complete combustion efficiency.

Cylinder-head gasket material is a carbon compound made to withstand high combustion temperatures and pressures. To further increase resistance to heat and wear a metal core is used on the gasket. The core is held in place with dowels that positively locate it over the cylinder bore.

The 2.5-liter engine uses overhead camshafts. Each cylinder head has a camshaft that regulates four valves for each cylinder. In turn, the four valves are arranged in a cross-flow design for compactness and enhanced cylinder filling and scavenging.

Camshaft timing uses a low valve overlap creating a smooth idle and good low-end torque.

Fuel and Ignition Systems

Eggenfellner Advanced Aircraft is determined to promote the latest in electronic engine operating systems. Solid state ignition and multi port fuel injection is used.

The injection system supplies the optimum air fuel mixture to the engine for all the various operating conditions through the use of the latest electronic technology. With this system, fuel, which is pressurized at a constant pressure, is injected into the intake air passage of the cylinder head. The injection quantity of fuel is controlled by an intermittent injection system with the electromagnetic injection valve (fuel injector) opening only for a short period of time, depending on the quantity of air required for one cycle operation. In actual operation, the injection quantity is determined by the duration of an electric pulse applied to the fuel injector and this permits simple, yet highly precise metering of the fuel.

Further, all the operating conditions of the engine are converted into electric signals which results in additional features of the system such as large improved adaptability, easier addition of compensating elements, etc.

The multi point fuel injection also has the following features: Reduced fuel consumption, increased engine output, superior acceleration and deceleration, superior stability and warm-up performance in cold weather since compensation is made for coolant and air intake temperature.

The heart of this system is the electronic control computer (ECU) which manages fuel injection and ignition systems and features learning, self-diagnosis, limp-home and anti-knock capabilities.

This 2.5 liter aircraft engine uses a sequential multi-point fuel-injection system for improved performance, better fuel efficiency, flyability and reliability. A mass air flow meter is used to sense air temperature, density and quantity.

The throttle control system uses a learning-control software that senses when the engine is idling and, in turn, adjusts operating parameters to ensure a smooth idle (most fuel injection systems use an on/off switch to signal an idle condition and to a set idle mode). This system has the ability to compensate for changes that naturally occur during an engine’s life.

Bi-directional fuel injectors are used to give a better fuel atomization for cold engine operation. They have the added benefit of increasing performance. At start-up the sequential injection system changes over to simultaneous injection for quick and efficient starts.

The ignition system is a distributor less design. Platinum-tipped spark plugs are used and don’t require any maintenance until 500 hrs. Both the fuel injection and ignition systems are controlled by a 16-bit engine-management computer with learn and self-diagnosis control.

The self-diagnosis system detects records and warns of faults in many input and output devices for the electronic control unit. A warning lamp illuminates on the panel to inform the pilot of a detected fault. A fail-safe (limp home) feature is employed if necessary to ensure flyability should a failure occur in some critical components. All engines use OBD-II standards for universal scan tool usage. (On Board Diagnostic)

Troubleshooting the engine is no problem with a modern scan tool. (about $299)  

Exhaust System

Equal-length exhaust tubes are used. This helps reduce exhaust noise and vibration while maximizing engine breathing efficiency. An oxygen sensor provides combustion efficiency feedback to the fuel injection computer.

Engine Electrical System

A lightweight 35A alternator is used. All engines use a reduction-type starter. The advantage of this type of starter is sure starts in all weather due to a very low current draw. By using a reduction gear in the starter its electrical demands are significantly reduced. Dual batteries are used for electrical redundancy.  

4 CYLINDER